A Molecular-Thermodynamic Lattice Model for Binary Mixtures

Abstract

Abstract Using a method originally proposed for describing a continuum-space polymer fluid, a new expression for the Helmholtz energy of mixing is proposed for a binary lattice mixture. Molecular size asymmetry and nonrandomness due to segment–segment interactions are taken into account. An expression proposed by Yan, Liu and Hu for a binary lattice mixture of monomers, based on the Ising model, is used as a reference system. Calculated critical constants and liquid–liquid coexistence curves are in good agreement with Monte Carlo simulations for lattice mixtures with modest size asymmetry. Because lattice spacing rises with increasing temperature, comparison of calculated binary liquid–liquid equilibria with experiment requires that calculations take into account that the interchange energy falls as temperature rises. While the new expression for the Helmholtz energy of mixing provides much improvement over the Flory–Huggins equation, calculated liquid–liquid equilibria for three binary systems are similar to those from Guggenheim's quasi-chemical theory.