THE STATE OF WATER IN NAFION-117 OF VARIOUS CATION FORMS

The state of water in Nafion 117 membranes of various alkali and alkal ine earth metal cation forms was investigated by means of differential scanning calorimetry (DSC) and infrared spectroscopy (IR). Two kinds of water molecules existed in the membranes: freezing water molecules, which interact with the metal cations and ion exchange sites weakly a nd could freeze at around -20 degrees C, behaving in a manner close to but not identical to that of bulk water, nonfreezing water molecules which could not transfer to ice even at -120 degrees C, and are consid ered as both the ones binding strongly with the cations and the ion ex change sites and the ones exposed to fluorocarbon environment. It was found that the fraction of freezing water of the membranes decreased w ith decreasing the hydrophility of the metal cations in the membranes and that the number of freezing water molecules were almost the same a s that of the water molecules pumped by the cations during the cation transport. The water permeability of these membranes were measured usi ng the streaming potential method as well. The results revealed that t he water permeability increased with increasing water content of the m embrane, and that the membrane equilibrated with alkali metal cations possessed a higher water permeability compared with the membrane equil ibrated with alkaline earth metal cations with same water contents, Th e impedance measurement showed that the membrane conductivity seems to depend on both the hydrophilicity and the valence of the cations. All these results indicated that the hydrophilic domain in Nafion membran es is the determining factor for both cation and water transports in t he membranes.