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

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 μm typical of “weakly bonded” water molecules are 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 investigating, even with optical techniques, the properties of confined water in an exceptionally wide temperature range.