Ion Exchange and the Swelling of Ion-exchange Resins

The review deals with the modern hypotheses concerning the physicochemical properties of ion-exchange resins and their effects in the interaction between resins and solvents and solutes, mainly electrolytes. The problems associated with the permeability of resins, the steric accessibility of the functional centres, and their screening are discussed. Particular attention has been devoted to the interaction between resins and solvents. The thermodynamic conditions of such interaction (the changes in the integral and differential thermodynamic functions of swelling) have been elucidated. The isotherms for the absorption of water vapour have been discussed as a function of the content of the cross-linking agent, the ionic form of the resin, and the hydrophilicity of the matrix. In the section dealing with the comparison of the isotherms for the absorption of water vapour by resins with different degrees of cross-linking, attempts to describe them on the basis of osmotic concepts are discussed. The methods of the osmotic theory have been examined in detail: the characteristics of the hydration of ion-exchange resins which follow from the analysis of the differential and integral free energies of swelling and also from data derived by other methods, such as infrared and nuclear spin resonance spectroscopy. Certain model approaches to the study of the selectivity of ion exchange with participation of the solvent have been described. The thermodynamic methods for the analysis of the relation between the selectivity and swelling of resins are quoted. The bibliography includes 126 references.