The glass transition and relaxation behavior of bulk water and a possible relation to confined water

Dynamical Susceptibilities of Confined Water from Room Temperature to the Glass Transition

We confine water to narrow silica pores, where crystallization is suppressed, and determine the dynamical susceptibilities of the liquid from room temperature down to the glass transition by combining broadband dielectric spectroscopy (BDS) with ¹H and ²H nuclear magnetic resonance (NMR), in particular, by establishing NMR field-cycling relaxometry. For the correlation times, derivative analysis reveals Vogel-Fulcher-Tammann and Arrhenius regimes at T ≥ 215 K and T ≤ 160 K, respectively, which are separated by a broad crossover region. The continuous transition in the temperature dependence is accompanied by a gradual change from asymmetric high-temperature shapes of the dynamical susceptibilities to symmetric low-temperature ones and by a steady decrease of the dielectric relaxation strength. In the Arrhenius regime (E_a = 0.48 eV) at T ≤ 160 K, 2D ²H NMR spectra reveal quasi-isotropic water reorientation. We rationalize these results in terms of a crossover to an interface-affected, noncooperative relaxation involving both rotational and translational motions.

Breakdown of the Stokes-Einstein relation in supercooled water: the jump-diffusion perspective

Although water is the most ubiquitous liquid it shows many thermodynamic and dynamic anomalies. Some of the anomalies further intensify in the supercooled regime. While many experimental and theoretical studies have focused on the thermodynamic anomalies of supercooled water, fewer studies explored the dynamical anomalies very extensively. This is due to the intricacy of the experimental measurement of the dynamical properties of supercooled water. Violation of the Stokes-Einstein relation (SER), an important relation connecting the diffusion of particles with the viscosity of the medium, is one of the major dynamical anomalies. In absence of experimentally measured viscosity, researchers used to check the validity of SER indirectly using average translational relaxation time or α-relaxation time. Very recently, the viscosity of supercooled water was accurately measured at a wide range of temperatures and pressures. This allowed direct verification of the SER at different temperature-pressure thermodynamic state points. An increasing breakdown of the SER was observed with decreasing temperature. Increasing pressure reduces the extent of breakdown. Although some well-known theories explained the above breakdown, a detailed molecular mechanism was still elusive. Recently, a translational jump-diffusion (TJD) approach has been able to quantitatively explain the breakdown of the SER in pure supercooled water and an aqueous solution of methanol. The objective of this article is to present a detailed and state-of-the-art analysis of the past and present works on the breakdown of SER in supercooled water with a specific focus on the new TJD approach for explaining the breakdown of the SER.

The peculiar size and temperature dependence of water diffusion in carbon nanotubes studied with 2D NMR diffusion-relaxation D -T 2eff spectroscopy

It is well known that water inside hydrophobic nano-channels diffuses faster than bulk water. Recent theoretical studies have shown that this enhancement depends on the size of the hydrophobic nanochannels. However, experimental evidence of this dependence is lacking. Here, by combining two-dimensional nuclear magnetic resonance diffusion-relaxation ( D - T 2 e f f ) spectroscopy in the stray field of a superconducting magnet and molecular dynamics simulations, we analyze the size dependence of water dynamics inside Carbon Nanotubes (CNTs) of different diameters ( 1.1 - 6.0  nm), in the temperature range of 265 - 305  K. Depending on the CNT diameter, the nanotube water is shown to resolve in two or more tubular components acquiring different self-diffusion coefficients. Most notably, a favorable CNT diameter range ( 3.0 - 4.5  nm) is experimentally verified for the first time, in which water molecule dynamics at the center of the CNTs exhibits distinctly non-Arrhenius behavior, characterized by ultrafast diffusion and extraordinary fragility, a result of significant importance in the efforts to understand water behavior in hydrophobic nanochannels.

Control of Solvent Dynamics around the B12-Dependent Ethanolamine Ammonia-Lyase Enzyme in Frozen Aqueous Solution by Using Dimethyl Sulfoxide Modulation of Mesodomain Volume

The temperature-dependent structure and dynamics of two concentric solvent phases, the protein-associated domain (PAD) and the mesodomain, that surround the ethanolamine ammonia-lyase (EAL) protein from Salmonella typhimurium in frozen polycrystalline aqueous solution are addressed by using electron paramagnetic resonance spectroscopy of the paramagnetic nitroxide spin probe, TEMPOL, over the temperature ( T) range 190-265 K. Dimethyl sulfoxide (DMSO), added at 0.5, 2.0, and 4.0% v/v and present at the maximum freeze concentration at T ≤ 245 K, varies the volume of the interstitial aqueous DMSO mesodomain ( V_meso) relative to a fixed PAD volume ( V_PAD). The increase in V_meso/ V_PAD from 0.8 to 6.0 is quantified by the partitioning of TEMPOL between the two phases. As V_meso/ V_PAD is increased, the Arrhenius parameters for activated TEMPOL rotational motion in the mesodomain remain uniform, whereas the parameters for TEMPOL in the PAD show a progressive transformation toward the mesodomain values (higher mobility). An order-disorder transition (ODT) in the PAD is detected by the exclusion of TEMPOL from the PAD into the mesodomain. The ODT T value is systematically lowered by increased V_meso/ V_PAD (from 215 to 200 K), and PAD ordering kinks the mesodomain Arrhenius dependence. Thus there is reciprocity in PAD-mesodomain solvent coupling. The results are interpreted as a dominant influence of ice-boundary confinement on the PAD solvent structure and dynamics, which is transmitted through the mesodomain and which decreases with mesodomain volume at increased added DMSO. The systematic tuning of PAD and mesodomain solvent dynamics by the variation of added DMSO is an incisive approach for the resolution of contributions of protein-solvent dynamical coupling to EAL catalysis.

Possible relations between supercooled and glassy confined water and amorphous bulk ice

A proposed relaxation scenario of bulk water based on studies of confined water and low density amorphous ice.

Rotational dynamics and dynamical transition of water inside hydrophobic pores of carbon nanotubes

Water in a nanoconfined geometry has attracted great interest from the viewpoint of not only basic science but also nanofluidic applications. Here, the rotational dynamics of water inside single-walled carbon nanotubes (SWCNTs) with mean diameters larger than ca. 1.4 nm were investigated systematically using ²H nuclear magnetic resonance spectroscopy with high-purity SWCNTs and molecular dynamics calculations. The results were compared with those for hydrophilic pores. It was found that faster water dynamics could be achieved by increasing the hydrophobicity of the pore walls and decreasing the pore diameters. These results suggest a strategy that paves the way for emerging high-performance filtration/separation devices. Upon cooling below 220 K, it was found that water undergoes a transition from fast to slow dynamics states. These results strongly suggest that the observed transition is linked to a liquid-liquid crossover or transition proposed in a two-liquid states scenario for bulk water.

Disruption of Hydrogen-Bonding Network Eliminates Water Anomalies Normally Observed on Cooling to Its Calorimetric Glass Transition

The calorimetric glass-transition temperature of water is 136 K, but extrapolation of thermodynamic and relaxation properties of water from ambient temperature to below its homogeneous nucleation temperature T_H = 235 K predicts divergence at T_S = 228 K. The "no-man's land" between the T_H and glassy water crystallization temperature of 150 K, which is encountered on warming up from the vitrified state, precludes a straightforward reconciliation of the two incompatible temperature dependences of water properties, above 235 K and below 150 K. The addition of lithium chloride to water allows bypassing both T_H and T_S on cooling, resulting in the dynamics with no features except the calorimetric glass transition, still at 136 K. We show that lithium chloride prevents hydrogen-bonding network completion in water on cooling, as manifested, in particular, in changing microscopic diffusion mechanism of the water molecules. Thus thermodynamic and relaxation peculiarities exhibited by pure water on cooling to its glass transition, such as the existence of the T_H and T_S, must be associated specifically with the hydrogen-bonding network.

Does water belong to the homologous series of hydroxyl compounds H(CH2)nOH?

The main objective of this paper is to find a source of anomalously high value of the equilibrium permittivity of water. The source is identified to be the unusually high deformation polarizability. The conclusion follows from the analysis of the behavior of the orientational entropy increment induced by an external electric field applied to the liquids belonging to the homologous series of hydroxyl compounds H(CH₂)_nOH at the end of which water is located. The finding reflects the "indecision" of water about its dielectric relationship with the alcohol family: the value of the permittivity of water absolutely does not fit into alcohols (is too high), while the dipolar orientation effects (which normally determine the permittivity level) fit into alcohols quite well. It results from the presented experimental data that among all the diversity of intermolecular hydrogen-bonded structures existing in liquid water, predominant are the polar entities, i.e. the structures which more or less resemble the chains. Otherwise, the dipolar orientational effects would behave in a quite different way than what is observed in the experiment. The result is convergent with the conclusion of Wernet et al., based on the high-performance X-ray studies of water (Science, 2004).

On the hydrogen-bond network and the non-Arrhenius transport properties of water

We study the structural and dynamic transformations of SPC/E water with temperature, through molecular dynamics (MD), and discuss the non-Arrhenius behavior of the transport properties and orientational dynamics, and the magnitude of the breakdown of the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations, in the light of these transformations. Our results show that deviations from Arrhenius behavior of the self-diffusion at low temperatures cannot be exclusively explained by the reduction of water defects (interstitial waters) and the increase of the local tetrahedrality, thus, suggesting the importance of the slowdown of collective rearrangements. Interestingly we find that at high temperatures (T  ⩾  340 K) water defects lead to a slight increase of the tetrahedrality and a decrease of the self-diffusion, opposite to water at low temperatures. The relative magnitude of the breakdown of the SE and the SED relations is found to be in accord with recent experiments (Dehaoui et al 2015 Proc. Natl Acad. Sci. USA 112 12020) resolving the discrepancy with previous MD results. Further, we show that SPC/E hydrogen-bond (HB) lifetimes deviate from Arrhenious behaviour at low temperatures in contrast with some previous MD studies. This deviation is nevertheless much smaller than that observed for the orientational dynamics and the transport properties of water, consistent with the relaxation times measured by several experimental methods. The HB acceptor exchange dynamics defined here by the acceptor switch and reform (librational dynamics) frequencies exhibit similar Arrhenius deviations, thus explaining to some extent the non-Arrhenius behavior of the transport properties and of the orientational dynamics of water. Our results also show that the fraction of HB switches through a bifurcated pathway follow a power law with the temperature decrease. Thus, at low temperatures HB acceptor switches are less frequent but occur on a faster time scale consistent with the temperature dependence of the ratio of the rotational relaxation times for the different Legendre polynomial ranks.

Evidence of Coupling between the Motions of Water and Peptides

Studies of protein dynamics at low temperatures are generally performed on hydrated powders and not in biologically realistic solutions of water because of water crystallization. However, here we avoid the problem of crystallization by reducing the size of the biomolecules. We have studied oligomers of the amino acid l-lysine, fully dissolved in water, and our dielectric relaxation data show that the glass transition-related dynamics of the oligomers is determined by the water dynamics, in a way similar to that previously observed for solvated proteins. This implies that the crucial role of water for protein dynamics can be extended to other types of macromolecular systems, where water is also able to determine their conformational fluctuations. Using the energy landscape picture of macromolecules, the thermodynamic criterion for such solvent-slaved macromolecular motions may be that the macromolecules need the entropy contribution from the solvent to overcome the enthalpy barriers between different conformational substates.

Reentrant condensation of lysozyme: Implications for studying dynamics of lysozyme in aqueous solutions of lithium chloride

Recent studies have outlined the use of eutectic solutions of lithium chloride in water to study microscopic dynamics of lysozyme in an aqueous solvent that is remarkably similar to pure water in many respects, yet allows experiments over a wide temperature range without solvent crystallization. The eutectic point in a (H2O)R(LiCl) system corresponds to R ≈ 7.3, and it is of interest to investigate whether less-concentrated aqueous solutions of LiCl could be used in low-temperature studies of a solvated protein. We have investigated a range of concentrations of lysozyme and LiCl in aqueous solutions to identify systems that do not show phase separation and avoid solvent crystallization on cooling down. Compared to the lysozyme concentration in solution, the concentration of LiCl in the aqueous solvent plays the major role in determining systems suitable for low-temperature studies. We have observed interesting and rich phase behavior reminiscent of reentrant condensation of proteins.

Confined Water as Model of Supercooled Water

Water in confined geometries has obvious relevance in biology, geology, and other areas where the material properties are strongly dependent on the amount and behavior of water in these types of materials. Another reason to restrict the size of water domains by different types of geometrical confinements has been the possibility to study the structural and dynamical behavior of water in the deeply supercooled regime (e.g., 150-230 K at ambient pressure), where bulk water immediately crystallizes to ice. In this paper we give a short review of studies with this particular goal. However, from these studies it is also clear that the interpretations of the experimental data are far from evident. Therefore, we present three main interpretations to explain the experimental data, and we discuss their advantages and disadvantages. Unfortunately, none of the proposed scenarios is able to predict all the observations for supercooled and glassy bulk water, indicating that either the structural and dynamical alterations of confined water are too severe to make predictions for bulk water or the differences in how the studied water has been prepared (applied cooling rate, resulting density of the water, etc.) are too large for direct and quantitative comparisons.

Pf1 bacteriophage hydration by magic angle spinning solid-state NMR

High resolution two- and three-dimensional heteronuclear correlation spectroscopy ((1)H-(13)C, (1)H-(15)N, and (1)H-(13)C-(13)C HETCOR) has provided a detailed characterization of the internal and external hydration water of the Pf1 virion. This long and slender virion (2000 nm × 7 nm) contains highly stretched DNA within a capsid of small protein subunits, each only 46 amino acid residues. HETCOR cross-peaks have been unambiguously assigned to 25 amino acids, including most external residues 1-21 as well as residues 39-40 and 43-46 deep inside the virion. In addition, the deoxyribose rings of the DNA near the virion axis are in contact with water. The sets of cross-peaks to the DNA and to all 25 amino acid residues were from the same hydration water (1)H resonance; some of the assigned residues do not have exchangeable side-chain protons. A mapping of the contacts onto structural models indicates the presence of water "tunnels" through a highly hydrophobic region of the capsid. The present results significantly extend and modify results from a lower resolution study, and yield a comprehensive hydration surface map of Pf1. In addition, the internal water could be distinguished from external hydration water by means of paramagnetic relaxation enhancement. The internal water population may serve as a conveniently localized magnetization reservoir for structural studies.

Dynamics of deeply supercooled interfacial water

In this review we discuss the relaxation dynamics of glassy and deeply supercooled water in different types of systems. We compare the dynamics of such interfacial water in ordinary aqueous solutions, hard confinements and biological soft materials. In all these types of systems the dielectric relaxation time of the main water process exhibits a dynamic crossover from a high-temperature non-Arrhenius temperature dependence to a low-temperature Arrhenius behavior. Moreover, at large enough water content the low-temperature process is universal and exhibits the same temperature behavior in all types of systems. However, the physical nature of the dynamic crossover is somewhat different for the different types of systems. In ordinary aqueous solutions it is not even a proper dynamic crossover, since the water relaxation decouples from the cooperative α-relaxation of the solution slightly above the glass transition in the same way as all secondary (β) relaxations of glass-forming materials. In hard confinements, the physical origin of the dynamic crossover is not fully clear, but it seems to occur when the cooperative main relaxation of water at high temperatures reaches a temperature where the volume required for its cooperative motion exceeds the size of the geometrically-confined water cluster. Due to this confinement effect the α-like main relaxation of the confined water seems to transform to a more local β-relaxation with decreasing temperature. Since this low-temperature β-relaxation is universal for all systems at high water content it is possible that it can be considered as an intrinsic β-relaxation of supercooled water, including supercooled bulk water. This possibility, together with other findings for deeply supercooled interfacial water, suggests that the most accepted relaxation scenarios for supercooled bulk water have to be altered.

Distinct Properties of Nanofibrous Amorphous Ice

We make glassy water in the form of nanofibers by electrospraying liquid water into a hyperquenching chamber. It is measured with means of differential scanning calorimetry, wide angle X-ray diffraction and Raman spectroscopy. It is found that two apparent glass transitions at T_g1 = 136 K and T_g2 = 228 K are detected and non-crystallized water is observed at temperatures up to 228 K. This finding may expand the research objects for liquid water at low temperatures.

Dynamic Crossovers and Stepwise Solidification of Confined Water: A (2)H NMR Study

(2)H NMR reveals two dynamic crossovers of supercooled water in nanoscopic (∼2 nm) confinement. At ∼225 K, a dynamic crossover of liquid water is accompanied by formation of a fraction of solid water. Therefore, we do not attribute the effect to a liquid-liquid phase transition but rather to a change from bulk-like to interface-dominated dynamics. Moreover, we argue that the α process and β process are observed in experiments above and below this temperature, respectively. Upon cooling through a dynamic crossover at ∼175 K, the dynamics of the liquid fraction becomes anisotropic and localized, implying solidification of the corresponding water network, most probably, during a confinement-affected glass transition.

NMR studies on the temperature-dependent dynamics of confined water

NMR studies of water in nanoscopic confinements of various sizes reveal two dynamical crossovers related to a partial solidification of internal molecules and a glass transition of interfacial molecules, respectively.

Hydroxylamine-doping effect on the Tg of 160 K for water confined in silica-gel nanopores

The glass transition behavior of hydroxylamine (HA) aqueous solutions in bulk and confined in silica-gel nanopores with average width of 1.1 nm was studied by means of differential scanning calorimetry measurements and adiabatic calorimetry. The glass transition temperature (Tg) of the confined solution with high HA mole-fraction (xHA) was essentially the same as the value of the bulk. This suggests that the nano-size confinement affects the Tg of HA aqueous solution little. Meanwhile, the bulk solution with xHA < 0.3 revealed partial crystallization on cooling and, on the other hand, the confined solution with the same xHA did not crystallize. The Tg of the xHA = 0.076 confined solution was 174 K which is higher than the value of 160 K for pure water confined in the same silica-gel pores. This demonstrates that HA doping leads to no abrupt Tg-decrease, unlike doping of all the other second components reported so far, suggesting that HA is set neatly in a hydrogen-bond network formed by water molecules. We discuss the xHA dependence of Tg for the HA aqueous solutions from a viewpoint related to peculiar phase-behavior of pure water. Considering that the xHA = 0.076 aqueous solution revealed no anomaly compared with pure water, it was recognized as corresponding to the high-temperature phase of pure water.

Dynamics of water in synthetic saponite clays: effect of trivalent ion substitution

Saponite clay belongs to the phyllosilicate family and is comprised of layers of Si(IV) tetrahedra and Al(III) or Mg(II) octahedra with definite interlayer spacing. In these systems, the trivalent ion substitutions in the tetrahedral layers lead to negative charge on the layers. Here we report the dynamics of water contained in [Si(6.97)Al(1.03)][Ni(6.00)]O(20)(OH)(4)[Na(1.03)]·28H(2)O (SAP-1) and [Si(7.13)Fe(0.86)][Ni(6.00)]O(20)(OH)(4)[Na(0.86)]·14H(2)O (SAP-2) saponite clays in the temperature range 200-310 K as studied by quasielastic neutron scattering technique. Particularly the effect of the ion substitution towards the dynamics of water is addressed here. Data analysis is carried out using the relaxing cage model. The existence of distribution in relaxation times indicated that the water molecules in saponite clay have a different local environment which leads to complex diffusion behavior. It is found that water exists in a supercooled state in the temperature range up to 235 K. However, some of the water molecules are found to be immobile in the temperature range 240-285 K. The fraction of immobile water decreases with increase in temperature. At higher temperatures, some of the water molecules in the hydration shells or those near the surface start participating in the diffusion process and at 293 K, almost all water molecules contribute to the dynamics. Diffusivity of water in both SAP-1 and SAP-2 are found to be lower in comparison to the bulk, and within the two samples of saponite clay diffusivity in SAP-1 is found to be lower compared to SAP-2; this has been explained on the basis of the charge on the tetrahedral layers and the charge balancing cations in the interlayer spacing.

Slow dynamics of water molecules in an aqueous solution of lithium chloride probed by neutron spin-echo

Aqueous solutions of lithium chloride are uniquely similar to pure water in the parameters such as glass transition temperature, Tg, yet they could be supercooled without freezing down to below 200 K even in the bulk state. This provides advantageous opportunity to study low-temperature dynamics of water molecules in water-like environment in the bulk rather than nano-confined state. Using high-resolution neutron spin-echo data, we argue that the critical temperature, Tc, which is also common between lithium chloride aqueous solutions and pure water, is associated with the split of a secondary relaxation from the main structural relaxation on cooling down. Our results do not allow distinguishing between a well-defined separate secondary relaxation process and the "excess wing" scenario, in which the temperature dependence of the secondary relaxation follows the main relaxation. Importantly, however, in either of these scenarios the secondary relaxation is associated with density-density fluctuations, measurable in a neutron scattering experiment. Neutron scattering could be the only experimental technique with the capability of providing information on the spatial characteristics of the secondary relaxation through the dependence of the signal on the scattering momentum transfer. We propose a simple method for such analysis.

Water dynamics in a lithium chloride aqueous solution probed by Brillouin neutron and x-ray scattering

We studied the collective excitations in an aqueous solution of lithium chloride over the temperature range of 270-205 K using neutron and x-ray Brillouin scattering. Both neutron and x-ray experiments revealed the presence of low- and high-frequency excitations, similar to the low- and high-frequency excitations in pure water. These two excitations were detectable through the entire temperature range of the experiment, at all probed values of the scattering momentum transfer (0.2 Å(-1) < Q < 1.8 Å(-1)). A wider temperature range was investigated using elastic intensity neutron and x-ray scans. Clear evidence of the crossover in the dynamics of the water molecules in the solution was observed in the single-particle relaxational dynamics on the µeV (nanosecond) time scale, but not in the collective dynamics on the meV (picosecond) time scale.

Computer simulations of dynamic crossover phenomena in nanoconfined water

In order to study dynamic crossover phenomena in nanoconfined water we performed a series of molecular dynamics (MD) computer simulations of water clusters adsorbed in zeolites, which are microporous crystalline aluminosilicates containing channels and cavities of nanometric dimensions. We used a sophisticated empirical potential for water, including the full flexibility of the molecule and the correct response to the electric field generated by the cations and by the charged atoms of the aluminosilicate framework. In addition, the full flexibility of the aluminosilicate framework was included in the calculations. Previously reported and new simulations of water confined in a number of different types of zeolites in the temperature range 100-300 K and at various coverage are discussed in connection with the experimental data. Dynamic crossover phenomena are found in all the considered cases, in spite of the different shape and size of the clusters, even when the confinement hinders the formation of tetrahedral hydrogen bonds for water molecules. Hypotheses about the possible dynamic crossover mechanisms are proposed.

Secondary water relaxation in a water/dimethyl sulfoxide mixture revealed by deuteron nuclear magnetic resonance and dielectric spectroscopy

We exploit the potential of a combined dielectric spectroscopy (DS) and deuteron nuclear magnetic resonance ((2)H NMR) approach to investigate the molecular dynamics in a supercooled 2:1 molar mixture of deuterated water (D(2)O) and dimethyl sulfoxide (DMSO). While DS probes the rotational motion of both components, application of (2)H NMR allows us to single out the dynamical behavior of the water molecules. Combining the results of both methods, we can follow the slowdown of the α-process of the mixture over more than 10 orders of magnitude in time, revealing that the Vogel-Fulcher-Tammann (VFT) equation describes well its temperature dependence down to the glass transition temperature, T(g) = 146 K. While the (2)H NMR data do not provide evidence for a secondary relaxation process in the weakly supercooled regime, they indicate that, in the deeply supercooled regime, T(g) ≤ T ≤ 160 K, the water molecules do show a secondary dynamical process, which is faster and exhibits a weaker temperature dependence than the α-process of the mixture. Consistently, the shape of the dielectric spectra changes in this temperature range. (2)H NMR rotational correlation functions reveal that this faster secondary water process destroys essentially all orientational correlation. In addition, these data show that the water reorientation process is characterized by a mean elementary jump angle smaller than 13°. Possible origins of the faster secondary water process in the deeply supercooled mixture are discussed.

Spatially inhomogeneous bimodal inherent structure of simulated liquid water

In the supercooled regime at elevated pressure two forms of liquid water, high-density (HDL) and low-density (LDL), have been proposed to be separated by a coexistence line ending at a critical point, but a connection to water at ambient conditions has been lacking. Here we perform large-scale molecular dynamics simulations and demonstrate that the underlying potential energy surface gives a strictly bimodal characterization of the molecules at all temperatures and pressures, including the biologically and technologically important ambient regime, as spatially inhomogeneous either LDL- or HDL-like with a 3 : 1 predominance for HDL under ambient conditions. The Widom line in the supercooled regime, where maximal structural fluctuations take place, coincides with a 1 : 1 distribution. Although our results are based on molecular dynamics force-field simulations the close agreement with recent analyses of experimental X-ray spectroscopy and scattering data indicates a unified description also of real liquid water covering supercooled to ambient conditions.

Findings of Cp maximum at 233 K for the water within silica nanopores and very weak dependence of the Tmax on the pore size

How low-temperature water develops the formation of strong hydrogen bonds with some network structure is still open to a question. Heat capacities of the water confined within silica MCM-41 nanopores with different diameters in the range 1.7-4.2 nm were measured by adiabatic calorimetry. They revealed a hump with its maximum at 233 and 240 K for ordinary and heavy water, respectively. The maximum temperatures were essentially independent of the pore diameter, whereas the maximum values increased only in proportion to the fraction of the internal water molecules within the pores. It was concluded that the manner in which the hydrogen-bond formation progresses in bulk water is essentially the same as that in nanopore water and that strong hydrogen bonds are formed on cooling by arranging the neighboring water molecules at tetrahedral positions but keeping their network structure irregular to make striking contrast with ice structure.