CHE P10 M28 etext - epgp.inflibnet.ac.inepgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/... · CHEMISTRY PAPER No.10 : TITLE: Physical Chemistry MODULE No.28 : TITLE: Factors affecting

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CHEMISTRY

PAPER No.10 : TITLE: Physical Chemistry MODULE No.28 : TITLE: Factors affecting CMC of Micelle

Subject Chemistry

Paper No and Title X; Physical Chemistry –III (Classical Thermodynamics, Non-Equilibrium Thermodynamics, Surface Chemistry, Fast Kinetics)

Module No and Title 28 Factors affecting CMC of Micelle

Module Tag CHE_P10_28

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TABLE OF CONTENTS

1. Learning outcomes 2. Introduction

3. Temperature Dependence 4. Chemical Structure effect

5. Electrolytes Effect 6. Summary

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1. Learning Outcomes

After studying this module, you shall

• Learn about some factors that affect micellization, temperature, chemical structure, electrolytes etc.

2. INTRODUCTION

The shape and size of the micelles can be controlled by changing chemical structure of the surfactant as well as by changing the solution conditions such as temperature, and electrolytes addition.

3. EFFECT OF TEMPERATURE

The temperature effect varies the CMC value with the type of surfactant molecules. The temperature has less effect on the micellar properties of ionic surfactants. This is very well shown from the graph of sodium dodecyl sulfate(SDS) that CMC versus temperature in Fig.1. The CMC varies in a irregular way by 10-20% over a wide range. The shallow minimum around 25oC can be related with a similar minimum in the dissolution of hydrocarbons in water. The decrease in solubility of hydrocarbon increases the capacity of respective molecules of surfactant to micellize.

Fig.1 The CMC temperature dependence of Sodium Dodecyl Sulfate (top) and penta

(ethylene glycol) monodactyl ether. From P.H. Elworthy, A.T. Florence and C.B.

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Macfarlane, Solubilization of Surface-Active Agents, Chapman & Hall, London, 1968.

The polyoxyethylene non-ionic surfactants type deviate from this behavior and

show typically a monotonic, and much more pronounced decrease in CMC with increase in temperature. Aqueous solutions of many non-ionic surfactants are turbid at their fixed temperature that is known as the cloud point (The cloud point of a fluid is the temperature at which solids dissolved are partially soluble, giving precipitate of a second phase) . At temperatures above the cloud point, micellar size increases and there is a corresponding decrease in CMC. 3.1 The Surfactants solubility can be strongly depend on temperature There are many important and intriguing temperature effects in surfactant self-assembly. One, which is of great practical significance, is the dramatic temperature-dependent solubility displayed notably by various ionic surfactant. The solubility may be very low at cold temperatures and then increase by the order of its magnitude in a relatively narrow temperature range. The phenomenon is generally denoted as KRAFFT phenomenon with the temperature for the onset of the strongly increasing solubility being known as the KRAFFT POINT or Krafft temperature. The dependence of temperature on surfactant solubility in the region of the Krafft point is illustrated inFig.2.

The Krafft point may vary dramatically with slight difference in the surfactants chemical structure. Some general remarks can be put for alkyl chain surfactant:

1. The Krafft point increases strongly as the length of alkyl chain increases. The increase is not regular but displays an odd-even effect.

2. The Krafft point is strongly dependent on the counter ion and the head group. But there are no general trends for the counter ion dependence. Salt addition typically raises the Krafft point, while many other consolutes decrease it.

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Fig. 2

4 EFFECT OF CHEMICAL STRUCTURE

Effect of chemical structure on the CMC values for few selected surfactants is given in Table 1 and a similar list for some non-ionic surfactant is depicted in Table 2

Table 1 List of the CMC values for the selected surfactants

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Surfactant aCMC Dodecyl ammonium chloride 21.47 10 M−× Dodecyltrimethylammonium chloride 22.03 10 M−× Decyltrimethylammomium bromide 26.5 10 M−× Dodecyltrimethylammonium bromide 21.56 10 M−× Hexadecyltrimethylammonium bromide 49.2 10 M−× Dodecylpyridinium chloride 21.47 10 M× Sodium tetradecyl sulfate 32.1 10 M−× Sodium dodecyl sulfate 38.3 10 M−× Sodium decyl sulfate 23.3 10 M−× Sodium octyl sulfate 11.33 10 M−× Sodium octanoate 14 10 M−× Sodium nonanoate 12.1 10 M−× Sodium decanoate 11.09 10 M−× Sodium undecanaote 25.6 10 M−× Sodium dodecanoate 22.78 10 M−× Sodium p-octylbenzene sulfonate 21.47 10 M−× Sodium p-dodecylbenzene sulfonate 31.20 10 M−× Dimethyloddecylamineoxide 32.1 10 M−×

( ) ( )3 2 29 6CH CH OCH CH OH 49 10 M−×

( ) ( )3 2 29 9CH CH OCH CH OH 31.3 10 M−×

( ) ( )3 2 211 6CH CH OCH CH OH 58.7 10 M−×

( ) ( )3 2 6 4 2 27 6CH CH C H CH CH O 42.05 10 M−× Potassium perfluorooctanoate 22.88 10 M−×

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Table 2 List of CMC values for non-ionic surfactants

Surfactant ( )MµCMC

6 3C E 410 10×

8 4C E 38.5 10×

8 5C E 39.2 10×

8 6C E 39.9 10×

10 5C E 29.0 10×

10 6C E 29.5 10×

10 8C E 210 10×

12 5C E 6.5 10×

12 6C E 6.8 10×

12 7C E 6.9 10×

12 8C E 7.1 10×

14 8C E 9.0 10×

16 9C E 2.1 10×

16 12C E 2.3 10×

16 21C E 3.9 10×

8 9C Eφ 23.4 10×

8 10C Eφ 23.4 10×

12C NO 32.2 10×

8D Cβ− − glucoside 42.5 10×

10D Cβ− − glucoside 32.2 10×

12β− −D C glucoside 21.9 10×

Ethoxylates of fatty alcohol are referred as CmEn, where m is equal to the the number of atoms of carbon in alkyl chain and n is equal to the number of oxy-ethylene units.

Some general remarks about the variation of the CMC with the chemical structure of

surfactant can be made: 1. The CMC decreases rapidly with increase in the alkyl chain length of the

surfactant (Figs. 3 and 4). For eg. the ethylene oxide chain length of a non-ionic

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surfactant increase which makes the molecule more hydrophilic and the CMC value increases (Table 3).

Fig.3 The logarithm of the CMC amount linearly with the carbon atoms in the alkyl

chain of the surfactant. The slope is larger for a non-ionic surfactant or an ionic with added salt than for an ionic surfactant without added electrolyte. From B. Lindeman and H. Wennerström, Topics of Current Chemistry, Vol. 87, Springer- Verlag, Germany, 1980, p.8

Fig.4 The logarithm of CMC (molar concentration) versus the carbon atoms in the

alkyl chain for octa(ethylene glycol) monoalkylethers ate different temperatures. From top to bottom, the temperatures are 15.0, 20.0, 25.0, 30.0 and 40.0oC. From K. Meguro, M. Ueno and K. Esumi, in Nonionic Surfactants. Physical Chemistry, M.J. Schick(Ed.), Marcel Dekker, New York, 1987, p.134

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2. Non-ionic surfactants mostly have low CMC values and have higher aggregation numbers than their corresponding ionic counterparts of similar hydrocarbon chains.

3. A decrease in CMC, which for compounds with identical polar head groups is represented by the linear equation: log [CMC] = A – Bnc where nc is the number of carbon atoms in the chain and A and Bare constants for a homologous series.

Table 3 The CMC value decreases firmly with the alkyl chain length. The decrease follows, to a good approximation, the relationship log ,= − cCMC A Bn where nc represent

the number of carbons in the alkyl chain.

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Surfactant series Temperature( )°C

A B

Na carboxylates (soaps) 20 1.85 0.30 K carboxylates (soaps) 25 1.92 Na (K)n-alkyl 1-sulfates or –sulfonates

25 1.51 0.29

Na n-allame-1-sulfonates 40 1.59 0.30 Na n-allame-1-sulfonates 55 1.15 0.29 Na n-allame-1-sulfonates 60 1.42 0.26 Na n-allame-1-sulfonates 45 1.42 0.28 Na n-allame-1-sulfonates 60 1.35 0.30 Na n-allame-1-sulfonates 55 1.28 0.28 Na p-n-alkylbenzene sulfonates 55 1.68 0.27 Na p-n-alkylbenzene sulfonates 70 1.33 0.29 n- Alkylammonium chlorides 25 1.25 0.27 n-Alkylammonium chlorides 45 1.79 0.30 n- Alkyltrimthylammonium bromides

25 1.77 0.30

n- Alkyltrimethylammonium chlorides (in 0.1 M NaCl)

25 1.23 0.33

n-Alkyltrimethylammonium bromides

60 1.77 0.29

n-Alkylpyridinium bromides 30 1.72 0.31 n- ( )2 1 2 4 6+n nC H OC H OH 25 1.82 0.49

4. Besides the major difference between ionics and non-ionics, the head group effects are moderate. Cationics typically have slightly higher CMCs than anionics. For non-ionics of the oxyethylene variety, a moderate increase in the CMC as the polar head becomes larger.

5. The valency of the counter-ion is significant. While simple monovalent inorganic counter ions give roughly the same CMC, increasing the valency to 2 gives a reduction of the CMC by roughly a factor of 4.

6. Micellar size for extremely cationic surfactant increases as thecounter-ion is changing according to the series Cl−< Br−< I−, and for a distinctly anionic surfactant according to the series Na+< K+< Cs+.

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7. Ionic surfactants includes organic counter-ions (e.g. maleates) have lower CMCs and high aggregation numbers as compared to the inorganic counter-ions.

8. While alkyl chain branching and double bonds, aromatic groups or some other polar character of the hydrophobic part produce sizeable turn in the CMC, a considerable lowering of the CMC (one or two series of magnitude) results from prefluorination of the alkyl chain. Partial fluorination interestingly may increase the CMC, e.g. fluorination of the methyl group at terminal roughly doubles the CMC value. The anomalous behavior of fractionally fluorinated surfactants is by deleterious interactions of hydrocarbon and fluorocarbon groups.

5 ELECTROLYTE EFFECT

A most important matter is of added electrolyte effect on the CMC of ionics. This is illustrated in Fig. 5 for the addition of 1:1 inert electrolyte to a solution of a monovalent surfactant.

Fig.5 Effect of sodium chloride addition on the CMC of different sodium alkyl sulfates. The dark lines are predictions of electrostatic theory; cs gives the salt concentration.Corrected with permission from G. Gunnarsson, B. Jönsson and H. Wennerström, J. Phys. Chem., 84(1980)3114. Copyright (1980) American Chemical society

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The addition of electrolyte to the solutions of ionic surfactants decreases the CMC and the micellar size increases. This is because at high concentration of electrolyte themicelles of ionic surfactants may become non-spherical. Salt addition gives a considerable lowering of the CMC, which may amount to an order of magnitude. The effect is moderate for short-chain surfactants but is much larger for long-chain ones. As a consequence, at high concentrations of salt the variation of CMC with the number of carbons in the alkyl chain is much stronger than without addition of the salt. The rate of change at high salt concentrations becomes similar to that of non- ionics.

6.Variation in Micelle Size and Structure

The driving force of micelle formation is the elimination of the contact between the alkyl chains and water. The larger a spherical micelle, then the more efficient this is, since the volume-to-area ratio increases. Decreasing the micelle size always leads to an increased hydrocarbon-water contact. However, if the spherical micelle was made so large that no surfactant molecule would reach from the micelle surface to the centre, one would either have to create a void or some surfactant molecules would lose the contact with the surface, introducing polar groups in the center. Both alternatives are unsatisfactory. We should note that the fact that the micelle radius equals the length of an extended surfactant molecule does not mean that the surfactant molecules are all extended. Only one molecule needs to be extended (in an all-trans state) to fulfil the requirements mentioned, and the majority of the surfactant molecules are in a disordered state with many gauche conformations. Spectroscopic studies have been used to characterize the state of the alkyl chains in micelles in detail. This state indeed is very close to that of the corresponding alkane in pure liquid oil. At the surface of the micelle we have the associated counter ions, which in number amount to 50-80% of the surfactant ions; as noted above, a number quite invariant to the conditions. Simple inorganic counter ions are very loosely associated with the micelle. The counter ions are very mobile and there is no specific complex formed with a definite counter ion-head group distance. Rather, the counter ions are associated by long-range electrostatic interactions to the micelle as a whole. They remain hydrated to a great extent; cations especially tend to keep their hydration shell.

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Some water of hydration is thus accounted for by the associated counter ions and, furthermore, the polar head groups are extensively hydrated. The micelle size, as expressed by the radius of a spherical aggregated, may be obtained inter alia from scattering experiments and from micelle self-diffusion. A related and equally important characteristic of a micelle is the micelle aggregation number, i.e. the number of surfactant molecules in one micelle. This is best determined in fluorescence quenching experiments. To take an example, the aggregation number of SDS micelles at is 60-70. The aggregation numbers deviating markedly from the average the probability is very small. A Geometrical Consideration of Chain Packing is Useful Based on the geometry of various micellar shapes and the space occupied by the hydrophilic and hydrophobic groups of the surfactants, it is possible to estimate the structure of a micelle. Since the cross-section area per chain decreases radially towards the centre, only one chain can be fully extended while the others are more or less folded. The aggregation number, N, can be expressed as the ratio between the micellar core volume, ,micV and the volume, v, corresponds to the volume of the hydrophobic group in

the micellar core (i.e.volume of one chain): 34

3mic micV RNv v

= = π

Where, Rmic is the radius of micelle. We can also express the aggregation number as the ratio between the micellar area, ,micA and the cross-sectional area, a, of one surfactant

molecule, as follows: 2

4mic micA RNa a

= = π

Putting these aggregation numbers as equal, we obtain:

( )13mic

vR a

=

Since micR cannot exceed the extended length of the surfactant alkyl chain, lmax:

max 1.5 1.265 cl n= + Therefore,

( )13max

vl a

≤ for a spherical micelle

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The ratio ( )max

,vl a

which gives a geometric characterization of a surfactant molecule, will

be seen to be very useful when discussing the type of structure formed by a given amphiphile. This is denoted as the critical packing parameter (CPP) or the surfactant number. Accordingly, the parameter v/lmaxacan determine the shape of the micelle, where, v corresponding to the volume of the hydrophobic group in the micellar core, lmaxis the length of the hydrophobic group in the core and a the cross-sectional area occupied by the hydrophilic group at the micelle-solution interface. According to Tanford (Tanford, C. The hydrophobic effect: Formation of micelles and biological membranes. Wiley, New York,1980.), v = 27.4 + 26.9ncÅ, where n is the number of carbon atoms in the chain less one, and lmax= 1.5 + 1.265ncÅ, depending upon the extension of the chain. Therefore, for a fully extended chain, lmax = 1.5 + 1.265n Å. Table 1 Correlation between the parameter v/lmaxaand the structure of the micelle. v/lmaxa Micellar structure in aqueous media

0-1/3 Spherical 1/3-1/2 Cylindrical 1/2-1 Laminar

v/lmaxa>1 Reversed micelles in nonpolar media

7. Summary

1. There are many factors that controlled the shape, size and CMC of micelles. The temperature effect on the CMC varies with the type of surfactant molecules. CMC for ionic surfactant varies in non monotonically, whereas for Non-ionic surfactants it varies monotonic way that is decrease in CMC with increasing temperature.

2. The CMC decreases potently with increase in alkyl chain length of the surfactant. Non-ionic surfactants have low CMC values and have higher aggregation numbers than their ionic counterparts with similar hydrocarbon chains.

3. The addition of electrolyte to the solutions of ionic surfactants decreases the CMC and increases the micelle size.

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CHE P10 M28 etext - epgp.inflibnet.ac.inepgp.inflibnet.ac.in/epgpdata/uploads/epgp_content/... · CHEMISTRY PAPER No.10 : TITLE: Physical Chemistry MODULE No.28 : TITLE: Factors affecting