Different Distributions of Counterions in the Surface Layer of Cationic Surfactant Solutions

We have separately determined the surface tension of pure aqueous solutions of cetyltrimethylammonium chloride and cetyltrimethylammonium bromide and their surface potentials by a Kelvin probe system. With the help of Gibbs equation, the surface excess has been determined through approximating the chemical activity of the surfactant by their dilute bulk concentration. In the following, the surface potential—surface excess isotherms were established. Those potential isotherms evidence that cetyltrimethylammonium chloride solution has a higher value compared to that of cetyltrimethylammonium bromide under equal surface excess. This phenomenon is supposed to be owed to the different distributions of chloride and bromide ions within the adsorption layer of the solutions, which can be attributed to the different properties of those two anions.