WATER AND THE SILICA SURFACE AS STUDIED BY VARIABLE-TEMPERATURE HIGH-RESOLUTION H-1-NMR

The interaction of water with the silica surface is explored using H-1 NMR techniques. The silica is examined at various levels of hydration/dehydration and temperature, using line narrowing by magic-angle spinning, with or without multiple-pulse averaging of H-1-H-1 dipolar interactions. Water physisorbed on the silica surface exhibits a freezing point depression of roughly 40 K. Molecular reorientation of physisorbed water at rates in the tens of kilohertz range is observed through interference with multiple-pulse line narrowing. These results are correlated with variable-temperature H-1 NMR spectra of pure water. Isotropic H-1 chemical shifts for pure ice are also reported.