The temperature dependence of the density of water, ρ( T ), is obtained by means of optical scattering data, Raman and Fourier transform infrared, in a very wide temperature range, 30 < T < 373 K. This interval covers three regions: the thermodynamically stable liquid phase, the metastable supercooled phase, and the low-density amorphous solid phase, at very low T . From analyses of the profile of the OH stretching spectra, we determine the fractional weight of the two main spectral components characterized by two different local hydrogen bond structures. They are, as predicted by the liquid–liquid phase transition hypothesis of liquid water, the low- and the high-density liquid phases. We evaluate contributions to the density of these two phases and thus are able to calculate the absolute density of water as a function of T . We observe in ρ( T ) a complex thermal behavior characterized not only by the well known maximum in the stable liquid phase at T = 277 K, but also by a well defined minimum in the deeply supercooled region at 203 ± 5 K, in agreement with suggestions from molecular dynamics simulations.

It is becoming common practice to partition glass-forming liquids into two classes based on the dependence of the shear viscosity η on temperature T . In an Arrhenius plot, ln η vs 1/ T , a strong liquid shows linear behavior whereas a fragile liquid exhibits an upward curvature [super-Arrhenius (SA) behavior], a situation customarily described by using the Vogel–Fulcher–Tammann law. Here we analyze existing data of the transport coefficients of 84 glass-forming liquids. We show the data are consistent, on decreasing temperature, with the onset of a well-defined dynamical crossover η _× , where η _× has the same value, η _× ≈ 10 ³ Poise, for all 84 liquids. The crossover temperature, T _× , located well above the calorimetric glass transition temperature T _g , marks significant variations in the system thermodynamics, evidenced by the change of the SA-like T dependence above T _× to Arrhenius behavior below T _× . We also show that below T _× the familiar Stokes–Einstein relation D / T ∼ η ^-1 breaks down and is replaced by a fractional form D / T ∼ η ^{- ζ} , with ζ ≈ 0.85.

Results of Raman scattering measurements performed on aqueous solutions of the homologous disaccharides (trehalose, maltose, and sucrose) are reported. To get some insight into the effects of disaccharides on the hydrogen bond network of water, and to clarify the reasons that make trehalose the most effective in protecting organisms from dehydration and freezing, we investigate the intramolecular OH stretching mode. To carry out this study, two different approaches are employed: namely, a decomposition of the isotropic spectra into an “open” and a “closed” contribution, and a spectral stripping procedure to extract the “collective” contribution from the polarized spectra. Both procedures agree in suggesting that disaccharides promote, with a different strength, a destructuring effect on the tetrahedral H-bond network of pure water. This result makes plausible the hypothesis that disaccharides obstruct the crystallization process reducing the amount of freezable water, namely destroying the network of water compatible with that of ice. What conclusively emerges from this Raman scattering study is that the greater bioprotective action of trehalose on biological structures is to be connected with its greater destructuring effect on the tetrahedral H-bond network of water.