Near-Infrared Spectra of Water Confined in Silica Hydrogels in the Temperature Interval 365−5 K

Abstract

We have used a sol−gel technique to obtain optically transparent hydrogels in which water is trapped within a tridimensional disordered silica matrix. A suitable aging of these hydrogels enables to have transparent noncracking samples down to cryogenic temperatures. We report the optical absorption spectra, in the near-infrared region, of water trapped in our silica hydrogels, measured in the temperature range 365−5 K, and we compare them with the same spectra of liquid water, measured in the temperature range 365−263 K. The data show that it is possible to have noncrystallizing water even at 5 K: indeed, the overtone bands at ∼1.41 μm and at ∼1.155 μmtypical of “weakly bonded” water moleculesare still detected at liquid helium temperature. Deconvolution of the ∼1.45 μm water band in terms of Gaussian components enables to have a van't Hoff plot for the “weakly bonded” component which, contrary to liquid water, does not show any critical behavior at 228 K. The reported data open the possibility of invest...