Phase stability of fluid hard-sphere mixtures interacting through an attractive Yukawa tail

Abstract

The mean spherical approximation has been applied to study both the thermodynamical and structural properties of fluid hard-sphere mixtures interacting through an attractive Yukawa tail. The solution scheme is the same as the one previously adopted by the authors for the case of hard-sphere mixtures. The problem of phase stability is more specifically treated in the paper. In the absence of computer simulation results to compare the theory with, reference is made to Monte Carlo calculations for Lennard-Jones (LJ) potentials, performed by other authors, and to experimental results for an Ar-Kr mixture in the fluid phase. A procedure is described which allows us to mimic these two different systems in terms of a proper adjustment of the parameters specifying the Yukawa potential, and of the diameter ratio of the hard spheres. The theory is able to locate the boundary of the phase stability for the LJ mixture rather accurately, and in good agreement with the computer simulation results. It also predicts pressure-concentration phase diagrams which reproduce all the qualitative features of the experimental data for the noble-gas mixture. Due to the simplicity of the solution scheme, one can explore the behavior of the system in a wide range of concentrations, potential strengths, and diameter ratios. It is thus possible to determine how the phase instability temperature depends on these parameters and to discuss the general conditions under which phase separation can take place.