ELECTROLYTE SURFACE-TENSION IN THE MODIFIED POISSON-BOLTZMANN APPROXIMATION

The surface tension of aqueous electrolytes is studied within the framework of the primitive model of the planar electric double layer, the surface being treated as a hard wall of relative permittivity unity, characteristic of gas. The structure of the model interface is described by the modified Poisson-Boltzmann (MPB) theory while the electrolyte activity is obtain from the bulk MPB approximation. The excess surface tension is determined by numerical integration of the Gibbs adsorption isotherm. The calculated wall/ion density profiles and the mean activity coefficients are compared with the corresponding Monte Carlo data for 1:1 and 2:2 valency systems, and the results for the excess surface tension are compared with the experimental values for several monovalent and divalent salt solutions. It is seen that (i) the MPB theory gives viable thermodynamics for the proposed model system (ii) the primitive model can mimic the surface properties of real systems to a remarkable degree.