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

Mutual diffusion coefficients, Dm, have been measured for mixed surfactant systems, N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C14DAPS)+potassium octadecanesulfonate (C18SO3K) and C14DAPS+octadecyltrimethylammonium chloride (C18TACl) in water at 298.2 K, where essentially all ionic surfactants are solubilized in C14DAPS micelles. By measuring diffusion coefficients of pyrene solubilized in micelles, tracer diffusion coefficients, Dt, have also been obtained for the mixed micelles. The Dt values decrease while Dm values increase with increasing concentration of the ionic surfactants. At the same concentration of the ionic surfactants, Dm values are larger for the C14DAPS+C18SO3K system than for the previously measured C14DAPS+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+, Dm values for C14DAPS+C18TACl are smaller than those for C14DAPS+C18SO3K. Micellar aggregation numbers were determined by measuring fluorescence decays of pyrene solubilized in micelles and were found to be smaller for C14DAPS+C18TACl than for C14DAPS+C18SO3Na. Thus, in the C14DAPS+C18TACl system, the larger counter-ion diffusivity that increases the Dm value is counteracted by the smaller number of counter ion per micelle. Using ion-selective electrodes, concentrations of free C18TA+ and Cl− ions were determined. A comparison of the observed Dm values with those predicted by a theory for the mutual diffusion coefficient of the polyion has been made.