IONIC HYDRATION AND WATER SORPTION ISOTHERMS OF ION-EXCHANGE RESINS

Water vapor sorption isotherms of Al3+, Mg2+, and Cs+ forms of variously cross-linked Dowex 50W ion exchange resins have been determined at 298 +/- 2 K using the isopiestic technique. Swelling pressures (pi) and swelling free energies (DELTAG(sw)) for the respective ionic forms have been computed from the isotherms by conventional approach. These data as well as the water sorption data on the Li+, Na+, and K+ forms of variously cross-linked Dowex 50W resins reported earlier from this laboratory have been analyzed using the D'Arcy and Watt equation and its modified forms to distinguish between the strong and weak water sorption sites and multilayer formation. The ionic hydration numbers obtained from cross-linking effects and vapor pressure isotope effects of cationites are found to be comparable to those derived from the present analysis based on the D'Arcy and Watt equation. These in turn have been compared with the hydration numbers of cations as obtained by various techniques using electrolyte solutions. It is found that the D'Arcy and Watt equation is unable to distinguish between a secondary hydration shell and the bulk water in the case of monovalent cations but can identify the secondary shell water molecules in the case of the Al3+ form very clearly. Multilayer formation in the water sorption isotherm is found to be related to swelling pressures, and swelling free energies are essentially free energies of dilution of cations.