Influence of the counterion in aqueous solutions of cationic surfactants .1. Electromotive force measurements

EMF measurements have been performed to study the influence of the counterion in aqueous solution of ionic surfactants in which the amphiphile is n-allcyltrimethylammonium cation. General rules of variation were observed for the activities of the amphiphilic ion and the counterion, as a fonction of the global concentration of surfactant salt. This study also allows us to measure the critical micellar concentration (CMC) and to have access to the degree of counterion binding beta over a wide range of counterion (bromide, chloride, fluoride, hydroxide, nitrate, acetate, benzoate). A systematic correlation between the concentration of the amphiphilic cation and the counterion in solution has been established when the micellization process occurs. This correlation is also verified when adding a background electrolyte with a common anion to the surfactant solution, and applied to the CMC, can be identified to the well-known relation of Corrin Harkin.