Collective Intermolecular Motions of Water in Sodium Montmorillonite

By tracking temperature-induced changes in vibrational spectra, inelastic neutron scattering revealed how confined water governs the phase behavior of the sodium montmorillonite gels. Translational acoustic phonons hardened (shifted to higher energy), while librational modes softened, corresponding to an increased distortion and confinement of water with decreased water content. For a wet sample (∼166 wt % water content) with well-developed pore structure, vibrational modes underwent a phase change on heating from hexagonal ice-like spectral features to those of bulk-like liquid water near 270 K. For a drier sample hydrated to ∼25 wt %, in which water was confined to the interlayer, only amorphous ice was observed in the vibrational density of states spectra and only a minor phase change occurred below 255 K. Temperature-dependent chemical shifts in the energy transfer of translational and librational modes of ice in frozen sodium montmorillonite were found to be strongly dependent on the size of water-filled pores as influenced by the hydration state.