LIQUID-VAPOR INTERFACE OF AN IONIC FLUID

We investigate the liquid-vapor interface of the restricted primitive model (RPM) for an ionic fluid using a density-functional approximatio n based on correlation functions of the homogeneous fluid as obtained from the mean-spherical approximation. The ionic interfacial density p rofiles, which for the RPM are identical for both species, have a shap e similar to those of simple atomic fluids in that the decay towards t he bulk values is more rapid on the vapor side than on the liquid side . This is the opposite asymmetry of the decay to that found in earlier calculations for the RPM based on a square-gradient theory. The width of the interface is, for a wide range of temperatures, approximately four times the second moment correlation length of the liquid phase. W e discuss the magnitude and temperature dependence of the surface tens ion and argue that for temperatures near the triple point the ratio of the dimensionless surface tension and critical temperature is much sm aller for the RPM than for simple atomic fluids.