PNNL

Abstract

Confinement of water in sub-nanometer pores strongly alters its vibrational dynamics from that of bulk water. The effect of confinement can, furthermore, be finely tuned by small changes in the size and symmetry of the confining pore. Using inelastic neutron scattering (INS), we recently studied the dynamics of water confined in the channels of beryl and cordierite in which, at low temperatures, water shows similar intramolecular mode behavior, very soft intermolecular librational and translational bands, and neutron-recoil scattering at large momentum and energy transfer perpendicular to the channels. All these features indicate an absence of hydrogen bonds acting on the water molecule at low temperature and a shallow water potential. In addition, we observed multiple tunneling modes (between 0.66 and 14.7 meV) in the INS spectra of beryl, due to transitions between the split ground-state of the water protons. Here we present a study of: (i) the effect of pressure on the dynamics of water in beryl, (ii) the dynamics of water in beryl containing alkali metals (which results in changing the orientation of the water molecule in the crystal), and (iii) the dynamics of water in cordierite at low energies. We found a shift in the tunneling and vibrational modes of water in beryl to higher energies at 22 kbar relative to 1 bar. No tunneling modes were observed for water in cordierite and type-II water in beryl. Therefore, we conclude that very small differences in the size and structure of the pores and the orientation of the water molecule in these minerals result in changes in the potential of the water protons and drastic changes in the confined water dynamics.