Adsorption of ions in the diffuse part of an electrical double layer at ionic surfactant solution-air interfaces in the presence of background electrolytes

The equations are derived for the calculation of adsorption values Gamma(d)(+/-) of coions and counterions in the diffuse part of an electrical double layer characterized by psi(d) potential in the presence of a background electrolyte. The case of arbitrary |psi(d)| values is considered. Based on the known experimental data, the contributions of adsorption values Gamma(d)(+/-) to the surface excesses of ions, as determined by the Gibbs method for the solution-air interfaces, are quantitatively estimated. It is shown that the adsorption of counterions in the diffuse part of the electrical double layer is significantly lower than that in its dense part; however, the orders of these values are comparable. At potentials |psi(d)| > 25 mV, surface-active ions are mainly located near the interface, and their adsorption values Gamma(d)(-) cannot noticeably affect the calculated surface excesses.