Confined Water as Model of Supercooled Water

Dynamics of Uncrystallized Water, Ice, and Hydrated Polymer in Partially Crystallized Poly(vinylpyrrolidone)-Water Mixtures

In this study, we investigated the cooperative molecular dynamics of poly(vinylpyrrolidone) (PVP), ice, and uncrystallized water (UCW) in partially crystallized PVP-water mixtures by means of broadband dielectric spectroscopy. Three relaxation processes, denoted I, II, and III, were observed at temperatures ranging from immediately below the crystallization temperature (T_c) to approximately 200 K. At temperatures of 173-193 K, processes I and II cannot be distinguished. Below 168 K, process II separates into two processes: process IV at higher frequencies and process V at lower frequencies. Process I contributes to process V. In partially crystallized mixtures, process I originates from UCW in an uncrystallized phase with PVP. Process II is attributed to ice in the mixture, with a relaxation time that is 2 orders of magnitude smaller than that of pure ice. The concentration dependence of the strength of process II and the relaxation time relative to that of ice in bovine serum albumin (BSA)-water and gelatin-water mixtures strongly support this conclusion. Observation of processes IV and V indicates the presence of multiple ice relaxation processes. Process III is attributed to the α process of PVP in the uncrystallized phase in 40 and 50 wt % PVP mixtures. For mixtures with 30 wt % PVP or less, process III is attributed not only to the α process of PVP but also to interfacial polarization.

Quasi-Elastic Neutron Scattering Studies on Hydration Water in Phospholipid Membranes

The dynamic behavior of hydration water in phospholipid membranes has been investigated to understand the relationship between water and biological molecules using various experimental techniques. Quasi-elastic neutron scattering (QENS) is an effective method for this purpose because the dynamic behaviors of both water and lipid molecules could be identified by using selective deuteration. In addition, the measurable ranges from the 10⁻¹² to 10⁻⁹ s time scale and the 10⁻¹¹ to 10⁻⁸ m length scale are suitable to investigate the slowing down of water molecules due to their interaction with lipid membranes. In this mini-review, QENS experiments on the dynamic behavior of hydration water molecules in neighboring phospholipid membranes are summarized.

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure-volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime.

Specific Heat and Transport Functions ofWater

Numerous water characteristics are essentially ascribed to its peculiarity to form strong hydrogen bonds that become progressively more stable on decreasing the temperature. However, the structural and dynamical implications of the molecular rearrangement are still subject of debate and intense studies. In this work, we observe that the thermodynamic characteristics of liquid water are strictly connected to its dynamic characteristics. In particular, we compare the thermal behaviour of the isobaric specific heat of water, measured in different confinement conditions at atmospheric pressure (and evaluated by means of theoretical studies) with its configurational contribution obtained from the values of the measured self-diffusion coefficient through the use of the Adam–Gibbs approach. Our results confirm the existence of a maximum in the specific heat of water at about 225 K and indicate that especially at low temperature the configurational contributions to the entropy are dominant.

Tuning Water Networks via Ionic Liquid/Water Mixtures

Water in nanoconfinement is ubiquitous in biological systems and membrane materials, with altered properties that significantly influence the surrounding system. In this work, we show how ionic liquid (IL)/water mixtures can be tuned to create water environments that resemble nanoconfined systems. We utilize molecular dynamics simulations employing ab initio force fields to extensively characterize the water structure within five different IL/water mixtures: [BMIM + ][BF 4 - ], [BMIM + ][PF 6 - ], [BMIM + ][OTf - ], [BMIM + ][NO 3 - ]and [BMIM + ][TFSI - ] ILs at varying water fraction. We characterize water clustering, hydrogen bonding, water orientation, pairwise correlation functions and percolation networks as a function of water content and IL type. The nature of the water nanostructure is significantly tuned by changing the hydrophobicity of the IL and sensitively depends on water content. In hydrophobic ILs such as [BMIM + ][PF 6 - ], significant water clustering leads to dynamic formation of water pockets that can appear similar to those formed within reverse micelles. Furthermore, rotational relaxation times of water molecules in supersaturated hydrophobic IL/water mixtures indicate the close-connection with nanoconfined systems, as they are quantitatively similar to water relaxation in previously characterized lyotropic liquid crystals. We expect that this physical insight will lead to better design principles for incorporation of ILs into membrane materials to tune water nanostructure.

Static field gradient NMR studies of water diffusion in mesoporous silica

NMR diffusometry is used to ascertain the pore-size dependent water diffusion in MCM-41 and SBA-15 silica over broad temperature ranges. Detailed analysis of 1H and 2H NMR stimulated-echo decays reveals that fast water motion through voids between different silica particles impairs such studies in the general case. However, water diffusion inside single pores is probed in the present approach, which applies high static field gradients to enhance the spatial resolution of the experiment and uses excess water in combination with subzero temperatures to embed the silica particles in an ice matrix and, thus, to suppress interparticle water motion. It is found that the diffusion of confined water slows down by almost two orders of magnitude when the pore diameter is reduced from 5.4 nm to 2.1 nm at weak cooling. In the narrower silica pores, the temperature dependence of the self-diffusion coefficient of water is well described by an Arrhenius law with an activation energy of Ea = 0.40 eV. The Arrhenius behavior extends over a broad temperature range of at least 207-270 K, providing evidence against a fragile-to-strong crossover in response to a proposed liquid-liquid phase transition near 225 K. In the wider silica pores, partial crystallization results in a discontinuous temperature dependence. Explicitly, the diffusion coefficients drop when cooling through the pore-size dependent melting temperatures Tm of confined water. This finding can be rationalized by the fact that water can explore the whole pore volumes above Tm, but is restricted to narrow interfacial layers sandwiched between silica walls and ice crystallites below this temperature. Comparing our findings for water diffusion with previous results for water reorientation, we find significantly different temperature dependencies, indicating that the Stokes-Einstein-Debye relation is not obeyed.

Synthesis of microporous silica nanoparticles to study water phase transitions by vibrational spectroscopy

Silica can take many forms, and its interaction with water can change dramatically at the interface. Silica based systems offer a rich tapestry to probe the confinement of water as size and volume can be controlled by various templating strategies and synthetic procedures. To this end, microporous silica nanoparticles have been developed by a reverse microemulsion method utilizing zinc nanoclusters encapsulated in hydroxyl-terminated polyamidoamine (PAMAM-OH) dendrimers as a soft template. These nanoparticles were made tunable within the outer diameter range of 20-50 nm with a core mesopore of 2-15 nm. Synthesized nanoparticles were used to study the effects of surface area and microporous volumes on the vibrational spectroscopy of water. These spectra reveal contributions from bulk interfacial/interparticle water, ice-like surface water, liquid-like water, and hydrated silica surfaces suggesting that microporous silica nanoparticles allow a way to probe silica water interactions at the molecular scale.

Dielectric response of hydrated water as a structural component of nanofibrillated cellulose (NFC) from different plant sources

The current work illuminates the interplay between nanofibrillated cellulose (NFC) films and hydrated water. The NFC films from three sources of technological importance, i.e. cotton, wood and flax, are compared. It is shown that cellulose materials present slight variations in supramolecular structure depending on the plant origin. The structural differences determine both quantity and state of the water adsorbed by cellulose. Dielectric spectroscopy was employed to study the state of hydrated water as a probe of both the overall and specific marks of NFCs' structure. The measurements, carried out in the wide frequency (10^-2Hz -10⁶Hz) and temperature (123 K-293 K) ranges, revealed the formation of non-interactive water clusters at low water content. At high water content, additional states of water were identified: Water in saturated glass-forming solution and bulk. These water states were shown to be determined by the NFC's structure and morphology.

The Proton Density of States in Confined Water (H2O)

The hydrogen density of states (DOS) in confined water has been probed by inelastic neutron scattering spectra in a wide range of its P–T phase diagram. The liquid–liquid transition and the dynamical crossover from the fragile (super-Arrhenius) to strong (Arrhenius) glass forming behavior have been studied, by taking into account the system polymorphism in both the liquid and amorphous solid phases. The interest is focused in the low energy region of the DOS (E<10 meV) and the data are discussed in terms of the energy landscape (local minima of the potential energy) approach. In this latest research, we consider a unit scale energy (EC) linked to the water local order governed by the hydrogen bonding (HB). All the measured spectra, scaled according to such energy, evidence a universal power law behavior with different exponents (γ) in the strong and fragile glass forming regions, respectively. In the first case, the DOS data obey the Debye squared-frequency law, whereas, in the second one, we obtain a value predicted in terms of the mode-coupling theory (MCT) (γ≃1.6).

Hydrogen bonding structure of confined water templated by a metal-organic framework with open metal sites

Water in confinement exhibits properties significantly different from bulk water due to frustration in the hydrogen-bond network induced by interactions with the substrate. Here, we combine infrared spectroscopy and many-body molecular dynamics simulations to probe the structure and dynamics of confined water as a function of relative humidity within a metal-organic framework containing cylindrical pores lined with ordered cobalt open coordination sites. Building upon the agreement between experimental and theoretical spectra, we demonstrate that water at low relative humidity binds initially to open metal sites and subsequently forms disconnected one-dimensional chains of hydrogen-bonded water molecules bridging between cobalt atoms. With increasing relative humidity, these water chains nucleate pore filling, and water molecules occupy the entire pore interior before the relative humidity reaches 30%. Systematic analysis of rotational and translational dynamics indicates heterogeneity in this pore-confined water, with water molecules displaying variable mobility as a function of distance from the interface.

The properties of water under confinement are significantly altered with respect to the bulk phase. Here the authors use infrared spectroscopy and many-body molecular dynamics simulations to show the structure and dynamics of confined water as a function of relative humidity within a metal-organic framework.

Water in Mesoporous Confinement: Glass-To-Liquid Transition or Freezing of Molecular Reorientation Dynamics?

The first mechanical relaxation measurements (f = 400 Hz) of water confined in micro-porous silica were performed more than 40 years ago. The authors reported a so called “capillary transition” (here denoted as P3) of water in the core of the pores and a second one at a lower temperature, which they called the “adsorbate transition” (P1 in present work) related to water near the surface of the pores. The capillary transition was identified with the freezing of water in the centre of the pores. However, even 40 years later, the origin of the adsorbate transition is not yet clear. One study relates it to the liquid-to-glass transition of the supercooled water in the pores, and another study to the freezing of the proton reorientations at the lattice defects. The present work shows the data from extensive dynamic mechanical analysis (DMA) measurements (f = 0.1 Hz–70 Hz) of water confined in mesoporous silica (d = 2.5, 5 and 10 nm), which are in favour of a liquid-to-glass scenario.

Dynamics of hydration water in gelatin and hyaluronic acid hydrogels

We employed broadband dielectric spectroscopy (BDS), for the investigation of the water dynamics in partially hydrated hyaluronic acid (HA), and gelatin (Gel), enzymatically crosslinked hydrogels, in the water fraction ranges [Formula: see text]. Our results indicate that at low hydrations ([Formula: see text]), where the dielectric response of the hydrogels is identical during cooling and heating, water plasticizes strongly the polymeric matrix and is organized in clusters giving rise to [Formula: see text]-process, secondary water relaxation and to an additional slower relaxation process. This later process has been found to be related with the dc charge conductivity and can be described in terms of the conduction current relaxation mechanism. At slightly higher hydrations, however, always below the hydration level where ice is formed during cooling, we have recorded in HA hydrogel a strong water dielectric relaxation process, [Formula: see text], which has Arrhenius-like temperature dependence and large time scale resembling relaxation processes recorded in bulk low density amorphous solid water structures. This relaxation process shows a strong-to-fragile transition at [Formula: see text]C and our data suggest that the VTF-like process recorded at [Formula: see text]C is controlled by the same molecular process like long range charge transport. In addition, our data imply that the crossover temperature is related with the onset of structural rearrangements (increase in configurational entropy) of the macromolecules. In partially crystallized hydrogels ([Formula: see text]) HA exhibits at low temperatures the ice dielectric process consistent with the bulk hexagonal ice, whereas Gel hydrogel exhibits as main low temperature process a slow relaxation process that refers to open tetrahedral structures of water similar to low density amorphous ice structures and to bulk cubic ice. Regarding the water secondary relaxation processes, we have shown that the [Formula: see text]-process and the [Formula: see text] process are activated in water hydrogen bond networks with different structures.

Some considerations on the water polymorphism and the liquid-liquid transition by the density behavior in the liquid phase

The bulk liquid water density data (ρ) are studied in a very large temperature pressure range including also the glass phases. A thorough analysis of their isobars, together with the suggestions of recent thermodynamical studies, gives evidence of two crossovers at T^* and P^* above which the hydrogen bond interaction is unable to arrange the tetrahedral network that is at the basis of the liquid polymorphism giving rise to the low density liquid (LDL). The curvatures of these isobars, as a function of T, are completely different: concave below P^* (where maxima are) and convex above. In both the cases, a continuity between liquid and glass is observed with P^* as the border of the density evolution toward the two different polymorphic glasses (low and high density amorphous). The experimental data of the densities of these two glasses also show a markedly different pressure dependence. Here, on the basis of these observations in bulk water and by considering a recent study on the growth of the LDL phase, by decreasing temperature, we discuss the water liquid-liquid transition and evaluate the isothermal compressibility inside the deep supercooled regime. Such a quantity shows an additional maximum that is pressure dependent that under ambient conditions agrees with a recent X-ray experiment. In particular, the present analysis suggests the presence of a liquid-liquid critical point located at about 180 MPa and 197 K.

Translational and rotational dynamics of high and low density TIP4P/2005 water

We use molecular dynamics simulations using TIP4P/2005 to investigate the self- and distinct-van Hove functions for different local environments of water, classified using the local structure index as an order parameter. The orientational dynamics were studied through the calculation of the time-correlation functions of different-order Legendre polynomials in the OH-bond unit vector. We found that the translational and orientational dynamics are slower for molecules in a low-density local environment and correspondingly the mobility is enhanced upon increasing the local density, consistent with some previous works, but opposite to a recent study on the van Hove function. From the analysis of the distinct dynamics, we find that the second and fourth peaks of the radial distribution function, previously identified as low density-like arrangements, show long persistence in time. The analysis of the time-dependent interparticle distance between the central molecule and the first coordination shell shows that particle identity persists longer than distinct van Hove correlations. The motion of two first-nearest-neighbor molecules thus remains coupled even when this correlation function has been completely decayed. With respect to the orientational dynamics, we show that correlation functions of molecules in a low-density environment decay exponentially, while molecules in a local high-density environment exhibit bi-exponential decay, indicating that dynamic heterogeneity of water is associated with the heterogeneity among high-density and between high-density and low-density species. This bi-exponential behavior is associated with the existence of interstitial waters and the collapse of the second coordination sphere in high-density arrangements, but not with H-bond strength.

Hysteresis in the temperature dependence of the IR bending vibration of deeply cooled confined water

Using Fourier Transform Infrared (FTIR) spectroscopy, we investigate the temperature dependence of the bending vibrations of water confined in the pores of a silica hydrogel in the temperature interval of 270-180 K. We also investigate the presence of thermal hysteresis by cooling and reheating temperature scans. The results clearly show the presence, at about 230 K, of a crossover in the temperature dependence of the IR spectra; moreover, the presence of hysteresis is clearly demonstrated. By comparing FTIR data with neutron diffraction data and previous calorimetric data on the same samples, we conclude that the crossover and the hysteretical behavior do not involve a water glass transition or crystallization but are related to a first-order-like liquid-liquid transition.

Acyclic Janus-AT Nucleoside Host Channels Precisely Lock Water into Single-File Wires with Local Rotational Flexibility

Confining polar water molecules to particular geometries demands sophisticated intermolecular interactions, and not many small synthetic molecules have accomplished such a task. Herein, regioisomeric acyclic Janus-AT nucleosides (1 and 2), with a self-complementary fused genetic alphabet and conformationally flexible side chains, have been selectively synthesized. 1 and 2 adopt disparate base-pair motifs from the π-π stacked hydrophobic base moieties and distinct hydrogen bond (HB) interconnections from the hydrophilic sugar residues, which in turn lead to divergent, intricate intermolecular interaction networks with different capacities to confine water molecules. Under the precise control of the host framework of the N⁸ -regioisomer, separate ordered single-file water wires can be locked through special three-HB clamps into unique inter- and intra-wire geometrical alignments. Localized dynamic synchronized rotations within the fixed framework coordinated by both the host hydroxy groups and guest water molecules were observed in a temperature-induced reversible single-crystal-to-single-crystal transition (SCSCT).

Soft biomimetic nanoconfinement promotes amorphous water over ice

Water is a ubiquitous liquid with unique physicochemical properties, whose nature has shaped our planet and life as we know it. Water in restricted geometries has different properties than in bulk. Confinement can prevent low-temperature crystallization of the molecules into a hexagonal structure and thus create a state of amorphous water. To understand the survival of life at subzero temperatures, it is essential to elucidate this behaviour in the presence of nanoconfining lipidic membranes. Here we introduce a family of synthetic lipids with designed cyclopropyl modifications in the hydrophobic chains that exhibit unique liquid-crystalline behaviour at low temperature, which enables the maintenance of amorphous water down to ~10 K due to nanoconfinement. The combination of experiments and molecular dynamics simulations unveils a complex lipid-water phase diagram in which bicontinuous cubic and lamellar liquid crystalline phases that contain subzero liquid, glassy or ice water emerge as a competition between the two components, each pushing towards its thermodynamically favoured state.

Crystallization and Dynamics of Water Confined in Model Mesoporous Silica Particles: Two Ice Nuclei and Two Fractions of Water

The crystallization and dynamics of water confined in model mesoporous silica particles (pore diameters ranging from 2.1 to 5 nm; pore length ≈ 1 μm) are studied in homogeneous aqueous suspensions by dielectric spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance (NMR) techniques. We establish the phase diagram ( T vs 1/ d) of confined water covering a broad range of pore diameters. A linear dependence of the heterogeneous and the homogeneous nucleation temperatures on the inverse pore diameter is shown. The two lines converge at a pore diameter of ∼2.6 nm, below which formation of stable crystals is suppressed. By combining dielectric spectroscopy and different NMR techniques, we determine the dynamics of water within mesoporous silica over broad temperature and frequency ranges. Both techniques identify two dynamically distinguishable fractions of confined water coexisting within the pores. We attribute the two fractions to an interfacial water layer at the pore walls and confined water in the pore interior. Two alternative scenarios are proposed to rationalize the coexistence of two dynamically distinguishable water fractions. In the first scenario, two liquid fractions of water coexist under extreme confinement conditions for a range of temperatures; we discuss similarities with the two ultraviscous liquids (high-density liquid and low-density liquid) put forward for supercooled bulk water. In the second scenario, a liquid and a solid phase coexist; we conjecture that highly distorted and unstable crystal nuclei exist under extreme confinement that exhibit reorientation dynamics with time scales intermediate to the surrounding confined liquid and to bulk ice.

Assembly of peptides in mica-graphene nanocapillaries controlled by confined water

Water in nanoscale-confined geometries has unique physicochemical properties in contrast to bulk water, and is believed to play important roles in biological processes although there is less direct information available in the literature. Here, we report the self-assembly behaviors of a neurodegenerative disease related peptide termed GAV-9 encapsulated in mica-graphene nanocapillaries interacting with water nanofilms condensed under ambient conditions, based on atomic force microscopy (AFM) imaging and molecular dynamics (MD) simulations. The results revealed that, upon increase in the humidity, the GAV-9 peptide monomers adsorbed the confined water molecules and transitioned to unexpected hydrogel-like structures. Our MD simulations also suggested that in the confined mica-graphene nanocapillaries, the GAV-9 peptide monomers would indeed form water-rich hydrogel structures instead of highly ordered nanofilaments. The interfacial water confined in the mica-graphene nanocapillary is found to be crucial for such a transition. Moreover, the distribution of confined water layers largely depended on the locations of the preformed peptide nanofilaments, and the peptide nanofilaments further assembled into nanosheets with the water layer increasing, but depolymerized to amorphous peptide assemblies with the water layer decreasing. The polymerization and depolymerization of the peptide nanofilaments could be controlled in a reversible manner. Our results have supplied a simplified model system to uncover the effects of the confined interfacial water on the biological process at the molecular level.

Nanoscale dynamics of water confined in ordered mesoporous carbon

The single particle dynamics of water confined within two ordered mesoporous carbon matrices was investigated in the temperature range from 290 K to 170 K by quasielastic neutron scattering using three high resolution neutron spectrometers. Thus, it was possible to investigate the mobility of water confined in model hydrophobic cavities at the nanoscale. Models developed for the nanoscale dynamics of supercooled water and water confined within hydrophilic matrices were able to describe the collected data but remarkable differences with analogous silica confined matrices were observed in these carbon samples. A significant fraction of the water molecules was immobile on the nanosecond timescale, even at room temperature. As the temperature was lowered, the mobility of the water molecules slowed down, but the strongly non-Arrhenius behavior observed in bulk water and for fully hydrated hydrophilic confinement was absent, which indicates frustration of the hydrogen bond network formation. The obtained results were relevant for applications of mesoporous carbon materials.

Link between molecular mobility and order parameter during liquid-liquid transition of a molecular liquid

Liquid-liquid transition (LLT) is the transformation of one liquid to another via first-order phase transition. For example, LLT in a molecular liquid, triphenyl phosphite, is macroscopically the transformation from liquid I in a supercooled state to liquid II in a glassy state. Reflecting the transformation from the liquid to glassy state, the LLT is accompanied by considerable slowing down of overall molecular dynamics, but little is known about how this proceeds at a molecular level coupled with the evolution of the order parameter. We report such information by performing time-resolved simultaneous measurements of dielectric spectroscopy and phase contrast microscopy/Raman spectroscopy by using a dielectric cell with transparent electrodes. We find that the temporal change in molecular mobility crucially depends on whether LLT is nucleation growth type occurring in the metastable state or SD type occurring in the unstable state. Furthermore, our results suggest that the molecular mobility is controlled by the local order parameter: more specifically, the local activation energy of molecular rotation is controlled by the local fraction of locally favored structures formed in the liquid. Our study sheds light on the temporal change in the molecular dynamics during LLT and its link to the order parameter evolution.

Motions of water and solutes-Slaving versus plasticization phenomena

It is well-accepted that hydration water is crucial for the structure, dynamics, and function of proteins. However, the exact role of water for the motions and functions of proteins is still debated. Experiments have shown that protein and water dynamics are strongly coupled but with water motions occurring on a considerably faster time scale (the so-called slaving behavior). On the other hand, water also reduces the conformational entropy of proteins and thereby acts as a plasticizer of them. In this work, we analyze the dynamics (using broadband dielectric spectroscopy) of some specific non-biological water solutions in a broad concentration range to elucidate the role of water in the dynamics of the solutes. Our results demonstrate that at low water concentrations (less than 5 wt. %), the plasticization phenomenon prevails for all the materials analyzed. However, at higher water concentrations, two different scenarios can be observed: the slaving phenomenon or plasticization, depending on the solute analyzed. These results generalize the slaving phenomenon to some, but not all, non-biological solutions and allow us to analyze the key factors for observing the slaving behavior in protein solutions as well as to reshaping the slaving concept.

Structural Relaxation Processes and Collective Dynamics of Water in Biomolecular Environments

In this simulation study, we investigate the influence of biomolecular confinement on dynamical processes in water. We compare water confined in a membrane protein nanopore at room temperature to pure liquid water at low temperatures with respect to structural relaxations, intermolecular vibrations, and the propagation of collective modes. We observe distinct potential energy landscapes experienced by water molecules in the two environments, which nevertheless result in comparable hydrogen bond lifetimes and sound propagation velocities. Hence, we show that a viscoelastic argument that links slow rearrangements of the water-hydrogen bond network to ice-like collective properties applies to both, the pure liquid and biologically confined water, irrespective of differences in the microscopic structure.

Effects of water on the stochastic motions of propane confined in MCM-41-S pores

Quasi-elastic neutron scattering (QENS) and molecular dynamics simulations (MDS) reveal the effects of water on the structure and dynamics of propane confined in 1.5 nm wide cylindrical pores of MCM-41-S.

Common microscopic structural origin for water's thermodynamic and dynamic anomalies

Water displays a vast array of unique properties, known as water's anomalies, whose origin remains subject to hot debate. Our aim in this article is to provide a unified microscopic physical picture of water's anomalies in terms of locally favored structures, encompassing both thermodynamic and dynamic anomalies, which are often attributed to different origins. We first identify locally favored structures via a microscopic structural descriptor that measures local translational order and provide direct evidence that they have a hierarchical impact on the anomalies. At each state point, the strength of thermodynamic anomalies is directly proportional to the amount of locally favored structures, while the dynamic properties of each molecule depend on the local structure surrounding both itself and its nearest neighbors. To incorporate this, we develop a novel hierarchical two-state model. We show by extensive simulations of two popular water models that both thermodynamic and kinetic anomalies can be almost perfectly explained by the temperature and pressure dependence of these local and non-local versions of the same structural descriptor, respectively. Moreover, our scenario makes three unique predictions in supercooled water, setting it apart from other scenarios: (1) Presence of an "Arrhenius-to-Arrhenius" crossover upon cooling, as the origin of the apparent "fragile-to-strong" transition; (2) maximum of dynamic heterogeneity around 20 K below the Widom line and far above the glass transition; (3) violation of the Stokes-Einstein-Debye relation at ∼2T _g, rather than 1.2T _g typical of normal glass-formers. These predictions are verified by recent measurement of water's diffusion at very low temperatures (point 1) and discoveries from our extensive simulations (points 2-3). We suggest that the same scenario may generally apply to water-like anomalies in liquids tending to form locally favored structures, including not only other important tetrahedral liquids such as silicon, germanium, and silica, but also metallic and chalcogenide liquids.

Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals

The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na⁺, K⁺, NMe₄⁺) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion-water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.

Fundamental Link between β Relaxation, Excess Wings, and Cage-Breaking in Metallic Glasses

In glassy materials, the Johari-Goldstein secondary (β) relaxation is crucial to many properties as it is directly related to local atomic motions. However, a long-standing puzzle remains elusive: why some glasses exhibit β relaxations as pronounced peaks while others present as unobvious excess wings? Using microsecond atomistic simulation of two model metallic glasses (MGs), we demonstrate that such a difference is associated with the number of string-like collective atomic jumps. Relative to that of excess wings, we find that MGs having pronounced β relaxations contain larger numbers of such jumps. Structurally, they are promoted by the higher tendency of cage-breaking events of their neighbors. Our results provide atomistic insights for different signatures of the β relaxation that could be helpful for understanding the low-temperature dynamics and properties of MGs.

DMA study of water's glass transition in nanoscale confinement

Dynamic mechanical analysis (DMA) measurements of water confined in nanoporous silica have been performed as a function of temperature and frequency for different pore sizes (2.5-10 nm) at heating and cooling. Most of the data show three processes, P1, P2 and P3, where P1 and P2 depend on measurement frequency and P3 does not. The characteristic shift of P3 with pore size shows that this process corresponds to freezing/melting of "internal water", i.e. in the core of the pores. Thermal expansion data indicate - in agreement with e.g. [A. Taschin, P. Bartolini and R. Torre, Meas. Sci. Technol., 2017, 28, 014009] - that in all our nanoporous systems about 2 layers of water remain liquid much below the freezing point. Dynamic elastic measurements show clear signatures of glass freezing of this supercooled water in the vicinity of P1. Extrapolating the DMA data to the timescale (103 s) of adiabatic calorimetry unveils a systematic behaviour: P1(T) shows a clear size dependence for a broad range of pore diameters, i.e. 2.5 nm ≤ d ≤ 52 nm, implying (together with the corresponding activation energy 0.5 eV) that P1 corresponds to the glass-liquid transition of a few layers of supercooled water at Tg(d). An extrapolation of Tg(d) to d → ∞ yields Tg(∞) ≈ 136 K, the traditional value for bulk water. The small (liquid like) value of Young's modulus in a temperature region above P1 is most naturally explained assuming that the supercooled water in this range is still liquid, implying that Tg values of 160 K or even 210 K - as suggested by various authors - are unlikely.

Origin of the emergent fragile-to-strong transition in supercooled water

Liquids can be broadly classified into two categories, fragile and strong ones, depending on how their dynamical properties change with temperature. The dynamics of a strong liquid obey the Arrhenius law, whereas the fragile one displays a super-Arrhenius law, with a much steeper slowing down upon cooling. Recently, however, it was discovered that many materials such as water, oxides, and metals do not obey this simple classification, apparently exhibiting a fragile-to-strong transition far above [Formula: see text] Such a transition is particularly well known for water, and it is now regarded as one of water's most important anomalies. This phenomenon has been attributed to either an unusual glass transition behavior or the crossing of a Widom line emanating from a liquid-liquid critical point. Here by computer simulations of two popular water models and through analyses of experimental data, we show that the emergent fragile-to-strong transition is actually a crossover between two Arrhenius regimes with different activation energies, which can be naturally explained by a two-state description of the dynamics. Our finding provides insight into the fragile-to-strong transition observed in a wide class of materials.

Molecular Dynamics Simulations of Water, Silica, and Aqueous Mixtures in Bulk and Confinement

Aqueous systems are omnipresent in nature and technology. They show complex behaviors, which often originate in the existence of hydrogen-bond networks. Prominent examples are the anomalies of water and the non-ideal behaviors of aqueous solutions. The phenomenology becomes even richer when aqueous liquids are subject to confinement. To this day, many properties of water and its mixtures, in particular, under confinement, are not understood. In recent years, molecular dynamics simulations developed into a powerful tool to improve our knowledge in this field. Here, our simulation results for water and aqueous mixtures in the bulk and in various confinements are reviewed and some new simulation data are added to improve our knowledge about the role of interfaces. Moreover, findings for water are compared with results for silica, exploiting that both systems form tetrahedral networks.

Water-like anomalies as a function of tetrahedrality

Water is the most common and yet least understood material on Earth. Despite its simplicity, water tends to form tetrahedral order locally by directional hydrogen bonding. This structuring is known to be responsible for a vast array of unusual properties, e.g., the density maximum at 4 °C, which play a fundamental role in countless natural and technological processes, with the Earth’s climate being one of the most important examples. By systematically tuning the degree of tetrahedrality, we succeed in continuously interpolating between water-like behavior and simple liquid-like behavior. Our approach reveals what physical factors make water so anomalous and special even compared with other tetrahedral liquids.

Tetrahedral interactions describe the behavior of the most abundant and technologically important materials on Earth, such as water, silicon, carbon, germanium, and countless others. Despite their differences, these materials share unique common physical behaviors, such as liquid anomalies, open crystalline structures, and extremely poor glass-forming ability at ambient pressure. To reveal the physical origin of these anomalies and their link to the shape of the phase diagram, we systematically study the properties of the Stillinger–Weber potential as a function of the strength of the tetrahedral interaction λ. We uncover a unique transition to a reentrant spinodal line at low values of λ, accompanied with a change in the dynamical behavior, from non-Arrhenius to Arrhenius. We then show that a two-state model can provide a comprehensive understanding on how the thermodynamic and dynamic anomalies of this important class of materials depend on the strength of the tetrahedral interaction. Our work establishes a deep link between the shape of the phase diagram and the thermodynamic and dynamic properties through local structural ordering in liquids and hints at why water is so special among all substances.

Properties of Hydrogen-Bonded Liquids at Interfaces

Effects of interfaces on hydrogen-bonded liquids play major roles in nature and technology. Despite their importance, a fundamental understanding of these effects is still lacking. In large parts, this shortcoming is due to the high complexity of these systems, leading to an interference of various interactions and effects. Therefore, it is advisable to take gradual approaches, which start from well designed and defined model systems and systematically increase the level of intricacy towards more complex mimetics. Moreover, it is necessary to combine insights from a multitude of methods, in particular, to link novel preparation strategies and comprehensive experimental characterization with inventive computational and theoretical modeling. Such concerted approach was taken by a group of preparative, experimentally, and theoretically working scientists in the framework of Research Unit FOR 1583 funded by the Deutsche Forschungsgemeinschaft (German Research Foundation). This special issue summarizes the outcome of this collaborative research. In this introductory article, we give an overview of the covered topics and the main results of the whole consortium. The following contributions are review articles or original works of individual research projects.

2H NMR Studies on the Dynamics of Pure and Mixed Hydrogen-Bonded Liquids in Confinement

2H NMR is used to ascertain dynamical behaviors of pure and mixed hydrogen-bonded liquids in bulk and in confinement. Detailed comparisons of previous and new results in broad dynamic and temperature ranges reveal that confinement effects differ for various liquids and confinements. For water, molecular reorientation strongly depends on the confinement size, with much slower and less fragile structural relaxation under more severe geometrical restriction. Moreover, a dynamical crossover occurs when a fraction of solid water forms so that the dynamics of the fraction of liquid water becomes even more restricted and, as a consequence, changes from bulk-like to interface-dominated. For glycerol, by contrast, confinement has weak effects on the reorientation dynamics. Mixed hydrogen-bonded liquids show even more complex dynamical behaviors. For aqueous solutions, the temperature dependence of the structural relaxation becomes discontinuous when the concentration changes due to a freezing of water fractions. This tendency for partial crystallization is enhanced rather than reduced by confinement, because different liquid-matrix interactions of the molecular species induce micro-phase segregation, which facilitates ice formation in water-rich regions. In addition, dynamical couplings at solvent-protein interfaces are discussed. It is shown that, on the one hand, solvent dynamics are substantially slowed down at protein surfaces and, on the other hand, protein dynamics significantly depend on the composition and, thus, the viscosity of the solvent. Furthermore, a protein dynamical transition occurs when the amplitude of water-coupled restricted backbone dynamics vanishes upon cooling.

2H NMR Studies on Water Dynamics in Functionalized Mesoporous Silica

Mesoporous silica MCM-41 is prepared, for which the inner surfaces are modified by 3-(aminopropyl)triethoxysilane (APTES) in a controlled manner. Nitrogen gas adsorpition yields a pore diameter of 2.2 nm for the APTES functionalized MCM-41. 2H nuclear magnetic resonance (NMR) and broadband dielectric spectroscopy (BDS) provide detailed and consistent insights into the temperature-dependent reorientation dynamics of water in this confinement. We find that a liquid water species becomes accompanied by a solid water species when cooling through ~210 K, as indicated by an onset of bimodal 2H spin-lattice relaxation. The reorientation of the liquid water species is governed by pronounced dynamical heterogeneity in the whole temperature range. Its temperature dependence shows a mild dynamic crossover when the solid water species emerges and, hence, the volume accessible to the liquid water species further shrinks. Therefore, we attribute this variation in the temperature dependence to a change from bulk-like behavior towards interface-dominated dynamics. Below this dynamic crossover, 2H line-shape and stimulted-echo studies show that water reorientation becomes anisotropic upon cooling, suggesting that these NMR approaches, but also BDS measurements do no longer probe the structural (α) relaxation, but rather a secondary (β) relaxation of water at sufficiently low temperatures. Then, another dynamic crossover at ~180 K can be rationalized in terms of a change of the temperature dependence of the β relaxation in response to a glassy freezing of the α relaxation of confined water. Comparing these results for APTES modied MCM-41 with previous findings for mesoporous silica with various pore diameters, we obtain valuable information about the dependence of water dynamics in restricted geometries on the size of the nanoscopic confinements and the properties of the inner surfaces.