Thermodynamic Modeling of Vapor−Liquid Equilibria in Mixed Aqueous−Organic Systems with Salts

Abstract

A thermodynamic model is proposed for prediction of vapor-liquid equilibria (VLE) in mixed aqueous-organic solvents containing salts. The model is based on thermodynamic relations that include explicitly the effect of a solute on the activity coefficients of the cosolvents in the mixture. These relations are derived by integrating the multicomponent Gibbs-Duhem equation. The model requires the difference between the Gibbs free energy of formation of the pure salt and the Gibbs free energy of formation of this salt corresponding to the standard state in each of the pure cosolvents. Activities for the cosolvents in the salt-free mixture are also required. The new formalism is illustrated with predictions of VLE for a number of mixtures of water with methanol, ethanol, and propanol in the presence of different salts over the entire range of concentration. The model is able to predict quantitatively azeotropic compositions and their shift as a function of salt concentration.