Hydration and interfacial water in nafion membrane probed by transmission infrared spectroscopy

Infrared spectroscopy was applied to probe water inside pores and channels of Nafion membrane exchanged with either proton (H⁺) or sodium ions (Na⁺). Transmission measurements were performed on freestanding Nafion 112 (∼50 μm thickness) in a cell that enabled adjustment of the relative humidity. Experiments that employed Na⁺-exchanged Nafion focused on relative humidity environments at or below about 32% generated through the use of humectants. Under these conditions, narrow features in the O-H stretching spectral region near 3650-3720 cm^-1, previously attributed to interfacial water, were detected and matched to bands in vibrational sum frequency (VSF) spectra of water/air, water/organic, and salt-solution/air interfaces. The features correspond to the stretching mode of the "free" OH group of water oriented with one hydrogen atom toward other water molecules and interacting through hydrogen bonding and the other straddling the interface extending into fluorocarbon-rich regions (∼3668 cm^-1) or air-filled segments (∼3700 cm^-1) in the membrane. For membrane exchanged with H⁺, -SO₃^- groups were easily shifted to -SO₃H as water was removed upon exposure to a few Torr of vacuum at 95°C. In contrast, residual water was retained by membrane exchanged with Na⁺ after exposure to these conditions for up to 72 h. The permeation of methanol and acetone into Na ⁺-exchanged Nafion 112 was also examined. The C-H and O-H stretching modes of methanol were perturbed in a manner that suggests the polymer disrupts hydrogen bonding interactions within the solvent, similar to the effect it exerts on pure water. For acetone, the C-H stretching modes were not shifted appreciably compared to those of the bulk liquid. However, the carbonyl band was affected, indicating the likely importance of dipolar interactions between solvent molecules and polar groups on the polymer. Control experiments performed with poly(hexafluoropropylene-co-tetrafluoroethylene) (FEP) membrane did not show evidence for water or methanol permeation, which demonstrates the critical role played by the ion-filled channels and pores in facilitating solvent transport within Nafion membrane.