CRITICAL SCALING LAWS AND AN EXCESS GIBBS ENERGY-MODEL

Excess Gibbs energy models which are widely used in modelling thermody namic properties of multicomponent liquid phases are based on the assu mption that long-range density or concentration fluctuations can be ne glected. This assumption is no longer valid near a system's critical p oint, where large density or concentration fluctuations effectively ma sk the identity of the system and produce universal phenomena which ha ve been well studied in simple liquid-vapor and liquid-liquid systems. Based on the Landau-Ginzburg-Wilson theory of fluctuations, a crossov er procedure has been developed to incorporate the effects of critical fluctuations into a classical excess Gibbs energy model. As an exampl e, we have applied our crossover procedure to the Non-Random Two-Liqui d (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 ener gy. The resulting transformed Gibbs energy has the universal scaling b ehavior near the consolute critical point and has a smooth crossover t o classical behavior far away from the consolute critical point. (C) 1 998 Elsevier Science B.V. All rights reserved.