A surface tension approach to the salting-out of nonpolar nonelectrolytes

Abstract

A simple approach is developed for the salting-out of nonpolar molecules by strongly solvated salts. The standard free energy of transfer of solute from pure solvent to salt solution, and hence the salting coefficient, k, is calculated in terms of the surface tension increment caused by the addition of salt to the solvent. In its simplest form the method gives k in terms of the molar volumes of solvent and solute, and the osmotic coefficient of the salt solution. It is more successful in the prediction of k than the McDevit and Long theory, and it also has advantages over the more complex scaled particle theory of salt effects. In addition to a range of nonpolar solutes, the salt effects on some alcohols are also considered but application to polar solutes generally requires a knowledge of experimentally determined surface tension increments which are not widely available. The correct order of magnitude for the temperature coefficient of k at room temperature for some alkanes and H2 in water is given, although minima in k as a function of temperature, which have sometimes been observed, are not reproduced.