FOURIER-TRANSFORM INFRARED-SPECTROSCOPY STUDY OF FULLY HYDRATED NAFION MEMBRANES OF VARIOUS CATION FORMS

The state of water in fully hydrated Nafion membranes of various alkal i, alkaline earth metal cation and organic aliphatic ammonium cation f orms was investigated by means of fourier transform infrared spectrosc opy (FT-IR). It was found that in addition to the water of hydration t o cations and to ionogenic sites, existence of water molecules which h ave an OH group (or occasionally both OH groups) exposed to the hydrop hobic domain (i.e., fluorocarbon environment) in the membrane was also confirmed and the fraction increased with decreasing hydrophilicity o f the cations which are equilibrated with the membranes. Furthermore, in almost all the cases, as the hydrophilicity of the cations increase d the absorption band of the water molecules arising from both OH stre tching mode and OH deformation or bending mode shifted toward the dire ction which corresponds to an enhancement in hydrogen bonding. This in dicated that most of the water molecules in the membrane located in th e hydration shell of the cations and sulfonate groups. The sequence of interactions between the cations and sulfonate groups in the membrane s appeared to be: Li>Na>K>Rb>Cs for the alkali metal and Ca>Sr>Ba>Mg f or the alkaline earth metal cation form membranes, respectively.