Phase diagram of symmetric binary mixtures at equimolar and nonequimolar concentrations: A systematic investigation

We consider symmetric binary mixtures consisting of spherical particles with equal diameters interacting via a hard-core plus attractive tail potential with strengths εᵢⱼ, $i,j=1,2,$ such that ε₁₁=ε₂₂>ε₁₂. The phase diagram of the system at all densities and concentrations is investigated as a function of the unlike-to-like interaction ratio δ=ε₁₂/ε₁₁ by means of the hierarchical reference theory. 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 δ is increased in the interval 0<δ<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 end point topology predicted by mean-field theory as δ approaches $1,$ at least up to δ=0.8, which is the largest value of δ 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.