A molecular level interpretation of ion exchange selectivity

Abstract

A molecular level interpretation of ion exchange selectivity has to be based on various types of interactions in the exchanger phase and the corresponding interactions in the solution phase. The various interactions in the resin phase, i.e. ionogenic group-water, ionogenic group counterion, counterion-water, and water-water interactions have been discussed in the light of available information in the literature on alkali metal cation/ lithium ion exchanges on Dowex 50W resins of different cross linking. Recently developed technique for studying counterion-water and water-water interactions in these resins (which uses hydrogen isotope effects in the dehydration of these resins as a probe for studying these interactions) has been described in some detail. Selectivity sequence for the alkali metal cations in these resins as well as the effect of cross linking on the selectivity have been interpreted in terms of the various molecular interactions involved. Selectivity sequence arises because of the greater hydration of more preferred cation in the resin phase compared to that in the solution phase. As the crosslinking decreases and the resins imbibe more water, the hydration shells of the preferred cations are progressively destroyed by the normal water structure formed by the free water present in the resins thus selectivity decreases with decreasing cross linking.