Critical micelle concentration anionic head groups

Most studies of micellar systems have been carried out on synthetic surfactants where the polar or ionic head group may be cationic, e.g. an ammonium or pyridinium ion, anionic, e.g. a carboxylate, sulfate or sulfonate ion, non-ionic, e.g. hydroxy-compound, or zwitterionic, e.g. an amine oxide or a carboxylate or sulfonate betaine. Surfactants are often given trivial or trade names, and abbreviations based on either trivial or systematic names are freely used (Fendler and Fendler, 1975). Many commercial surfactants are mixtures so that purity can be a major problem. In addition, some surfactants, e.g. monoalkyl sulfates, decompose slowly in aqueous solution. Some examples of surfactants are given in Table 1, together with values of the critical micelle concentration, cmc. This is the surfactant concentration at the onset of micellization (Mukerjee and Mysels, 1970) and can therefore be taken to be the maximum concentration of monomeric surfactant in a solution (Menger and Portnoy, 1967). Its value is related to the change of free energy on micellization (Fendler and Fendler, 1975 Lindman and Wennerstrom, 1980). 

In a recent paper, the interaction of various simple flavonoids with an anionic surfactant, sodium dodecyl sulfate (SDS) in aqueous solution, has been studied through absorption spectroscopy as a function of the concentration of the surfactant above and below the critical micelle concentration.The approximate number of additive molecules (flavonoids) incorporated per micelle was estimated at a particular concentration of SDS. Incorporation of flavonoids in micelles shifted the UV absorption bands toward higher wavelengths, and the bathochromic shifts also depended upon the nature of the surfactant head group. 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

Submicroscopic, colloidal aggregates can influence chemical reactivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behavior of ionic and zwitterionic micelles and their effects on chemical reactivity. 

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