Binary liquid-liquid equilibria from a double-lattice model

Abstract

Hu Y., Liu H., Soane D.S. and Prausnitz J.M., 1991. Binary liquid-liquid equilibria from a double-lattice model. Fluid Phase Equilibria, 67: 65-86. Freed's lattice field theory is used to establish a double-lattice model for the Helmholtz energy of mixing for strongly non-ideal binary liquid mixtures. The coefficients of Freed's expansion terms are adjusted such that the calculated coexistence curve for the simple Ising lattice is in excellent agreement with that calculated from the Padé-approximant coefficients for spontaneous magnetization proposed by Scesney. For a variety of binary systems, two parameters (ϵ/ k and r 2) are obtained from experimental critical consolute points; here (ϵ/ k is the interchange energy and r 2 is the number of lattice sites required by molecule 2, with r 1=1. Calculated coexistence curves are generally in good agreement with experiment for fluid mixtures without specific (oriented) interactions, much better than those calculated using Flory-Huggins theory, especially near critical consolute points. Introducing a secondary lattice to account for highly oriented interactions (hydrogen bonding), requires an additional energy parameter δϵ/ k and an empirical parameter c 10. With these parameters coexistence curves are fitted well for systems having a miscibility loop with both lower and upper critical solution temperatures.