Mutual and tracer diffusion coefficients for zwitterionic and ionic mixed micellar systems: effects of counter ions and micellar aggregation numbers

Abstract

Mutual diffusion coefficients, D m, have been measured for mixed surfactant systems, N-tetradecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate (C 14DAPS)+potassium octadecanesulfonate (C 18SO 3K) and C 14DAPS+octadecyltrimethylammonium chloride (C 18TACl) in water at 298.2 K, where essentially all ionic surfactants are solubilized in C 14DAPS micelles. By measuring diffusion coefficients of pyrene solubilized in micelles, tracer diffusion coefficients, D t, have also been obtained for the mixed micelles. The D t values decrease while D m values increase with increasing concentration of the ionic surfactants. At the same concentration of the ionic surfactants, D m values are larger for the C 14DAPS+C 18SO 3K system than for the previously measured C 14DAPS+C 18SO 3Na system. The result can be interpreted in terms of the larger diffusivity of K + than Na +. In spite of even larger diffusivity of Cl − than K +, D m values for C 14DAPS+C 18TACl are smaller than those for C 14DAPS+C 18SO 3K. Micellar aggregation numbers were determined by measuring fluorescence decays of pyrene solubilized in micelles and were found to be smaller for C 14DAPS+C 18TACl than for C 14DAPS+C 18SO 3Na. Thus, in the C 14DAPS+C 18TACl system, the larger counter-ion diffusivity that increases the D m value is counteracted by the smaller number of counter ion per micelle. Using ion-selective electrodes, concentrations of free C 18TA + and Cl − ions were determined. A comparison of the observed D m values with those predicted by a theory for the mutual diffusion coefficient of the polyion has been made.