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.
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.
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