Quantitative treatments of micellar effects in aqueous solution

Quantitative treatments of micellar rate effects in aqueous solution The development of quantitative models of micellar effects upon reaction rates and equilibria was based on the concept that normal micelles in aqueous, or similar associated, solvents behave as a separate medium from the body of the solvent. 

Therefore, provided that there is an equilibrium between the micellar and aqueous pseudophases, the problem resolves itself into estimation of the distribution of reactants between aqueous and micellar pseudophase and calculation of the rate constants of reaction in each pseudophase. Menger and Portnoy [70] developed an equation which successfully accounted for micellar inhibited saponification of 4-nitrophenyl alkanoates. This model was also applied to spontaneous, unimolecular, hydrolyses of dinitrophenyl sulfate monoanions [66] and phosphate dianions [68] which are speeded by cationic micelles in water. 

The equiUbrium constant can alternatively be written in terms of the concentration of micelles. The equilibrium constant is then NK, where N is the average aggregation number of the micelle, and equilibrium constants are often written in these terms [70]. 

The first order rate constants for reaction in the aqueous and micellar pseudophases are k and A , so that the overall first-order rate constant, k, is given by  

3 adequately fits data for unimolecular tnicellar-catalyzed reactions [66,68], and for micellar-inhibited reactions, where for bimolecular reactions, is usually small so that the second term in the numerator of Eqn. 3 can be neglected [70]. In some cases, for example with very hydrophobic substrates, micellar rate effects are observed at [D] cmc, so that in these cases we cannot equate the concentration of monomeric surfactant with the cmc, probably because the substrate promotes micellization or interacts with submicellar aggregates, and modified forms of Eqn. 3 have been used [71]. 

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