An Alternative Approach to Nonrandomness in Solution Thermodynamics

An alternative formulation of the configurational partition function of mixtures is proposed, which leads to analytical expressions for the basic thermodynamic quantities, including the local compositions or the nonrandomness factors in multicomponent mixtures. By implementing the new formalism into an equation-of-state framework, a simple and versatile nonrandom model of fluids and their mixtures is developed. The new model is tested against experimental data for both pure fluids and mixtures. Among others, the model calculates a significant degree of nonrandomness in the distribution of free volume, even in nonpolar systems. The new expressions for local compositions are compared with corresponding expressions from the literature, including Guggenheim's quasi-chemical expressions and Monte Carlo simulations. These expressions are similar to the well-known Wilson-type expressions. Thus, our approach provides, to some degree, a semi-theoretical basis for the Wilson expressions for local compositions, and their consistency problem is discussed.