Highly confined water: two-dimensional ice, amorphous ice, and clathrate hydrates

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.

Pressure-Induced Melting of Confined Ice

The classic regelation experiment of Thomson in the 1850s deals with cutting an ice cube, followed by refreezing. The cutting was attributed to pressure-induced melting but has been challenged continuously, and only lately consensus emerged by understanding that compression shortens the O:H nonbond and lengthens the H-O bond simultaneously. This H-O elongation leads to energy loss and lowers the melting point. The hot debate survived well over 150 years, mainly due to a poorly defined heat exchange with the environment in the experiment. In our current experiment, we achieved thermal isolation from the environment and studied the fully reversible ice-liquid water transition for water confined between graphene and muscovite mica. We observe a transition from two-dimensional (2D) ice into a quasi-liquid phase by applying a pressure exerted by an atomic force microscopy tip. At room temperature, the critical pressure amounts to about 6 GPa. The transition is completely reversible: refreezing occurs when the applied pressure is lifted. The critical pressure to melt the 2D ice decreases with temperature, and we measured the phase coexistence line between 293 and 333 K. From a Clausius-Clapeyron analysis, we determine the latent heat of fusion of two-dimensional ice at 0.15 eV/molecule, being twice as large as that of bulk ice.

Non-Continuum Intercalated Water Diffusion Explains Fast Permeation through Graphene Oxide Membranes

Recent experimental studies have revealed unconventional phase and transport behaviors of water confined within lamellar graphene oxide membranes, which hold great promise not only in improving our current understanding of nanoconfined water but also in developing high-performance filtration and separation applications. In this work, we explore molecular structures and diffusive dynamics of water intercalated between graphene or graphene oxide sheets. We identify the monolayer structured water between graphene sheets at temperature T below T_c = ∼315 K and an interlayer distance d = 0.65 nm, which is absent as the sheets are oxidized. The non-continuum collective diffusion of water intercalation between graphene layers facilitates fast molecular transport due to reduced wall friction. This solid-like structural order of intercalated water is disturbed as T or d increases to a critical value, with abnormal declines in the coefficients of collective diffusion. Based on a patched model of graphene oxide sheets consisting of spatially distributed pristine and oxidized regions, we conclude that the non-continuum collective diffusion of intercalated water can explain fast water permeation through graphene oxide membranes as reported in recent experimental studies, in stark contrast to the conventional picture of pressure-driven continuum flow with boundary slip, which has been widely adopted in literature but may apply only at high humidity or in the fully hydrated conditions.

Ice Squeezing Induced Multicolor Fluorescence Emissions from Polyacrylamide Cryogels

Being short of conventional chromophores, polyacrylamide is generally not regarded as a fluorescent material. Exactly the polymerization of dilute solutions of acrylamide and N,N'-methylenebisacrylamide led to thick liquids at 60 °C, showing no fluorescence. Things changed when the phase transition of water was involved. The squeezing effect of ice crystals not only created polymeric solids (cryogels) at - 20 °C, but also endowed them unexpected fluorescence emissions. The macroporous cryogels are mainly blue fluorescent polymers. However yellow and red fluorescence were also achieved by changing the ingredient ratios. A series of instrumental detections revealed that the multicolor fluorescence were based on exquisite amido stacking induced from ice squeezing. If people make good use of the squeezing effect of the heaven-sent molecule to manipulate the interactions of monomer functionalities, cryogenic polymerization can be a promising method to produce diverse polymeric materials.

CH4 Hydrate Formation between Silica and Graphite Surfaces: Insights from Microsecond Molecular Dynamics Simulations

Microsecond simulations have been performed to investigate CH₄ hydrate formation from gas/water two-phase systems between silica and graphite surfaces, respectively. The hydrophilic silica and hydrophobic graphite surfaces exhibit substantially different effects on CH₄ hydrate formation. The graphite surface adsorbs CH₄ molecules to form a nanobubble with a flat or negative curvature, resulting in a low aqueous CH₄ concentration, and hydrate nucleation does not occur during 2.5 μs simulation. Moreover, an ordered interfacial water bilayer forms between the nanobubble and graphite surface thus preventing their direct contact. In contrast, the hydroxylated-silica surface prefers to be hydrated by water, with a cylindrical nanobubble formed in the solution, leading to a high aqueous CH₄ concentration and hydrate nucleation in the bulk region; during hydrate growth, the nanobubble is gradually covered by hydrate solid and separated from the water phase, hence slowing growth. The silanol groups on the silica surface can form strong hydrogen bonds with water, and hydrate cages need to match the arrangements of silanols to form more hydrogen bonds. At the end of the simulation, the hydrate solid is separated from the silica surface by liquid water, with only several cages forming hydrogen bonds with the silica surface, mainly due to the low CH₄ aqueous concentrations near the surface. To further explore hydrate formation between graphite surfaces, CH₄/water homogeneous solution systems are also simulated. CH₄ molecules in the solution are adsorbed onto graphite and hydrate nucleation occurs in the bulk region. During hydrate growth, the adsorbed CH₄ molecules are gradually converted into hydrate solid. It is found that the hydrate-like ordering of interfacial water induced by graphite promotes the contact between hydrate solid and graphite. We reveal that the ability of silanol groups on silica to form strong hydrogen bonds to stabilize incipient hydrate solid, as well as the ability of graphite to adsorb CH₄ molecules and induce hydrate-like ordering of the interfacial water, are the key factors to affect CH₄ hydrate formation between silica and graphite surfaces.

Structures and thermodynamics of water encapsulated by graphene

Understanding phase behaviors of nanoconfined water has driven notable research interests recently. In this work, we examine water encapsulated under a graphene cover that offers an ideal testbed to explore its molecular structures and thermodynamics. We find layered water structures for up to ~1000 trapped water molecules, which is stabilized by the spatial confinement and pressure induced by interfacial adhesion. For monolayer encapsulations, we identify representative two-dimensional crystalline lattices as well as defects therein. Free energy analysis shows that the structural orders with low entropy are compensated by high formation energies due to the pressurized confinement. There exists an order-to-disorder transition for this condensed phase at ~480-490 K, with a sharp reduction in the number of hydrogen bonds and increase in the entropy. Fast diffusion of the encapsulated water demonstrates anomalous temperature dependence, indicating the solid-to-fluid nature of this structural transition. These findings offer fundamental understandings of the encapsulated water that can be used as a pressurized cell with trapped molecular species, and provide guidance for practical applications with its presence, for example, in the design of nanodevices and nanoconfined reactive cells.

A review of the structure and dynamics of nanoconfined water and ionic liquids via molecular dynamics simulation

During the past decade, the research on fluids in nanoconfined geometries has received considerable attention as a consequence of their wide applications in different fields. Several nanoconfined systems such as water and ionic liquids, together with an equally impressive array of nanoconfining media such as carbon nanotube, graphene and graphene oxide have received increasingly growing interest in the past years. Water is the first system that has been reviewed in this article, due to its important role in transport phenomena in environmental sciences. Water is often considered as a highly nanoconfined system, due to its reduction to a few layers of water molecules between the extended surface of large macromolecules. The second system discussed here is ionic liquids, which have been widely studied in the modern green chemistry movement. Considering the great importance of ionic liquids in industry, and also their oil/water counterpart, nanoconfined ionic liquid system has become an important area of research with many fascinating applications. Furthermore, the method of molecular dynamics simulation is one of the major tools in the theoretical study of water and ionic liquids in nanoconfinement, which increasingly has been joined with experimental procedures. In this way, the choice of water and ionic liquids in nanoconfinement is justified by applying molecular dynamics simulation approaches in this review article.

CO Separation from H2 via Hydrate Formation in Single-Walled Carbon Nanotubes

Hydrogen is an alternative fuel without generating greenhouse gas or other harmful emissions. Industrial hydrogen production, however, always contains a small fraction of carbon monoxide (CO) (∼0.5-2%) that must be removed for use in fuel cells. Here, we present molecular dynamics simulation evidence on facile separation of CO from H₂ at ambient pressure via the formation of quasi-one-dimensional (Q1D) clathrate hydrates within single-walled carbon nanotubes (SW-CNTs). At ambient pressure, Q1D CO (or H₂) clathrates in SW-CNTs are formed spontaneously when the SW-CNTs are immersed in CO (or H₂) aqueous solution. More interestingly, for the CO/H₂ aqueous solution, highly preferential adsorption of CO over H₂ occurs within the octagonal or nonagonal ice nanotubes inside of SW-CNTs. These results suggest that the formation of Q1D hydrates within SW-CNTs can be a viable and safe method for the separation of CO from H₂, which can be exploited for hydrogen purification in fuel cells.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.

Enhanced Configurational Entropy in High-Density Nanoconfined Bilayer Ice

A novel kind of crystal order in high-density nanoconfined bilayer ice is proposed from molecular dynamics and density-functional theory simulations. A first-order transition is observed between a low-temperature proton-ordered solid and a high-temperature proton-disordered solid. The latter is shown to possess crystalline order for the oxygen positions, arranged on a close-packed triangular lattice with AA stacking. Uniquely among the ice phases, the triangular bilayer is characterized by two levels of disorder (for the bonding network and for the protons) which results in a configurational entropy twice that of bulk ice.

Two Dimensional Ice from First Principles: Structures and Phase Transitions

Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here, we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression, the pentagonal structure becomes the most stable and persists up to ∼2  GPa, at which point the square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and the width.

Structural and configurational properties of nanoconfined monolayer ice from first principles

Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations.

AB-stacked square-like bilayer ice in graphene nanocapillaries

Water, when constrained between two graphene sheets and under ultrahigh pressure, can manifest dramatic differences from its bulk counterparts such as the van der Waals pressure induced water-to-ice transformation, known as the metastability limit of two-dimensional (2D) liquid.

Two-dimensional interlocked pentagonal bilayer ice: how do water molecules form a hydrogen bonding network?

The tradeoff between the conditions of an ideal hydrogen bonding network can serve as a generic guidance to understand the rich phase behaviors of nanoconfined water.

Effects of thermodynamic inhibitors on the dissociation of methane hydrate: a molecular dynamics study

We investigate the effects of methanol and NaCl, which are known as thermodynamic hydrate inhibitors, on the dissociation kinetics of methane hydrate in aqueous solutions by using molecular dynamics simulations. It is shown that the dissociation rate is not constant but changes with time. The dissociation rate in the initial stage is increased by methanol whereas it is decreased by NaCl. This difference arises from the opposite effects of the two thermodynamic inhibitors on the hydration free energy of methane. The dissociation rate of methane hydrate is increased by the formation of methane bubbles in the aqueous phase because the bubbles absorb surrounding methane molecules. It is found that both methanol and NaCl facilitate the bubble formation. However, their mechanisms are completely different from each other. The presence of ions enhances the hydrophobic interactions between methane molecules. In addition, the ions in the solution cause a highly non-uniform distribution of dissolved methane molecules. These two effects result in the easy formation of bubbles in the NaCl solution. In contrast, methanol assists the bubble formation because of its amphiphilic character.

Compression Limit of Two-Dimensional Water Constrained in Graphene Nanocapillaries

Evaluation of the tensile/compression limit of a solid under conditions of tension or compression is often performed to provide mechanical properties that are critical for structure design and assessment. Algara-Siller et al. recently demonstrated that when water is constrained between two sheets of graphene, it becomes a two-dimensional (2D) liquid and then is turned into an intriguing monolayer solid with a square pattern under high lateral pressure [ Nature , 2015 , 519 , 443 - 445 ]. From a mechanics point of view, this liquid-to-solid transformation characterizes the compression limit (or metastability limit) of the 2D monolayer water. Here, we perform a simulation study of the compression limit of 2D monolayer, bilayer, and trilayer water constrained in graphene nanocapillaries. At 300 K, a myriad of 2D ice polymorphs (both crystalline-like and amorphous) are formed from the liquid water at different widths of the nanocapillaries, ranging from 6.0 to11.6 Å. For monolayer water, the compression limit is typically a few hundred MPa, while for the bilayer and trilayer water, the compression limit is 1.5 GPa or higher, reflecting the ultrahigh van der Waals pressure within the graphene nanocapillaries. The compression-limit (phase) diagram is obtained at the nanocapillary width versus pressure (h-P) plane, based on the comprehensive molecular dynamics simulations at numerous thermodynamic states as well as on the Clapeyron equation. Interestingly, the compression-limit curves exhibit multiple local minima.

Mechanical instability of monocrystalline and polycrystalline methane hydrates

Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments.

Sediment-hosted gas hydrates may release vast quantities of methane upon failure, but destabilizing mechanisms at the molecular level are poorly understood. Here, the authors study the deformation using simulations and find that failure differs between single crystals and polycrystalline hydrates.

Water in Inhomogeneous Nanoconfinement: Coexistence of Multilayered Liquid and Transition to Ice Nanoribbons

Phase behavior and the associated phase transition of water within inhomogeneous nanoconfinement are investigated using molecular dynamics simulations. The nanoconfinement is constructed by a flat bottom plate and a convex top plate. At 300 K, the confined water can be viewed as a coexistence of monolayer, bilayer, and trilayer liquid domains to accommodate the inhomogeneous confinement. With increasing liquid density, the confined water with uneven layers transforms separately into two-dimensional ice crystals with unchanged layer number and rhombic in-plane symmetry for oxygen atoms. The monolayer water undergoes the transition first into a puckered ice nanoribbon, and the bilayer water transforms into a rhombic ice nanoribbon next, followed by the transition of trilayer water into a trilayer ice nanoribbon. The sequential localized liquid-to-solid transition within the inhomogeneous confinement can also be achieved by gradually decreasing the temperature at low liquid densities. These findings of phase behaviors of water under the inhomogeneous nanoconfinement not only extend the phase diagram of confined water but also have implications for realistic nanofluidic systems and microporous materials.