Counterion effects in aqueous solutions of cationic surfactants:: Electromotive force measurements and thermodynamic model

EMF measurements have been performed to study the influence of counterions in aqueous solutions of ionic surfactants in which the amphiphile is an n-alkyltrimethylammonium cation. General rules of variation were observed for the activities of the amphiphilic ion and the counterion, as a function of the global concentration of the surfactant salt. This study also allows us to have access to the degree of counterion binding beta over a wide range of counterions (bromide, chloride, fluoride, hydroxide, nitrate, acetate, benzoate). A systematic correlation between the concentrations of an amphiphilic cation and a counterion in solution has been established when the micellization process occurs. This correlation is also verified when a background electrolyte with a common anion is added to the surfactant solution. The mathematical relation translating the real link between concentrations of both ions of the micellar salt, deduced from this correlation, emphasizes the existence of a condition of micellization, in the form of a micellization product. A new thermodynamic model based on the electrochemical equilibrium of a dispersed phase (pseudo-phase) is developed, and we justify the existence of this condition of micellization or micellization product. Furthermore, this model is shown to be well adapted far the description of the whole phenomena observed in micellar solutions. (C) 1999 Academic Press.