Phase diagram of symmetric binary mixtures at equimolar and non-equimolar concentrations: a systematic investigation

TL;DR

This study systematically explores the phase behavior of symmetric binary mixtures across all densities and concentrations using hierarchical reference theory, revealing complex critical phenomena and phase coexistence features as interaction parameters vary.

Contribution

It provides a comprehensive phase diagram analysis of symmetric binary mixtures at various concentrations and interaction ratios, extending previous equimolar studies with new insights into critical points.

Findings

Identification of tricritical points at certain concentrations.

Observation of coexistence regions enclosing one-phase fluid islands.

No evidence of critical endpoint topology up to delta=0.8.

Abstract

We consider symmetric binary mixtures consisting of spherical particles with equal diameters interacting via a hard-core plus attractive tail potential with strengths epsilon_{ij}, i,j=1,2, such that epsilon_{11} = epsilon_{22} > epsilon_{12}. The phase diagram of the system at all densities and concentrations is investigated as a function of the unlike-to-like interaction ratio delta = epsilon_{12}/epsilon_{11} by means of the hierarchical reference theory (HRT). The results are related to those of previous investigations performed at equimolar concentration, as well as to the topology of the mean-field critical lines. As delta is increased in the interval 0 < delta < 1, we find first a regime where the phase diagram at equal species concentration displays a tricritical point, then one where both a tricritical and a liquid-vapor critical point are present. We did not find any clear evidence of the critical endpoint topology predicted by mean-field theory as delta approaches 1, at least up to delta=0.8, which is the largest value of delta investigated here. Particular attention was paid to the description of the critical-plus-tricritical point regime in the whole density-concentration plane. In this situation, the phase diagram shows, in a certain temperature interval, a coexistence region that encloses an island of homogeneous, one-phase fluid.