Closed-Loops of liquid-liquid immiscibility in binary mixtures of equal sized molecules predicted with a simple theoretical equation of state

Several types of closed-loop liquid-liquid immiscibility behavior in binary mixtures have been found with a simple isotropic model fluid. The calculations are based on an equation of state composed by the Carnahan-Starling hard sphere repulsion and the van der Waals-attraction. The extension to binary mixtures is accomplished with the usual quadratic one-fluid mixing rules. The results are presented in a global phase diagram as introduced by van Konynenburg and Scott. Several closed-loop phase diagrams similar to those discovered by Boshkov ten years ago with a Lennard-Jones equation of state have been found. The results are discussed in the context of the topological and physical origin of closed-loop behavior in systems of isotropic mixtures.