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Chapter 1. General overview
1
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
Chapter 1. General overview
2
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
Chapter 1. General overview
3
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.
Chapter 1. General overview
4
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
Chapter 1. General overview
5
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.
Chapter 1. General overview
6
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 head- group
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.
Chapter 1. General overview
7
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
Chapter 1. General overview
8
(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.
Chapter 1. General overview
9
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
Chapter 1. General overview
10
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
Chapter 1. General overview
11
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
Chapter 1. General overview
12
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.
Chapter 1. General overview
13
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
Chapter 1. General overview
14
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
Chapter 1. General overview
15
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
Chapter 1. General overview
16
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].
Chapter 1. General overview
17
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
Chapter 1. General overview
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
Chapter 1. General overview
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.
Chapter 1. General overview
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].
Chapter 1. General overview
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
Chapter 1. General overview
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)
Chapter 1. General overview
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).
Chapter 1. General overview
24
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