Quantum effects in the dynamics of deeply supercooled water

2004

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.

Nature, 2000

As with most liquids, it is possible to supercool water; this generally involves cooling the liquid below its melting temperature (avoiding crystallization) until it eventually forms a glass. The viscosity and related relaxation times (tau) of glass-forming liquids typically show non-Arrhenius temperature (T) dependencies. Liquids with highly non-Arrhenius behaviour in the supercooled region are termed 'fragile'. In contrast, liquids whose behaviour is close to the Arrhenius law (In tau infinity 1/T) are termed 'strong'. A unique 'fragile-strong' transition around 228 K has been proposed for supercooled water; however, experimental studies of bulk supercooled water in this temperature range are generally hampered because crystallization occurs. Here we use broad-band dielectric spectroscopy to study the relaxation dynamics of supercooled water in a wide temperature range, including the usually inaccessible temperature region. This is possible because the supe...

Physical Review E, 1999

We have made a high-resolution quasielastic incoherent neutron scattering ͑QENS͒ study of the translational dynamics of supercooled water contained in micropores of Vycor glass at different hydration levels. QENS spectra from the confined H 2 O are analyzed in terms of the ␣-relaxation dynamics predicted by mode-coupling theory of supercooled liquids and by a recent computer molecular-dynamics simulation of extended simple point charge model water. We verify that the stretched exponential relaxation description of the long-time test-particle dynamics is consistent with the measured QENS spectral line shape. We are thus able to determine the wave-number dependence of magnitudes of the structural relaxation rate 1/ and the stretch exponent ␤ as functions of temperature and coverage. A power-law dependence of the average relaxation time on the magnitude of the scattering vector Q is observed. In the Q range studied, the exponent starts out with nearly Ϫ2.0, at room temperature, indicating a continuous diffusion, and gradually becomes less negative as the temperature is decreased to below the freezing temperature.

Physical Review Letters, 2000

Intermediate scattering functions for density fluctuation in D 2 O contained in pores of a Vycor glass have been measured using an improved neutron spin-echo spectrometer at two supercooled temperatures. The measurements cover the time range from 1 to 2300 ps with the Q range spanning the first diffraction peak of water. The time correlation functions can be fitted to a stretched exponential relaxation function with a Q-dependent amplitude. Both the stretch exponent and the relaxation time peak approximately at the Q value corresponding to the first diffraction peak, confirming the validity of the mode coupling idea in supercooled water.

Physical Review Letters, 2004

Many of water's peculiar physical properties are still not well understood, and one of the most important unresolved questions is its glass transition related dynamics. The consensus has been to accept a glass transition temperature (T g ) around 136 K, but this value has been questioned and reassigned to about 165 K. We find evidence that the dielectric relaxation process of confined water that has been associated with the long accepted T g of water (130 -140 K) must be a local process which is not related to the actual glass transition. Rather, our data indicate a glass transition at 160 -165 K for bulk water and about 175 K for confined water (depending on the confining system).

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.

Journal of the American Chemical …, 2005

Quantum and classical simulations are carried out on model water systems over a wide range of temperatures, from 100 to-35°C. A detailed examination of the equilibrium and dynamical properties of liquid water is presented, together with a discussion of the interplay between quantum mechanical tunneling and dynamics. The study shows that quantum effects are essential for a description of the dynamical behavior of liquid water, particularly in the low-temperature (supercooled) region. The similarities and differences between quantum effects and the effects associated with increasing the temperature are explicitly characterized.

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 .

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.