Thursday, August 22, 2013

Micelles

Certain substances like soap and detergents behave as electrolytes at low concentration (the molecules of soap or detergent are smaller than colloidal particles). However, at high concentrations they constitute colloidal sol, which is known as Associated Colloids. The colloidal behaviour of such substances is due to formation of aggregates or clusters in solutions. Such clusters are of colloidal dimensions and are known as Micelles. Thus, micelles may be defined as the cluster or aggregated particles formed by associated colloids in solutions. The formation of micelles takes place above certain concentration called Critical Micellisation Concentration (CMC). Every micelle system has specific value of CMC.
Mechanism of Micelle Formation
Micelles are generally formed by the specific type of molecules which have lyophilic as well as lyophobic ends. Such molecules are known as Surface Active Molecules or Surfactant Molecules. Sodium oleate, C17 H33 COO Na+  is an example of such molecules.

The long hydrocarbon part of oleate radical (C17H-33) is lyophobic end while COO is lyophilic end. When the concentration of the solution is below CMC (3 x 10–3 Mol L–1) sodium oleate behaves as an electrolyte and ionises to Na+ and C17 H33 COO ions. As the concentration exceeds CMC, the lyophobic part starts moving away from solvent (it is solvent repelling) and are made to approach each other. However the polar COO part tends to interact with water (solvent). This ultimately lead to formation of cluster having dimensions of the colloidal particles. In each such cluster a large number of oleate groups (usually 100 or more) are clumped together in a spherical manner so that their hydrocarbon part interact with one another to reduce contact with solvent but COO part remains projected in water.

2 comments:

  1. Dear Santosh,

    could you please give a reference for CMC oleate = 3 mM
    and do you agree that the CMC for oleic acid is 6 µM?
    Best regards

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  2. CMC of 6 µM is from
    Richieri et al., J Lipid Res 1992, 23495-23501

    ReplyDelete