The liquid-vapor interface of an ionic fluid

TL;DR

This paper models the liquid-vapor interface of an ionic fluid using a density-functional approach based on correlation functions, revealing differences from previous theories and analyzing surface tension behavior near the triple point.

Contribution

It introduces a density-functional approximation for the ionic fluid interface based on MSA correlation functions, providing new insights into interface shape and surface tension.

Findings

Interface decay is more rapid on vapor side than liquid side.

Interface width is about four times the correlation length.

Surface tension ratio near the triple point is smaller than in atomic fluids.

Abstract

We investigate the liquid-vapor interface of the restricted primitive model (RPM) for an ionic fluid using a density-functional approximation based on correlation functions of the homogeneous fluid as obtained from the mean-spherical approximation (MSA). In the limit of a homogeneous fluid our approach yields the well-known MSA (energy) equation of state. The ionic interfacial density profiles, which for the RPM are identical for both species, have a shape similar to those of simple atomic fluids in that the decay towards the 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. We discuss the magnitude and temperature dependence of the surface tension, and argue that for temperatures near the triple point the ratio of the dimensionless surface tension and critical temperature is much smaller for the RPM than for simple atomic fluids.