Dynamical Behaviour of H₂O in Silica Gel

The vibrational spectra of silica gel with low concentrations of water, 3% and 10%, were studied with inelastic neutron scattering at 5 K. The experimental spectrum for 3% water in silica gel shows dramatic changes compared to the spectra for the other known ice phases in the energy transfer region of 2-140 meV. Its intermolecular librational band is shifted to lower energy by about 20 meV (or by a factor 1.43) compared to ice Ih, and shows a similar librational band with liquid at 290K. A significant excess of low frequency vibration modes (< 7meV) over normal ice Ih and very wide acoustic peak (4-15 meV) were observed. Experimental data indicate that little H₂O (3%) in silica gel exhibits a strong softening of hydrogen bonds compared to bulk normal ice and all the other known ice phases.