INFLUENCE OF THE COUNTERION IN AQUEOUS-SO LUTIONS OF CATIONIC SURFACTANTS .1. ELECTROMOTIVE-FORCE MEASUREMENTS

EMF measurements have been performed to study the influence of the cou nterion in aqueous solution of ionic surfactants in which the amphiphi le is n-allcyltrimethylammonium cation. General rules of variation wer e observed for the activities of the amphiphilic ion and the counterio n, as a fonction of the global concentration of surfactant salt. This study also allows us to measure the critical micellar concentration (C MC) and to have access to the degree of counterion binding beta over a wide range of counterion (bromide, chloride, fluoride, hydroxide, nit rate, acetate, benzoate). A systematic correlation between the concent ration of the amphiphilic cation and the counterion in solution has be en established when the micellization process occurs. This correlation is also verified when adding a background electrolyte with a common a nion to the surfactant solution, and applied to the CMC, can be identi fied to the well-known relation of Corrin Harkin.