Relevance of the local-density approximation to interfacial properties of ionic fluids within gradient theory

Abstract

The effect of employing different approximations to the local density on properties of inhomogeneous systems as obtained from a gradient expansion of the local free energy (gradient theory) is investigated. We present liquid - vapour density profiles, surface tensions, correlation lengths, and Ginzburg temperatures for an ionic model fluid, the restricted primitive model (RPM), treated in the framework of Debye - Hückel theory. The dependence of the interfacial properties, density profile and surface tension, on the type of local-density approximation is comparatively weak (at most a factor of four in the surface tension). In contrast, the much more sensitive Ginzburg temperature, which estimates the extent of the critical region, varies over a range of four orders of magnitude depending on which local-density approximation is applied.