A new phase diagram of water under negative pressure: The rise of the lowest-density clathrate s-III

Assisted Self-Assembly of Nanoporous Ices via Carbon Nanomaterial Templates

Self-assembly is a widely used synthetic method in nanoscience to assemble well-organized structures. Self-assembly processes usually occur in a water solvent environment. However, the self-assembly of water molecules is rarely studied. Herein, we show a strategy to fabricate porous ice via carbon nanomaterial-assisted self-assembly. Diverse frameworks of nanoporous ice are formed by using orthorhombic and tetragonal arrays of carbon nanotubes or carbon-atom chains as templates. In contrast to many bulk ices discovered in nature, nanoporous ices are shown to be stable only under negative pressure. Hence, nanoporous ices cannot be produced through the direct nucleation of water at negative pressure. The template-assisted self-assembly method is shown to be the most effective method to fabricate nanoporous ice in quantity. Several key factors for the self-assembly of nanoporous ices are identified, including proper gap spacings in the carbon nanomaterial template and suitable interactions between water and the carbon nanomaterials.

Hydrophobic hydration of the hydrocarbon adamantane in amorphous ice

Hydrophobic molecules are by definition difficult to hydrate. Previous studies in the area of hydrophobic hydration have therefore often relied on using amphiphilic molecules where the hydrophilic part of a molecule enabled the solubility in liquid water. Here, we show that the hydrophobic adamantane (C10H16) molecule can be fully hydrated through vapour codeposition with water onto a cryogenic substrate at 80 K resulting in the matrix isolation of adamantane in amorphous ice. Using neutron diffraction in combination with the isotopic substitution method and the empirical potential structure refinement technique, we find that the first hydration shell of adamantane is well structured consisting of a hydrogen-bonded cage of 28 water molecules that is also found in cubic structure II clathrate hydrates. The four hexagonal faces of the 51264 cage are situated above the four methine (CH) groups of adamantane whereas the methylene (CH2) groups are positioned below the edges of two adjoining pentagonal faces. The oxygen atoms of the 28 water molecules can be categorised on the basis of symmetry equivalences as twelve A, twelve B and four C oxygens. The water molecules of the first hydration shell display orientations consistent with those expected for a clathrate-hydrate-type cage, but also unfavourable ones with respect to the hydrogen bonding between the water molecules. Annealing the samples at 140 K, which is just below the crystallisation temperature of the matrix, removes the unfavourable orientations and leads to a slight increase in the structural order of the first hydration shell. The very closest water molecules display a tendency for their dipole moments to point towards the adamantane which is attributed to steric effects. Other than this, no significant polarisation effects are observed which is consistent with weak interactions between adamantane and the amorphous ice matrix. FT-IR spectroscopy shows that the incorporation of adamantane into amorphous ice leads to a weakening of the hydrogen bonds. In summary, the matrix-isolation of the highly symmetric adamantane in amorphous ice provides an interesting test case for hydrophobic hydration. Studying the structure and spectroscopic properties of water at the interface with hydrophobic hydrocarbons is also relevant for astrophysical environments, such as comets or the interstellar medium, where amorphous ice and hydrocarbons have been shown to coexist in large quantities.

Evidence of Spontaneous Formation of Two-Dimensional Amorphous Clathrates on Superhydrophilic Surfaces

Clathrate hydrates reserved in the seabed are often dispersed in the pores of coarse-grained sediments; hence, their formation typically occurs under nanoconfinement. Herein, we show the first molecular dynamics (MD) simulation evidence of the spontaneous formation of two-dimensional (2D) clathrate hydrates on crystal surfaces without conventional nanoconfinement. The kinetic process of 2D clathrate formation is illustrated via simulated single-molecule deposition. 2D amorphous patterns are observed on various superhydrophilic face-centered cubic surfaces. Notably, the formation of 2D amorphous clathrate can occur over a wide range of temperatures, even at room temperature. The strong water-surface interaction, the characteristic properties of guest-gas molecules, and the underlying surface structure dictate the formation of 2D amorphous clathrates. Semiquantitative phase diagrams of 2D clathrates are constructed where representative patterns of 2D clathrates for characteristic gas molecules on prototypical Pd(111) and Pt(111) surfaces are confirmed by independent MD simulations. A tunable pattern of 2D amorphous clathrates is demonstrated by changing the lattice strain of the underlying substrate. Moreover, ab initio MD simulations confirm the stability of 2D amorphous clathrate. The underlining physical mechanism for 2D clathrate formation on superhydrophilic surfaces is elucidated, which offers deeper insight into the crucial role of water-surface interaction.

Advances in single ice crystal shaping materials: From nature to synthesis and applications in cryopreservation

Antifreeze (glyco) proteins [AF(G)Ps], which are widely present in various extreme microorganisms, can control the formation and growth of ice crystals. Given the significance of cryogenic technology in biomedicine, climate science, electronic energy, and other fields of research, scientists are quite interested in the development and synthesis high-efficiency bionic antifreeze protein materials, particularly to reproduce their dynamic ice shaping (DIS) characteristics. Single ice crystal shaping materials, a promising class of ice-controlling materials, can alter the morphology and growth rate of ice crystals at low temperatures. This review aims to highlight the development of single ice crystal shaping materials and provide a brief comparison between a series of natural and bionic synthetic materials with DIS ability, which include AF(G)Ps, polymers, salts, and nanomaterials. Additionally, we summarize their applications in cryopreservation. Finally, this paper presents the current challenges and prospects encountered in developing high-efficiency and practical single ice crystal shaping materials. STATEMENT OF SIGNIFICANCE: The formation and growth of ice crystals hold a significant importance to an incredibly broad range of fields. Therefore, the design and fabrication of the single ice crystal shaping materials have gained the increasing popularity due to its key role in dynamic ice shaping (DIS) characteristics. Especially, single ice crystal shaping materials are considered one of the most promising candidates as ice inhibitors, presenting tremendous prospects for enhancing cryopreservation. In this work, we focus on the molecular characteristics, structure-function relationships, and DIS mechanisms of typical natural and biomimetic synthetic materials. This review may provide inspiration for the design and preparation of single ice crystal shaping materials and give guidance for the development of effective cryopreservation agent.

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.

Puckered Zigzag Monolayer Ice: Does a Confined Flat Four-Coordinated Monolayer Ice Always Have a Corresponding Puckered Phase?

We note that a flat, four-coordinated monolayer ice under confinement always has a corresponding puckered phase. Recently, a monolayer ice consisting of an array of zigzag water chains (ZZMI) predicted by first-principles calculations of water under confinement is a flat four-coordinated monolayer ice. Herein, to investigate whether puckered ZZMI exists stably, we perform molecular dynamics simulations of two-dimensional (2D) ice formation for water constrained in graphene nanocapillaries. We find a novel monolayer ice structure that can be viewed as the ZZMI puckered along the direction perpendicular to the zigzag chain (pZZMI). Unlike ZZMI that does not satisfy the ice rule, each water molecule in pZZMI can form four hydrogen bonds (HBs) via forming two stable intersublayer HBs and two intrasublayer HBs. This work provides a fresh perspective on 2D confined ice, highlighting the intrinsic connections between 2D confined ices.

Computational investigations of stable multiple-cage-occupancy He clathrate-like hydrostructures

One of the several possibilities offered by the interesting clathrate hydrates is the opportunity to encapsulate several atoms or molecules, in such a way that more efficient storage materials could be explored or new molecules that otherwise do not exist could be created. These types of applications are receiving growing attention from technologists and chemists, given the future positive implications that they entail. In this context, we investigated the multiple cage occupancy of helium clathrate hydrates, to establish stable novel hydrate structures or ones similar to those predicted previously by experimental and theoretical studies. To this purpose, we analyzed the feasibility of including an increased number of He atoms inside the small (D) and large (H) cages of the sII structure through first-principles properly assessed density functional approaches. On the one hand, we have computed energetic and structural properties, in which we examined the guest-host and guest-guest interactions in both individual and two-adjacent clathrate-like sII cages by means of binding and evaporation energies. On the other hand, we have carried out a thermodynamical analysis on the stability of such He-containing hydrostructures in terms of changes in enthalpy, ΔH, Gibbs free energy, ΔG, and entropy, ΔS, during their formation process at various temperature and pressure values. In this way, we have been able to make a comparison with experiments, reaffirming the ability of computational DFT approaches to describe such weak guest-host interactions. In principle, the most stable structure involves the encapsulation of one and four He atoms inside the D and H sII cages, respectively; however, more He atoms could be entrapped under lower temperature and/or higher pressure thermodynamic conditions. We foresee such accurate computational quantum chemistry approaches contributing to the current emerging machine-learning model development.

Nanoporous ices: an emerging class in the water/ice family

The history of scientific research on diverse ice structures dates back to more than a century. To date, 20 three-dimensional crystalline ice phases (ice I-ice XX) have been identified in the laboratory, among which ice XVI and ice XVII belong to a class of low-density nanoporous ices. Nanoporous ices can also be viewed as a special class of porous materials or water ice, as they possess a relatively high fraction of nano-cavities and/or nano-channels built into the hydrogen-bonded water framework. As such, like the prototypical class of porous materials (e.g., MOFs and COFs), nanoporous ices can be named as water oxygen-vertex frameworks (WOFs). Because of their large surface-to-volume ratio, WOFs may be potential media for gas storage, gas purification and separation. They may be applied to the biomedical field owing to their excellent biocompatibility. The field of porous ices is still emerging, as many porous ice structures that are predicted to be stable by computer simulations require future experimental confirmation. For future theoretical/computational studies, as the machine-learning method becomes an increasingly popular research tool in the material science and chemical science fields, more reliable porous ice structures and phase diagrams will be predicted with the development of more accurate machine-learning force fields.

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.

Delving into guest-free and He-filled sI and sII clathrate hydrates: a first-principles computational study

The dynamics of the formation of a specific clathrate hydrate as well as its thermodynamic transitions depend on the interactions between the trapped molecules and the host water lattice. The molecular-level understanding of the different underlying processes benefits not only the description of the properties of the system, but also allows the development of multiple technological applications such as gas storage, gas separation, energy transport, etc. In this work we investigate the stability of periodic crystalline structures, such as He@sI and He@sII clathrate hydrates by first-principles computations. We consider such host water networks interacting with a guest He atom using selected density functional theory approaches, in order to explore the effects on the encapsulation of a light atom in the sI/sII crystals, by deriving all energy components (guest-water, water-water, guest-guest). Structural properties and energies were first computed by structural relaxations of the He-filled and empty sI/sII unit cells, yielding lattice and compressibility parameters comparable to experimental and theoretical values available for those hydrates. According to the results obtained, the He enclathration in the sI/sII unit cells is a stabilizing process, and both He@sI and He@sII clathrates, considering single cage occupancy, are predicted to be stable whatever the XDM or D4 dispersion correction applied. Our results further reveal that despite the weak underlying interactions the He encapsulation has a rather notable effect on both lattice parameters and energetics, with the He@sII being the most energetically favorable in accord with recent experimental observations.

Two-dimensional partitioned square ice confined in graphene/graphite nanocapillaries

As one of the most fascinating confined water/ice phenomena, two-dimensional square ice has been extensively studied and experimentally confirmed in recent years. Apart from the unidirectional homogeneous square icing patterns considered in previous studies, the multidirectional partitioned square icing patterns are discovered in this study and characterized by molecular dynamics (MD) simulations. Square icing parameters are proposed to quantitatively distinguish the partitioned patterns from the homogeneous patterns and the liquid water. The number of graphene monolayers n is varied in this study, and the results show that it is more energetically favorable to form partitioned square icing patterns when the water molecules are confined between graphite sheets (n ≥ 2) compared to graphene (n = 1). This phenomenon is insensitive to n as long as n ≥ 2 because of the short-range nature of the interaction between water molecules and the carbon substrate. Moreover, it is energetically unfavorable to form partitioned square icing patterns for a single layer of water molecules even for n ≥ 2, verifying that the interaction between layers of water molecules is another dominant factor in the formation of partitioned structures. The conversion from partitioned structure to homogeneous square patterns is investigated by changing the pressure and the temperature. Based on the comprehensive MD simulations, this study unveils the formation mechanism of the partitioned square icing patterns.

To be or not to be? that is the entropic, enthalpic, and molecular interaction dilemma in the formation of (water)20 clusters and methane clathrate

A detailed analysis under a comprehensive set of theoretical and computational tools of the thermodynamical factors and of the intermolecular interactions behind the stabilization of a well known set of (water)20 cavities and of the methane clathrate is offered in this work. Beyond the available reports of experimental characterization at extreme conditions of most of the systems studied here, all clusters should be amenable to experimental detection at 1 atm and moderate temperatures since 280 K marks the boundary at which, ignoring reaction paths, formation of all clusters is no longer spontaneous from the 20H2O → (H2O)20 and CH4 + 20H2O → CH4@512 processes. As a function of temperature, a complex interplay leading to the free energy of formation occurs between the destabilizing entropic contributions, mostly due to cluster vibrations, and the stabilizing enthalpic contributions, due to intermolecular interactions and the PV term, is best illustrated by the highly symmetric 512 cage consistently showing signs of stronger intermolecular bonding despite having smaller binding energy than the other clusters. A fluxional wall of attractive non-covalent interactions, arising because of the cumulative effect of a large number of tiny individual charge transfers to the interstitial region, plays a pivotal role stabilizing the CH4@512 clathrate.

Formation of porous ice frameworks at room temperature

Bulk crystalline ices with ultralow densities have been demonstrated to be thermodynamically metastable at negative pressures. However, the direct formation of these bulk porous ices from liquid water at negative pressures is extremely challenging. Inspired by approaches toward porous media based on host-guest chemistry, such as metal-organic frameworks and covalent organic frameworks, we herein demonstrate via molecular dynamics simulations that a class of ultralow-density porous ices with upright channels can be formed spontaneously from liquid water at 300 K with the assistance of carbon nanotube arrays. We refer to these porous ice polymorphs as water oxygen-vertex frameworks (WOFs). Notably, our simulations revealed that the liquid-WOF phase transition is first-order and occurs at room temperature. All the WOFs exhibited the unique structural feature that they can be regarded as assemblies of nanoribbons of hexagonal bilayer ice (2D ice I) at their armchair or zigzag edges. Based on density functional theory calculations, a comprehensive phase diagram of the WOFs was constructed considering both the thermodynamic and thermal stabilities of the porous ices at negative pressures. Like other types of porous media, these WOFs may be applicable to gas storage, purification, and separation. Moreover, these biocompatible porous ice networks may be exploited as medical-related carriers.

The everlasting hunt for new ice phases

Water ice exists in hugely different environments, artificially or naturally occurring ones across the universe. The phase diagram of crystalline phases of ice is still under construction: a high-pressure phase, ice XIX, has just been reported but its structure remains ambiguous.

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.

Formation of hot ice caused by carbon nanobrushes. II. Dependency on the radius of nanotubes

Stable crystalline structures of confined water can be different from bulk ice. In Paper I [T. Yagasaki et al., J. Chem. Phys. 151, 064702 (2019)] of this study, it was shown, using molecular dynamics (MD) simulations, that a zeolite-like ice structure forms in nanobrushes consisting of (6,6) carbon nanotubes (CNTs) when the CNTs are located in a triangle arrangement. The melting temperature of the zeolite-like ice structure is much higher than the melting temperature of ice I_h when the distance between the surfaces of CNTs is ∼0.94 nm, which is the best spacing for the bilayer structure of water. In this paper, we perform MD simulations of nanobrushes of CNTs that are different from (6,6) CNTs in radius. Several new porous ice structures form spontaneously in the MD simulations. A stable porous ice forms when the radius of its cavities matches the radius of the CNTs well. All cylindrical porous ice structures found in this study can be decomposed into a small number of structural blocks. We provide a new protocol to classify cylindrical porous ice crystals on the basis of this decomposition.

Quantum-mechanical exploration of the phase diagram of water

The set of known stable phases of water may not be complete, and some of the phase boundaries between them are fuzzy. Starting from liquid water and a comprehensive set of 50 ice structures, we compute the phase diagram at three hybrid density-functional-theory levels of approximation, accounting for thermal and nuclear fluctuations as well as proton disorder. Such calculations are only made tractable because we combine machine-learning methods and advanced free-energy techniques. The computed phase diagram is in qualitative agreement with experiment, particularly at pressures ≲ 8000 bar, and the discrepancy in chemical potential is comparable with the subtle uncertainties introduced by proton disorder and the spread between the three hybrid functionals. None of the hypothetical ice phases considered is thermodynamically stable in our calculations, suggesting the completeness of the experimental water phase diagram in the region considered. Our work demonstrates the feasibility of predicting the phase diagram of a polymorphic system from first principles and provides a thermodynamic way of testing the limits of quantum-mechanical calculations.

Identification of synthesisable crystalline phases of water – a prototype for the challenges of computational materials design

We discuss the identification of experimentally realisable crystalline phases of water to outline and contextualise some of the diverse building blocks of a computational materials design process.

Hydrogen Inter-Cage Hopping and Cage Occupancies inside Hydrogen Hydrate: Molecular-Dynamics Analysis

The inter-cage hopping in a type II clathrate hydrate with different numbers of H2 and D2 molecules, from 1 to 4 molecules per large cage, was studied using a classical molecular dynamics simulation at temperatures of 80 to 240 K. We present the results for the diffusion of these guest molecules (H2 or D2) at all of the different occupations and temperatures, and we also calculated the activation energy as the energy barrier for the diffusion using the Arrhenius equation. The average occupancy number over the simulation time showed that the structures with double and triple large-cage H2 occupancy appeared to be the most stable, while the small cages remained with only one guest molecule. A Markov model was also calculated based on the number of transitions between the different cage types.

Hydrogen Storage in Propane-Hydrate: Theoretical and Experimental Study

There have been studies on gas-phase promoter facilitation of H2-containing clathrates. In the present study, non-equilibrium molecular dynamics (NEMD) simulations were conducted to analyse hydrogen release and uptake from/into propane planar clathrate surfaces at 180–273 K. The kinetics of the formation of propane hydrate as the host for hydrogen as well as hydrogen uptake into this framework was investigated experimentally using a fixed-bed reactor. The experimental hydrogen storage capacity propane hydrate was found to be around 1.04 wt% in compare with the theoretical expected 1.13 wt% storage capacity of propane hydrate. As a result, we advocate some limitation of gas-dispersion (fixed-bed) reactors such as the possibility of having un-reacted water as well as limited diffusion of hydrogen in the bulk hydrate.

Computational Prediction of Novel Ice Phases: A Perspective

Although computational prediction of new ice phases is a niche field in water science, the scientific subject itself is representative of two important areas in physical chemistry, namely, statistical thermodynamics and molecular simulations. The prediction of a variety of novel ice phases has also attracted general public interest since the 1980s. In particular, the prediction of low-dimensional ice phases has gained momentum since the confirmation of a number of low-dimensional "computer ice" phases in the laboratory over the past decade. In this Perspective, the research advancements in computational prediction of novel ice phases over the past few years are reviewed. Particular attention is placed on new ice phases whose physical properties or dimensional structures are distinctly different from conventional bulk ices. Specific topics include the (i) formation of superionic ices, (ii) electrofreezing of water under high pressure and in a high external electric field, (iii) prediction of low-density porous ice at strongly negative pressure, (iv) ab initio computational study of two-dimensional (2D) ice under nanoscale confinement, and (v) 2D ices formed on a solid surface near ambient temperature without nanoscale confinement. Clearly, the formation of most of these novel ice phases demands certain extreme conditions. Ongoing challenges and new opportunities for predicting new ice phases from either classical molecular dynamics simulation or high-level ab initio computation are discussed.

He Inclusion in Ice-like and Clathrate-like Frameworks: A Benchmark Quantum Chemistry Study of Guest-Host Interactions

Energetics and structural properties of selected type and size He@hydrate frameworks, e.g., from regular structured ice channels to clathrate-like cages, are presented from first-principles quantum chemistry methods. The scarcity of information on He@hydrates makes such complexes challenging targets, while their computational study entails an interesting and arduous task. Some of them have been synthesized in the laboratory, which motivates further investigations on their stability. Hence, the main focus is to examine the performance and accuracy of different wave function-based electronic structure methods, such as MP2, CCSD(T), their explicitly correlated (F12) and domain-based local pair-natural orbital (DLPNO) analogs, as well as modern and conventional density functional theory (DFT) approaches, and analytical model potentials available. Different structures are considered, starting from the "simplest system" formed by a noble gas atom (such as He) and one water molecule, followed by the study of the "fundamental units" present in all ice-like and clathrate-like frameworks (such as pentamers and hexamers) and finally the description of interactions in the "building blocks" of three-dimensional (3D) ice channels (e.g., horizontal and perpendicular ice II and I_h) and clathrate-like cages, such as the 5¹² present in the most common sI, sII, and sH clathrate-hydrate structures. The idea is to provide well-converged DLPNO-CCSD(T) and DFMP2/CBS reference datasets that in turn are used to validate how DFT functionals (in total, 29 approaches from generalized-gradient approximation (GGA), meta-GGA, to hybrid and range-separated functionals, including dispersion correction treatments, were checked) and analytical semiempirical/ab initio-based potentials perform compared with high-level alternatives. Within all tested approaches, those best-performing were identified and classified. Most of the DFT/DFT-D functionals, as well as available analytical pairwise model potentials, face difficulties in describing both hydrogen-bonded water frameworks and dispersion bound He-water interactions. Including dispersion corrections yields an overall well-balanced performance for LCωPBE-D3BJ and PBE0-D4 functionals. Such benchmark datasets can benefit research into the development of new cheminformatics models, as can serve to guide and cross-check methodologies, lending increased predicted power to future molecular simulations for investigating the role of structures and phase transitions from nanoscale clusters to macroscopic crystalline structures.

Nanopumping of water via rotation of graphene nanoribbons

In this paper, we perform molecular dynamics simulations to propose a novel bio-inspired nanopumping mechanism that is achieved through the rotation of graphene nanoribbons. Due to the rotation and interaction with water, the graphene nanoribbons undergo morphological transformation. It is shown that with appropriate geometrical and spatial parameters, the resulting morphology is twisted ribbon, which is efficient in pumping of water through a channel. This mimics the propulsive behavior of bacterial flagella through continual rotation at the base and causing morphology of the geometry into twisted ribbons, thus driving flow. It was observed that the maximum flux rate decreases upon reaching the optimal configuration even with increasing rotational speed and graphene width. This is due to the development of cavitation near the region of the nanoribbon with tip velocities approaching the speed of sound in water. The simulation shows promising results where the flux rate of the driven flow outperforms various nanopump configurations that have been reported in recent literature by more than one order.

Mechanical Response of Nanocrystalline Ice-Contained Methane Hydrates: Key Role of Water Ice

Water ice and gas hydrates can coexist in the permafrost and polar regions on Earth and in the universe. However, the role of ice in the mechanical response of ice-contained methane hydrates is still unclear. Here, we conduct direct million-atom molecular simulations of ice-contained polycrystalline methane hydrates and identify a crossover in the tensile strength and average compressive flow stress due to the presence of ice. The average mechanical shear strengths of hydrate-hydrate bicrystals are about three times as large as those of hydrate-ice bicrystals. The ice content, especially below 70%, shows a significant effect on the mechanical strengths of the polycrystals, which is mainly governed by the proportions of the hydrate-hydrate grain boundaries (HHGBs), the hydrate-ice grain boundaries (HIGBs), and the ice-ice grain boundaries (IIGBs). Quantitative analysis of the microstructure of the water cages in the polycrystals reveals the dissociation and reformation of various water cages due to mechanical deformation. These findings provide molecular insights into the mechanical behavior and microscopic deformation mechanisms of ice-contained methane hydrate systems on Earth and in the universe.

Phase Transition of Ice at High Pressures and Low Temperatures

The behavior of ice under extreme conditions undergoes the change of intermolecular binding patterns and leads to the structural phase transitions, which are needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H2O-rich exoplanets. Such extreme conditions limit the structural explorations in laboratory but open a door for the theoretical study. The ice phases IX and XIII are located in the high pressure and low temperature region of the phase diagram. However, to the best of our knowledge, the phase transition boundary between these two phases is still not clear. In this work, based on the second-order Møller–Plesset perturbation (MP2) theory, we theoretically investigate the ice phases IX and XIII and predict their structures, vibrational spectra and Gibbs free energies at various extreme conditions, and for the first time confirm that the phase transition from ice IX to XIII can occur around 0.30 GPa and 154 K. The proposed work, taking into account the many-body electrostatic effect and the dispersion interactions from the first principles, opens up the possibility of completing the ice phase diagram and provides an efficient method to explore new phases of molecular crystals.

Compressive behavior and electronic properties of ammonia ice: a first-principles study

We performed systematic ab initio calculations to explore the structures and electronic properties of ammonia ice by hydrostatic compression.

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.

An ultralow-density porous ice with the largest internal cavity identified in the water phase diagram

The recent back-to-back findings of low-density porous ice XVI and XVII have rekindled the century-old field of the solid-state physics and chemistry of water. Experimentally, both ice XVI and XVII crystals can be produced by extracting guest atoms or molecules enclosed in the cavities of preformed ice clathrate hydrates. Herein, we examine more than 200 hypothetical low-density porous ices whose structures were generated according to a database of zeolite structures. Hitherto unreported porous EMT ice, named according to zeolite nomenclature, is identified to have an extremely low density of 0.5 g/cm3 and the largest internal cavity (7.88 Å in average radius). The EMT ice can be viewed as dumbbell-shaped motifs in a hexagonal close-packed structure. Our first-principles computations and molecular dynamics simulations confirm that the EMT ice is stable under negative pressures and exhibits higher thermal stability than other ultralow-density ices. If all cavities are fully occupied by hydrogen molecules, the EMT ice hydrate can easily outperform the record hydrogen storage capacity of 5.3 wt % achieved with sII hydrogen hydrate. Most importantly, in the reconstructed temperature-pressure (T-P) phase diagram of water, the EMT ice is located at deeply negative pressure regions below ice XVI and at higher temperature regions next to FAU. Last, the phonon spectra of empty-sII, FAU, EMT, and other zeolite-like ice structures are computed by using the dispersion corrected vdW-DF2 functional. Compared with those of ice XI (0.93 g/cm3), both the bending and stretching vibrational modes of the EMT ice are blue-shifted due to their weaker hydrogen bonds.

Room temperature electrofreezing of water yields a missing dense ice phase in the phase diagram

Water can freeze into diverse ice polymorphs depending on the external conditions such as temperature (T) and pressure (P). Herein, molecular dynamics simulations show evidence of a high-density orthorhombic phase, termed ice χ, forming spontaneously from liquid water at room temperature under high-pressure and high external electric field. Using free-energy computations based on the Einstein molecule approach, we show that ice χ is an additional phase introduced to the state-of-the-art T–P phase diagram. The χ phase is the most stable structure in the high-pressure/low-temperature region, located between ice II and ice VI, and next to ice V exhibiting two triple points at 6.06 kbar/131.23 K and 9.45 kbar/144.24 K, respectively. A possible explanation for the missing ice phase in the T–P phase diagram is that ice χ is a rare polarized ferroelectric phase, whose nucleation/growth occurs only under very high electric fields.

Water can crystallize in different ice polymorphs according to temperature and pressure conditions. Here the authors predict by molecular dynamics simulations a new ice phase spontaneously forming at room temperature under high pressure and high electric field.

Advances in the experimental exploration of water's phase diagram

Water's phase diagram displays enormous complexity with currently 17 experimentally confirmed polymorphs of ice and several more predicted computationally. For almost 120 years, it has been a stomping ground for scientific discovery, and ice research has often been a trailblazer for investigations into a wide range of materials-related phenomena. Here, the experimental progress of the last couple of years is reviewed, and open questions as well as future challenges are discussed. The specific topics include (i) the polytypism and stacking disorder of ice I, (ii) the mechanism of the pressure amorphization of ice I, (iii) the emptying of gas-filled clathrate hydrates to give new low-density ice polymorphs, (iv) the effects of acid/base doping on hydrogen-ordering phase transitions as well as (v) the formation of solid solutions between salts and the ice polymorphs, and the effect this has on the appearance of the phase diagram. In addition to continuing efforts to push the boundaries in terms of the extremes of pressure and temperature, the exploration of the "chemical" dimensions of ice research appears to now be a newly emerging trend. It is without question that ice research has entered a very exciting era.

Phase Diagram of Methane Hydrates and Discovery of MH-VI Hydrate

Methane hydrate is not only the predominant natural deposits of permafrost and continental margins of Earth but also the dominant methane-containing phase in the nebula and major moons of gas giants. Depending on the surrounding environment (mainly pressure), seven methane hydrate phases have been discovered by experiment or predicted by computer simulation, such as clathrate methane hydrates I, II, H, and K, and filled-ice methane hydrates III, IV, and V. Using extensive Monte Carlo packing algorithm and density functional theory optimization, here we predict a partial clathrate methane hydrate VI built by basic units of 4²6² water bowl encapsulating a methane molecule, which is dynamically stable from the computed phonon dispersion. Its density and structural characteristics are comparable to that of filled-ice methane hydrate III. By calculating the formation enthalpies of a variety of candidate phases at different pressures, a phase diagram of methane hydrates is constructed. As pressure rises, phase transitions occur  among the methane hydrates along with decreasing water/methane molecular ratios. The newly predicted methane hydrate VI emerges as the most stable phase in the region between clathrate phase II and filled-ice phase III, suggesting that methane hydrate VI might be synthesized in a laboratory under accessible conditions.

Mapping uncharted territory in ice from zeolite networks to ice structures

Ice is one of the most extensively studied condensed matter systems. Yet, both experimentally and theoretically several new phases have been discovered over the last years. Here we report a large-scale density-functional-theory study of the configuration space of water ice. We geometry optimise 74,963 ice structures, which are selected and constructed from over five million tetrahedral networks listed in the databases of Treacy, Deem, and the International Zeolite Association. All prior knowledge of ice is set aside and we introduce "generalised convex hulls" to identify configurations stabilised by appropriate thermodynamic constraints. We thereby rediscover all known phases (I-XVII, i, 0 and the quartz phase) except the metastable ice IV. Crucially, we also find promising candidates for ices XVIII through LI. Using the "sketch-map" dimensionality-reduction algorithm we construct an a priori, navigable map of configuration space, which reproduces similarity relations between structures and highlights the novel candidates. By relating the known phases to the tractably small, yet structurally diverse set of synthesisable candidate structures, we provide an excellent starting point for identifying formation pathways.

Adsorption of Kinetic Hydrate Inhibitors on Growing Surfaces: A Molecular Dynamics Study

We investigate the mechanism of a typical kinetic hydrate inhibitor (KHI), polyvinylcaprolactam (PVCap), which has been applied to prevent hydrate plugs from forming in gas pipe lines, using molecular dynamics simulations of crystal growth of ethylene oxide hydrate. Water-soluble ethylene oxide is chosen as a guest species to avoid problems associated with the presence of the gas phase in the simulation cell such as slow crystal growth. A PVCap dodecamer adsorbs irreversibly on the hydrate surface which grows at supercooling of 3 K when the hydrophobic part of two pendent groups are trapped in open cages at the surface. The amide hydrogen bonds make no contribution to the adsorption. PVCap can adsorb on various crystallographic planes of sI hydrate. This is in contrast to antifreeze proteins, each of which prefers a specific plane of ice. The trapped PVCap gives rise to necessarily the concave surface of the hydrate. The crystal growth rate decreases with increasing surface curvature, indicating that the inhibition by PVCap is explained by the Gibbs-Thomson effect.

Assessing Intermolecular Interactions in Guest-Free Clathrate Hydrate Systems

Recently, empty hydrate structures sI, sII, sH, and others have been proposed as low-density ice structures by both experimental observations and computer simulations. Some of them have been synthesized in the laboratory, which motivates further investigations on the stability of such guest-free clathrate structures. Using semiempirical and ab initio-based water models, as well as dispersion-corrected density functional theory approaches, we predict their stability, including cooperative many-body effects, in comparison with reference data from converged wave function-based DF-MP2 electronic structure calculations. We show that large basis sets and counterpoise corrections are required to improve convergence in the interaction/binding energies for such systems. Therefore, extrapolation schemes based on triple/quadruple and quadruple/quintuple ζ quality basis sets are used to reach high accuracy. Eleven different water structures corresponding to dodecahedron, edge sharing, face sharing, and fused cubes, as a part of the WATER27 database, as well as cavities from the sI, sII, and sH clathrate hydrates formed by 20, 24, 28, and 36 water molecules, are employed, and new benchmark energies are reported. Using these benchmark sets of interaction energies, we assess the performance of both analytical models and direct DFT calculations for such clathrate-like systems. In particular, seven popular water models (TIP4P/ice, TIP4P/2005, q-TIP4P/F, TTM2-F, TTM3-F, TTM4-F, and MB-pol) available in the literature, and nine density functional approximations (3 meta-GGAs, 3 hybrids, and 3 range separated functionals) are used to investigate their accuracy. By including dispersion corrections, our results show that errors in the interaction energies are reduced for most of the DFT functionals. Despite the difficulties faced by current water models and DFT functionals to accurately describe the interactions in such water systems, we found some general trends that could serve to extend their applicability to larger systems.

Clathrate ice sL: a new crystalline phase of ice with ultralow density predicted by first-principles phase diagram computations

A new crystalline ice phase (clathrate ice sL) with ultralow density under negative pressure predicted by the first-principles phase diagram computations.

Endowing Hydrochromism to Fluorans via Bioinspired Alteration of Molecular Structures and Microenvironments and Expanding Their Potential for Rewritable Paper

Interest and effort toward new materials for rewritable paper have increased dramatically because of the exciting advantages for sustainable development and better nature life cycle. Inspired by how nature works within living systems, herein, we have used fluorans, as a concept verification, to endow original acidochromic, basochromic or photochromic molecules with broader properties, such as switchable with solvent, water, heat, electricity, stress, other force, etc., via simplified methods (i.e., via variation of submolecular structure or microenvironments). The hydrochromic visual change and reversible behavior of selected molecules have been explored, and the primary mechanism at the atomic or subatomic level has been hypothesized. In addition, several newly demonstrated hydrochromic fluorans have been utilized for water-jet rewritable paper (WJRP), which exhibit great photostability, high hydrochromic contrast, and fast responsive rate and which can be reused at least 30 times without significant variation. The water-jet prints have good resolution and various colors and can keep legibility after a few months or years. This improved performance is a major step toward practical applications of WJRP.

Dynamics and unsteady morphologies at ice interfaces driven by D2O-H2O exchange

The growth dynamics of D2O ice in liquid H2O in a microfluidic device were investigated between the melting points of D2O ice (3.8 °C) and H2O ice (0 °C). As the temperature was decreased at rates between 0.002 °C/s and 0.1 °C/s, the ice front advanced but retreated immediately upon cessation of cooling, regardless of the temperature. This is a consequence of the competition between diffusion of H2O into the D2O ice, which favors melting of the interface, and the driving force for growth supplied by cooling. Raman microscopy tracked H/D exchange across the solid H2O-solid D2O interface, with diffusion coefficients consistent with transport of intact H2O molecules at the D2O ice interface. At fixed temperatures below 3 °C, the D2O ice front melted continuously, but at temperatures near 0 °C a scalloped interface morphology appeared with convex and concave sections that cycled between growth and retreat. This behavior, not observed for D2O ice in contact with D2O liquid or H2O ice in contact with H2O liquid, reflects a complex set of cooperative phenomena, including H/D exchange across the solid-liquid interface, latent heat exchange, local thermal gradients, and the Gibbs-Thomson effect on the melting points of the convex and concave features.

Phase Diagram of TIP4P/2005 Water at High Pressure

We report a new ice phase that forms spontaneously at the interface between ice VII and liquid water in molecular dynamics simulations of TIP4P/2005 water. The new phase is structurally quite similar to an ice phase originally found to be a precursor in the course of the homogeneous nucleation of ice VII in liquid water. A part of the water molecules in these ice phases can rotate easily because the number of hydrogen bonds in them is less than four, and thus they can be regarded as partial plastic phases. A rough estimate suggests that these phases are thermodynamically more stable than either ice VI or ice VII for 3 GPa < P < 18 GPa at T = 300 K. Although the partial plastic phases would be metastable states at any point in the phase diagram of real water, they might be realized experimentally with the aid of dopants and/or solid surfaces.