Water−Ethylene Glycol Alkyltrimethylammonium Bromide Micellar Solutions as Reaction Media: Study of the Reaction Methyl 4-Nitrobenzenesulfonate + Br-

Abstract

The SN2 substitution reaction between methyl 4-nitrobenzenesulfonate and bromide ions has been studied in several water−ethylene glycol, EG, cationic micellar solutions, the surfactants being hexadecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyltrimethylammonium bromide. Together with the kinetic experiments, conductivity measurements gave information about the critical micelle concentration and the micellar ionization degree of the water−EG micellar solutions used as reaction media. They also permitted us to gain thermodynamic information about the influence of added EG on the micellization process of the surfactants investigated. A simple pseudophase model was shown to be useful in the discussion of the kinetic results. The importance of the working surfactant concentration range in the reliability of the adjustable parameters obtained from the fittings was stressed. It was found that the reaction was faster in cationic micelles than in water, in the absence as well as in the presence of ethylene glycol. The influence of the surfactant nature as well as of the ethylene glycol content on the equilibrium binding constant describing the distribution of the methyl 4-nitrobenzenesulfonate between the micellar and water−EG bulk phase and on the second-order rate constant for the reaction in the micellar pseudophase was rationalized by considering the polarity of the surface micellar region of the aggregates present in the different reaction media.