atkin cationic adsorption acis 2003

Advances in Colloid and Interface Science103 (2003) 219–304

0001-8686/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0001-8686(03)00002-2

Mechanism of cationic surfactant adsorption atthe solid–aqueous interface

R. Atkin , V.S.J. Craig , E.J. Wanless *, S. Biggsa b c, d

School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UKa

Department of Applied Mathematics, Research School of Physical Sciences and Engineering,b

Australian National University, Canberra, ACT 0200, AustraliaDiscipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle,c

Callaghan, NSW 2308, AustraliaSchool of Process, Environmental and Materials Engineering, University of Leeds,d

Leeds LS2 9JT, UK

Abstract

Until recently, the rapid time scales associated with the formation of an adsorbed surfactantlayer at the solid–aqueous interface has prevented accurate investigation of adsorptionkinetics. This has led to the mechanism of surfactant adsorption being inferred fromthermodynamic data. These explanations have been further hampered by a poor knowledgeof the equilibrium adsorbed surfactant morphology, with the structure often misinterpreted assimple monolayers or bilayers, rather than the discrete surface aggregates that are present inmany surfactant–substrate systems. This review aims to link accepted equilibrium data withmore recent kinetic and structural information in order to describe the adsorption process forionic surfactants. Traditional equilibrium data, such as adsorption isotherms obtained fromdepletion approaches, and the most popular methods by which these data are interpreted areexamined. This is followed by a description of the evidence for discrete aggregation on thesubstrate, and the morphology of these aggregates. Information gained using techniques suchas atomic force microscopy, fluorescence quenching and neutron reflectivity is then reviewed.With this knowledge, the kinetic data obtained from relatively new techniques with hightemporal resolution, such as ellipsometry and optical reflectometry, are examined. On thisbasis the likely mechanisms of adsorption are proposed.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Surfactant aggregates; Adsorption to silica; Surfactant adsorption; Adsorption mechanism;Adsorption kinetics

*Corresponding author. Tel.:q61-2-4921-8846; fax:q61-2-4921-5472.E-mail addresses: [email protected](E.J. Wanless),

[email protected](R. Atkin), [email protected](V.S.J. Craig),[email protected](S. Biggs).

220 R. Atkin et al. / Advances in Colloid and Interface Science 103 (2003) 219–304

Contents

1. Introduction ............................................................................................ 2212. General surfactant and substrate properties..................................................... 2232.1. Silica surface chemistry....................................................................... 2232.2. Surfactant properties........................................................................... 225

3. Adsorption isotherms................................................................................ 2253.1. Introduction...................................................................................... 2253.2. Traditional analysis............................................................................. 2253.2.1. The two-step model..................................................................... 2273.2.2. The four-region model.................................................................. 2293.2.3. Similarities between models........................................................... 230

3.3. The influence of surfactant chain length.................................................. 2303.4. The role of surface charge................................................................... 2303.4.1. Increases in surface charge with adsorption........................................ 2313.4.2. The common intersection point....................................................... 2333.4.3. Adsorption model based on the cip.................................................. 2353.4.4. The influence of surface preparation................................................ 2353.4.5. Comparison of adsorption mechanisms on raw and acid washed silica..... 2363.4.6. Surface charge and gemini surfactant adsorption................................. 2383.4.6.1. The importance of the spacer group........................................... 2383.4.6.2. Gemini surfactant adsorption isotherms...................................... 240

3.5. Evidence for discrete aggregation from adsorption isotherms........................ 2423.6. Calorimetry ...................................................................................... 2433.6.1. The importance of surface water..................................................... 2443.6.2. Calorimetry and adsorption mechanism............................................. 2443.6.3. Interactions between the hydrocarbon tail and the surface..................... 246

3.7. Summary of adsorption isotherms.......................................................... 2474. Atomic force microscopy........................................................................... 2484.1. Introduction...................................................................................... 2484.2. The earliest images of surfactant aggregation: CTAB on graphite.................. 2494.3. Graphite strongly orientates surfactant aggregates...................................... 2514.4. Adsorption studies on mica.................................................................. 2514.4.1. Alkyltrimethylammonium halides on mica......................................... 2524.4.2. The influence of electrolyte on aggregate morphology.......................... 2524.4.3. Gemini surfactants on mica............................................................ 253

4.5. Adsorption studies on silica.................................................................. 2544.5.1. The influence of electrolyte and counterion type................................. 2544.5.2. Adsorption kinetics measured by AFM............................................. 2544.5.3. The influence of counterion polarisability.......................................... 2564.5.4. Gemini surfactant aggregates on silica.............................................. 256

4.6. Model hydrophobic substrates............................................................... 2594.7. Summary of AFM investigations........................................................... 262

5. Fluorescence quenching experiments............................................................. 2635.1. Introduction...................................................................................... 263

221R. Atkin et al. / Advances in Colloid and Interface Science 103 (2003) 219–304

5.2. Time resolved fluorescence quenching.................................................... 2645.3. Determination of aggregation numbers.................................................... 2655.4. The earliest fluorescence probe studies.................................................... 2655.5. Time resolved fluorescence quenching of adsorbed aggregates...................... 2655.6. Summary of fluorescence quenching experiments...................................... 266

6. Reflectance techniques............................................................................... 2686.1. Introduction and underlying principles.................................................... 2686.2. Neutron reflectivity and adsorbed layer structure....................................... 2686.2.1. Limitation of NR......................................................................... 2686.2.2. Contrast control........................................................................... 2696.2.3. NR studies of cationic surfactants on silica........................................ 269

6.3. Ellipsometry, optical reflectometry and adsorption kinetics............................ 2726.3.1. Dynamic aspects of surfactant adsorption.......................................... 2726.3.2. Principles of optical techniques....................................................... 2726.3.3. Hydrodynamic considerations......................................................... 2736.3.4. Ellipsometric measurements of nonionic surfactant adsorption................ 2736.3.5. OR studies of nonionic surfactant adsorption...................................... 2766.3.6. Adsorption kinetics of CTAB on silica............................................. 2776.3.6.1. Ellipsometry......................................................................... 2776.3.6.2. Optical reflectometry.............................................................. 278

6.3.7. The role of micelles in adsorption................................................... 2786.3.8. The influence of electrolyte on adsorption......................................... 2806.3.9. The influence of co-ion type on CTAC adsorption.............................. 2826.3.10. Effect of chain length on adsorption............................................... 2836.3.11. The slow adsorption region.......................................................... 2856.3.12. Adsorption of gemini surfactants to silica by OR............................... 2916.3.13. Adsorption of ionic surfactants to a charged hydrophobic substrate........ 293

6.4. Summary of reflectance observations...................................................... 2967. Summary................................................................................................ 2987.1. Mechanism of adsorption and the adsorption isotherm................................ 2987.1.1. The electrostatic concentration span................................................. 3007.1.2. The electrostatic and hydrophobic concentration span........................... 3007.1.3. The hydrophobic concentration span................................................ 300

7.2. Adsorption kinetics and the adsorption isotherm......................................... 301References.................................................................................................. 301

1. Introduction

The adsorption of a solute at the solid–aqueous interface results in an increase inthe local concentration or surface concentration. When the interaction is favourablethe local concentration will exceed the concentration of the bulk solution. This iscommonly referred to as asurface excess. For simple solutes, adsorption behaviouris generally uncomplicated, and can be modelled accurately on the basis of theinteractions between the adsorbing species and the surface of the substrate. This


type of adsorption is generally interpreted using the Langmuir isotherm, whichadequately describes adsorption behaviour up to a monolayer level of coverage.The behaviour of amphiphilic surfactant molecules at such an interface is more

complex, and has been of considerable scientific interest since the concept of a‘hemimicelle’ was first proposed by Gaudin and Fuerstenauw1x. This study showedthat the level of adsorption of both cationic and anionic surfactants on quartzincreased only slightly up to a certain critical concentration, but once this concen-tration was exceeded, the surface excess increased markedly, indicating a cooperativeadsorption process. This increase was attributed to the formation of adsorbedaggregates, termed hemimicelles, and the concentration at which a rapid increase inthe surface excess occurs became known as the hemimicelle concentration(hmc).The aim of this manuscript is to review contemporary studies of surfactant

adsorption at the solid–aqueous interface in order to develop the most likelyadsorption mechanism. In classical chemistry, all but the simplest chemical reactionsare a consequence of several steps, or elementary reactions. Thereaction mechanism,which describes the process by which the reactants are converted into products,generally consists of a series of such elementary reactions. For a given chemicalreaction, there may be several plausible reaction mechanisms. In order to determinewhich mechanism best describes what actually occurs, the theoretical rate laws forthe proposed elementary steps and overall reaction are compared to experimentallydetermined reaction rates. That is, the reaction mechanism is elucidated by studyingthe reaction kinetics. Analogously, an understanding of the adsorption kinetics isimportant to understanding the mechanisms of adsorption.The fast kinetics associated with the formation of an adsorbed surfactant layer at

the solid–aqueous interface has, until recently, prevented accurate investigation.This has led to the mechanism of surfactant adsorption being inferred fromthermodynamic data or, in the analogy of a classical chemical reaction, byconsidering only the products. These explanations have been further hampered by apoor knowledge of the equilibrium adsorbed surfactant structures(the products). Inmany cases, these have been misinterpreted as simple monolayers or bilayers, ratherthan the discrete surface aggregates that are present in many surfactant–substratesystems.To summarise, experimental limitations associated with kinetic and structural

measurement have, until recently, hindered any determination of a satisfactorymechanism of surfactant adsorption at the solid–aqueous interface. Here we aim tolink the equilibrium and kinetic information in order to describe the adsorptionprocess. In order to accomplish this, we shall first examine traditional equilibriumdata and the most popular methods by which these data are interpreted. This isfollowed by a description of the evidence for discrete aggregation on the substrate,and the morphology of the aggregates formed. With this knowledge, the kineticdata obtained from relatively new ellipsometric methods of investigation will beexamined, and on this basis the likely mechanism or mechanisms of adsorption willbe proposed.To this end, particular attention will be given to literature that is representative

of the present understanding in the field. These results will be supplemented with


other relevant works where appropriate. Nonionic surfactant adsorption will only beexamined when the results give insight into the nature of ionic surfactant adsorption.This generally occurs when the interactions between the hydrocarbon chains and ahydrophobic surface are considered, but also may be of interest when examining aspecific area of investigation, such as the adsorption kinetics.The head-group charge present on ionic surfactants results in a more complicated

adsorption process when compared to nonionic amphiphiles. Ionic surfactant adsorp-tion is particularly sensitive to the interactions of counter- and co-ions with thecharged groups of the surface. Adjustment of the solution pH may also affectseveral factors in the surfactantysubstrate system. These can include the level ofdissociation of surface groups, the degree of counterion binding to micelles, and theoverall ionic strength. If the affinity of co-ions for surface groups is sufficientlyhigh, then the co-ions can compete for adsorption sites at the surface. All thesefactors have implications, not only for the surface excess, but also for the morphologyof the surface aggregates formed. For all of these reasons, systematic studies arerequired to quantify the nature of the various factors that control surfactantadsorption.The structure of the adsorbed layer has been elucidated by innovative experimental

techniques, such as atomic force microscopy(AFM), neutron reflectivity(NR), andfluorescence spectroscopy. Knowledge of the substrate structure allows the generalfeatures of adsorption phenomena to be equated with the morphology of thesurfactant aggregate present on the substrate. The intermolecular energetics willinfluence the type of structure formed and the macroscopic properties of theinterface, which in turn affects the suitability of the surfactant for practicalapplications. These include ore flotation, stabilisation of foams and emulsions,wetting control, and detergency, amongst others.

2. General surfactant and substrate properties

2.1. Silica surface chemistry

A significant proportion of the available literature concerning surfactant adsorptionat the solid–aqueous interface concerns amorphous silicaw1–4x. As a result, wewill briefly discuss the chemistry associated with silica. Specific differences betweenthe silica surface and other substrates will be discussed when applicable. Note thatmany of the features described here for silica are relevant to other mineral oxideinterfaces. Silica is by far the major constituent of the earth’s crust and as a result,the chemistry associated with the silica surface has been widely studiedw5x. Bulksilica consists of siloxane units joined together in a tetrahedral lattice. Severaldifferent functional groups can be present at the surface, depending on thepreparation of the surface and, if in solution, the nature of that solution. Functionalgroups commonly associated with the silica surface are depicted schematically inFig. 1.Like other mineral oxide surfaces, silica has a surface charge character that is

defined by the relative concentrations of H and OH(the potential determiningq y


Fig. 1. Schematic representation of the types of functional groups that occur on the silica surface.(a)Hydrated and(b) anhydrous silanol groups are associated with the hydroxylated surface whereas(c)siloxane-dehydrated groups occur mainly on the pyrogenic surface. Redrawn after Ref.w5x(a).

ions) in solution, as shown by the following equations.

q qSiOHqH mSiOH (1)2K1

y ySiOHqOH mSiO qH O (2)2K2

It is the relative magnitude of the equilibrium constantsK andK in Eqs. (1)1 2

and (2) that determine the charge on the silica surface. The isoelectric point(iep)for silica occurs at approximately pH 2w5x, and is somewhat dependent on theexact nature of the surface. The density of negative charges remains low until thesolution pH reaches 6, but increases sharply between pH 6 and 11w6x. Whencompared to other well-characterised mineral oxide surfaces, the charge vs. pHcurve for silica is unusualw2,6,7x, and modeling studies indicate that the surfacepotential of silica as a function of pH is highly non-Nernstianw8x.While solution depletion studies use silica particles(which typically have a high

sodium content) the silica surfaces used in reflective techniques and AFM studiesare often produced from silicon wafers. High purity silicon wafers are readilyavailable commercially. The simplest method for preparing oxide layers on thesurface of a wafer is to bake the wafer at high temperature in an oxygen atmosphere.By controlling the length of time that the wafer is baked, the oxide film thicknesscan be easily controlled. This process produces wafers of pyrogenic silica. Hydrox-ylated silica surfaces are prepared by rehydrolysing the surface, either by soakingthe wafers in water or treatment with basic solution.When analysing the silica surface charge, the structure of the oxide layer must

be considered. Hydroxylated silica has a high density of hydroxyl groups(;4.5OH nm ) w5x that are in close proximity to one another. This leads to hydrogeny2

bonding between the hydrogen of one hydroxyl group and the oxygen of theneighbouring group, as depicted in Fig. 1b. Consequently, the hydroxyl hydrogenatoms are strongly bound at normal pH levels, resulting in the hydroxylated silicahaving a low surface charge. Conversely, pyrogenic silica has a lower density of


hydroxyl groups(;0.7 OH nm ) w5x and a higher net charge than hydroxylatedy2

silica. The presence of numerous siloxane-dehydrated groups(Fig. 1c) will renderthe pyrogenic surface partially hydrophobic.

2.2. Surfactant properties

Table 1 lists the name, structural formula, the most commonly used abbreviationand cmc of the surfactants covered in this review. In some cases, particularly forthe alkyltrimethylammonium bromide surfactants, more than one acronym is usedto refer to the same surfactant. In this review, the abbreviation used in the paperunder consideration will be used so that the text corresponds to the reproducedfigure.

3. Adsorption isotherms

3.1. Introduction

Adsorption isotherms are traditionally determined by solution depletion methodsw9x. Depletion experiments are accomplished by mixing a surfactant solution with agiven mass of adsorbate of known surface area. After equilibration, the surfaceexcess is determined by the change in the solution surfactant concentration. In orderto facilitate measurement of solution concentrations, surfactants containing spectro-scopically active groups are often, but not always, employed. A series of experimentsconducted at appropriate surfactant concentrations allows the adsorption isotherm tobe resolved.

3.2. Traditional analysis

Much of the literature concerning adsorption isotherms predatesin situ methodsof probing the adsorbed layer morphologyw9x. As a consequence, models proposedto reconcile the features of the isotherm, particularly the saturation surface excess,often describe simple monolayers and bilayers. This is in stark contrast to morerecent data that in many cases suggests discrete surface aggregation. However, thisdoes not discount isotherm analysis in developing an understanding of the adsorptionprocess, particularly below the critical surface aggregation concentration(csac). Inthis pre-aggregation region of the isotherm, even the most recent experimentalmethods yield only inconclusive indications of adsorbed layer structure. Adsorptionisotherms can provide particularly useful information concerning the electrostaticinteractions that occur at low surfactant concentrations and also probe the mannerin which the surface charge adapts as the solution conditions and surface excess arealtered. In this section we will examine two of the more durable explanations foradsorption at a charged interface: the ‘two-step’ and ‘four-region’ adsorption models.Detailed attention will also be given to the influence of chain length, surface chargeeffects, the relevance of the common intersection point(cip) between isotherms

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Table 1Characteristics of surfactants reviewed

Surfactant name Structural formula Acronymyabbreviation cmc (mM) cmc (mM)10 mM salt

Cetyltrimethylammonium bromide C H N Me Brq y16 33 3 CTAB or HTAB 0.9 0.15

Cetyltrimethylammonium chloride C H N Me Clq y16 33 3 CTAC 1.1 0.3

Cetylpyridinium bromide C H N (C H ) CHBrq y16 33 2 2 2 CPBr 0.7 0.1

Cetylpyridinium chloride C H N (C H ) CHClq y16 33 2 2 2 CPC 0.8 0.15

Tetradecyltrimethylammonium bromide C H N Me Brq y14 29 3 MTAB, C TAB or TTAB14 3.6 2.1

Dodecyltrimethylammonium bromide C H N Me Brq y12 25 3 DTAB or C TAB12 15.3 11

Dodecylpyridinium chloride C H N (C H ) CHClq y12 25 2 2 2 DPC 14.7 10.5

Sodium dodecylsulphate C H SO Nay q12 25 4 SDS 8.1 6.5

Didodecyldimethylammonium bromide (C H ) N Me Brq y12 25 2 2 DDAB 0.05 –

Benzyldimethyloctylammonium bromide C H N CH C H Me Brq y8 17 2 6 5 2 BDOAB – –

Benzyldimethyldodecylammonium bromide C H N CH C H Me Brq y12 25 2 6 5 2 BDDAB 5.6 –

Ethyl-a,v-bis (dodecyldimethylammonium bromide) C H (C H N Me Br )q y2 4 12 25 2 2 12-2-12 0.84 –

Propyl-a,v-bis dodecyldimethylammonium bromide) C H (C H N Me Br )q y3 6 12 25 2 2 12-3-12 0.9 –

Butyl-a,v-bis (dodecyldimethylammonium bromide) C H (C H N Me Br )q y4 8 12 25 2 2 12-4-12 1.09 –

Hexyl-a,v-bis (dodecyldimethylammonium bromide) C H (C H N Me Br )q y6 12 12 25 2 2 12-6-12 1.01 –

Octyl-a,v-bis (dodecyldimethylammonium bromide) C H (C H N Me Br )q y8 16 12 25 2 2 12-8-12 0.83 –

Decyl-a,v-bis (dodecyldimethylammonium bromide) C H (C H N Me Br )q y10 20 12 25 2 2 12-10-12 0.63 –

Dodecyl-a,v-bis (dodecyldimethylammonium bromide) C H (C H N Me Br )q y12 24 12 25 2 2 12-12-12 0.37 –

Methyl groups(CH ) are abbreviated to Me.3


Fig. 2. Adsorption isotherms of DTA (s) and Br (h) ions measured on particulate silica at pH 8.q y

Note the adsorption of bromide ions only occurs once the second increase has commenced for theDTA ions. The solid and dashed lines were drawn by hand to guide the eye. The solution cmc isq

indicated by a dashed vertical line. Reproduced from Ref.w10x.

measured at different salt concentrations and the consequences of different methodsof substrate preparation.In general, adsorption isotherms are interpreted by discerning changes in the rate

of increase in the surface excess with concentration. This allows the isotherm to bedivided into regions, and the most likely conformation of adsorbed surfactant ineach region ascertained. The most common approaches for this type of interpretationare the two-step and four-region adsorption isotherms. At first glance these modelsmay appear to be fundamentally different, but in actual fact they have much incommon. Both models divide the isotherm into four sections, and there is goodagreement regarding the orientation of surfactant adsorbed at the interface in mostregions. The primary difference between the models pertains to the region in whichhemimicellar aggregation is initiated. The four-region model predicts that hemimi-celle formation takes place in the second region, whilst the two-step model hashemimicelle formation occurring at higher solution concentrations, in the thirdregion.In more recent studies, isotherm data are often combined with other information

allowing more precise determination of the nature of adsorption. The surface charge,zeta potential, counterion concentration, solution pH and solution conductivity havebeen monitored with surface excess. As we shall see below, studies that combinetechniques allow the adsorption mechanism to be commented upon with muchgreater certainty.

3.2.1. The two-step modelWhen expressed on a linear scale, adsorption isotherms typically display two

plateau regionsw10x, and a sharp increase in the surface excess near the cmc. For aclassical example see Fig. 2.Many descriptions of two-step isotherms are available in the literature for a wide

variety of surfactant–substrate combinationsw11–16x. In works published beforethe application of the AFM to imaging of adsorbed surfactant layers in 1994w17x,


Fig. 3. The two-step model for cationic surfactants adsorbed to silica.(a) The general shape of theadsorption isotherm. Thex axis indicates residual surfactant concentration and they axis indicates adsorp-tion density.(b) The proposed model of adsorption. Adapted from Ref.w15x.

the shape of the isotherm is often interpreted as being indicative of a monolayer onhydrophobic surfaces and a bilayer on hydrophilic surfaces e.g. Fig. 2.A notable exception is the work of Gao et al.w15x. In their study of the adsorption

of alkylpyridinium halides to silica, they determined two plateau regions in theadsorption isotherm. The plateau regions were at low surfactant concentrations(pre-hmc) and the saturation level plateau observed above the cmc. This led to theproposal of a two-step model for adsorption as shown in Fig. 3. The regionssuggested were a low surface excess region(I), a first plateau region(II), ahydrophobic interaction region(III ), and a second plateau(IV).It was suggested that in region(I) the surfactant is adsorbing via electrostatic

interactions with the silica substrate. The surface excess is determined mainly bythe surface charge. Adsorption is sparse, so interactions between adsorbed surfactantmolecules are negligible. In region(II), the substrate surface charge has beenneutralised. However, the solution activity of the surfactant is not sufficient to lead


Fig. 4. The four-region or reverse orientation model of adsorption. Proposed adsorption isotherm andsurfactant aggregates on solid substrates. Adapted from Ref.w19x.

to any form of aggregation at the interface thus surfactants are still adsorbed asmonomers. The abrupt increase in adsorption at the hmc denotes the onset of region(III ). In this region, the solution surfactant concentration is sufficient to lead tohydrophobic interactions between monomers. The monomers electrostaticallyadsorbed in region(II) are thought to act as anchors(or nucleation sites) for theformation of hemimicelles. In this article, a hemimicelle was defined as a sphericalstructure with surfactant head-groups facing both towards the substrate and intosolution w15x. In more recent times this type of structure has been redefined as anadmicelle. In region(III ) the admicellar structure was not necessarily fully formed,allowing for further adsorption. Region(IV) occurred above the cmc, with theformation of fully formed aggregates and saturation levels of surface coverage.

3.2.2. The four-region modelWhilst this type of ‘two-step’ analysis adequately explains many of the common

features of adsorption isotherms, it is not the only method of evaluation available.Somasundaran and Fuerstenau proposed the four-region or reverse orientation modelfor interpretation of surfactant adsorption isotherms when plotted on a log–log scalew18x. This method has been shown to be particularly successful for modelingadsorption behaviour on alumina and rutilew4,18x. The primary advantage of usinga log–log plot is that it amplifies the features of the isotherm at low surface excessvalues. The general form of isotherms plotted in this manner, and the morphologyof adsorbed structures associated with each region are depicted schematically in Fig.4.In region I of the isotherm, surfactant monomers are electrostatically adsorbed to

the substrate, with head-groups in contact with the surface. Hydrocarbon tail-groupsmay interact with any hydrophobic regions of the substrate. Region II involvesstrong lateral interaction between adsorbed monomers, resulting in the formation ofprimary aggregates. Using techniques such as Raman spectroscopy, fluorescencespectroscopy, electron spin resonance and contact angle measurement, Somasundaranet al. w19–22x have shown that the surfactants are adsorbed with head-groups facingtowards the surface while the hydrocarbon tail-groups protrude into solution. Thiscreates hydrophobic patches on the surface. In the four-region model, this type ofaggregate is known as a hemimicelle. Increases in the surface excess in region III


are thought to result from growth of the structures formed in region II, without anyincrease in the number of surface aggregates. The presence of head-groups facinginto solution renders the surface hydrophilic once more. The transition betweenregion II and region III is thought to be due to neutralisation of the surface charge.Finally, in region IV, the surface morphology is assumed to be a fully formedbilayer. Further increases in the solution surfactant concentration do not lead to anyfurther increases in the surface excess.

3.2.3. Similarities between modelsClearly, these traditional types of analysis have a good deal in common. The

most obvious difference between the models is a lack of hydrophobic interaction inthe second region for the two-step model. Interestingly, in light of recent data, thestructures proposed in the four-step model below charge neutralisation, and theadmicellar structures predicted by the two-step model above charge neutralisation,may yet prove to be correct.

3.3. The influence of surfactant chain length

As the hydrocarbon chain length of a surfactant molecule is increased, themonomer is essentially rendered ‘more hydrophobic’. That is, an increased numberof clathrate bound water molecules are required to solubilise successively longertail-groups, which lowers the overall entropy of the system. As a result, surfactantswith longer hydrocarbon chains have a much greater driving force for aggregation,and this dramatically reduces the solution cmc, cf. Table 1.Chain length is also of critical importance in determining the adsorption behaviour

of a surfactant. Fig. 5 shows that increasing the chain length by four methylenegroups, from C to C (i.e. DPC to CPC) lowers the concentration at which the12 16

features of the isotherm occur by approximately an order of magnitude, in line withthe reduction in solution cmcw23x. The ‘shifting’ of the isotherm to lowerconcentrations for longer chained surfactants is a result of the increased hydropho-bicity imparted by longer tail-groups. At the solid–aqueous interface, hydrophobicinteractions may exist between the surfactant and the surface, and also laterallybetween adsorbed surfactants. Some evidence for this is apparent in Fig. 5. As theincrease in surface excess in regions II and III of the isotherm is dependent onlateral hydrophobic interactions, it would be expected that the surface excess shouldincrease more rapidly with concentration for the surfactant bearing the longerhydrocarbon chain. This is indeed what is observed in Fig. 5b, as the slopes ofregions II and III are clearly steeper for the C surfactant. In region IV the16

saturation surface excess is clearly greater for CPC than for DPC, but whether thiswas due to an increased level of hydrophobic interaction, or a change in the structureof the aggregate formed at the interface, could not be ascertained from this study.

3.4. The role of surface charge

A major limitation of the solution depletion method for studying surfactantadsorption was observed by Goloub et al.w2x, who argued that the silica surface


Fig. 5. Adsorption isotherms for CPC and DPC with 0.001 M KCl at pH 9.(a) Presents the data on alog–log scale and the four-regions of the isotherm are indicated.(b) Shows the data on a linear-logscale.� 1997 ACS. Reproduced with permission from Ref.w23x.

charge varies not only with pH, but also with surfactant adsorption. Ionisation ofsurface groups will alter the pH of the solution. This means that without careful pHcontrol, pH changes may occur not only from isotherm to isotherm, but also alongan isotherm. However, most studies report only the initial pH.

3.4.1. Increases in surface charge with adsorptionIn order to overcome this difficulty, Goloub et al. conducted a systematic study

of the variation of surface charge with surfactant adsorptionw2x. The solution pHwas adjusted throughout equilibration of the surfactant and substrate until no furtherchanges in pH were observed. The results obtained(an example of which is shownin Fig. 6) give valuable insight into the mechanism of the adsorption process.Fig. 6 shows that at both low electrolyte and low surfactant concentrations the

adsorption and surface charge isotherms are practically identical. The greatestincrease in surface charge occurs within this initial region of the isotherm and thiseffect is more pronounced with increased pH. The correlation between surfacecharge and adsorption at low concentrations suggests that whenever a surfactant isadsorbed to the surface a proton is displaced, which indicates that the surfactanthead-group is in close proximity to the surface. The surfactant concentration atwhich the substrate surface charge is neutralised is denoted as the charge compen-


Fig. 6. Adsorption of DPC and surface charge of silica at 0.001 M KCl as a function of the DPCconcentration at(a) pH 7 and(b) pH 9. To facilitate the comparison, the surface charge is expressed asG ss yF, wereF is the Faraday constant. In this way surface ‘charge’ and the adsorbed amount of0 0

surfactant,G , are both expressed in micromole per square metre.� 1996 ACS. Reproduced with per-s

mission from Ref.w2x.

sation point(ccp). Increasing the surfactant concentration above the ccp had littleeffect on the surface charge even though the surface excess continues to increase.This suggests adsorption of a second layer on top of the electrostatically adsorbedlayer, with surfactant head-groups facing into solution.This interesting result was expanded upon by examining a plot of the surface

charge vs. the surface excess of the surfactant, reproduced in Fig. 7, which showsthat at low surfactant concentrations the surface excess is less than the native surfacecharge. In view of this it is somewhat surprising that the surface charge began toincrease as soon as surfactant adsorption commenced. That is, rather than surfactantmonomers first adsorbing to existing charged sites on the substrate, then creatingadditional charges, the adsorption of surfactant molecules causes nearby hydroxylgroups to immediately become more acidic, inducing further surface ionisation. Asthe charge neutralisation point was approached(intersection with the line of unitslope), the gradient of the surface charge isotherm was close to unity, which shows


Fig. 7. Surface charge of silica as a function of the surface excess of DPC measured at 0.001 M KCland pH 7 and 9, and the line of unit gradient.� 1996 ACS. Reproduced with permission from Ref.w2x.

that the number of surfactant molecules adsorbing and the number of surface sitescreated are nearly equal.Somewhat different results were obtained at high electrolyte concentrations. In

100 mM KCl, the initial surface charge was much higher, and the increase in surfacecharge on surfactant adsorption was much decreased. The surfactant ions werecompeting for charged sites on the substrate with the potassium ions of theelectrolyte; hence adsorption did not reach measurable levels until much highersolution surfactant concentrations compared with the low electrolyte case. However,at high electrolyte concentrations, the Coulombic repulsions between the monomerhead-groups was greatly decreased. As a consequence, once adsorption is com-menced the isotherm increased steeply. Other authors have reported similar resultsw16x. The implications of high electrolyte concentration for the adsorbed morphologywill be elucidated below in the discussion of AFM imaging.

3.4.2. The common intersection pointDe Keizer et al.w24x showed that the cip between adsorption isotherms measured

at different electrolyte concentrations was a useful method of analysing adsorptionisotherms. Further examples of the cip effect are provided in the work of Golouband Koopalw23x. An example of the cip for DPC at pH 7 at two salt concentrationsis presented in Fig. 8.At the surfactant concentration at which the cip occurs, added electrolyte has no

effect on the surface excess. This condition may not hold in the case of longerchain surfactants that adsorb strongly at low concentrations. In the case of DPC,however, this observation allowed the effect of electrolyte on the adsorption processabove and below the cip to be commented upon. It was suggested that, providing


Fig. 8. Isotherms for DPC adsorption on silica at pH 7 for two salt concentrations on(a) a log–log and(b) a linear-log scale.� 1997 ACS. Reproduced with permission from Ref.w23x.

there are no specific interactions between the electrolyte and the substrate, the cipcorresponds to the iep of the substrate. In its simplest form, the cip represents thepoint where the orientation of surfactants adsorbing at the surface changes fromheads facing towards the substrate to heads facing towards solution, formingbilayered aggregates.In order to test the validity of the cip as a means of identifying the iep of the

adsorbent, the variation in electrophoretic mobility in the presence of surfactant wasalso investigated. This result is reproduced and shown in Fig. 9 for two pH valuesand salt concentrationsw23x. The iep results for DPC and its C analogue CPC16

both correlate with the cip.These data are directly comparable to the surface charge isotherm already

discussed in Section 3.4.1. Not only did the cip correspond to the iep, but also tothe ccp, cf. Fig. 6. The fact that these three points occurred at the same bulksurfactant concentration showed that there is little or no specific adsorption of theelectrolyte to the substrate i.e. no adsorption beyond the level dictated by Coulombicattraction.Thus, the cip represents the point where the electrostatic contribution tothe adsorption process changes from attractive to repulsive.


Fig. 9. Electrophoretic mobilities of silica particles as a function of DPC concentration at two saltconcentrations and(a) pH 7 and(b) pH 9.� 1997 ACS. Reproduced with permission from Ref.w23x.

3.4.3. Adsorption model based on the cipIt has been postulated that, below the cip where adsorption is primarily electro-

statically driven, adsorption is decreased with increasing electrolyte concentrationbecause of competition between electrolyte co-ions and the surfactant monomers.As with the other models described, surfactant adsorption in this region is orientatedwith head-groups towards the surface due to electrostatic attraction. Above the cip,hydrophobic interactions between surfactant tail-groups provide the driving forcefor further adsorption. As electrolyte reduces the Coulombic repulsions between thesurfactant head-groups, increasing the electrolyte concentration above the cipenhances surfactant adsorption. This can be observed in Fig. 9.

3.4.4. The influence of surface preparationIt has been shown that the solution conditions are not the only factors that

influence the surface charge of the substrate. The effect of different methods ofsurface preparation was investigated by Chorro et al.w25x, who found that acidtreatment of the substrate prior to adsorption could reduce the maximum surfaceexcess by almost 50%. In this depletion study, differences in the adsorption isothermson raw and HCl washed particulate silica, designated SiNa and SiH, respectively,


were investigated for DTAB and the gemini surfactant 12-2-12. For the moment weshall direct our attention towards the general implications of this study. A morethorough treatment of gemini surfactant adsorption and surface charge will then beundertaken in Section 3.4.6.This study was performed under ‘free’ conditions i.e. no attempt was made to

control the solution conditions. Thus, the ionic strength of the major species in thesolution (surfactant ions, counterions, H , Na and OH) regulated the behaviourq q y

of the system. This may appear to be precisely the type of study that Goloub et al.w2x had described as flawed due to a lack of control over solution conditions.However, in this case, as the level of surfactant adsorption increased, the propertiesof the supernatant(pH, electrophoretic mobility, conductivity and counterionconcentration) were closely monitored. This allowed the effect of surfactantadsorption on the system as a whole to be monitored and is therefore a valuablemeans of investigation.Upon immersion of the silica and surfactant in solution, the presence of sodium

ions was noted. It was shown that the sodium content for acid washed silica(7ppm) was considerably less than that of raw silica(67 ppm). Thus, the HCl washingtechnique was particularly successful for the removal of sodium from the surfaceand the majority of residual sodium ions were strongly surface bound. These ionswere released during equilibration, and not during washing, due to the much longertime period of the equilibration.The adsorption isotherms for DTAB and 12-2-12 are reproduced in Fig. 10, with

the concentration axis presented as a function of the cmc. The cmc of DTAB in thesupernatant is 12.8 and 11.8 mM for the SiH and SiNa systems, respectively. For12-2-12 the cmc is significantly reduced at 0.8 and 0.4 mM for the SiH and SiNasystems. All of these cmc values are less than that of the corresponding surfactantin pure water, reflecting the contribution of the released sodium ions to the solutionionic strength. The most startling difference between the two substrates is that, forboth surfactants, almost double the surface excess was obtained on raw silica asopposed to the acid treated silica. These differences were attributed to the differentcharging properties of the surface. When sodium ions were released from thesurface, this resulted in the formation of ionised sites i.e. the raw silica surface isconsiderably more charged than the acid washed substrate. The released sodiumions also raised the ionic strength of both systems, but obviously this effect is muchgreater for the raw silica system. The marked difference in plateau surface excessvalues reflects not only the effect of increased ionic strength, but also the importanceof the number of initial surface charges on the adsorption process. This workhighlights the significant effects that changes in surface chemistry induced bysurfactant adsorption can have on adsorption behaviour and highlights a majorweakness in many depletion studies.

3.4.5. Comparison of adsorption mechanisms on raw and acid washed silicaSurfactant monomers initially adsorb to pre-existing charged sites electrostatically,

and these act as nucleation points for further surfactant adsorption. The acidity ofnearby hydroxyl groups increases, releasing H ions into solution i.e. adsorptionq


Fig. 10. The adsorption of DTAB and 12-2-12 on(a) SiNa and on(b) SiH reproduced from Ref.w25x.

that leads to ionisation of surface groups is detectable by a decrease in the solutionpH. This was observed for all surfactant–substrate combinations but was moreobvious for the acid washed silica due to the increased number of protons associatedwith this substrate. Thus, when a free system is under investigation, the equilibriumconcentration of sodium ions in solution relates to the initial surface charge, whilechanges in the H concentration are a measure of the number of surface chargeq

sites that are induced by surfactant adsorption.The acid washed surface in Fig. 10b is obviously chemically different from the

silica used in the studies described previously. Nonetheless, the results obtained areinteresting, particularly when contrasted with the results for raw silica. For theDTABySiH system, the pH of the supernatant was decreasing gently up to the pointwhere the surface excess reached 25% of the maximum value i.e. the end of thefirst pseudo-plateau in the isotherm. Electrophoretic data indicated that the pzc wasreached at this surface excess and the adsorption of bromide ions at the surface wasshown to be very low. These data suggest that up to the end of the first pseudo-


plateau, the DTAB was adsorbed head-group towards the substrate due to electro-static interactions.In the region of rapid increase in surface excess, the fraction of surface bound

bromide ions increased. A rapid decrease in the supernatant pH accompanied thisbromide ion adsorption, but only until the solution H concentration reached 60%q

of the maximum value. This suggests that above this surface excess, surfactant wasadsorbed with head-groups facing towards solution. Increases in the electrophoreticmobility of the particles supported a change in the orientation of the adsorbingsurfactant at this surface excess. It was postulated that surface bound aggregateswere the most likely adsorbed morphology to account for these observations. Theseaggregates could be loosely packed initially, but as saturation levels of coveragewere approached, aggregates would become tightly packed.The major difference between the acid washed and raw silica surface was in the

ionisation of surface groups. For the raw silica surface, the solution pH reached aplateau level at approximately 25% coverage of the surface i.e. the concentrationwhere surfactant is adsorbing exclusively with head-groups facing into solution isreached much earlier for the raw silica system. Thus, the model for adsorption thatthe data on raw silica implies shows good agreement with that described by Golouband Koopalw23x.

3.4.6. Surface charge and gemini surfactant adsorptionGemini surfactantsw26x are a relatively new genre of amphiphilic molecules, first

appearing in the literature in 1974w27x. They have recently become a topic ofrevived scientific interest, due in part to their effectiveness in the modification ofinterfacial properties, but also because their unusual molecular geometries lead tointeresting aggregation structures.A gemini surfactant consists of two identical surfactant molecules joined by an

alkyl spacer group. The spacer group can be flexible or rigidw28x, hydrophilic orhydrophobicw29x and generally connects the two surfactant moieties at, or near, thehead-group. The attachment of the spacer group increases the hydrophobicity of thedimeric surfactant relative to the constituent monomeric units. As a consequence,the cmc of the gemini can be up to 100 times lower than that of the monomer unitsw30x.For simplicity, shorthand nomenclature of gemini surfactants is often employed,

based on the number of carbon atoms in the surfactant chain and the spacer group,and is best illustrated by example. For alkanediyl-a,v-bis (dodecyldimethylammon-ium bromide) dimeric surfactants with the alkanediyl spacer groups C H , or2 4

C H , the corresponding surfactants are referred to as 12-2-12, and 12-8-12,8 20

respectively. The molecular structure of a typical gemini surfactant is depicted inFig. 11. All discussion is limited to this family(12-s-12) of gemini surfactants.

3.4.6.1. The importance of the spacer group. The properties of gemini surfactantsare greatly dependent on the length of the spacer group. The spacer group controlsthe separation between the two head-groups and may be greater or less than theaverage separation of the corresponding monomers in an aggregate. This changes


Fig. 11. The structural formula of a typical gemini surfactant. The molecule represented is 12-4-12.Counterions have not been included.

the mobility and the packing geometry of the gemini within a micelle, whether insolution or at an interface.Danino et al.w31x have demonstrated that the structure of the micelles formed in

solution varies significantly with spacer length. For quaternary ammonium surfac-tants of the form 12-s-12, spacer groups of length less than or equal to fivemethylene groups dictate that the head-groups are in close proximity. As the Bjerrumlength in water at 258C is equal to 0.7 nmw32x, for gemini surfactants with shortspacer lengths, charge condensation must occur, and the effective charge of thesurfactant is less than 2. The resultant monomer geometry leads to aggregates oflower curvature than that of the corresponding monomer. Fors values between 6and 10, the distance between head-groups induced by the spacer is similar to thatof the monomer in a micellar aggregate, and similar structures result. Fors valuesgreater than 14, it is suggested that the spacer adopts a looped conformation withinthe aggregate, thus acting like additional hydrocarbon chains. The structure formedin this case is similar to those of dimeric surfactants. More specifically, forss2worm-like micelles result,ss3 gives rise to extended micelles, while fors greaterthan 4 essentially spherical micelles are formedw31,33–35x.The effect of variation in the length of the spacer group has been extensively

investigated at the solution–air interface. Perhaps not surprisingly, it has beendemonstrated that the surface area occupied per surfactant molecule increases withthe size of the spacer fors between 3 and 10w36x. The behaviour of geminisurfactants at the solid–liquid interface has been shown to follow similar trends.The first adsorption isotherms for a gemini surfactant at the solid–aqueous

interface were determined by Esumi et al. The adsorption of 12-2-12 was investigated


at the silica–aqueous interfacew37x, the laponite clay–aqueous interfacew38x andthe titanium–aqueous interfacew39x. It was shown that the amount of 12-2-12adsorbed at the silica–aqueous interface, was lower than that of the monomericanalogue, DTAB. However, while some of the macroscopic properties of theadsorbed aggregates were elucidated, no consideration was given to the adsorptionmechanism.This initial study motivated subsequent researchw25,40,41x. A systematic inves-

tigation of the effect of spacer length on the adsorption isotherm, and the mechanismof adsorption was initially undertakenw40x. A subsequent paper dealt with theeffects of a variation in surface preparation, which has already been discussed inSection 3.4.5w25x. We shall review the effect of spacer length and mechanism ofadsorption first, before returning to the effects of acid washing the substrate priorto adsorption of gemini surfactants.

3.4.6.2. Gemini surfactant adsorption isotherms. Adsorption isotherms for 12-2-12,12-4-12, 12-6-12 and 12-10-12 on acid washed silica are presented in Fig. 12w40x.As the size of the spacer group increased the maximum surface excess of surfactantwas decreased. Corresponding results have been reported at the air–water interfacew36x, and have been attributed to increasing head-group area. Similarly, at the silica–aqueous interface it is likely that the adsorbed morphology alters as the spacerlength is varied. This will be demonstrated in Section 4.5.4. Interestingly, althoughEsumi et al.w37x did not pre-treat their surface to remove sodium ions, the maximumsurface excess attained for 12-2-12 was similar in both studies. This suggests thatall of the surface bound sodium ions were exchanged by the surfactant.Electrophoretic data showed that the silica was substantially negatively charged

at the beginning of adsorption, and that the amount of surfactant adsorbedat thepoint of zero charge was the same irrespective of the spacer length and correspondedto an area occupied per surfactant of 25 nm . This equates to an average distance2

between monomers of 5 nm. As this value is much larger than the length of a fullyextended spacer group(;1.4 nm for 12-10-12) each surfactant can only neutraliseone surface charge site. The observed zero net charge implied that the second head-group, which is not surface bound, had a bromide ion associated with it. Alterna-tively, the close proximity of the unattached head-group may have resulted in theformation of a charged site at the surface, which was then associated with the head-group. A third possibility is that the number of unassociated surfactant head-groupsmatched the number of free charged sites on the surface, thereby achievingelectroneutrality.All of the gemini surfactant adsorption isotherms exhibited a plateau after the

point of zero charge was reached. As can be seen in Fig. 12, this plateau was muchnarrower for 12-10-12 than for the other surfactants. Surprisingly, the surface excessof gemini surfactant required for charge neutralisation was 5 times less than that forDTAB cf. Fig. 10. Recall that both electrostatic and hydrophobic interactions wereinvolved in the first step of the adsorption process for DTAB. This result suggeststhat the surface charge is indeed redistributed once gemini surfactantswith shortspacers were adsorbed to the surface. Thus, for spacer lengths less than or equal to


Fig. 12. Adsorption isotherms of 12-2-12(circles); 12-4-12(triangles); 12-6-12(squares); and 12-10-12 (diamonds) on silica at 258C. The surface excess is expressed in micromole of surfactant per gramof silica. The concentration scale has been normalised by the appropriate surfactant cmc. Solid lines areguides for the eye. The surface area per gram was determined by the BET method to be 29 m . Repro-2

duced from Ref.w40x.

6, the adsorption mechanism operating in the first step is different to that of themonomeric analogue. Once the gemini is adsorbed at the surface, the second chargedhead-group is brought into close proximity with the surface, an effect which becomesmore pronounced as the spacer length is decreased. The acidity of nearby silanolgroups is increased, raising the likelihood of local ionisation. As a decrease in thesolution pH was not noted at this time, it was postulated that the charged sites werecreated by desorption of surface bound sodium ions that were not removed duringthe washing process.Increasing the spacer length allows the second head-group to be positioned further

from the surface. In this case, the second head-group is more likely to be neutralisedby a bromide ion. Thus, gemini surfactants with long spacer groups behave morelike their monomeric analogue, with an overlap between the first and second stepsin the adsorption process. The decreased size of the first plateau for 12-10-12suggested that it was acting more like a monomeric surfactant than its counterpartswith shorter spacer lengths.The second step in the adsorption process is thought to be due to lateral

hydrophobic interactions. As the solution surfactant concentration is increased,interactions between electrostatically adsorbed monomers and the adsorbing surfac-tants are more likely, which leads to aggregate formation. It was expected that theconcentration at which the surface excess rises appreciably should be lowest for thesurfactant with the lowest solution cmc. This is indeed what was observed, with thesteeply rising region of the isotherm occurring in the order 12-10-12-12-2-12-12-6-12f12-4-12, cf. Table 1. A drop in the pH of the supernatant accompaniedthis step, which showed that the high positive charge density of the surfactant


Fig. 13. Physicochemical characteristics of 12-2-12 adsorption on SiH: the degree of surface coverageu; the conductivity of the equilibrated supernatant, the pH, and the degree of bromide ion associationto the surfaceb are plotted against the concentration of surfactant in the equilibrated supernatant. Repro-duced from Ref.w25x.

aggregates induced the formation of surface charge sites. Recent optical reflectometry(OR) studies reported somewhat different findings as will be discussed in Section6.3.12.We now return to the adsorption of 12-2-12 on acid washed and raw silica. As

the study described above was for acid washed silica, the mechanism for the SiHy12-2-12 system has already been outlined above. Variation in the physicochemicalproperties of the SiHy12-2-12 system with solution surfactant concentration can beseen in Fig. 13. It is important to note that along with the decrease in solution pHthat accompanies the sharp increase in surface excess, the degree of bromide ionadsorption reaches a plateau at approximately 0.5 mM. This value is similar to thatobtained for 12-2-12 micelles in solutionw42x, and is indicative of an adsorptionprocess that was hydrophobically driven.12-2-12 was proposedw25x to have the same mechanism of adsorption on raw

silica. The quantitative differences in the adsorption isotherms are thought to be duemostly to the greater capacity of the raw silica surface to release sodium ions,thereby increasing the ionic strength of the supernatant. The pzc for the SiNay12-2-12 system was significantly higher than that of the SiH surface, and correspondedto a smaller area per adsorbed molecule(only 2.7 nm). However, once the initial2

charged surface sites were neutralised, there was no further decrease in the pH ofthe supernatant. This shows that the second adsorption step had already begun andaggregation was occurring at the surface.

3.5. Evidence for discrete aggregation from adsorption isotherms

Adsorption isotherms yield significant information concerning the nature of theinteractions between the surface and the surfactant, particularly in the initial stages


of adsorption. However, it is difficult to infer adsorbed structures from this type ofdata, and the traditional monolayerybilayer interpretation has been the favouredmodel until recently. Despite this, some indirect evidence for admicelle structuresmay be inferred from adsorption isotherms. When one compares isotherms obtainedwith different surfactants, counterions, and degrees of surface modification, itbecomes difficult to rationalise the results simply in terms of mono- and bi-layeraggregation.Furthermore the surface excess values obtained usually do not correlatewith the levels that would be expected for complete monolayers or bilayers. Thishas been rationalised as being due to ‘patchy’ coverage, which of course in itselfinfers a more discrete structure.Some of the most persuasive isotherm data for the presence of interfacial

aggregates is that obtained with the salicylate ion. Several studies have shown thatthe surface excess of surfactants with pyridinium-based head-groups on silicadepends strongly on the counterion. Leimbach et al.w43x first demonstrated fortetradecylpyridinium that a sixfold increase in the plateau surface excess occurswhen the counterion is changed from the weakly binding chloride ion to the stronglybinding salicylate ion. Although this result may be deceptive due to the reportedspecific interactions between the salicylate ion and pyridinium head-groupsw44–47x, it is nonetheless very difficult to explain this type of increase only in terms ofmono- and bi-layer aggregation. However, it is relatively easy to envisage a situationof increased aggregate growth around electrostatically bound surfactants, given thewell-known increase in solution aggregation number as the degree of counterionbinding is increased.Further support for this aggregation model can be found in isotherm data obtained

on hydrophobically modified silica. Leimbach and Rupprechtw48x covalentlyattached a low concentration of octadecyl groups to a silica surface, thereby creatinganchor sites for surface aggregation. The concentration of the hydrophobic groupswas such that only 7% of surface hydroxyl sites were occupied. Thus the surfacewas negatively charged and hydrophobic. The adsorption of the anionic surfactantSDS to the treated and untreated silica was investigated. On the untreated silica,SDS did not adsorb to detectable levels. However, on the modified silica a one stepisotherm was obtained both with and without added electrolyte. The saturationadsorption density was 0.4 and 1.4mmol m for the no added electrolyte and 0.1y2

mM NaCl system, respectively. The increase in adsorption density in electrolyte wasjustified on the basis of decreased repulsions between adsorbed surfactant head-groups, and the surfactant and the substrate, which carried the same charge. Thismodel provides convincing evidence for electrostatically adsorbed surfactant mono-mers acting as nucleation sites for further adsorption. This hypothesis will be furtherprobed in Section 5.

3.6. Calorimetry

Adsorption isotherms can be complemented by measurement of the heat ofadsorption (calorimetry w49x). This allows the energetics of adsorption to bemonitored throughout an adsorption isotherm. Much useful information has been


obtained by calorimetry concerning the mechanism of adsorption, without actuallyrevealing a definitive surface structure. Comparative measurements can allow thenature of the exchange of cations, the conformation of adsorbed water and theorientation of the surfactant with respect to the surface to be commented upon,amongst other issues.There are several methodologies by which heats of adsorption may be obtained

w50–52x. Flow calorimetryw52x is one technique that illustrates the fundamentalsassociated with the measurement of adsorption energetics, and is basically anextension of ion exchange chromatography. The surfactant solution is passed througha column containing the powdered form of the substrate, for example silica oralumina. This column is contained within a microcalorimetry chamber, and aconstant temperature is achieved by means of a feedback loop that controls thepower supply to a heating coil. Initially the column is equilibrated under flow ofpure solvent; then a known quantity of surfactant is passed into the column. Themagnitude of the energy required to maintain the temperature of the column ismonitored, and a plot of heat flow vs. time deduced. The net heat transfer is simplythe area under this curve. As this process is isothermal, this area directly equates tothe heat of adsorption, which can be converted to the molar enthalpy of adsorptionby dividing by the molar surface excess.

3.6.1. The importance of surface waterThe role of water in the energetics of adsorption cannot be underestimated. In

order to adsorb to a substrate, an incoming surfactant may need to displace waterof hydration at the solid surface. The influence of the solid on the arrangement ofthe adjacent water molecules will depend upon the properties of that surface. Forexample, sodium cations specifically bound at the substrate attract free watermolecules. This leads to a local ordering of the water molecules at the interface.The ability of small metal ions in bulk solution to induce structure in nearby watermolecules has been recognised for some timew53x. The likelihood of an analogouseffect at the solid–liquid interface has also been discussedw54x. Thus, endothermiccontributions to the heat of adsorption will depend on the co-ion concentration bothat the surface and in the bulk. Other specific interactions between water and silicamay limit the ability of the surfactant to adsorb to the surface other than byCoulombic interactions.

3.6.2. Calorimetry and adsorption mechanismFig. 14a shows the adsorption isotherms for DTA and TTA ions on silica atq q

pH 8.3 w55,56x. It is worth noting that the TTA isotherm was shifted to the leftq

relative to DTA , but the general shape of the isotherm was similar. This shift wasq

due to the longer tail-group of TTA . A longer hydrocarbon chain provides aq

greater driving force for aggregation(evidenced by a lower solution cmc cf. Table1), and results in the entire isotherm being compressed relative to DTA . Bothq

surfactants reach their saturation surface excess slightly below their respectivesolution cmc values. The corresponding heats of adsorption are presented in Fig.14b.


Fig. 14. The adsorption of DTA (squares) and TTA (circles) ions measured on silica at 258C andq q

pH 8.2. (a) The adsorption isotherms; and(b) the heats of adsorption. The solid lines were drawn byhand to aid the eye.� 1994 ACS. Reproduced with permission from Ref.w55x.

The change in enthalpy during the adsorption process was dependent on theproperties of the bulk and adsorbed phases. Fig. 14b shows that the enthalpy initiallydecreased as the surface excess increases. This was due to the displacement ofsurface cations and water molecules from successively more strongly bound sites.As the surface excess increased two important effects came into play. Firstly, therate of ion exchange was reduced, as there were fewer exchangeable ions presenton the surface. Secondly, strong lateral interactions between the tail-groups of theadsorbed surfactants led to a perpendicular orientation of the hydrocarbon chainsrelative to the surface. These two effects led to the observed minimum, thensubsequent increase in the heat of adsorption. The energetic state of interfacial watermolecules was less affected and the overall heat of adsorption eventually becameendothermic.The region of monotonic increase of enthalpy with the degree of surface coverage

corresponds well with the sharply increasing region of the adsorption isotherm.Adsorption in this region is entropically driven and is dominated by intermolecularinteractions, similar to those that lead to micellisation in the bulk. Bulk micellisationis also an endothermic process for most surfactants, with the driving force derivedfrom the entropy gained upon aggregationw57x. This entropy increase is acquiredfrom the release of clathrate bound water molecules associated with the tail-groupsinto the bulk solution upon micellisation. In some cases it has been shown that theheat of adsorption at moderate to high concentrations is remarkably similar to thatof bulk micellisationw58x. Moreover, at high surface excess values, the temperaturevariation in the adsorption enthalpy mirrors that of the micellisation enthalpyw59x.These results have been used to argue for the presence of surface bound micelles.


Fig. 15. The influence of the hydrophobic tail length on the adsorption energetics for benzyldimethy-lalkylammonium bromides onto precipitated silica from aqueous solutions at 298 K with an initial pHof 8.3: the differential molar enthalpies of displacement against the quantity of adsorption in a limitedadsorption range.� 1996 ACS. Reproduced with permission from Ref.w60x.

3.6.3. Interactions between the hydrocarbon tail and the surfaceZajac et al.w60x investigated the effect of surfactant chain length on the enthalpy

of adsorption at low surface excess values. Specific interactions between thesurfactant tail-groups and the substrate brought about the desorption of structuredinterfacial water. This provided a significant endothermic contribution due to theheat of adsorption. The heats of adsorption of benzyltrimethylammonium bromide(BTMAB), benzyldimethyloctylammonium bromide(BDOAB) and benzyldime-thyldodecylammonium bromide(BDDAB) are reproduced in Fig. 15.For surface excess values of up to 20mmol g , enthalpies of adsorption of they1

C tailed BDOAB and its head-group BTMAB were negative and indistinguishable.8

This suggested that the short C alkyl chain did not interact with the silica surface8

and the most likely orientation for the tail-group was perpendicular to the substrate.Conversely, the C BDDAB had a positive enthalpy of adsorption throughout the12

same range of surface excess values. It was suggested that interactions between thelonger C tail-group and the substrate led to disruption of the structured interfacial12

water. This de-wetted the silica and made a significant contribution to the energeticsof the adsorption process. These interactions were made possible by the additionalconformations available to a C tail-group over a C chain. Thus, at low surface12 8

coverage values, the C tail-group is oriented parallel to the surface to some degree.12

It would be expected that the surfactant tail-group would interact more stronglywith a hydrophobic graphite substrate. The recent calorimetric study of Kiraly andFindeneggw59x used heat of adsorption data to determine whether the most likelyconformation of C TAB adsorbed to graphite was the classical reorientation model12


(where the adsorbed surfactant molecules are reoriented from horizontal to a verticalposition, accompanied by further adsorption from solution), or that of an interfacialaggregate(formation of hemicylindrical admicelles templated by an epitaxiallybound surfactant monolayer). The calorimetric evidence, based on the displacementof water from the interface, showed that the adsorption process had two distinctphases. The first phase, in which the surfactant molecules are horizontally adsorbedas a monolayer, was strongly exothermic and, surprisingly, appeared to be independ-ent of the ambient temperature(in the range 288–318 K) and the surface coverage.The second phase was less exothermic than the first phase and weakly dependenton the level of surface coverage. The second stage was, however, inversely dependenton temperature. This important result strongly suggests a high degree of intermole-cular cooperativity between neighbouring adsorbate molecules, which was extremelydifficult to reconcile on the basis of the reorientation model. On this basis, theauthors concluded that the most concordant aggregate morphology was of hemicy-lindrical aggregates, as suggested by AFM imaging studies. These AFM studies,and more detail of the structure of the adsorbed surfactant layer at the graphite–water interface is discussed below in Section 4.3.

3.7. Summary of adsorption isotherms

The study of adsorption isotherms by depletion methods continues to be aneffective means of studying surfactant adsorption at the most fundamental level,however, future efforts must in all cases consider the possibility that changes in thesurface chemistry during surfactant adsorption will influence the bulk concentrationof various species and this in turn will influence the surfactant adsorption. Modelshave been available for some time to explain the features of adsorption isotherms,and it would seem that the most durable is the ‘four-region’ model for surfactantadsorption. However, in light of recent evidence the interpretation of the final stepin the four-step isotherm must be modified to account for aggregate formation.Based on the data reviewed in Section 3, the ‘two-step’ model proposed by Gu etal. w15,16x (cf. Fig. 3) would seem to be invalid, as it fails to account for theincrease in surface charge and lateral hydrophobic interactions that occur in thesecond region of the isotherm.It was demonstrated in Section 3.3 that increasing the hydrocarbon chain length

of the surfactant, which increases the hydrophobicity of the monomer, displaces theadsorption isotherm to lower bulk concentrations. The rate of increase of surfaceexcess with concentration in regions II and III of the isotherm, in which adsorptionis partially or wholly hydrophobically driven, respectively, is more rapid forsurfactants with longer tail-groups. Both of these effects become more pronouncedas the surfactant chain length is successively increased.The critical intersection point(cip) between isotherms of the same surfactant at

different salt concentrations denotes the bulk concentration at which the electrostaticcontribution to adsorption changes from attractive to repulsive. At this concentrationthe orientation of adsorbing surfactant molecules switches from head-groups facingtowards the substrate to head-groups facing into solution. Below the cip, the addition


of electrolyte lowers adsorption by competing with monomers for charged surfacesites. Above the cip added electrolyte lowers the electrostatic repulsions experiencedbetween surfactants in aggregates, permitting tighter packing. This increases thesurface excess relative to the no added electrolyte case. Acid washing of the silicasubstrate prior to surfactant adsorption lowers the number of charged surface sitesand decreases the number of surfactant molecules adsorbed in region I. Thus, thereare fewer nucleation sites for continued adsorption lowering the saturation surfaceexcess for the acid washed substrate.Gemini surfactants are a relatively new class of surfactants that exhibit interesting

adsorption behaviour. Most importantly, the saturation surface excess has beenshown to be strongly dependent on the length of the spacer group, with shorterspacer length gemini surfactants having the highest saturation surface excess values.The adsorption mechanism for gemini surfactants differs from that of their mono-meric analogue only in the initial stages(region I). For spacer lengths greater than6, it appears that the gemini surfactant is adsorbed to the substrate by one head-group, with the other head-group electrostatically bound to a bromide co-ion somedistance from the interface. For spacer lengths less than 6, the second head-groupis held in sufficiently close proximity to the substrate to induce ionisation of asurface hydroxyl group, and both head-groups are bound to the substrate.Calorimetric investigations show that the first step in the adsorption process

involves significant displacement of adsorbed water. The energetics of subsequentsurfactant adsorption involve a significant cooperative hydrophobic interaction, suchthat the heat of adsorption is similar to bulk micellisation. Calorimetry also showsthat surfactant molecules with chain lengths of 12 or more carbon atoms interactwith ‘hydrophobic’ areas on the silica substrate presumably by adopting a flatconformation on the surface at low coverage levels.While some indirect evidence for the presence of discrete surface aggregates can

be obtained from adsorption isotherms and calorimetry, the major disadvantage ofthese techniques is the lack of information they provide concerning the adsorbedmorphology. Without knowledge of the adsorbed layer structure, surfactant adsorp-tion models must always be somewhat speculative. The remainder of this reviewwill essentially deal with techniques that aid in elucidating the precise nature of theadsorbed aggregate. With this knowledge, the most likely adsorption mechanismand the structures present in each region of the isotherm will be presented inSection 7.

4. Atomic force microscopy

4.1. Introduction

Perhaps the greatest advance in the study of surfactant adsorption at the solid–liquid interface of the last decade is the development of techniques to obtain in situimages of the adsorbed aggregates using the AFM. Imaging has allowed adsorptionisotherms to be analysed with knowledge of the aggregate morphology andcomplemented model dependent techniques such as neutron reflectivity to provide


greater accuracy. AFM is particularly well suited to detecting the periodicity ofdiscrete surface aggregates i.e. a peak to peak separation. However, AFM is notwithout its limitations. In order to image aggregates, a surfactant layer must havehead-groups facing solution to provide a repulsive force. Thus meaningful AFMdata are generally obtained only above the csac, and most AFM experiments arecarried out at concentrations greater than the cmc. As only the adsorbed layer facingsolution is scanned, for a bilayered aggregate the nature of the underlying layerremains speculative. AFM provides no information concerning the surface excessand the density of adsorption between aggregates is also open to speculation. As aconsequence, several possible morphologies can adequately justify a given image,particularly for bilayered aggregates. However, when AFM results are analysed withinformation from other forms of adsorption data, the most likely structure can bepredicted with reasonable confidence. Thus, AFM is most useful when used inconjunction with other measurements.Traditionally, the AFM has been used with the probe in contact with the substrate.

While this is suitable for the imaging of a hard surface, it is inappropriate forimaging adsorbed surfactants. Surfactant aggregates are generally fragile and hardcontact measurements will destroy the adsorbed morphology. However, the sensitivityof the AFM is such that it is possible to sense the repulsive electrical double layerassociated with the adsorbed species. By using a ‘fly height’ of approximately 1nm above the adsorbed layer, sufficient contrast is obtained to image the aggregatestructure. This type of imaging has been termed soft contactw61x and lowers, butdoes not eliminate, the risk of aggregate deformation.A considerable volume of surfactant adsorption research predates AFM. Some

AFM experimentation has been aimed at better understanding known surface andsolution effects. For example, for decades it has been observed that the addition ofsalt increases the surface excess of surfactant, and the AFM has been used toelucidate the precise response of the adsorbed aggregates to electrolyte. However,other AFM investigations have been motivated solely by a desire to manipulateaggregate morphology, accomplished by studying surfactants with unusual geome-tries or by specific surface modification. Both types of experimentation have greatlyadded to our understanding of adsorption phenomena.The size, shape and spacing of adsorbed surfactant aggregates are dependent upon

the intermolecular and surfactant–substrate interactionsw62x. These interactions arestrongly influenced by solution conditions such as the ionic strength and pH. Thesolution micellar size and shape is strongly related to the geometry of the monomer,however at a surface this structure must be reconciled with the confinementsimposed by the substrate. Nonetheless, the arrangement of aggregates at an interfaceis often analogous to solution structure observed at higher concentrationsw63x.

4.2. The earliest images of surfactant aggregation: CTAB on graphite

The first direct imaging of interfacial aggregation concerned the adsorption ofCTAB on the cleavage plane of highly ordered pyrolytic graphitew17x. Graphite isa frequently used substrate for AFM investigations because it is available in


Fig. 16. Adsorption of CTAB on graphite(a) an AFM image obtained in soft-contact mode using thedouble layer forces between the tip and sample. The adsorbed layer structure appears as stripes whichare spaced 4.2"0.4 nm apart(about twice the length of the adsorbed surfactant). Image size 240=240nm , z-range 1.2 nm.(b) The proposed hemicylindrical structure of CTAB on graphite in cross-section.2

The base molecules(shaded) were probably strongly bound epitaxially by the graphite surface, whilethe rest of the monomers in the hemimicelle are more labile.� 1994 ACS. Reproduced with permissionfrom Ref. w17x.

atomically smooth crystalline form, making it ideal for AFM investigations, andgraphon, a form of particulate graphite, is an often-used hydrophobic adsorbate. Thegraphite lattice consists of three equivalent symmetry axes and the interactionsbetween graphite and surfactants are primarily hydrophobic. The images obtained(one of which is reproduced in Fig. 16a) showed parallel stripes spaced 4.2 nmapart for CTAB concentrations between 0.8 and 5 mM. The orientation wasperpendicular to the symmetry axes of the substrate. As the period between thestripes was slightly greater than twice that of the extended monomer, the authors


suggested that the most likely surface conformation of the surfactant was ahemicylindrical arrangement, as reproduced schematically in Fig. 16b. Adsorptionisotherm data acquired more than 30 years prior to the AFM investigation hadrevealed a two-step adsorption isothermw64x. Without knowledge of the adsorbedstructure it was proposed that the shape of the isotherm resulted from the monomerbeing adsorbed with its alkyl chain extended in the substrate plane at lowconcentration, and as the concentration increased the monomers interacted laterallyto orient themselves perpendicularly to the substrate. This was thought to result inmonolayer formation. However, with knowledge of the ultimate equilibrium struc-ture, the new mechanism proposed by Manne et al. of hemimicelle formation hasrightly gained widespread acceptance, this is described beloww17x.

4.3. Graphite strongly orientates surfactant aggregates

The first step in the isotherm results from surfactant monomer binding stronglyto the surface via the hydrophobic interaction, forming a film that has a relativelylow rate of exchange with solution surfactants. The adsorbed surfactants alignthemselves with the graphite symmetry axis, with the tail-groups oriented towardsthe interior of the hemimicelle. This first layer acts as a template for subsequentsurfactant adsorption, as the solution concentration is increased. Aggregation isdriven by hydrophobic interactions between the exposed surfactant tail-groups andthe tail-groups of surfactants in the bulk.Graphite exhibits the highest degree of control over adsorbed aggregate structure

of any substrate investigated to date, due to its large interaction area with thesurfactant. Hemicylindrical aggregation has been observed on graphite for ionicw17,62,63,65,66x (conventional and gemini), nonionicw67–69x and zwitterionicw70xsurfactants on graphite with hydrocarbon tail-groups longer than 12 carbon atoms.Surfactants with tail-groups of 10 or fewer carbon atoms in length form a featurelessmonolayer on graphite. This is most likely due to the tail length failing to reach acritical length to successfully adsorb epitaxially and act as a template for hemicy-lindrical aggregation.

4.4. Adsorption studies on mica

Considerably different aggregate morphologies have been determined for thecrystalline mica substrate. As mica is hydrophilic, it is the head-group of surfactantmonomers that interact strongly with the substrate and thus, relative to graphite, thearea of interaction per surfactant molecule is greatly reduced. Mica has exchangeablesurface cations, the presence of which can bring about higher levels of coveragethan the less charged silica substrate. The density of the charges on the micasubstrate(0.48 nm) w61x is such that surfactants are adsorbed to the surface with2

their head-groups closer together than the equilibrium bulk separation(for compar-ison, the head-group area for CTAB in a micelle is 0.64 nm) w26x. This leads to2

adsorbed aggregates having a lower degree of curvature than the correspondingsolution micelles. The negative surface sites on mica are arranged precisely in the


Fig. 17. Adsorption of CTAB on mica in the presence of 10 mM KBr.(a) AFM image in 1.8 mMCTAB. (b) Schematic representation of the cylindrical structure of adsorbed CTAB. Part of the cylinderon the right has been cut away to reveal the interior. The cylinders may be flattened on the bottombecause of an attractive electrostatic interaction between the surfactant head-groups and the mica.�1999 ACS. Reproduced with permission from Ref.w72x.

surface lattice of the substrate, which is expected to influence aggregate morpholo-gies. However, relative to graphite, the mica surface only weakly orients the axialdirections of any adsorbed aggregatesw47,61,62x.

4.4.1. Alkyltrimethylammonium halides on micaDTAB, MTAB and CTAB have been shown to form flattened cylindrical

aggregates on mica. The period of these aggregates is similar to the diameter of asolution micelle, whilst the length of the aggregate was much largerw62,71,72x. Thelong axis of neighbouring aggregates was aligned locally in one of three directions,presumably due to the crystallinity of the substrate, giving a striped appearance.Interestingly, it was shown that for CTAB at twice the cmc the cylindrical structuresobserved 2 h after surfactant was passed into the fluid cell had been transformedinto a bilayer after approximately 24 h. Addition of 10 mM KBr had little effect onthe type of aggregate formed initially, but with added electrolyte the cylindricalstructure was stable up to at least 30 h. An AFM image of adsorbed CTAB and aschematic representation of the structure of the aggregate have been reproduced inFig. 17.

4.4.2. The influence of electrolyte on aggregate morphologyDucker and Wanlessw72x investigated the effect of added electrolyte on aggregate

shape. AFM imaging was used to demonstrate that the thermodynamically stableflat bilayer formed by CTAB was transformed to cylinders on the addition ofelectrolyte(KBr). As the electrolyte concentration was increased further, the longaxis of the cylinders was shortened, resulting in the formation of discrete aggregateswith no directional orientation. It was postulated that the addition of salt affected


Fig. 18. AFM images(150=150 nm) of gemini surfactant aggregates on the cleavage plane of mica.2

(a) Spherical admicelles of the asymmetric gemini surfactant 12-3-1(3.0 mM solution). (b) Cylindricalaggregates of the symmetric gemini surfactant 12-4-12(2.2 mM solution, or 2= cmc); (c) Adsorbedbilayer of the symmetric gemini surfactant 12-2-12(1.0 mM solution, or 1.3= cmc). � 1997 ACS.Reproduced with permission from Ref.w63x.

aggregation in two ways. Firstly, once the co-ion concentration reached sufficientlyhigh levels, it competes effectively with surfactant cations for surface charged sites.Additionally, the free counterion was electrostatically attracted to the head-groupsof the surfactant, lowering the tendency of the surfactant to bind to the surface.This released sufficient surfactant from the surface to reduce, then eliminate, thetemplating effect. This explanation is supported by the fact that more defects areproduced in the layer with increasing potassium ion concentration. Additionally, theaddition of H , which is known to be more strongly surface binding than K ,q q

produces a greater number of defects at the same concentration.

4.4.3. Gemini surfactants on micaGemini surfactants have proved particularly useful in the investigation of the

relationship between surfactant geometry and substrate interaction on aggregatestructure. Gemini surfactants offer the opportunity to systematically vary surfactantgeometry by changing chain andyor spacer lengths, which allows the importance ofsurfactant geometry on aggregate shape to be investigated. Manne et al.w63x studiedthe adsorption of gemini surfactants of various geometries to graphite and micawith interesting results. On the strongly orientating graphite, where adsorption occursvia hydrophobic interactions, hemicylindrical aggregates formed regardless of thesurfactant geometry. As with conventional surfactants, these structures were orien-tated perpendicular to the underlying symmetry of the substrate. However, oncrystalline mica, the surfactant geometry was shown to have a stronger influenceon the aggregate formed. This is shown eloquently in Fig. 18.The spherical structures formed by the asymmetric gemini 18-3-1 were also

present for 16-3-1 and 12-3-1, although the spacing between aggregates decreasedwith chain length. Cylinders formed in the presence of 12-4-12 and 12-6-12.


However, a reduction in the spacer length to 2 resulted in the formation of afeatureless bilayer. These results show the same general trend as solution aggregates(where the curvature of aggregates is lowered as the spacer length is decreased),and demonstrate the dependence of structure morphology on geometry, even whenthe surface exerts a templating effect.

4.5. Adsorption studies on silica

In contrast to the study of adsorption isotherms relatively few AFM studies haveemployed amorphous silica, perhaps primarily because it is more difficult to obtainclear images. In general, straight chain cationic surfactants have been shown toform essentially spherical admicelles with no long range ordering on the silicasubstratew62,73–75x. This morphology agrees well with the mechanism predictedby adsorption isotherms: that electrostatically adsorbed monomers act as nucleationsites for further surfactant adsorption. As with other substrates, factors that affectaggregation in solution often have consequences for structures at the silica–waterinterface.

4.5.1. The influence of electrolyte and counterion typeThe influence of surfactant and electrolyte concentration and type of counterion

on the adsorbed morphology was investigated by Velegol et al.w73x. For CTAB,when the surfactant concentration was increased from 0.9= cmc to 10= cmc theadsorbed morphology changed from short rods to ‘worm-like’. This occurredregardless of whether 10 mM KBr was present or absent, although added electrolytedid reduce the aggregate period. When the counterion was changed from bromideto chloride, the aggregate morphology was predominately spheroidal for bothconcentrations, and once again the presence of electrolyte did not significantlychange the aggregate structure. These results are reproduced in Fig. 19.Fig. 19 clearly shows the dependence of adsorbed layer morphology on the type

of counterion. These results were rationalised by the greater binding efficiency ofbromide over chloride allowing bromide to better stabilise the lower curvaturecylindrical structures. Using OR, it was shown that the change in structure fromspheroids to ‘worm-like’(for CTAC and CTAB, respectively) corresponded to thesurface excess increasing by a factor of two thirds.

4.5.2. Adsorption kinetics measured by AFMIn a recent study of the coadsorption of surfactants and polymers, Liu et al.w74x

made an interesting observation concerning the adsorption and desorption kineticsof CTAC. Although the kinetics of adsorption were not measured directly, forcecurves were analysed to give some indication of the adsorption rate. Immediatelyafter surfactant solution was passed into the AFM cell a steep repulsive force andadhesion was evident. This indicated the presence of bilayered surfactant aggregates,which were successfully imaged. When the cell was rinsed with water, the repulsivebarrier disappeared, and was replaced by an attractive force and larger adhesiveforce. The length of time that these forces persisted was found to be dependent on


Fig. 19. AFM images of a range of cationic surfactants on silica:(a) CTAB 10= cmc.(b) CTAB 0.9=cmc.(c) CTAC 10= cmc (d) CTAC 0.9= cmc, with push through the adsorbed layer to the underlyingsilica substrate shown in the bottom of the image. The peak-to-peak distance between CTAC aggregateswith and without 10 mM kBr is 10"2 nm at 10= cmc and 13"2 nm at 0.9= cmc. For CTAB, thepeak to peak distances are 10"1 nm without salt and 8"1 nm with 10 mM KBr at 10= cmc.� 2000ACS. Reproduced with permission from Ref.w73x.

how long the CTAC solution was exposed to the substrate. The general features ofthis effect are shown in Fig. 20.These results suggested that the molecules that serve to reverse the surface charge

are easy to desorb; whereas those that are electrostatically bound to the surface areretained by the surface much longer upon rinsing. The time dependence of thisprocess was justified using a model proposed by Chen et al.w76x. This adsorptionmodel proposed that the surfactant ion is initially adsorbed with a counterion bound,and that this counterion is expelled from the film over time, allowing the surfactantto be electrostatically bound to the surface. The electrostatic nature of this adsorptionserves to extend the desorption process. Desorption was found to proceed more


Fig. 20. Schematic representation of the desorption of CTAC from silica, showing the dependence onthe length of exposure of the surfactant to the surface.� 2001 ACS. Reproduced with permission fromRef. w74x.

slowly at higher pH values. As increasing the pH should increase the level ofelectrostatic binding, this result supports the proposed model.

4.5.3. The influence of counterion polarisabilitySubramanian and Duckerw75x investigated the effect of counterion on the adsorbed

structure of CTA on the basis of the ‘hardness’ of the ion. It was found that soft,q

polarisable (e.g. Br ) ions were more effective than hard counterions(e.g.y

CH COO , Cl ) at inducing shape changes in admicelles, namely a sphere-to-y y3

cylinder transition. It was suggested that as hard anions strongly bind water theyare relatively unavailable for binding to surfactant ions. Soft counterions, whichweakly interact with water, associate more readily with surfactant. Binding ofcounterions lowers the repulsive force between head-groups, permitting the formationof the less curved cylindrical aggregates. This study reported that surface micelleswere clearly observed at 0.5= cmc in the absence of electrolyte. This is becauseelectrostatic adsorption of surfactant to the substrate lowers the activation energy ofaggregate formation and permits the formation of admicelles below the solutioncmc.

4.5.4. Gemini surfactant aggregates on silicaThe adsorbed morphology of gemini surfactants at the silica–aqueous interface

has been investigated by Atkin et al.w77x. An image of 12-2-12 at approximately2= cmc is presented in Fig. 21 revealing flattened ellipsoidal structures. Similarstructures were imaged for 12-3-12. The adsorption density was found to beapproximately 150 aggregates per 10 000 nm and the aggregate thickness is2


Fig. 21. An AFM image of the silica–solution interface immersed in an aqueous solution of 2= cmc12-2-12 revealing adsorbed aggregates with approximately circular profiles in the upper portion of theimage. The slow scan direction is down the page. The underlying silica surface is imaged in the lowerportion of the page when a higher imaging force is applied.� 2003 ACS. Reproduced with permissionfrom Ref. w77x.

estimated to be 3.5 nm, from the push through distance obtained in the force curve.As these surfactants have been shown to form worm-like aggregates in solution, thesmaller surface aggregates must be attributed to the influence of the substrate. Thephysical dimensions and spacing of the admicellar structures presented here aresimilar to those observed for monomeric quaternary ammonium surfactants on silica(Section 4.5.1).In this study we reported that obtaining clear images of 12-3-12 was considerably

more difficult than for 12-2-12. This was attributed to the decreased adsorptiondensity of 12-3-12. The number of adsorbed molecules of 12-3-12 was 25% lowerthan that of 12-2-12. As the spacer length increased, the adsorption density continuedto fall as per Fig. 12. Fors)3, AFM images of the adsorbed surfactant layer couldnot be obtained. This was a consequence of the change in the nature of theinteraction force between the AFM tip and the surface, resulting fromproximaldesorption of surfactant.Proximal adsorption or desorption describes a change in surface excess as a

function of surface separation. Ducker et al.w78,79x recently demonstrated that themagnitude of proximal desorption was significant for cationic surfactants adsorbed


to silica surfaces. At concentrations where the surface charge is reversed due tosurfactant adsorption, surfactant monomers desorb from the surfaces upon approachin order to lower the electrostatic repulsion. Subramanian and Ducker has shownthat, for CTAB concentrations slightly higher than the cmc, the proximal desorptionincreases dramatically at surface separations of less than 10 nmw78x. At approxi-mately 9 nm the change in adsorption is;0.25 molecule nm . If the change iny2

adsorption was similar for gemini surfactants then this represents a significantproportion of the total surface excess, up to 50% for the larger spacer groups(asthere is a significant decrease in surface excess with increasing spacer length, cf.Fig. 12 and Section 6.3.12). Thus, fewer surfactant monomers are required to desorbfrom the substrate for the longer spacer sizes in order to expose a hydrophobicsurface. Additionally, the electrostatic repulsion is greater for the longer spacerlengths, therefore the driving force for desorption is likely to be greater. Both ofthese influences strongly suggest that proximal desorption during imaging will havea greater effect on the adsorbed structures as the spacer length increases.This proximal desorption process is depicted schematically in Fig. 22. It is

expected that solution-facing monomers will be more likely to desorb from theinterface as the tip approaches. As a consequence, a portion of the tail-groups ofthe monomers adsorbed with head-groups facing the silica substrate will be exposed.This will produce hydrophobic surfaces and consequently the surfaces jump togetherunder the influence of the long-range hydrophobic attraction, which precludesimaging of surface structures. As similar force vs. distance data were obtained forspacer lengths between 4 and 12, a similar process is likely occurring for all ofthese spacer lengths.As images of the adsorbed structures for spacers of length greater than 3 could

not be obtained, the likely structures of the adsorbed aggregates were inferred fromthe images of the 12-2-12 aggregates and the linear increase in the area per adsorbedmoleculew77x with spacer size. It is known that the same surface excess in termsof number of monomers is required to neutralise the surface charge for the geminisurfactants(cf. Section 3.4.6). Further it is generally accepted that surface aggregatesform around electrostatically adsorbed monomers(cf. Section 3). Therefore approx-imately the same number of surface aggregates should be formed for 12-2-12 up to12-12-12. This was supported by AFM imaging of the 12-2-12 and 12-3-12 geminisurfactants, which reveal similar numbers of aggregates per unit area despite a 25%difference in surface excess. As the same number of aggregates were present perunit area, and the surface area occupied by the aggregates did not change withspacer length, by assuming the mass density of the aggregates does not change, theaggregate volumes and shapes fors)3 were determined. It was shown that theadsorbed aggregates become more flattened as the spacer length is increased.Schematic representations of the variation in adsorbed structure with spacer lengthare shown in Fig. 23. Clearly as the head-group area increases(larger s), thesurfactant chains interdigitate to a greater degree in order to maximise chain–chaininteractions. This leads to the flattening of the surface aggregates. These flattenedaggregates appear to be less than optimal in terms of surfactant packing as they arevery easily disrupted.


Fig. 22. Schematic representation of the release of gemini surfactant molecules from an adsorbed aggre-gate upon approach of the AFM tip apex(represented by a large filled triangle) for spacer lengths offour or more. The surfactant concentration is sufficient to lead to surface aggregation. No attempt hasbeen made to represent counterions or surfactants that are not adsorbed either at the silica substrate orAFM tip in the first instance, and for simplicity all surfactants are represented in the cis conformationwith straight hydrocarbon chains. In(a) the tip is effectively far from the interface, at a separation of;100 nm. The AFM senses an electrostatic repulsion, but this repulsion is not sufficient to inducerelease of surfactant from the adsorbed aggregate.(b) Depicts the situation as the AFM tip continues toapproach the interface. At these smaller separations the magnitude of the electrostatic interaction issufficient to force surfactant monomers out of the admicelle. However, the overall electrostatic force isgreater than the hydrophobic attraction at these separations.(c) Represents the situation immediatelyprior to the jump into contact. The outer layer of the surfactant aggregates on both the AFM tip and thesubstrate have largely diffused into solution, resulting in the residual electrostatic repulsion being con-siderably reduced relative to the initial situation, and a significant amount of hydrophobic material beingexposed. The hydrophobic attraction between the surfactant adsorbed to the AFM tip and the substrateleads to the jump to contact observed in the force curve, the magnitude of which is indicated by thearrow. (d) Shows the expected configuration of surfactant adsorbed to the tip and the substrate in theconstant compliance region. Depending on the stiffness of the adsorbed layer, the AFM tip may pushthrough to contact directly with the silica substrate. The situation depicted also leads to the significantadhesion observed for all spacer sizes greater than 3 as the AFM tip is retracted from the substrate.�2003 ACS. Reproduced with permission from Ref.w77x.

4.6. Model hydrophobic substrates

Model substrates can be used to study the influence of factors such as surfacecharge, roughness, crystallinity and hydrophobicity on the adsorption process.Grant et al.w80x systematically investigated the influence of substrate hydropho-

bicity on the adsorption of a nonionic surfactant, namely octa(oxyethylene) n-dodecyl ether(C E ). In the absence of a charged head-group, the driving force12 8

for the adsorption of nonionic surfactants is derived from hydrophobic and van derWaals attractions. In this study, altering the ratio of chemisorbed hexadecane thioland thiohexadecanol to a gold substrate controlled the substrate hydrophobicity. As


Fig. 23. Schematic representation of the variation in the adsorbed gemini surfactant layer as the spacersizes increased. The diagrams presented are two-dimensional cross sections of three-dimensional aggre-gates present at the interface. Counterions are not represented. The lateral dimensions of the aggregateare similar at all spacer sizes, and it is the increased size of the surfactant head-group as the spacerlength increases that results in reduced aggregation numbers, a lower surface excess and a reducedaggregate thickness. As the adsorbed aggregates cannot be imaged for spacer sizes of four or more, thestructures depicted are proposed on the basis of area per molecule and force data.� 2003 ACS. Repro-duced with permission from Ref.w77x.

the relative amount of thiohexadecane was increased, so was the substrate hydro-phobicity. This allowed the preparation of five increasingly hydrophobic surfaces.The contact angle varied almost linearly with the proportion of chemisorbedhexadecane thiol, from 258 at 0%, to 1108 at 100% hexadecane thiol(the threeother surfaces prepared were 25, 50 and 75% hexadecane thiol). Force curves andimages were collected at a solution surfactant concentration of 2= cmc.Analysis of AFM images and force curves allowed the variation of substrate

hydrophobicity on the structure of the adsorbed layer to be commented upon. Thegeneral results are summarised in Fig. 24w80x. On the most hydrophilic surfaces(0and 25% CH terminated) the force curves showed that the surfactant was easily3

displaced from the surface. This indicated that the interactions between the surfactantand the substrate are relatively weak. On these surfaces, there were many sites atwhich water could be hydrogen bonded, allowing water to effectively compete forthe surface. Imaging of the adsorbed layer indicated diffuse micellar coverage, withan aggregate period of approximately 10 nm. Adsorption was attributed to hydro-phobic interactions and a comparatively weak van der Waals contribution, shown inFig. 24a.On the 50% methyl terminated surface, the adsorbed aggregate period was

decreased to 6 nm. A more stable repulsive barrier was present in the force curve,indicating a layer thickness of 6 nm. This value compared well to the solutionmicellar diameter of 6.2 nm. Increasing the substrate hydrophobicity had increasedthe strength of the hydrophobic interactions between the surface and the surfactant.The morphology of the surfactants was found to be close packed micellar aggregates


Fig. 24. Schematic representation of the influence of the variation in the percentage of CH terminated3

alkyl chains on C E adsorption. The remainder of the surface is composed of CH OH terminated12 8 2

groups.� 2000 ACS. Reproduced with permission from Ref.w80x.

(Fig. 24b). When the degree of methyl group termination was increased to 75%force imaging could reveal no discrete aggregation. However, once again the forcecurve indicated that the adsorbed layer was 6 nm thick. In this case the adsorbedlayer morphology was interpreted to be a classical bilayer, represented in Fig. 24c.When the degree of alkyl termination was increased to 100%, a featureless layerwas also imaged. However, in this case the layer thickness interpreted from theforce curve was approximately 4 nm. This indicates the presence of a surfactantmonolayer on the surface, see Fig. 24d.These results also serve to demonstrate the importance of interfacially adsorbed

water. The degree of water hydrogen bonded to the surface increases as the level ofalkyl termination is reduced from 100 to 0%. Where water is not bound at theinterface in great amounts, as on the 100 and 75% alkyl terminated surfaces, mono-and bi-layered aggregation results. The 50% alkyl terminated surface provides agreater quantity of sites for water binding, and discrete aggregates are observed.One could interpret this result on the basis of the adsorbed water preventing thebilayered aggregation, leading to the formation of curved aggregates. The 0 and25% alkyl terminated surfaces were even more hydrophilic, which allowed water tocompete more strongly for surface area. This led to the aggregates being spacedfurther apart and suggested a lower surface excess. Thus, increasing the level ofsubstrate hydrophobicity not only increases the attraction between the surface andthe surfactant, it also lowers the level of water interacting with the surface. Both ofthese factors have implications for structure of the adsorbed surfactant.


Several other AFM studies have used modified hydrophobic substrates and havegenerally reported adsorption of single layered aggregates with adsorption dominatedby substrate tail-group interactions. Briefly, Grant and Duckerw81x have investigatednonionic surfactant aggregation at an amorphous hydrophobic surface, prepared bycovalently attaching diethyloctylchlorosilane(DEOS) to a silica substrate. Therepulsive forces observed, together with AFM imaging of the adsorbed surfactant,were deemed to be consistent with the formation of a uniform monolayer at thesurface, with head-groups facing solution and the surfactant tail-groups and DEOSinteracting hydrophobically. Wolgemuth et al.w82x studied the adsorption of variousionic, nonionic and zwitterionic surfactants to an amorphous silica surface, hydro-phobised by the covalent attachment of trimethylchlorosilane(TMCS). It was foundthat the interfacial aggregates formed were roughly hemispherical, in contrast to thehalf-cylindrical structures formed on crystalline hydrophobic substrates. Similarsurfactant morphologies were observed regardless of the type of head-group, leadingthe authors to suggest that adsorption is driven by hydrophobic interactions betweenthe substrate and the surfactant tail-groups. SDS adsorption was not noted consis-tently on the TMCS treated substrate, which was attributed to a residual negativecharge associated with the silica after reaction with TMCS.

4.7. Summary of AFM investigations

AFM has provided direct evidence for the presence of discrete aggregates at thesolid–aqueous interface and has led to a slow revolution in our interpretation ofsurfactant adsorption data from all sources. AFM imaging is only useful above thecsac, where surfactant head-groups are facing towards solution, imparting a repulsion(electrostatic for ionic surfactants) that allows soft contact imaging to be accom-plished. The clarity of the AFM images obtained varies with the surfactant and thesubstrate under investigation. As the AFM is most useful at detecting periodicity,surfactant–substrate combinations that produce highly regular morphologies producethe clearest AFM images.The hydrophobic cleavage plane of graphite orients the adsorbed surfactant

structures more strongly than any other substrate. This is primarily due to a highlevel of (hydrophobic) interaction with initially adsorbing monomers, which tem-plates the subsequently formed hemicylindrical structures for surfactant chains withgreater than 10 carbon atoms. Chains of less than 10 carbon atoms are not templatedby the substrate and result in a laterally featureless adsorbed layer.The hydrophilic, crystalline mica substrate also orients the adsorbed surfactant

structures, but not as strongly as graphite. The alkyltrimethylammonium surfactantsform cylindrical aggregates, which were in some cases transformed into a featurelesslayer. However, the addition of electrolyte stabilises the cylindrical admicelles. Theadsorbed morphology of gemini surfactants on mica can be manipulated by alteringthe geometry of the surfactant monomer.Surfactants are less strongly oriented on silica surfaces, due to a lower surface

charge and the amorphous state of the substrate and perhaps also due to an increasein surface roughness. The adsorbed structure of CTAB and CTAC on silica has


been investigated. The adsorbed configuration of CTAB was concentration depend-ent, with short rods at 0.9= cmc and worms at 10= cmc. These structures wereinsensitive to the presence or absence of electrolyte. For CTAC, the adsorbedmorphology was insensitive to both the surfactant concentration and the presenceor absence of electrolyte, with spheroidal structures present at both 0.9= cmc and10= cmc in the presence and absence of electrolyte. In all cases there was little orno long range ordering of the adsorbed aggregates. Similar structures have beenobserved for MTAB and DTAB on silica.By altering the substrate hydrophobicity the adsorbed morphology of nonionic

surfactants can be controlled. By sequentially increasing the substrate hydrophobicitythe structure of adsorbed nonionic surfactants can be changed in a controlled mannerfrom diffuse adsorbed micelles on the least hydrophobic surface, to densely packedmicelles, to a bilayer, to a monolayer on the most hydrophobic interface.It is clear that adsorbed aggregate structures are often formed upon surfactant

adsorption at the solid–aqueous interface, and that the properties of the substratemay exert a high degree of influence on the type of aggregate formed. Thesestructures can be reconciled with the adsorption isotherm data presented in Section3, and this will be discussed in Section 7. The intervening discussion is aimed atmore completely characterising the structure of the adsorbed layer.

5. Fluorescence quenching experiments

5.1. Introduction

Fluorescence quenching studies statistically analyse the fluorescence emission ofa micelle bound probe molecule. A second molecule, known as a quencher,suppresses this emission. Fluorescence quenching is a well accepted method for thedetermination of micelle aggregation numbers in bulk solutionw83,84x. The fluoro-phore probe, typically pyrene, is excited at a particular wavelength and thefluorescence intensity monitored at a second wavelength. All fluorescence quenchingstudies assume that both the probe and quencher molecules are entirely containedwithin micelles, that there is no exchange between aggregates, and that the micellesare unperturbed by the presence of the probe and quencher.A random distribution of the fluorophore and the quencher throughout the system

under investigation is assumed. Therefore, some aggregates will contain both theprobe and the quencher, some contain either the probe or the quencher, and someaggregates will contain neither. Statistical analysis will allow the average numberof quencher molecules per micelle in the system to be calculated. As only micellescontaining the fluorophore are probed, the aggregation number of the micelles canbe determined from the intensities of the fluorescence emission in the absence andpresence of a known concentration of quencher. This relatively simple analysis isknown as the static emission method and for this method to be successful it isessential that quenching is rapid. It is assumed that there is no fluorescence frommicelles containing quencher, thus any emission from these micelles alters theaggregation number determined. This effect is expected to be more pronounced for


Fig. 25. Representative fluorescence decay curves for pyrene fluorescence from admicelles on silica.Two curves are shown for each surfactant: one with no quencher present and one in the presence ofquencher. The concentration of surfactant is as follows TTAB(Cs1.01 mM) and CTAB (Cs0.801mM). The amount of pyrene in each sample is approximately 1 pyrene per 50 surface-bound micelles.� 2000 ACS. Reproduced with permission from Ref.w85x.

large aggregates, where the rate of quenching will be dependent on the diffusionrate of the probe and quencher throughout the micelle core.

5.2. Time resolved fluorescence quenching

The time resolved fluorescence quenching experiment is not as dependent on theassumption of rapid quenching. This experiment uses a delta pulse excitation of theprobe molecules and the decay of the fluorescence emission with time is recorded.As the rate of quenching is dependent on molecular collisions between the probeand the quencher molecules, this gives rise to a fast initial decay, followed by aslower unquenched decay. Examples of the form of these curves are presented inFig. 25, reproduced from Strom et al.w85x. These curves were obtained from¨admicelles on a silica surface.For time resolved fluorescence quenching experiments, the aggregation number

can be determined from the curved portion of these data, or by extrapolation of thelong time data back tots0. This gives the intensity at time 0 in the presence andabsence of quencher molecules, and allows the aggregation number to be determinedin the same manner as for the static emission method.


5.3. Determination of aggregation numbers

In order to determine aggregation numbers, the concentration of aggregate boundsurfactant must be known. In solution, this can be determined from the cmc. Whenstudying the properties of aggregates adsorbed at an interface, the surface concen-tration must be determined by other methods, and it is assumed that all surfactantis aggregate bound. If aggregates are present in solution and at the interface, theprobe molecule will be distributed through both solution and surface aggregates,leading to results that are inaccurate. However, if the surfactant concentration ischosen such that it is below the solution cmc, but above the concentration at whichaggregates form on the surface(csac), then the probe molecule will be fully surfacebound. This allows the adsorbed aggregates to be probed directly.

5.4. The earliest fluorescence probe studies

The first fluorescence probe studies of adsorbed aggregates were performed byLevitz et al. w86,87x, using the static emission method. The adsorption of nonionicsurfactants such as Triton X-100, were investigated. It was shown that theaggregation numbers for adsorbed surfactant determined by fluorescence emissionfollowed similar trends to bulk micelles. It was thus concluded that the most likelyform of surface aggregate was that of a small surface admicelle. Fan et al.w19xperformed similar studies for ionic surfactant adsorption. The aggregation ofalkyltrimethylammonium bromides on an alumina substrate was investigated andthe presence of small surface bound aggregates was also determined. The size ofthese aggregates appeared to grow as the surface excess of surfactant increased.This result is shown in Fig. 26.These results agree well with the adsorption model for ionic surfactants to an

oppositely charged substrate suggested by the adsorption isotherm data described inSection 3. That is, electrostatically adsorbed monomers act as nucleation sites forfurther surfactant adsorption, leading to the formation of surface aggregates. Thisaccounts for the observed increase in aggregation number. These data also showthat fluorescence probe quenching gives insight into the structure of the adsorbedlayer at surface excess values below that where aggregates are fully formed, andare thus not readily investigated using AFM.

5.5. Time resolved fluorescence quenching of adsorbed aggregates

The first time resolved fluorescence quenching study of adsorbed aggregates wasperformed by Strom et al.w85x. The adsorption of DTAB, TTAB, CTAB and¨dodecyl-1,3-propylene-pentamethyl-bis(ammonium chloride) or DoPPDAC, a diva-lent cationic surfactant, was studied. The quencher molecules used in this studywere alkyl pyridinium surfactants. This choice was deemed suitable in light of thestructural similarity between the quaternary ammonium surfactants under study andthe alkyl pyridinium quencher, which minimised the degree to which the systemwas perturbed. Emission data typical of that obtained by this study are shown in


Fig. 26. Aggregation numbers for adsorbed surfactant aggregates on alumina formed using alkyltrime-thylammonium bromide surfactants of three different chain lengths. The aggregation numbers corre-sponding to a given surface excess are given on the figure for the steeply increasing region of theisotherms.� 1997 ACS. Reproduced with permission from Ref.w19x.

Fig. 25. In particular, note the initial curved decay, which results from micellescontaining both quencher and probe molecules, and note that the exponential portionof the curve, due to micelles containing probe only, has the same slope as the resultrecorded without quencher present. These features provide strong evidence for theformation of discrete surface aggregates, which was the principal finding of thisstudy. This result agrees well with AFM imaging results, and cannot be rationalisedsimply in terms of mono- and bi-layer aggregation. The evolution of aggregationnumber with surfactant concentration and with increasing electrolyte concentrationis reproduced in Fig. 27. It was judged that the uncertainty in these values was atmost 20%.The increase in aggregation number with surface excess observed in Fig. 27a

supports the trend observed by Fan et al. for the adsorption of alkyltrimethylam-monium bromides on aluminaw19x. Increasing the electrolyte concentration wasalso shown to increase the aggregation number. This was rationalised on electrostaticgrounds, with added electrolyte decreasing the electrostatic repulsions betweenmonomers within adsorbed aggregates. This facilitates tighter packing withinadmicelles, leading to the observed increase in aggregation number.

5.6. Summary of fluorescence quenching experiments

For the purposes of this review, fluorescence quenching experiments provide twoimportant results. Firstly, fluorescence quenching provides strong evidence for the


Fig. 27. Aggregation numbers of micelles on silica for DTAB(circles), TTAB (squares), DoPPAC(triangles), and CTAB(cross). Aggregation numbers are presented as a function of(a) bulk concentra-tion and(b) as a function of added electrolyte.� 2000 ACS. Reproduced with permission from Ref.w85x.

presence of discrete aggregates. This supports the results obtained using AFM, thatdiscrete admicellar structures are present at the substrate as summarised in Section4. Perhaps more importantly, several fluorescence quenching studies have shownthat the aggregation number of the adsorbed structures increases with surface excess.


This result provides the strongest evidence for the nucleation mechanism ofsurfactant adsorption, described previously in Section 3.

6. Reflectance techniques

6.1. Introduction and underlying principles

The principle of reflectivity is essentially the same with respect to scattering,regardless of whether light, X-rays or neutrons are used as the radiation source.Whereas X-rays are sensitive to the electron density, and hence the atomic numberof the species under investigation, the scattering of neutrons is dependant on thenuclear properties of the molecules being probed. Optical techniques such asellipsometry and optical reflectometry(OR) derive their sensitivity from thedependence of reflection and refraction on refractive index and interference effectsthat give phase information. These effects allow calculation of the adsorbed layerthickness or surface excess. The significantly shorter wavelengths associated withX-rays and neutrons allow the structure of the adsorbed layer to be examined moreintimately.

6.2. Neutron reflectivity and adsorbed layer structure

NR provides sufficient resolution to study the orientation of surfactant moleculesin the adsorbed layer, as different nuclei scatter neutrons with different amplitudes.In the case of protons and deuterons, neutrons are scattered with opposite phases,which allows contrast variation to be achieved by use of a combination of protonatedand deuterated adsorbates. This type of substitution alters the reflectivity of theadsorbed layer, but leaves the surfactant morphology unaltered. Additionally, bycareful adjustment of the proton to deuterium ratio of the solvent, the contrastbetween the solvent and the surface can be set to 0, resulting in the recordedreflectivity profile being dependent only on the adsorbed layer. The resolution, orthe smallest length scale able to be probed, is a function of the wavelength. Theshort wavelength of neutrons gives increased resolution over that of opticaltechniques. This, combined with the contrast control available using isotopicsubstitution, allows NR to elucidate concentration profiles close to the surface, andeven the orientation of the adsorbed species.

6.2.1. Limitation of NRThe major limitation of NR is that interpretation of the results is model dependent.

Conventionally, the adsorbed layer is treated as a laterally unstructured filmw88x.This allows the interference fringe that results from the reflections at the solid–layer and layer–solvent interfaces to be characterised by a thickness and a scatteringlength density profilew89x. In practice, the adsorbed film is often more complicatedthan a homogenous layer, and the only satisfactory method for determination of theperpendicular composition profile is to divide the adsorbed film into a series oflayers with separate density profiles using the optical matrix formulation of


reflectivities. Each layer is characterised by its scattering length density, thickness,and if necessary an interfacial roughness factor. The division of layers generallyreflects the different scattering length densities of the surfactant head-groups andalkyl chains.

6.2.2. Contrast controlThe additional contrast achieved by isotopic substitution provides information

that aids in selecting the most appropriate model. In spite of this detail, a determinedprofile is not necessarily a unique solution i.e. several models of the adsorbed layerstructure may fit the experimental result. For the most part, neutron reflection resultsfor surfactant layers have been interpreted as a monolayer on hydrophobic solidsand bilayered or a patchy bilayer morphology on hydrophilic substrates.Whilst these results are somewhat surprising in light of AFM imaging, if one

takes into account the limitations of the technique, useful information can be derivedfrom these studies. Neutron reflection gives the most direct measurement of thethickness of adsorbed aggregates, and provides the orientation of molecules perpen-dicular to the surface. However, it provides no data concerning the lateral orderingof molecules, and hence one cannot make definitive statements concerning the in-plane dimensions of aggregatesw90x.

6.2.3. NR studies of cationic surfactants on silicaSeveral NR studies have examined the adsorption of CTAB to silicaw89–91x.

The consensus of these studies is that the surface is incompletely covered. This isrationalised by either a defective bilayer, or by the adsorption of discrete micelle-like aggregates. The AFM and fluorescence data presented above indicate that theadmicelle morphology is the correct interpretation.McDermott et al.w92x showed that whilst the fraction of the surface covered

increased from 51% at 1y3= cmc to 65% at 1= cmc, the layer thickness of 3.4nm was consistent at both concentrations. From this result the authors suggestedthat the increase in the level of surface coverage was unlikely to be derived from achange in the aggregate structure. Due to the incomplete coverage of the surface itwas postulated that the water and surfactant occupy separate domains on the surface,with ‘islands’ of surfactants suggested. It was noted that these islands would mostlikely have head-groups protruding from the sides in order to shield the alkyl chainsfrom the water intermittent to the islands. It was shown that when the dimensionsof these islands were sufficiently small the head-groups at the sides of the aggregateswould become important to the fitting of a consistent optical model, in that bothhead-groups and chains would be present in the midplane of the aggregates.Secondly, inclusion of the intermittent water in the optical model suggested a lateralsize of 9 nm at the higher surfactant concentration. This result is consistent withthe dimensions observed using AFM in Section 4.5.1. Further, the fluorescencequenching studies show that the aggregation number increases with surface excess.This result is inconsistent with the above interpretation.These results were clarified by Fragneto et al.w90x, who also concluded that if

the adsorbed aggregates were micelle-like, then they must be strongly flattened.


This result has been corroborated by several authors using various techniques, andhas now become well accepted. This study also investigated the effect of surfaceroughness on adsorption. As does surface preparation, surface roughness couldcontribute to the variation in results for surfactant adsorption on silica substrates.The authors reached several important conclusions. Firstly, the level of surfactantadsorption decreased as the surface roughness increased. This result may seemcounter-intuitive, as increasing roughness should increase the available surface area.It was suggested that this effect could be derived from an inability of the roughersurface to match the curvature of the adsorbed aggregate. Additionally, the thicknessof the surfactant aggregates was increased on the rougher surface. This could beattributed to reduced electrostatic interactions between the substrate and the aggregateleading to a greater aggregate curvature. Alternatively the surface roughness couldadd to the thickness of the layer by varying the position of the centre of theaggregate relative to the surface.It has been consistently noted that adsorption studies using two or more

experimental techniques have a far greater probability of producing unequivocalresults. This is also the case with NR. On the basis of the numerous discreteadsorbed aggregates determined by AFM imaging, Schulz et al.w93,94x haverecently produced reflectivity models using unit cells consisting of spheres andcylinders. The authors point out that adsorbed films consisting of discrete aggregateswill naturally produce a level of surface coverage that is less than 100%. This is animportant observation, as the ‘patchy bilayer’ or ‘island’ interpretations suggestedby many NR studies are simply corrections made to account for fractional surfacecoverage, with little supporting evidence and can be seen as originating fromoutdated interpretations of adsorption isotherms where surface excess values consid-erably below that required for bilayer adsorption were interpreted as ‘patchybilayers’.The most recent investigation of Schulz et al. combined AFM and NR studies of

surfactant adsorption onto crystalline quartz and showed that NR can correctlydistinguish between adsorbed morphologies by using bulk solution contrast variationw94x. Three surfactant-quartz combinations were studied, and the structure of theadsorbed layer was determined using AFM imaging. At concentrations above thesolution cmc, TTAB was shown to form spherical admicelles, TTAB with 200 mMNaBr formed cylindrical structures and the double chained surfactant DDABproduced a laterally unstructured film. These morphologies are consistent with thesolution structures, where spheres, cylinders and bilayers have been determined forTTAB, TTAB and 200 mM NaBr, and DDAB, respectively.The authors produced theoretical scattering models for spherical, cylindrical and

bilayer morphologies for each surfactant. The best fit parameters used for each ofthe models is reproduced in Table 2. While each model had a unique scatteringlength density profile, it was not possible to determine the structure of the adsorbedlayer solely on the basis of goodness of fit, as all structures seemed equally likelyfor each surfactant.However, the structure of the adsorbed surfactant layer can be ascertained by

comparing surface excess values obtained in pure D O, and in a system where the2

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Table 2Fits to adsorbed layer structure for cationic surfactant systems on quartz from neutron reflectometry

NR in D O2 NR in contrast matched D OyH O2 2 AFM

G d t G d t d(mmol m )y2 (nm) (nm) (mmol m )y2 (nm) (nm) (nm)

TTAB Spheres 6.6"0.6 5.3"0.3 4.6"0.4 6.5"0.4 5.2"0.3 4.6"0.3 6.9"0.5Cylinders 6.1"0.6 5.1"0.5 3.6"0.4 6.4"0.3 6.0"0.3 4.0"0.2Bilayer 5.4"0.5 2.7"0.3 6.4"0.3 3.3"0.1

TTABq200 mM NaBr Spheres 7.3"0.3 5.2"0.2 4.7"0.1 6.9"0.3 4.7"0.2 4.4"0.2Cylinders 7.0"0.4 5.5"0.3 4.0"0.2 6.9"0.3 5.2"0.2 3.9"0.2 4.9"0.5Bilayer 6.3"0.4 3.0"0.2 6.8"0.3 3.2"0.1

DDAB Spheres 4.7"0.3 3.0"0.2 3.2"0.2 4.9"0.2 3.5"0.2 3.6"0.2Cylinders 4.5"0.3 2.5"0.2 2.6"0.2 4.8"0.2 3.3"0.1 3.1"0.1Bilayer 4.2"0.2 2.1"0.2 4.8"0.2 2.6"0.1

Consistent fits to NR and AFM images for each system are set in bold, showing agreement between adsorbed amounts in the D O and quartz contrast-2

matched D OyH O measurements. Also listed are the film thicknessest and nearest-neighbour spacingsd, from NR fitting and AFM images, showing2 2

agreement between the two techniques for the best fit NR case. Redrawn from Ref.w94x.


solution is contrast matched to the quartz substrate. This is achieved by varying theratio of H O to D O in the bulk. For the correct model, the surface excess(and the2 2

fitted parameters) must be equivalent at both contrasts. As can be seen in Table 2,the surface excess values(G) are indeed independent of the model used when thequartz and the bulk solution are contrast matched. In the unmatched system, thesurface excess values show considerable variation. The interfacial model which bestfits these data, between these limits in contrast, is shown in bold in Table 2. Theresults obtained agree well with those obtained with AFM imaging. This led theauthors to suggest that this method of adsorbed layer structure determination maybe of particular use when the necessary conditions for soft contact AFM imagescannot be obtained. This elegant work indicates that AFM and NR data arecompatible provided an appropriate model is employed to interpret the NR data.

6.3. Ellipsometry, optical reflectometry and adsorption kinetics

The evolution of the adsorbed layer structure with concentration is oftencommented upon when studying surfactant adsorption using equilibrium techniques.These studies imply a series of processes that occur as the interfacial concentrationincreases. As we have seen previously for ionic surfactants, the general consensusis that surfactant adsorption is by electrostatic means at low concentrations, and asthe surface excess increases lateral hydrophobic interactions lead to induction ofsurface charge and hemimicelle formation. This is followed by the formation ofaggregates at higher surface excess values. This interpretation is quite well estab-lished and accepted. However, whether these processes occur kinetically is open toquestion. That is, for a concentration where aggregates result, is the surface chargefirst neutralised, followed by hemimicelle formation, then by bilayered aggregateformation, or do these processes occur simultaneously? Additionally, the contributionor otherwise of micelles to the adsorption process is often questioned. Investigationsof surfactant adsorption kinetics should elucidate these issues.

6.3.1. Dynamic aspects of surfactant adsorptionIn contrast to equilibrium adsorption characteristics, the dynamic aspects of

surfactant adsorption have been far less studied and are consequently not as wellunderstood. This is in spite of the importance of adsorption kinetics to many real-world applications such as wetting, lubrication and spreading. The difficulty inmonitoring the expected fast kinetics of the adsorption process is the primary reasonfor the lack of investigation of adsorption rates. However, the recent developmentof ellipsometric techniques with high temporal resolution has made it possible tostudy the kinetics of adsorption and desorption with adequate precision.

6.3.2. Principles of optical techniquesOR and ellipsometry both monitor the variation in the reflectivity of an interface

upon adsorption. These variations are induced by the change in the refractive indexprofile of the substrate upon the adsorption of a surfactant layer. Both techniquesuse a linearly polarised light source that is reflected off the substrate to which


adsorption takes place. The polarisation characteristics of the reflected light aremonitored. These polarisation changes are highly sensitive to the presence of asurfactant layer. Ellipsometry allows the average thickness and refractive index ofthe layer throughout the adsorption process to be calculated by monitoring theellipsometric anglesc andD with time. For an extensive explanation of ellipsometry,see Ref.w95x. OR records the amplitudes of the perpendicularly polarised compo-nents of the reflected beam. Upon adsorption, the ratio of these two intensities isaltered and this allows for calculation of the surface excess. OR is described indetail in Ref. w96x. Unlike NR, reflectance techniques using light as the radiationsource cannot give information concerning the molecular structure of the adsorbedlayer.

6.3.3. Hydrodynamic considerationsIn order to make quantitative statements concerning the adsorption kinetics it is

critical that the hydrodynamics of the measurement be well defined. Whether theadsorption process is transport-limited or limited by the rate of layer organizationcannot be discerned unless the rate of diffusion of surfactant to the substrate isknown. Several studies have suffered from a lack of hydrodynamic controlw97–101x. This has complicated the accompanying attempts at modeling the observedkinetics. However, the qualitative results obtained are still of value. When hydro-dynamic conditions are well defined, such as that provided by use of a stagnantpoint flow w102x, the character of the adsorption process can be commented uponwith much greater certainty.

6.3.4. Ellipsometric measurements of nonionic surfactant adsorptionSeveral ellipsometry studies have investigated the adsorption of nonionic surfac-

tantsw98–101x. Whilst the hydrodynamics of the interface were not well controlled,interesting results were nonetheless obtained. For a nonionic surfactant adsorbing tosilica, only weak interactions are expected between the surfactant and the substrate.In this case, a transport limited adsorption process is expected, and the resultsobtained were in good agreement with this.Tiberg et al.w98x used in situ ellipsometry to study the adsorption and desorption

kinetics of polyethylene glycol alkyl ethers at the silica–aqueous interface. A typicaladsorptionydesorption cycle was determined that comprised of five separate kineticregions. It was found that the rate of adsorption was strongly dependent on the bulksurfactant concentration. The rate of adsorption continued to increase above thecmc, clearly demonstrating the involvement of micelles in the adsorption process.Representative results are presented in Fig. 28.The increasing section of the adsorption profile was divided into two separate

regions: first, a linear region where adsorption increases monotonically with time,followed by a transition region where the adsorption rate decreases. The linearregion of the result was attributed to the rate of diffusion of monomers and micellesfrom the bulk through the stagnant layer in the immediate vicinity of the surfacebeing lower than the rate of adsorption to the substrate. That is, in this portion ofthe data the adsorption rate was thought to be transport limited. As the surface

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Fig. 28.Left: Time evolution of the surface excess(G, open diamonds) and mean optical thickness(d , filled diamonds) for C E at concentrations, fromf 14 6

bottom, 0.007, 0.009 and 0.01 mM. All concentrations are below the cmc.Right: Time evolution of the surface excess(G, open diamonds) and mean opticalthickness(d , filled diamonds) for C E at concentrations, from bottom, 0.025, 0.1 and 0.25 mM. All concentrations are above the cmc.� 1994 ACS.f 14 6

Reproduced from Ref.w98x.


Fig. 29. Desorption kinetics from silica for three different hexaethylene glycol monoalkyl ethers:C E , (filled diamonds); C E (open diamonds); C E (filled triangles). The inset shows the corre-12 6 14 6 16 6

sponding process for different polyethylene glycol monododecyl ethers: C E(open triangles); C E12 5 12 6

(filled triangles); C E (open diamonds). � 1994 ACS. Reproduced with permission from Ref.w98x.12 8

excess increased, the concentration gradient between the surface and the bulkdecreased. This lowered the rate of adsorption to the surface and led to the curvedregion of the adsorption data. Throughout this transition region, the rate of adsorptionsteadily decreased, presumably as the surface became more saturated. This eventuallyled into the plateau region of adsorption. For surfactant concentrations above thecsac, it was shown that the equilibrium surface excess increased as the size of thehydrocarbon moiety of the surfactant increased. Similar results have been reportedfor ionic surfactants by solution depletion methods cf. Section 3.3.Interestingly, for all surfactant concentrations studied, the measured mean optical

thickness was found to rapidly increase to a steady value of approximately 4.7 nm.This was interpreted as indicating that the adsorbed layer was built up of micellarlike structures, which had a well-defined thickness even in the initial stages of theadsorption process. Higher surface excess values were achieved by the formation ofmore surface aggregates that eventually saturated the surface, or by increasing thepacking density of monomers in the existing aggregates.When the adsorption plateau was reached using a bulk concentration above the

cmc, no desorption was apparent until the solution surfactant concentration fellbelow the csac. This demonstrated the cooperative nature of nonionic surfactantadsorption. The general features of the desorption kinetics are given in Fig. 29,showing the effect of alkyl chain length and head-group size.Two regions were also identified in the desorption results. The surface excess

initially decreases in a linear fashion with time. It was suggested that the rate ofdiffusion away from the surface was the rate-limiting step in this region. As shownin Fig. 29, the rate of desorption increases by an order of magnitude when thelength of the alkyl chain is increased by two carbon atoms. However, variation in


Fig. 30. Reversibility of the adsorption–desorption cycle of C E adsorbed on polystyrene coated silica.12 5

C s0.17 mM. First desorption upon dilution with Cs0.0006 mM. Further desorption with water.1 2

Second cycle: adsorption of a 0.17 mM surfactant solution. Desorption by water. The soluteysolventswitches are indicated by arrows.� 2000 ACS. Reproduced with permission from Ref.w103x.

the head-group size had only a slight effect on the desorption rate. These effectswere thought to be directly correlated with the corresponding changes in the csac.That is, as surfactant diffuses away from the surface, the adsorbed equilibriumadjusts such that in the region of the solution immediately adjacent to the surfacethe surfactant concentration is kept constant at the csac. The lower csac of thesurfactants with longer tail-groups was therefore able to bring about the shorterdesorption times observed. The increased strength of hydrophobic interactionsbetween adsorbed surfactants was not considered.The decrease in surface excess eventually becomes non-linear. This slowing of

desorption suggested that the monomer concentration could no longer be maintainedat the csac. This shows that as the surface excess decreases, the rate of dissociationof surface micelles becomes the rate-determining step.

6.3.5. OR studies of nonionic surfactant adsorptionThe adsorption of nonionic surfactants to synthetic hydrophobic substrates has

also been investigated. Geffroy et al.w103x used OR with stagnant point flowhydrodynamics to study the adsorption of ethylene oxide surfactants to a polystyrenecoated silica surface. Interestingly, it was shown that a portion of the surfactantlayer was irreversibly bound to the substrate.The hydrodynamics associated with the stagnant point flow make it particularly

suitable for cycling experiments, where solvent, then surfactant solution is succes-sively flowed toward the substrate. An example of such an experiment is shown inFig. 30, which shows that the initial adsorption with a surfactant concentration of0.17 mM rapidly leads to a surface excess of;1 molecule nm . This was firsty2


desorbed with a much lower concentration of surfactant, 0.0006 mM, which led tothe surface excess being reduced to;0.4 molecule nm . When pure water wasy2

passed into the cell, no change in the surface excess was detectable. Uponreintroduction of the higher surfactant concentration the original surface excess wasobtained, and rinsing with water returned the surface excess to the lower adsorptiondensity. This led the authors to suggest that an adsorbed amount of;0.4molecule nm would be obtained at very low surfactant concentrations. The authorsy2

suggested that this irreversibly bound surfactant was adsorbed flat at the surfacebased on the form of the desorption kinetics and molecular area considerations.In this study the kinetics of adsorption were relatively fast, and the equilibrium

surface excess values were reached within a few minutes. Below the cmc, the rateof adsorption increased linearly with concentration. Above the cmc the rate ofadsorption continued to increase albeit more slowly. In this system, it was notexpected that the micelles would adsorb to the surface directly, as the head-groupsare quite hydrophilic. Nonetheless, the observed increase in the rate of adsorptionabove the cmc suggested that the micelles were acting as a source for monomersadsorbing to the surface, which increased the adsorption rate.

6.3.6. Adsorption kinetics of CTAB on silicaSeveral studies have investigated the adsorption kinetics of CTAB at the silica–

aqueous interface. The presence of the charged head-group should provide a greaterinitial driving force for adsorption. However, the native charge associated with thesilica surface will be neutralised at low surface excess values. Goloub et al.w2xhave shown that the surface charge will compensate for the charge associated withthe adsorbed surfactant to a significant extent, but the maximum potential of thesurface has been reached well below the saturation surface excess. Thus, for asurfactant concentration above the csac, adsorption is against an electrostaticrepulsive barrier throughout most of the adsorption process.

6.3.6.1. Ellipsometry. Eskilsson and Yaminskyw104x used in situ ellipsometry tostudy the adsorption of CTAB to silica. The adsorption isotherms were determinedfor both CTAB and CTA , reproduced in Fig. 31. These isotherms showed that theq

level of surfactant adsorption began to increase significantly at approximately 0.1mM, with the most rapid increase in the level of adsorption occurring between 0.4and 0.8 mM. In this concentration range the surface excess rose from 1 to 4mmol m , which was the saturation surface excess. At the beginning of this regiony2

of sharp increase in surface excess, the level of counterion adsorption becameappreciable. This indicates that surfactant is starting to be adsorbed with the head-group facing away from the surface. These results are in reasonable agreement withthose obtained by Goloub et al.w2x for a similar cationic surfactant.For solution surfactant concentrations above 0.4 mM the thickness of the adsorbed

layer was determined to be approximately 2.5 nm. This value did not increase withsurfactant concentration, even though at this concentration the surface excess wasonly 25% of the saturation value. It was suggested that the Coulombic repulsionsbetween adsorbed surfactant serve to orientate the surfactants normal to the surface.


Fig. 31. Adsorption isotherms for CTAB(crosses) and CTA (circles) at the silica–aqueous interface.q

� 1998 ACS. Reproduced with permission from Ref.w104x.

The kinetics of adsorption were found to be strongly dependent on the bulkconcentration. For surfactant concentrations less than 0.1= cmc up to 2 h wasrequired for equilibrium to be obtained at the surface. However, as the surfactantconcentration approached and exceeded the cmc, adsorption was complete withinminutes. For all surfactant concentrations studied, the rate of desorption was fast,complete within a few minutes and no concentration dependence was found.Examples of adsorption and desorption experiments are shown in Fig. 32.

6.3.6.2. Optical reflectometry. Pagac et al.w105x investigated the adsorption ofCTAB to silica using OR with laminar flow hydrodynamics. Unfortunately, thisstudy was influenced by a trace impurity that caused large overshoots in the surfaceexcess values for surfactant concentrations below the cmc. This impurity was latershown to be due to leaching from PVC tubing contained within a peristaltic pumpw73x. Results obtained above the cmc appear to be unaffected, and display kineticssimilar to those obtained by Eskilsson and Yaminskyw104x. The equilibrium wasreached within approximately 1 min of adsorption commencing. Once again, theinvolvement of micelles in the adsorption process was indicated by an increase inthe rate of adsorption above the cmc.

6.3.7. The role of micelles in adsorptionThe role of micelles in the adsorption process for CTABw106x and CPBrw107x

was elucidated by our group using an optical reflectometer with stagnant point flowhydrodynamics. The well defined hydrodynamic flow fields associated with stagnantpoint flow permits the measured initial rate of adsorption to be compared to the


Fig. 32. Surface excess vs. time during adsorption from 1 mM CTAB solution. The surfactant is injectedat time 0. Rinsing of the sample cell with pure water is initiated atts1420 s. The inset shows theadsorption from 0.1 mM CTAB.� 1998 ACS. Reproduced with permission from Ref.w104x.

theoretically derived diffusion limited flux to the surfacew102x. The quotient ofthese two values is recorded as the sticking ratiow106x, and is simply the numberof surfactant molecules that are adsorbed to the surface normalised by thetheoretically derived diffusion limited flux to the surface. Note that this is anoversimplification of the actual process, as adsorbed surfactant molecules are freelyexchanging with surfactant in the bulk. However, trends in the sticking ratio cangive valuable insight into the nature of the adsorption process. An increase insticking ratio with concentration indicates cooperativity, a decreasing sticking ratiosuggests competitive adsorption, while a constant sticking ratio infers that surfactantmolecules are adsorbing independently.The sticking ratios for CTAB and CTAB with 10 mM KBr are shown in Fig. 33.

In this figure the concentration of surfactant has been normalised by the cmc foreach system(1.25=10 M with 10 mM KBr, w108x, 9.0=10 M without salty4 y4

w109x). For concentrations above the cmc the theoretical flux consists of twocomponents, the flux due to monomers and the flux due to micelles. Any increasein the concentration above the cmc is reflected only in the flux due to micelles asthe monomer concentration is assumed to be constant.The important result from Fig. 33 is the marked increase in the sticking ratio for

both systems at the cmc, indicating an increase in the efficiency of the adsorptionprocess above the cmc. If surfactant molecules were competing for adsorption sites,this would be reflected in a reduction in the sticking ratio. In the absence of salt,the sticking ratio is essentially constant up to the cmc indicating that the monomersare adsorbing independently. With 10 mM KBr present the sticking ratio is higher


Fig. 33. The sticking ratio(determined from the initial rate of adsorption) vs. concentration for solutionsof CTAB, normalised by the corresponding cmc, with(squares) and without(diamonds) added 10 mMKBr. The sticking ratio increases at the cmc for both systems.� 2000 ACS. Reproduced with permissionfrom Ref. w106x.

than in the absence of salt and is seen to gradually increase even below the cmc,indicating that adsorption of monomers is cooperative in this case. Cooperativeadsorption is clearly aided by the screening of the head-group charge by electrolyte.Perhaps such screening allows hydrophobic interactions to play a greater role inadsorption? The sticking ratio increases sharply at the cmc, both in the presenceand absence of electrolyte, which clearly indicates that above the cmc adsorptionhas become cooperative. Similar trends were determined for CPBrw107x.The most obvious explanation for cooperativity is that micelles are directly

adsorbing to the surface, either partially or wholly. For this to lead to the observedtrend in sticking ratio the increased success with which micelle bound monomersadsorb to the substrate must be due to more effective penetration of the electricaldouble layer of the interfacial layer by micelles compared to monomers. Opposingadsorption is the electrostatic repulsion of this double layer, and as a micelle has asignificant number of counterions associated with it, we propose that surfactantcontained within a micelle will more effectively penetrate the double layer than amonomer. This is due to the reduced charge per monomer contained in a micelle.

6.3.8. The influence of electrolyte on adsorptionThe effect of changing the counterion from bromide to chloride on the surface

excess, kinetics of adsorption and adsorbed layer structure was investigated usingOR and AFM in the study of Velegol et al.w73x. The implications for the adsorbedlayer structure have already been discussed in Section 4.5.1. For now we shallconcern ourselves primarily with the adsorption kinetics. Below the cmc, the surfaceexcess reached 80% of its maximum value within 2 min, while above the cmc,equilibrium was attained within this time interval. When the surface was rinsed withwater, the surfactant was completely desorbed within seconds.


Fig. 34. Response of the adsorbed layer to a change in the type of counterion in solution. All solutionscontain 2 mM C TA X in 10 mM KX where X is either Br or Cl . The composition is varied asq y y y

16

follows: (1) CTAB in KBr; (2) CTAC in KCl; (3) CTAC in KBr; (4) CTAB in KCl; (5) CTAB inKBr. The surface excess for the upper curve is calculated from the optical data by using the refractiveindex increment of CTAC and for the lower curve by using the refractive index increment of CTAB.�2000 ACS. Reproduced with permission from Ref.w73x.

The suitability of OR for performing cycling experiments is demonstrated by Fig.34. This experiment utilised a 2 mM surfactant solution with 10 mM electrolyte,which was continuously flowed over the substrate. The composition of the solutionwas varied between combinations of CTAB, CTAC, KBr and KCl. In any combi-nation, the added electrolyte provided 83% of the total counterions in solution. ForCTAC with 10 mM KBr, the surface excess obtained is the same as that of CTABwith 10 mM KBr. However, when the chloride concentration was greater than thatof bromide, as with CTAB with KCl, the surface excess is intermediate to thatreached for CTAC with KCl and CTAB with KBr. These results show that whenboth the chloride and bromide counterions are present, there is competition forbinding to the aggregate surface, both in solution and adsorbed at the interface. Thisinfluenced the surface excess obtained. The results obtained in this experiment wereconsistent with the bromide ion having a much higher affinity for the aggregatethan the more hydrated chloride ion. This was evident in the higher surface excessvalues obtained for the bromide counterion.


Fig. 35. Adsorption Isotherms for CTAC; without added electrolyte(closed diamonds), with 10 mMKCl (open diamonds), with 10 mM NaCl (open triangles) and with 10 mM LiCl (open circles). Formore information see Ref.w112x.

6.3.9. The influence of co-ion type on CTAC adsorptionWe have also investigated the effect of variation in the co-ion type on adsorption

behaviour. The CTAC system was used to investigate this effect, as it has beendemonstratedw73,75,110,111x that the concentration of chloride ions has littleinfluence on the surface aggregate structure formed upon adsorption of CTAC.Thus, if variation in adsorption is observed, it is likely to be a consequence ofaltering the electrolyte co-ion.The adsorption isotherms for CTAC and CTAC in 10 mM NaCl, KCl and LiCl

(henceforth referred to as CTAC and CTACqNaCl, CTACqKCl and CTACqLiCl,or collectively as CTACqXCl) are presented in Fig. 35. The adsorption isothermsare shifted to lower concentrations in the presence of electrolyte. Importantly, thesize of this shift is independent of the co-ion identity. In fact, the adsorptionisotherms for the different salts are remarkably similar, indicating that the co-ionhas little effect on the surface excess above or below the cmc. It is possible thatthe different co-ions cause different adsorption behaviour at very low surfactantconcentrations where electrostatics provide the driving force for adsorption. How-ever, the surface excess values at these adsorption levels are below the sensitivityof the reflectometer. The rates of adsorption for the three electrolyte types were alsoalike at all surfactant concentrations, indicating that the co-ion has little effect onthe adsorption rate.These results agree well with the AFM study of Subramanian and Duckerw78x

(Section 4.5.3) that suggested that the addition of LiCl up to a concentration of 500mM had little effect on the adsorbed aggregate morphology of CTAC. The resultsobtained here extend this finding, and show that KCl, NaCl and LiCl have no


Fig. 36. (a) Adsorption isotherms for CTAB(open squares), MTAB (closed diamonds) and DTAB(open triangles) in the presence of 10 mM KBr(b) demonstrates the similarities in the form of theadsorption isotherms when the same data are plotted as a function of the solution cmc. For more infor-mation see Ref.w112x.

measurable effect on the saturation surface excess up to 10 mM XCl. Thus, it couldbe expected that similar surface aggregate structures would be present for CTAC inthe presence or absence of 10 mM XCl.

6.3.10. Effect of chain length on adsorptionWe have also studied the influence of the length of the hydrocarbon chain of

quaternary ammonium surfactants using ORw112x. The isotherms in Fig. 36demonstrate that the data are qualitatively similar for DTAB, MTAB and CTAB inthe presence of 10 mM KBr. At concentrations below 0.1= cmc, where surface


Fig. 37. Initial adsorption rate for CTAB(open squares), MTAB (closed diamonds) and DTAB(closedtriangles) in the presence of 10 mM KBr. The dashed vertical lines represent the solution cmc for eachsurfactant system. Reproduced from Ref.w112x.

excess values are low, the isotherms are essentially the same within instrumentallimitations. For concentrations greater than 0.1= cmc the isotherms separate, withthe surface excess increasing in the order DTAB-MTAB-CTAB for a givenfraction of the cmc. The increase in the surface excess is too large to be simply dueto the increased mass of individual surfactants i.e. the number of surfactant moleculesadsorbed at the interface increases in the order DTAB-MTAB-CTAB.The kinetics of adsorption for DTAB, MTAB and CTAB in 10 mM KBr are

presented in Fig. 37. The general form of the data are the same for the threesurfactants. Below the cmc the rate of adsorption increases steadily with concentra-tion. In the vicinity of the solution cmc there is an abrupt increase in the rate ofadsorption, as noted previously in Section 6.3.7. This is due to the direct adsorptionof micelles, which facilitates rapid and effective surface aggregate formation.Comparison of the adsorption kinetics for the three surfactants is not as simple

as it may at first appear. If one chooses a surfactant concentration and compares theadsorption rates, unless all three surfactants are below or above their cmc values,different surface and bulk structures will be present. This will affect the rate atwhich the adsorbed layer is created and thus the initial adsorption rate. It may seemthat the solution to this problem is to normalise the data by the solution cmc sothat the surface structures and hydrophobic driving force at a given fraction of thecmc are similar. However, in doing this the absolute concentration of the surfactantsbeing compared will be vastly different, which will have a similarly large effect onthe flux of surfactant to the substrate. This obviously hampers any comparison.However, it is possible to make some direct comparisons in certain regions of theisotherms, and several more general observations.Below 0.02 mM, where the surfactants are far below their solution cmc values,

the initial adsorption rates are low for all three surfactants. At these concentrations


it is expected that electrostatic interactions between the surfactant and the substrateprovide the driving force for adsorption and that hydrophobic interactions betweenthe tail-groups will have little, if any, effect on the adsorption rate. Thus, the initialadsorption rates are all quite similar.At 0.1 mM the three surfactants are still below their respective cmc values, but

there is a clear trend with the adsorption rate increasing in the order DTAB-MTAB-CTAB. If one examines the adsorption isotherms(Fig. 36) at thisconcentration, we note that the surface excess values increase in the same order.That is, not only does an increased hydrophobic driving force lead to the assemblyof a more complete adsorbed layer, it also leads to significantly faster adsorptionrates.Above 1 mM CTAB and MTAB are both above their solution cmc values. Once

again CTAB has the faster adsorption rate. The increased rate of CTAB adsorptionin this case is most likely due to the fact that the CTAB solution will have a muchhigher proportion of its surfactant contained within micelles. As micelles areconsiderably more effective at covering the substrate than monomer unitsw106,107x,this brings about a faster adsorption rate. Above the cmc for DTAB no data wereobtained for CTAB. However, it is still possible to compare data for DTAB andMTAB. The same effect occurs in this case, with MTAB having a significantlyfaster adsorption rate for the concentration corresponding to the DTAB cmc.In general, once the surface excess is appreciable(i.e. above 0.2 mM) the rate

of adsorption is always greater for the surfactant with the longer hydrocarbon chain.When the surfactants are all below their solution cmc values it is the increasedhydrophobic interactions between monomers that leads to the faster adsorption rate.A longer tail-group surfactant will also have a greater proportion of surfactantcontained within micelles(when present). As micelles are more effective in creatingan adsorbed layer on the substrate, this means that the longer chain surfactant willalso always have a faster adsorption rate above the cmc. Thus, at any concentration,the adsorption rate always increases in the same order as the tail-group size.

6.3.11. The slow adsorption regionOur OR investigations have revealed a concentration range in the vicinity of the

csac where long-term increases in adsorption are observedw106,107,111,112x. Thisregion is known as the slow adsorption region(SAR), and was first noted for 0.6mM CTAB in the absence of salt, where the adsorption of surfactant occurred overa period of several hours. This is shown in Fig. 38. The surface excess rapidlyincreased up to;0.6 mg m , but then proceeded very slowly until it reached they2

final equilibrium adsorption value of;1.6 mg m which was equivalent to thaty2

reached at all higher concentrations of CTAB. Similar long-term increases in surfaceexcess have also been elucidated for CTACw111x for surfactant concentrationsbetween 0.6 and 0.9 mM.These unusual results were investigated in more detail using CPBr. As CPBr is

UV active, solution surfactant concentrations could be determined with highprecision spectrophotometrically. This allowed the effect of small increases insurfactant concentration on slow adsorption phenomena to be ascertained. As shown


Fig. 38. Surface excess of CTAB at the pyrogenic silica–solution interface vs. time for 0.6 mM CTAB.Adsorption occurs over a much greater time period than for other concentrations. The surface excessinitially increases rapidly to 0.6 mg m (which equals the equilibrium excess achieved at slightly lowery2

concentrations) then over a number of hours adsorption increases to the plateau level obtained for slightlyhigher concentrations. At these higher concentrations equilibrium adsorption levels are obtained withinminutes Upon introduction of pure water at 12 000 s the adsorption returns to baseline levels. Reproducedwith permission from Ref.w106x.

in Fig. 39, solution CPBr concentrations of 0.2 and 0.55 mM were sufficient to leadto a plateau coverage of 0.4 and 1.6 mg m , respectively. It was between thesey2

two boundaries, and only between these boundaries, that long-term increases inCPBr adsorption were noted.For CPBr concentrations between 0.274 and 0.306 mM, the surface excess rapidly

increased to approximately 0.4 mg m , and then continued to increase at a greatlyy2

reduced rate. Changes in solution CPBr concentrations of only 0.01 mM weresufficient to bring about notable increases in the rate of the secondary adsorption.The rate of this secondary increase in adsorption was proportional to the surfactantconcentration. Increasing the solution surfactant concentration to 0.336 mM CPBrled to a fast increase in the level of surface coverage to 0.9 mg m , followed byy2

a similar slow increase in the level of adsorption. The rate of the secondaryadsorption in this case was decreased over that observed for concentrations between0.274 and 0.306 mM. For each of these experiments the surface excess had notreached plateau levels after 30 h. Limitations associated with data collectionprevented longer observation times.These results suggest that in the secondary phase of the adsorption process

adsorbing surfactants are filling surface adsorption sites stochastically. As theconcentration is increased, the probability of a monomer adsorbing is also increased,leading to a faster rate of secondary adsorption. The initial level of surface excessis higher for 0.336 mM(0.9 mg m ), which suggests that the adsorbed surfactanty2

layer is more complete in the initial stages of adsorption than that obtained withslightly lower bulk concentrations. Consequently, there was a reduced number of


Fig. 39. Measured surface excess of CPBr at the hydroxylated silica–solution interface vs. time for CPBrconcentrations of 0.202(filled circles), 0.274 (filled diamonds), 0.284 (filled squares), 0.306 (filledtriangles), 0.315(open squares), 0.336(open diamonds) and 0.554 mM(open triangles) with no addedelectrolyte. Adsorption occurs over a much greater time period than for concentrations outside this range.The surface excess initially increases rapidly to 0.4 mg m(or 0.9 mg m for 0.336 mM CPBr) theny2 y2

continues to increase over many hours.� 2000 ACS. Reproduced with permission from Ref.w107x.

adsorption sites available on the surface, which led to the observed reducedsecondary rate of adsorption at this concentration.In this work AFM imaging was used to investigate the structural changes that

must accompany these slow increases in adsorption. AFM images of the silicasurface were obtained 20 min and 22.5 h after the injection of 0.3 mM CPBr. After20 min had elapsed there was no evidence of any structure present in the adsorbedlayer on the silica surface, despite a surface coverage of;0.4 mg m obtained byy2

reflectometry. However, after 22.5 h the presence of an adsorbed layer structureconsisting of elongated admicelles is clearw107x. Changes in the force vs. distancedata collected over this time period were also consistent with the slow formation ofsurface aggregates.It appears that the SAR is a consequence of kinetic barriers to the formation of

the thermodynamically stable arrangement of adsorbed surfactant. As the SARoccurs at concentrations below the solution cmc, adsorption is due only to monomers.These monomers must participate in an aggregation process at the interface thatresults in the formation of surface structures analogous to bulk micelles. During thisprocess, a monomer may have to sample many sites before it is successfullyincorporated into a surface aggregate. Additionally, an incoming monomer must alsoovercome an electrostatic barrier to reach the surface, as the surface excess of 0.4mg m is sufficient to cause charge reversal of the silica surface. Thus, wey2

concluded that the SAR is a result of kinetic barriers to the adsorption process, inthe form of structural and electrostatic components.The boundaries of the SAR were determined by both the surface structure and

coverage, and by the aggregate structure of surfactant in solution. At concentrationsbelow the SAR the concentration in bulk is not sufficient to raise the chemicalpotential of the monomer to a level where surface aggregation is favourable. The


surface excess therefore indefinitely remains below that required to give rise tosurface aggregates. At concentrations above the SAR surface aggregates are clearlyforming and are doing so rapidly. One explanation is that at concentrations abovethe SAR but below the cmc a number of transient semi-formed aggregates arepresent in solution and these premature aggregates adsorb to the surface, negatingany requirement for monomer units to slowly pack into a preformed aggregatestructure. Alternatively, as the bulk substrate concentration is increased the electro-static repulsion between the monomer and the surface will be screened moreeffectively. This will decrease the energy barrier to adsorption for monomers andfacilitate more rapid accumulation at the surface.The electrostatic screening interpretation is supported by the presence of a SAR

for CTAC in the presence of 10 mM LiClw112x. This important result shows thatlong-term increases in adsorption can occur in the presence of electrolyte, providedthe electrolyte does not influence the surface structure. Recall that the addition of10 mM XCl to CTAC solutions has little effect on the equilibrium surface excessw112x (cf. Fig. 35) or the adsorbed structure(cf. Section 4.5.3). Thus, while thepresence of 10 mM XCl diminishes the electrostatic repulsion an adsorbing monomerexperiences in the secondary adsorption phase, the same structural barriers toadsorption remain in the presence of electrolyte. This permits long-term increasesin adsorption for CTACq10 mM LiCl and indicates that it is the structural barrierto adsorption that is critical for the evolution of slow adsorption effects.Using OR in conjunction with AFM, similar secondary increases in surface excess

have been observed for low surfactant concentrations at high pHw111x. Theadsorption kinetics were monitored using OR, and it was shown that for 0.11 mMCTAB at pH 9.4, a rapid increase in adsorption occurred in the first 10 s, followedby a slow rise(2–3 h) after which the equilibrium surface excess was reached.This result was complemented by variation in force vs. apparent separation data asa function of time and AFM imaging. Immediately after surfactant was passed intothe AFM cell, the substrate-tip interaction was dominated by an attractive jump intocontact starting from an apparent separation of approximately 10 nm. This originatesfrom a hydrophobic attraction between the tail-groups of the surfactant adsorbed tothe tip and surface. With additional time, a steeply repulsive force was observed,typically from a separation of approximately 5 nm. This permitted images of theadsorbed aggregates to be obtained, suggesting an adsorbed morphology similar tothat obtained at much higher surfactant concentrations, cf. Section 4.5.1.These results show that a critical density of surfactant molecules adsorbed at the

interface is required to initiate spontaneous aggregation on the surface, rather thana certain bulk surfactant concentration. Increasing the pH of aqueous solutions incontact with silica leads to a substantial increase in surface charge. This promotesthe adsorption of cationic surfactants from dilute solution. Once a certain surfaceexcess is reached(0.6 mg m for CTAB on silica), the density of hydrophobe ony2

the surface is such that bilayered aggregation is initiated i.e. the adsorption of moresurfactant via hydrophobic interactions. The successful imaging of these aggregatesindicates that the csac for silica at pH 9.6"0.4 is in the vicinity of 0.055 mM


Fig. 40. Surface excess of CTAB on hydroxylated silica vs. time for(a) sequentially increasing and(b)sequentially decreasing CTAB concentrations in the absence of electrolyte. A stable baseline wasobtained in water; the surfactant solutions were passed into the cell in series. Surfactant concentrationsused were 0.05 mM(filled circles), 0.15 mM(open diamonds), 0.3 mM(open triangles), 0.5 mM(opencircles), 0.6 mM (closed squares), 0.7 mM (closed diamonds), and 0.8 mM(open squares). Note thelong-term increase observed for 0.6 mM. Surfactant concentrations of 1, 3 and 5 mM were also used.These concentrations all produced an equilibrium surface excess of 1.4 mg m within experimentaly2

limitations and are not shown here for clarity. Water(crosses) is passed into the cell at 16 500 s andthe non-zero surface excess obtained is due to baseline drift. Only representative data points are provided.However, the line to guide the eye provides a reasonable indication of the form of the experiment.Reproduced from Ref.w113x.

CTAB, about an order of magnitude lower than at neutral pHw111x. As such it isno longer logical to normalise the csac as a function of the bulk concentration butto acknowledge that a certain surface concentration of adsorbed surfactant isrequired to initiate surface aggregation. This approach not only considers thesurfactant–surfactant interactions which are correlated with the bulk behaviour butincludes the influence of the substrate.


OR enables different surfactant concentrations to be consecutively analysed, thisallowed the SAR to be further probed by cycling experimentsw113x. Fig. 40a showsthe increasing concentration cycle and Fig. 40b the decreasing concentration cycle,where surfactant concentrations were sequentially equilibrated at the interface. Asexpected, a stepwise increase in surface excess is noted for all concentrations in theincreasing concentration cycle, except for 0.6 mM, where long-term increases insurface excess are noted. In the decreasing concentration cycle of this experimentthe surfactant solutions were passed into the cell in reverse order and a stepwisedecrease in surface excess results. When water was passed into the cell at 16 500 s,a surface excess of approximately 0.15 mg m results. This reflects drift in they2

baseline over the course of the experiment, rather than residual surfactant adsorbedat the surface. We note that the drift in these experiments is greater than for othersas the temperature control is adversely influenced by the large number of solutionchanges.The long-term increase for 0.6 mM observed in this concentration cycling

experiment(Fig. 40a) is somewhat different to that described above in Fig. 38, asin this case the surface excess does not reach saturation levels of coverage. Whilstthis may seem problematic, there are important differences between the experimentspresented in Figs. 38 and 40a. In the first case, 0.6 mM CTAB is introduced to asubstrate which is initially bare and a fast increase in surface excess to 0.6mg m results. It is assumed that, in the period of fast adsorption, the surfactanty2

adsorbs in a manner which is favourable for continued adsorption, albeit slow. Thisallows the adsorption to continue, and saturation surface excess is eventuallyreached. In the concentration cycling experiment, when the 0.6 mM surfactant ispassed into the cell, it encounters a pre-adsorbed surfactant concentration of 0.6mg m that has been sequentially built up over a period of approximately 1 h.y2

Thus, in the cycling experiment when 0.6 mM is passed into the cell, the structureinitially present at the interface is the equilibrium structure for 0.5 mM CTAB. Thispre-adsorbed structure leads to a different structural adsorption path that becomeskinetically trapped at a surface excess below the equilibrium surface excess. Becauseof this, it is not until the surfactant concentration reaches 0.8 mM that saturationlevels of coverage are reached.Surprisingly the desorption cycle reveals an increase in surface excess for

concentrations below 0.5 mM. Some of this discrepancy can be attributed to thedrift in the baseline of;0.1 mg m , but not all of it. This suggests that in they2

increasing cycle, equilibrium is not achieved at the lower concentrations despite theobservation that the surface excess is stable. Perhaps this is an extreme case ofkinetically trapped conformations and suggests that the SAR may be indicative of amore general effect at low concentrations and thataccurate equilibrium adsorptionisotherms may only be obtained by desorption from higher concentrations.

These results highlight the importance of the surfactant structure in slowadsorption, and also serve to illustrate the folly of implying kinetic informationfrom equilibrium data.


Fig. 41. The adsorption of 12-s-12 gemini surfactants at the silica–solution interface at 1= cmc: 12-2-12 (filled diamonds), 12-3-12(closed squares), 12-4-12(closed triangles), 12-6-12(closed circles), 12-8-12 (open squares), 12-10-12(crosses) and 12-12-12(open triangles). A typical adsorption result for1 mM CTAB (open circles) is included for comparison. The surfactant solution is first passed into thecell at;15.5 s leading to surfactant adsorption. The plateau level of adsorption is maintained whilstthe surfactant solution is flowing into the cell.� 2003 ACS. Reproduced with permission from Ref.w77x.

6.3.12. Adsorption of gemini surfactants to silica by ORThe adsorption kinetics of 12-s-12 gemini surfactants to silica has been investi-

gated w77x, and is presented in Fig. 41. The surfactant concentrations shown are;1= cmc. For a given spacer length there was no variation in the form of the dataover the range of surfactant concentrations studied. For each surfactant a baselinewas obtained for;15 s prior to surfactant being introduced into the cell, at whichtime the surface excess rapidly increases. For all spacer lengths, equilibriumadsorption levels are attained within 4 s of adsorption commencing. The surfaceexcess rises rapidly to approximately 70% of the equilibrium surface excess,followed by a slower increase to the final value. The equilibrium surface excessremained constant whilst the surfactant concentration was maintained. For compar-ison, the adsorption curve of CTAB, also at 1= cmc, is included. CTAB has alonger alkyl chain(C ) than the gemini surfactants under investigation here, but a16

similar cmc(0.9 mM), and is therefore an appropriate comparison. The form of theadsorption for CTAB is similar to that of all the gemini surfactants and identical tothat obtained for 12-3-12. Upon water being passed into the cell, desorption wascomplete and rapid for all surfactants(not shown).This study reported the most complete set of adsorption isotherms obtained for

gemini surfactants at the silica–aqueous interface, with adsorption isotherms forseven different spacer sizes investigated, presented in Fig. 42. Below 0.3= cmc,the adsorption isotherms are coincident. This indicates that electrostatic interactionsbetween the substrate and the adsorbed surfactant play a dominant role in theadsorption process up to this concentration. The saturation surface excess decreases


Fig. 42. Adsorption isotherms for 12-s-12 gemini surfactants at the silica–solution interface with theconcentration axis as a function of the cmc: 12-2-12(filled diamonds), 12-3-12(closed squares), 12-4-12 (closed triangles), 12-6-12(closed circles), 12-8-12(open squares), 12-10-12(crosses) and 12-12-12 (open triangles). Below 0.3= cmc there is no apparent variation in surface excess with spacer size.Above 0.3= cmc the isotherms begin to separate, and the saturation surface excess decreases withincreasing spacer size.� 2003 ACS. Reproduced with permission from Ref.w77x.

with increasing spacer size. The implications of this decrease for the adsorbedmorphology are detailed in Section 4.5.4.There are two important differences between the data obtained in this study and

the depletion studies examined in Section 3.4.6. Firstly, solution depletion studies(cf. Fig. 12) show a substantial increase in surface excess above the solution cmc,an effect that was more pronounced for short spacers. No evidence of this effectwas detected in the current study for any spacer length. The isotherms for all spacerlengths reached saturation at or slightly below the solution cmc. The same behaviouris found for conventional cationic surfactants adsorbing to silicaw106,107x. Thecause of the discrepancy between this study and those previous is unclear. Thesecond discrepancy relates to the concentration required to lead to the secondadsorption stage. Previous adsorption isotherms reported that the second adsorptionstage for the longest spacer investigated(12-10-12) commenced at lower concentra-tions than for those with shorter spacer groups. This was attributed to the increasedhydrophobicity of the longer chain surfactant. In this study the concentration leadingto the second adsorption step was independent of the spacer length. The differencebetween the two studies may be related to a change in the ionic strength of thesolution on surfactant adsorption that can occur in depletion studies. Surfactantadsorption to charged surface sites on silica can induce nearby hydroxyl groups tobecome more acidicw2x. For longer spacer groups this may result in both head-groups interacting with the substrate and a consequent increase in solution ionicstrength w41x. An increased ionic strength will result in the solution cmc beingdecreased, shifting the features of the adsorption isotherm to lower concentrations.


Fig. 43. Adsorption isotherms for CTAB(closed squares), DTAB in no added electrolyte(closed tri-angles) and DTAB in 10 mM NaBr(open triangles) on the AAPP substrate. The filled lines are drawnto guide the eye. The dashed lines separate the regions of the adsorption isotherms. Within instrumentallimitations, the adsorption isotherms for CTAB and DTAB with no added electrolyte are indistinguishablebelow 0.1 mM, as are the DTAB no added electrolyte and DTAB with 10 mM NaBr isotherms aboveapproximately 3 mM.� 2003 ACS. Reproduced with permission from Ref.w114x.

In the experiments using the reflectometer, the ionic strength of the solution isunaffected by surface ionisation.

6.3.13. Adsorption of ionic surfactants to a charged hydrophobic substrateFor comparison, surfactant adsorption at a negatively charged, hydrophobic,

plasma polymer substrate was used to study the influence of the substrate onadsorption behaviourw114x. The acetaldehyde plasma polymer(henceforth denotedAAPP) surface used is similar to silica in charge density but differs in that theregions between the charge sites are hydrophobic. The adsorption isotherms forCTAB, DTAB and DTAB with 10 mM NaBr are presented in Fig. 43 and for SDSand SDS with 10 mM NaBr in Fig. 44. The isotherms can easily be divided intofour-regions. Region I is a low surface excess region where the level of adsorptionwas independent of the bulk concentration. Regions II and III occurred where thesurface excess was increasing linearly with concentration, the latter increasing moresteeply, and a final plateau region where no further adsorption is observed as thebulk concentration of surfactant is increased(region IV). It was noted that the firstregion was absent for the anionic surfactant systems. This difference was attributedto the negative surface charge on the AAPP substrate, which drives electrostaticadsorption of cationic surfactant and hinders adsorption of anionic surfactant.In Fig. 43 it can be seen that up to a surfactant concentration of;0.02 mM the

adsorption isotherms of CTAB and DTAB are indistinguishable. This indicates thatthe adsorption in the first region is independent of the hydrophobicity of thesurfactant monomer, and is thus driven purely by Coulombic attractions between


Fig. 44. Adsorption isotherms for SDS in no added electrolyte(closed circles) and in 10 mM NaBr(open circles) on the AAPP substrate. The lines are drawn to guide the eye with regard to the trend inthe data series. The dashed lines separate the regions of the adsorption isotherms. Note that region I isabsent. Within experimental limitations, the isotherms are indistinguishable above 3 mM.� 2003 ACS.Reproduced with permission from Ref.w114x.

the surfactants and the charged sites on the surface. In the DTABq10 mM NaBrsystem the surface excess is much lower in this region of the isotherm. We attributethis difference to screening of the surfactant–substrate electrostatic interaction andyor reduction in the number of charged sites suitable for adsorption on the surfacedue to adsorption of Na ions. Similar effects have been observed on silica for theq

adsorption of CPBrw107x. Note no measurable adsorption occurs for the SDSsystems. That is, the first region is absent in Fig. 44.For all isotherms, further adsorption beyond region I takes place against a

repulsive electrostatic interaction and therefore must be driven by hydrophobicinteractions. The concentration which separates the first and second regions of theisotherm is analogous to the cacw115x for surfactant–polymer interactions insolution and is denoted thesurface cac. In region II, all systems exhibit a linearincrease of surface excess with the log of concentration above the surface cac. Theslope of the isotherm in this region is indicative of the magnitude of the favourablehydrophobic surface–monomer interactions. The slope of the CTAB isothermexceeds that of the DTAB isotherm. This is a direct consequence of the greaterhydrophobicity of the C vs. C hydrocarbon chain. Importantly, all the systems16 12

remain linear until the same surface excess of;0.3 mg m is reached. Saturationy2

of this adsorption process occurs at this concentration and strongly suggests that thearrangement of surfactants on the surface is the same in all systems. It is postulatedthat coverage of the substrate in this region is by random sequential adsorption.Adsorbed surfactant chains are confined to the plane of the surface by hydrophobicinteractions whilst the head-groups may protrude from the surface in order tomaximise hydration. For the cationic surfactants, the surfactants adsorb opposite a


surface charge, orientated with head-groups electrostatically bound to the chargedsurface group, and hydrocarbon chains adsorbed to the substrate. Between thecharged substrate groups, surfactants are adsorbed with hydrocarbon chains againstthe hydrophobic surface, while the head-groups may be orientated towards solution.For SDS, it is anticipated that the surfactant will be adsorbed in the same manneras for DTAB and CTAB but lie between charged surface sites.The next sub-region occurs for surface excess values from 0.3 mg m up to they2

saturation level of coverage for each system(;0.5 mg m for C systems andy212

0.7 mg m for CTAB). The surface excess is now too large for all the surfactanty2

monomers to be confined to a prostrate layer against the substrate; a more complexstructure must exist. This region of the isotherm exhibits the greatest rate of increasein surface excess with increasing bulk concentration. This rapid increase is aconsequence of cooperative hydrophobic interactions between adsorbed and adsorb-ing monomers. The end result of this process is the plateau region of the adsorptionisotherm, and further increases in concentration have no effect on surface excess.The level of adsorption beyond the prostrate monolayer coverage achieved at the

end of region II is not large(;40% of the total). The proposed structure is one ofmonomers adsorbed to the most hydrophobic patches of the prostrate monolayer.This will leave the more hydrophilic charged sites exposed to water. Thus theoverall structure is one of a prostrate monolayer with a partially(;65%) completesecond prostrate monolayer attached to it. The sparseness of the ‘second’ layerensures that head-group interactions are not important. Note the ‘second’ layershould not be considered a distinct layer as it will be anchored in the prostrate layerand very incomplete. The disorder in the structure ensures that AFM images cannotbe obtained.The times to obtain equilibrium for surfactant adsorption on the AAPP substrate

were also reported. In regions II, III and IV of the isotherms when electrolyte isabsent, the adsorption of both SDS and DTAB to the AAPP substrate typicallyrequired approximately 2500 s to achieve equilibrium. On the addition of electrolyte,these equilibration times were considerably reduced, to approximately 100 s. It issuggested that the presence of added salt permits more rapid coverage of thesubstrate by reducing the range of electrostatic repulsions between head-groups.Lengthening the hydrocarbon chain from 12 to 16 carbon atoms(i.e. DTAB toCTAB) resulted in a similar reduction in equilibration time. In this case, thereduction could only be due to the increased hydrophobicity of the monomerimparted by a longer tail-group, as electrostatic influences are not expected to besignificantly different between DTAB and CTAB in the absence of electrolyte.The initial adsorption rates were investigated, and it was shown that CTAB had

the fastest initial adsorption rate of the surfactants studied. This was also attributedto the greater hydrophobic driving force imparted by a longer tail-group. Theadsorption rates for the DTAB, DTABq10 mM NaBr and SDSq10 mM NaBrsystems were quite similar at all concentrations. The SDS no added electrolytesystem had the lowest initial adsorption rate, due to the electrostatic repulsionsbetween the substrate and the monomer.


6.4. Summary of reflectance observations

Results from NR provide the most accurate measure of the thickness of theadsorbed layer of CTAB on silica, of approximately 3.5 nm, as well as a lateralsize of discrete aggregates of approximately 9 nm. It was suggested that if theseaggregates were micelle-like, then they must be strongly flattened. This result agreeswell with push through distances measured using AFM, and the flattening of theaggregate is most likely due to the surface charge of the substrate.SAR on silica have been identified for CTAB and CPBr at normal(unadjusted)

pH in the absence of electrolyte, for CTAC both in the presence and absence ofadded electrolyte, and for CTAB at pH 10 using OR. We propose that the nature ofthe surfactant arrangement in solution and on the surface at equilibrium determinesif an SAR region is likely. An SAR can only occur if the packing constraints of amonomer in a surface aggregate is very much greater than the packing constraintsin solution. In the absence of electrolyte, the equilibrium surface structure presentis small micelle-like surface aggregates adsorbed on the surface, whereas in solutiononly monomers are present. It is this process of building order at the surface thatgives rise to the slow kinetics. The presence of electrolyte increases the aggregationin solution and at the same time results in surface aggregates with less curvature,thus removing the mismatch in packing constraints between the bulk and the surface.The evolution of these structures were followed by AFM. As the SAR correspondsto the first concentration where aggregates are formed at the interface, the lowerconcentration boundary of the SAR is equivalent to the csac. Slow adsorption effectshave not been elucidated in the presence of the electrolyte of a strongly binding co-ion i.e. the addition of 10 mM KBr to CTAB or CPBr solutions. Slow adsorptioneffects have been observed in the presence of the weakly binding chloride counterion.This supports a structural interpretation for the origin of slow adsorption. Thegemini surfactants did not exhibit slow adsorption for any spacer size.For surfactants in the presence of the electrolyte of a strongly binding co-ion,

equilibrium is rapidly reached at all surface excess values and increases in thesurface excess above the cmc are noted. This is a result of a higher packing densityof the surface aggregates and a change in aggregate morphology. Thus, for thesesystems, the csac is actually greater than the cmc. In these systems aggregates occurin bulk before they are present on the surface indicating that the energy cost of thegeometrical constraints of the surface on the aggregate structure exceed theadsorption energy.In both the presence and absence of electrolyte, chloride surfactants exhibit

similar features to bromide surfactants in the absence of electrolyte, due to thechloride ion having a much lower affinity for surfactant aggregates than the bromideion. The decreased binding efficiency of the chloride ion results in the saturationsurface excess of the chloride system being approximately 50% less than that of thecorresponding bromide surfactant. For concentrations up to 10 mM, the identity ofthe electrolyte co-ion does not influence adsorption behaviour within the experimen-tal limitations of OR.


For constant solution conditions, lengthening the hydrocarbon tail-group of thesurfactant monomer leads to two important effects. Firstly, the entire adsorptionisotherm is ‘shifted’ to lower concentrations, in line with the reduction in solutioncmc, and the width of the increasing region of the isotherm is decreased. Secondly,the saturation surface excess is increased for the longer chain surfactants. Calculationof the number of surfactant monomers per unit area reveals that this increase insurface excess is due to the presence of a greater number of molecules adsorbed atthe interface, and not merely a result of the increased mass of the monomer units.For 12-s-12 gemini surfactants adsorbing at the silica–aqueous interface it has

been shown that below 0.3= cmc, the surface excess is independent of the spacersize. At these concentrations attractive electrostatic interactions between the adsorbedsurfactants and the substrate determine the level of adsorption. At increasedsurfactant concentrations further adsorption is due to hydrophobic interactionsbetween adsorbed monomers. The saturation surface excess is a function of thespacer size, with the shortest spacer size producing the largest saturation surfaceexcess.Whereas the equilibrium surface structures determined for cationic surfactants on

silica have been shown to be elongated spherical structures, the adsorbed morphologyof the surfactant layer on the AAPP substrate is a prostrate monolayer with apartially (;65%) complete second prostrate monolayer attached to it. The ‘second’layer is not a distinct layer as it will be anchored in the prostrate layer and veryincomplete. CTAB has the highest saturation surface excess of the surfactantsinvestigated on this substrate. However, the area per molecule data shows thatapproximately the same numbers of molecules are adsorbed for all surfactant–substrate combinations at saturation and so the increase in surface excess is due tothe increased mass of the CTAB monomer. On this substrate, DTAB and SDS havethe same saturation surface excess both in the presence and absence of added 10mM NaBr, despite the surface having a substantial negative charge. This demon-strates the dominance of hydrophobic interactions for surfactant adsorption ontohydrophobic substrates.Studies of surfactant adsorption kinetics give insight into the mechanism of

surfactant adsorption. Comparison of the measured kinetics of adsorption(forcationic surfactants adsorbing to silica) to the theoretical diffusion limited flux ofsurfactant to the substrate has revealed a cooperative character in the adsorptionprocess above the solution cmc. This cooperativity is due to the direct adsorptionof micelles to the substrate.The kinetic aspects of gemini surfactant adsorption have been shown to be similar

to that of conventional surfactants. The adsorption process is complete withinseconds for all spacer sizes, at all concentrations. Thus the slightly slower micellarmonomer exchange rates for gemini surfactants has little influence on the adsorptionkinetics.For a constant head-group type, a longer tail-group will have a higher initial

adsorption rate at any concentration, on both silica and the AAPP substrate. This isdue to the increased hydrophobicity of the monomer providing a greater driving


force for adsorption at pre-aggregate concentrations. This increased hydrophobicityis evidenced by the fact that longer tail-group surfactants form micelles at lowerconcentrations. As micelles facilitate more effective substrate coverage, this leadsto faster adsorption rates.The addition of electrolyte to surfactant solutions increases the rate of adsorption

at a given concentration for both the silica and AAPP substrate. In addition toscreening electrostatic interactions the addition of electrolyte lowers the solubilityof the hydrocarbon chain, rendering the surfactant molecule more hydrophobic. Thisresults in the adsorption rate being increased as described above. As the increase inthe rate of adsorption is not dependent on the binding efficiency of the counterionto the surfactant, in the absence of specific interactions, the species of the electrolytedoes not influence the rate of adsorption.In general, the kinetics of adsorption is slower on the AAPP substrate than for

silica. This is most likely due to the difference in the adsorbed conformation on thetwo substrates. The mixed nature of the surface may also play a role, in thatfavourable electrostatic and hydrophobic interactions are possible on the AAPPsurface but the strength of these interactions may be reduced. The adsorption ofDTAB and SDS in the absence of electrolyte to the AAPP substrate is comparativelyslow to reach equilibrium, but does not correspond to a SAR. For these systems,adsorption at all concentrations is slow, not just in a discrete portion of the isotherm.Secondly, the increase in surface excess with time does not correspond to a structuraltransition in the adsorbed aggregates. The role of the substrate in the pace ofadsorption should not be underestimated.

7. Summary

Adsorption isotherms are the traditional method for investigating surfactantadsorption phenomena, and they continue to yield much valuable information. Bycomparing isotherm data with the more recent structural information provided bytechniques such as AFM, calorimetry, fluorescence probe studies, NR, and ellipso-metry, changes in the interfacial conformation of the surfactant as a function of thesolution concentration can now be determined. In a further advance, analysis ofadsorbed aggregate structures in the light of new and detailed kinetic data indicatesthat it is the type of aggregates that form on the surface that determines both thesurface excess and the rate of adsorption.

7.1. Mechanism of adsorption and the adsorption isotherm

The new knowledge now available allows us to comment on the adsorptionprocess with greater precision than ever before. This is particularly true for cationicsurfactants on oxide surfaces. Adsorption is controlled by electrostatic and hydro-phobic interactions. The relative influence of these interactions is determined byboth the properties of the substrate and the nature and concentration of the surfactant.We divide the adsorption isotherm into threeconcentration spans. In each span a

new adsorption process is possible. The spans are named on the basis of the


Fig. 45. Proposed Mechanism of surfactant adsorption. Each span is described in detail in Section 7.

adsorption mechanism that is newly available in that concentration range. They are,in increasing concentration, the electrostatic concentration span, the electrostatic–hydrophobic concentration span and the hydrophobic concentration span. Themechanism of adsorption in each span differs and is depicted schematically in Fig.45. Note that the hydrophobic concentration span may be further divided into above-


cmc and below-cmc spans to reflect the direct adsorption of micelles. We provideour description of the adsorption mechanisms in order of increasing concentrationbut this should not be taken to imply that the kinetics of adsorption operate in thismanner. That is, adsorption at a concentration in, for example, the hydrophobic spandoes not imply that the adsorption process occurs sequentially through the electro-static span, the electrostatic–hydrophobic span then the hydrophobic span. Ratherin the hydrophobic span the mechanisms applicable to all regions may be operatingsimultaneously and at different rates. For example, as we have evidence that micellesadsorb directly to the surface, in this case the hydrophobic interactions are essentiallyin operation before the micelle approaches the surface, at which time the electrostaticmechanisms begin to operate. The mechanism we have assigned to each span isavailable to surfactant adsorption at greater concentrations butnot at lesserconcentrations.

7.1.1. The electrostatic concentration spanIn the first span surfactant molecules are electrostatically adsorbed to the charged

surface sites. The presence of a positively charged head-group at the interfacerenders nearby hydroxyl groups more acidic, which induces more charged sites inthe vicinity of the initial charge site.

7.1.2. The electrostatic and hydrophobic concentration spanSurfactant tail-groups will interact with any hydrophobic regions that are present

on the substrate. Regardless, the hydrophobic tails of the surfactant, along with thenewly induced sites of surface charge, act as nucleation points for further surfactantadsorption. The adsorption is thus driven both by hydrophobic interactions andelectrostatic attraction. Throughout the second span the charge on the underlyingsubstrate continues to increase. At the end of this concentration span the adsorbedmorphology is described as a ‘teepee’ structure, the substrate ionisation is at amaximum and the overall surface charge is neutralised.

7.1.3. The hydrophobic concentration spanAny further adsorption is purely hydrophobically driven, and will be against a

repulsive electrostatic barrier that arises as a result of overcompensation of thesurface charge by the adsorbed surfactant. Thus span three is characterised by thehydrophobic adsorption of surfactant molecules to the ‘teepees’ present at theinterface, with head-groups oriented away from the surface. In the third concentrationspan, the level of counterion adsorption becomes appreciable. The kinetic evidenceevaluated in this review suggests that the explanation of the hydrophobic concentra-tion span is relevant for all surfactant concentrations above charge neutralisation.The only variation in the adsorption mechanism is the direct adsorption of micellesabove the solution cmc.Not all systems will exhibit different adsorption regions corresponding to the

three concentration spans described. We have seen that for an anionic surfactantadsorbing to a partially hydrophobic anionic surfacew114x, there is no electrostatic


adsorption, as such the first span is not present in this isotherm. Highly hydrophobicsurfaces give a similar result.

7.2. Adsorption kinetics and the adsorption isotherm

We have demonstrated that the adsorption process can continue over long periodsdue to ‘kinetic trapping’. The concentration range over which this occurs is calledthe SAR. The SAR spans concentrations that are sufficient to lead to aggregatelevels of coverage on the substrate, but where no aggregates are present in solutionand is a consequence of kinetic barriers to the formation of the thermodynamicallystable arrangement of adsorbed surfactant. This phenomenon may yet prove to bewidespread. We have seen that adsorption levels obtained on dilution exceed thosethat are reached when the surfactant concentration is increased in a step-wisemanner. Further, the adsorption levels obtained by increasing the concentration in astepwise fashion were less than those obtained when the concentration is increaseddirectly, in one step. These differences exist despite the fact that in all cases thesurface excess had stabilised and apparent equilibrium had been reached. The factthat stepwise introduction of surfactant led to values of surface excess less thanthose seen at the same concentration for stepwise reductions in surfactant concen-trations can be interpreted as evidence that equilibrium has not been reached in theformer case, and that this is due to kinetic trapping akin to that which gives rise tothe SAR. However in this case the kinetic trapping is so severe that continuedadsorption has ceased. Thus, equilibrium adsorption isotherms that are determinedby stepwise changes in surfactant concentration should always be conducted undera dilution regime to prevent false equilibrium values being obtained.An assessment of the available literature on the adsorption of cationic surfactants

to silica surfaces has led to a new understanding of the mechanisms operatingduring adsorption. The mechanisms available are determined by which concentrationspan of the adsorption isotherm the solution surfactant concentration lies in. Theadsorbed surfactant morphology determines both the kinetics of adsorption and thesurface excess. The influence of the kinetics of adsorption can be dramatic, leadingto very slow adsorption processes and possibly complete kinetic trapping at surfaceexcess values below the true equilibrium surface excess.

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