An infrared study of water-ion interactions in perfluorosulfonate (Nafion) membranes

We have studied the infrared spectra of Nafion membranes in twenty different cationic forms, in order to characterize the water-anion-cation interactions at low water contents. Spectra in the OH stretching region and in the HOH bending region allow the distinction between hydrogen-bonded and non-hydrogen-bonded water molecules, and yield information on the mode of attachment of water molecules to the anion-cation pairs. When there is a good match between the Lewis acid strength of the cation and the Lewis base strength of the anion, ion pairs are relatively stable and water molecules tend to attach on the outside. When the acid strength of the cation and the base strength of the anion are mismatched, as is the case for small cations of high charge or for large cations of low charge, water molecules tend to be inserted between the ion pairs even at the lowest water contents.