Critical micelle concentration cationic 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). 

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. 

Molecular interactions between two surfactants at an interface or in micelles are frequently described through the so-called parameters, which can be obtained from surface (or interfacial) tension or from critical micellar concentration data [13]. Attractive interactions are characterized by negative values of this parameter and, specifically, it has been found that it becomes less negative as the mole fraction of the co-surfactant increases. It has also been previously observed [14] that this tendency, for different mixed surfactant systems, can be explained by the role played by the interactions of the cationic surfactants head groups in the stability of the mixed micelles. Desai and Dixit [15] have found similar variations depending on the mixtures composition of cationic and polyoxyethylenic non-ionic surfactants. 

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