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 ${\ensuremath{\epsilon}}_{\mathrm{ij}},$ $i,j=1,2,$ such that ${\ensuremath{\epsilon}}_{11}={\ensuremath{\epsilon}}_{22}>{\ensuremath{\epsilon}}_{12}.$ The phase diagram of the system at all densities and concentrations is investigated as a function of the unlike-to-like interaction ratio $\ensuremath{\delta}={\ensuremath{\epsilon}}_{12}/{\ensuremath{\epsilon}}_{11}$ 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 $\ensuremath{\delta}$ is increased in the interval $0<\ensuremath{\delta}<1,$ 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 end point topology predicted by mean-field theory as $\ensuremath{\delta}$ approaches $1,$ at least up to $\ensuremath{\delta}=0.8,$ which is the largest value of $\ensuremath{\delta}$ 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.