VAPOR-LIQUID-EQUILIBRIA OF MIXED-SOLVENT ELECTROLYTE-SOLUTIONS - ION-SIZE EFFECTS BASED ON THE MSA THEORY

We present a new method for calculating the vapor-liquid equilibria of mixed-solvent electrolyte systems. We use the water-methanol-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 gamma +/-. 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 gamma +/-, 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.