Dynamics Of Supercooled Water Research Articles

2H NMR studies on the dynamics of supercooled water in a metal-organic framework.

We use 2H nuclear magnetic resonance (NMR) to study water (D2O) reorientation and diffusion in the metal-organic framework MFU-4l, which features a regular three-dimensional network of nearly spherical pores with diameters of 1.2 and 1.9nm. We observe that the rotational correlation times follow Vogel-Fulcher-Tammann and Arrhenius (Ea = 0.48eV) relations above ∼225K and below ∼170K, respectively, whereas the temperature dependence continuously evolves from one to the other behavior in the broad crossover zone in between. In the common temperature range, the present NMR results are fully consistent with previous broadband dielectric spectroscopy (BDS) data on water (H2O) in a very similar framework. Several of our observations, e.g., rotational-translational coupling, indicate that a bulk-like structural (α) relaxation is observed above the crossover region. When cooling through the crossover zone, a quasi-isotropic reorientation mechanism is retained, while 2H spin-lattice relaxation evolves from exponential to nonexponential, implying that the water dynamics probed at low temperatures does no longer fully restore ergodicity on the time scale of this experiment. We discuss that the latter effect may result from bulk-like and/or confinement-imposed spatially heterogeneous water properties. Comparison with previous NMR and BDS results for water in other confinements reveals that, for confinement sizes around 2nm, water reorientation depends more on the pore diameter than on the pore chemistry, while water diffusion is strongly affected by the connectivity and topology of the pores.

Self-Diffusion in Confined Water: A Comparison between the Dynamics of Supercooled Water in Hydrophobic Carbon Nanotubes and Hydrophilic Porous Silica.

Confined liquids are model systems for the study of the metastable supercooled state, especially for bulk water, in which the onset of crystallization below 230 K hinders the application of experimental techniques. Nevertheless, in addition to suppressing crystallization, confinement at the nanoscale drastically alters the properties of water. Evidently, the behavior of confined water depends critically on the nature of the confining environment and the interactions of confined water molecules with the confining matrix. A comparative study of the dynamics of water under hydrophobic and hydrophilic confinement could therefore help to clarify the underlying interactions. As we demonstrate in this work using a few representative results from the relevant literature, the accurate assessment of the translational mobility of water molecules, especially in the supercooled state, can unmistakably distinguish between the hydrophilic and hydrophobic nature of the confining environments. Among the numerous experimental methods currently available, we selected nuclear magnetic resonance (NMR) in a field gradient, which directly measures the macroscopic translational self-diffusion coefficient, and quasi-elastic neutron scattering (QENS), which can determine the microscopic translational dynamics of the water molecules. Dielectric relaxation, which probes the re-orientational degrees of freedom, are also discussed.

Structure and dynamics of supercooled water in the hydration layer of poly(ethylene glycol).

The statics and dynamics of supercooled water in the hydration layer of poly(ethylene glycol) (PEG) were studied by a combination of quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. Two samples, that is, hydrogenated PEG/deuterated water (h-PEG/D2O) and fully deuterated PEG/hydrogenated water (d-PEG/H2O) with the same molar ratio of ethylene glycol (EG) monomer to water, 1:1, are compared. The QENS data of h-PEG/D2O show the dynamics of PEG, and that of d-PEG/H2O reveals the motion of water. The temperature-dependent elastic scattering intensity of both samples has shown transitions at supercooled temperature, and these transition temperatures depend on the energy resolution of the instruments. Therefore, neither one is a phase transition, but undergoes dynamic process. The dynamic of water can be described as an Arrhenius to super-Arrhenius transition, and it reveals the hydrogen bonding network relaxation of hydration water around PEG at supercooled temperature. Since the PEG-water hydrogen bond structural relaxation time from MD is in good agreement with the average relaxation time from QENS (d-PEG/H2O), MD may further reveal the atomic pictures of the supercooled hydration water. It shows that hydration water molecules form a series of pools around the hydrophilic oxygen atom of PEG. At supercooled temperature, they have a more bond ordered structure than bulk water, proceed a trapping sites diffusion on the PEG surface, and facilitate the structural relaxation of PEG backbone.

Consistency of geometrical definitions of hydrogen bonds based on the two-dimensional potential of mean force with respect to the time correlation in liquid water over a wide range of temperatures

We examine the temperature dependence of the hydrogen-bond (H-bond) dynamics in supercooled water through the molecular dynamics (MD) simulations using the TIP4P/2005 model. First, we investigate the two-dimensional (2D) potential of mean force (PMF) for the H-bond definition using a pair of distance and angle coordinates between two water molecules. Specifically, we utilize four combinations of the geometrical variables, R-β, r-α, r-γ, and r-ψ, where R and r are the intermolecular O-O and O-H distances, respectively. β and α represent the angles of O-OH and O-HO, respectively, while ψ and γ are angles introduced in terms of the intermolecular O-H vector and the HOH plane of the H-bond acceptor. The consistency among the four definitions of H-bond was then examined by identifying the saddle point on the 2D PMF and analyzing the time correlation function of H-bond. It was found over a wide range of temperatures from the ambient to supercooled that the H-bond definition does not affect the long-time H-bond correlation function and correspondingly, the H-bond lifetime τHB.

Dynamics of supercooled water in nanotubes: cage correlation function and diffusion coefficient.

Dynamics of low-temperature water in nanostructured materials is important to a variety of phenomena, ranging from transport in cement and asphaltene, to conformational dynamics of proteins in "crowded" cellular environments, survival of microorganisms at very low temperatures, and diffusion in nanogeoscience. Using silicon-carbide nanotubes as a prototype of nanostructured materials, extensive molecular dynamics simulations were carried out to study the cage correlation function C(t) and self-diffusivity D of supercooled water in the nanotubes. C(t), which measures changes in the atomic surroundings inside the nanotube, follows the Kohlrausch-Williams-Watts law, C(t)∼exp[-(t/τ)^{β}], where τ is a relaxation time and β is a topological exponent. For the temperature range 220K<T≤273 K, we find β≃0.438, in excellent agreement with and confirming the prediction by Phillips [Rep. Prog. Phys. 59, 1133 (1996)]RPPHAG0034-488510.1088/0034-4885/59/9/003, β=3/7. The self-diffusivity manifests a transition around 230 K, very close to 228 K, the temperature at which a fragile-to-strong dynamic crossover is supposed to happen. Thus the results indicate that water does not freeze in the nanotube over the studied temperature range, and that the Stokes-Einstein relation breaks down.

Microscopic origin of temporal heterogeneities in translational dynamics of liquid water.

Liquid water is known to reorient via a combination of large angular jumps (due to exchange of hydrogen bonding (H-bond) partners) and diffusive orientations. Translation of the molecule undergoing the orientational jump and its initial and final H-bond acceptors plays a key role in the microscopic reorientation process. Here, we partition the translational dynamics into those occurring during intervals when rotating water molecules (and their initial and final H-bonding partners) undergo orientational jump and those arising when molecules wait between consecutive orientational jumps. These intervals are chosen in such a way that none of the four possible H-bonds involving the chosen water molecule undergo an exchange process within its duration. Translational dynamics is analysed in terms of the distribution of particle displacements, van Hove functions, and its moments. We observe that the translational dynamics, calculated from molecular dynamics simulations of liquid water, is fastest during the orientational jumps and slowest during periods of waiting. The translational dynamics during all temporal intervals shows an intermediate behaviour. This is the microscopic origin of temporal dynamic heterogeneity in liquid water, which is mild at 300 K and systematically increases with supercooling. Study of such partitioned dynamics in supercooled water shows increased disparity in dynamics occurring in the two different types of intervals. Nature of the distribution of particle displacements in supercooled water is investigated and it reveals signatures non-Gaussian behaviour.

Dynamics of supercooled water in a biological model system of the amino acid L-lysine.

The dynamics of supercooled water in aqueous solutions of the single amino acid L-lysine has been studied by broadband dielectric spectroscopy. The chosen biological system is unique in the sense that the water content is high enough to fully dissolve the amino acid, but low enough to avoid crystallisation to ice at any temperature. This is not possible to achieve for proteins or other larger biomolecules, where either hydrated samples without ice or solutions with large quantities of ice, or a cryoprotectant sugar, have to be studied at low temperatures. Thus, it is a key finding to be able to study water and biomolecular dynamics in a non-crystallized and biologically realistic solution at supercooled temperatures. Here, we focus on the water dynamics in this unique biological solution of L-lysine and water. We show that this unique system also gives rise to unique water dynamics, since, for the first time, a continuation of a cooperative (α-like) water relaxation is observed after a crossover to a more local β-like water relaxation has occurred with decreasing temperature. This implies that the supercooled water in the biological solution shows a twofold relaxation behaviour, with one relaxation identical to the main relaxation of water in hard confinements and one relaxation almost identical to the main water relaxation in ordinary aqueous solutions.

Structure and dynamics of supercooled water in neutral confinements

We perform molecular dynamics simulations to study the structure and dynamics of liquid water in neutral nanopores, which are generated by pinning a suitable subset of water molecules in an equilibrium configuration of a bulk system. It is found that such neutral confinement does not disturb the structure of water, in particular, the local tetrahedral order, while it imposes a pronounced spatial inhomogeneity on the dynamics of water. Specifically, when the pore wall is approached, hopping motion sets in and water dynamics slows down. We show that the logarithm of the correlation time is an exponential function of the distance to the wall, indicating a tremendous gradient of water mobility across the confinement. Upon cooling, the length scale associated with this exponential distance dependence and, thus, the range of the wall effect increases, at least down to the critical temperature of mode coupling theory, T(c). Also, the temperature dependence of water dynamics varies across the pore, i.e., fragility is high in the pore center, while it is low near the pore wall. Due to all these effects, time-temperature superposition is violated. Our observations for a neutral confinement reveal that specific interactions at hydrophilic or hydrophobic walls are not the main cause of spatially inhomogeneous dynamics of confined water. In view of similarities with the behavior of Lennard-Jones liquids in neutral confinements, one may rather speculate that the effects observed for confined water are general and result from the existence of a static contribution to the energy landscape, which is imprinted by an immobile environment.

Corresponding states for mesostructure and dynamics of supercooled water

Water famously expands upon freezing, foreshadowed by a negative coefficient of expansion of the liquid at temperatures close to its freezing temperature. These behaviors, and many others, reflect the energetic preference for local tetrahedral arrangements of water molecules and entropic effects that oppose it. Here, we provide theoretical analysis of mesoscopic implications of this competition, both equilibrium and non-equilibrium, including mediation by interfaces. With general scaling arguments bolstered by simulation results, and with reduced units that elucidate corresponding states, we derive a phase diagram for bulk and confined water and water-like materials. For water itself, the corresponding states cover the temperature range of 150 K to 300 K and the pressure range of 1 bar to 2 kbar. In this regime, there are two reversible condensed phases - ice and liquid. Out of equilibrium, there is irreversible polyamorphism, i.e., more than one glass phase, reflecting dynamical arrest of coarsening ice. Temperature-time plots are derived to characterize time scales of the different phases and explain contrasting dynamical behaviors of different water-like systems.

Rotational dynamics in supercooled water from nuclear spin relaxation and molecular simulations

Structural dynamics in liquid water slow down dramatically in the supercooled regime. To shed further light on the origin of this super-Arrhenius temperature dependence, we report high-precision (17)O and (2)H NMR relaxation data for H(2)O and D(2)O, respectively, down to 37 K below the equilibrium freezing point. With the aid of molecular dynamics (MD) simulations, we provide a detailed analysis of the rotational motions probed by the NMR experiments. The NMR-derived rotational correlation time τ(R) is the integral of a time correlation function (TCF) that, after a subpicosecond librational decay, can be described as a sum of two exponentials. Using a coarse-graining algorithm to map the MD trajectory on a continuous-time random walk (CTRW) in angular space, we show that the slowest TCF component can be attributed to large-angle molecular jumps. The mean jump angle is ∼48° at all temperatures and the waiting time distribution is non-exponential, implying dynamical heterogeneity. We have previously used an analogous CTRW model to analyze quasielastic neutron scattering data from supercooled water. Although the translational and rotational waiting times are of similar magnitude, most translational jumps are not synchronized with a rotational jump of the same molecule. The rotational waiting time has a stronger temperature dependence than the translation one, consistent with the strong increase of the experimentally derived product τ(R) D(T) at low temperatures. The present CTRW jump model is related to, but differs in essential ways from the extended jump model proposed by Laage and co-workers. Our analysis traces the super-Arrhenius temperature dependence of τ(R) to the rotational waiting time. We present arguments against interpreting this temperature dependence in terms of mode-coupling theory or in terms of mixture models of water structure.

Water at Interface with Proteins

Water is essential for the activity of proteins. However, the effect of the properties of water on the behavior of proteins is only partially understood. Recently, several experiments have investigated the relation between the dynamics of the hydration water and the dynamics of protein. These works have generated a large amount of data whose interpretation is debated. New experiments measure the dynamics of water at low temperature on the surface of proteins, finding a qualitative change (crossover) that might be related to the slowing down and stop of the protein’s activity (protein glass transition), possibly relevant for the safe preservation of organic material at low temperature. To better understand the experimental data several scenarios have been discussed. Here, we review these experiments and discuss their interpretations in relation with the anomalous properties of water. We summarize the results for the thermodynamics and dynamics of supercooled water at an interface. We consider also the effect of water on protein stability, making a step in the direction of understanding, by means of Monte Carlo simulations and theoretical calculations, how the interplay of water cooperativity and hydrogen bonds interfacial strengthening affects the protein cold denaturation.

ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water.

Using electron spin resonance spectroscopy (ESR), we measure the rotational mobility of probe molecules highly diluted in deeply supercooled bulk water and negligibly constrained by the possible ice fraction. The mobility increases above the putative glass transition temperature of water, T(g) = 136 K, and smoothly connects to the thermodynamically stable region by traversing the so called "no man's land" (the range 150-235 K), where it is believed that the homogeneous nucleation of ice suppresses the liquid water. Two coexisting fractions of the probe molecules are evidenced. The 2 fractions exhibit different mobility and fragility; the slower one is thermally activated (low fragility) and is larger at low temperatures below a fragile-to-strong dynamic cross-over at approximately 225 K. The reorientation of the probe molecules decouples from the viscosity below approximately 225 K. The translational diffusion of water exhibits a corresponding decoupling at the same temperature [Chen S-H, et al. (2006) The violation of the Stokes-Einstein relation in supercooled water. Proc Natl Acad Sci USA 103:12974-12978]. The present findings are consistent with key issues concerning both the statics and the dynamics of supercooled water, namely the large structural fluctuations [Poole PH, Sciortino F, Essmann U, Stanley HE (1992) Phase behavior of metastable water. Nature 360:324-328] and the fragile-to-strong dynamic cross-over at approximately 228 K [Ito K, Moynihan CT, Angell CA (1999) Thermodynamic determination of fragility in liquids and a fragile-to-strong liquid transition in water. Nature 398:492-494].

Dynamics of supercooled water in highly compacted clays studied by neutronscattering

The freezing behavior of water confined in compacted charged and unchargedclays (montmorillonite in Na- and Ca-forms, illite in Na- and Ca-forms, kaoliniteand pyrophyllite) was investigated by neutron scattering. Firstly, the amount offrozen (immobile) water was measured as a function of temperature at the IN16backscattering spectrometer, Institute Laue–Langevin (ILL). Water in uncharged, partlyhydrophobic (kaolinite) and fully hydrophobic (pyrophyllite) clays exhibited asimilar freezing and melting behavior to that of bulk water. In contrast, waterin charged clays which are hydrophilic could be significantly supercooled. Toobserve the water dynamics in these clays, further experiments were performedusing quasielastic neutron scattering. At temperatures of 250, 260 and 270 K thediffusive motion of water could still be observed, but with a strong reduction in thewater mobility as compared with the values obtained above 273 K. The diffusioncoefficients followed a non-Arrhenius temperature dependence well described by theVogel–Fulcher–Tammann and the fractional power relations. The fits revealed that Na- andCa-montmorillonite and Ca-illite have similar Vogel–Fulcher–Tammann temperatures(TVFT, often referred to as the glass transition temperature) of∼120 K and similar temperatures at which the water undergoes the ‘strong–fragile’ transition,Ts∼210 K. On the other hand, Na-illite had significantly larger values ofTVFT∼180 Kand Ts∼240 K. Surprisingly, Ca-illite has a similar freezing behavior of water to that ofmontmorillonites, even though it has a rather different structure. We attribute this to thestronger hydration of Ca ions as compared with the Na ions occurring in the illiteclays.

Metabasin dynamics and local structure in supercooled water

We employ the distance matrix method to investigate metabasin dynamics in supercooled water. We find that the motion of the system consists in the exploration of a finite region of configuration space (enclosing several distinct local minima), named metabasin, followed by a sharp crossing to a different metabasin. The characteristic time between metabasin transitions is comparable to the structural relaxation time, suggesting that these transitions are relevant for the long-time dynamics. The crossing between metabasins is accompanied by very rapid diffusional jumps of several groups of dynamically correlated particles. These particles form relatively compact clusters and act as cooperative relaxing units responsible for the density relaxation. We find that these mobile particles are often characterized by an average coordination larger than four, i.e., they are located in regions where the tetrahedral hydrogen bond network is distorted.

Dynamics of supercooled water in various mesopore sizes

Double-quantum-filtered NMR and T(1) inversion-recovery spectroscopy were employed to exploit the temperature-dependent dynamics of D(2)O confined in MCM-41. Samples with three pore sizes of 1.58, 2.03, and 2.34 nm and two D(2)O contents were investigated. The reorientation correlation times of confined D(2)O in variously sized pores exhibit different temperature dependencies. The results reveal that the D(2)O molecules at fast motion site remain mobile below approximately 225 K and a liquid-liquid phase transition occurs around this temperature for all samples studied. This temperature is thought to be unreachable for supercooled D(2)O. Particularly, in 20 wt % D(2)O loaded samples with pore diameters of 1.58 and 2.03 nm, the reorientational correlation times of D(2)O at fast motion site exhibit Arrhenius behavior between 225 and 290 K, while other samples show power law dependency. Thus, a liquid phase of the fragile type in bigger pores changes to the strong type in samples with smaller pores.

Local order and dynamics in supercooled water: a study by IR spectroscopy and molecular dynamic simulations.

Micron-sized water droplets in a cryogenic flow tube were probed by IR spectroscopy. The analysis of the IR spectra suggests that there is a relative increase of about 30% in the fraction, f(L), of low density domains in water on cooling over the temperature range between 300 and 240 K. The results derived from the experiments agree qualitatively with those of molecular dynamics (MD) simulations in terms of the increase in the f(L) values. The MD simulations show that the intensities of the mode at about 100 cm(-1) for the molecules in the low density domains are reduced in comparison to the average, while the intensities and frequencies of the librational mode at 600 cm(-1) are increased. Furthermore, the reorientations (dielectric relaxation times) in these domains are found to be somewhat slower, pointing to the fact that these low density "cages" live longer than the average local molecular environments in supercooled water.

Dynamics of Supercooled Water and Glassy Water

Quasielastic neutron scattering of normal and supercooled water was measured in the temperature range 253-330K to investigate the possible anomaly expected to occur around 230K. Anomalous temperature dependence of self-diffusion coefficient, which may suggest a critical slowing-down, was found for the first time. Vapor-deposited glassy water (amorphous ice) and those doped with methanol (hydrogen-bond breaking impurity) and Xe and SF6 (both hydrogen-bond forming impurities) were prepared in a novel cryostat designed for this experiment. The in-situ neutron scattering of these samples was measured on a total scattering, smallangle scattering, and inelastic scattering spectrometers. A strong correlation among the ordering of hydrogenbond system, volume expansion, homogenization in a nano-scale range, and decrease of the excess lowenergy excitation was found in the experiments for the annealing and impurity effects. The future study on supercooled and glassy water was discussed along with high-intensity spectrometers at J-PARC.

Dynamics of supercooled water in mesoporous silica matrix MCM-48-S.

Using three different quasielastic neutron spectrometers with widely different resolutions, we have been able to study the microscopic translational and rotational dynamics of water, in a mesoporous silica matrix MCM-48-S, from T=300 K to 220 K, with a single consistent model. We formulated our fitting routine using the relaxing cage model. Thus, from the fit of the experimental data, we extracted the fraction of water bound to the surface of the pore, the characteristic relaxation times of the long-time translational and rotational decays, the stretch exponent describing the shape of the relaxation processes, and the power exponent determining the Q-dependence of the translational relaxation time. A tremendous slowing down of the rotational relaxation time, as compared to the translational one, has been observed.

Recent results on the connection between thermodynamics and dynamics in supercooled water

We review recent results on the connection between thermodynamics and dynamics in a model for water. We verify the Adam–Gibbs relation between entropy and dynamic properties using computer simulations, which allow direct access to the relevant properties. We combine experimental measurements of entropy with the Adam–Gibbs hypothesis to predict dynamic properties in deeply supercooled states, which are difficult to access experimentally. We find evidence suggesting that the glass transition temperature of water may be significantly higher than previously reported, but is still consistent with recent measurements. Finally, we discuss the hypothesis that the dynamical behavior of deeply supercooled water undergoes a crossover from ‘fragile’ to ‘strong’ behavior.

Hydration water dynamics of a completely hydrophobic oligopeptide

Abstract The dynamics of hydration water of a completely deuterated penta-alanine peptide has been studied by incoherent quasi-elastic neutron scattering. Measurements have been made at different hydration levels (7%, 30%, 50%, 90%), and on the dried powder (0%) which contains one structural water molecule. The dynamical contribution of this first hydration molecule of water is characteristic of a slow rotational motion with a relaxation time, τ1, of 2.2 ps, similar to what is found in supercooled water dynamics. Adding two more hydration water molecules (7%) the rotational motion of the first water is coupled with the new diffusive motion and the dynamics profile can be, in first approximation, described through a rotational jump model. The results suggest a behavior similar to that of bulk water at 2 °C. At higher levels of hydration, the mobility of new molecules of water approaches that of bulk water, with a rotation relaxation time of 1 ps and a confined diffusing motion. However the residence time value, τ0, is of the same order of magnitude as supercooled water at T=−10 ° C.