Why Micelles are Formed and Remain in Solution

Micelles are loose aggregates of amphiphiles in water or organic solvents which form above a certain temperature (Krafft point) and concentration (critical micellar concentration, cmc). Below the Krafft temperature, clear micellar solutions become turbid and the amphiphile forms three-dimensional hydrated crystals. Below the cmc, micelles dissociate into monomers and small aggregates. Above the cmc, the micelles of an aggregation number n are formed n then remains stable over a wide concentration range . Table 1 gives some typical cmcs and three Krafft point values. 

Micellar solutions are stable and remain clear over years, although the individual micelle usually explodes and reforms within a few milliseconds. The question arises as to why such aggregates of a limited number of molecules (usually between fifty and one hundred) form at all  

The first reason lies in the fact that the interaction between solvent molecules (usually water) is stronger than the interaction between the solvent and the solute. This effect alone would lead to a precipitation of the solute. In the case of amphiphiles which form micelles, however, the head groups are strongly hydrated and repulse each other. The hydration forces and steric forces which are made responsible for this repulsion effect prevent crystallization above the Krafft point and also above the cmc. Where the formation of 3D crystals is impeded, the smallest possible droplet is formed, removing the alkyl chains from the solvent. The interactions between solvent molecules are therefore disturbed to a minimal extent, allowing the head groups to be solvated with a minimal entropy loss. It is irrelevant whether the solvent contains clusters or not. Micelle formation only occurs as a result of a solvation of head groups and non-solvation of a solvophobic core.  

Ishikawa, H. Kuwahara, T. Kunitake, J. Am. Chem. Soc., 1994, 116, 5579 J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985 

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