Some regularities in the behavior of salting constants for polar molecules in aqueous electrolyte solutions

The salting effect of polar molecules in aqueous electrolyte solutions has been examined in the case of 22 solute molecules. It is observed that the salting constants of polar and nonpolar molecules of the same size in a given salt solution are close to each other and are generally greater than a critical diameter of the solute molecules. Using the available data from the literature, the salting constants ks of neutral polar molecules by univalent cation electrolytes has been classified in three groups: (1) Organic molecules and inorganic salts: for a given electrolyte ks is a monotonous function of the solute size and the nonelectrolyte dipole moment plays no role. (2) Organic molecules and organic salts: again a smooth variation of ks with size is observed whatever the polar mono-functional molecule. (3) Inorganic molecules and inorganic salts: a correlation is exhibited between ks and the solute dipole moment. These observations may be interpreted by assuming that both ends of a dipole are to be available to solute + solvent interactions for a dipole effect to be observed.Furthermore it is shown that in case 1, the scaled-particle theory with incorporation of a preferential solvation phenomenon may be useful for predicting salting out effects of polar molecules in inorganic salt solutions. In case 2 an approach based on microscopic surface tension and molecular surface area is also considered.