The temperature dependence of the density of water, (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 highdensity 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 (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.infrared and Raman scattering ͉ liquid-liquid phase transition ͉ supercooled and amorphous water ͉ Widom line in water U nderstanding the fundamental role that water plays on Earth and in all aspects of life phenomena represents one of the most challenging research problems in science and technology. In comparison with other simple molecular liquids, the thermodynamic properties of water (H 2 O) are characterized by a counterintuitive trend as temperature is lowered: examples are the isothermal compressibility, the isobaric heat capacity, the isobaric expansivity, and the density (1-3). The latter quantity, as is well known, exhibits a maximum at 277 K. Such a maximum is the only one occurring in liquids in their stable liquid phases just above the melting point. Over the years, many plausible explanations for these unusual behaviors have been proposed. In all of these explanations, the anomalies of water are invariably attributed to the role played by the hydrogen bond (HB) formation between water molecules (1). More precisely, the formation of HBs governs the overall structure and dynamics (1) of water, giving rise to, on decreasing T, a clustering process from which an open tetrahedrally coordinated HB network around each water molecule is gradually developed. It is such an increase in the HB structure that expands the liquid, compensating for the normal tendency of a liquid to contract as it is cooled. This finding is the basic reason for the occurrence of the density maximum phenomenon at 277 K (1-3).Among the many theoretical approaches (4-6) developed to explain water properties in a supercooled state, there is the liquid-liquid phase transition (LLPT) hypothesis (6), which has received the most substantial support from various theoretical (7-10) and experimental studies (11,12). For the LLPT model of water, the liquid state of water above the critical point is a mixture of two different local structures, ch...

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 glassforming 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. dynamical arrest | dynamic transition | supercooled liquids

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.

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We report viscosity and compressibility measurements of trehalose, maltose, and sucrose aqueous solutions at different concentration and temperature values. What emerges from the concentration dependence of viscosity and compressibility is that trehalose, in comparison to maltose and sucrose, shows a higher interaction strength with water, which gives rise to a greater value of the hydration number throughout the investigated temperature range. Furthermore, viscosity measurements reveal that at high concentration values, trehalose shows a "stronger" kinetic character than the other disaccharides, namely, a lower structural sensitivity to temperature changes in the investigated temperature range. This result could explain the greater cryptobiotic attitude of trehalose at high concentrations. The present work, which consists of quantifying experimentally the basic hydration behavior and solution structure of the investigated disaccharides as a function of concentration and temperature, allows the working hypothesis of the several existing simulation approaches to be tested.