chemistry paper no.10 : physical chemistry-iii · chemistry paper no.10 : physical chemistry-iii...

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PAPER No.10 : Physical Chemistry-III MODULE No.27 : Micelle and Critical micelle concentration

Subject Chemistry

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

Module No and Title 27: Micelle and Critical micelle concentration

Module Tag CHE_P10_M27

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TABLE OF CONTENTS 1. Learning Outcomes

2. Introduction: Micelle 2.1 Micellization

3. Critical Micelle Concentration

3.1 Determination of CMC for a Surfactant

4. The Shape and Size of Micelle

5. Structure and Composition

5.1 Structure of micelle formed by ionic surfactants

5.2 Structure of micelle formed by nonionic surfactants

5.3 Structure of micelle in non aqueous solution

6. Variation of micelle size and structure

7. Summary

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1. Learning Outcomes

After studying this module, you shall be able to

• Know about the micelle and critical micelle concentration (CMC). • Know about the shape and composition of micelle produced by various kinds of surfactants

and factors that affects the shape and size of the micelles.

2. INTRODUCTION: MICELLE

A surface active agent (surfactant), when sent in less concentration in a system, assimilate at the surfaces or interfaces extremely by varying the surface or interfacial free energy. When molecules of surfactant are water soluble at the concentrations more than the critical micelle concentration (CMC), they form aggregates termed as micelles, that are having importance in science and pharmacy because they are having competency of increasing the dissolution of mildly soluble substances in water. In a micelle, the hydrophobic tails attached to the inner core to decrease their link with water, while hydrophilic heads remain at the outer surface to maximize their interaction with water. They are movable structures and are repeatedly formed and dissociated in solution (they must not be thought of as solid spheres). There exist a state of equilibrium between micelle and free molecules of surfactant in solution. This is illustrated in the figure as shown.

Fig. 1 Representative model of the reversible monomer-micelle in thermodynamic equilibrium. The

head of surfactant is represented by dark circle (hydrophilic portion ) and the curved dark lines represent the tail of surfactant (hydrophobic moieties).

A pile of the molecules of surfactant called micelle spread in a colloid of liquid . In liquefied solution, an ideal micelle forms an aggregate with the hydrophilic "head" regions which interacts with the surrounding solvent that is forming an outer shell, the sequestering the non-polar or hydrophobic single-tail regions at centre.of micelle.

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Hence, the micelle nucleus, which is consists of long non polar tails, resembles an oil or gasoline drop that we see in the figure. The non -polar tail length , the nature and the size of the polar or ionic head, the acidity of the solution, the temperature, and the presence of salts that are added are the essential factors in determining the kind of the product obtained. If these domains are changed, it is viable to change the shape and size of the micelles. A number of amphiphilic molecules form an aggregate gives aggregation number; it is the way to explain the size of the micelle.

Fig. 2. (a Representation of a micelle in aqueoussolution. (b) Representation of a spherical micelle for dodecyl sulphate. The process of micellization in water execute mild balancing of intermolecular forces containing hydrophobic, steric, electrostatic, hydrogen bonding and van der Waals interactions. The leading force which results from the hydrophobic effect is formed with the non-polar surfactant tails, and the leading adversing repulsive force which results from the interactions i.e steric and electrostatic between the surfactant polar heads as represented in figure. Whether micellization occurs and, if, at which flocking of monomeric surfactant, build upon the uniformity of the the forces promoting micellization and some of those opposing it. We can mention l that it builds on the balancing of two main effects i.e the readiness of the non polar tails to reject the contacts with water and the repulsion between the polar or charged heads, an effect of destabilizing at the aggregated process. Hydrocarbon tails reject the interactions with the solvent molecules which is moving toward the aggregate/micelle interior, which have low water capacity. Inspite of this , the repulsion between the charged heads on the micelle surface is governed by the existence of oppositely charged ions (counter-ions). The suitable association of the non polar tails in the micelle interior occur through the hydrophobic interaction, that is the prevailing effect in the formation process of these aggregates.

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Fig. 3

Amphiphilic molecules structure micelles in water, as well as in non-polar organic solvents. In such cases, micelle aggregates are called inverse micelles because the situation is inverted as respect to water as depicted in the fig .In this case, hydrocarbon tails are exposed to the solvent, whereas the polar heads indicate towards the micelle interior so as to remove the contacts with the non-polar solvent.

Fig. 4 Reverseible micelles are able to control huge amount of water in its interior. In this way, a "pocket" is formed which is only suitable for the dissolution and transportation of polar solutes through a non polar solvent.

3. CRITICAL MICELLE CONCENTRATION

CMC i.e critical micelle concentration. The critical micelle concentration (CMC) is explained as the concentration of surfactants above which micelles are formd and all additional surfactants are added in the system so as to go to the micelles. The CMC is the accurate flocking of surfactants at which aggregates become thermodynamically soluble in an liquid solution. There is no high density of surfactant below the CMC which immediately precipitate into a separate t phase. Above the CMC, the solubility of the surfactant in the aqueous solution go beyond.. The energy which is required to keep the surfactant in the solution is no longer in the e lowest energy state. To decrease the system free energy the surfactant is precipitated out which is depicted in the fig. Now when we go from frame 1à4,

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with increase of surfactant concentration in water, slowly a layer is formed at the surface and eventually micelles are formed at or above the CMC.

= Fig. 5. You must be wondering how the CMC of surfactants is determined? It is done by establishing inflection points for pre-determined surface tension of surfactants in solution. The design of inflection point against the surfactant concentration will give picture of the critical micelle concentration by representing the stabilization of phases.

Fig. 6

In the setup as depicted in Fig. three phases ae shown which are

1. At very low concentrations of surfactant less changes in surface tension are detected. 2. Additional surfactants decreases the surface tension. 3. Surface become full weighted , no more change in the surface tension.

(1)

(2)

(3)

(4)

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DETERMINATION OF CMC OF THE SURFACTANT The CMC for a surfactant can be obtained by measuring physical properties, such as surface tension (γ), conductivity (κ) ( ionic surfactants), osmotic pressure (π), detergency, etc. When these properties are setup as a function of surfactant concentration (or its logarithm, in case of surface tension), a sharp rupture is seen in the curves attain by signalling the formation of micelles at this point (explain figure) Another important parameter that featured is the aggregation number, Nagg, that correlates to the average number of monomers of surfactant in each micelle.. Normally , in a micelle solution the aggregation number is relatively constant for a broad range of total concentration (up to 100 times the CMC), with increasing number of micelles. Infact, under undisturbed conditions micelles can grow with the aggregation number which differs with the surfactant concentration.

Fig. 7: Change in the physical properties of detergency, conductivity (κ), osmotic pressure (π) and surface

tension (γ) of an aqueous solution of surfactant which is depicted as a function of surfactant concentration. The break in the curve of each property indicates the Critical Micelle Concentration (CMC).

There are some other physical properties which can be used to induce the CMC value that shows some change near CMC such as

a. Electrical conductivity b. Molar (Equivalent) conductivity c. Light scattering

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d. Refractive index

4 MICELLE SHAPE AND SIZE

Micelles are the labile entities which are outlined by the non-covalent combination of single surfactant monomers. Therefore, they may be spherical, cylindrical, or planar (discs or bilayers) as depicted in Figure the shape and size of micelle can be controlled by changing chemical structure of the surfactant as well as by altering solution conditions such as temperature, overall surfactant concentration, surfactant composition (in the case of mixed surfactant systems), ionic strength and pH. Depending on the type of surfactant and the solution conditions, the spherical micelles can grow one-dimensionally in cylindrical micelles or two-dimensionally to bilayers or discoidal micelles. The growth of micelle is controlled generally by the heads of surfactant , as the two i.e (one-dimensional and two-dimensional) growth requires taking the surfactant heads closer to each other to reduce the available area per surfactant molecule at the micelle surface, and also e the bend of the micelle surface Surfactant aggregates:

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Fig. 10. Surfactants self-assemblies. Self-assembly of surfactant leads to a domain of different structures as we will discuss now:

(a) Spherical micelles with its interior is formed of the hydrocarbon chains and the polar head group surface which is covering water . The radiusof the hydrocarbon core is close to alkyl chain length .

(b) Cylindrical micelles with its interior is formed of the hydrocarbon chains and polar head groups surface water surface . The cross-section of hydrocarbon core is identical r to that of spherical micelles. The micelle length is fluctuating so these micelles are polydisperse.

(c) Surfactants bilayers which is build up of lamellar liquids crystals have surfactant-water system having a hydrocarbon core of thickness of 80% of the length of two extended alkyl chains.

(d) Reversed or inverted micelle has core of water that is covered by the polar head groups surfactants . The alkyl chains together with a non-polar solvent make up the continuous medium. Like normal micelle they can reproduce into cylindrical form.

(e) A bicontinuous structure having the surfactant molecules assembled into connected films.

(f) Vesicles are built from bilayer like those of the lamellar phase and are featured by two distinct water compartments, one producing the core and one the external medium. Vesicles may have different shape and there are also reversed-type vesicles.

In particular, depend on the type of surfactant and on the solution conditions, spherical micelles which are one-dimensionally can grow to cylindrical micelles or two-dimensionally

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to bilayers or discoidal micelles. The growth of micelle is controlled basically by the surfactant heads, as both one-dimensional and two-dimensional growth require the surfactant heads closer to each other to reduce the available area per surfactant molecule at the surface of the micelle , and also the bend of the micelle surface. For all the micelle structures in aqueous media, the surfactant molecules are turned with their polar heads in the water phase and their tail away from it.

5. STRUCTURE AND COMPOSITION

(a) The inner part of the micelle, i.e a hydrophobic core is formed of the

hydrocarbon chain of the surfactant molecule. It’s radius is approximately of the range of fully extended hydrophobic chain shown in the fig

(b) A Stern layer covering the core, is part of electrical double layer, which have coordinated shell of hydrophilic head groups with (1 - α )N counter ions, where α is the degree of ionisation and N is the aggregation number (number of molecules in the micelle). the degree of ionisation for most of the ionic micelles is between 0.2 and 0.3; that is, 70 - 80% of the counter ions may be treated as they are bound to the micelles .

(c) The Gouy-Chapman portion of electrically two layer is diffused and surrounds the Stern layer, it comprises of the N counter ions that are needed to neutralise the charge on the micelle. The thickness of the double layer is the ionic strength function the solution and is can be highly reduced in the presence of electrolyte.

Fig. 11a

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Fig. 11b

5.2 Structure of micelle formed by non-ionic surfactants (a) The structure is almost identical , except the counter ions which are not available

t in the outer region but rather are the loops of hydrated polyethylene oxide chain.

(b) They are bigger than than its ionic counterparts and occasionally be stretched into an ellipsoid or rod-like structure.

(c) They contain a hydrophobic core which is formed of the hydrocarbon chains of the surfactant molecules that is enclosed by a shell (the palisade layer) composed of the oxyethylene chains of the surfactant which is heavily hydrated as depicted in the figure.

Fig. 12

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Another important features of micelles is the aqueous phase that diffuse into the micelle besides the hydrophilic head groups, and the methylene groups that are adjoining to the head are considered in the hydration sphere. Therefore, we can isolate micelle inner region into outer core that is penetrated by water and the internal core is completely removed from water 5.3 Structure of micelle in non-aqueous solution in non-aqueous solution the micelles are formed (reverse or inverted micelles) with their core formed of the hydrophilic groups which is surrounded by a shell of the hydrocarbon chains

Fig. 13 positioned on the geometry of the various micelle shapes and the space occupied by the

hydrophilic and hydrophobic groups of the surfactants, it is possible to conclude the structure of a micelle.

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7. Summary

Micelle is surfactant aggregate which is formed where surfactant concentration in solution is lie above the critical micelle concentration (CMC). In aqueous solution, formation of micelle takes place where polar or ionic head groups cast an outer shell and non-polar hydrophobic tails are separated in the internal . In non polar solvents, hydrophobic tails are reveal to the solvent, while polar head indicate to the inner of the aggregate, called inverse micelle. The process of micelle formation lean on the balancing of intermolecular force i.e. hydrophobic, steric, electrostatic, hydrogen bond and van der waals interactions. CMC is the accurate concentration at which a aggregate become thermodynamically soluble in liquid solution and can be observed as inflection point when the physicochemical properties are outlined as function of concentration.

Structure of micelle formation by the ionic surfactants are composed of (i) hydrophobic core formed by hydrocarbon chain, (ii) An electrical twinlayer divided into (a) stern layer and (b) Gouy-chapman layer. Stern layer is composed of the hydrophilic head group whereas gouy-chapman layer contains the counter-ions that neutralize the charge on the micelle.

Micelle formed by the non ionic surfactant have hydrophobic core that is covered by the shell which is composed of polyethylene oxide chain, no counter- ions are present in it. The shape and structure of micelle can be estimated by the relationship of v/lmaxa .

chemistry paper no.10 : physical chemistry-iii · chemistry paper no.10 : physical chemistry-iii...

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