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Micelle

Micelles are aggregates of surfactants in a liquid medium which are formed when the surfactant concentration exceeds the critical micelle concentration(CMC). It must be mentioned that this definition is only for normal micelles, for the case of reversed micelles it is not necessary to have a critical micelle concentration(CMC).

In the normal micelle the surfactant is oriented in such a way that the hydrophobic hydrocarbon chains are towards the interior of the micelle, leaving the hydrophilic groups in contact with the aqueous medium.

The term "micelle" should designate any soluble aggregate spontaneously and reversibly formed from amphiphilic molecules or ions. As the concentration of aqueous solutions of many amphiphilic substance increases, there is a pronounced change in the physical properties of the solution.

 

Micelle Formation

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The solvent polarity which determines the interactions of its molecules with polar and non-polar regions of surfactant molecules, plays an important role in the formation of micelles in non-aqueous medium.

For micelle formation to take place the medium has to be a "good solvent" for hydrocarbon chains only. Micelles do not form in the medium of nature similar to both parts of di-philic surfactant molecules; the surfactant reveal only true solubility in such medium. Low alcohols which are good solvents for both polar and non polar regions of surfactant molecules are the typical examples for such media.
Micelle

Theories of Micelle Formation

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The most important characteristic of micellization is that an appropriate theory should explain are
  1. The formation of aggregates of limited size.
  2. The dependence of micelle size and shape upon head group type, temperature and concentration.
  3. The free energy and enthalpy of micelle formation, particularly the incremental changes per methylene group.

Hartley recognized the strong interaction between water molecules and their attraction for ionic groups as important factors in the process as follows

"The association of amphipathic ions in aqueous solution is therefore to be considered, from the molecular force point of view, as an association of water molecules leaving clusters of non-water-attracting groups of the largest possible dimensions in order that there should be least instruction amongst the water molecule".
Micelles are classified into direct and reverse micelles. Direct micelles are formed in water and in other highly polar media. Their polar heads stretch out and the assembled hydrophobic tails form the low-polar 'nano-phase', which can solubilize low-polar molecules.

On the other hand, reverse micelles formed in low polar solvents the low-polar tails stretch out and polar heads assemble. They can solubilize water and other highly polar molecules.

Schematic representation of direct and reverse micelles are shown below.
Direct MicelleReverse Micelle

Characteristics of Micelle Formation

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The unique characteristic of micelles is that they provide a micro environment that is distinct different from the bulk environment. This hydrophobic micro environment provides sites of interaction for solutes in or on the micellar aggregates, thus enhancing the solubility of insoluble non polar compounds in aqueous media.

The more polar compound remain in the bulk aqueous medium; non polar solutes are partitioned into the micelle. The stronger the interaction, the longer the solutes migrate with the micelle, thereby separating the various components of a mixture based on their partition coefficient.

The main characteristic of micelles are the micellar mass (the sum of the masses of the involved molecules), the aggregation number (number of molecules), critical micelle concentration ($C_{m}$) and the charge and degree of binding of ions $\beta $ (for micelles of ionic surfactants).

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