Ion Size Dependences of the Salting-Out Effect: Reversed Order of Sodium and Lithium Ions

A general trend of the salting-out effect on hydrophobic solutes in aqueous solution is that the smaller the size of a dissolved ion, the larger the effect of reducing the solubility of a hydrophobe. An exception is that Li+, the smallest in alkali metal ions, has a notably weaker effect than Na+. To understand the reversed order in the cation series, we performed molecular dynamics simulations of aqueous solutions of salt ions and calculated the Setschenow coefficient of methane with the ionic radius of either a cation or an anion varied in a wide range. It is confirmed that the Setschenow coefficient is correlated with the packing fraction of salt solution, as observed in earlier studies, and also correlated with the partial molar volume of an ion. Analyses of correlation function integrals, packing fractions of solvation spheres, and orientations of water molecules surrounding an ion reveal the key differences in microscopic properties between the cation and anion series, which give rise to the reversed order in the cation series of the partial molar volumes of ions and ultimately that of the Setschenow coefficients.