On the orientational mobility of water molecules in proton and sodium terminated nafion membranes

We study the dielectric properties of proton and sodium terminated Nafion at varying degrees of hydration with a frequency-domain coaxial-line reflectometric method (1.5–20 GHz) and with time-domain THz-pulse spectroscopy (0.7–1.5 THz). The water content is varied from 1 to 12 water molecules per Nafion polymer unit. At low hydration levels, we observe that the orientational mobility of the water molecules embedded in the Nafion membrane channels is strongly slowed down in comparison to that of bulk water. Only at the highest hydration level does the rotational mobility of the water molecules become similar to that of bulk water. We also observe that the mobilities of protons and sodium ions strongly depend on the degree of hydration of the Nafion membranes and that the mobility of the proton is more strongly decreased than that of sodium upon lowering the hydration level.