Effect of electrolytes on surface tension of ionic surfactant solutions

An improved model of ionic surfactant adsorption was developed by considering penetration of counterions into the interfacial Stern layer. Accordingly, the surface layer was treated as quasi two-dimensional electrolyte in which the electroneutrality condition was not fulfilled. Moreover, in the model the finite size of surfactant head-groups and counterions were taken into account as well as lateral electrostatic interactions. The model was confronted with experimental data obtained for cetyltrimethylammonium bromide in different electrolytes. It was found, that the model correctly reflected the variation in surface tension isotherms with the ionic strength of the solution. Contrary to previous models of ionic surfactant adsorption, our model allowed to account for the strong dependence of adsorption on type of counterion present in solution. Using the model a dependence of Stern layer potential and diffuse double layer potential on the concentration of surfactant in solution also was predicted.