Hydration of ion exchangers: Thermodynamics and quantum chemistry calculations. IV. The state of ions and water molecules in alkali forms of sulfostyrene resins

Abstract

New information on the state of water molecules and ions in the alkali ion forms of sulfostyrene resins have been obtained by joint application of thermodynamic and quantum chemistry methods to treatment of experimental water sorption isotherms taken from the literature. The theoretical water sorption isotherms have been computed from suggested earlier model of Predominant Hydrates. The model parameters were obtained with the help of quantum chemical calculations of structures of the swollen ion exchangers. Ab initio non-empirical SCF MO LCAO method has been used. The molecular models included fragments of the polystyrene chain with two sulfonic groups and the number of water molecules identical to that in the resins with 10% divinylbenzene. The theoretical water sorption isotherms for all alkali metal forms accurately describe the experimental data in the assumption of presence in the resin three (Li+ and Na+) or two (K+, Rb+ and Cs+) predominant hydrates. The calculations allowed obtaining the following characteristics of the molecular models: binding energy of the water molecules to the swollen ion exchanger, Gibbs' energy of the hydrates formation, distances between the water molecules and ions, lengths of the hydrogen bonds and some other parameters. These data allowed explaining physical reasons for the difference in properties of the alkali ions related to the water “structure forming” and “structure breaking” subgroups. The results of the quantum chemistry calculations clearly indicate separation of water molecules in the resin into several groups differing in the binding energy and the distances to the cations. The number of hydrogen bonds per water molecule in the ion exchanger is about 25% less than in the bulk water, but the total number of intermolecular bonds (hydrogen and ion-molecular) is approximately the same as in the bulk water. Intermolecular bonds in the ion exchanger form an integrated net in which the mobile and the fixed ions function as the knots.