Liquid to quasicrystal transition in bilayer water

Formation of a two-dimensional helical square tube ice in hydrophobic nanoslit using the TIP5P water model

In extreme and nanoconfinement conditions, the tetrahedral arrangement of water molecules is challenged, resulting in a rich and new phase behavior unseen in bulk phases. The unique phase behavior of water confined in hydrophobic nanoslits has been previously observed, such as the formation of a variety of two-dimensional (2D) ices below the freezing temperature. The primary identified 2D ice phase, termed square tube ice (STI), represents a unique arrangement of water molecules in 2D ice, which can be viewed as an array of 1D ice nanotubes stacked in the direction parallel to the confinement plane. In this study, we report the molecular dynamics (MD) simulations evidence of a novel 2D ice phase, namely, helical square tube ice (H-STI). H-STI is characterized by the stacking of helical ice nanotubes in the direction parallel to the confinement plane. Its structural specificity is evident in the presence of helical square ice nanotubes, a configuration unseen in both STI and single-walled ice nanotubes. A detailed analysis of the hydrogen bonding strength showed that H-STI is a 2D ice phase diverging from the Bernal-Fowler-Pauling ice rules by forming only two strong hydrogen bonds between adjacent molecules along its helical ice chain. This arrangement of strong hydrogen bonds along ice nanotube and weak bonds between the ice nanotube shows a similarity to quasi-one-dimensional van der Waals materials. Ab initio molecular dynamics simulations (over a 30 ps) were employed to further verify H-STI's stability at 1 GPa and temperature up to 200 K.

Ice structures assembled from cubic water clusters of D2d and S4 symmetry

The study of self-assembly processes is of key importance for fundamental science and modern technologies. Cubic water clusters of D2d and S4 symmetry show great potential as building blocks for self-assembly. The objective of this paper is to construct possible ice structures formed by hydrogen bonding of these very stable water clusters. A number of such structures are herein presented, including quasi-2D and 3D ices as well as spatial layered and tubular ices. The energetics and structure of many configurations differing in the arrangement of hydrogen atoms in hydrogen bonds have been studied. It was established that the proton disorder of all such ices is of island type. The residual entropy of these ices is equal to ln(3)/4 in dimensionless form. For layered structures formed by the stacking of multiple bilayers, the determining role of the van der Waals interactions is shown. Note that, for all considered ices, the lowest-energy configurations are formed only by clusters of D2d symmetry.

Robust Estimation of Position-Dependent Anisotropic Diffusivity Tensors from Molecular Dynamics Trajectories

Confinement breaks translational and rotational symmetry in materials and makes all physical properties functions of position. Such spatial variations are key to modulating material properties at the nanoscale, and characterizing them accurately is therefore an intense area of research in the molecular simulations community. This is relatively easy to accomplish for basic mechanical observables. Determining spatial profiles of transport properties, such as diffusivity, is, however, much more challenging, as it requires calculating position-dependent autocorrelations of mechanical observables. In our previous paper (Domingues, T.S.; Coifman, R.; Haji-Akbari, A. J. Phys. Chem. B 2023, 127, 5273 10.1021/acs.jpcb.3c00670), we analytically derive and numerically validate a set of filtered covariance estimators (FCEs) for quantifying spatial variations of the diffusivity tensor from stochastic trajectories. In this work, we adapt these estimators to extract diffusivity profiles from MD trajectories and validate them by applying them to a Lennard-Jones fluid within a slit pore. We find our MD-adapted estimator to exhibit the same qualitative features as its stochastic counterpart, as it accurately estimates the lateral diffusivity across the pore while systematically underestimating the normal diffusivity close to hard boundaries. We introduce a conceptually simple and numerically efficient correction scheme based on simulated annealing and diffusion maps to resolve the latter artifact and obtain normal diffusivity profiles that are consistent with the self-part of the van Hove correlation functions. Our findings demonstrate the potential of this MD-adapted estimator in accurately characterizing spatial variations of diffusivity in confined materials.

The pressure induced phase diagram of double-layer ice under confinement: a first-principles study

Here, we present double-layer ice confined within various carbon nanotubes (CNTs) using state-of-the-art pressure induced (-5 GPa to 5 GPa) dispersion corrected density functional theory (DFT) calculations. We find that the double-layer ice exhibits remarkably rich and diverse phase behaviors as a function of pressure with varying CNT diameters. The lattice cohesive energies for various pure double layer ice phases follow the order of hexagonal > pentagonal > square tube > hexagonal-close-packed (HCP) > square > buckled-rhombic (b-RH). The confinement width was found to play a crucial role in the square and square tube phases in the intermediate pressure range of about 0-1 GPa. Unlike the phase transition in pure bilayer ice structures, the relative enthalpies demonstrate that the pentagonal phase, rather than the hexagonal structure, is the most stable ice polymorph at ambient pressure as well as in a deep negative pressure region, whereas the b-RH phase dominates under high pressure. The relatively short O⋯O distance of b-RH demonstrates the presence of a strong hydrogen bonding network, which is responsible for stabilizing the system.

First-Principles Molecular Dynamics Simulations of the Spontaneous Freezing Transition of 2D Water in a Nanoslit

As with bulk ices, two-dimensional (2D) ices exhibit diverse crystalline structures, and the majority of these 2D structures have been predicted based on classical molecular dynamics (MD) simulations. Here, the spontaneous freezing transition of 2D liquid water within hydrophobic nanoslits is demonstrated for the first time using first-principles MD simulations. Various 2D ices are observed under different lateral pressure and temperature conditions. Notably, the liquid water confined to a 6.0 Å-wide nanoslit can spontaneously freeze into a monolayer ice consisting of an array of zigzag water chains at 2.5 GPa and 250 K. Moreover, within an 8.0 Å-wide nanoslit and at 4.0 GPa and 300 K, a previously unreported bilayer ice forms spontaneously that has a structure resembling that of the double surface layers of bulk ice-VII. Both 2D crystalline ices do not obey the ice rule, suggesting first-principles simulation can access a certain phase space that is not easily approached using classical simulations.

Two Liquid-Liquid Phase Transitions in Confined Water Nanofilms

The stories behind supercooled bulk and confined water can be different. Bulk water has a metastable liquid-liquid phase transition at deeply supercooled conditions, but the existence of such a phenomenon in confined water is in question. Herein we show simulation results of first-order phase transitions between high- and low-density liquid (HDL and LDL) in confined water in both positive and negative pressures. A mid-density state between these two local states appears, which lets the transition show the hysteresis loop with transiently stable intermediate states. On the basis of Landau theory that we have adapted for mixing of moieties with high- and low-density states, we explain the phase transitions with the order parameter-dependent free energy change which is governed by second- to higher-order interactions among those moieties.

Gas hydrates in confined space of nanoporous materials: new frontier in gas storage technology

Gas hydrates (clathrate hydrates, clathrates, or hydrates) are crystalline inclusion compounds composed of water and gas molecules. Methane hydrates, the most well-known gas hydrates, are considered a menace in flow assurance. However, they have also been hailed as an alternative energy resource because of their high methane storage capacity. Since the formation of gas hydrates generally requires extreme conditions, developing porous material hosts to synthesize gas hydrates with less-demanding constraints is a topic of great interest to the materials and energy science communities. Though reports of modeling and experimental analysis of bulk gas hydrates are plentiful in the literature, reliable phase data for gas hydrates within confined spaces of nanoporous media have been sporadic. This review examines recent studies of both experiments and theoretical modeling of gas hydrates within four categories of nanoporous material hosts that include porous carbons, metal-organic frameworks, graphene nanoslits, and carbon nanotubes. We identify challenges associated with these porous systems and discuss the prospects of gas hydrates in confined space for potential applications.

Is Ice Nucleation by Organic Crystals Nonclassical? An Assessment of the Monolayer Hypothesis of Ice Nucleation

Potent ice nucleating organic crystals display an increase in nucleation efficiency with pressure and memory effect after pressurization that set them apart from inorganic nucleants. These characteristics were proposed to arise from an ordered water monolayer at the organic-water interface. It was interpreted that ordering of the monolayer is the limiting step for ice nucleation on organic crystals, rendering their mechanism of nucleation nonclassical. Despite the importance of organics in atmospheric ice nucleation, that explanation has never been investigated. Here we elucidate the structure of interfacial water and its role in ice nucleation at ambient pressure on phloroglucinol dihydrate, the paradigmatic example of outstanding ice nucleating organic crystal, using molecular simulations. The simulations confirm the existence of an interfacial monolayer that orders on cooling and becomes fully ordered upon ice formation. The monolayer does not resemble any ice face but seamlessly connects the distinct hydrogen-bonding orders of ice and the organic surface. Although large ordered patches develop in the monolayer before ice nucleates, we find that the critical step is the formation of the ice crystallite, indicating that the mechanism is classical. We predict that the fully ordered, crystalline monolayer nucleates ice above -2 °C and could be responsible for the exceptional ice nucleation by the organic crystal at high pressures. The lifetime of the fully ordered monolayer around 0 °C, however, is too short to account for the memory effect reported in the experiments. The latter could arise from an increase in the melting temperature of ice confined by strongly ice-binding surfaces.

Pauli Repulsion Enhances Mobility of Ultraconfined Water

Water is ubiquitous on Earth and dominates chemical and biological processes in daily life. However, how water behaves under some critical conditions is not fully understood. In this paper, we employed quantum first-principles calculations and dynamics simulations to reveal the unexpectedly high mobility of water molecules in ultraconfined spaces. The water molecules rotated more freely in the (4, 4) carbon nanotube than in the (5, 5) carbon nanotube, which is induced by the Pauli repulsion from the wall of the narrower channel when reducing the size of the channel from general confinement to ultraconfinement. Moreover, this quantum effect facilitates the transport of water molecules into the space within their van der Waals diameter easily, which is in contrast to the general understanding. Thus, the conventional concept that the tighter the confined space, the more difficult the motion of the confined object is not always correct. This quantum-induced enhancement of water mobility by Pauli repulsion calls us to pay more attention to the existence and the function of water in neglected ultraconfined spaces (e.g., cells and the Earth's crust) in the future.

Replica exchange MD simulations of two-dimensional water in graphene nanocapillaries: rhombic versus square structures, proton ordering, and phase transitions

The hydrogen bond patterns, proton ordering, and phase transitions of monolayer ice in two-dimensional hydrophobic confinement are fundamentally different from those found for bulk ice. To investigate the behavior of quasi-2D ice, we perform molecular dynamics simulations of water confined between fixed graphene plates at a distance of 0.65 nm. While experimental results are still limited and theoretical investigations are often based on a single, often empirically based force field model, this work presents a systematic study modeling the water-graphene interaction by effective Lennard-Jones potentials previously derived from high-level ab initio CCSD(T) calculations of water adsorbed on graphene [Phys. Chem. Chem. Phys., 2013, 15, 4995]. For the water-water interaction different water force fields, i.e. SPCE, TIP3P, TIP4P, TIP4P/ICE, and TIP5P, are used. The water occupancy of the graphene capillary at a pressure of 1000 MPa is determined to be between 13.5 and 13.9 water molecules per square nanometer, depending on the choice of the water force field. Based on these densities, we explore the structure and dynamics of quasi-2D water for temperatures ranging from 200 K to about 600 K for each of the five force fields. To ensure complete sampling of the configurational space and to overcome the barriers separating metastable structures, these simulations are based on the replica exchange molecular dynamics technique. We report different tetragonal hydrogen bond patterns, which are classified as nearly square or as rhombic. While many of these arrangements are essentially flat, in some cases puckered arrangements are found, too. Also the proton ordering of the quasi-2D water structures is considered, allowing us to identify them as ferroelectric, ferrielectric or antiferroelectric. For temperatures between 200 K and 400 K we find several second-order phase transitions from one ice structure to another, changing in many cases both the arrangements of the oxygen atoms and the proton ordering. For temperatures between 400 K and 600 K there are melting-like transitions from a monolayer of ice to a monolayer of liquid water. These first-order phase transitions have a latent heat between 3.4 and 4.0 kJ mol^-1. Both the values of the transition temperatures and of the latent heats display considerable model dependence for the five different water models investigated here.

Direct observation of 2-dimensional ices on different surfaces near room temperature without confinement

Water-solid interfaces play important roles in a wide range of fields, including atmospheric science, geochemistry, electrochemistry, and food science. Herein, we report simulation evidence of 2-dimensional (2D) ice formation on various surfaces and the dependence of the 2D crystalline structure on the hydrophobicity and morphology of the underlying surface. Contrary to the prevailing view that nanoscale confinement is necessary for the 2D liquid-to-bilayer ice transition, we find that the liquid-to-bilayer hexagonal ice (BHI) transition can occur either on a model smooth surface or on model fcc-crystal surfaces with indices of (100), (110), and (111) near room temperature. We identify a critical parameter that characterizes the water-surface interaction, above which the BHI can form on the surface. This critical parameter increases as the temperature increases. Even at temperatures above the freezing temperature of bulk ice (Ih ), we find that BHI can also form on a superhydrophilic surface due to the strong water-surface interaction. The tendency toward the formation of BHI without confinement reflects a proper water-surface interaction that can compensate for the entropy loss during the freezing transition. Furthermore, phase diagrams of 2D ice formation are described on the plane of the adsorption energy versus the fcc lattice constant (Eads-afcc), where 4 monolayer square-like ices are also identified on the fcc model surfaces with distinct water-surface interactions.

Anomalous Stability of Two-Dimensional Ice Confined in Hydrophobic Nanopores

The freezing of water mostly proceeds via heterogeneous ice nucleation, a process in which an effective nucleation medium not only expedites ice crystallization but also may effectively direct the polymorph selection of ice. Here, we show that water confined within a hydrophobic slit nanopore exhibits a freezing behavior strongly distinguished from its bulk counterpart. Such a difference is reflected by a strong, non-monotonic pore-size dependence of freezing temperature but, more surprisingly, by an unexpected stacking ordering of crystallized two-dimensional ice containing just a few ice layers. In particular, confined trilayer ice is found to exclusively crystallize into a well-ordered, hexagonal stacking sequence despite the fact that nanopore exerts no explicit constraint on stacking order. The absence of cubic stacking sequence is found to be originated from the intrinsically lower thermodynamic stability of cubic ice over hexagonal ice at the interface, which contrasts sharply the nearly degenerated stability of bulk hexagonal and cubic ices. Detailed examination clearly reveals that the divergence is attributed to the inherent difference between the two ice polymorphs in their surface phonon modes, which is further found to generically occur at both hydrophobic and hydrophilic surfaces.

Novel pressure-induced topological phase transitions of supercooled liquid and amorphous silicene

This molecular dynamics (MD) simulation carries a detailed analysis of a pressure-induced structural transition supercooled liquid and amorphous silicene (a-silicene). Low-density models of supercooled liquid and a-silicene containing 10 000 atoms are obtained by rapid cooling processes from the melts. Then, an a-silicene model at T  =  1000 K, a supercooled liquid model at T  =  1500 K and a liquid silicon model at T  =  2000 K have been isothermally compressed step by step up to a high density in order to observe the pressure-induced structural changes. Specifically 'Cairo tiling' pentagonal and square lattices of silicene are discovered in our calculations. Structural properties of those penta-silicene and tetra-silicene models have been carefully analyzed through the radial distribution functions, interatomic distances, bond-angle distributions under high-pressure condition. The dependence of pressure on formation behaviors is calculated via pressure-volume and energy-density relationships. The first order transition from low-density supercooled liquid/amorphous silicene to high-density penta-silicene and continuous transition from low-density liquid to high-density tetra-silicene are discussed. Atomic mechanism and sp³/sp² hybridization evolution are inspected whereas the role of low-membered ring defects/boundary promises remarkable application and advanced research in future.

Two-dimensional dry ices with rich polymorphic and polyamorphic phase behavior

Both carbon dioxide (CO2) and water (H2O) are triatomic molecules that are ubiquitous in nature, and both are among the five most abundant gases in the Earth's atmosphere. At low temperature and ambient pressure, both CO2 and H2O form molecular crystals--dry ice I and ice I h Because water possesses distinctive hydrogen bonds, it exhibits intricate and highly pressure-dependent phase behavior, including at least 17 crystalline ice phases and three amorphous ice phases. In contrast, due to its weak van der Waals intermolecular interactions, CO2 exhibits fewer crystalline phases except at extremely high pressures, where nonmolecular ordered structures arise. Herein, we show the molecular dynamics simulation results of numerous 2D polymorphs of CO2 molecules in slit nanopores. Unlike bulk polymorphs of CO2, 2D CO2 polymorphs exhibit myriad crystalline and amorphous structures, showing remarkable polymorphism and polyamorphism. We also show that depending on the thermodynamic path, 2D solid-to-solid phase transitions can give rise to previously unreported structures, e.g., wave-like amorphous CO2 structures. Our simulation also suggests intriguing structural connections between 2D and 3D dry ice phases (e.g., Cmca and PA-3) and offers insights into CO2 polyamorphic transitions through intermediate liquid or amorphous phases.

Ice-Liquid Oscillations in Nanoconfined Water

Nanoscale confinement has a strong effect on the phase behavior of water. Studies in the last two decades have revealed a wealth of novel crystalline and quasicrystalline structures for water confined in nanoslits. Less is known, however, about the nature of ice-liquid coexistence in extremely nanoconfined systems. Here, we use molecular simulations to investigate the ice-liquid equilibrium for water confined between two nanoscopic disks. We find that the nature of ice-liquid phase coexistence in nanoconfined water is different from coexistence in both bulk water and extended nanoslits. In highly nanoconfined systems, liquid water and ice do not coexist in space because the two-phase states are unstable. The confined ice and liquid phases coexist in time, through oscillations between all-liquid and all-crystalline states. The avoidance of spatial coexistence of ice and liquid originates on the non-negligible cost of the interface between confined ice and liquid in a small system. It is the result of the small number of water molecules between the plates and has no analogue in bulk water.

Structure, mechanism, and performance evaluation of natural gas hydrate kinetic inhibitors

Ice-like crystal compounds, which are formed in low-temperature and high-pressure thermodynamic conditions and composed of a combination of water molecules and guest gas molecules, are called gas hydrates. Since its discovery and recognition as the responsible component for blockage of oil and gas transformation line, hydrate has been under extensive review by scientists. In particular, the inhibition techniques of hydrate crystals have been updated in order to reach the more economically and practically feasible methods. So far, kinetic hydrate inhibition has been considered as one of the most effective techniques over the past decade. This review is intended to classify the recent studies regarding kinetic hydrate inhibitors, their structure, mechanism, and techniques for their performance evaluation. In addition, this communication further analyzes the areas that are more in demand to be considered in future research.

Phase behaviors of deeply supercooled bilayer water unseen in bulk water

Akin to bulk water, water confined to an isolated nanoslit can show a wealth of new 2D phases of ice and amorphous ice, as well as unusual phase behavior. Indeed, 2D water phases, such as bilayer hexagonal ice and monolayer square ice, have been detected in the laboratory, confirming earlier computational predictions. Herein, we report theoretical evidence of a hitherto unreported state, namely, bilayer very low density amorphous ice (BL-VLDA), as well as evidence of a strong first-order transition between BL-VLDA and the BL amorphous ice (BL-A), and a weak first-order transition between BL-VLDA and the BL very low density liquid (BL-VLDL) water. The diffusivity of BL-VLDA is typically in the range of 10-9 cm2/s to 10-10 cm2/s. Similar to bulk (3D) water, 2D water can exhibit two forms of liquid in the deeply supercooled state. However, unlike supercooled bulk water, for which the two forms of liquid can coexist and merge into one at a critical point, the 2D BL-VLDL and BL high-density liquid (BL-HDL) phases are separated by the highly stable solid phase of BL-A whose melting line exhibits the isochore end point (IEP) near 220 K in the temperature-pressure diagram. Above the IEP temperature, BL-VLDL and BL-HDL are indistinguishable. At negative pressures, the metastable BL-VLDL exhibits a spatially and temporally heterogeneous structure induced by dynamic changes in the nanodomains, a feature much less pronounced in the BL-HDL.

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.

Is Water at the Graphite Interface Vapor-like or Ice-like?

Graphitic surfaces are the main component of soot, a major constituent of atmospheric aerosols. Experiments indicate that soots of different origins display a wide range of abilities to heterogeneously nucleate ice. The ability of pure graphite to nucleate ice in experiments, however, seems to be almost negligible. Nevertheless, molecular simulations with the monatomic water model mW with water-carbon interactions parameterized to reproduce the experimental contact angle of water on graphite predict that pure graphite nucleates ice. According to classical nucleation theory, the ability of a surface to nucleate ice is controlled by the binding free energy between ice immersed in liquid water and the surface. To establish whether the discrepancy in freezing efficiencies of graphite in mW simulations and experiments arises from the coarse resolution of the model or can be fixed by reparameterization, it is important to elucidate the contributions of the water-graphite, water-ice, and ice-water interfaces to the free energy, enthalpy, and entropy of binding for both water and the model. Here we use thermodynamic analysis and free energy calculations to determine these interfacial properties. We demonstrate that liquid water at the graphite interface is not ice-like or vapor-like: it has similar free energy, entropy, and enthalpy as water in the bulk. The thermodynamics of the water-graphite interface is well reproduced by the mW model. We find that the entropy of binding between graphite and ice is positive and dominated, in both experiments and simulations, by the favorable entropy of reducing the ice-water interface. Our analysis indicates that the discrepancy in freezing efficiencies of graphite in experiments and the simulations with mW arises from the inability of the model to simultaneously reproduce the contact angle of liquid water on graphite and the free energy of the ice-graphite interface. This transferability issue is intrinsic to the resolution of the model, and arises from its lack of rotational degrees of freedom.

Modelling the atomic arrangement of amorphous 2D silica: a network analysis

The recent experimental discovery of a semi two-dimensional silica glass has offered a realistic description of the random network theory of a silica glass structure, initially discussed by Zachariasen.

Non-close-packed three-dimensional quasicrystals

Quasicrystals are frequently encountered in condensed matter. They are important candidates for equilibrium phases from the atomic scale to the nanoscale. Here, we investigate the computational self-assembly of four quasicrystals in a single model system of identical particles interacting with a tunable isotropic pair potential. We reproduce a known icosahedral quasicrystal and report a decagonal quasicrystal, a dodecagonal quasicrystal, and an octagonal quasicrystal. The quasicrystals have low coordination number or occur in systems with mesoscale density variations. We also report a network gel phase.

Self-assembly of two-dimensional binary quasicrystals: a possible route to a DNA quasicrystal

We use Monte Carlo simulations and free-energy techniques to show that binary solutions of penta- and hexavalent two-dimensional patchy particles can form thermodynamically stable quasicrystals even at very narrow patch widths, provided their patch interactions are chosen in an appropriate way. Such patchy particles can be thought of as a coarse-grained representation of DNA multi-arm 'star' motifs, which can be chosen to bond with one another very specifically by tuning the DNA sequences of the protruding arms. We explore several possible design strategies and conclude that DNA star tiles that are designed to interact with one another in a specific but not overly constrained way could potentially be used to construct soft quasicrystals in experiment. We verify that such star tiles can form stable dodecagonal motifs using oxDNA, a realistic coarse-grained model of DNA.

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.

Continuous melting through a hexatic phase in confined bilayer water

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquidlike throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems.

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.

Thermodynamic and kinetic anisotropies in octane thin films

Confinement breaks the translational symmetry of materials, making all thermodynamic and kinetic quantities functions of position. Such symmetry breaking can be used to obtain configurations that are not otherwise accessible in the bulk. Here, we use computer simulations to explore the effect of substrate-liquid interactions on thermodynamic and kinetic anisotropies induced by a solid substrate. We consider n-octane nano-films that are in contact with substrates with varying degrees of attraction, parameterized by an interaction parameter ϵS. Complete freezing of octane nano-films is observed at low temperatures, irrespective of ϵS, while at intermediate temperatures, a frozen monolayer emerges at solid-liquid and vapor-liquid interfaces. By carefully inspecting the profiles of translational and orientational relaxation times, we confirm that the translational and orientational degrees of freedom are decoupled at these frozen monolayers. At sufficiently high temperatures, however, free interfaces and solid-liquid interfaces close to loose (low-ϵS) substrates undergo "pre-freezing," characterized by mild peaks in several thermodynamic quantities. Two distinct dynamic regimes are observed at solid-liquid interfaces. The dynamics is accelerated in the vicinity of loose substrates, while sticky (high-ϵS) substrates decelerate dynamics, sometimes by as much as two orders of magnitude. These two distinct dynamical regimes have been previously reported by Haji-Akbari and Debenedetti [J. Chem. Phys. 141, 024506 (2014)] for a model atomic glass-forming liquid. We also confirm the existence of two correlations-proposed in the above-mentioned work-in solid-liquid subsurface regions of octane thin films, i.e., a correlation between atomic density and normal stress, and between atomic translational relaxation time and lateral stress. Finally, we inspect the ability of different regions of an octane film to explore the potential energy landscape by performing inherent structure calculations, and observe no noticeable difference between the free surface and the bulk in efficiently exploring the potential energy landscape. This is unlike the films of model atomic glass formers that tend to sample their respective landscape more efficiently at free surfaces. We discuss the implications of this finding to the ability of octane-and other n-alkanes-to form ultrastable glasses.

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.

Self-assembly of the decagonal quasicrystalline order in simple three-dimensional systems

Using molecular dynamics simulations we show that a one-component system can be driven to a three-dimensional decagonal (10-fold) quasicrystalline state just by purely repulsive, isotropic and monotonic interaction pair potential with two characteristic length scales; no attraction is needed. We found that self-assembly of a decagonal quasicrystal from a fluid can be predicted by two dimensionless effective parameters describing the fluid structure. We demonstrate stability of the results under changes of the potential by obtaining the decagonal order for three particle systems with different interaction potentials, both purely repulsive and attractive, but with the same values of the effective parameters. Our results suggest that soft matter quasicrystals with decagonal symmetry can be experimentally observed for the same systems demonstrating the dodecagonal order for an appropriate tuning of the effective parameters.

Molecular simulations of heterogeneous ice nucleation. II. Peeling back the layers

Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for a face centered cubic (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.

Solid-liquid critical behavior of a cylindrically confined Lennard-Jones fluid

Extensive molecular dynamics simulations have been performed to study the phase behavior of Lennard-Jones particles confined in a quasi-one-dimensional hydrophobic nanopore. We provide unambiguous evidence for a solid-liquid critical point by investigating (i) isotherms in the pressure-volume plane, (ii) the spontaneous solid-liquid phase separation below a certain temperature, (iii) diverging heat capacity and isothermal compressibility as a certain point is approached, (iv) continuous change of dynamical and structural properties above the point, (v) the finite-size scaling analysis of the density distribution below and above the point. The result combined with earlier studies of confined water suggests that the solid-liquid critical point is not uncommon in quasi-one- and quasi-two-dimensional fluids.

Subcellular neuronal quasicrystals: Implications for consciousness

Neuron neurotransmitter receptors are in general pentameric. This enables them to form pentagonal components in biological quasicrystals (similar to mathematical aperiodic tilings). As quasicrystals have been proposed to require quantum effects to exist this might introduce such effects as a component of neurotransmission and thus consciousness. Microtubules may play a role in the clustering of the receptors into quasicrystals, thus modulating their function and may even form quasicrystals themselves. Other quaiscrystals in neurons are potentially formed by water, cholera toxin complexes, and the cytoskeletal components actin and ankyrin.