Fourier Transform Infrared Spectroscopy Study of Fully Hydrated Nafion Membrane of Various Cation Forms

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

Fig. 1. FT-IR spectra of the recast Nafion memebranes of H⁺ and alkai metal cation forms.