Critical scaling laws and an excess Gibbs energy model

Abstract

Excess Gibbs energy models which are widely used in modelling thermodynamic properties of multicomponent liquid phases are based on the assumption that long-range density or concentration fluctuations can be neglected. This assumption is no longer valid near a system's critical point, where large density or concentration fluctuations effectively mask the identity of the system and produce universal phenomena which have been well studied in simple liquid–vapor and liquid–liquid systems. Based on the Landau–Ginzburg–Wilson theory of fluctuations, a crossover procedure has been developed to incorporate the effects of critical fluctuations into a classical excess Gibbs energy model. As an example, we have applied our crossover procedure to the Non-Random Two-Liquid (NRTL) excess Gibbs energy model. This crossover procedure involves the use of transformed variables for temperature and concentration and the addition of a fluctuation term to the classical excess Gibbs energy. The resulting transformed Gibbs energy has the universal scaling behavior near the consolute critical point and has a smooth crossover to classical behavior far away from the consolute critical point.