Liquid-like Water Research Articles

Rotationally invariant local bond order parameters for accurate determination of hydrate structures

Averaged local bond order parameters based on spherical harmonics, also known as Lechner and Dellago order parameters, are routinely used to determine crystal structures in molecular simulations. Among different options, the combination of the q ¯ 4 and q ¯ 6 parameters is one of the best choices in the literature since distinguishes between solid- and liquid-like particles and between different crystallographic phases, including cubic and hexagonal phases. Recently, Algaba et al. [J. Colloid Interface Sci. 623, 354, (2022)] have used the Lechner and Dellago order parameters to distinguish hydrate- and liquid-like water molecules in the context of determining the carbon dioxide hydrate-water interfacial free energy. According to the results, the preferred combination previously mentioned is not the best option to differentiate between hydrate- and liquid-like water molecules. In this work, we revisit and extend the use of these parameters to deal with systems in which clathrate hydrates phases coexist with liquid phases of water. We consider carbon dioxide, methane, tetrahydrofuran, nitrogen, and hydrogen hydrates that exhibit sI and sII crystallographic structures. We find that the q ¯ 3 – q ¯ 12 combination is the best option possible between a large number of possible different pairs to distinguish between hydrate- and liquid-like water molecules in all cases.

Ion-specific ice provides a facile approach for reducing ice friction

HypothesisIce friction plays a crucial role in both basic study and practical use. Various strategies for controlling ice friction have been developed. However, one unsolved puzzle regarding ice friction is the effect of ion–ice interplay on its tribological properties. Experiments and SimulationsHere, we conducted ice friction experiments and summarized the specific effects of hydrated ions on ice friction. By selecting cations and anions, the coefficient of ice friction can be reduced by more than 70 percent. Experimental spectra, low-field nuclear magnetic resonance (LF-NMR), density functional theory (DFT) calculations, and Molecular dynamics (MD) simulations demonstrated that the addition of ions could break the H-bonds in water. FindingsThe link between the charge density of ions and the coefficients of ice friction was revealed. A part of the ice structure was changed from an ice-like to a liquid-like interfacial water structure with the addition of ions. Lower charge density ions led to weaker ionic forces with the water molecules in the immobilized water layer, resulting in free water molecules increasing in the lubricating layer. This study provides guidance for preparing ice-making solutions with low friction coefficients and a fuller understanding of the interfacial water structure at low temperatures.

Facilitating methane conversion and hydrogen evolution on platinized gallium oxide photocatalyst through liquid-like water nanofilm formation

The dehydrogenative coupling of methane, an approach aimed at converting methane into valuable chemicals like ethane and hydrogen, can be facilitated under ambient thermal conditions using semiconductor photocatalysts. Herein, we present an efficient continuous-flow photocatalytic system for methane conversion by inducing the formation of physically adsorbed water nanofilms on the surface of a Pt-loaded Ga2O3 photocatalyst. The microscopic properties of the adsorbed water were examined by diffuse reflectance infrared (IR) spectroscopy and ab initio molecular dynamics (AIMD) calculations. In comparison to the photocatalytic non-oxidative coupling of methane operated in the absence of water vapor, the presence of the interfacial water resulted in a 90-fold and 500-fold enhancement in the production rate of hydrogen and ethane, respectively. The rate of ethane formation significantly increased with increasing water vapor pressure (PH2O) under a high methane pressure (PCH4) of 200 kPa. The selectivity for ethane reached 65 % based on carbon content at PH2O = 3 kPa and PCH4 = 200 kPa. Through diffuse reflectance IR spectroscopy, volumetric water vapor adsorption analysis, and AIMD calculations conducted under the reaction conditions, we observed the formation of a liquid-like water nanolayer on the Ga2O3 surface at 300 K. Thus, this strategy of inducing water nanofilms affords a highly efficient photocatalytic system for hydrogen and ethane production.

Nanoconfined Water Clusters in Zinc White Oil Paint.

Pigments in oil paint are bound by a complex oil polymer network that is prone to water-related chemical degradation. We use cryo-Fourier-transform infrared spectroscopy and differential scanning calorimetry to study how water distributes inside zinc white oil paint. By measuring water freezing and melting transitions, we show that water-saturated zinc white oil paint contains both liquid-like clustered water and nonclustered water. A comparison of titanium white paint and nonpigmented model systems indicates that water clustering happens near the pigment-polymer interface. The cluster size was estimated in the nanometer range based on the ice melting and freezing temperatures and on the position of the O-D vibration band. As liquid-like water can play a crucial role in the dissolution and transport of ions and molecules, understanding the factors that favor this phenomenon is essential for establishing safe conditions for the conservation of painted works of art.

Molecular Changes in Spider Viscid Glue As a Function of Relative Humidity Revealed Using Infrared Spectroscopy.

Spider aggregate glue can absorb moisture from the atmosphere to reduce its viscosity and become tacky. The viscosity at which glue adhesion is maximized is remarkably similar across spider species, even though that viscosity is achieved at very different relative humidity (RH) values matching their diverse habitats. However, the molecular changes in the protein structure and the bonding state of water (both referred to here as molecular structure) with respect to the changes in RH are not known. We use attenuated total reflectance-infrared (ATR-IR) spectroscopy to probe the changes in the molecular structure of glue as a function of RH for three spider species from different habitats. We find that the glue retains bound water at lower RH and absorbs liquid-like water at higher RH. The absorption of liquid-like water at high RH plasticizes the glue and explains the decrease in glue viscosity. The changes to protein conformations as a function RH are either subtle or not detectable by IR spectroscopy. Importantly, the molecular changes are reversible over multiple cycles of RH change. Further, separation of glue constituents results in a different humidity response as compared to pristine glue, supporting the standing hypothesis that the glue constituents have a synergistic association that makes spider glue a functional adhesive. The results presented in this study provide further insights into the mechanism of the humidity-responsive adhesion of spider glue.

Investigating the quasi-liquid layer on ice surfaces: a comparison of order parameters.

Ice surfaces are characterized by pre-melted quasi-liquid layers (QLLs), which mediate both crystal growth processes and interactions with external agents. Understanding QLLs at the molecular level is necessary to unravel the mechanisms of ice crystal formation. Computational studies of the QLLs heavily rely on the accuracy of the methods employed for identifying the local molecular environment and arrangements, discriminating between solid-like and liquid-like water molecules. Here we compare the results obtained using different order parameters to characterize the QLLs on hexagonal ice (Ih) and cubic ice (Ic) model surfaces investigated with molecular dynamics (MD) simulations in a range of temperatures. For the classification task, in addition to the traditional Steinhardt order parameters in different flavours, we select an entropy fingerprint and a deep learning neural network approach (DeepIce), which are conceptually different methodologies. We find that all the analysis methods give qualitatively similar trends for the behaviours of the QLLs on ice surfaces with temperature, with some subtle differences in the classification sensitivity limited to the solid-liquid interface. The thickness of QLLs on the ice surface increases gradually as the temperature increases. The trends of the QLL size and of the values of the order parameters as a function of temperature for the different facets may be linked to surface growth rates which, in turn, affect crystal morphologies at lower vapour pressure. The choice of the order parameter can be therefore informed by computational convenience except in cases where a very accurate determination of the liquid-solid interface is important.

Effect of Size and Temperature on Water Dynamics inside Carbon Nano-Tubes Studied by Molecular Dynamics Simulation.

Water transport inside carbon nano-tubes (CNTs) has attracted considerable attention due to its nano-fluidic properties, its importance in nonporous systems, and the wide range of applications in membrane desalination and biological medicine. Recent studies show an enhancement of water diffusion inside nano-channels depending on the size of the nano-confinement. However, the underlying mechanism of this enhancement is not well understood yet. In this study, we performed Molecular Dynamics (MD) simulations to study water flow inside CNT systems. The length of CNTs considered in this study is 20 nm, but their diameters vary from 1 to 10 nm. The simulations are conducted at temperatures ranging from 260 K to 320 K. We observe that water molecules are arranged into coaxial water tubular sheets. The number of these tubular sheets depends on the CNT size. Further analysis reveals that the diffusion of water molecules along the CNT axis deviates from the Arrhenius temperature dependence. The non-Arrhenius relationship results from a fragile liquid-like water component persisting at low temperatures with fragility higher than that of the bulk water.

Water structure at the multilayers of palladium deposited at nanostructured Au electrodes

In situ ATR-SEIRAS experiments were carried out to describe water structure at the surface of multilayers of Pd modified nanopatterned gold film electrodes in contact with aqueous 0.1 M HClO 4 solution. The results show that the surface water becomes more ordered when the thickness of the Pd film increases. The “ice-like water” band becomes stronger and for the electrode with 5 ML Pd@Au, the ratio of “ice-like water” to “liquid-like water” becomes larger than that in bulk water. Concomitantly, the population of “monomers” or small water clusters diminishes. All surface water bands increase when hydrogen adsorption takes place. The bending band corresponding to “monomers” or “multimers” water is observed chiefly at the potentials of hydrogen adsorption.

Influence of Pore Surface Chemistry on the Rotational Dynamics of Nanoconfined Water

We have investigated the dynamics of water confined in mesostructured porous silicas (SBA-15, MCM-41) and four periodic mesoporous organosilicas (PMOs) by dielectric relaxation spectroscopy. The influence of water-surface interaction has been controlled by the carefully designed surface chemistry of PMOs that involved organic bridges connecting silica moieties with different repetition lengths, hydrophilicity and H-bonding capability. Relaxation processes attributed to the rotational motions of non-freezable water located in the vicinity of the pore surface were studied in the temperature range from 140 K to 225 K. Two distinct situations were achieved depending on the hydration level: at low relative humidity (33% RH), water formed a non-freezable layer adsorbed on the pore surface. At 75% RH, water formed an interfacial liquid layer sandwiched between the pore surface and the ice crystallized in the pore center. In the two cases, the study revealed different water dynamics and different dependence on the surface chemistry. We infer that these findings illustrate the respective importance of water-water and water-surface interactions in determining the dynamics of the interfacial liquid-like water and the adsorbed water molecules, as well as the nature of the different H-bonding sites present on the pore surface.

“Liquid-like” Water in Clathrates Induced by Host–Guest Hydrogen Bonding

Clathrate structures are sustained by a host lattice formed by hydrogen-bonding water molecules, which encapsulates guest molecules. Up to now, all water molecules in the host lattice are considered ice-like crystallized. Here, we discovered the occurrence of “liquid-like” water molecules and the resulting defects in (polar) tetrahydrofuran clathrates by liquid-state 1H NMR experiments. The liquid-like water molecules start occurring at 271 K, well below the apparent dissociation point (277 K) of the clathrate matrix, via extracting water molecules from the host lattice by host–guest H-bonding. We found an intriguing two-stage dissociation of the clathrate: Partial dissociation at 271 K converting one-third of water molecules into liquid-like followed by complete dissociation at 277 K. The clathrate structure is molecularly heterogeneous in the region between 271 K and 277 K. No liquid-like water exists in (nonpolar) cyclopentane clathrates. This work uncovers the essentiality of host–guest interaction for clathrate structures and the ability to tune their stability using polar molecules.

Influence of hydration/dehydration on adsorbed molecules: Case of phthalate on goethite

The behavior of adsorbed species on metal (hydr)oxides under water unsaturated conditions has been seldom studied despite its major implications in several domains such as geochemistry and heterogeneous catalysis. In this study, chemical forms of phthalate adsorbed on iron hydroxide (goethite) were investigated by infrared spectroscopy upon hydration and dehydration of goethite particle aggregates. Experimental results indicated important changes with relative humidity (RH) on chemical forms of phthalates adsorbed on goethite. With increasing RH from the dried state, bidentate phthalate surface complex is partially desorbed to form a protonated monodentate phthalate complex by using protons from water molecules. This protonated monodentate phthalate is still kept adsorbed on the goethite surface but becomes deprotonated at high RH values by solvating in liquid-like water covering on the goethite surface. DFT calculations made on a simplified system support these adsorption models. These results indicate that further studies are needed on the behavior of adsorbate in water unsaturated systems including hydration/dehydration processes.

Effect of Water Adsorption on the Frictional Properties of Hydrogenated Amorphous Carbon Films in Various Relative Humidities.

The tribological properties of hydrogenated amorphous carbon (a-C:H) films in ambient air were investigated from the microstructural point of view. a-C:H films with various microstructures (polymer-like, diamond-like, and graphite-like structures) were prepared, and the thickness of water adsorption layers on the films was measured. The adsorption behavior of water molecules on a-C:H films could be expressed with the Brunauer-Emmett-Teller (BET) isotherm, while the thicknesses of icelike and liquidlike water layers adsorbed on the films could be determined using the BET parameters C and nma. The polymer-like films exhibited the thickest icelike and liquidlike water adsorption layers, which decreased as the film structure changed to a diamond-like or a graphite-like structure. A strong relationship was observed between the thickness of water adsorption layers and the surface oxidation of the a-C:H films. The friction coefficient of the films in ambient air can be well explained by the surface oxidation and the thickness of water adsorption layers. Polymer-like films showed high friction coefficients due to the formation of a thick water layer on the films originated from the high surface oxidation of the film surface, whereas the graphite-like film exhibited a low friction coefficient due to low oxidation and a thin water adsorption layer. Furthermore, friction tests between the a-C:H films with different microstructures under ambient air were performed to determine the lowest friction pair in various relative humidities (RHs).

The effect of relative humidity on CaCl2 nanoparticles studied by soft X-ray absorption spectroscopy.

Ca- and Cl-containing nanoparticles are common in atmosphere, originating for example from desert dust and sea water. The properties and effects on atmospheric processes of these aerosol particles depend on the relative humidity (RH) as they are often both hygroscopic and deliquescent. We present here a study of surface structure of free-flying CaCl2 nanoparticles (CaCl2-NPs) in the 100 nm size regime prepared at different humidity levels (RH: 11–85%). We also created mixed nanoparticles by aerosolizing a solution of CaCl2 and phenylalanine (Phe), which is a hydrophobic amino acid present in atmosphere. Information of hydration state of CaCl2-NPs and production of mixed CaCl2 + Phe nanoparticles was obtained using soft X-ray absorption spectroscopy (XAS) at Ca 2p, Cl 2p, C 1s, and O 1s edges. We also report Ca 2p and Cl 2p X-ray absorption spectra of an aqueous CaCl2 solution. The O 1s X-ray absorption spectra measured from hydrated CaCl2-NPs resemble liquid-like water spectrum, which is heavily influenced by the presence of ions. Core level spectra of Ca2+ and Cl− ions do not show a clear dependence of % RH, indicating that the first coordination shell remains similar in all measured hydrated CaCl2-NPs, but they differ from aqueous solution and solid CaCl2.

Ion Solvation and Water Structure in an Aqueous Sodium Chloride Solution in the Gigapascal Pressure Range.

The structure of a 3 m (= mol/kg) NaCl aqueous solution at 1.3 and 1.7 GPa and 300 K, as well as at an ambient condition, is determined by synchrotron X-ray diffraction measurements combined with an empirical potential structure refinement (EPSR) modeling. When the solution is pressurized to the gigapascal pressure range, the ice-like hydrogen-bonded water network at 300 K/0.1 MPa is drastically perturbed to give rise to a simple, liquid-like water molecules arrangement retaining the hydrogen bonds. The coordination number of the chloride ion increases from around 6 at 0.1 MPa to about 16 at 1.7 GPa, accompanied by the extended solvation shells' evolution. On the other hand, the sodium ion's solvation structure does not change significantly with pressure and consists of 6-fold water molecules' coordination. We discuss a structure makers/breakers' concept for the ion solvation concerning the water structure in the gigapascal pressure range.

Influence of gas aggregation on water-solid interface: molecular simulation

ABSTRACT In this study, we conducted a series of molecular dynamics simulations of gas accumulation on homogeneous graphite substrates with a focus on the structure of fluid in the region near the substrate. The fluid density distribution revealed that epitaxial layers and gas enrichment layer arise from the adsorption of fluid particles on solids. Our analysis suggests that interfacial gas molecules transform into a liquid-like state and water molecules tend to cluster within the thin region adjacent to the substrate as the strength of gas–solid interaction increases. We then calculated the argon-water interfacial tension using three different methods: the Young-Laplace equation, direct integration method, and force balance method. Our analysis suggests that higher pressure within the gaseous domain tends to reduce tension at the curved argon-water interface. Although widely adopted to describe tension at gas–liquid interfaces, our results indicate that Bakker's equation is not necessarily suitable for the calculation of tension at fluid-solid interfaces. Our results indicate that fluid-solid interfacial properties and the morphology of the gaseous domain are influenced by the liquid-like epitaxial gas layer and gas enrichment layer, which could be attributed to the highly energetic nature of gas molecules and the tendency of gasses to adsorb on solids.

Two-dimensional sub-aerial, submerged, and transitional granular slides

The slide of granular material in nature and engineering can happen under air (subaerial), under a liquidlike water (submerged), or a transition between these two regimes, where a subaerial slide enters a liquid and becomes submerged. Here, we experimentally investigate these three slide regimes (i.e., subaerial, submerged, and transitional) in two dimensions, for various slope angles, material types, and bed roughness. The goal is to shed light on the complex morphodynamics and flow structure of these granular flows and also to provide comprehensive benchmarks for the validation and parametrization of the numerical models. The slide regime is found to be a major controller of the granular morphodynamics (e.g., shape evolution and internal flow structure). The time history of the runout distance for the subaerial and submerged cases present a similar three-phase trend (with acceleration, steady flow, and deceleration phases) tough with different spatiotemporal scales. Compared to the subaerial cases, the submerged cases show longer runout time and shorter final runout distances. The transitional trends, however, show additional deceleration and reacceleration. The observations suggest that the impact of slide angle, material type, and bed roughness on the morphodynamics is less significant where the material interacts with water. Flow structure, extracted using a granular particle image velocimetry technique, shows a relatively power-law velocity profile for the subaerial condition and strong circulations for the submerged condition. An unsteady theoretical model based on the µ(I) rheology is developed and is shown to be effective in the prediction of the average velocity of the granular mass.

Visualization of supercritical water pseudo-boiling at Widom line crossover

Supercritical water is a green solvent used in many technological applications including materials synthesis, nuclear engineering, bioenergy, or waste treatment and it occurs in nature. Despite its relevance in natural systems and technical applications, the supercritical state of water is still not well understood. Recent theories predict that liquid-like (LL) and gas-like (GL) supercritical water are metastable phases, and that the so-called Widom line zone is marking the crossover between LL and GL behavior of water. With neutron imaging techniques, we succeed to monitor density fluctuations of supercritical water while the system evolves rapidly from LL to GL as the Widom line is crossed during isobaric heating. Our observations show that the Widom line of water can be identified experimentally and they are in agreement with the current theory of supercritical fluid pseudo-boiling. This fundamental understanding allows optimizing and developing new technologies using supercritical water as a solvent.

Composite Membranes for High Temperature PEM Fuel Cells and Electrolysers: A Critical Review.

Polymer electrolyte membrane (PEM) fuel cells and electrolysers offer efficient use and production of hydrogen for emission-free transport and sustainable energy systems. Perfluorosulfonic acid (PFSA) membranes like Nafion® and Aquivion® are the state-of-the-art PEMs, but there is a need to increase the operating temperature to improve mass transport, avoid catalyst poisoning and electrode flooding, increase efficiency, and reduce the cost and complexity of the system. However, PSFAs-based membranes exhibit lower mechanical and chemical stability, as well as proton conductivity at lower relative humidities and temperatures above 80 °C. One approach to sustain performance is to introduce inorganic fillers and improve water retention due to their hydrophilicity. Alternatively, polymers where protons are not conducted as hydrated H3O+ ions through liquid-like water channels as in the PSFAs, but as free protons (H+) via Brønsted acid sites on the polymer backbone, can be developed. Polybenzimidazole (PBI) and sulfonated polyetheretherketone (SPEEK) are such materials, but need considerable acid doping. Different composites are being investigated to solve some of the accompanying problems and reach sufficient conductivities. Herein, we critically discuss a few representative investigations of composite PEMs and evaluate their significance. Moreover, we present advances in introducing electronic conductivity in the polymer binder in the catalyst layers.

Water Vapor Binding on Organic Matter-Coated Minerals.

Atmospheric water vapor binding to soils is a key process driving water availability in unsaturated terrestrial environments. Using a representative hydrophilic iron oxyhydroxide, this study highlights key mechanisms through which water vapor (i) adsorbs and (ii) condenses at mineral surfaces coated with Leonardite humic acid (LHA). Microgravimetry and vibrational spectroscopy showed that liquid-like water forms in the three-dimensional array of mineral-bound LHA when present at total C/Fe ratios well exceeding ∼73 mg C per g Fe(26 C atoms/nm2). Below these loadings, minerals become even less hydrophilic than in the absence of LHA. This lowering in hydrophilicity is caused by the complexation of LHA water-binding sites to mineral surfaces, and possibly by conformational changes in LHA structure removing available condensation environments for water. An empirical relationship predicting the dependence of water adsorption densities on LHA loadings was developed from these results. Together with the molecular-level description provided in this work, this relationship should guide efforts in predicting water availability, and thereby occurrences of water-driven geochemical processes in terrestrial environments.

Why Is It So Difficult to Identify the Onset of Ice Premelting?

Premelting of ice at temperatures below 0 °C is of fundamental importance for environmental processes. Various experimental techniques have been used to investigate the temperature at which liquid-likewaterfirst appears at the ice-vapor interface, reporting onset temperatures from -160 to -2 °C. The signals that identify liquid-likeorderat the ice-vapor interface in these studies, however, do not show a sharp initiation with temperature. That is at odds with the expected first-order nature of surface phase transitions, and consistent with recent large-scale molecular simulations that show the first premelted layer to be sparse and to develop continuously over a wide range of temperatures. Here we perform a thermodynamic analysis to elucidate the origin of the continuous formation of the first layer of liquid at the ice-vapor interface. We conclude that a negative value of the line tension of the ice-liquid-vapor three-phase contact line is responsible for the continuous character of the transition and the sparse nature of the liquid-like domains in the incomplete first layer.