A neutron spin-echo study of confined water

We report neutron spin-echo (NSE) results of essentially two-dimensional water confined in a fully hydrated Navermiculite clay. The two layers of water molecules exhibit a broad distribution of relaxation times with a pronounced non-Arrhenius temperature dependence, typical for so-called fragile liquids, in the range 254-323 K. This high temperature behaviour is in contrast to an earlier dielectric study, where an Arrhenius dependence was observed for T < 215 K. The apparently contradictory NSE and dielectric results can be explained in terms of a 'fragile-strong' transition somewhere in the temperature range 215-254 K.

The dynamics of supercooled confined water has recently been shown to have a pronounced, apparent fragile-to-strong transition (FST). Here we use broadband dielectric spectroscopy (10 −2 -10 9 Hz) to study the dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21.4 and 36.1Å, respectively. The local dynamics of water molecules and the dynamics of the hydroxyl groups on the inner wall of the pores are followed up to over 240 K. We argue that the reported FST for confined water is due to the vanishing of the cooperative α relaxation, which implies that it should not be interpreted as a true FST.

For a number of decades, the anomalous behavior of water, e.g., the increase of density upon heating and the increase of diffusivity upon compression, has been the subject of intense research. 1À7 More than 80 anomalies of water have been discovered in experiments. Some of these anomalies concern static thermodynamic properties, e.g., the increase of compressibility and specific heat when the temperature is decreased, and others concern dynamic properties, e.g., the breakdown of the StokesÀEinstein relation 8,9 and the non-Arrhenius to Arrhenius dynamic crossover at low temperatures. 9À11 To explain the anomalous behavior of water, the existence of a liquidÀliquid (LL) phase transition has been proposed, 12,13 but this hypothesized LL transition lies in a region of the pressureÀ temperature phase diagram inaccessible to experimentation on bulk water due to crystallization of the liquid within experimental time scales. Fortunately, the crystallization within this temperature region occurs at microsecond time scales, but the density relaxation of liquid is in the range of tens of nanoseconds ( . Thus, because of this rapid relaxation time, it is possible to study the metastable equilibrium behavior of liquid water at low temperatures using computer simulation. 12,14À16 Recent neutron scattering studies of liquid water by the groups of S.-H. Chen and F. Mallamace 8,17À20 support the possible existence of a first-order phase transition in liquid water that ends at a liquidÀliquid critical point. 9,10,21 By confining water in MCM-41, a matrix of hexagonal silica pores, Chen and his colleagues were able to measure the density relaxation time of confined water at temperatures as low as T = 180 K. In their experiments, they found that water undergoes a dynamic crossover at T ≈ 225 K, where the dynamics change from non-Arrhenius at high temperatures to Arrhenius at low temperatures. Subsequent studies of different molecular and coarse-grained models suggest that the observed dynamic crossover occurs when the line of maximum correlation length is crossed due to decreasing temperatures. Specifically, it was shown that, as the temperature is decreased at a constant

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.

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.

Physical Review Letters

In this work we investigate, by means of Elastic Neutron Scattering (ENS), the pressure dependence of Mean Square Displacements (MSD) of hydrogen atoms of deeply cooled water confined in the pores of a 3-dimensional disordered SiO2 xerogel; experiments have been performed at 250 and 210K from atmospheric pressure to 1200 bars. The ”pressure anomaly” of supercooled water (i.e. an MSD increase with increasing pressure) is observed in our sample at both temperatures; however, contrary to previous simulation results and to the experimental trend observed in bulk water, the pressure effect is smaller at lower (210K) than at higher (250K) temperature. ENS results are complemented by differential scanning calorimetry data that put in evidence, besides the glass transition at about 170K, a first order-like endothermic transition occurring at about 230K that, in view of the neutron scattering results, can be attributed to a liquid-liquid crossover. Our results give experimental evidence for ...

The European Physical Journal E - Soft Matter, 2003

In many relevant situations, water is not in its bulk form but is instead attached to some substrates or filling some cavities. We shall call water in the latter environment "confined or interfacial water" as opposed to bulk water. This confined water is essential for the stability and function of biological macromolecules. In this review paper, we present the more recent up to date account of the dynamics of confined water as compared with that of bulk water. Various techniques are used to study the dynamics of confined water. Among them, quasi-elastic and inelastic neutron scattering is a powerful tool to study translational and rotational diffusion as well as vibrational density of states of confined water. Various examples involving water confined in porous media, adsorbed on surface of ionic crystals, in the presence of organic solutes and at the surface of biological molecules are presented. The combined effects of the hydration level and the temperature on the retardation of the water molecules motions are discussed on the basis of phenomenological models as well as of power law fits based on the Mode Coupling Theory.

Physical review. E, Statistical, nonlinear, and soft matter physics, 2015

Despite its simple chemical structure, water remains one of the most puzzling liquids with many anomalies at low temperatures. Combining neutron scattering and dielectric relaxation spectroscopy, we show that quantum fluctuations are not negligible in deeply supercooled water. Our dielectric measurements reveal the anomalously weak temperature dependence of structural relaxation in vapor-deposited water close to the glass transition temperature T_{g}∼136K. We demonstrate that this anomalous behavior can be explained well by quantum effects. These results have significant implications for our understanding of water dynamics.

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 in Physics

The main characteristic of liquid water is the formation of dynamic hydrogen bond networks that occur over a broad range of time scales from tens of femtoseconds to picoseconds and are responsible for water’s unique properties. However, in many important processes water does not exist in its bulk form, but in confined nanometer scale environments. The investigation of this confined water dynamics is challenging since the intermediate strength of the hydrogen bonds makes it possible to alter the structure and dynamics of this constrained water. Even if no single experimental technique can give a full picture of such intricate dynamics, it is well established that quasielastic neutron scattering (QENS) is a powerful tool to study the modification of hydrogen bonds in confinement in various materials. This is possible because neutrons tell us where the atoms are and what they are doing, can detect hydrogen, are penetrative and non-destructive. Furthermore, QENS is the only spectroscopi...