THE SOLVATION FACTOR IN SPECIFICITY OF ION ADSORPTION AT ELECTRODES

The microscopic situations of ions, especially anions, adsorbed at metal/water and air/water interfaces are comparatively examined with regard to changes of ionic hydration that must be involved in so-called specific adsorption where short-range electronic chemisorptive interactions arise with the electrode-metal surface. Specific adsorption of non-hydrophobic ions takes place usually with significant charge-transfer related to the electrosorption valency. In order for such electronic effects to arise through metal/lone-pair electron interactions, it is necessary for accompanying changes of solvation energy and of related geometry of the inner-shell solvent coordination of the ions to take place so that lone-pair orbitals eg on anions, can have access to emergent surface orbitals of the metal electrode. Correspondingly, the ion solvation factor is demonstrated by consideration of ion adsorption at air-water interfaces where the ion/metal electronic interaction is absent. In relation to the solvation factors, individual hydrated-ion properties are plotted vs, ionic crystal radii and show that such properties are differently sensitive to radii of anions in comparison with those of cations.