The Donnan Distribution of Salts in the Case of Ionic Micelles

Abstract

We examined the Donnan distribution of small ions for the NaCl solutions of tetradecyltrimethyl-ammonium chloride (C 14 TAC) and hexadecyltrimethylammonium chloride (C 16 TAC) micelles with the nonequilibrium method. The salt concentration of the external solution in equilibrium with the internal solution was determined from the condition of zero emf. In the external solution, the surfactant was added to a range of the critical micelle concentration (cmc). Values of the apparent salt exclusion parameter Γ app did not depend on the external surfactant concentrations m e ' for a narrow range of m e '. The constant Γ app value was taken as the proper salt exclusion parameter value Γ and the minimum value of m e ' giving this constant value was taken as the cmc', the cmc of the external solution. We propose, in this way, a procedure to determine experimentally the salt exclusion parameter for the ionic micelle solutions. The Γ values for both C 14 TAC and C 16 TAC solutions scarcely depended on the surfactant concentration m e , at constant m s except a narrow range (0.5-1) of m e /m s in the case of C 14 TAC. Also, the Γ values for both C 14 TAC and C 16 TAC solutions were determined by the ratio m e /m s and did not depend on m s . These behaviors differ from those found on linear polyelectrolytes. The Γ values for the C 14 TAC solutions were greater than those for the C 16 TAC solutions.