Vapor-liquid equilibria of mixed-solvent electrolyte solutions: ion-size effects based on the MSA theory

Abstract

Abstract Wu R.-S. and Lee L.L., 1992. Vapor-liquid equilibria of mixed-solvent electrolyte solutions: ion-size effects based on the MSA theory. Fluid Phase Equilibria, 78: 1-24. We present a new method for calculating the vapor-liquid equilibria of mixed-solvent electrolyte systems. We use the water-methahol-lithium chloride system as an example to demonstrate the approach. In this approach, a statistical mechanical theory, the mean spherical approximation (MSA), is used together with the Gibbs-Duhem relation of thermodynamics and Furter's linear volatility theory of solvation to form a complete theory of phase equilibria. The MSA theory gives the salt activity coefficients In γ±. The Gibbs-Duhem relation gives a relation between the activities of the solvent a, the cosolvent b and the salt s. A third equation that relates the relative affinity of the solvents for the salt completes the conditions needed for calculating the vapor-liquid equilibria of ternary systems. The MSA formulas are sensitive to ion-size variations: a requirement for modeling “salting-out” behavior. We have shown that up to 20% variations in in γ± could be caused by ion-size changes in some sample systems. We show that to achieve “complete” scale conversion for properties from the McMillan-Mayer scale to the Lewis-Randall scale, one must have a good theory of solvation. The solvent-ion and solvent-solvent correlations enter the conversion in a non-trivial way. The sample system water-methanol-LiCl can be predicted to within 0.01 in mole fractions and 5 mmHg in pressures over the concentration ranges 10–15 molal. This method introduces molecular theory into the formulation without postulating an excess free energy. It represents an alternative to the conventional excess free energy models.