Dynamics of supercooled confined water measured by deep inelastic neutron scattering

In this paper, we present the results of deep inelastic neutron scattering （DINS） measurements on supercooled water confined within the pores （average pore diameter - 20A） 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 semi-empirical 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.

Kinetic energy and radial momentum distribution of hydrogen and oxygen atoms of water confined in silica hydrogel in the temperature interval 170-325 K

Water is an ubiquitous liquid and it is necessary for life;studies on water are therefore of obvious scientific and technological relevance.In view of its peculiar physical properties(the so-called water anomalies,particularly relevant at low temperatures[1]),studies on water structure and dynamics in ample temperature intervals,covering also the supercooling region,have attracted much interest in recent years.In particular,studies focused on the supercooled phase are important in order to test theories and hypotheses[2,3],including the liquid-liquid phase transition hypothesis[4-6]and the related fragile-to-strong crossover observed in water confined in silica matrices and in the hydration water of proteins[7,8].In this context,water confined within nanometer-sized porous hydrophilic/hydrophobic matrices has been investigated both to extend the supercooling temperature range accessible to experiment and to mimic the crowding/confined conditions experienced by water molecules in biological systems relevant to biophysics,bio-preservation,and pharmaceutics.In view of the above arguments,studies on the short-time dynamics of hydrogen and oxygen atoms of supercooled water(bulk or confined)are of great relevance.

The onset of the tetrabonded structure in liquid water

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.