Critical Micelle Concentration

Many properties of a detergent in aqueous solution depend on its critical micelle concentration ) cmc), which in turn is determined by the chemical structure of the detergent. In aqueous solution, detergents remain either in aggregated (micellar) or monomeric states, and the cmc is the maximum concentration at which the detergent molecules remain as monomers. An increase in detergent concentration above the cmc leads to the formation of micelles or mixed micelles with the solubilized lipid and protein molecules. Although the formation of mixed micelles of detergents with lipid and protein molecules is an important requirement in detergent solubilization of plasma membrane proteins and lipids, a low cmc may not be essential for efficient membrane solubilization because the cmc value alone does not always reflect a detergent's ability to dissociate membranes. For example, sodium dodecyl sulfate (SDS) is strongly ionic and has a cmc of ~0.23% at 25°C, whereas MEGA-10 is polar but nonionic and has virtually the same cmc of ~0.22%. Yet SDS is probably the strongest denaturing detergent known, whereas MEGA-10, just like MEGA-8, is much less disruptive to biomembranes (1, 2). A very similar picture emerges from comparing sodium cholate, which is strongly ionic and denaturing, with CHAPS, which is moderately zwitterionic and much less denaturing, even though both have a cmc of ~0.5% (2). Because the cmc values are also the result of certain more fundamental factors, such as structural features and ionic properties of the detergents (ie, nonionic, moderately ionic, or strongly ionic, and the membrane-dissociating property of a detergent is directly related to such factors), one effective way of classifying detergents is based on their structure and ionizability, rather than their cmc values.

References 

1. M. Hanatani, K. Nishifuji, M. Futai, and T. Tsuchiya (1984) J. Biochem. 95, 1349–1353. 

2. A. C. Newby, A. Chrambach, and E. M. Bailyes (1982) Techniques in Lipid and Membrane Biochemistry, B409, Elsevier/North-Holland, pp. 1–22.