The Adsorption of Dodecyldimethylammonium Chloride on Aqueous Surfaces of Concentrated NaCl Solutions

Abstract

Abstract Surface tension of aqueous NaCl solutions of dodecyldimethylammonium chloride has been measured at high NaCl concentrations. Surface excesses of surfactant cation, Na+ and Cl− are derived by means of the Gibbs adsorption isotherm from the derivatives of surface tension with respect to surfactant concentration and NaCl concentration. The rational mean activity coefficients of surfactant and NaCl are both approximated by a common expression satisfying the Brönsted principle for mixed electrolytes but including the ionic strength as a variable. Saturated value of surface excess of surfactant cation remains constant at low NaCl concentrations up to 0.10 M, while it increases as the NaCl concentration exceeds 0.50 M. The adsorption of Na+ is negative at low NaCl concentrations, but it increases rapidly and is positive above 0.50 M NaCl. Surfactant and NaCl are subject to the strong salting-out effect when the NaCl concentration is higher than 0.50 M. The Corrin-Harkins plot of the critical micelle concentration at high NaCl concentrations deviates from the straight line.