Studies on the hydration process of cation-exchange membrane with the isopiestic equilibrium and dielectric methods

Abstract

The state of water hydrating counterions of the -SO 3H functional group in PES ion-exchange membrane (sulfonated polyethylene) of low water content ( p po < 0.5) has been quantitatively and qualitatively evaluated by the isopiestic equilibrium and dielectric methods at T = 293 K. The BET and Bradley isotherms are useful for describing water sorption on a PES ion-exchange membrane. From the straight-line part of the experimental isotherm the following parameters were determined: n m = 2.0 (number of water molecules per one functional group corresponding to monolayer) and Q 1 = 48,219 kJ/ mol (the adsorption heat). Results of the isopiestic equilibrium measurements have been interpreted in relation to the results of the dielectric method. The hydration number of the -SO 3H functional group in the cationite PES membrane, as determined by the dielectric method, is n h = 2. The differential specific polarization of adsorbed water p n for both stages of polyelectrolyte macrochain ionization (first stage p n = 1.04; second stage p n = 2.47 and 2.68) in the PES membrane has been calculated. For the PES cation-exchange membrane, n h = n m = 2.0, but for other cation-exchange resins with -SO 3H groups, n h = 2.5 and n m = 2.2). There is similar agreement between n h and m values for carboxyl ion-exchange resins with -COOH groups (Wofatit CP and Zerolit 226 H +), suggesting that a greater physical significance might be attributed to the n m value (parameter). Values of n m [determined for sulfonated polyethylene (PES membrane) from the BET isotherm] and n h (determined by the dielectric method) show the number of water molecules that are bound to the functional group in polyelectrolyte macrochains until the moment of its dissociation.