The structure and surface tension of the liquid-vapour interface of a model of a molten salt

Abstract

We present results of calculations of the equilibrium density profile and surface tension for the liquid-vapour interface of the restricted primitive model of a molten alkali halide. In this model the anion and cation are represented by hard spheres of equal diameter R and opposite charges; there is no electrical double layer at the interface. Our calculations are based on a square gradient approximation to the free energy of an inhomogeneous charged fluid. We find that the shape of the total density profile is different from that calculated for a Lennard-Jones fluid and that the interfacial thickness is somewhat sharper at low temperatures; the ‘10–90’ thickness is ≈ R near the melting temperature. The surface tension decreases linearly with temperature until near the critical point, in a manner similar to that found for the Lennard-Jones fluid. Our results are in qualitative agreement with the results of a recent computer simulation for a model of KCl. We have also compared our result for the surface tension with experimental data for those alkali halides where the anion and cation are of similar size. This involves specifying R for each salt. The theory gives a reasonable description of both the magnitude and the temperature dependence of the measured surface tension.