Equations of State for Mixtures of Square-Well Molecules: Perturbation and Local Composition Theories for Binary Mixtures of Equal-Sized Molecules

Abstract

In this paper two classes of equations of state for binary mixtures of equal-sized square-well spherical molecules are compared. The first class is perturbation theory, and the second includes several models based on local composition arguments. It is shown that second-order perturbation theory generally is in better agreement with configurational energy and compressibility factor simulation data than the local composition models. However, perturbation theory is written in a form that requires three different mixing rules. A new, simple equation of state for square well mixtures is proposed. This model is a closed-form version of perturbation theory which has the exact second virial coefficient behavior for both pure components and mixtures. This model also converges to the mean-field limit at the correct rate, and it provides very good agreement with Monte Carlo simulation data over the entire density range.