Theory of Gas Solubility and Hydrophobic Interaction in Aqueous Electrolyte Solutions.

Ion-specific effects on the solubility of nonpolar solutes and on the solute-solute hydrophobic interaction in aqueous electrolyte solutions are studied on the basis of a continuum theory that incorporates the excluded volume of the molecules using the four-component (water, cations, anions, and solutes) Boublı́k-Mansoori-Carnahan-Starling-Leland model and ion hydration (electrostriction) using the Born model. We examine how the ordering of ions in the salt effect on the solubility as measured by the Sechenov coefficient changes with varying sizes of ions and solutes. Our calculation reproduces the general trend of experimentally measured and also provides insight into the irregular behavior of for lithium ion. The correlation between and the salt effect on the hydrophobic interaction that has been pointed out earlier is accounted for by an explicit connection between and the salt-enhanced-association coefficient in the expansion of the second osmotic virial coefficient () = (0) - + ··· in powers of the salt density at fixed pressure and temperature. The quadratic relation is derived for ions and solutes that are not very large.