Reactivity in surfactant systems

The transfer of a reactive solute into a micelle changes several features of its existence that can alter its inherent stability. In the micelle the molecular environment of the solute molecules will have changed drastically, from an aqueous to a relatively non-polar milieu, depending on the depth of insertion into the micelle. The simple fact that the solute may be protected from attacking species such as or OH ions will give rise to stabilization of labile molecules such as esters in some charged micelles, the surface characteristics will be such that there is a concentration of ions which would result in a more rapid breakdown than in simple aqueous solution. 

The micellar environment is sufficiently different from the simple aqueous environment that reaction rates may sometimes be dramatically changed because of this there has built up a school of chemistry in which micellar systems are used deliberately to alter the rates and directions of chemical reactions. In pharmaceutical formulations the influence of surfactant on the stability of the pharmaceutical is generally secondary to its main purpose, but surfactants may be used to stabilize labile pharmaceuticals, and an understanding of the mode of action and interactions can avoid the problem of destabilization which might unwittingly occur. 

In this chapter we wish to explore not only the influence of micelles on reaction rates and the course of reactions, both chemical and photochemical, but also the stability of surfactants themselves and how aggregation can affect their stability. The chemical modification of surface-active agents and attempts to polymerize surfactant micelles will also be covered. The literature on reactivity in micellar systems has grown enormously since 1968 when an account of the pharmaceutical aspects was given in the first edition of this book [1], to the extent that a book has been devoted to the subject reviewing and collating the data in the literature prior to mid-1974 [2]. Here we can probably only hope to extract some of the salient features of the subject, and could certainly not claim to be comprehensive. The reference list, however, contains several reviews which should be consulted for more detailed treatments. The analytical consequences of solubilization of chromophoric species and change in the apparent dissociation constants of compounds in the presence of surfactants is also discussed at the end of the chapter. 

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