Infrared investigation of ionic hydration in ion-exchange membranes. Part 2.—Alkaline earth salts of grafted polystyrene sulphonic acid membranes

Abstract

Infrared spectroscopic studies have been performed on thin ion-exchange membranes which consist of alkaline earth salts of polystyrene sulphonic acid grafted on a Teflon FEP matrix. The membranes were placed at isopiestic equilibrium with water vapour in the sampling cell. The water absorption isotherm at 25 °C has been determined by measuring the integrated absorbance of the sorbed-water bending vibration. In the case of Mg²⁺, Ca²⁺ and Sr²⁺ salts the isotherms exhibit an absorption step with a hysteresis. The frequency of the symmetrical stretching band of the SO^–₃ group passes through a minimum, and the OH stretching band of water in the membrane passes through a maximum at the relative water vapour pressure corresponding to the step displayed by the absorption isotherm.

The dependence of the bending vibration band of water is also analysed in terms of solvent structure in the membrane. A model of hydration of alkaline earth salts of an FEP–PSSA membrane which emerges from the spectral analysis is suggested.