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 K_S changes with varying sizes of ions and solutes. Our calculation reproduces the general trend of experimentally measured K_S and also provides insight into the irregular behavior of K_S for lithium ion. The correlation between K_S and the salt effect on the hydrophobic interaction that has been pointed out earlier is accounted for by an explicit connection between K_S and the salt-enhanced-association coefficient C_I in the expansion of the second osmotic virial coefficient B(n_s) = B(0) - C_In_s + ··· in powers of the salt density n_s at fixed pressure and temperature. The quadratic relation $$ is derived for ions and solutes that are not very large.