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Chapter 1. General overview

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Surfactants are one of the multipurpose chemicals that find applications in

almost every chemical industry and also at domestic level. The soaps, we use for

bathing, the detergents we use in cleaning our laundry and our homes (toiletries), in

cosmetics, shampoos, in tanning of leather, in the motor oils in our automobiles, in

drug delivery systems, in oil industry e.g. in enhanced and tertiary oil recovery, as

flotation agents used in ores purification. Surfactants are extensively used in

detergents, paints, dyestuffs, paper coatings, inks, corrosion inhibition, plastics and

fibers, personal care and household products, agrochemicals, pharmaceuticals, food

processing, etc. as emulsifiers, demulsifiers, dispersants, foaming agents, wetting

agents, solubilizers and viscosity modifiers [1, 2]. Apart from this, surfactants are

very important for formation of biological membranes and have function in living

cells. The study of surfactant solution behaviour is an important facet of colloid and

interface science. The main aspects are to explore ability of surfactant molecules to

aggregate, and to search surfactant or combination of surfactants or surfactant in

presence of different organic/inorganic additives for optimized specific applications.

Surfactant is a diminutive form of the phrase Surface Active Agent. These are

amphiphilic molecules. The word amphiphile is derived from the Greek αφι (amphi

= both) and φιλιοζ (philios = friend). Thus, the combination of these two words

provides the meaning of amphiphilic, which is something that is friendly to both.

From chemistry viewpoint amphiphilic molecules have affinity for both polar ‘water’

and nonpolar ‘oil’. The amphiphilic nature of surfactant is because of the presence of

two distinctly different polar and nonpolar molecules in the same molecule. Thus, a

surfactant molecule has both hydrophilic (water-loving/oil-hating) and hydrophobic

(water-hating/oil loving) groups [3]. Symbolically, a surfactant molecule can be

represented as having a polar “head” and a non-polar "tail" as shown below (Fig. 1.1).

The hydrophobic group in a surfactant is usually a hydrocarbon chain but may

be a fluorocarbon or polymeric chain of appropriate length. The hydrophilic group is

polar, which contains heteroatoms such as O, S, P, or N, included in functional groups

(such as alcohol, thiol, ether, ester, acid, sulfate, sulfonate, phosphate, amine, amide

etc.) may be ionic, zwitterionic or nonionic, and accompanied by counterions in the

first two cases. The hydrocarbon chain interacts weakly with the water molecules in


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an aqueous environment, whereas the polar or ionic head group interacts strongly with

water molecules via dipole or ion-dipole interactions or hydrogen bonding. The

cooperative action of dispersion and hydrogen bonding between the water molecules

not allows the hydrocarbon chain to remain in water and hence these chains are

referred to as hydrophobic [4].

Oil loving tail Water loving head

Hydrophobic (Lipophilic) part Hydrophilic part

(non-polar) (polar or ionic)

Fig. 1.1 Schematic representation of a surfactant molecule

1.1 Classification of Surfactant

The most common classification of surfactant is based on the nature of

charge on polar head group and are thus classified as anionic, cationic , zwitterionic

and nonionic surfactants.

(i) Anionic surfactants have negatively charged polar head (hence anionic) and a

positively charged counterion. Anionics are amongst most used surfactants for their

low cost and easy manufacturing. Examples of anionic surfactants are sulfonates,

alcohol sulfates, alkyl benzene sulfonates, phosphoric acid esters, and carboxylates.

As a counterion sodium, potassium, calcium, ammonium and various protonated alkyl

amines are possible. The most common uses of anionic surfactant are in detergent

formulations. Anionic surfactants tend to be good solubilizers and are relatively

nontoxic. They have been used in petroleum oil recovery operations as well as in

contaminant hydrogeology remediation applications. They are the oldest class of


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surfactant and still used extensively. Soaps are most popular anionic surfactant (alkali

metal salt of carboxylic acid obtained from animal fats or vegetable oils).

(ii) Cationic surfactants yield a positively charged surfactant ion and a negatively

charged counterion. Cationic surfactants first became important when Domagk

recognized the commercial potential of their bacteriostatic properties in 1935. From

this came a proliferation of hundreds of commercial products. Two common types of

cationic surfactants are long chain amines and quaternary ammonium salts. The long

chain amine types are made from natural fats and oils or from synthetic amines. They

function, as surfactant in protonated state only hence cannot be used at high pH. On

the other hand, quaternary amine type cationic surfactants are not pH sensitive and are

very important as fabric conditioners and softeners. The majority of surfaces viz.,

metals, minerals, plastics, fibers, cell membranes, etc., are negatively charged.

Cationics adsorb on these surfaces with their hydrophobic groups oriented away.

These characteristics allow use of cationic surfactant as anticorrosion agent, flotation

collector, antistatic agents, antiseptic agents, anticaking agents, bactericides, fabric

softener and conditioner.

(iii) Nonionic surfactants are characterized by hydrophilic head groups that do not

ionize appreciably in water hence have no charge on head group. Examples include

alcohol ethoxylates, alkylphenol ethoxylates, and alkanolamides. Nonionic surfactants

tend to be good solubilizers and are relatively nontoxic. They are usually easily

blended with other types of surfactants (i.e., used as cosurfactants) and therefore have

found widespread use in petroleum and environmental applications. The performance

of nonionic surfactants, unlike anionic surfactants, is relatively insensitive to the

presence of salts in solution. The physicochemical properties of nonionic surfactant

soltions are very much temperature dependent. Contrary to ionic surfactant, nonionics

often become less water soluble with increase in temperature. However, for sugar

based nonionic surfactant solubility increase with temperature.

(iv) Zwitterionic surfactants are those for which the charge on the polar head group

can be either positive or negative depending upon the pH of the solution. At the

isoelectric point the physicochemical behavior of zwitterionic is analogous to

nonionic. Slow shift towards anionic and cationic character is seen on pH variation.


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Examples of zwitterionic surfactants are RN+H2CH2COO- (longchain amino acid),

RN+(CH3)2CH2CH2SO3-(sulfobetaine).

Beside the common classes anionic/cationic/zwitterionic/nonionic, following

surfactants have gained important attention.

(i) Gemini Surfactants are one of the most exciting developments in the field of

surfactant chemistry. The term gemini surfactant, coined by Menger and Littau [5] has

become accepted in the surfactant literature for describing dimeric surfactants, that is,

surfactant molecules that have two hydrophilic (chiefly ionic) groups and two tails per

surfactant molecule [5,6]. A spacer group of varying length (most commonly a

methylene spacer or an oxyethylene spacer) links these twin parts of the surfactants.

Gemini surfactants of the type N,N’-didodecyl-N,N,N’,N’-tetramethylalkane-α,ω-

diammonium dibromide, are generally referred as “m-s-m”, where m and s represent

the carbon numbers present in the alkyl chain of surfactant tail and in the

polymethylene group in spacer, respectively. A schematic diagram of m-s-m gemini

surfactant is shown in Fig. 1.2. They may be different kind depending on the charge

on head group viz. (i) Anionic Gemini, (ii) Cationic Gemini, (iii) Nonionic Gemini,

(iv) Zwitterinoinc Gemini and (v) Hetero Gemini.

Br - Br -

N

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 3

C H 3

CH 3 N

C H 3

C H 3

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

C H 3

(CH 2 ) S+ +

Fig. 1.2 Structure of a 12-s-12 type cationic gemini surfactant


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A number of reviews cover the properties of cationic and anionic gemini

surfactants. [7,8,9,10]. Gemini surfactants possess superior properties when compared

to conventional single-headed and single-tailed surfactants; geminis exhibit lower

CMC values (by about an order of magnitude), increased surface activity and lower

surface tension at the CMC, enhanced solution properties such as hard-water

tolerance, superior wetting times, and lower Krafft points. Given these performance

advantages of gemini surfactants, one can anticipate their use in a myriad of surfactant

applications (e.g., soil remediation, oil recovery and commercial detergents) given a

favorable cost/performance ratio. Recently, gemini surfactants have attracted much

attention as potential agents in gene therapy and their properties as vehicle for gene

delivery into cells (transfection) were reported.

Silicone Surfactants [11-14]: These are surfactants based upon silicone as a

hydrophobe that contains other functional groups, similar to those seen in traditional

surfactants. In some instances, silicone is incorporated into a surface-active agent,

with a polyoxyalkylene portion of the molecule and or a hydrocarbon portion of the

molecule. This results in several unique properties of the surfactant. They consist of a

methylated siloxane hydrophobic group (polydimethyl siloxane) coupled to one or

more polar groups. The silicone surfactant, if composed of silicone- and water-soluble

groups, will lower the surface tension of water to around 20 mNm-1. Some of them are

used in pharmacy as antiflatulent surfactants since they are biologically inert.

Fluorinated Surfactants [15]: These can be polyfluorinated or fluorocarbon-based

(per fluorinated) [16]. As a surfactant, they are more effective at lowering the surface

tension of water than comparable hydrocarbon surfactants. They have a fluorinated

"tail" and a hydrophilic "head." Some fluorosurfactants, such as PFOS

(Perfluorooctanesulfonic acid), are detected in humans and wildlife. Perfluorinated

carboxylates and sulfonates produce monolayers with less lateral interactions than

their hydrocarbon counterparts. They are able to turn a surface non-wettable to both

water and organic solvents. They produce a superficial (air-aqueous solution) tension

downto 15 mNm-1, i.e. twice as low as the value reachable with the best tension

reducing hydrocarbon surfactants.


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Bolaform Surfactants [17]: They have hydrophilic head groups at both ends of a

sufficiently long hydrophobic hydrocarbon chain (Bola is a long cord with heavy bolls

at each ends used by gauchos in the Argentinean Pampa to capture cattle [18].

Compared to single- headed amphiphiles, the introduction of a second headgroup

generally induces a higher aqueous solubility, an increase in the CMC, and a decrease

in aggregation number. They come with wide variety of ionic and nonionic head

groups. e.g hexadecanediyl-1,16-bis(trimethylammonium bromide), potassium

hexadecanedioate, dodecanediyl -1,12-bis(tetrkis-(oxyethylene)monoether)[19 ,20]

Naturally occurring/ Biological Surfactant: One example of these groups is

phospholipids; the main components of biological membranes and include

phosphatidylcholine (lecithin), lysolecithin, phosphatidylethanolamine and

phosphatidyl inositol [4]. The amphiphilic nature of these molecules defines the way

in which they form membranes. They arrange themselves into bilayers, by positioning

their polar groups towards the surrounding aqueous medium, and their lipophilic

chains towards the inside of the bilayer, these lipids are also used as emulsifiers for

intravenous fat emulsions, anaesthetic emulsions as well as for production of

liposomes or vesicles for drug delivery[21,22]. Although phospholipids are principal

constituents of biological membranes there are other amphiphilic molecules, such as

cholesterol, glycolipids and bile salts which are also included in these structures and

give them different physical and biological properties. Bile salts are synthesized in the

liver and consist of alicyclic compounds possessing hydroxyl and carboxyl groups.

1.2 Surfactant solution behavior

1.2.1 Krafft point (KP) and Cloud point (CP)

The solubility of surfactants shows a strong increase above a certain

temperature, termed the Krafft point (KP). The explanation is that at low temperatures,

the low monomer solubility determines the total solubility; while at higher temperatures

when the monomer solubility has reached the CMC, it is determined by the micelle

solubility, which is much higher. A particular surfactant in particular solvent is

characterized by a definite Krafft point. The solubility at Krafft point is called the CMC

of the surfactant at that temperature.


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The Krafft point of ionic surfactant may be defined as the temperature at which

the solubility of surfactant becomes equal to its CMC. Krafft point represents the

temperature at which the alkyl chains melt resulting in the dissolution of surfactant

crystals into micelles and monomers as illustrated in Fig. 1.3a. The Krafft temperature

is an essential parameter that must be known for a surfactant before putting it into use.

Since micelles form only at temperature above the Krafft point, a surfactant is often

required to possess a KP value lower than the temperature at which it is used.

Nonionic surfactants usually do not exhibit Krafft points. Instead, the solubility

of nonionic surfactants decreases with increasing temperature and these surfactants may

begin to lose their surface active properties above a transition temperature referred to as

Cloud point (Fig.1.3b). This occurs because above the cloud point a surfactant rich

phase of swollen micelles separates and the transition is usually accompanied by a

marked increase in dispersion turbidity.

Fig. 1.3 Krafft Point (KP) and Cloud Point (CP)

1.2.2 Micelle formation and critical micelle concentration

Due to presence of both hydrophilic and hydrophobic parts, the most suitable

place for surfactant molecules is at surface or interface, which separate two

immiscible phases. In aqueous solutions, surfactant molecules adsorb at the surface


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(air-water interface) where the forces of both attraction and repulsion to water can be

satisfied. Adsorption of surfactant molecule is such that non-polar tail stays away

from water at air side of interface and polar head in contact with water (as shown in

Fig. 1.4). At certain concentration, the surface gets saturated with surfactant

molecules; then a new phenomenon comes in to action characterized as micellization;

surfactant molecules aggregate in bulk phase by forming micelles (Fig. 1.4). In

micelles, the surfactant hydrophobic groups are directed towards the interior of the

aggregate where hydrophobic tails interact with one another and the polar head groups

are directed towards the solvent. Surfactant micelles are considered as associated

colloids but these are not a stable association since micelles are in dynamic

equilibrium with unassociated molecules.

Fig. 1.4 Adsorption, saturation, micellization and decrease in surface tension as a

function of surfactant concentration.


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The driving force for surfactant adsorption is the lowering of the free energy

of the interface or in other words surface or interfacial tension (γ). This tension is

related with the amount of work required to expand surface/interface. Adsorption of

surfactant at the interface lowers the γ of solvent until it saturates the interface. After,

saturation surfactant molecules aggregates in bulk phase to form micelles hence γ

remains almost constant (see Fig. 1.4). Due to formation of micelles weak contact

between hydrocarbon tail and bulk water decrease that reduces the free energy of

system. Thus, the formation of micelle (micellization) is spontaneous process. The

concentration at which micelles are formed is designated as the critical micelle

concentration (CMC). The CMC and degree of reduction of γ depend on surfactant

structure and nature of two immiscible phase that meet at interface [4,23].

When surfactant molecules are present in aqueous solution, they reduce the

hydrogen bonding possibilities of adjacent water molecules by breaking its tetrahedral

structure. The decrease in entropy is thought to be the result of the breakdown of the

normal hydrogen-bonded structure of water accompanied by the formation of

clathrate like structure of water around the hydrocarbon chain of surfactant often

termed icebergs. These highly ordered clathrate cages result in the loss of normal

hydrogen bonded structure and cause a decrease in entropy of the water, and hence a

decrease in the total entropy of the system. To compensate for this entropy change the

surfactant hydrophobic chain associate and decrease the number of water molecule

involved in formation of clathrate cage. This overall process is enthalpically favoured,

but entropically unfavourable.

At certain concentration of surfactant, the unfavorable entropy changes bring

the hydrophobic tails together, which is known as the hydrophobic interaction [24].

Thus, molecular interactions, arising from the tendency for the water molecules to

regain their normal tetrahedral structure, and the attractive dispersion forces between

hydrocarbon chains, act cooperatively to remove the hydrocarbon chain from the

water “icebergs”, leading to an association of hydrophobic chains (i.e., formation of

micelle). The formation of these surfactant micelles has a positive and negative

contribution to the entropy of the system. The micelle itself is a highly ordered

structure and thus decreases the entropy of the system. More important however, is


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the increase in entropy due to the breaking of the clathrate cages and the return of the

water molecules to the bulk solution which increases the overall entropy of system

hence micellization is spontaneous and entropy driven process.

1.2.3 Micellar aggregation number (Nagg)

The number of surfactant monomers/ions present in a micelle is called

“aggregation number” (Nagg). Its value depends upon nature of the involved as well as

on several external parameters such as the presence of electrolytes, nonpolar

solubilizates, temperature, pressure, pH, etc [25]

The CMC and Nagg run anti-parallel. Any increase in the CMC is often

associated with a decrease in the Nagg. Thus, for a class of surfactant with the same

polar head group, increase in the length of the hydrocarbon chain favors aggregation

and hence Nagg.

For a given chain length, increase in the size or charge on the polar head

groups of an ionic surfactant brings about intramicellar repulsions and increases free

energy of the system. Such systems generally possess low Nagg.

For nonionic polyoxyethylenated surfactants, an increase in the

polyoxyethylene block causes a decrease in the Nagg. This could be thought of as due

to greater steric hindrances arising due to the hydration of the additional oxyethylene

groups. Addition of an electrolyte to ionic surfactant systems leads to charge

neutralization and enables closer packing of surfactants in the micelle, thereby

increasing the CMC. The influence of electrolytes on the Nagg of nonionic surfactants

is not very clear with both increase and decrease in the aggregation number being

observed.

Increase in temperature generally causes considerable increase in the Nagg of

polyoxyethylenated nonionic surfactants as high temperature causes the dehydration

of the polyoxyethylene chains thereby increasing the hydrophobicity. Ionic surfactants

usually show slight decrease in the Nagg with temperature.

1.3 Mixed micellesThe surfactants used in a multitude of industrial products, processes and other

practical applications almost always consist of a mixture of surfactants. Therefore, in

recent years, much research has been directed towards the study of mixed surfactant


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systems Mixed systems usually exhibit interfacial and micellar/rheological properties

significantly different from those of the individual components. Mixed surfactant

systems exhibit greater surface activity (lower CMC values) than that obtained with

any of the individual components of the mixture at the same concentration (i.e., mixed

surfactant shows synergistic behavior) and therefore mixed systems are less expensive

[26,27]. Such systems are commonly used in medical and pharmaceutical

formulations, in industries in various personal care and house hold products, in

enhanced oil recovery processes, in detergency, cleaning, emulsification, and

dispersion. Mixed surfactant systems that contain surfactants with different structures

are therefore of great theoretical and industrial interest [28,29].

The interaction between different surfactants can lead to synergism or to

antagonism, depending on the type of surfactants. When there is no net interaction

between two surfactants with similar head groups, the CMC of the mixture is an

average of the CMCs of the pure surfactants. However, for many surfactant mixtures

such as mixtures of nonionics and anionics or anionics and cationics, there is a fairly

strong interaction between the both types of surfactants. Interaction between anionic-

cationic surfactants is very strong but such systems often lead to

precipitation/coacervation because of the coulombic interactions between oppositely

charged species. To get information on the extent of interaction between two

surfactants in a micelle, regular solution theory has often been applied to estimate the

molecular interaction parameter (β) [30-34]. The β has positive or negative values

when the interaction between the surfactant is respectively repulsive (antagonism) or

attractive (synergism). Typical β values for mixtures of non-ionic and anionic are

around –2 and -5 for mixtures of cationic and anionic between –10 and –20.

Various theoretical models have been proposed to interpret the formulation of

mixed micelles. The first model proposed by Lange and Beck [35] and used by Clint

[36] assumes ideal mixing of the surfactants in the micellar phase, which is based on

the phase separation model. Rubingh and Holland [37] based on regular solution

theory (RST) proposed a model for nonideal mixed systems and extensively used by

different workers [38, 39]. Rosen et al. [40, 41] have extended the nonideal solution

treatment for mixed micelle formation by binary surfactant systems to estimate, from


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surface tension data, the surfactant molecular interaction and composition in adsorbed

mixed monolayer at air/water interface. Apart from this, Maeda’s[42] approach has

been used to explain the stability of mixed ionic nonionic surfactant systems.

Mixed surfactant systems, as is the case with single surfactant systems, exhibit

preferential adsorption at interfaces at low concentration forming mixed monolayers

leading to lowering in surface tension. At concentrations above CMC, surfactant

monomers undergo co-operative self-association in the bulk to form mixed micelles.

Different techniques have been used to collect structural information on mixed

micelle formation, and to obtain their CMC [43-46].

Within the last decade, studies of gemini surfactants with conventional

surfactants (mainly ionic and non-ionic), in mixed systems, have become increasingly

popular. The existence of synergistic effects between these surfactants may render the

use of such mixtures even more attractive. Several studies of gemini + conventional

surfactant mixtures did focus on synergism in both micelle and monolayer formation

at the air /water interface [47-49]. The role of hydrophobic chain length and spacer

group length in the interactions between monomeric and dimeric surfactants in mixed

micelles and non-ideality of binary monomeric- dimeric surfactant mixtures was

investigated.

1.4 Micelle size and shapeSurfactant molecules are well known to spontaneously self-assemble into

many different shapes [50,51] (Table 1.1). Ionic surfactants form smaller micelles

(aggregation number ~10-100) than nonionic surfactants (aggregation number > 100).

This is because the electrostatic repulsion between ionic head-groups is greater than

the steric repulsion between non-ionic head groups. In general, the shape of the

micelle is depends on the structure of the surfactant, typically the relative size of the

head group and tail group. The competition between the propensity of the head

group(s) for the solvent, and the tendency of hydrocarbon tails to minimize solvent

contact dictate the equilibrium structure. The structure of a micelle could vary from

spherical to rod- or disclike to lamellar in shape.


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Table 1.1 Different shape of Surfactant monomers and corresponding shape of

micelle

Packingparameter

P

Criticalpacking

shape

General Surfactanttype

Expected micelleshape

< 0.33

Single-chain surfactantswith large head groups.

Spherical

0.33- 0.5

Single-chain surfactantswith small head groups, orionics in the presence oflarge amounts ofelectrolyte. Cyllindrical

0.5-1.0

Double-chain surfactantswith large head groups andflexible chains.

Vesicle (liposom)

1.0

Double-chain surfactantswith small head groups orrigid, immobile chain

Bilayers

> 1.0

Double-chain surfactantswith small head groups,very large and bulkyhydrophobic groups

Reverse

Israelachvili [52] derived the shapes assumed by nanostructures based on

packing arguments on the hydrocarbon tails in the surfactant assembly. Accordingly

the theory of the packing parameter [53] presents the best explanation to understand

in which form surfactants will self aggregate. The central parameter in this

development is the dimensionless ratio defined as packing parameter P; the ratio

between the volume ν of the hydrophobic tail of the surfactant and the product of the


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area occupied by the polar head a with the chain length l of the hydrophobic tail of the

surfactant.

P =la

v

For P approximately ~ 0.33 the amphiphile molecule has the shape of a cone and it

tends to form spherical micelles. If the shape is more similar to a truncated cone, then

it forms cylindrical micelles with a P value between 0.33 and 0.5. When amphiphile

molecules have cylindrical-shaped, P > 0.5 and bilayer micelles are predicted.

Consequently, in disk-shaped structures and vesicles the value of P should be

somewhere in the range 0.5 - 1 but closer to unity. If the surfactant has the shape of a

truncated inverted cone, then it tends to form reverse structures for which P > 1.

Table1.1 summarizes the different shapes of surfactant molecule and resultant

micellar structure. Solution conditions such as electrolyte concentration; pH etc. also

can bring about transformations in micellar aggregates from one form to another.

1.5 Factors affecting CMCIn aqueous medium, the CMC decreases as the number of carbon atoms in the

hydrophobic tail increases. As a general rule for ionic surfactants with one

hydrophilic group, the CMC is halved by the addition of one methylene (-CH2) group.

But for nonionic and zwitterionic surfactants, the magnitude of decrease in CMC is

much larger. Chain branching, unsaturation of hydrophobic chain also has an

influence on CMC. The CMC values increase with increase in the number of head

groups of surfactant.With respect to nature of the charged headgroup, the values of

CMCs show variation. The CMC of dodecyltrimethylammonium chloride (DTAC) is

20 mM, while for a 12 carbon non-ionic surfactant, hexaethylene glycol mono n-

dodecyl ether (C12E6), the CMC is about 0.09 mM;[ 54-56]. The CMC for sodium

dodecylsulfate (SDS) is about 8 mM, while that for disodium 1,2-dodecyl disulfate

(1,2-SDDS) is 40 mM.[57]. Thus, the main factor that affects CMC is the charge of

the hydrophilic head group. In addition, CMCs show dependence on the nature of the


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counterion. It is mainly the valence number of the counterion that affects the CMC.

As an example, the CMC value for copper bis(dodecylsulfate) Cu(DS)2 is about 1.2

mM, while the CMC for SDS is about 8 mM [58].

Temperature increase decreases hydration of the hydrophilic group, which

favors micellization. However, temperature increase also disrupts the structured water

surrounding the hydrophobic group, an effect that disfavors micellization. Ionic

surfactants show an increase in CMC (sometimes with a minimum) with temperature,

while nonionic shows an opposite trend. In general, CMC exhibits a weak

dependence on temperature and pressure [59-61]. For nonionic surfactants with EO-

based hydrophilic groups exhibit a monotonic decrease in CMC with increase in

temperature. The hydrophilicity of nonionics is solely based on the hydration of the

EO blocks and increases in temperature results in their progressive dehydration

thereby increasing the hydrophobic nature of the surfactant and hence decrease the

CMC.

The nature of the counterions and their degree of dissociation influences the

CMCs of ionic surfactants to a considerable extend. The magnitude of the CMC value

reflects the degree of counterion dissociations with high CMCs observed for

surfactants with high degree of counterion dissociation. Lower the dissociation of

counterions from the polar (ionic) head groups results in reduction/neutralization of

the head group charge thereby favoring aggregation. An increase in the valency as

well as the polarizability of the counterions decrease the degree of dissociations while

an increase in hydrated radius enhances the same. It has been observed that for a

given hydrophobic tail and anionic head group, the CMC decreases in the order:

NH4+ > K+ > Na+ > Li+ > 1/2 Ca2+ 1/2 Mg2+

In the case of cationic surfactants, CMCs are found to decrease in the order:

F -> Cl- > Br- > I ->CN-

In most applications, surfactants are used in the presence of additives in order

to improve their performance properties. In aqueous solution, the presence of

electrolyte alters the CMC. The effect of electrolytes (salts) is more pronounced for

anionic and cationic than for zwitterionic surfactants and more pronounced for

zwitterionics than for nonionics. For ionic surfactants presence of salt markedly


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decreases the CMC. Main reason behind it is neutralization of charge on ionic head

groups which reduces the repulsion between head groups and favores micellization.

An increase in the concentration of electrolytes progressively contracts the electrical

double layer around the micelle and in the process stabilizes the micelle. This is

manifested as a strong reduction in CMC with electrolyte concentration.

The effect of electrolytes on the CMC of nonionic surfactants is much less

significant. The CMC of a nonionic surfactant may increase or decrease depending

upon the nature of the electrolyte additive. Some electrolytes on addition to water

disrupt the water structure and this enables the stay of the hydrocarbon portion of the

surfactant more favorable and hence its solubility increases with a corresponding rise

in the CMC. This is referred to as the ‘salting in’ of the surfactant. On the contrary, in

the presence of an electrolyte, which makes the water structure, less work is required

for the removal of the hydrophobic part from the solvent resulting in a decrease of the

CMC (‘salting out’). A comparison of the salting out effect of some anions shows the

following order: SO42- > Cl- > Br- > NO3

-

Small hydrated ions are more effective in salting out than are larger ones. For

alkali metal cations the order is: Na+ > K+ > Li+

Lithium ions in spite of their low lyotropic number (small hydrated ionic size)

are found to be least effective as a salting out agent as they are capable of forming

complexes with the ether linkages of nonionic surfactants, resulting in an extent of

salting in.

Amongst the organic additives in surfactant solutions, alcohols have been

most frequently studied. This is because; alcohols are the most common cosurfactant /

cosolvent which are added to generate microemulsion. Suggested use of

microemulsion in the field of oil recovery has stimulated the research on surfactant -

alcohol - water systems [59-61]. Short chain alcohols are retained mainly in the

aqueous phase, affect the micellization process by modifying the solvent properties,

and generally increase the CMC while medium chain alcohols typically decrease the

CMC. When the alcohol molecules partition between water and the pseudo-micellar

phase, the fraction bound to the micelles replaces water molecules at the interface

leading to increased electrostatic repulsion between surfactant head groups [62,63].


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Moderately hydrophobic alcohols in low concentration promote micellization

probably by residing at the micellar surface and reducing unfavorable water

hydrocarbon contacts [64]. However, at higher concentrations these alcohols

destabilize micelles by displacing water from the surface, therefore decreasing its

effective dielectric constant, increasing head group repulsions, and disrupting

surfactant packing. Higher alcohols are known to reduce the CMC of ionic surfactants

in aqueous solutions [65]. To elucidate the mechanism responsible for this

observation, studies on properties such as aggregation number, partitioning of

alcohols between micellar and aqueous phase are reported [66, 67].The change of

micellar size as a function of alcohol concentration is still a subject of some

controversy.

Other organic additives that frequently used in combination with

surfactants are glycols, their oligomers and mono-, di- alkyl ethers. The characteristic

property of ethylene glycol and its oligomers is the presence of two hydroxyl groups

and increasing number of ether groups (-CH2CH2O-) or ethereal oxygen (-O-). The

presences of hydroxyl and ether group in glycols provide ability to form intra- and

intermolecular hydrogen bonds with water molecules and thus behave as water

structure breaker [68]. With increase in number of ethereal oxygen dielectric constant

(ε) decreases whereas dipole moment (μ) increases. These manifest the decrease in the

orientational polarization and increase in the polarity of the glycol molecules as a

function of number of ethereal oxygen [69].Thus, due to these characteristic features

of glycols, their presence greatly influences the micellar behavior of surfactant in

aqueous solutions.

The influence of pH on the CMC depends upon the nature of the head group

present in the surfactant. The pH does not have significant influence when the

surfactants are salts of strong acids such as the sulfates or the sulfonates as they

undergo almost complete ionization is solution. In such cases, acids or bases when

present at high concentrations behaves as ordinary electrolytes. For surfactants which

are salts of weak acids such as carboxylates, where the ionization is not complete in

solution, pH influences the degree of ionization and hence the CMC. Similar is the

case for surfactants with ionizable groups such as -NH2 and - (CH3) N0, where


18

again the ionization is pH dependent. Amphoteric surfactants such as those based on

amino acids may behave like anionic (high pH) or cationic (low pH) species

depending upon the pH of the solution while surfactants such as

dodecyldimethylamine oxide (DMDAO) may behave as a cationic at low pH or as a

nonionic at higher pH.

There is considerable interest in the determination of CMC because in

practice, it is the lowest concentration of the surfactant offering the optimum benefits.

The physico-chemical properties of surfactants vary markedly above and below the

CMC value. Below the CMC value, the physicochemical properties of ionic

surfactants resemble those of a strong electrolyte. Above the CMC value, these

properties change dramatically, indicating a highly cooperative association process is

taking place. Thus, due to their characteristic aggregation behavior in solutions, a

study of bulk properties of these materials such as surface tension, electrical

conductivity, sound velocity, dye solubilization and light scattering etc. show

dramatic changes over very small concentration ranges. These properties when

measured as a function of concentration of the surfactant show curves with sharp

inflections at the CMCs. This is illustrated by Preston’s [70] classic graph, shown in

Fig. 1.5


19

Fig. 1.6 Determination of CMC using different methods

1.6 Techniques used for characterization of surfactant

solutionsRecent studies on aqueous solution behaviour of surfactants involve

micellization and phase behavior and several techniques have been employed to

investigate formations and characteristic of micelles formed.Physico-chemical

methods like conductance, potentiometry, surface tension, viscometry dye

solubilization, calorimetry [71-75], chromatographic [76,77], Microcalorimetry [76-

79], velocity/ultrasonic absorption method [80,81] etc. are used.


20

Table 1.2 Instrumental techniques for characterization of surfactant micelles.

Technique Measurement Information Obtain

Physicochemical methodsConductometry Specific conductantce CMC, counterion binding, only

useful for ionic surfactantTensiometry Surface tension CMC, effectiveness and efficiency,

area per molecule, of surfactant,Viscometry Viscosity Change in micelle shape

Thermal Methods

Isothermal titration calorimetry(ITC)

Enthalpy change(∆H) Enthalpy of association, CMC

High-sensitivity differentialscanning calorimetry (HSDSC)

Heat flow Enthalpy of association, CMT, Cloudpoint(CP)

Spectral MethodsNuclear magnetic resonance(NMR)and Two-dimensional nuclearoverhauser enhancementspectroscopy (2D-NOESY)

Chemical shift Dynamics on segmental level, CMT,locations of additives and theirdiffusion in micelle.

Infrared Spectroscopy (IR) &Fourier transfer infrared (FTIR)

Stretching vibrationalfrequency

Dynamics on segmental level, CMT

Steady state & Time resolvedfluorescence

Decay curves andquenching rate

Micellar equilibrium dynamics,Aggregation number

Ultra-violet (UV-visible) Optical density Drug/dye solubilization, CMC

Scattering MethodsStatic Light Scattering (SLS) Rayleigh scattering or

light intensityWeight average molar Mass, Radius

of gyraionDynamic Light Scattering (DLS) Intensity-intensity

autocorrelation function.Diffusion co-efficient,

Hydrodynamic radius

Small Angle X-ray Scattering(SAXS) & Small Angle NeutronScattering (SANS)

Scattering intensity I(Q)vs. scattering vector(Q)

Weight average molar mass, Radiusof gyration, Core radius, Macrolattice structure, Size and aggregationnumber of micelle.

Spectroscopic/scattering/microscopic methods are very useful to determine the

local structure and the environment of the component system. These include static and

dynamic light scattering [82-84], Small angle neutron scattering [85-,87,], static and

time resolved fluorescence [88-90], Fourier transfer infrared [91,92], Pulsed field

gradient spin-echo nuclear magnetic resonance [93], transmission electron

microscopy (TEM) [94]. Cryogenic-TEM (Cryo-TEM) [95].


21

1.7 Work PlanThe outlook of the thesis is given in nine chapters. The experimental

techniques used have been physicochemical methods (like electrical conductivity,

tensiometry and viscometry), scattering (SANS), specrtral (1H NMR, 2D NOESY,

DOSY,13C NMR) and microscopy (Cryo-TEM). The entire experimental work has

been grouped into chapters 2-9. Each chapter constitutes one manuscript.

This chapter introduces the world of surfactant science and the scope of

amphiphilic substances is presented. Current references along with the previous

studies on micellar behavior viz. CMC, aggregation number, micelle size and

intermicellar interaction and influences of various factors and organic and inorganic

additives on the surface and colloid chemical behavior of surfactants are been

reported.

Chapter 2 provides studies on micellization of for series of symmetrical

cationic gemini surfactants of the type N,N’-didodecyl-N,N,N’,N’-tetramethylalkane-

α,ω-diammonium dibromide “12-s-12” (s = 2, 4, 6, 8, 10, and 12). Specific

conductivity as a function of surfactant concentration was measured and critical

micelle concentration (CMC), degree of counterion dissociation (α) of the micelle,

and thermodynamic parameters, namely, Gibbs energy (ΔGm), enthalpy (ΔHm), and

entropy (ΔSm) of micellization were evaluated at various temperatures. Surface

tension studies at 298.15 K provided similar CMCs as given by conductometry along

with information on the efficiency/effectiveness and the area occupied per molecule.

Small-angle neutron scattering (SANS) experiments were performed in order to

investigate effect of spacer length on micelle size of 12-s-12 gemini surfactants. (This

work is published in J. Chem. Eng. Data 56 (2011) 2647–2654.)

Chapter 3 deals with comparison of the effect of 1-butanol and 1,4-butanediol

on micellization of two cationic surfactants (conventional and gemini). The CMC and

α as a function of alcohol concentration are determined. Relative viscosity was used

to investigate change in micelle size. Micellar aggregation number (Nagg) and micelle

size have been evaluated by using SANS. Location and diffusion of alcohols between


22

bulk and micellar phase was evaluated from 2D NMR (NOESY and DOSY)

experiments. (This work has been published in Colloids and Surfaces A:

Physicochem. Eng. Aspects 378 (2011) 79–86).

Chapter 4 focuses on solution behaviour of a cationic surfactant

tetradecyltrimethyl-ammonium bromide (TTAB) in water containing four α,ω-

alkanediols (diols) viz. 1,2-ethanediol(ED), 1,4-butanediol(BD), 1,6-hexanediol(HD)

and 1,8-octanediol(OD). Electrical conductance was used to evaluate the effect of

these diols on CMC and α. In order to find out interaction of long chain diol with

hydrophobic alkyl tail of surfactant, 13C NMR chemical shift changes (∆δ) of different

carbon of diols were compared. SANS experiments provided data of Nagg and size of

micelle in presence of diols.(This work is under review in J. Surfact. Deterg.).

Chapter 5 describes the effect of glycols viz., ethylene glycol (EG),

diethylene glycol (DEG), triethylene glycol (TEG) and teraethylene glycol (TeEG) on

micellization of teradecyletrimethylammonium bromide. The critical micelle

concentration (CMC), degree of counterion dissociation (α) and Gibbs energy of

micellization (ΔGm) as well as Gibbs energy of transfer (ΔGt) of alkyl chain from bulk

phase to micellar phase as a functions glycol concentration at 303.15K were

evaluated. In order to obtain enthalpy (ΔHm) and entropy (ΔSm) of micellization,

CMCs were determined in 10% glycol solutions in temperature range from 303.15 K

to 323.15 K. All the data obtained were correlated with number of ethereal oxygen in

glycol molecules. (This work is under review in J. Solution Chem.).

Chapter 6 describes micellization of the gemini surfactant didodecyl-

N,N,N’,N’-tetramethylbutane-α,ω-diammonium dibromide (12-4-12) in aqueous

solutions in the presence of alkanols viz ethanol, isomeric butyl alcohols, 1-hexanol

and alkanediols (ethanediol, 1,4-butanediol, 1,2-hexanediol, 2,5-hexanediol 1,6-

hexanediol, and 1,8-octanediol). The results are explained on the basis of the structure

and hydrophobicity of alcohols that determine their effect as cosolvent or cosurfactant

(partitioning in micelles). 2D-NOESY was used to examine the location for 1-butanol

and 1,4-butanediol in micellar systems as representative additives from alkanols and

alkanediols.(This work has been published in J. Mol. Liq. 161 (2011) 72–77)


23

Chapter 7 is about the effect of ethylene glycol (EG), diethylene glycol

(DEG) and their monoalkyl (C1-C6) ethers ( cellosolves and carbitols) and dimethyl

ethers (glyme and diglyme) on micellar behavior of tetradecyltrimethylammonium

bromide in aqueous solution.The effect of these additives on critical micelle

concentration (CMC) and degree of counterion dissociation (α) is discussed. 2D

NOESY experiments were used to confirm the location of glycol/glycol ethers in

micellar solution. (This work is under review in J. Mol. Liq.).

In chapter 8 mixed micellar behavior of the nonionic surfactant Triton X-100

with three cationic gemini surfactants (12-s-12,s=2,4 and 6) is investigated. The CMC

of mixed surfactant system were determined tensiometrically. The extent of

interacting forces in mixed micelles formed in 0.10, 0.25, 0.35, 0.50, 0.65, 0.75 and

0.90 mole fraction was evaluated by regular solution theory. The CMC, molecular

interaction parameter (β) and the mole fraction (X) of the two components in the

mixed micelles were calculated. On the basis of Maeda’s approach, the

thermodynamic stability of mixed micelle was quantified using ΔGm. The mixed

micelles were characterized for their shape and size by SANS measurements. (This

work has been published in J. Surfact. Deterg. 14 (2011) 353–362).

In Chapter 9 the effect of bile salts viz. sodium cholate (NaC), sodium

taurocholate (NaTC) and sodium deoxycholate (NaDC) on the size and transition of

worm like cetyltrimethylammonium toluene sulfonate (CTAT) micelles was

examined by viscosity. Cryogenic transmission electron microscopy (Cryo-TEM)

provided clear picture of micelle and change in micelle morphology with increase in

concentration of different bile salts. SANS study provided data on Nagg and size of

CTAT micelles as a function of bile salt concentration. To get information about

location of bile salts in CTAT micelle and to characterize the interaction between

CTAT and bile salts 2D NOESY experiments were performed. (A manuscript based

on this work is under preparation and will be communicated soon to Langmuir).


24

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