Tuning the free energy of host-guest encapsulation by cosolvent

Supramolecular hosts create unique microenvironments which enable the tuning of reactions via steric confinement and electrostatics. It has been shown that "solvent shaping inside hydrophobic cavities" is an important thermodynamic driving force for guest encapsulation in the nanocage host. Here, we show that even small (5%) changes in the solvent composition can have a profound impact on the free energy of encapsulation. In a combined THz, NMR and ab initio MD study, we reveal that the preferential residing of a single DMSO molecule in the cavity upon addition of ≥5% DMSO results in a considerable change of ΔS from 63-76 cal mol-1 K-1 to 23-24 cal mol-1 K-1. This can be rationalized by reduction of the cavity volume due to the DMSO molecule which resides preferentially in the cavity. These results provide novel insights into the guest-binding interactions, emphasizing that the entropic driving force is notably influenced by even small changes in the solvent composition, irrespective of changes in metal ligand vertices. Having demonstrated that the local solvent composition within the cage is essential for regulating catalytic efficiency, solvent tuning might enable novel applications in supramolecular chemistry in catalysis and chemical separation.

Multi-Site Red-Edge Excitation Shift Reveals the Residue-Specific Solvation Dynamics during the Native to Amyloid-like Transition of an Amyloidogenic Protein

Changes in water-protein interactions are crucial for proteins to achieve functional and nonfunctional conformations during structural transitions by modulating local stability. Amyloid-like protein aggregates in deteriorating neurons are hallmarks of neurodegenerative disorders. These aggregates form through significant structural changes, transitioning from functional native conformations to supramolecular cross-β-sheet structures via misfolded and oligomeric intermediates in a multistep process. However, the site-specific dynamics of water molecules from the native to misfolded conformations and further to oligomeric and compact amyloid structures remain poorly understood. In this study, we used the fluorescence method known as red-edge excitation shift (REES) to investigate the solvation dynamics at specific sites in various equilibrium conformations en route to the misfolding and aggregation of the functional domain of the TDP-43 protein (TDP-43tRRM). We generated three single tryptophan-single cysteine mutants of TDP-43tRRM, with the cysteines at different positions and tryptophan at a fixed position. Each sole cysteine was fluorescently labeled and used as a site-specific fluorophore along with the single tryptophan, creating four monitorable sites for REES studies. By investigating the site-specific extent of REES, we developed a residue-specific solvation dynamics map of TDP-43tRRM during its misfolding and aggregation. Our observations revealed that solvation dynamics progressively became more rigid and heterogeneous to varying extents at different sites during the transition from native to amyloid-like conformations.

Role of hydrogen-bond coordination defects in the structural relaxation of supercooled water

In this work, we shall study the role of threefold and fivefold coordination defects in the structure and dynamics of the hydrogen bond network of liquid water, with special emphasis on the glassy regime. A significant defect clusterization propensity will be made evident, with a prevalence of mixed pairs, that is, threefold- and fivefold-coordinated defects being first neighbors of each other. This structural analysis will enable us to determine the existence of defective and nondefective regions compatible with the high local density and low local density molecular states of liquid water, respectively. Hydrogen bond coordination defects will also be shown to promote water's structural relaxation, with the undercoordinated ones playing a main role in driving glassy relaxation dynamics. Moreover, we shall show that the three-foldcoordinated molecules together with their first neighbors present at the initial configuration act as markers of the dynamical heterogeneities that would emerge at later times commensurate with the structural relaxation of the supercooled system.

Confinement Effects on Reorientation Dynamics of Water Confined within Graphite Nanoslits

Molecular dynamics simulations were used to investigate the reorientation dynamics of water confined within graphite nanoslits of size less than 2 nm, where molecules formed inner and interfacial layers parallel to the confining walls. Significantly related to molecular reorientations, the hydrogen-bond (HB) network of nanoconfined water therein was scrutinized by HB configuration fractions compared to those of bulk water and the influences on interfacial-molecule orientations relative to a nearby C atom plate. The second-rank orientation time correlation functions (OTCFs) of nanoconfined water were calculated and found to follow stretched-exponential, power-law, and power-law decays in a time series. To understand this naïve behavior of reorientation relaxation, the approach of statistical mechanics was adopted in our studies. In terms of the orientation Van Hove function (OVHF), an alternative meaning was given to the second-rank OTCF, which is a measure of the deviation of the OVHF between a molecular system and free molecules in random orientations. Indicated by the OVHFs at related time scales, the stretched-exponential decay of the second-rank OTCF resulted from molecules evacuating out of HB cages formed by their neighbors. After the evacuations, the inner molecules relaxed at relatively fast rates toward random orientations, but the interfacial molecules reoriented at slow rates due to restrictions by hydrophobic interactions with graphite walls. The first power-law decay of the second-rank OTCF was attributed to the distinct relaxation rates of inner and interfacial molecules within a graphite nanoslit. When the inner molecules were completely random in orientation, the second-rank OTCFs changed to another power law decay with a power smaller than the first one.

A structural determinant of the behavior of water at hydration and nanoconfinement conditions

The molecular nature of the phases that conform the two-liquid scenario is elucidated in this work in the light of a molecular principle governing water structuring, which unveils the relevance of the contraction and reorientation of the second molecular shell to allow for the existence of coordination defects in water's hydrogen bond network. In turn, such principle is shown to also determine the behavior of hydration and nanoconfined water while enabling to define conditions for wettability (quantifying hydrophobicity and predicting drying transitions), thus opening the possibility to unravel the active role of water in central fields of research.

Characterizing the Orderliness of Interfacial Water through Stretching Vibrations

Spatial orderliness, which is the orderly structure of molecules, differs significantly between interfacial water and bulk water. Understanding this property is essential for various applications in both natural and engineered environments. However, the subnanometer thickness of interfacial water presents challenges for direct and rapid characterization of its structural orderliness. Herein, through molecular dynamics simulations and infrared spectral analysis of interfacial water in a graphene slit pore, we reveal a hyperbolic tangent relationship between the water ordering and its O-H stretching information in the infrared spectrum. Specifically, O-H symmetric stretching dominated in the highly ordered water structure, while a transition to the asymmetric stretching corresponded to an increase in the degree of disorder. Thus, the O-H stretching behavior could serve as a useful and quick assessment of the orderliness of interfacial water. This work provided insights into interfacial water's unique molecular network and structural dynamics and identified the stretching vibrations' key role in its degree of order, providing insight for fields such as nanotechnology, biology, and material science.

Water and ions in electrified silica nano-pores: a molecular dynamics study

Solid-liquid interfaces (SLIs) are ubiquitous in science and technology from the development of energy storage devices to the chemical reactions occurring in the biological milieu. In systems involving aqueous saline solutions as the liquid, both the water and the ions are routinely exposed to an electric field, whether the field is externally applied, or originating from the natural surface charges of the solid. In the current study a molecular dynamics (MD) framework is developed to study the effect of an applied voltage on the behaviour of ionic solutions located in a ∼7 nm pore between two uncharged hydrophilic silica slabs. We systematically investigate the dielectric properties of the solution and the organisation of the water and ions as a function of salt concentration. In pure water, the interplay between interfacial hydrogen bonds and the applied field can induce a significant reorganisation of the water orientation and densification at the interface. In saline solutions, at low concentrations and voltages the interface dominates the whole system due to the extended Debye length resulting in a dielectric constant lower than that for the bulk solution. An increase in salt concentration or voltage brings about more localized interfacial effects resulting in dielectric properties closer to that of the bulk solution. This suggests the possibility of tailoring the system to achieve the desired dielectric properties. For example, at a specific salt concentration, interfacial effects can locally increase the dielectric constant, something that could be exploited for energy storage.

Water molecules in boron nitride interlayer space: ice and hydrolysis in super confinement

Development of nano-sized channels and filters in the recent years has made the role of water immensely important as water molecules affect their performance and durability. Here, we take advantage of molecular dynamics and density functional theory methods to demonstrate the shift in behavior of water molecules confined between hexagonal boron nitride (HBN) sheets spaced at 3.0 to 6.5 Å. Our results demonstrate that lower interlayer spaces cause higher amounts of charge transferred between the species, while at extreme degrees of confinement, these interactions cause the disintegration of trapped water molecules. Consequently, the inner face of the HBN sheets is functionalized with hydroxyl groups, releasing hydrogens in the form of protons that travel the interlayer space by Grotthuss mechanism. This is the first-hand evidence of a mechanical form of hydrolysis that corresponds with a nucleophilic attack (on boron atoms) to relieve water from extreme confined conditions. This process unveils a previously unknown behavior of water within extremely confined spaces and reveals new considerations concerning nanofilters and nanochannels.

Water at the nanoscale: From filling or dewetting hydrophobic pores and carbon nanotubes to "sliding" on graphene

In this work, we study the effect of nanoconfinement on the hydration properties of model hydrophobic pores and carbon nanotubes, determining their wetting propensity and the conditions for geometrically induced dehydration. By employing a recently introduced water structural index, we aim at two main goals: (1) to accurately quantify the local hydrophobicity and predict the drying transitions in such systems, and (2) to provide a molecular rationalization of the wetting process. In this sense, we will further discuss the number and strength of the interactions required by the water molecules to promote wetting. In the case of graphene-like surfaces, an explanation for their unexpectedly significant hydrophilicity will also be provided. On the one hand, the structural index will show that the net attraction to the dense carbon network that a water molecule experiences through several simultaneous weak interactions is sufficient to give rise to hydrophilic behavior. On the other hand, we will show that an additional effect is also at play: the hydrating water molecule is retained on the surface by a smooth exchange of such simultaneous weak interactions, as if "sliding" on graphene.

A water structure indicator suitable for generic contexts: Two-liquid behavior at hydration and nanoconfinement conditions and a molecular approach to hydrophobicity and wetting

In a recent work, we have briefly introduced a new structural index for water that, unlike previous indicators, was devised specifically for generic contexts beyond bulk conditions, making it suitable for hydration and nanoconfinement settings. In this work, we shall study this metric in detail, demonstrating its ability to reveal the existence of a fine-tuned interplay between the local structure and energetics in liquid water. This molecular principle enables the establishment of an extended hydrogen bond network, while simultaneously allowing for the existence of network defects by compensating for uncoordinated sites. By studying different water models and different temperatures encompassing both the normal liquid and the supercooled regime, this molecular mechanism will be shown to underlie the two-state behavior of bulk water. In addition, by studying functionalized self-assembled monolayers and diverse graphene-like surfaces, we shall show that this principle is also operative at hydration and nanoconfinement conditions, thus generalizing the validity of the two-liquid scenario of water to these contexts. This approach will allow us to define conditions for wettability, providing an accurate measure of hydrophobicity and a reliable predictor of filling and drying transitions. Hence, it might open the possibility of elucidating the active role of water in the broad fields of biophysics and materials science. As a preliminary step, we shall study the hydration structure and hydrophilicity of graphene-like systems (parallel graphene sheets and carbon nanotubes) as a function of the confinement dimensionality.

Acetylation of Nanocellulose: Miscibility and Reinforcement Mechanisms in Polymer Nanocomposites

The improvement of properties in nanocomposites obtained by topochemical surface modification, e.g., acetylation, of the nanoparticles is often ascribed to improved compatibility between the nanoparticle and the matrix. It is not always clear however what is intended: specific interactions at the interface leading to increased adhesion or the miscibility between the nanoparticle and the polymer. In this work, it is demonstrated that acetylation of cellulose nanocrystals greatly improves mechanical properties of their nanocomposites with polycaprolactone. In addition, molecular dynamics simulations with a combination of potential of mean force calculations and computational alchemy are employed to analyze the surface energies between the two components. The work of adhesion between the two phases decreases with acetylation. It is discussed how acetylation can still contribute to the miscibility, which leads to a stricter use of the concept of compatibility. The integrated experimental-modeling toolbox used has wide applicability for assessing changes in the miscibility of polymer nanocomposites.

Relationships between Water's Structure and Solute Affinity at Polypeptoid Brush Surfaces

Excellent antifouling surfaces are generally thought to create a tightly bound layer of water that resists solute adsorption, and highly hydrophilic surfaces such as those with zwitterionic functionalities are of significant current interest as antifoulant strategies. However, despite significant proofs-of-concept, we still lack a fundamental understanding of how the nanoscopic structure of this hydration layer translates to reduced fouling, how surface chemistry can be tuned to achieve antifouling through hydration water, and why, in particular, zwitterionic surfaces seem so promising. Here, we use molecular dynamics simulations and free energy calculations to investigate the molecular relationships among surface chemistry, hydration water structure, and surface-solute affinity across a variety of surface-decorated chemistries. Specifically, we consider polypeptoid-decorated surfaces that display well-known experimental antifouling capabilities and that can be synthesized sequence specifically, with precise backbone positioning of, e.g., charged groups. Through simulations, we calculate the affinities of a range of small solutes to polypeptoid brush surfaces of varied side-chain chemistries. We then demonstrate that measures of the structure of surface hydration water in response to a particular surface chemistry signal solute-surface affinity; specifically, we find that zwitterionic chemistries produce solute-surface repulsion through highly coordinated hydration water while suppressing tetrahedral structuring around the solute, in contrast to uncharged surfaces that show solute-surface affinity. Based on the relationship of this structural perturbation to the affinity of small-molecule solutes, we propose a molecular mechanism by which zwitterionic surface chemistries enhance solute repulsion, with broader implications for the design of antifouling surfaces.

Ice-Water Equilibrium in Nanoscale Confinement

We show that 2D ^{2}H NMR spectra enable valuable insights into the nature of an ice-water equilibrium in nanoscale confinement, which extends over a broad temperature range. In particular, 2D ^{2}H NMR line-shape analysis allows us to determine the timescale on which the coexisting ice and water phases exchange molecules. For D_{2}O in a silica nanopore with a diameter of 5.4 nm, we find that the residence time of a water molecule in either phase is characterized by an NMR exchange time of τ_{X}=5.7  ms at 220 K. Thus, the ice-water equilibrium is highly dynamic, which is an important aspect for an understanding of deeply cooled confined and, possibly, bulk waters.

Water confinement in small polycyclic aromatic hydrocarbons

The confinement of water molecules is vital in fields from biology to nanotechnology. The conditions allowing confinement in small finite polycyclic aromatic hydrocarbons (PAHs) are unclear, yet are crucial for understanding confinement in larger systems. Here, we report a computational study of water cluster confinement within PAHs dimers. Our results serve as a model for larger carbon allotropes and for understanding molecular interactions in confined systems. We identified size and structural motifs allowing confinement and demonstrated the motifs in various PAHs systems. We show that optimal OH⋯π interactions between water clusters and the PAH dimer permit optimal confinement to occur. However, the lack of such interactions leads to the formation of CH⋯O interactions, resulting in less ideal confinement. Confinement of layered clusters is also possible, provided that the optimal OH⋯π interactions are conserved.

Different Temperature- and Pressure-Effects on the Water-Mediated Interactions between Hydrophobic, Hydrophilic, and Hydrophobic-Hydrophilic Nanoscale Surfaces

Water-mediated interactions (WMI) are responsible for diverse processes in aqueous solutions, including protein folding and nanoparticle aggregation. WMI may be affected by changes in temperature and pressure and hence, they can alter chemical/physical processes that occur in aqueous environments. Traditionally, attention has been focused on hydrophobic interactions while, in comparison, the role of hydrophilic and hybrid (hydrophobic-hydrophilic) interactions have been mostly overlooked. Here we study the role of T and P of the WMI between nanoscale (i) hydrophobic-hydrophobic, (ii) hydrophilic-hydrophilic, and (iii) hydrophilic-hydrophobic pairs of (hydroxylated/non-hydroxylated) graphene-based surfaces. We find that hydrophobic, hydrophilic and hybrid interactions are all sensitive to P. However, while hydrophobic interactions [case (i)] are sensitive to T-variations, hydrophilic [case (ii)] and hybrid interactions [case (iii)] are practically T-independent. An analysis of the entropic and enthalpic contributions to the PMF for cases (i)-(iii) is also presented. Our results are important in understanding T- and P-induced protein denaturation, and the interactions of biomolecules in solution, including protein aggregation andphase separation processes. From the computational point of view, the results presented here are relevant in the design of implicit water models for the study of molecular and colloidal/nanoparticle systems at different thermodynamic conditions.

Structural and Dynamical Properties of Liquids in Confinements: A Review of Molecular Dynamics Simulation Studies

Molecular dynamics (MD) simulations are a powerful tool for detailed studies of altered properties of liquids in confinement, in particular, of changed structures and dynamics. They allow, on one hand, for perfect control and systematic variation of the geometries and interactions inherent in confinement situations and, on the other hand, for type-selective and position-resolved analyses of a huge variety of structural and dynamical parameters. Here, we review MD simulation studies on various types of liquids and confinements. The main focus is confined aqueous systems, but also ionic liquids and polymer and silica melts are discussed. Results for confinements featuring different interactions, sizes, shapes, and rigidity will be presented. Special attention will be given to situations in which the confined liquid and the confining matrix consist of the same type of particles and, hence, disparate liquid-matrix interactions are absent. Findings for the magnitude and the range of wall effects on molecular positions and orientations and on molecular dynamics, including vibrational motion and structural relaxation, are reviewed. Moreover, their dependence on the parameters of the confinement and their relevance to theoretical approaches to the glass transition are addressed.

Structure, Properties, and Phase Transformations of Water Nanoconfined between Brucite-like Layers: The Role of Wall Surface Polarity

The interaction of water with confining surfaces is primarily governed by the wetting properties of the wall material-in particular, whether it is hydrophobic or hydrophilic. The hydrophobicity or hydrophilicity itself is determined primarily by the atomic structure and polarity of the surface groups. In the present work, we used molecular dynamics to study the structure and properties of nanoscale water layers confined between layered metal hydroxide surfaces with a brucite-like structure. The influence of the surface polarity of the confining material on the properties of nanoconfined water was studied in the pressure range of 0.1-10 GPa. This pressure range is relevant for many geodynamic phenomena, hydrocarbon recovery, contact spots of tribological systems, and heterogeneous materials under extreme mechanical loading. Two phase transitions were identified in water confined within 2 nm wide slit-shaped nanopores: (1) at p₁ = 3.3-3.4 GPa, the liquid transforms to a solid phase with a hexagonal close-packed (HCP) crystal structure, and (2) at p₂ = 6.7-7.1 GPa, a further transformation to face-centered cubic (FCC) crystals occurs. It was found that the behavior of the confined water radically changes when the partial charges (and, therefore, the surface polarity) are reduced. In this case, water transforms directly from the liquid phase to an FCC-like phase at 3.2-3.3 GPa. Numerical simulations enabled determination of the amount of hydrogen bonding and diffusivity of nanoconfined water, as well as the relationship between pressure and volumetric strain.

Nanoconfined Fluids: Uniqueness of Water Compared to Other Liquids

Nanoconfinement can drastically change the behavior of liquids, puzzling us with counterintuitive properties. It is relevant in applications, including decontamination and crystallization control. However, it still lacks a systematic analysis for fluids with different bulk properties. Here we address this gap. We compare, by molecular dynamics simulations, three different liquids in a graphene slit pore: (1) A simple fluid, such as argon, described by a Lennard-Jones potential; (2) an anomalous fluid, such as a liquid metal, modeled with an isotropic core-softened potential; and (3) water, the prototypical anomalous liquid, with directional HBs. We study how the slit-pore width affects the structure, thermodynamics, and dynamics of the fluids. All the fluids show similar oscillating properties by changing the pore size. However, their free-energy minima are quite different in nature: (i) are energy-driven for the simple liquid; (ii) are entropy-driven for the isotropic core-softened potential; and (iii) have a changing nature for water. Indeed, for water, the monolayer minimum is entropy driven, at variance with the simple liquid, while the bilayer minimum is energy driven, at variance with the other anomalous liquid. Also, water has a large increase in diffusion for subnm slit pores, becoming faster than bulk. Instead, the other two fluids have diffusion oscillations much smaller than water, slowing down for decreasing slit-pore width. Our results, clarifying that water confined at the subnm scale behaves differently from other (simple or anomalous) fluids under similar confinement, are possibly relevant in nanopores applications, for example, in water purification from contaminants.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Crowding effects on water-mediated hydrophobic interactions

Understanding the fundamental forces such as hydrophobic interactions in a crowded intracellular environment is necessary to comprehensively decipher the mechanisms of protein folding and biomolecular self-assemblies. The widely accepted entropic depletion view of crowding effects primarily attributes biomolecular compaction to the solvent excluded volume effects exerted by the "inert" crowders, neglecting their soft interactions with the biomolecule. In this study, we examine the effects of chemical nature and soft attractive energy of crowders on the water-mediated hydrophobic interaction between two non-polar neopentane solutes using molecular dynamics simulations. The crowded environment is modeled using dipeptides composed of polar and non-polar amino acids of varying sizes. The results show that amongst the non-polar crowders, Leu₂ strengthens the hydrophobic interactions significantly, whereas the polar and small-sized non-polar crowders do not show significant strengthening. Distinct underlying thermodynamic driving forces are illustrated where the small-sized crowders drive hydrophobic interaction via a classic entropic depletion effect and the bulky crowders strengthen it by preferential interaction with the solute. A crossover from energy-stabilized solvent-separated pair to entropy-stabilized contact pair state is observed in the case of bulky non-polar (Leu₂) and polar (Lys₂) crowders. The influence of solute-crowder energy in affecting the dehydration energy penalty is found to be crucial for determining the neopentane association. The findings demonstrate that along with the entropic (size) effects, the energetic effects also play a crucial role in determining hydrophobic association. The results can be extended and have implications in understanding the impact of protein crowding with varying chemistry in modulating the protein free energy landscapes.

Mechanical hydrolysis imparts self-destruction of water molecules under steric confinement

Decoding behavioral aspects associated with the water molecules in confined spaces such as an interlayer space of two-dimensional nanosheets is key for the fundamental understanding of water-matter interactions and identifying unexpected phenomena of water molecules in chemistry and physics. Although numerous studies have been conducted on the behavior of water molecules in confined spaces, their reach stops at the properties of the planar ice-like formation, where van der Waals interactions are the predominant interactions and many questions on the confined space such as the possibility of electron exchange and excitation state remain unsettled. We used density functional theory and reactive molecular dynamics to reveal orbital overlap and induction bonding between water molecules and graphene sheets under much less pressure than graphene fractures. Our study demonstrates high amounts of charge being transferred between water and the graphene sheets, as the interlayer space becomes smaller. As a result, the inner face of the graphene nanosheets is functionalized with hydroxyl and epoxy functional groups while released hydrogen in the form of protons either stays still or traverses a short distance inside the confined space via the Grotthuss mechanism. We found signatures of a new hydrolysis mechanism in the water molecules, i.e. mechanical hydrolysis, presumably responsible for relieving water from extremely confined conditions. This phenomenon where water reacts under extreme confinement by disintegration rather than forming ice-like structures is observed for the first time, illustrating the prospect of treating ultrafine porous nanostructures as a driver for water splitting and material functionalization, potentially impacting the modern design of nanofilters, nanochannels, nano-capacitators, sensors, and so on.

Simulations of the IR and Raman spectra of water confined in amorphous silica slit pores

Water in nano-scale confining environments is a key element in many biological, material, and geological systems. The structure and dynamics of the liquid can be dramatically modified under these conditions. Probing these changes can be challenging, but vibrational spectroscopy has emerged as a powerful tool for investigating their behavior. A critical, evolving component of this approach is a detailed understanding of the connection between spectroscopic features and molecular-level details. In this paper, this issue is addressed by using molecular dynamics simulations to simulate the linear infrared (IR) and Raman spectra for isotopically dilute HOD in D₂O confined in hydroxylated amorphous silica slit pores. The effect of slit-pore width and hydroxyl density on the silica surface on the vibrational spectra is also investigated. The primary effect of confinement is a blueshift in the frequency of OH groups donating a hydrogen bond to the silica surface. This appears as a slight shift in the total (measurable) spectra but is clearly seen in the distance-based IR and Raman spectra. Analysis indicates that these changes upon confinement are associated with the weaker hydrogen-bond accepting properties of silica oxygens compared to water molecules.

Role of Nanoscale Interfacial Proximity in Contact Freezing in Water

Contact freezing is a mode of atmospheric ice nucleation in which a collision between a dry ice nucleating particle (INP) and a water droplet results in considerably faster heterogeneous nucleation. The molecular mechanism of such an enhancement is, however, still a mystery. While earlier studies had attributed it to collision-induced transient perturbations, recent experiments point to the pivotal role of nanoscale proximity of the INP and the free interface. By simulating the heterogeneous nucleation of ice within INP-supported nanofilms of two model water-like tetrahedral liquids, we demonstrate that such nanoscale proximity is sufficient for inducing rate increases commensurate with those observed in contact freezing experiments, but only if the free interface has a tendency to enhance homogeneous nucleation. Water is suspected of possessing this latter property, known as surface freezing propensity. Our findings therefore establish a connection between the surface freezing propensity and kinetic enhancement during contact nucleation. We also observe that faster nucleation proceeds through a mechanism markedly distinct from classical heterogeneous nucleation, involving the formation of hourglass-shaped crystalline nuclei that conceive at either interface and that have a lower free energy of formation due to the nanoscale proximity of the interfaces and the modulation of the free interfacial structure by the INP. In addition to providing valuable insights into the physics of contact nucleation, our findings can assist in improving the accuracy of heterogeneous nucleation rate measurements in experiments and in advancing our understanding of ice nucleation on nonuniform surfaces such as organic, polymeric, and biological materials.

Bulk supercooled water versus adsorbed films on silica surfaces: specific heat by Monte Carlo simulation

Between 150 and 230.6 K, bulk supercooled water freezes upon cooling, and amorphous ice crystallizes upon heating: bulk water thus exists only in its stable ice form. To circumvent this problem, experiments are generally performed on water adsorbed in SiO2 based porous systems. In this work, we take advantage of Monte Carlo simulations to explore this metastable supercooled region inaccessible to experiments. Using three rigid, non-polarizable water models, namely SPC, TIP4P and TIP4P/2005, we investigate the isobaric specific heat capacity (Cp), between 100 and 300 K, of bulk water and water films of few monolayers adsorbed on different SiO2 surfaces: a smooth surface, a non-hydroxylated (0001) surface of quartz, and a fully hydroxylated (001) surface of cristobalite. As Cp is directly related to the entropy fluctuations and we focus on low temperatures, the convergence of the Monte Carlo simulations is a critical point of this work. Also, due to the small mass of the hydrogen atoms, quantum corrections are taken into account, and lead to an excellent agreement of the simulated and experimental Cp values at low temperature (100 K region). Altogether, we conclude that, in bulk, Cp is shown to exhibit a broad peak around 225 K for the SPC and TIP4P models, and around 250 K for the TIP4P/2005 model, in qualitative agreement with the experimentally observed features in Cp measurements. For interfacial water, in all cases, the broad Cp peak disappears. This result, at odds with experimental observations, suggests that disorder and hydrogen bonding at the interface (not yet taken into account) have a fundamental role in confined water transitions.

Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution

Performance of membranes for water purification is highly influenced by the interactions of solvated species with membrane surfaces, including surface adsorption of solutes upon fouling. Current efforts toward fouling-resistant membranes often pursue surface hydrophilization, frequently motivated by macroscopic measures of hydrophilicity, because hydrophobicity is thought to increase solute-surface affinity. While this heuristic has driven diverse membrane functionalization strategies, here we build on advances in the theory of hydrophobicity to critically examine the relevance of macroscopic characterizations of solute-surface affinity. Specifically, we use molecular simulations to quantify the affinities to model hydroxyl- and methyl-functionalized surfaces of small, chemically diverse, charge-neutral solutes represented in produced water. We show that surface affinities correlate poorly with two conventional measures of solute hydrophobicity, gas-phase water solubility and oil-water partitioning. Moreover, we find that all solutes show attraction to the hydrophobic surface and most to the hydrophilic one, in contrast to macroscopically based hydrophobicity heuristics. We explain these results by decomposing affinities into direct solute interaction energies (which dominate on hydroxyl surfaces) and water restructuring penalties (which dominate on methyl surfaces). Finally, we use an inverse design algorithm to show how heterogeneous surfaces, with multiple functional groups, can be patterned to manipulate solute affinity and selectivity. These findings, importantly based on a range of solute and surface chemistries, illustrate that conventional macroscopic hydrophobicity metrics can fail to predict solute-surface affinity, and that molecular-scale surface chemical patterning significantly influences affinity-suggesting design opportunities for water purification membranes and other engineered interfaces involving aqueous solute-surface interactions.

An isolated water droplet in the aqueous solution of a supramolecular tetrahedral cage

Water under nanoconfinement at ambient conditions has exhibited low-dimensional ice formation and liquid-solid phase transitions, but with structural and dynamical signatures that map onto known regions of water's phase diagram. Using terahertz (THz) absorption spectroscopy and ab initio molecular dynamics, we have investigated the ambient water confined in a supramolecular tetrahedral assembly, and determined that a dynamically distinct network of 9 ± 1 water molecules is present within the nanocavity of the host. The low-frequency absorption spectrum and theoretical analysis of the water in the Ga₄L₆ ^12- host demonstrate that the structure and dynamics of the encapsulated droplet is distinct from any known phase of water. A further inference is that the release of the highly unusual encapsulated water droplet creates a strong thermodynamic driver for the high-affinity binding of guests in aqueous solution for the Ga₄L₆ ^12- supramolecular construct.

Thermodynamic Contribution of Water in Cryptophane Host-Guest Binding Reaction

A detailed examination of binding thermodynamics is undertaken for the interaction between rubidium ion and a water-soluble cryptophane molecule using isothermal titration calorimetry. The equilibrium-binding quotient for this host-guest pair decreases with increasing product formation. When analyzed with a thermodynamic framework that considers water explicitly in the governing equation, the shift in equilibrium is ascribed to an unfavorable change in the free energy of solvation upon formation of the inclusion complex. A van't Hoff analysis of the binding data, as well as an observation of aggregation between inclusion complexes, suggests that charge-charge interactions between rubidium ion and the phenolate groups of the cryptophane host provide the driving force for association in water that overcomes a large and unfavorable change in solvent enthalpy.

Nanoconfinement-Mediated Water Treatment: From Fundamental to Application

Safe and clean water is of pivotal importance to all living species and the ecosystem on earth. However, the accelerating economy and industrialization of mankind generate water pollutants with much larger quantity and higher complexity than ever before, challenging the efficacy of traditional water treatment technologies. The flourishing researches on nanomaterials and nanotechnologies in the past decade have generated new understandings on many fundamental processes and brought revolutionary upgrades to various traditional technologies in almost all areas, including water treatment. An indispensable step toward the real application of nanomaterials in water treatment is to confine them in large processable substrate to address various inherent issues, such as spontaneous aggregation, difficult operation and potential environmental risks. Strikingly, when the size of the spatial restriction provided by the substrate is on the order of only one or several nanometers, referred to as nanoconfinement, the phase behavior of matter and the energy diagram of a chemical reaction could be utterly changed. Nevertheless, the relationship between such changes under nanoconfinement and their implications for water treatment is rarely elucidated systematically. In this Critical Review, we will briefly summarize the current state-of-the-art of the nanomaterials, as well as the nanoconfined analogues (i.e., nanocomposites) developed for water treatment. Afterward, we will put emphasis on the effects of nanoconfinement from three aspects, that is, on the structure and behavior of water molecules, on the formation (e.g., crystallization) of confined nanomaterials, and on the nanoenabled chemical reactions. For each aspect, we will build the correlation between the nanoconfinement effects and the current studies for water treatment. More importantly, we will make proposals for future studies based on the missing links between some of the nanoconfinement effects and the water treatment technologies. Through this Critical Review, we aim to raise the research attention on using nanoconfinement as a fundamental guide or even tool to advance water treatment technologies.

Real-Time Visualization and Dynamics of Boron Nitride Nanotubes Undergoing Brownian Motion

We report the first real-time imaging of individualized boron nitride nanotubes (BNNTs) via stabilization with a rhodamine surfactant and fluorescence microscopy. We study the rotational and translational diffusion and find them to agree with predictions based on a confined, high-aspect-ratio rigid rod undergoing Brownian motion. We find that the behavior of BNNTs parallels that of individualized carbon nanotubes (CNTs), indicating that BNNTs could also be used as model rigid rods to study soft matter systems, while avoiding the experimental disadvantages of CNTs due to their strong light absorption. The use and further development of our technique and findings will accelerate the application of BNNTs from material engineering to biological studies.

Wetting Transition of Ionic Substrate by Modulating Surface Charge Distribution

Surface wettability regulation plays a crucial role in antifouling and related applications. For regulating surface wettability, one of the effective approaches is to modulate the surface charge distribution. Herein, we report a theoretical study for unraveling the mechanistic relation between surface charge distribution and ionic substrate wettability. Specifically, acetonitrile liquids at ambient condition in contact with various ionic substrates are considered. At different surface charge distributions, the interfacial thermodynamic properties are investigated by means of molecular density functional theory. We find that the variation of the spatial interval among the discrete charges strongly alters the substrate-acetonitrile interaction and leads to an oscillation in the interfacial tension, indicating that the substrate can be tuned from a solvophobic one to a solvophilic one. This trend can be further enhanced by increasing the charge quantity. The underlying mechanisms are extensively discussed and expatiated. Our work provides theoretical guidance to engineer and regulate surface wettability.

Comparative Study of Water-Mediated Interactions between Hydrophilic and Hydrophobic Nanoscale Surfaces

Self-assembly processes in aqueous solutions, such as protein folding and nanoparticle aggregation, are driven by water-mediated interactions (WMIs). The most common of such interactions are the attractive forces between hydrophobic units. While numerous studies have focused on hydrophobic interactions, WMIs between hydrophilic moieties and pairs of hydrophilic-hydrophobic surfaces have received much less attention. In this work, we perform molecular dynamics simulations to study the WMI between nanoscale (i) hydrophobic-hydrophobic, (ii) hydrophilic-hydrophilic, and (iii) hydrophilic-hydrophobic pairs of (hydroxylated/nonhydroxylated) graphene-based surfaces. We find that in all cases, the potential of mean force (PMF) between the plates exhibits oscillations as a function of the plate separations r, up to r ≈ 1-1.5 nm. The local minima of the PMF, which define the stable/metastable states of the system, correspond to plates' separations at which water molecules arrange into n = 0, 1, 2, ... layers between the plates. In case (i), the stable state of the system corresponds to the plates in contact with one another. Instead, in cases (ii) and (iii), water is never removed between the plates. The free-energy barriers separating the stable/metastable states of the system vary with the hydrophilicity/hydrophobicity of the interacting plates. However, the effective forces between the plates are comparable in magnitude. This strongly suggests that hydrophilic-hydrophilic and hydrophilic-hydrophobic interactions can play a relevant role in self-assembly processes in aqueous solutions, alike hydrophobic interactions. Interestingly, we find that the WMIs between hydrophilic-hydrophilic and hydrophilic-hydrophobic plates are similar, suggesting that only one hydrophilic surface is sufficient to induce hydrophilic-like WMI. We also briefly discuss the role of surface polarity on the WMI. In particular, we show that depending on the surface polarity, WMI can exhibit mixed features characteristic of hydrophobic and hydrophilic interactions. Our results suggest that the forces between hydrophobic, hydrophilic, and hydrophobic/hydrophilic surfaces are all relevant in driving a self-assembly system toward its final state, but it is the hydrophobic interaction that provides stability to such a final state.

Surface Topography Effects of Globular Biomolecules on Hydration Water

Hydration water serves as a microscopic manifestation of structural stability and functions of biomolecules. To develop bio-nanomaterials in applications, it is important to study how the surface topography and heterogeneity of biomolecules result in their diversity of the hydration dynamics and energetics. We here performed molecular dynamics simulations combined with the steered molecular dynamics and umbrella sampling to investigate the dynamics and escape process associated with the free energy change of water molecules close to a globular biomolecule, i.e., hemoglobin (Hb) and G-quadruplex DNA (GDNA). The residence time, power of long-time tail, and dipole relaxation time were found to display drastic changes within the averaged hydration shell of 3.0-5.0 Å. Compared with bulk water, in the inner hydration shell, the water dipole moment displays a slower relaxation process and is more oriented toward GDNA than toward Hb, forming a hedgehog-like structure when it surrounds GDNA. In particular, a spine water structure is observed in the GDNA narrow groove. The water isotope effect not only prolongs the dynamic time scales of libration motion in the inner hydration shell and the dipole relaxation processes in the bulk but also strengthens the DNA spine water structure. The potential of the mean force profile reflects the integrity of the hydration shell structure and enables us to obtain detailed insights into the structures formed by water, such as the caged H-bond network and the edge bridge structures; it also reveals that local hydration shell free energy (LHSFE) depends on H-bond rupture processes and ranges from 0.2 to 4.2 kcal/mol. Our results demonstrate that the surface topography of a biomolecule influences the integrity of the hydration shell structure and LHSFE. Our studies are able to identify various further applications in the areas of microfluid devices and nano-dewetting on bioinspired surfaces.

On the origin of the extremely different solubilities of polyethers in water

The solubilities of polyethers are surprisingly counter-intuitive. The best-known example is the difference between polyethylene glycol ([-CH₂-CH₂-O-]_n) which is infinitely soluble, and polyoxymethylene ([-CH₂-O-]_n) which is completely insoluble in water, exactly the opposite of what one expects from the C/O ratios of these molecules. Similar anomalies exist for oligomeric and cyclic polyethers. To solve this apparent mystery, we use femtosecond vibrational and GHz dielectric spectroscopy with complementary ab initio calculations and molecular dynamics simulations. We find that the dynamics of water molecules solvating polyethers is fundamentally different depending on their C/O composition. The ab initio calculations and simulations show that this is not because of steric effects (as is commonly believed), but because the partial charge on the O atoms depends on the number of C atoms by which they are separated. Our results thus show that inductive effects can have a major impact on aqueous solubilities.

Temperature regulation of the contact angle of water droplets on the solid surfaces

We investigate theoretically the stability of the wetting property, i.e., the contact angle values, as a function of the temperature. We find that the estimated temperature coefficient of the contact angle for the water droplets on an ordered water monolayer on a 100 surface of face-center cubic (FCC) is about one order of magnitude larger than that on a hydrophobic hexagonal surface in the temperature range between 290 K and 350 K, using molecular dynamics simulations. As temperature rises, the number of hydrogen bonds between the ordered water monolayer and the water droplet will increase, which therefore enhances the hydrophilicity of the ordered water monolayer at the FCC model surface. Our work thus provides an easily controllable and reversible way to control the degree of hydrophobicity of various solid surfaces exhibiting a similar wetting property of water droplets on the ordered water monolayer as such particular FCC (100) surfaces.

Molecular Origin of Superlubricity between Graphene and a Highly Hydrophobic Surface in Water

Graphene is efficient to provide ultralow friction after the formation of an incommensurate interface but is limited to dry contact conditions and specific lattice structures. In this Letter, a new strategy is proposed to achieve the superlubricity of graphene through the creation of a sliding interface between graphene and a highly hydrophobic surface of self-assembled fluoroalkyl monolayers (SAFMs) in water. A superlow friction coefficient of μ = 0.0003 was obtained, demonstrating the extremely low shear stress between graphene and hydrophobic SAFMs in water. Molecular dynamics (MD) simulation shows that a nanometer-thick water layer is intercalated between graphene and hydrophobic SAFMs, and the weak interactions between water molecules and graphene provide a small energy barrier for water molecules sliding on graphene, which contributes to superlubricity. This finding reveals how to form a superlubricity interface by water intercalation, which has implications for minimizing the friction of layered materials and hydrophobic surfaces in water.

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.

Molecular dynamics simulations of nano-confined methanol and methanol-water mixtures between infinite graphite plates: Structure and dynamics

Molecular dynamics simulations are used to investigate microscopic structures and dynamics of methanol and methanol-water binary mixture films confined between hydrophobic infinite parallel graphite plate slits with widths, H, in the range of 7-20 Å at 300 K. The initial geometric densities of the liquids were chosen to be the same as bulk methanol at the same temperature. For the two narrowest slit widths, two smaller initial densities were also considered. For the nano-confined system with H = 7 Å and high pressure, a solid-like hexagonal arrangement of methanol molecules arranged perpendicular to the plates is observed which reflects the closest packing of the molecules and partially mirrors the structure of the underlying graphite structure. At lower pressures and for larger slit widths, in the contact layer, the methanol molecules prefer having the C-O bond oriented parallel to the walls. Layered structures of methanol parallel to the wall were observed, with contact layers and additional numbers of central layers depending on the particular slit width. For methanol-water mixtures, simulations of solutions with different composition were performed between infinite graphite slits with H = 10 and 20 Å at 300 K. For the nanoslit with H = 10 Å, in the solution mixtures, three layers of molecules form, but for all mole fractions of methanol, methanol molecules are excluded from the central fluid layer. In the nanopore with H = 20 Å, more than three fluid layers are formed and methanol concentrations are enhanced near the confining plates walls compared to the average solution stoichiometry. The self-diffusion coefficients of methanol and water molecules in the solution show strong dependence on the solution concentration. The solution mole fractions with minimal diffusivity are the same in confined and non-confined bulk methanol-water mixtures.

Characterizing Solvent Density Fluctuations in Dynamical Observation Volumes

Hydrophobic effects drive diverse aqueous assemblies, such as micelle formation or protein folding, wherein the solvent plays an important role. Consequently, characterizing the free energetics of solvent density fluctuations can lead to important insights into these processes. Although techniques such as the indirect umbrella sampling (INDUS) method can be used to characterize solvent fluctuations in static observation volumes of various sizes and shapes, characterizing how the solvent mediates inherently dynamic processes, such as self-assembly or conformational change, remains a challenge. In this work, we generalize the INDUS method to facilitate the enhanced sampling of solvent fluctuations in dynamical observation volumes, whose positions and shapes can evolve. We illustrate the usefulness of this generalization by characterizing water density fluctuations in dynamical volumes pertaining to the hydration of flexible solutes, the assembly of small hydrophobes, and conformational transitions in a model peptide. We also use the method to probe the dynamics of hard spheres.

Water flow modeling through a graphene-based nanochannel: theory and simulation

Understanding the behavior of water molecule transport through artificial nano-channels is essential in designing novel nanofluidic devices that could be used especially in nanofiltration processes. In this study, using nonequilibrium molecular dynamics (MD) simulations, we simulated the water flow through different graphene-based channels to investigate the influences of some key factors such as the channel thickness and applied pressure on the water flow. It was demonstrated that the water flow was enhanced by increasing the applied pressure and channel thickness. Our results indicated that a third order polynomial curve could describe the variation of the water flow as a function of the channel thickness and the applied pressure. In addition, we improved the hydrodynamics equation used to consider the water flow through nano-channels, by adding two terms to describe the slip effect and the entrance/exit effect, in which the first term increased the water flow rate, while the second term reduced it. This study may be helpful in designing high-performance graphene-based membranes with some practical applications such as desalination.

Comparative Study of the Effects of Temperature and Pressure on the Water-Mediated Interactions between Apolar Nanoscale Solutes

We perform molecular dynamics simulations to study the effects of temperature and pressure on the water-mediated interaction (WMI) between two nanoscale (apolar) graphene plates at 240 ≤ T ≤ 400 K and -100 ≤ P ≤ 1200 MPa. These are thermodynamic conditions relevant to, for example, cooling-, heating-, compression-, and decompression-induced protein denaturation. We find that at all ( T, P) studied, the potential of mean force between the graphene plates, as a function of plate separation r, exhibits local minima at specific plate separations r = r _n that can accommodate n water layers ( n = 0,1,2,3). In particular, our results show that isobaric cooling and isothermal compression have a similar effect on WMI between the plates; both processes tend to suppress the attraction and ultimate collapse of the graphene plates by kinetically trapping the plates at the metastable states with r = r _n ( n > 0). In addition, isobaric heating and isothermal decompression also have a similar effect; both processes tend to reduce the range and strength of the interactions between the graphene plates. Interestingly, at low temperatures, the WMI between the plates is affected by crystallization. However, crystallization depends deeply on the water model considered, SPC/E and TIP4P/2005 water models, with the crystallization occurring at different ( T, P) conditions, into different forms of ice.

Non-monotonous Wetting of Graphene-Mica and MoS2-Mica Interfaces with a Molecular Layer of Water

Hydration of interfaces with a layer of water is a ubiquitous phenomenon, which has important implications for numerous natural and technologically important processes. Nevertheless, at the nanoscale, the understanding of the wetting process is still limited, since it is experimentally difficult to follow. Here, graphene and monolayers of MoS₂ deposited on dry mica are used to investigate wetting of the two-dimensional (2D) material-mica interfaces with a molecularly thin layer of water employing scanning force microscopy in different modes. Wetting occurs non-monotonously in time and space for both types of interfaces. It starts at relative humidities (RH) of 10-17% for graphenes and 8-9% for MoS₂ and concludes with a homogeneous layer at 25-30 and 15-20%, respectively. Investigation of the process at the graphene-mica interface indicates that up to about 25% RH, initially a highly compliant and unstable layer of water spreads, which subsequently stabilizes by developing labyrinthine nanostructures. Moreover, these nanostructures exhibit distinct mechanical deformability and dissipation, which is ascribed to different densities of the confined water layer. The laterally structured morphology is explained by the interplay of counteracting long-range dipole-dipole repulsion and short-range line tension, with the latter causing at least in part by the mechanical deformation of the 2D material. The proposed origins of the interactions are common for thin layers of polar molecules at interfaces, implying that the lateral structuring of thin wetting layers at submonolayer concentrations may also be a quite general phenomenon.

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.

Temperature Effects on Water-Mediated Interactions at the Nanoscale

We perform molecular dynamics simulations to study the effects of temperature on the water-mediated interactions between nanoscale apolar solutes. Specifically, we calculate the potential of mean force (PMF) between two graphene plates immersed in water at 240 ≤ T ≤ 400 K and P = 0.1 MPa. These are thermodynamic conditions relevant to cooling- and heating-induced protein denaturation. It is found that both cooling and heating tend to suppress the attraction, and ultimate collapse, of the graphene plates. However, the underlying role played by water upon heating and cooling is different. Isobaric heating reduces the strength and range of the interactions between the plates. Instead, isobaric cooling stabilizes the plates separations that can accommodate an integer number of water layers between the graphene plates. In particular, the energy barriers separating these plate separations increase linearly with 1/ T. We also explore the sensitivity of the plates PMF to the water model employed. In the case of the TIP4P/2005 model, water confined between the plates crystallizes into a defective bilayer ice at low temperatures, whereas in the case of the SPC/E model, water remains in the liquid state at same thermodynamic conditions. The effects of varying water-graphene interactions on the plates PMF are also studied.

Lubricating properties of single metal ions at interfaces

The behaviour of ionic solutions confined in nanoscale gaps is central to countless processes, from biomolecular function to electrochemistry, energy storage and lubrication. However, no clear link exists between the molecular-level behaviour of the liquid and macroscopic observations. The problem mainly comes from the difficulty to interrogate a small number of liquid molecules. Here, we use atomic force microscopy to investigate the viscoelastic behaviour of pure water and ionic solutions down to the single ion level. The results show a glassy-like behaviour for pure water, with single metal ions acting as lubricants by reducing the elasticity of the nano-confined solution and the magnitude of the hydrodynamic friction. At small ionic concentration (<20 mM) the results can be quantitatively explained by the ions moving via a thermally-activated process resisted by the ion's hydration water (Prandtl-Tomlinson model). The model breaks down at higher salt concentrations due to ion-ion interaction effects that can no longer be neglected. The correlations are confirmed by direct sub-nanometre imaging of the interface at equilibrium. The results provide a molecular-level basis for explaining the tribological properties of aqueous solutions and suggest that ion-ion interactions create mesoscale effects that prevent a direct link between nanoscale and macroscopic measurements.

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.

Phase Diagram of Water Confined by Graphene

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Remarkably, nanoscale confinement drastically alters the properties of water. Using molecular dynamics simulations, we determine the phase diagram of water confined by graphene sheets in slab geometry, at T = 300 K and for a wide range of pressures. We find that, depending on the confining dimension D and density σ, water can exist in liquid and vapor phases, or crystallize into monolayer and bilayer square ices, as observed in experiments. Interestingly, depending on D and σ, the crystal-liquid transformation can be a first-order phase transition, or smooth, reminiscent of a supercritical liquid-gas transformation. We also focus on the limit of stability of the liquid relative to the vapor and obtain the cavitation pressure perpendicular to the graphene sheets. Perpendicular cavitation pressure varies non-monotonically with increasing D and exhibits a maximum at D ≈ 0.90 nm (equivalent to three water layers). The effect of nanoconfinement on the cavitation pressure can have an impact on water transport in technological and biological systems. Our study emphasizes the rich and apparently unpredictable behavior of nanoconfined water, which is complex even for graphene.

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.