DYNAMICS OF SUPERCOOLED WATER STUDIED BY NEUTRON SCATTERING

The Journal of Chemical Physics, 2013

The OH stretching vibrational spectrum of water was measured in a wide range of temperatures across the triple point, 269 K <T < 296 K, using Inelastic Neutron Scattering (INS). The hydrogen projected density of states and the proton mean kinetic energy, E K OH , were determined for the first time within the framework of a harmonic description of the proton dynamics. We found that in the liquid the value of E K OH is nearly constant as a function of T, indicating that quantum effects on the OH stretching frequency are weakly dependent on temperature. In the case of ice, ab initio electronic structure calculations, using non-local van der Waals functionals, provided E K OH values in agreement with INS experiments. We also found that the ratio of the stretching ( E K OH ) to the total ( E K exp ) kinetic energy, obtained from the present measurements, increases in going from ice, where hydrogen bonding is the strongest, to the liquid at ambient conditions and then to the vapour phase, where hydrogen bonding is the weakest. The same ratio was also derived from the combination of previous deep inelastic neutron scattering data, which does not rely upon the harmonic approximation, and the present measurements. We found that the ratio of stretching to the total kinetic energy shows a minimum in the metastable liquid phase. This finding suggests that the strength of intermolecular interactions increases in the supercooled phase, with respect to that in ice, contrary to the accepted view that supercooled water exhibits weaker hydrogen bonding than ice. © 2013 AIP Publishing LLC. [http://dx.

Frontiers of Physics, 2017

In this paper, we present the results of deep inelastic neutron scattering (DINS) measurements on supercooled water confined within the pores (average pore diameter ∼ 20 Å) of a disordered hydrophilic silica matrix obtained through hydrolysis and polycondensation of the alkoxide precursor Tetra-Methyl-Ortho-Silicate via the sol-gel method. Experiments were performed at two temperatures (250 K and 210 K, i.e., before and after the putative liquid-liquid transition of supercooled confined water) on a "wet" sample with hydration h ∼ 40% w/w, which is high enough to have water-filled pores but low enough to avoid water crystallization. A virtually "dry" sample at h ∼ 7% was also investigated to measure the contribution of the silica matrix to the neutron scattering signal. As is well known, DINS measurements allow the determination of the mean kinetic energy and the momentum distribution of the hydrogen atoms in the system and therefore, allow researchers to probe the local structure of supercooled confined water. The main result obtained is that at 210 K the hydrogen mean kinetic energy is equal or even slightly higher than at 250 K. This is at odds with the predictions of a semiempirical harmonic model recently proposed to describe the temperature dependence of the kinetic energy of hydrogen in water. This is a new and very interesting result, which suggests that at 210 K, the water hydrogens experience a stiffer intermolecular potential than at 250 K. This is in agreement with the liquid-liquid transition hypothesis.

The Journal of Chemical Physics, 1996

A detailed study of the single-particle dynamics of liquid water in normal and supercooled regime has been carried out by comparing molecular dynamics ͑MD͒ simulation results with now available high resolution quasielastic neutron scattering ͑QENS͒ data. Simulation runs have been performed at 264, 280, 292, and 305 K, using the extended simple point charge model, well suited for reproducing single-particle properties of H 2 O. The microscopic dynamics has been probed over a wide range of times and distances. The MD results indicate that a substantial coupling between translational and rotational dynamics exists already at about 1 ps. The decay of the translational dynamic correlations has been phenomenologically analyzed in terms of three exponential components, and the agreement between the parameters thus obtained from experimental and simulation derived datasets is quite satisfactory. Both QENS and MD data can not be described with sufficient accuracy by simple diffusion models over the entire range of examined wave vectors.

Journal of Molecular Liquids, 2007

Neutron diffraction measurements with isotopic substitution (NDIS) and Deep In-elastic Neutron Scattering (DINS) experiments have been performed on bulk liquid water in the supercooled regime. Supercooling of ultra-pure water has been obtained thanks to a PTFE coating of the sample container. From the structural point of view the effect of supercooling at ambient pressure results in a slight change of the water coordination shells with respect to the structure of ambient water, although similarities with Low Density Water can be observed. As a matter of fact, the present data compare well with previous results obtained at about the same temperature, applying external pressure and can be interpreted within the second critical point scenario. DINS measurements have been carried out on this system with the aim of determining the anharmonic character of the momentum distribution of the protons, complementing the structural information on supercooled bulk water.

Brazilian Journal of Physics, 2009

A strong temperature dependence of proton mean kinetic energy was observed for liquid water around the density maximum and for moderately supercooled water. Line shape analysis of proton momentum distribution, determined from deep inelastic neutron scattering measurements, shows that there are two proton kinetic energy maxima, one at the same temperature of the macroscopic density maximum at 277 K, and another one in the supercooled phase located around 270 K. The maximum at 277 K is a microscopic quantum counterpart of the macroscopic density maximum, where energetic balance giving rise to the local water structure is manifest in the temperature dependence of kinetic energy. The maximum in the supercooled phase, with higher kinetic energy with respect to stable phases, is associated to changes in the proton potential as the structure evolves with a large number of H-bond units providing both stronger effective proton localization, as well as proton quantum delocalization.

We study hydrogen-bond dynamics in liquid water at low temperatures using molecular dynamics simulations, and find results supporting the hypothesized continuity of dynamic functions between the liquid and glassy states of water. We find that average bond lifetime ͑ϳ1 ps͒ has Arrhenius temperature dependence. We also calculate the bond correlation function decay time ͑ϳ1 ns͒ and find powerlaw behavior consistent with the predictions of the mode-coupling theory, suggesting that the slow dynamics of hydrogen bonds can be explained in the same framework as standard transport quantities.

Journal of Molecular Structure, 1991

This paper reviews the more recent results obtained on the dynamics of water by neutron scattering and shows that some information can be obtained by this technique at the microscopic level of the hydrogen bond. It also accounts for some very recent results obtained with the hydrated protein C-phycocyanin.

Journal of Non-Crystalline Solids, 2015

Here we discuss the structure of water in terms of a temperature-dependent balance between two classes of hydrogen-bonded structures. At high and down to mildly supercooled temperatures most molecules favor a closer packing than tetrahedral, with strongly distorted hydrogen bonds. This allows the quantized librational modes to be excited and contribute to the entropy while the loss of enthalpy due to breaking hydrogen bonds is compensated by enhanced van der Waals interactions. Tetrahedral hydrogen bonding is of lower enthalpy resulting in tetrahedrally bonded water patches appearing, but only as fluctuations with size and life-time increasing at lower temperatures. Measurements of the structure at deeply supercooled conditions show a continuous increase in tetrahedrality which becomes accelerated below the temperature of homogeneous ice nucleation. The two local structures are connected to the liquid-liquid critical point (LLCP) hypothesis in supercooled water and correspond to high density liquid (HDL) and low density liquid (LDL). We propose that both HDL and LDL behave as normal liquids and that the anomalous properties of water result from the transition between them, which occurs over a wide temperature range at ambient pressure. The key issue is the competition between incompatible conditions for maximizing the entropy, favored in HDL, and minimizing the enthalpy, favored in LDL, which leads to the instability in the liquid and is the fundamental origin of the proposed LLCP.

Neutron Compton Scattering measurements presented here of the momentum distribution of hydrogen in water at temperatures slightly below freezing to the supercritical phase show a dramatic change in the distribution as the hydrogen bond network becomes more disordered. Within a single particle interpretation, the proton moves from an essentially harmonic well in ice to a slightly anharmonic well in room temperature water, to a deeply anharmonic potential in the supercritical phase that is best described by a double well potential with a separation of the wells along the bond axis of about .3 Angstroms. Confining the supercritical water in the interstices of a C60 powder enhances this anharmonicity. The changes in the distribution are consistent with gas phase formation at the hydrophobic boundaries.

Science China Physics, Mechanics & Astronomy

Water properties are dominated by the hydrogen bond interaction that gives rise in the stable liquid phase to the formation of a dynamical network. The latter drives the water thermodynamics and is at the origin of its well known anomalies. The HB structural geometry and its changes remain uncertain and still are challenging research subjects. A key question is the role and effects of the HB tetrahedral structure on the local arrangement of neighboring molecules in water. Here the hydrogen dynamics in bulk water is studied through the combined use of Neutron Compton Scattering and NMR techniques. Results are discussed in the framework of previous studies performed in a wide temperature range, in the liquid, solid, and amorphous states. For the first time this combined studies provide an experimental evidence of the onset of the water tetrahedral network at T∼315 K, originally proposed in previous studies of transport coefficients and thermodynamical data; below this temperature the local order in water changes and the lifetime of local hydrogen bond network becomes long enough to gradually develop the characteristic tetrahedral network of water. water, neutron compton scattering, NMR