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

Mutual diffusion coefficients, D-m, have been measured for mixed surfactant systems, N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C(14)DAPS) + potassium octadecanesulfonate (C18SO3K) and C(14)DAPS + octadecyltrimethylammonium chloride (C(18)TACl) in water at 298.2 K, where essentially all ionic surfactants are solubilized in C(14)DAPS 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(14)DAPS + C18SO3K system than for the previously measured C(14)DAPS+C18SO3Na 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(14)DAPS + C(18)TACl are smaller than those for C(14)DAPS + C2(18)SO(3)K. Micellar aggregation numbers were determined by measuring fluorescence decays of pyrene solubilized in micelles and were found to be smaller for C(14)DAPS + C(18)TACl than for C(14)DAPS + C18SO3Na. Thus, in the C(14)DAPS + C(18)TACl 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(18)TA(+) 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. (C) 2000 Elsevier Science B.V. All rights reserved.