Relaxational dynamics of supercooled water in porous glass

Philosophical Magazine B

An inelastic neutron scattering experiment on water con®ned in Vycor glass has been performed to test the behaviour of the hydrogen dynamics against the predictions of the mode coupling theory, in the b-relaxation region.

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.

An inelastic neutron scattering experiment on water con®ned in Vycor glass has been performed to test the behaviour of the hydrogen dynamics against the predictions of the mode coupling theory, in the b-relaxation region.

Le Journal de Physique Colloques, 1984

Nous présentons des r é s u l t a t s d'expériences de d i f f u s i o n incohér e n t e de neutrons, quasi-élastique e t i n é l a s t i q u e , par de l ' e a u en phase surfondue. L'analyse du spectre quasi-élastique permet de déterminer deux temps c a r a c t é r i s t i q u e s e t l e u r dépendance en température. La d i f f u s i o n e s t expliquée par l e modèle de saut e t un mécanisme de r u p t u r e de l a l i a i s o n hydrogène e s t proposé. Le spectre i n é l a s t i q u e e s t étendu jusqu'à 600 meV montrant, pour l a première f o i s , l a r a i e due aux v i b r a t i o n s intramoléculaires de s t r e t c h i n g . Abstract -Incoherent q u a s i -e l a s t i c and i n e l a s t i c neutron s c a t t e r i n g by water was performed i n a temperature range extending t o t h e supercooled s t a t e . The analysis o f the q u a s i -e l a s t i c snectrum separates two main components and gives two c h a r a c t e r i s t i c times. T h e i r temperature analysis j u s t if i e s t h e use o f t h e Jump D i f f u s i o n mode1 and suggests a mechanism f o r t h e hydrogen bond breaking. The i n e l a s t i c spectra extend u n t i l 600 meV, i .e. covering the intramolecular v i b r a t i o n r e g i o n showing, f o r t h e f i r s t time, the s t r e t c h i n g band.

2004

Journal of Physics: Condensed Matter, 2006

We report a set of dynamical data of confined water measured in a very deeply supercooled regime (290-190 K). Water is contained in silica matrices (MCM-41-S) which consist of 1D cylindrical pores with diameters d = 14, 18 and 24Å. When confined in these tubular pores, water does not crystallize, and can be supercooled well below 200 K. We use the NMR technique to obtain the characteristic proton relaxation time-constants (the spin-lattice relaxation time-constant T1 and the spin-spin relaxation time-constant T2) and a direct measurement of the self-diffusion coefficient in the whole temperature range. We give evidence of the existence of a fragile-to-strong dynamic crossover (FSC) at T L = 225 K from the temperature dependence of the self-diffusion coefficient. A combination of the NMR self-diffusion coefficient with the average translational relaxation time, as measured by quasi-elastic neutron scattering, shows a well defined decoupling of transport coefficients, i.e. the breakdown of the Stokes-Einstein relation, on approaching the crossover temperature T L .

Entropy, 2017

We review our simulation results on properties of supercooled confined water. We consider two situations: water confined in a hydrophilic pore that mimics an MCM-41 environment and water at interface with a protein. The behavior upon cooling of the α relaxation of water in both environments is well interpreted in terms of the Mode Coupling Theory of glassy dynamics. Moreover, we find a crossover from a fragile to a strong regime. We relate this crossover to the crossing of the Widom line emanating from the liquid-liquid critical point, and in confinement we connect this crossover also to a crossover of the two body excess entropy of water upon cooling. Hydration water exhibits a second, distinctly slower relaxation caused by its dynamical coupling with the protein. The crossover upon cooling of this long relaxation is related to the protein dynamics.

Physica A: Statistical Mechanics and its Applications, 2002

Studies of single particle dynamics of water conÿned in silica pores performed with computer molecular dynamics and inelastic neutron scattering are presented. In the computer study for the highest hydrations two dynamical regimes are found: close to the hydrophilic substrate molecules are below the mode coupling crossover temperature, TC , already at ambient temperature (bound water). The water closer to the center of the pore (free water) approaches TC upon supercooling as predicted by mode coupling theories. Inelastic neutron scattering data are analyzed upon supercooling for hydration levels of 12% and 8%. Also these data are discussed in the framework of the mode coupling theory, in the region of the ÿ relaxation. Strong deviations from the theoretical predictions are found and ascribed to the existence of a low-frequency scattering excess also visible in the simulation.

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.

Physical Review Letters, 2006

We show that the viscosity related main (a) relaxation of confined water vanish at a temperature where the volume required for the cooperative a relaxation becomes larger than the size of the geometrically confined water cluster. This occurs typically around 200 K, implying that above this temperature we observe a merged a-b relaxation, whereas below it only a local (b) relaxation remains. This also means that such confined supercooled water does not exhibit any true glass transition, in contrast to other liquids in similar confinements. Furthermore, it implies that deeply supercooled water in biological systems, such as membranes and proteins, generally shows only a local b relaxation, a finding of importance for low temperature properties of biological materials.