Micellar kinetic effects in gemini micellar solutions:: Influence of sphere-to-rod transitions on kinetics

The reactions 4-nitrobenzenesulfonate + Br- and methyl naphthalene-2-sulfonate + Br- were studied in various water-ethylene glycol, EG, [C12H25(CH3)(2)N-(CH2)(s)N(CH3)(2)C12H25]Br-2 micellar solutions (12-s-12,2Br(-) with s = 3-5 methylene groups). Results showed that the observed rate constant of the two reactions varied when a sphere-to-rod transition occurs. This morphological transition is accompanied by changes in the interfacial region water content and in its polarity. The micellar ionization degree is also altered, making the counterion interfacial concentration change. Finally, variations on the molar surfactant volume, V-m, also follow the sphere-to-rod transitions. A simple pseudophase kinetic model is inadequate for quantitatively discussing the kinetic micellar effects observed, since the changes accompanying micellar growth affect the second-order rate constant in the micellar pseudophase, the equilibrium binding constant and the surfactant molar volume, neither of them remaining constant in the whole surfactant concentration range. However, this simple model can be helpful in the treatment of kinetic data for surfactant concentrations below the morphological transitions, providing some interesting, although approximate, information. (c) 2007 Elsevier Inc. All rights reserved.