Liquid-vapor phase behavior of a symmetrical binary fluid mixture

Using Monte Carlo simulation and mean-field calculations, we study the liquid-vapor phase diagram of a square-well binary fluid mixture as a function of a parameter $\ensuremath{\delta}$ measuring the relative strength of interactions between particles of dissimilar and similar species. The results reveal a rich variety of liquid-vapor coexistence behaviors as $\ensuremath{\delta}$ is tuned. Specifically, we uncover critical end point behavior, a triple point involving a vapor and two liquids of different density, and tricritical behavior. For a certain range of $\ensuremath{\delta},$ the mean-field calculations also predict a ``hidden'' (metastable) liquid-vapor binodal.