Water-ionomer interfacial interactions investigated by infrared spectroscopy and computational methods

Structures for interfacial water condensed in pores and channels of the fluorinated ionomer Nafion from low relative humidity atmosphere were probed through the use of Fourier transform infrared (FTIR) spectroscopy and support from classical and quantum chemical calculations. Modern FTIR spectra of H2O and the O-H stretching region for the deuterium-substituted HOD species interacting at the water-ionomer interface in Nafion exchanged by sodium cations are reported and compared to characteristics observed in the earlier studies that employed a dispersive infrared spectrometer and unspecified spectral resolution. Molecular simulations that examine the orientations of water molecules in the vicinity of ionomer were applied to understand the appearance of multiple free O-H stretching bands and the effect of HOD addition. One computational approach was based on a classical force field model, and the other employed density functional theory (DFT) to investigate atomic-scale interactions of water with regions of different hydrophobicity and charge on a perfluorosulfonate ionomer segment. The results suggest hydrogen bonding stabilizes the types of water-ionomer environments that can lead to multiple free O-H stretching vibrational features in experimental spectra. The studies shed light on the structure of H2O at interfaces inside ion conducting membrane materials and have potential for application in elucidating structure at different types of water interfaces.