A neutron scattering technique was developed to measure the density of heavy water confined in a nanoporous silica matrix in a temperature-pressure range, from 300 to 130 K and from 1 to 2,900 bars, where bulk water will crystalize. We observed a prominent hysteresis phenomenon in the measured density profiles between warming and cooling scans above 1,000 bars. We interpret this hysteresis phenomenon as support (although not a proof) of the hypothetical existence of a first-order liquid-liquid phase transition of water that would exist in the macroscopic system if crystallization could be avoided in the relevant phase region. Moreover, the density data we obtained for the confined heavy water under these conditions are valuable to large communities in biology and earth and planetary sciences interested in phenomena in which nanometer-sized water layers are involved.confined water | equation of state | liquid-liquid critical phenomenon I n many biological and geological systems, water resides in pores of nanoscopic dimensions, or close to hydrophilic or hydrophobic surfaces, comprising a layer of water, one or two molecules thick, with properties often different from the bulk. Such "confined" or "interfacial" water has attracted considerable attention, due to its fundamental importance in many processes, such as protein folding, concrete curing, corrosion, molecular and ionic transport, etc. (1-3). However, our understanding of the numerous physicochemical anomalies of confined water, and indeed of bulk water, is still incomplete. Basic gaps persist, among which the most interesting one is the origin of the unusual behavior of water in the supercooled region where water remains in the liquid state below the melting point (4-7). Recent studies have aimed at explaining anomalies such as the density maximum and minimum (8-10), and the apparent divergence of the thermodynamic response functions at 228 K at ambient pressure (11). The three major hypothesized scenarios currently under scrutiny are the "singularity-free (SF) scenario" (12, 13), the "liquidliquid critical point (LLCP) scenario" (14, 15), and the "critical point-free (CPF) scenario" (16). It is hypothesized, by all these three scenarios, that in the low temperature range bulk water is composed of a mixture of two structurally distinct liquids: the low-density liquid (LDL) and the high-density liquid (HDL). They are respectively the thermodynamic continuation of the low-density amorphous ice (LDA) and high-density amorphous ice (HDA) into the liquid state. Evidence of a first-order phase transition between LDA and HDA has been reported since 1985 (17-20). Subsequently, several experimental findings have been interpreted as support of the hypothetical existence of two different structural motifs of liquid water (21-27). However, some of the interpretations have been questioned (28,29). So far, direct evidence of a first-order liquid-liquid phase transition between LDL and HDL, as a thermodynamic extension of the first-order transition established in the am...

The single particle dynamics of water confined in a hydrophobically modified MCM-41-S sample has been studied using three high resolution quasielastic neutron scattering spectrometers in the temperature range from 300 to 210 K. A careful modeling of the dynamics allowed us to obtain good agreement among the results obtained with the three instruments, which have very different energy resolutions. The picture arising from the data is that, because of the heterogenous environment experienced by the water molecules, the dynamics show a broad distribution of relaxation times. However, the Fickian diffusive behavior is retained. In the investigated temperature range we found no evidence of the dynamic crossover, from a non-Arrhenius to an Arrhenius behavior, which was detected for water confined in hydrophilic MCM-41-S. This finding is in agreement with what was reported by Chu et al. [Phys. Rev. E 76, 021505 (2007)] for water confined in other hydrophobic confining media that the dynamic crossover takes place at a much lower temperature. The results reported in the paper help clarify the role that the chemical interaction between the water molecules and the walls of the confining host plays in determining the characteristics of the water dynamics, as compared to purely geometric constraints such as the size and shape of the pores.

The surface effect on the peculiar dynamic and thermodynamic properties of supercooled water, such as the density, has been puzzling the scientific community for years. Recently, using the small angle neutron scattering method, we were able to measure the density of H(2)O confined in the hydrophobic mesoporous material CMK-1-14 from room temperature down to the deeply supercooled temperature 130 K at ambient pressure. We found that the well-known density maximum of water is shifted 17 K lower and, more interestingly, that the previously observed density minimum in hydrophilic confinement disappears. Furthermore, the deduced thermal expansion coefficient shows a much broader peak spanning from 240 to 180 K in comparison with the sharp peak at 230 K in hydrophilic confinement. These present results may help in the understanding of the effect of hydrophobic/hydrophilic interfaces on the properties of supercooled confined water.