The phase-behavior of Lennard-Jones mixtures with nonadditive hard cores: Comparison between molecular dynamic calculations and perturbation theory

Abstract

An analytical equation of state for Lennard-Jones mixtures has recently been derived using a perturbation theory with an additive hard sphere mixture (i.e., for the collision diameter d12=(d11+d22)/2) as a reference system. Here we generalize this equation of state using a nonadditive hard sphere mixture as a reference system. Even for Lennard-Jones mixtures that obey the Lorentz–Berthelot mixing rules [σ12=(σ11+σ22)/2 and ε12 =√ε11ε22 ], we find that our generalized theory shows an improvement in the predictions of the excess Gibbs free energy and the excess volume compared to the old version of the theory. For several non-Lorentz–Berthelot mixtures the phase diagrams predicted by the equations of state with recent Gibbs-ensemble Monte Carlo and new molecular dynamics results were compared. In this comparison the van der Waals 1-fluid model as well as an effective hard sphere model were considered. In this work only the fluid–fluid phase behavior was studied. For mixtures characterized by non-Lorentz–Berthelot energy parameters the generalization of the original equation of state gives the best predictions. For a mixture characterized by a relatively large nonadditivity in the repulsion parameters the 1-fluid approximation is best. As a by-product this study yields a generalization of the MIX1 equation of state for mixtures of nonadditive hard spheres with d11≠d22.