Concluding remarks for Faraday Discussion on Water at Interfaces

Water at interfaces is a fascinating and multifaceted topic that has garnered significant attention in various scientific fields due to its relevance and implications. This Faraday Discussion explored the complexity of water at different interfaces. Many of the reports highlight the need for a molecular-level understanding. The Discussion was lively and constructive. In these summarizing remarks, I do not aim to be complete, but will rather try to sketch the status of the field, highlight the progress that we as a community have made, and present eclectic examples of where more work needs to be done.

Diffusion, Interactions, and Disparate Kinetic Trapping of Water-Hydrocarbon Mixtures in Nanoporous Solids

Mixed fluids confined in porous solid hosts present challenges for the accurate characterization of individual-component behavior. NMR diffusometry with chemical resolution is used to identify unexpected loading- and composition-dependent anomalous diffusion in water/cyclohexane mixtures confined to solid nanoporous glass (NPG) hosts. Diffusion NMR results indicate that data obtained on pure-component liquids in confinement cannot be extrapolated to their nonideal liquid mixtures confined in the same solid host. Loading-dependent data must be obtained on each component in the confined mixture in order to determine which of the liquid components exhibits chemical affinity for the host and, conversely, which of the components exhibits anomalous diffusivity. Most notably, NMR diffusometry revealed that cyclohexane diffusivity varied by 2 orders of magnitude in a water-rich mixture depending on the total fluid loading in the NPG host, ranging from anomalously high diffusivities that significantly exceeded that for pure cyclohexane in NPG at low fluid loadings to kinetically trapped sequestration at high fluid loadings. NMR diffusometry indicates that nonideal solution behavior in fluids confined within nanoporous hosts may have practical implications for enhanced oil recovery methods. Specifically, kinetic trapping of hydrocarbons in water-flooding regimes can result from complex liquid-vapor equilibrium that is significantly perturbed from that which exists in bulk or microporous confinement.

In Situ Study of Molecular Structure of Water and Ice Entrapped in Graphene Nanovessels

Water is ubiquitous in natural systems, ranging from the vast oceans to the nanocapillaries in the earth crust or cellular organelles. In bulk or in intimate contact with solid surfaces, water molecules arrange themselves according to their hydrogen (H) bonding, which critically affects their short- and long-range molecular structures. Formation of H-bonds among water molecules designates the energy levels of certain nonbonding molecular orbitals of water, which are quantifiable by spectroscopic techniques. While the molecular architecture of water in nanoenclosures is of particular interest to both science and industry, it requires fine spectroscopic probes with nanometer spatial resolution and sub-eV energy sensitivity. Graphene liquid cells (GLCs), which feature opposing closely spaced sheets of hydrophobic graphene, facilitate high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) measurements of attoliter water volumes encapsulated tightly in the GLC nanovessels. We perform in situ TEM and EELS analysis of water encased in thin GLCs exposed to room and cryogenic temperatures to examine the nanoscale arrangement of the contained water molecules. Simultaneous quantification of GLC thickness leads to the conclusion that H-bonding strengthens under increased water confinement. The present results demonstrate the feasibility of nanoscale chemical characterization of aqueous fluids trapped in GLC nanovessels and offer insights on water molecule arrangement under high-confinement conditions.

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.

Phase Transition in Monolayer Water Confined in Janus Nanopore

The ubiquitous nature of water invariably leads to a variety of physical scenarios that can result in many intriguing properties. We investigate the thermodynamics and associated phase transitions for a water monolayer confined within a quasi-two-dimensional nanopore. An asymmetric nanopore constructed by combining a hydrophilic (hexagonal boron nitride) sheet and a hydrophobic (graphene) sheet leads to an ordered water structure at much higher temperatures compared to a symmetric hydrophobic nanopore consisting of two graphene sheets. The discontinuous change in the thermodynamic quantities, potential energy ( U), and entropy ( S) of confined water molecules computed from the all-atom molecular dynamics simulation trajectories, uncovers a first-order phase transition in the temperature range of T = 320-330 K. Structural analysis reveals that water molecules undergo a disorder-to-order phase transformation in this temperature range with a 4-fold symmetric phase persisting at lower temperatures. Our findings predict a novel confinement system which has the melting transition for monolayer water above the room temperature, and provide a microscopic understanding which will have important implications for other nanofludic systems as well.

Insights on magnesium and sulfate ions' adsorption on the surface of sodium alumino-silicate hydrate (NASH) gel: a molecular dynamics study

The movement of water and ions in sodium alumino-silicate hydrate gel (NASH) influences the physical and chemical properties of the geopolymer material. In this paper, in order to better understand the structure and dynamics of water and ions in the interfacial region of the NASH gel, molecular dynamics was utilized to model Na2SO4 and MgSO4 solutions (both at 0.44 mol L-1) near the NASH surface. The broken silicate-aluminate surface network, with predominant percentage of randomly connected Q1 and Q2 silicate and aluminate species, provides plenty of non-bridging oxygen sites to accept the H bond from the surface water molecules, contributing toward a strongly adsorbed hydration layer with a thickness of around 5 Å. Consequently, the water molecule in the hydration layer exhibits increased density, increased dipole moment magnitude, orientation preference, and slow diffusivity. In contrast, up to 36.4% of the counter sodium ions, originally caged in the vacancies on the NASH surface, gradually dissociate from the silicate-aluminate skeleton and migrate into the bulk solution, which is consistent with the experimentally observed leaching process of alkali ions in the geopolymer material. In the MgSO4 solution, the magnesium ions-with a smaller ionic radius-penetrate into the silicate-aluminate skeleton vacancy, have 1.8 to 2.5 coordinated solid oxygen atoms, and remain on the NASH surface for a fairly longer time due to the stable Mg-O bonds. Mg species adsorbed on the inner sphere got rooted onto the hydroxyl layer, healing the damaged silicate-aluminate structures and stabilizing the network by inhibiting Na ion immigration into the solution. Mg ions in the outer layer, on average, associated with around one neighboring SO4 ion, forming ionic pairs and accumulating into large Mg-SO4 clusters, to help the immobilization of sulfate ions on the NASH surface.

Positron annihilation and nuclear magnetic resonance study of the phase behavior of water confined in mesopores at different levels of hydration

We investigated the molecular origin of the phase behavior of water confined in MCM 41 mesopores at different levels of hydration using positron annihilation spectroscopic and nuclear magnetic resonance techniques. The level of hydration influenced the phase behavior of the nanoconfined water. Two transitions above and below the bulk freezing temperature were observed depending on the level of hydration. At the highest level of hydration, nucleation seemed to predominate over the effect of confinement, leading to the complete freezing of water, whereas disrupted H-bonding dominated at the lowest level of hydration, leading to the disappearance of the transitions. A transition at c. T = 188 K (close to the reported glass transition temperature of interface-affected water) was observed at intermediate hydration level. This study suggests that the H-bonding network within nanoconfined water, which can be tampered by the degree of hydration, is the key factor responsible for the phase behavior of supercooled water. This study on the phase behavior and associated transitions of nanoconfined water has implications for nanofluidics and drug-delivery systems, in addition to understanding the fundamentals of water in confinement.

Neutron diffraction study of aqueous Laponite suspensions at the NIMROD diffractometer

The process of dynamical arrest, leading to formation of different arrested states such as glasses and gels, along with the closely related process of aging, is central for both basic research and technology. Here we report on a study of the time-dependent structural evolution of two aqueous Laponite clay suspensions at different weight concentrations. Neutron diffraction experiments have been performed with the near and intermediate range order diffractometer (NIMROD) that allows studies of the structure of liquids and disordered materials over a continuous length scale ranging from 1 to 300 Å, i.e., from the atomistic to the mesoscopic scales. NIMROD is presently a unique diffractometer, bridging the length scales traditionally investigated by small angle neutron scattering or small angle x-ray scattering with that accessible by traditional diffractometers for liquids. Interestingly, we have unveiled a signature of aging of both suspensions in the length scale region of NIMROD. This phenomenon, ascribed to sporadic contacts between Laponite platelets at long times, has been observed with the sample arrested as gel or as repulsive glass. Moreover, water molecules within the layers closest to Laponite platelets surface show orientational and translational order, which maps into the crystalline structure of Laponite.

Model atomistic protrusions favouring the ordering and retention of water

The ordering of water molecules near model linear atomistic protrusions is studied using classical molecular dynamics simulations.

Ice polyamorphism in the minimal Mercedes-Benz model of water

We investigate ice polyamorphism in the context of the two-dimensional Mercedes-Benz model of water. We find a first-order phase transition between a crystalline phase and a high-density amorphous phase. Furthermore, we find a reversible transformation between two amorphous structures of high and low density; however, we find this to be a continuous and not an abrupt transition, as the low-density amorphous phase does not show structural stability. We discuss the origin of this behavior and its implications with regard to the minimal generic modeling of polyamorphism.

First Principles Calculations of NMR Chemical Shifts of Liquid Water at an Amorphous Silica Interface

We investigate the anomalous structure and hydrogen bond network of water molecules confined inside a silica nanopore (MCM-41 type). In addition to geometric data, we use proton NMR chemical shifts as a measure for the strength of the H-bonding network. We compute the 1H NMR shifts of confined water based on a first principle approach in the framework of density functional perturbation theory under periodic boundary conditions. The hydrophilic character of the silica is well manifested in the water density profile. Our calculations illustrate both the modifications of the 1H NMR chemical shifts of the water with respect to bulk water and a considerable slowing down of water diffusion. In the vicinity of silanols, weakly hydrogen bonded liquid water is observed, while at the center region of the pore, the hydrogen bonding network is enhanced with respect to bulk water.

Dynamics and energetics of hydrophobically confined water

The effects of water confined in regions between self-assembling entities is relevant to numerous contexts such as macromolecular association, protein folding, protein-ligand association, and nanomaterials self-assembly. Thus assessing the impact of confined water, and the ability of current modeling techniques to capture the salient features of confined water is important and timely. We present molecular dynamics simulation results investigating the effect of confined water on qualitative features of potentials of mean force describing the free energetics of self-assembly of large planar hydrophobic plates. We consider several common explicit water models including the TIP3P, TIP4P, SPC/E, TIP4P-FQ, and SWM4-NDP, the latter two being polarizable models. Examination of the free energies for filling and unfilling the volume confined between the two plates (both in the context of average number of confined water molecules and "depth" of occupancy) suggests TIP4P-FQ water molecules generally occupy the confined volume at separation distances larger than observed for other models under the same conditions. The connection between this tendency of TIP4P-FQ water and the lack of a pronounced barrier in the potential of mean force for plate-plate association in TIP4P-FQ water is explored by artificially, but systematically, populating the confined volume with TIP4P-FQ water at low plate-plate separation distances. When the critical separation distance [denoting the crossover from an unoccupied (dry) confined interior to a filled (wet) interior] for TIP4P-FQ is reduced by 0.5 Å using this approach, a barrier is observed; we rationalize this effect based on increased resistant forces introduced by confined water molecules at these low separations. We also consider the dynamics of water molecules in the confined region between the hydrophobes. We find that the TIP4P-FQ water model exhibits nonbulklike dynamics, with enhanced lateral diffusion relative to bulk. This is consistent with the reduced intermolecular water-water interaction indicated by a decreased molecular dipole moment in the interplate region. Analysis of velocity autocorrelation functions and associated power spectra indicate that the interplate region for TIP4P-FQ at a plate separation of 14.4 Å approaches characteristics of the pure water liquid-vapor interface. This is in stark contrast to the other water models (including the polarizable SWM4-NDP model).

Adhesion of alumina surfaces through confined water layers containing various molecules

When two surfaces confine water layers between them at the nanoscale, the behavior of these confined water molecules can deviate significantly from the behavior of bulk water, and it could reflect on the adhesion of such surfaces. Thus, the aim of this study is to assess the role of confined water layers on the adhesion of hydrophilic surfaces and how sensitive this adhesion is to the presence of contaminants. Our methodology used under water AFM force measurements with an alumina-sputtered sphere-tipped cantilever and a flat alumina single crystal and then added fractions of ethanol, dimethylformamide, formamide, trimethylamine, and trehalose to water as contaminants. Such solutions were designed to illuminate the influences of dielectric constant, molecular size, refractive index, and number of hydrogen bonds from donors and acceptors of solutes to water. Apart from very dilute solutions of dimethylformamide, all solutions decreased the ability of confined water to give adhesion of the alumina surfaces. The predicted theoretical contribution of van der Waals and electrostatic forces was not observed when the contaminants distorted the way water organizes itself in confinement. The conclusion was that adhesion was sensitive mostly to the hydrogen-bonding network within water layers confined by the hydrophilic alumina surfaces.

Temperature dependence of the structure of protein hydration water and the liquid-liquid transition

We study the temperature dependence of the structure and orientation of the first hydration layers of the protein lysozyme and compare it with the situation for a model homogeneous hydrophobic surface, a graphene sheet. We show that in both cases these layers are significantly better structured than bulk water. The geometrical constraint of the interface makes the water molecules adjacent to the surface lose one water-water hydrogen bond and expel the fourth neighbors away from the surface, lowering local density. We show that a decrease in temperature improves the ordering of the hydration water molecules, preserving such a geometrical effect. For the case of graphene, this favors an ice Ih-like local structuring, similar to the water-air interface but in the opposite way along the c axis of the basal plane (while the vicinal water molecules of the air interface orient a hydrogen atom toward the surface, the oxygens of the water molecules close to the graphene plane orient a lone pair in such a direction). In turn, the case of the first hydration layers of the lysozyme molecule is shown to be more complicated, but still displaying signs of both kinds of behavior, together with a tendency of the proximal water molecules to hydrogen bond to the protein both as donors and as acceptors. Additionally, we make evident the existence of signatures of a liquid-liquid transition (Widom line crossing) in different structural parameters at the temperature corresponding to the dynamic transition incorrectly referred to as "the protein glass transition."

Density minimum of confined water at low temperatures: a combined study by small-angle scattering of X-rays and neutrons

A simple explanation is given for the low-temperature density minimum of water confined within cylindrical pores of ordered nanoporous materials of different pore size. The experimental evidence is based on combined data from in-situ small-angle scattering of X-rays (SAXS) and neutrons (SANS), corroborated by additional wide-angle X-ray scattering (WAXS). The combined scattering data cannot be described by a homogeneous density distribution of water within the pores, as was originally suggested from SANS data alone. A two-step density model reveals a wall layer covering approximately two layers of water molecules with higher density than the residual core water in the central part of the pores. The temperature-induced changes of the scattering signal from both X-rays and neutrons are consistent with a minimum of the average water density. We show that the temperature at which this minimum occurs depends monotonically on the pore size. Therefore we attribute this minimum to a liquid-solid transition of water influenced by confinement. For water confined in the smallest pores of only 2 nm in diameter, the density minimum is explained in terms of a structural transition of the surface water layer closest to the hydrophilic pore walls.

Glassy nature of water in an ultraconfining disordered material: the case of calcium-silicate-hydrate

We present the structural and dynamic nature of water ultraconfined in the quasi-two-dimensional nanopores of the highly disordered calcium-silicate-hydrate (C-S-H), the major binding phase in cement. Our approach is based on classical molecular simulations. We demonstrate that the C-S-H nanopore space is hydrophilic, particularly because of the nonbridging oxygen atoms on the disordered silicate chains which serve as hydrogen-bond acceptor sites, directionally orienting the hydrogen atoms of the interfacial water molecules toward the calcium-silicate layers. The water in this interlayer space adopts a unique multirange structure: a distorted tetrahedral coordination at short range up to 2.7 Å, a disordered structure similar to that of dense fluids and supercooled phases at intermediate range up to 4.2 Å, and persisting spatial correlations through dipole-dipole interactions up to 10 Å. A three-stage dynamics governs the mean square displacement (MSD) of water molecules, with a clear cage stage characteristic of the dynamics in supercooled liquids and glasses, consistent with its intermediate-range structure identified here. At the intermediate time scales corresponding to the β-relaxation of glassy materials, coincident with the cage stage in MSD, the non-Gaussian parameter indicates a significant heterogeneity in the translational dynamics. This dynamic heterogeneity is induced primarily because of the heterogeneity in the distribution of hydrogen bond strengths. The strongly attractive interactions of water molecules with the calcium silicate walls serve to constrain their motion. Our findings have important implications on describing the cohesion and mechanical behavior of cement from its setting to its aging.

Dynamics of water clusters confined in proteins: a molecular dynamics simulation study of interfacial waters in a dimeric hemoglobin

Water confined in proteins exhibits dynamics distinct from the dynamics of water in the bulk or near the surface of a biomolecule. We examine the water dynamics at the interface of the two globules of the homodimeric hemoglobin from Scapharca inaequivalvis (HbI) by molecular dynamics (MD) simulations, with focus on water-protein hydrogen bond lifetimes and rotational anisotropy of the interfacial waters. We find that relaxation of the waters at the interface of both deoxy- and oxy-HbI, which contain a cluster of 17 and 11 interfacial waters, respectively, is well described by stretched exponentials with exponents from 0.1 to 0.6 and relaxation times of tens to thousands of picoseconds. The interfacial water molecules of oxy-HbI exhibit slower rotational relaxation and hydrogen bond rearrangement than those of deoxy-HbI, consistent with an allosteric transition from unliganded to liganded conformers involving the expulsion of several water molecules from the interface. Though the interfacial waters are translationally and rotationally static on the picosecond time scale, they contribute to fast communication between the globules via vibrations. We find that the interfacial waters enhance vibrational energy transport across the interface by ≈10%.

The effect of confinement on water structure

Neutron diffraction experiments with hydrogen isotope substitution on water confined in MCM41-S15 have been performed at temperatures of 300 and 210 K. Data are analyzed at a microscopic level using a revised version of the empirical potential structure refinement technique. It is found that the influence of the substrate on the water structure is not negligible and depends on the temperature: owing to the geometrical constraints and the symmetry breaking induced by the wall, comparison with the corresponding bulk phases is not straightforward and standard analysis should be replaced by a more suitable one.

Anomalous dynamics of water confined in MCM-41 at different hydrations

We present the results of molecular dynamics simulations on water confined in a silica pore of 15 Å modelled to mimic the MCM-41 structure. We focus on the dynamical properties of water for different hydration levels of the pore. Density profiles show a well-defined double-layer structure close to the surface. From the analysis of the layers we find that water molecules close to the substrate show an anomalous diffusion which is connected to the behaviour of the residence time distribution. The interaction with the substrate induces temporal disorder. As a consequence strong deviations are found from the Markovian processes that usually determine the long-time diffusion properties. The residence time distribution of the water molecules in the inner part of the pore, far from the hydrophilic surface, is compatible with a Markovian process.

Structural and dynamical aspects of water in contact with a hydrophobic surface

By means of molecular dynamics simulations we study the structure and dynamics of water molecules in contact with a model hydrophobic surface: a planar graphene-like layer. The analysis of the distributions of a local structural index indicates that the water molecules proximal to the graphene layer are considerably more structured than the rest and, thus, than the bulk. This structuring effect is lost in a few angstroms and is basically independent of temperature for a range studied comprising parts of both the normal liquid and supercooled states (240K to 320K). In turn, such structured water molecules present a dynamics that is slower than the bulk, as a consequence of their improved interactions with their first neighbors.