Chapter - 2 Critical Micelle Concentration of Surfactant, Mixed ...€¦ · Critical Micelle Concentration (CMC) is the key point for surface chemistry as well as chemist and systematic

KINETIC STUDIES OF SOME ESTERS AND AMIDES IN PRESENCE OF MICELLES

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Chapter - 2

Critical Micelle Concentration of Surfactant, Mixed Surfactant and Polymer By Different Methods at Room Temperatur e And

Its Importance

Abstract

Critical Micelle Concentration (CMC) is the key point for surface chemistry as

well as chemist and systematic data collection and day-to-day development in

this regard is very essential. Hence, we have analyzed the CMC of surfactant,

mixed surfactant and polymer at room temperature (280C), with the change in

surfactant, mixed surfactant and polymer concentration CMC values were

observed by two methods, surface tension and conductometric method. The

result of CMC values of surfactant, mixed surfactant and polymer show

different characteristics and out comes. The CMC values follows below order:

Polymer >Surfactant–Polymer > Mixed Surfactant > Surfactant

Introduction

The role of micellar catalysis in recent years has been no need to say its

importance in different area such as pharmaceuticals, oil recovery industry,

environmental as well as Nano technological system (Proceedings ICCE

Indore 2005). The role of micellar catalysis may not be understood without its

critical micelle concentration. This is the concentration where surfactant will

work as micelle. Therefore, it is very interested as well as important to know

this factor very correctly and accurately. Our group had done some work in

this area. Nevertheless, systematic and new out comes develop this work.

The present work is an investigation of critical micelle concentration of


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different surfactant, polymer, and mixed micelle at room temperature (280C)

with reaction condition. Groundwater aquifers, facilitate/enhance residual oil

recovery and in soil-cleanup operations is well established, and both anionic

and nonionic surfactants have been used to remediate land polluted with oils

and hydrocarbons as well as many other organic contaminants. A

fundamental property of surfactants is their ability to form micelles (colloidal

sized clusters) in solution. This property is due to the presence of both

hydrophobic and hydrophilic groups in each surfactant molecule. It is the

formation of micelles in solution which gives surfactants their excellent

detergency and solubilization properties. Hence, the most important

parameter in terms of the ability of a surfactant to mobilize or solubilize

hydrophobic contaminants in contaminated soil is the surfactant CMC. The

CMC of a surfactant solution can be determined by any physical property (e.g.

surface tension used in this work) that shows a distinct transition around the

CMC. Based on this particle nucleation mechanism, the well-known Smith-

Ewart theory predicts that the number of particles nucleated is proportional to

the surfactant concentration to the 0.6 power (Romsted 1977). This is

reasonable because for a given surfactant the number of micelles formed

generally increases with an increase in the surfactant concentration.

Surfactants are organic substances, which significantly decrease the surface

tension of water at relatively low concentrations, atleast partially water

soluble. Because surfactants are absorbed mainly on the surface of the

solution, creating a thin monolayer, they are called surface active substances.

When dissolved them, after they reach a certain value of concentration,

molecules or ions of surfactants begin to associate and to organize

themselves into more complex units, also called micelles. The characteristic

concentration value, where the association process begins, is called the

critical micelle concentration and is labeled with symbol cc or abbreviation

CMC. The CMC is one of the most useful physicochemical characteristics of

many biologically active substances and drugs. From the chemical point of

view, surfactants are mostly low-molecular compounds, so when dissolved,

form the true solutions in concentration ranges below the CMC. Micelles are


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aggregates of larger number of simple molecules or ions of surfactants (e.g.

several dozens), so the resulting size of such structures is in the colloidal

range. For this reason the micelle solutions of surfactants are regarded as

association colloids. The molecular structure of surfactants is amphiphilic

(biphilic), it consists of both nonpolar (hydrophobic, lipophilic) and polar

(hydrophilic) parts (Figure 2.1).

Figure 2.1. Amphiphilic structure of surfactants An illustration of a

spherical micelle (of dodecyl sulfate) emphasizing the liquid-like

character with a disordered hydrocarbon core and a rough surface.

(Redrawn from Jonsson, B., Lindman, B., Holmberg, K . and Kronberg,

B., Surfactants and Polymers in Aqueous Solution), ©1998 John Wiley &

Sons Ltd., Chichester, Reprinted in April 2001, p. 36, Reproduced with

Permission)

The lipophilic part of the structure usually consists of hydrocarbon groups, in a

simple case, it is one long alkyl chain, typically with more than ten carbon


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atoms. The hydrophilic part of the structure may be represented by non-ionic

polar groups or ionic groups and in this sense

we distinguish between non-ionic surfactants and ionic surfactants. The ionic

groups are much more hydrophilic than the non-ionic polar groups. The non-

ionic surfactants are dissolved as electroneutral molecules, e.g. higher fatty

alcohols or cholesterol which have polar hydroxyl groups. Ionic surfactants

dissociate in aqueous solutions into pairs of 2 cations and anions, but usually

only one kind of these ions are surface active, and ions with the opposite

charge are called counterions. Based on the charge and the nature of the ion

which generates surface activity, we can divide the ionic surfactants into

following classes:

1. anion-active (anionic) surfactants - e.g. sodium or potassium salts of

higher fatty acids (soaps), salts like sodium dodecyl sulfate, sodium

tetradecyl sulfate etc.

2. cation-active (cationic) surfactants - e.g. quaternary ammonium salts –

hexadecylpyridinium bromide, carb ethopendecinium bromide (Septonex)

etc.

3. ampholytic surfactants – e.g. long-alkyl amino acids, with pH dependent

charges.

Surfactants form a numerous group of various natural and synthetic

compounds, In common practice, their mixtures are used as soaps,

saponates and detergents. Non-ionic neutral and some anionic surfactants

are used in pharmacy as useful excipients - additives, emulgators,

solubilizers, constituents of creams and ointments. Among the cationic

surfactants we can find a large number of especially antimicrobial substances.

Ionic surfactants are generally well soluble in water and at concentrations

below the corresponding CMC. They behave like fully dissociated strong

electrolytes. Anionic surfactant - sodium dodecyl sulfate - is dissociated into

the surface active CH3(CH2)11OSO3 - anión and the sodium cation Na+

(counterion) in the aqueous solution.


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The surface active amphiphilic anions are absorbed on the water surface

where they create a characteristic monolayer. The lipophilic dodecyl alkyls -

CH3(CH2)11 are oriented outside from the water surface, while the hydrophilic -

OSO3 - headgroups are directed into the aqueous environment. When the

concentration of sodium dodecyl sulfate reaches its corresponding CMC

value, the dodecyl sulfate anions starts to aggregate into the negatively

charged globular micelles. The lipophilic core of the micelles is built up from

the non-polar hydrocarbon alkyl chains while anionic groups localized on the

micelle surface with orientation to the polar aqueous environment. Negatively

charged micelles bind by electrostatic forces certain fraction of the Na+

counterions or other cations which may be present in the solution. Cationic

surfactant - carbethopendecinium bromide (Septonex) - is a well known

quaternary ammonium salt with strong antimicrobial activity. Its

carbethopendecinium cations form positively charged micelles in aqueous

solution at concentrations above its CMC value. The carbethopendecinium

cation has more complex structure, the nonpolar group is represented by an

alkyl chain and its positive ionic charge is on the quaternary nitrogen atom.

Critical Micellar Concentration (CMC)

The CMC of surfactants in recent literature, has been defined as the

concentration of the surfactant solution at which the molecules selfaggregate

to form spherically shaped micelles.(SHAH 1983) The Critical Micelle

Concentration indicates the usually narrow range of concentrations separating

the limits,at below which most of the surfactant is in the monomeric state and

above which virtually all additional surfactants enters the micellar

state.(Desando 1986) .

The variation of the CMC with chemical and physical parameters provides a

good insights into the nature of the surfactant self-association. There are quite

abrupt changes in the concentration dependence of a larger number of

physico chemical properties at a particular concentration , this led to CMC

concept(Lindman 1980, Moroi 1988) shown diagrammatically in the Figure

2.1. Below the CMC negligible aggregation of the surfactants takes place


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which in turn occurs as extensive aggregates above the CMC (Lindman

1984). For surfactants containing long chain alkyl groups the value of CMC is

usually between 10-4 and 10-2 M. The Sharpness of the break in the physical

properties depends on the nature of the micelle and on the method of the

CMC determination. Among 70 known methods (Mukherjee 1971), there are

however are a great

Figure 2.2. Schematic representation of the concent ration dependence

of some physical properties of solutions of a micel le-forming surfactant.

(Redrawn from Jönsson, B., Lindman, B., Holmberg, K . and Kronberg,

B., Surfactants and Polymers in Aqueous Solution), ©1998 John Wiley &

Sons Ltd., Chichester, Reprinted in April 2001, p. 36, Reproduced with

Permission)

differences in the sensitivity and reliability. The physical methods for CMC

determination includes conductivity, solubility, viscosity, light scattering,

measuring the surface tension by Wiebelmy slide method or by the method of

maximum bubble pressure , measurement of ion activity and by dye


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incorporation method, Gel filtration spectrophtometrically and counterion

magnetic resonance (Elworthy 1968, Cermakova 1986, Beg 1984).

Examination of the literature shows that the concentration of the monomers

present in the solutions increases after the CMC and there is a possibility of a

second marked aggregation, known as second CMC (Robertz and Jones

1973). These, undoubtly, reflect change in size, shape, polydispersity and

degree of concentration binding to binding to the micelle and also change rate

of hydration (Kubota 1973). A second CMC exists where the aggregates gain

positional order due to increased electrical repulsions among the micelles

(Kale 1980). According to Porte et al (1984), the second CMC arises from a

sphere to rod transition of the micelle geometry. Studies on micellization in

potassium n-Octanoate in deuterium oxide, over wide concentration and

temperature intervals , reveals that a second critical micelle concentration

exists around 1.0 M potassium at Ca-30-40 oC (Desando 1986).

It is noteworthy that micelles when formed are not independent (Lawrence

1958). The half life is about ten million seconds for pure ionic surfactants

(Elworthy 1968). The half life for the micelle of SDS and dodecyl pyridium

bromide is about 10-2 seconds (Jaycock 1964). Different NMR(Florence 1971)

and ultrasonic measurements (Craber 1970) reveal that the rate constant for

the dissociation of the monomer from a micelle is in the region 10-2-10-9

seconds-1.

Micelles donot have an indefinite lifetime but are constantly formed and

destroyed in the solution by kinetic processes (Kielmann 1976). The life time

of micelles depend in a very complicated way on the hydrocarbon chain

lengths, the dissociation degree, the aggregation numbers and additives

(Hoffmann 1985). There are two mechanisms, by which micelles can be

formed and destroyed, i.e, at low salt concentrations, ionic micelles change

their aggregation number in a regular fashion, while for high salt

concentration, micelles can also coalesce and break into pieces (Lebney

1981) . The micelles must therefore be considered as an extremely complex

dynamic entity which is capable of rapid break down and formation.


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The micelle formation is invariably affected by the effect of hydrophobic

length and the nature of the additive added. The CMC decreases strongly as

the hydrophobic part of the surfactant is increased; the decrease is more rapid

for nonionic than for ionic surfactants (Lindman 1984). As a general rule for

ionic surfactants, the CMC is halved when the length of the straight

hydrocarbon chain is increased by one methylene group (Attwood 1970). An

even more pronounced decrease in CMC with increase of hydrocarbon length

has been noted with non-ionic surfactants, the addition of one more

methylene group causes the CMC to decrease to one third of its original

value.

The CMCs of the various homolog’s of surfactants with a linear alkyl chain

follow the equation:

Log CMC = A – Bn (1)

Where, n is the alkyl chain and A and B are constants

Figure 2.2 shows a linear relationship between log CMC and the no. of carbon

chain length. For Chain length above 16 carbon, this pronounced effect

gradually loss its significance and no further appreciable effect of CMC are

observed because of the coiling of the long chains is observed (Mukerjee

1967). In case of branched hydrocarbon chains, the effect on the CMC (with

increase in the number of carbon atom) is not higher than in the straight

chain.


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Figure 2.3. Variation of critical micelle concentr ation with hydrocarbon

chain length for (A) sodium alkyl sulphate and alky l trimethyl ammonium

bromide (B) Hexaoxy ethylene monoalkyl ethers. (Aft er Attwood and

Florence 1983).

Where A and B are constants for a homologous series. Branching of a

hydrocarbon chain causes an increase in CMC (Janjjap 1988). Since the

decrease in free energy arising from the aggregation of branch chain

molecules, is less than that obtained with linear molecules with the same

number of carbon atoms.

For a given alkyl chain, CMCs increase in the order nonionic (Polyoxyethylene

glycol monoethers)< Zwitterionic< ionic (Cationic or anionic) (Winterborn

1972).

Effect of Additives

Examination of the literature shows that additives have a marked effect on the

CMCs of both ionic and nonionic surfactants, the more ionized groups present

in the surfactant, the higher the CMC, due to the increase in electrical work to

form the micelles.


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A detailed study of the effect of the nature of the polar head group of the ionic

surfactants on the micellar properties has been reported by Anacker and

Coworkers. The effect of different ionic groups on CMC is small, provided

complex dissociation occurs, as the extent of work necessary to overcome

electrical repulsion between ions of same charge is similar. Since the effect of

electrical repulsion is absent in non-ionic surface active agents, aggregation is

facilitated and the CMC are much lower than those ionic surface active

agents.

As far as the effect of number of hydrophilic groups are concerned, increase

in the number of any hydrophilic group increases the solubility of surface

active agents, leading to increase in CMC(36). The position of ionic groups

also affect the micellar properties. A marked increase in the CMC of Sodium

Alkyl Sulphate was observed with change in position of sulphate from terminal

position to a medial position along the chain (Evans). A similar increase in

CMC was observed for the polyoxy ethylated either type of non-ionic

surfactant with increase in the chain length of polyoxy ethylene chain and

decrease in micelle size.

For a compound to be qualified as a surfactant, it should also exhibit surface

activity. It means that when the compound is added to a liquid at low

concentration, it should be able to absorb on the surface or interface of the

system and reduce the surface or interfacial excess free energy. The surface

is a boundary between air and liquid and the interface is a boundary between

two immiscible phases (liquid–liquid, liquid–solid and solid–solid).

Surface activity is achieved when the number of carbon atoms in the

hydrophobic tail is higher than 8. Surfactant activities are at a maximum if the

carbon atoms are between 10 and 18 at which level a surfactant has good but

limited solubility in water. If the carbon number is less than 8 or more than 18,

surfactant properties become minimal. Below 8, a surfactant is very soluble

and above 18, it is insoluble. Thus, the solubility and practical surfactant

properties are somewhat related.


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Determining Critical Micelle Concentration (Surface Tension

Method )

The molecules of a liquid attract each other due to dispersion, dipole-dipole

and dipole-induced-dipole forces, as well as hydrogen bonding. In the bulk

liquid, a molecule senses the same attractive and repellent forces in all

directions, while for a molecule at the surface, those forces are lacking in one

direction. This asymmetry of forces is the origin of the surface energy or

equivalent the surface tension. For the reasons mentioned above, a

compound will possess higher surface tension, the more polar it is. Likewise,

two liquids are immiscible if the surface tension between them is large, but for

entropic reasons they are miscible if that tension is low enough. Then

practically, what a surfactant does, to lower the surface tension. This leads to

increased solubility of non-polar molecules in aqueous solutions (e.g.

solubilization of micelles). Surfactants can also improve the stability of

dispersions, emulsions and foams. There is, of course, a practical limit to how

low the surface tension can be. With special formulations, so-called ultra low

interfacial tension, values in the range of 10-3 mN/m or lower can be reached.

Such microemulsion systems are of interest for the purpose of enhanced oil

recovery (EOR). A summary of the surface tension values of some liquids is

found in Table 2.1.


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Table 2.1 Surface tension values (in mN/m) of some liquids at 25 °C

Solvent Surface Tension

water 72-73

10% aq. NaOH 78

Aqueous surfactant solution

28-30 (at or above CMC)

Ethanol 22

Chloroform 27

Bromoform 45

Hexane 18

Octane 22

Dodecane 25

Hexadecane 27

Diethyl ether 17

Mercury 480

The properties of surfactant at low concentration in water are similar to those

of simple electrolytes except that the surface tension decreases sharply with

increase in concentration. At a certain concentration, surfactant monomers

assemble to form a closed aggregate (micelle) in which the hydrophobic tails

are shielded from water while the hydrophilic heads face water. The critical

aggregation concentration is called the critical micelle concentration (CMC)

when micelles form in an aqueous medium. The CMC is a property of the

surfactant. It indicates the point at which monolayer absorption completes and

the surface active properties are at an optimum. Above the CMC, the

concentrations of monomers are nearly constant. Hence, there are no

significant changes in the surfactant properties of the solution, since the


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monomers are the cause of the surface activity. Micelles have no surface

activity and any increase in the surfactant concentration does not affect the

number of monomers in the solution but affects the structure of micelles. The

typical CMC values at room temperature are 10−3 to 10−2M for anionic

surfactants, 10−3 to 10−1M for amphoteric and cationic surfactants and 10−5 to

10−4Mfor non-ionic surfactants. The CMC of several surfactants in aqueous

media can be found in (Rosen 2004, Mukherjee 1971). Surfactant structure,

temperature, the presence of electrolyte, existence of organic compounds and

the presence of a second liquid has an effect on the CMC. The following

factors contribute CMC decrease (Porter 1994, Rosen 2004, Evans 1956,

Schick 1962, Ray 1971, Schick 1957, 1965, Herzfeld 1950, Hunter 1987):

(a) An increase in the number of carbon atoms in the hydrophobic tails

(b) The existence of polyoxypropylene group

(c) Fluorocarbon structure

(d) An increased degree of binding of the counterions

(e) The addition of electrolyte to ionic surfactants

(f) The existence of polar organic compounds (such as alcohols and

amides)

(g) The addition of xylose and fructose

The following factors contribute to CMC increase (Porter 1994, Rosen 2004,

Evans 1956, Schick 1962, Ray 1971, Schick 1957, 1965, Herzfeld 1950,

Hunter 1987):

(a) Branch hydrophobic structure

(b) Double bonds between carbon atoms

(c) Polar groups (O or OH) in hydrophobic tail

(d) Strongly ionised polar groups (sulphates and quaternaries)


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(e) Hydrophilic groups placed in the surfactant molecule centre

(f) Increase in the number of hydrophilic head

(g) Trifluoromethyl groups

(h) An increase in the effective size of hydrophilic head

(i) An increase in the pH of weak acids (such as soap)

(j) A decrease in pH from isoelectric region and increase in pH from

isoelectric region foramphoteric surfactants (low CMC at the isoelectric

region and high CMC outside the isoelectric region)

(k) addition of urea, formamide, and guanidinium salts, dioxane,

ethylene glycol and water soluble esters

The CMC decreases with temperature to a minimum and then increases with

further increase in temperature. The minimum appears to be around 25◦C for

ionic surfactants and 50◦C for non-ionic surfactants (Flow chart 1961, Crook

1967).

Several empirical correlations are available for the estimation of CMC values.

For straight and saturated single tail ionic surfactants, the CMC can be

calculated from (Klevens 1953)

Log CMC = A − Bn

Where, n is the number of carbon atoms in the hydrophobic tail, and A and B

are temperature dependent constants for a given type of surfactant. The value

of B is around 0.3 is equivalent to log102 ) for the ionic surfactants because

the CMC of the ionic surfactants is halved for each carbon atom added to the

hydrophobic tail. B value is about 0.5(= 0.5 log 10) for the non-ionic and

amphoteric surfactants because the CMC will decrease by a factor of 10 for

each of the two methylene groups added to the hydrophobic tail. The values

of A and B for some surfactants can be found (Kreshech 1975).


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The effect of electrolyte concentration on the CMC of ionic surfactant is given

by (Corrin et al 1947).

logCMC = a − b log C

where a and b are constants for a given ionic hydrophilic head at a certain

temperature and C is the total counter ion concentration in equivalent per litre.

The effect of electrolyte concentration on the CMC of non-ionic and

amphoteric surfactants is given by Ray (1971) and Shinoda (1961).

logCMC = x − yCe Where Ce < 1

where x and y are constants for a given surfactant, electrolyte and

temperature, and Ce is theconcentration of electrolyte in moles per litre.

Further discussion of the theoretical CMC equations can be found in [Rossen

2004, Hobbs 1951, Molyneux 1965).

In non-polar solvents, hydrophilic head groups interact due to dipole–dipole

attractions and produce aggregates called reverse micelles.With this

structure, head groups of surfactant molecules orientate towards the

interiorandthehydrophobic tails orientate towards the nonpolar solvents. In the

absence of additives such as water, the aggregation numbers of reverse

micelles are small (mostly less than 10). On the other hand, in polar solvents

such as glycol, glycerol and formamide, surfactant aggregates are thought to

be similar to the aggregates in water since these polar solvents havemultiple

hydrogen bonding capacity. In general, the CMC of ionic and non-ionic

surfactants is higher in nonaqueous solvent than in water (Kaler 1994). The

CMC is a useful tool for the selection of surfactants for specific applications or

properties. For example, surfactants with a low CMC are less of an irritant

than those with high CMC. The CMC can be determined by measuring the

changes in physical properties such as electrical conductivity, turbidity,

surface tension, interfacial tension, solubilisation and auto diffusion. Detail

evaluation of different methods for the determination of CMC can be found in

(Mukherjee et al 1971). Amongst these methods, the surface tension method

is most commonly used in practice and ISO Standard 4311 describes this


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method which is applicable to all types of surfactants in both commercial and

pure forms. It requires the strict control of test temperature for precise and

reproducible values. According to the procedure, 16 surface tension values

are measured over the range of surfactant concentrations; among these, six

values should be in the region close to CMC. Each value is repeated three

times and measurements are made within 3 h of solution preparation. The

average of each set of three values is plotted as surface tension versus the

log of the surfactant concentration. For a pure surfactant, the break point at

the CMC is sharp and well defined. The concentration at the minimum surface

tension gives the CMC value. Most formulators use more than one surfactant

to improve the properties of products. In addition, commercial surfactants are

mixtures because they are made from mixed chain length feedstock and are

mixtures of isomers and by-products depending on their synthesis. Purifying

the surfactant to a great extent is not economically feasible. Furthermore, a

mixture of surfactants was found to perform better than single surfactants in

many applications such as emulsion formation, detergents and enhanced oil

recovery. The CMC of the mixture is either the intermediate value between

theCMCvalues of each surfactant, less than any of the surfactant CMC

(positive synergism) or larger than any of the surfactant CMC (negative

synergism). The CMC of the mixture, if the mixture contains two surfactants

and mixed micelles, is an ideal mixture (activity coefficients of free surfactant

monomers for each surfactant type in the mixture are equal to unity.

CMCM = x1 CMC1 + (1 − x1) CMC2

where x1 is the mole fraction of surfactant 1 in solution on a surfactant base,

and CMC1 and CMC2 are the critical micelle concentrations of pure

surfactants 1 and 2, respectively. Details and the equations for the non ideal

surfactant mixtures can be found in (Scamehorn 1986).


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Micellar Aggregate Structures and Shapes

A theory for the aggregate structure was developed, based on the area

occupied by the hydrophilic and hydrophobic groups of surfactant (Tanford

1980, Israelachvili 1992). For a stable formation of a surfactant aggregate

structure in an aqueous system, the internal part of the aggregate should

contain the hydrophobic part of the surfactant molecule while the surface of

the aggregate should be made up of the hydrophilic heads. The polar head

groups in water, if ionic, will repel each other because of same charge

repulsion. The larger the charge, the greater the repulsion and the lower the

tendency to form aggregates. The hydrophilic heads have also strong affinity

for water and they space out to allow water to solvate the head groups. On

the other hand, hydrophobic tails attract one another due to hydrophobic

effect. When the surfactant concentration is high enough, the surfactant

molecules pack together due to the interaction of the two opposing forces

between the surfactant molecules. The shape and the size of the aggregate

can be determined by using the surfactant packing parameter which is the

ratio of the hydrophobic group area to the hydrophilic head area. The ν and lc

are the volume and length of the hydrophobic tail in the surfactant aggregate:

ν = 27.4 + 26.9n

lc ≤ 1.5 + 1.265n

where n is the total or one less than the total number of carbon atoms of the

hydrophobic tail in the surfactant aggregate, ν is in cubic Angstrom (˚A3) and

lc is in A˚ . For saturated straight chain, lc is 80% of the fully extended chain

(Tanford 1940). The structures of surfactant aggregates as a function of

surfactant packing parameter and shape are shown in Figure 2.3 (Israelachvili

1992).

Spherical micelles are formed, where the value of surfactant packing

parameter is less than 1/3 (single chain surfactants with large head group

areas such as anionic surfactants). The spherical aggregates are extremely


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small and their radius is approximately equal to the maximum stretched out,

length of the surfactant molecule.

Cylindrical micelles are formed where the surfactant packing parameter is

between 1/3 and 1/2 (single chain surfactants with small head group areas

such as non-ionic surfactants and ionic surfactants in high salt concentration).

Any change in solution properties which causes a reduction in the effective

size of hydrophilic head groups will change the aggregate size and shape

from spherical to cylindrical form. For example, the addition of electrolyte

reduces the effective hydrophilic area of ionic surfactants because the

increased counterions reduce the repulsion between ionic polar head groups.

Addition of co-surfactant with a smaller head group size also contributes to

mixed micelle formation of cylindrical shape. Increasing the temperature,

reduces the ethoxylated non-ionic head groups. Furthermore, changing the

pH changes the degree of protonation of amphoteric surfactants and affects

the head size. Vesicles, liposomes and flexible bilayers are formed where the

surfactant packing parameter is between 1/2 and 1 (double chain surfactants

with large head group areas such as phospholipids, surfactants with bulky or

branched tail groups and the mixture of anionic and cationic surfactants with

single chain at nearly equimolar concentration). These types of surfactants

cannot pack themselves into a close micelle and form bilayers (lamellar

structure). As the packing parameter approaches unity, the lamella becomes

flat and planar (double chain anionic surfactants in high salt concentration).

Only the flexible lamellar bilayer bends around and joins in a sphere (vesicle).

This structure keeps aqueous solution both inside and outside of the sphere.

Liposomes are concentric spheres of vesicles (layers of an onion

arrangement) they are more than a micrometer in size and formed by gentle

shaking of surfactant in water. The internal bilayer structures of the liposomes

are optically active. Hence, they can easily be identified with a polarising light

microscope. Vesicles are formed from liposomes by ultrasonication,

ultrafiltration or microfluidisation. They are nanometre in size and can only be

detected by electron microscopy. Vesicles are used as drug delivery agents,

model components for cell membranes and cationic softeners in detergency.


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Inverted or reverse micelles are formed where the surfactant packing

parameter is greater than 1 (surfactants with small head groups or large tail

groups such as double tailed anionic surfactants). These structures are

formed in non-polar solvents. In these structures, head groups are clustered

together and tails are extended towards the solvent. They have the capacity

to take water into their cores and hence, form water-in-oil microemulsions.

Hydrophilic materials can also be solubilised into the reverse micellar core

(engine oil additives, hydraulic oils and cutting oils). Inverse micelles are often

used for the separation of biological molecules such as proteins. The

surfactant phase diagrams for several surfactants have been developed in

order to understand the phase tructureof surfactants in solution at high

concentration. With these


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Figure2.4. Schematic structures of surfactant self- assemblies as a

function of surfactant packing parameters and shape .


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CMC and Micellar Growth

When added to an aqueous solution, surfactant molecules minimize their

energy by creating a monolayer on the air-water surface. This is a slow

process with large molecules, for example, polymers (equilibrium times can

vary from seconds to days), but considerably faster for surfactants (ms-s). In

order to minimize their free energy, the hydrophobic parts of the surfactants

are directed towards the less polar air, while the hydrophilic groups are

directed towards the polar water molecules. Upon increased surfactant

concentration, the surface becomes increasingly saturated by surfactant

molecules, which decreases the surface tension of the solution. In other

words, the surface tension will decrease upon the addition of surfactants. The

kinetic process can be followed by so called dynamic surface tension studies.

The low-concentration kinetics can be described by the Ward-Tordai equation,

which assumes diffusion from the bulk and absorption at the interface, here

NMR diffusion experiments can be useful for determining the diffusion

coefficient, using the Stokes-Einstein Equation of the surfactant. The

equilibrium situation is more commonly studied, and results in, among other

things, the acquisition of the CMC value (see below), which is an important

characteristic of the surfactants. The surface tension is commonly studied by

means of a Wilhelmy plate tensiometry apparatus. The minimum surface

tension reachable is determined by how effective the packing of the

surfactants is at the surface and on the interactions between the surfactants

at the interface) different hydrophobic and hydrophilic groups thus create

different minimum surface tensions. With concentrations above the saturation

point, called the critical micellization concentration, the CMC, the freely

dissociated surfactant molecules in the water bulk phase start to form closed

micellar structures, that can be spherical, oblate, prolate, tablet shaped or rod-

like, with a hydrophilic surface and a hydrophobic interior (Figure 2.1). Such

micelles usually have hydrodynamic radii, RH, ranging from 20 to 400 Å. High

accuracy measurements of the force between two solid interfaces coated with

surfactants can be made by means of a MASIF apparatus (Measurement and

Analysis of Surface Interaction Forces) or other surface force instruments.


76 | P a g e

The CMC of polyoxyethylene-based surfactants decreases with increasing

temperature. Sugar-based surfactants, as well as SDS, seem to exhibit

minimum CMC values at given temperatures. The favorable decrease in

energy gained from micelle formation further increases the solubility of the

surfactant molecules themselves, as it also does with any non-polar organic

compound trapped in the micellar interior. At the CMC, usually in the order of

µM-mM, almost all further surfactant molecules are incorporated into the

micellar structures and hence the absorption to surfaces is also saturated. No

noticeable decrease in surface tension is gained by increased surfactant

concentrations above the CMC. Thus, the CMC can easily be determined, by

application of the Gibbs isotherm, from a plot of the logarithmic surfactant

concentration (often in mM) vs. the surface tension (, often in mN/m) of the

solution. From such a plot it is also possible to calculate the absorbed amount

of surfactant at the interface, with the absorbed amount being proportional to

the slope of the surface tension isotherm (up to the CMC). For practical

formulations, the CMC itself can be lowered by the addition of a molecule that

is more hydrophobic than the surfactant itself, since the formation of micelles

is then stimulated. Small rather hydrophobic compounds such as short chain

alcohols also interact with surfactants. They affect the packing of the

surfactant at the interface (e.g. affecting the foam stability). So-called

hydrotropes are often small molecules with bulk hydrophobic groups and/or

charged polar groups. They interfere with the packing of the surfactant and

affects a range of properties, e.g. for ethylene oxide-based surfactants, they

increase the cloud point (the temperature at which the surfactant solution

turns turbid due to a liquid-liquid phase separation). The actual radii of the

micelles formed are determined by surfactant concentration and surfactant

geometry. The size and shape can be effectively measured, for example, by

using a SANS apparatus (Small Angle Neutron Scattering). With further

increased concentration the micelles increase in size, forming rod- or sponge-

like structures instead, and begin to pack into lamellar, hexagonal or cubic

structures Figure 2.5.


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This phenomenon, which is also temperature-sensitive is often described for

surfactant-water mixtures by means of binary or ternary phase diagrams.

Furthermore, increased temperature induces a decrease in the hydration

number of the head group of the ethylene oxide-based surfactants, rendering

it more hydrophobic. Sugar-based surfactants are less affected by

temperature variations. Thus, with increased hydrophobicity, the ethylene

oxide-based surfactant will, when mixed with oil, gradually go from promoting

oil droplets in water to forming water droplets in oil. This change in structure

can be described as a result of changes in the spontaneous curvature, H, a

parameter that describes surfactant packing at different

Figure 2.5. Surfactant self-assembly leads to a ran ge of different

structures, a few of which are shown here. (Redrawn from Jönsson, B.,

Lindman, B., Holmberg, K. and Kronberg, B., Surfact ants and Polymers

in Aqueous Solution), ©1998 John Wiley & Sons Ltd., Chichester,

Reprinted in April 2001, p. 34,

temperatures in a given system, with positive values giving oil-in-water

emulsions and negative values giving reversed micelles (water-in-oil

emulsions). The temperature at which the H-parameter is 0, meaning exact

balancing of hydrophobicity and hydrophilicity, is referred to as the phase


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inversion temperature (PIT) of the surfactant in that particular system.

Mixtures of surfactants working synergistically are often characterized in

ternary phase diagrams, showing the various phases of different molar ratios,

molar fractions, weight fractions or weight percentages of the three

components involved. A number of phenomena, including wetting, dispersion,

emulsification and foaming, can be explained by the formation of micelles and

the minimizing of free energy of the systems. Several physical properties of a

solution are concentration-dependent on the presence of a micelle-forming

surfactant. Reactive surfactants can covalently bind to the dispersed phase

and as such have a distinct advantage over conventional surfactants that are

only physically adsorbed and can be displaced from the interface by shear or

phase changes with the subsequent loss of emulsion stability. Depending

upon the chemical structure and effects, there are different types of reactive

surfactants:

1. Functionalized monomers

2. Surface active initiators (Inisurfs)

3. Surface active transfer agents (Transurfs)

4. Polymerizable surfactants (Surfmers)

Inisurfs, Transurfs and Surfmers may be used to reduce/avoid the use of

conventional surfactants in emulsion polymerization. However, when Inisurfs

and Transurfs are used, the stability of the system cannot be adjusted without

affecting either the polymerization rate (Inisurfs) or the molecular weight

distribution (Transurfs). Furthermore, the efficiency rate of Inisurfs is low due

to the cage effect.

Functionalized monomers

Functionalized monomers are sometimes regarded as polymerizable

surfactants. Vinyl or allyl monomers are reacted with ethylene oxide (EO),

propylene oxide (PO) or butylenes oxide (BO) in a sequential or random


79 | P a g e

addition mode. The terminal hydroxyl group can be optionally reacted with

methyl or benzyl chloride to produce Williamson ethers (if the hydroxyl group

has to be deactivated) or are further sulfated to deliver electrosteric

stabilization. Functionalized monomers can be copolymerized with other

ethylenically unsaturated monomers for permanent polymer modification. The

drawback of allylic, acrylic and vinylic polymerizable groups is their tendency

to homopolymerize. Allylic derivatives, furthermore, are susceptible to

degradative chain transfer.

Figure 2.6 Functionalized monomer.


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Derivatives from pure ethylene oxide are water soluble and result in silicon

polyols with water solubility or dispersibility. Propylene oxide and even more

butylene oxide allow for more compatibility with organic media, e.g. butylene

oxide gives compatibility with organic oils. Depending upon the demanded

balance of hydrophilicity/hydrophobicity, different proportions and order of

addition of the alkoxide can be used. Incertain instances blocking of the

terminal hydroxyl group may be required, e.g. by reaction with methyl or, less

commonly, benzyl chloride.

Propylene oxide undergoes rearrangement in the presence of base and forms

allyl alcohol, thus forming in situ initiators during the alkoxylation process.

Unless properly accounted for, this decreases the molecular weight of the

polyether produced. The hydrosylilation of the allyloxy polyether intermediate

by a siloxane hydride is catalyzed by e.g. chloroplatinic acid and is

exemplified in eqn. 1.

(Me3SiO)2MeSiH + H2C=CHCH2O(EO)xH → (Me3SiO)2MeSi(CH2)3O(EO)xH

Where, EO stands for ethylene oxide.

Surface active initiators

Surface active initiators or Inisurfs have the advantage of reducing the

number of ingredients in an emulsion polymerization recipe to water,

monomer and initiator, at least in the initial stages of the process. However,

the surface active properties of the Inisurfs may be reduced on formation of

the radicals and additional surfactant must be added to stabilize the latex if

high solid levels are wanted.

Inisurf molecules contain three moieties:

� The radical generating moiety, which can be azo or peroxy

� A hydrophobic moiety which is usually a hydrocarbon (alkyl or alkyl

phenyl), some times extended by the inclusion of propylene oxide


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� A hydrophilic moiety which can be anionic, cationic or nonionic

The molecules can be symmetrical, i.e. the structural moieties are the same

on both sides of the radical-generating group and two surface active radicals

are produced on decomposition. If the structure is asymmetrical only one

surface active radical is produced on decomposition. The key feature of

Inisurfs is their surfactant behavior. They form micelles and are absorbed at

interfaces, and as such they are characterized by a critical micelle

concentration (CMC) and an area/molecule in the adsorbed state. This

influences both the decomposition behavior and the radical efficiency, which

are much lower than those for conventional, low molecular weight initiators.

Tauer and Kosmella (1993) have observed that in the emulsion

polymerization of styrene, using an Inisurf concentration above the CMC,

resulted in an increase in the rate constant of the production of free radicals.

This was attributed to micellar catalysis effects as described, for example, by

Rieger (1986). Conversely, if the Inisurf concentration was below the CMC the

rate constant of the production of free radicals decreased with an increase in

the Inisurf concentration, which was attributed to enhanced radical

recombination. Also note that a similar effect of the dependence of initiator

efficiency on concentration was reported by Van Hook and Tobolsky for

azobisisobutyronitrile (AIBN) (Van Hook 1958).


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Table 2.2. Examples of functionalized monomers

Chemical Composition

Nature Feature Comments

Allyl Polyalkylene glycol ethers

Nonionic Copolymerizable emulsifiers for the emulsion polymerization of vinyl acetate, acrylates, styrene/acrylates Addition during emulsion polymerization improve latex stability and reduce grit levels Reduce water uptake of polymer films

Level of use 1-2% active materials based on monomers

Vinyl polyalkylene glycol ethers

Nonionic Copolymerizable emulsifier for the emulsion polymerization of vinyl axetate, acrylates, styrene/acrylates Improve grit levels Reduce water uptake of polymer films



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Allyl polyakkylene glycol ether sulfate ammonium salt

Anionic Copolymerizable emulsifiers for the emulsion polymerization of vinyl acetate, acrylates, styrene/acrylates Anionic monomer with surface activity can be used without additional emulsifier improve latex stability and reduce grit levels reduce water uptake of polymer films


Methacrylic acid esters of alky polyethylene glycol ethers Allyl polyethylene glycol ethers

Nonionic

Hydrophilic monomers for emulsion, inverse emulsion and solution polymerization

Alkyl can be methyl or lauryl

Allyl polyethylene glycol ether

Nonionic Used to produce surface active, water-soluble silicone ethers (antifoams)

Allyl polyethylene glycol ether

Nonionic Used in the copoly merization with acrylates to produce water soluble polymers


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Surface Active Transfer Agents (Transurfs)

Polymerizable surfactants are capable of working as transfer agents include

thiosulfonates, thioalkoxylates and methyl methacrylate dimer/trimer

surfactants. Thioalkoxylates with 17–90 ethylene oxide units were produced

from ethoxylated 11 bromo-undecanol by replacing the bromine with a thiol

group via the thiazonium salt route (Vidal 1995). In the presence of water-

soluble azo initiator the thio ended Transurfs (used at a concentration above

the CMC) gave monodispersed latex particles in emulsion polymerization of

styrene. However, the incorporation of the Transurf remained low, irrespective

of the process used for the polymerization (batch, semibatch, seeded). The

stability of the lattices when the surfactant and the transfer function were

incorporated in the same molecule was better than when they were

decoupled.


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Polymerizable surfactants (Surfmers)

Polymerizable surfactants may be considered as surface-active monomers

and essentially

consist of :

1 A hydrophilic moiety

2 A hydrophobic moiety

3 A polymerizable group

In common with conventional surfactants, Inisurfs and Transurfs, Surfmers

form micelles

in aqueous solutions above the CMC. The organized monomer aggregates of

colloidal dimension are microscopically heterogeneous and may affect

polymerization kinetics and polymer structure and properties.

HS – C11 – H22 – O –( CH2 – CH2 – O ) - H

n = 17 to 90

Advantages of polymerizable surfactants in emulsion polymerization

processes include latex stabilization and resistance to electrolyte addition and

to freeze-thaw cycles. In film forming polymers, the most interesting property

is, however, the superior water resistance achievable compared to

conventional surfactants. This manifests in an increase in the hydrophobicity

of the films because the covalent bonding of the Surfmer to the particles

reduces migration to the surface.

One important requirement in replacing a conventional, nonreactive surfactant

with a reactive one is that neither the molecular weight nor the particle size

distribution of the latex may significantly change. Also, the Surfmer reactivity

is important: if the Surfmer is too reactive compared to the other monomers in

the recipe, it will become partially buried inside the growing polymer particles.


86 | P a g e

This will cause poor stability during polymerization and broadening of the

particle size distribution.

Most of the reactive surfactants used for emulsion polymerization have the

reactive group at the end of the hydrophobic moiety of the molecule, on the

assumption that the polymerization process takes place in the latex particle.

Work of Ferguson et al. shows, indeed, a lower stability of lattices produced

with Surfmers with an acrylate group attached to the end of the hydrophilic

chain than those produced with the equivalent terminated with an thyl ester

group.

Figure 2.9 Ionic surfmer

Figure 2.10. Transurf Structure.


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Figure 2.11. Ionic Surfmer

The determination of CMC is generally based on the localization of the

position of a breaking point in the concentration dependencies of selected

physical or chemical properties of

Figure 2.12. The specific conductivity (a) and mola r conductivity (b) as a

function of concentration.

surfactant solutions. Because of the surface activity of there substances,

measurements of the surface tension of surfactant solutions represent the

principal method of CMCs determination. However, it is rather tedious and

time-consuming procedure. In the case of ionic surfactants, the utilization of

electrochemical measurements is much more convenient, especially the

measurements of the electrical conductivity of their solutions with varying

concentration. The conductometric method is based on the finding of a


88 | P a g e

breaking point on the curves, which describes the concentration dependence

of conductivity. It is well-known, that the conductivity of any solution is directly

proportional to the concentration of its ions. The point, where the micelle

formation starts, is indicated on the concentration dependence of specific

conductivity (Κ), as a breaking point. It is easy to find the breaking point,

because it marks a significant change of the linear slope of the dependence Κ

=f(c).The requested value of CMC is the intercept of two linear functions with

mutually different slopes Figure 2.12(a).

The dependence of the molar conductivity, (Λ) on the second root of

concentration (c) can be used for more precise determination of CMC of ionic

surfactants, Figure 2.12(b). The solution of surfactant, e.g. sodium dodecyl

sulfate, behaves as a strong univalent type of electrolyte in the concentration

range below the CMC and the linear function of dependence of the molar

conductivity on the second root has a slightly negative slope. This

concentration dependence of the molar conductivity is thus described by the

Onsager equation (8):

Ȝ = Ȝ0 - α√c

where Λ0 is the corresponding molar conductivity at the infinitive dilution and

c is the concentration of the studied surfactant. Values of the molar

conductivity are calculated from the experimental values of specific

conductivity (Κ) and the concentration of solution. The basic unit of this

quantity is S m2 mol-1 (Eq. 9)

Ȝ = k/c

Units and their transformation;

S m-1 is the main SI unit for specific conductivity. Because conductometers

often display the value in other subunits the following transformation

relationship could be useful:

1 µS cm -1 = 10-4 S m-1


89 | P a g e

Determination of CMC

The surface tension measurements were made by a Surface Tensiometer

(CBVP-A3, Face) and a Dynamic Contact Angle Analyzer (DCA322,Cahn). A

sandblasted platinum plate of dimensions 1.95 × 1.00 x 0.02 cm was used.

Solutions were contained by a double-walled Pyrex vessel thermostatted at

25.0±0.2°C or at 80.0± 0.5°C. To prevent the contam ination of solution from

dust in air during the operation, the vessel had a cover with a hole only

allowing a thin wire hanging the platinum plate to go through, and the whole

vessel was placed inside a closed sample chamber of the surface

Tensiometer or the dynamic contact-angle analyzer. Solutions were allowed

to equilibrate until the surface tension stabilized. Therefore, the surface

tension was measured as a function of time. It was found that the surface

tension decreases as time evolves. It is believed that this dynamic behavior of

surface tension is simply due to the rearrangement of surfactant molecular

configuration at the interface. For certain systems, this phenomenon is very

pronounced and it even took more than 20 h to reach equilibrium. The

equilibrium surface tension was then determined whenever the tension

revelled off to ensure the system reached fully equilibrium.

Material and Method

The stock solution of the surfactant sodium dodecyl sulfate with concentration

c = 0.012 mol.dm-3 (or Septonex c = 0.0012) in 250 ml volumetric flask: - The

calculated amount of the studied substance should be weighed using

analytical balance directly in the analytical flask. - Dissolve the substance by

adding a small amount of redistilled water, and when it is fully dissolved add

the remaining volume of redistilled water, as it is given with the marker on the

analytical flask.

slow mixing motions was used to avoid the formation of foam. The stirrer was

inserted and the solution was poured in the conductivity vessel which was

previously stirred , cleaned and dried. As the temperature reached 25°C, the

first conductivity of the various solutions were determined.


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The surfactant CTAB, CPC, CPB, SLS, TX-100, Brij –35, PEG –300 extra

pure, AR grade was obtained with a high grade of purity (99%) which were

used through out the experiment. All Solutions were prepared with weighing

accuracy 0.001gm Denver make balance at room temperature.

(Conductometer systronic make digital, glass surface tension was used).

Triton X-100 (TX100) extra pure, was purchased from Scharlau Chemie,

Spain. Sodium Dodecyl Sulphate (SDS) was obtained from Merck with a high

grade of purity (99%). All chemicals were used as received without further

purification. Selected physicochemical properties of the compounds are

presented in Table2.1.

The surfactant solutions were prepared in a standard 1000mL volumetric

flasks, surfactants were weighed on mass basis and emptied into the

volumetric flask and then double distilled water was used to complete the

solution to the final weight (1kg). After preparation of the stock solution, it was

diluted to obtain desired concentration. CTAB, CPC, CPB, SLS, TX-100, Brij –

35, PEG –300 Solutions were prepared at concentrations ranged from 1 x 10-

3M to 2.5 x 10-3. Mixed surfactant solutions were prepared by mixing the listed

solutions of the same weightas per the listed experimental use . Mixed

surfactant solutions were allowed to equilibrate for at least 5hrs before any

measurements were made.

Surface Tension Measurement

The surface tension technique was applied to determine the CMC in various

surfactant systems, applied in the work. The surface tension measurements

were carried out with Krüss tensiometer (Krüss GmbH, Hamburg, using a

platinum-iridium ring at constant temperature (25±1ºC). The Tensiometer was

calibrated using method described in ASTM Designation: D1331-89. Surface

tension measurements were undertaken according to the method described in

ASTM Designation: D1331-89. Krüss tensiometer operates on the Du Nouy

principle, in which a platinum-iridium ring is suspended from a torsion

balance, and the force (in mN/m) necessary to pull the ring free from the

surface film is measured. Surface tension value was taken when stable


91 | P a g e

reading was obtained for a given surfactant concentration, as indicated by at

least three consecutive measurements having nearly the same value. The

average of a series of consistent readings for each sample was then

corrected to account for the tensiometer configuration, yielding a corrected

surface tension value . A correction factor, F is multiplied by the average dial

reading in order to obtain the corrected value for surface tension (ST).

Zuidema and Waters, proposed the following empirical correlation to calculate

the correction factor:

Where F= the correction factor;

R =the radius of the ring, cm;

r =the radius of the wire of the ring, cm;

ST =the apparent value or dial reading, dyne/cm (mN/m);

Äñ =the density difference between the lower and upper phases, g/cc;

g= acceleration due to gravity, 980 cm/sec2 . The Equation is applicable only when 0.045 ≤∆ρgR3/(ST) ≤ 7.5

CMC measurement

The CMC values obtained through conventional plot of the surface tension

versus the surfactant concentration. The CMC concentration corresponds to

the point where the surfactant first shows the lowest surface tension. The

surface tension remains relatively constant after this point.

Result and Discussion

Critical micelle concentration (CMC) is the concentration above which

monomer surfactant molecule abruptly aggregate to form micelle(Romsted

1977, Bunton 1979). It is mostly determined by conduct metric & surface


92 | P a g e

tension method. Using these CMC data, is helpful to find shape, structure of

micelle, surface activity, solubilization, absorption, wetting phase behavior etc.

In the Table 2.2 and Table 2.3, the CMC values for different surfactant,

polymer, and polymer-Surfactant, mixed surfactant has been successfully

determined by surface tension method and the values are increasing with

increasing molar concentration. We can easily find out the drastic change in

surface tension in all cases. The points have been shown in following figures.

Table 2.3: Determination of CMC (Surfactant and Pol ymer PEG-300) by

Surface tension

MOLAR

CONC

Surface Tension

SLS CPC CTAB CPB TX-100 BRIJ35 PEG-300

0.21 47.5 41.2 49.6 32.2 31.9 45.6 74.5

0.32 49.2 42.3 50.3 33.0 32.5 48.3 75.9

0.44 50.2 45.6 52.3 34.2 33.6 49.6 77.4

0.58 51.2 48.2 55.3 35.2 34.2 51.2 78.9

0.68 55.6 49.2 56.8 35.6 34.5 55.6 80.5

0.72 56.5 51.6 59.0 36.6 34.8 56.3 83.8

0.8 57.5 58.6 65.2 41.0 35.1 58.9 87.5

1.5 55.9 60.3 66.3 41.9 35.4 59.3 93.7

2.5 53.7 65.2 67.5 42.9 36.1 61.2 98.3


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The measurement of critical micellar concentration of pure surfactants

through surface tension at varying molar concentration from 0.21 x 10-3 M to

2.5 x 10-3 M indicate a marked correlation of surfactants mainly CPC, CTAB

and CPB with increase in concentration however less than PEG.

Figure 2. 4 Correlation Coefficient of surfactant ( Pure/mixed) with PEG-

300.

MOLAR CONC x 10-3 SLS CPC CTAB CPB

TX-100 BRIJ35

PEG-300

MOLAR CONC x 10-

3 1.00

SLS 0.44 1.00

CPC 0.90 0.73 1.00

CTAB 0.84 0.80 0.99 1.00

CPB 0.87 0.73 0.99 0.99 1.00

TX-100 0.81 0.83 0.94 0.94 0.91 1.00

BRIJ35 0.81 0.88 0.96 0.97 0.94 0.97 1.00

PEG-300 0.95 0.65 0.98 0.96 0.97 0.90 0.92 1.00


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In a similar experiment of determining critical micellar concentration of

surfactants mixed with PEG showed enhanced values compared to pure or

mixed surfactant as depicted in Table 2.3 and correlation coefficient in Table

2.4

Determination of Critical micellar concentration th rough conductivity

Determination of critical micellar concentration through conductivity

measurements of pure surfactants (Table 2.5 ) and Surfactant-PEG-300

mixture (Table ) too reveal higher values for the mixture than in the individual

surfactants shown in graph () and ().

Table 2.5 visualizes critical micellar concentration through surfactant -PEG mixture which finds strong support for PEG combinat ion with PEG-CPC,CTAB

,TX-100,SLS shown in Table-3.

Molar

Conc.x10-

3

SLS+PEG CPB+PEG CPC+PEG CTAB+PEG BRIJ-

35+PEG

TX-100+

PEG

PEG

Molar

Conc.x10-3

1.00

SLS+PEG 0.02 1.00

CPB+PEG 0.98 0.18 1.00

CPC+PEG 0.99 0.14 1.00 1.00

CTAB+PEG 0.99 0.11 0.99 1.00 1.00

BRIJ-35+PEG 0.01 1.00 0.16 0.13 0.09 1.00

TX-100+PEG 0.99 0.14 1.00 1.00 0.99 0.12 1.00

PEG -0.04 0.98 0.11 0.08 0.03 0.99 0.07 1.00


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Table 2.6: Determination of CMC (Surfactant) by conductivity

Molar Conc.

10−−−−3 CONDUCTIVITY

S.L.S C.T.A.B. C.P.C. C.P.B. Brij -35 TX-100 S.L.S.+Brij -35 S.L.S.+TX-100

0.0 2.59 2.59 2.59 2.59 2.59 2.59 2.59 2.59

0.21 3.48 3.29 4.60 2.74 4.36 3.38 4.32 3.75

0.32 3.76 3.34 5.61 3.46 5.00 4.10 4.74 4.13

0.44 7.64 4.75 7.71 6.16 6.55 6.90 4.53 7.20

0.58 8.41 5.79 8.55 6.70 7.25 7.70 9.13 8.14

0.68 8.66 7.10 9.67 7.46 10.17 8.55 10.20 8.75

0.72 8.79 7.20 10.40 9.07 10.52 8.36 9.40 8.61

0.89 8.82 7.70 11.20 7.97 10.63 9.33 10.52 9.07

1.5 14.02 11.40 12.80 11.88 15.93 15.18 15.13 13.23

2.5 NA 16.36 16.00 16.89 19.99 NA NA 18.97

1.5 14.02 11.40 12.80 11.88 15.93 15.18 15.13 13.23

2.5 NA 16.36 16.00 16.89 19.99 NA NA 18.97

NA not analyzed


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Figure 2.7 Correlation coefficient of CMC determin ed by conductivity for Surfactant/Polymer combination

Molar conc. x 10-3M

PEG-300+ S.L.S

PEG-300+CPC

PEG-300+CTAB

PEG-300+CPB

PEG-300+ TX-100

S.L.S.+Brij-35

PEG-300

PEG+CPC 0.89 0.98 1.00

PEG+CTAB 0.86 0.97 0.99 1.00

PEG+CPB 0.79 0.93 0.95 0.94 1.00

PEG+TX-100 0.92 0.98 0.97 0.93 0.94 1.00

S.L.S.+Br ij -35 0.85 0.97 0.96 0.91 0.94 0.97 1.00

PEG-300 0.95 0.96 0.98 0.97 0.91 0.96 0.91 1.00


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Correlation coefficient of CMC values of surfactant mixed with PEG-300

through conductometric method too reveal the enhanced CMC for the

polymer polyethylene glycol-300 mixed with Brij-35, CPB, TX-100 ( corr.

coeff.= 0.98-1.00) as compared to those of pure surfactant(Table 2.5 & 2.7)

and their relative correlation coefficient values(Figure 1.6).

The value of CMC decreases with increase in the hydrophobic chain length of

the molecule. Hydrophobic interaction opposed by electrostatic repulsion

among the ionic head groups derives the process of micellization .A list of CMC

data of some surfactant, mixed surfactant, polymer, surfactant polymer is

determined by surface tension and conduct metric method shown in table 2.4.

The values of CMC for surfactant & mixed surfactant are allmost same with

surface tension & conductmetric. It proves the authenticity of result. However,

the CMC values of surfactant are low as compared to mixed surfactant due to

long hydrophobic group.


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Table 2.8 Observed CMC Values of surfactant, mixe d surfactant, polymer &surfactant –polymer By Surface tension and conduct ivity

Since the Critical micellar concentration values for polymer & mixed polymer

are all most different in both the methods of determination. In surface tension,

CMC values are almost half than the CMC values determined through

conductivity in maximum cases. The degree of correlation for CMC values

determined through surface tension and conductivity, was almost 50%( corr.

value = 0.49). Here we find that in simple polymer CMC values is higher than

or almost equal to the values of mixed in both case ,surface tension and

conductivity. This variation in values are also due to hydrophobicity Hence the

finally CMC values are increases as follows:

Surfactant < Mixed –surfactant < Polymer < Surfact ant –Polymer

Surfactant /Mixed surfactant

Surface Tension

Conductivity Polymer /

Surfactant mixed

Surface tension

Conductivity

SLS 0.44 0.44 SLS+PEG 0.72 0.51

CTAB 0.99 0.99 CPB+PEG 0.59 1.10

CPC 0.44 0.44 CPC+PEG 0.44 1.00

CPB 0.99 0.99 CTAB+PEG 0.44 1.00

SLS+Brij-35 0.58 0.58 Brij-35+PEG

0.68 1.00

SLS+TX-100 0.44 0.44 TX-100+PEG

0.32 0.99

Brij-35 0.99 1.50 PEG-300 0.78 0.81

TX-100 0.44 0.44


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Conclusion

The value of CMC decreases with increase in the hydrophobic chain length of

the molecule. Hydrophobic interaction opposed by electrostatic repulsion

among the ionic head groups derives the process of micellization .A list of CMC

data of some surfactant, mixed surfactant, polymer, surfactant polymer

determined by surface tension and conduct metric method shown in table 9.9.

The values of CMC for surfactant & mixed surfactant are allmost same with

surface tension & conduct metric. It proves the authenticity of result. However,

the CMC values of surfactant are low as compared to mixed surfactant due to

long hydrophobic group (Menger 1993)

Figure 2.12. Graphical representation for observed CMC Values of surfactant, mixed surfactant, polymer &surfactant –polymer By Surface

tension and conductivity

Hence the final trend of Critical micellar concentration CMC for the present

work is

Surfactant < Mixed –surfactant < Polymer < Surfactant –Polymer


100 | P a g e

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Chapter - 2 Critical Micelle Concentration of Surfactant, Mixed ...€¦ · Critical Micelle Concentration (CMC) is the key point for surface chemistry as well as chemist and systematic