A MOLECULAR THEORY OF SETCHENOV SALTING-OUT PRINCIPLE AND APPLICATIONS IN MIXED-SOLVENT ELECTROLYTE-SOLUTIONS

An exact molecular theory is developed to express the salting-out beha vior of solute species in mixed-solvent (more than one solvent) electr olyte solutions. The starting point is the Kirkwood-Buff solution theo ry. The Setchenov constant can be shown to be a special case (at infin ite dilution) of the general theory. The new formula involves the part ial molar volumes of the components and the isothermal compressibility of the medium. In addition, it contains the direct correlation functi on integrals for solvent-salt interactions. Furter's theory of relativ e volatilities is shown to be related to Setchenov's equation and the molecular-based equation. All these equations express the affinities o f the solvent and cosolvents toward the salt in the solution. Low affi nity (for a particular solvent-salt pair) means salting-out of this so lvent. When combined with the Gibbs-Duhem relation, the affinity equat ions result in a differential equation for the activity of a single so lvent which can be numerically solved. This affords a means of obtaini ng the activities of solvents individually in the mixture. We test the new approach for two ternary solutions, methanol-water-LiBr and metha nol-water-LiCl, by applying an empirical affinity equation. For the io nic activities, we use the mean spherical approximation expressions. C omparison with experimental data shows that the agreement is close for the vapor-liquid equilibria of these ternary systems. Generalization to higher-component systems is outlined. (C) 1997 Elsevier Science B.V .