Infrared spectroscopic study of the effect of electrostatic interaction on the molecular arrangement in the hydration shell around Li+ in a nafion membrane.

In the present paper we characterize the hydration shell around a Li ion, as -SO(3)(-)Li(+), from a series of infrared spectra of the hydrated Li salt in a sample of Nafion. The infrared spectrum significantly changes through prolonged dehydration under a controlled flow of drying gas, to a limiting dryness level. We found that the Nafion membrane contains a small population of the SO(3)(-)Li(+) group isolated in a hydrophobic matrix (denoted A) in addition to the main component of the clustered group (B). The second hydration shell around A evaporates at moderate levels of dehydration, followed by partial evaporation of the first hydration shell during the final stage. In contrast, in the hydration shell around B, which occupies a hydrophilic environment, the innermost layer of the second shell still remains unevaporated under full dehydration conditions. From the hydration shell around A, we were able to resolve the distinct fundamental bands nu(3), nu(1), and nu(2) of the water that is coordinated to Li(+) in the first hydration shell. From analysis of the infrared spectrum of the hydration shell around B, we found that the strong electrostatic field of Li(+) brings about a molecular arrangement that is less favorable for intermolecular hydrogen-bonding in the inner second hydration shell. This structural effect occurs to a minor extent for the second hydration shell around a Na ion, which has a significantly weaker electrostatic field.