Effect of aging on the water sorption and ion exchange studies on Nafion and Dowex resins: Transition metal ions-proton exchange systems

Transition metal ion-H+ exchanges on Nafion 117 membrane (pre-treated differently prior to use), Dowex 50W x 4 and Dowex 50W x 8 resins have been studied. The water sorption isotherms of these exchangers in different ionic forms generated over the entire range of water activity have been analyzed by the D'Arcy and Watt equation (DWE). The results show that the physical structure of the exchangers change due to long-storage or aging, and the amount of water sorbed becomes low. As a result, the counter-ions in the exchangers are not hydrated, resulting in discrepancies in the ion exchange studies. In the absence of any hydration of the transition metal ions in the resin phase, the driving force for the ion exchange reaction is the electrostatic interaction between the sulphonato group and transition metal ions. Pre-treatment of Nation 117 membrane with boiling acid/water modifies the membrane structure and arrangement of the ionogenic groups. These modifications influence the state of water present in the Nation H 7 membrane and the ion exchange equilibria. The results obtained in the present study indicate that the ion exchangers lose their elasticity or swelling characteristics and therefore show poor water sorption characteristics. (C) 2006 Published by Elsevier B.V.