Electrostatic effects and involvements of solvation in separation of ions

Abstract

This paper reviews the electrostatic and solvation effects, both of which play decisive roles in separation and recognition of ions. Charged interfaces are often utilized for ionic separation and recognition; ion-exchange chromatography, micellar partition, ion sensors, and solvent extraction are typical examples. Usual stoichiometric rules, which assume two discrete phases and the electroneutrality in both phases, do not apply to such systems where electrostatic energies dominate the entire interaction, because the electrostatic force is exerted beyond a long distance. We have evaluated the results of chromatographic separation of ions with models derived from electrostatic theories, and have revealed various phenomena that were not found by the conventional stoichiometric models. Considerations based on the electrostatic models strongly imply important involvements of ionic solvation in the determination of separation selectivity. X-ray absorption fine structure (XAFS) has thus been applied to elucidate the local solvation structures of ions at separation interfaces. XAFS has very high elemental selectivity, is hardly subject to the interference from matrices, and provides structural data for noncrystalline materials. The local structures of counteranions in anion-exchange resins have been, for example, studied in various solvents, suggesting that tight ion-associations are formed between the ion-exchange groups and halide anions in aprotic solvents, while they are partly dissociated from the ion-exchange sites by complete solvation in water and methanol. In addition, the total reflection of the incident X-ray at the surface of an aqueous solution allows us to study XAFS of ions attracted by surface monolayers. Some results obtained with this novel method are also discussed. © 2005 The Japan Society for Analytical Chemistry.