A potential model for the study of ices and amorphous water: TIP4P/Ice

Investigating Shear Stress of Ice Accumulated on Surfaces with Various Roughnesses: Effects of a Quasi-Water Layer

The investigation of the anti-icing/deicing is essential because the icing phenomenon deteriorates the natural environment and various projects. By conducting molecular dynamics simulation, this work analyzes the effect of the quasi-water layer on the ice shear stress over smooth and rough surfaces, along with the underlying physics of the quasi-water layer. The results indicate that the thickness of the quasi-water layer monotonically increases with temperature, resulting in a monotonic decrease in the ice shear stress on the smooth surface. Due to the joint effects of the smooth surface wettability and the quasi-water layer, the ice shear stress increases and then decreases to almost a constant value when the surface changes from a hydrophobic to a hydrophilic one. For rough surfaces with stripe nanostructures, when the width of the bump for one case equals the depression for the other case, the variations of shear stress with height for these two cases are almost the same. The rough surface is effective in reducing the ice shear stress compared to the smooth surface due to the thickening of the quasi-water layer. Each molecule in the quasi-water layer and its four nearest neighboring molecules gradually form a tetrahedral ice-like structure along the direction away from the surface. The radial distribution function also shows that the quasi-water layer resembles the liquid water rather than the ice structure. These findings shed light on developing anti-icing and deicing techniques.

Mode coupling behavior and fragile to strong transition of trehalose in a binary mixture with water upon supercooling

We perform molecular dynamics simulations of a binary mixture of water and trehalose with the TIP4P/Ice water model. We analyze the slow dynamics of trehalose molecules in the mildly supercooled region for concentrations of 3.66 and 18.57 wt. %. We previously studied the dynamics of water in the same mixtures. Supercooled TIP4P/Ice water solvating trehalose molecules was found to follow the Mode Coupling Theory (MCT) and to undergo a transition from a fragile to a strong behavior for both concentrations. Here, we show that also the dynamics of trehalose molecules follows the MCT and displays a fragile to strong crossover (FSC). The results show that trehalose in binary mixtures with water shares with it the dynamical behavior typical of glass forming liquids. Moreover, the FSC for trehalose structural relaxation times is found to occur at temperatures close to those previously obtained for water in the same solutions, showing that the dynamics of the solute is strongly coupled to that of the solvent. We also perform a MCT test showing that the trehalose dynamics obeys the MCT time-temperature superposition principle and that the exponents derived from the theory and the ones obtained from fitting procedure of the relaxation times are comparable, confirming that trehalose molecules in supercooled water solutions follow the MCT of glassy dynamics. Moreover, as predicted by the theory, trehalose particles have MCT parameters comparable to those of water in the same mixtures. This is an important result, given that MCT was originally formulated for monoatomic particles.

Water slowing down drives the occurrence of the low temperature dynamical transition in microgels

The protein dynamical transition marks an increase in atomic mobility and the onset of anharmonic motions at a critical temperature (T d), which is considered relevant for protein functionality. This phenomenon is ubiquitous, regardless of protein composition, structure and biological function and typically occurs at large protein content, to avoid water crystallization. Recently, a dynamical transition has also been reported in non-biological macromolecules, such as poly(N-isopropyl acrylamide) (PNIPAM) microgels, bearing many similarities to proteins. While the generality of this phenomenon is well-established, the role of water in the transition remains a subject of debate. In this study, we use atomistic molecular dynamics (MD) simulations and elastic incoherent neutron scattering (EINS) experiments with selective deuteration to investigate the microscopic origin of the dynamical transition and distinguish water and PNIPAM roles. While a standard analysis of EINS experiments would suggest that the dynamical transition occurs in PNIPAM and water at a similar temperature, simulations reveal a different perspective, also qualitatively supported by experiments. From room temperature down to about 180 K, PNIPAM exhibits only modest changes of dynamics, while water, being mainly hydration water under the probed extreme confinement, significantly slows down and undergoes a mode-coupling transition from diffusive to activated. Our findings therefore challenge the traditional view of the dynamical transition, demonstrating that it occurs in proximity of the water mode-coupling transition, shedding light on the intricate interplay between polymer and water dynamics.

A Molecular Dynamics Study on Xe/Kr Separation Mechanisms Using Crystal Growth Method

The separation of xenon/krypton gas mixtures is a valuable but challenging endeavor in the gas industry due to their similar physical characteristics and closely sized molecules. To address this, we investigated the effectiveness of the hydrate-based gas separation method for mixed Xe-Kr gas via molecular dynamics (MD) simulations. The formation process of hydrates facilitates the encapsulation of guest molecules within hydrate cages, offering a potential strategy for gas separation. Higher temperatures and pressures are advantageous for accelerating the hydrate growth rate. The final occupancy of guest molecules and empty cages within 512, 51264, and all hydrate cages were thoroughly examined. An increase in the pressure and temperature enhanced the occupancy rates of Xe in both 512 and 51264 cages, whereas elevated pressure alone improved the occupancy of Kr in 51264 cages. However, the impact of temperature and pressure on Kr occupancy within 512 cages was found to be minimal. Elevated temperature and pressure resulted in a reduced occupancy of empty cages. Predominantly, 51264 cages were occupied by Xe, whereas Kr showed a propensity to occupy the 512 cages. With increasing simulated pressure, the final occupancy of Xe molecules in all cages rose from 0.37 to 0.41 for simulations at 260 K, while the final occupancy of empty cages decreased from 0.24 to 0.2.

Unravelling the interactions between small molecules and liposomal bilayers via molecular dynamics and thermodynamic modelling

Lipid-based drug delivery systems hold immense promise in addressing critical medical needs, from cancer and neurodegenerative diseases to infectious diseases. By encapsulating active pharmaceutical ingredients - ranging from small molecule drugs to proteins and nucleic acids - these nanocarriers enhance treatment efficacy and safety. However, their commercial success faces hurdles, such as the lack of a systematic design approach and the issues related to scalability and reproducibility. This work aims to provide insights into the drug-phospholipid interaction by combining molecular dynamic simulations and thermodynamic modelling techniques. In particular, we have made a connection between the structural properties of the drug-phospholipid system and the physicochemical performance of the drug-loaded liposomal nanoformulations. We have considered two prototypical drugs, felodipine (FEL) and naproxen (NPX), and one model hydrogenated soy phosphatidylcholine (HSPC) bilayer membrane. Molecular dynamic simulations revealed which regions within the phospholipid bilayers are most and least favoured by the drug molecules. NPX tends to reside at the water-phospholipid interface and is characterized by a lower free energy barrier for bilayer membrane permeation. Meanwhile, FEL prefers to sit within the hydrophobic tails of the phospholipids and is characterized by a higher free energy barrier for membrane permeation. Flory-Huggins thermodynamic modelling, small angle X-ray scattering, dynamic light scattering, TEM, and drug release studies of these liposomal nanoformulations confirmed this drug-phospholipid structural difference. The naproxen-phospholipid system has a lower free energy barrier for permeation, higher drug miscibility with the bilayer, larger liposomal nanoparticle size, and faster drug release in the aqueous medium than felodipine. We suggest that this combination of molecular dynamics and thermodynamics approach may offer a new tool for designing and developing lipid-based nanocarriers for unmet medical applications.

Probing structural superlubricity of two-dimensional water transport with atomic resolution

Low-dimensional water transport can be drastically enhanced under atomic-scale confinement. However, its microscopic origin is still under debate. In this work, we directly imaged the atomic structure and transport of two-dimensional water islands on graphene and hexagonal boron nitride surfaces using qPlus-based atomic force microscopy. The lattice of the water island was incommensurate with the graphene surface but commensurate with the boron nitride surface owing to different surface electrostatics. The area-normalized static friction on the graphene diminished as the island area was increased by a power of ~-0.58, suggesting superlubricity behavior. By contrast, the friction on the boron nitride appeared insensitive to the area. Molecular dynamic simulations further showed that the friction coefficient of the water islands on the graphene could reduce to <0.01.

On the engulfment of antifreeze proteins by ice

Antifreeze proteins (AFPs) are remarkable biomolecules that suppress ice formation at trace concentrations. To inhibit ice growth, AFPs must not only bind to ice crystals, but also resist engulfment by ice. The highest supercooling, [Formula: see text], for which AFPs are able to resist engulfment is widely believed to scale as the inverse of the separation, [Formula: see text], between bound AFPs, whereas its dependence on the molecular characteristics of the AFP remains poorly understood. By using specialized molecular simulations and interfacial thermodynamics, here, we show that in contrast with conventional wisdom, [Formula: see text] scales as [Formula: see text] and not as [Formula: see text]. We further show that [Formula: see text] is proportional to AFP size and that diverse naturally occurring AFPs are optimal at resisting engulfment by ice. By facilitating the development of AFP structure-function relationships, we hope that our findings will pave the way for the rational design of AFPs.

Imaging surface structure and premelting of ice Ih with atomic resolution

Ice surfaces are closely relevant to many physical and chemical properties, such as melting, freezing, friction, gas uptake and atmospheric reaction1-8. Despite extensive experimental and theoretical investigations9-17, the exact atomic structures of ice interfaces remain elusive owing to the vulnerable hydrogen-bonding network and the complicated premelting process. Here we realize atomic-resolution imaging of the basal (0001) surface structure of hexagonal water ice (ice Ih) by using qPlus-based cryogenic atomic force microscopy with a carbon monoxide-functionalized tip. We find that the crystalline ice-Ih surface consists of mixed Ih- and cubic (Ic)-stacking nanodomains, forming 19 × 19 periodic superstructures. Density functional theory reveals that this reconstructed surface is stabilized over the ideal ice surface mainly by minimizing the electrostatic repulsion between dangling OH bonds. Moreover, we observe that the ice surface gradually becomes disordered with increasing temperature (above 120 Kelvin), indicating the onset of the premelting process. The surface premelting occurs from the defective boundaries between the Ih and Ic domains and can be promoted by the formation of a planar local structure. These results put an end to the longstanding debate on ice surface structures and shed light on the molecular origin of ice premelting, which may lead to a paradigm shift in the understanding of ice physics and chemistry.

In-layer inhomogeneity of molecular dynamics in quasi-liquid layers of ice

Quasi-liquid layers (QLLs) are present on the surface of ice and play a significant role in its distinctive chemical and physical properties. These layers exhibit considerable heterogeneity across different scales ranging from nanometers to millimeters. Although the formation of partially ice-like structures has been proposed, the molecular-level understanding of this heterogeneity remains unclear. Here, we examined the heterogeneity of molecular dynamics on QLLs based on molecular dynamics simulations and machine learning analysis of the simulation data. We demonstrated that the molecular dynamics of QLLs do not comprise a mixture of solid- and liquid water molecules. Rather, molecules having similar behaviors form dynamical domains that are associated with the dynamical heterogeneity of supercooled water. Nonetheless, molecules in the domains frequently switch their dynamical state. Furthermore, while there is no observable characteristic domain size, the long-range ordering strongly depends on the temperature and crystal face. Instead of a mixture of static solid- and liquid-like regions, our results indicate the presence of heterogeneous molecular dynamics in QLLs, which offers molecular-level insights into the surface properties of ice.

Computer Simulation of Three-Phase Equilibria for Some Water/n-Alkane Binary Systems

In this work, the vapor-liquid-liquid equilibrium (VLLE) of the water/n-pentane, water/n-hexane, water/n-octane, and water/n-decane binary systems is calculated by computer simulation using the NVT-Gibbs ensemble (in the version of three simulation boxes) combined with the configurational bias Monte Carlo method. The combination of both methods, the molecular potential models used, and the simulation details allowed us to calculate the triphasic equilibrium properties of the systems studied: the densities of the three phases in equilibrium, their compositions, and potential energies. In previous works, these simulations were not carried out at a temperature range nor water/n-alkanes systems simulated in this work, probably because they are highly nonideal systems; so, to the best of our knowledge, this is the first time that this phenomenon is studied in detail. The results from VLLE simulations of the water/n-pentane system for temperatures from 343.2 to 435 K, the water/n-hexane system for temperatures from 373.11 to 473.15 K, the water/n-octane system for temperatures from 310.9 to 500 K, and for the water/n-decane system for temperatures from 374.15 to 525 K are reported here. The temperature range was selected in concordance with the experimental data available for an adequate study of the VLLE simulation results. The subcritical densities (vapor and liquid rich in n-alkane phases) at various temperatures fit well with the scaling law and the law of rectilinear diameters, allowing the estimation of upper critical end point temperature and density of the VLLE. The simulation results show a good prediction with experimental data reports in the literature.

Mesomorphology of clathrate hydrates from molecular ordering

Clathrate hydrates are crystals formed by guest molecules that stabilize cages of hydrogen-bonded water molecules. Whereas thermodynamic equilibrium is well described via the van der Waals and Platteeuw approach, the increasing concerns with global warming and energy transition require extending the knowledge to non-equilibrium conditions in multiphase, sheared systems, in a multiscale framework. Potential macro-applications concern the storage of carbon dioxide in the form of clathrates, and the reduction of hydrate inhibition additives currently required in hydrocarbon production. We evidence porous mesomorphologies as key to bridging the molecular scales to macro-applications of low solubility guests. We discuss the coupling of molecular ordering with the mesoscales, including (i) the emergence of porous patterns as a combined factor from the walk over the free energy landscape and 3D competitive nucleation and growth and (ii) the role of molecular attachment rates in crystallization-diffusion models that allow predicting the timescale of pore sealing. This is a perspective study that discusses the use of discrete models (molecular dynamics) to build continuum models (phase field models, crystallization laws, and transport phenomena) to predict multiscale manifestations at a feasible computational cost. Several advances in correlated fields (ice, polymers, alloys, and nanoparticles) are discussed in the scenario of clathrate hydrates, as well as the challenges and necessary developments to push the field forward.

Molecular Insights into the Dual Promotion-Inhibition Effects of NaCl at Various Concentrations on the CO2 Hydrate Growth: A Molecular Simulation Study

Hydrate-based CO2 storage in the ocean is considered a potential method for mitigating the greenhouse effect. Numerous studies demonstrated that NaCl exhibited the dual effects of promotion and inhibition in the nucleation and growth processes of CO2 hydrate, whose mechanisms remain unclear. In this study, the effects of NaCl at various concentrations on the CO2 hydrate growth and crystal are investigated. The independent gradient model based on Hirshfeld partition, electrostatic potential, and binding energy is conducted to study the interaction between ions and water molecules. The motion trajectories of ions are observed at the molecular level to reflect the impact of ion motion on hydrate growth. The results show that the influence of NaCl on hydrate growth depends on a delicate balance of dual promotion-inhibition effects. NaCl can combine more water molecules and provide a transport channel of CO2 to promote hydrate growth at low concentrations. Meanwhile, the promoting effects shift toward inhibition with increasing NaCl concentrations. In a word, this paper proposes a novel mechanism for the dual promotion-inhibition effects of NaCl on hydrate growth, which is significant for further research on hydrate-based CO2 storage in the ocean.

Three-phase equilibria of hydrates from computer simulation. II. Finite-size effects in the carbon dioxide hydrate

In this work, the effects of finite size on the determination of the three-phase coexistence temperature (T3) of the carbon dioxide (CO2) hydrate have been studied by molecular dynamic simulations and using the direct coexistence technique. According to this technique, the three phases involved (hydrate-aqueous solution-liquid CO2) are placed together in the same simulation box. By varying the number of molecules of each phase, it is possible to analyze the effect of simulation size and stoichiometry on the T3 determination. In this work, we have determined the T3 value at 8 different pressures (from 100 to 6000 bar) and using 6 different simulation boxes with different numbers of molecules and sizes. In two of these configurations, the ratio of the number of water and CO2 molecules in the aqueous solution and the liquid CO2 phase is the same as in the hydrate (stoichiometric configuration). In both stoichiometric configurations, the formation of a liquid drop of CO2 in the aqueous phase is observed. This drop, which has a cylindrical geometry, increases the amount of CO2 available in the aqueous solution and can in some cases lead to the crystallization of the hydrate at temperatures above T3, overestimating the T3 value obtained from direct coexistence simulations. The simulation results obtained for the CO2 hydrate confirm the sensitivity of T3 depending on the size and composition of the system, explaining the discrepancies observed in the original work by Míguez et al. [J. Chem Phys. 142, 124505 (2015)]. Non-stoichiometric configurations with larger unit cells show a convergence of T3 values, suggesting that finite-size effects for these system sizes, regardless of drop formation, can be safely neglected. The results obtained in this work highlight that the choice of a correct initial configuration is essential to accurately estimate the three-phase coexistence temperature of hydrates by direct coexistence simulations.

Three-phase equilibria of hydrates from computer simulation. I. Finite-size effects in the methane hydrate

Clathrate hydrates are vital in energy research and environmental applications. Understanding their stability is crucial for harnessing their potential. In this work, we employ direct coexistence simulations to study finite-size effects in the determination of the three-phase equilibrium temperature (T3) for methane hydrates. Two popular water models, TIP4P/Ice and TIP4P/2005, are employed, exploring various system sizes by varying the number of molecules in the hydrate, liquid, and gas phases. The results reveal that finite-size effects play a crucial role in determining T3. The study includes nine configurations with varying system sizes, demonstrating that smaller systems, particularly those leading to stoichiometric conditions and bubble formation, may yield inaccurate T3 values. The emergence of methane bubbles within the liquid phase, observed in smaller configurations, significantly influences the behavior of the system and can lead to erroneous temperature estimations. Our findings reveal finite-size effects on the calculation of T3 by direct coexistence simulations and clarify the system size convergence for both models, shedding light on discrepancies found in the literature. The results contribute to a deeper understanding of the phase equilibrium of gas hydrates and offer valuable information for future research in this field.

Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates

In this work, the effect of the range of dispersive interactions in determining the three-phase coexistence line of the CO2 and CH4 hydrates has been studied. In particular, the temperature (T3) at which solid hydrate, water, and liquid CO2/gas CH4 coexist has been determined through molecular dynamics simulations using different cutoff values (from 0.9 to 1.6 nm) for dispersive interactions. The T3 of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T > T3. Conversely, if the hydrate grows, then T < T3. The effect of the cutoff distance on the dissociation temperature has been analyzed at three different pressures for CO2 hydrate: 100, 400, and 1000 bar. Then, we have changed the guest and studied the effect of the cutoff distance on the dissociation temperature of the CH4 hydrate at 400 bar. Moreover, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cutoff value of 0.9 nm. The results obtained in this work highlight that the cutoff distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T3 about 2-4 K depending on the system and the pressure.

Physical breakdown of CH4 hydrate under stress: a molecular dynamics simulation study

As a solid energy source, CH4 hydrate will inevitably break down physically as the result of geological movement or exploitation. Here, the molecular dynamics method was employed to simulate the uniaxial-deformation behavior of structure I (sI type) CH4 hydrate under stress. The stress increases regardless of whether the hydrate is stretched or squeezed, and other physical parameters also changed, such as hydrate cage numbers, order parameters, and the number of water molecules. A noticeable difference is observed between the two systems. Upon stretching, the stress immediately recovers to 0 GPa once the hydrate is completely stretched apart. During the squeeze process, the stress is ultimately not zero since solid and liquid are always in contact. When the hydrate is stretched apart, about 5% of water molecules change from solid to liquid, about 7.8% of CH4 molecules lose their shelter and become free due to the disintegration of water cages. While in the squeezing process, large cages (51262) are crushed more easily than small cages (512); in the end, about 93.5% of large cages and 73% of small cages are crushed, and approximately 87.5% CH4 is released from the cages. In mining CH4 hydrates, caution must be exercised, as if the hydrates break as a result of stress, a large release of CH4 may pose a security risk.

Modeling oceanic sedimentary methane hydrate growth through molecular dynamics simulation

The crystallization process of methane hydrates in a confined geometry resembling seabed porous silica sedimentary conditions has been studied using molecular dynamics simulations. With this objective in mind, a fully atomistic quartz silica slit pore has been designed, and the temperature stability of a methane hydrate crystalline seed in the presence of water and guest molecule methane has been analyzed. NaCl ion pairs have been added in different concentrations, simulating salinity conditions up to values higher than average oceanic conditions. The structure obtained when the hydrate crystallizes inside the pore is discussed, paying special attention to the presence of ionic doping inside the hydrate and the subsequent induced structural distortion. The shift in the hydrate stability conditions due to the increasing water salinity is discussed and compared with the case of unconfined hydrate, concluding that the influence of the confinement geometry and pore hydrophilicity produces a larger deviation in the confined hydrate phase equilibria.

Water Potential from Adaptive Force Matching for Ice and Liquid with Revised Dispersion Predicts Supercooled Liquid Anomalies in Good Agreement with Two Independent Experimental Fits

A revised version of the Water potential from Adaptive force matching for Ice and Liquid (WAIL) was developed by using the previous data set for fitting the WAIL model but with a dispersion term calculated using symmetry adapted perturbation theory (SAPT). The model has no adjustable parameters and relies solely on fitting first-principles information. The new model, named revised WAIL (rWAIL), shows improved predictions of most properties of water when compared to the previously published WAIL model. The rWAIL model also compares favorably to other first-principles-derived water models, such as MB-Pol, at only a fraction of the computational cost. The rWAIL model is used to study the properties of supercooled water. The model shows evidence of a liquid-liquid phase transition (LLPT) in the supercooled regimes with the liquid-liquid critical point (LLCP) at 203 K and 90 MPa. This estimate is in good agreement with a recent polynomial fit to the experimental density of water. Also, the fit to the surface tension of supercooled water based on the rWAIL model shows excellent agreement with the corresponding fit to the experimental data. Consistent with previously published molecular dynamics and experimental data, the surface tension of water exhibits exponential growth in the supercooled regime, which is likely a result of the emergence of a low-density liquid form of water. The simulation thus unites two separate experimental fits with one first-principles-based model, lending strong evidence of an LLPT in real water.

Comparing machine learning potentials for water: Kernel-based regression and Behler-Parrinello neural networks

In this paper, we investigate the performance of different machine learning potentials (MLPs) in predicting key thermodynamic properties of water using RPBE + D3. Specifically, we scrutinize kernel-based regression and high-dimensional neural networks trained on a highly accurate dataset consisting of about 1500 structures, as well as a smaller dataset, about half the size, obtained using only on-the-fly learning. This study reveals that despite minor differences between the MLPs, their agreement on observables such as the diffusion constant and pair-correlation functions is excellent, especially for the large training dataset. Variations in the predicted density isobars, albeit somewhat larger, are also acceptable, particularly given the errors inherent to approximate density functional theory. Overall, this study emphasizes the relevance of the database over the fitting method. Finally, this study underscores the limitations of root mean square errors and the need for comprehensive testing, advocating the use of multiple MLPs for enhanced certainty, particularly when simulating complex thermodynamic properties that may not be fully captured by simpler tests.

Mechanical properties of amorphous CO2 hydrates: insights from molecular simulations

Understanding physicochemical properties of amorphous gas hydrate systems is of great significance to reveal structural stabilities of polycrystalline gas hydrate systems. Furthermore, amorphous gas hydrates can occur ordinarily in the nucleation events of gas hydrate systems. Herein, the mechanical properties of amorphous carbon dioxide hydrates are examined by means of all-atom classical molecular dynamic simulations. Our molecular simulation results reveal that mechanical strengths of amorphous carbon dioxide hydrates are evidently governed by temperatures, confining pressures, and ratios of water to carbon dioxide molecules. Notably, under compressive loads, amorphous carbon dioxide hydrates firstly exhibit monotonic strain hardening, followed by an interesting distinct phenomenon characterized by a steady flow stress at further large deformation strains. Furthermore, structural evolutions of amorphous carbon dioxide hydrates are analyzed on the basis of the N-Hbond DOP order parameter. These important findings can not only contribute to our understanding of the structural stabilities of amorphous gas hydrate systems, but also help to develop fundamental understandings about grain boundaries of gas hydrate systems.

A robust thin-film droplet-induced electricity generator

The pursuit of cost-effective, high-voltage electricity generators activated by droplets represents a new frontier in hydropower technology. This study presents an economical method for crafting droplet generators using common materials such as solid polytetrafluoroethylene (PTFE) films and readily available tapes, eliminating the need for specialized cleanroom facilities. A thorough investigation into voltage-limiting factors, encompassing device capacitance and induced electrode charges, reveals specific areas with potential for optimization. A substantial enhancement in the open-circuit voltage (Voc) was achieved, reaching approximately 282.2 ± 27.9 V-an impressive increase of around 60 V compared to earlier benchmarks. One device showcased its capability to power 100 LEDs concurrently, underscoring its efficacy. Ten such devices created diverse luminous patterns with uniform light intensity for each LED, showcasing the practical potential of the approach. The methodology's cost-effectiveness results in a remarkable cost reduction compared to solution-based materials, paving the way for the widespread adoption of large-scale water droplet energy harvesting.

Liquid-Vapor Coexistence and Spontaneous Evaporation at Atmospheric Pressure of Common Rigid Three-Point Water Models in Molecular Simulations

Molecular dynamics (MD) simulations are widely used to investigate molecular systems at atomic resolution including biomolecular structures, drug-receptor interactions, and novel materials. Frequently, MD simulations are performed in an aqueous solution with explicit models of water molecules. Commonly, such models are parameterized to reproduce the liquid phase of water under ambient conditions. However, often, simulations at significantly higher temperatures are also of interest. Hence, it is important to investigate the equilibrium of the liquid and vapor phases of molecular models of water at elevated temperatures. Here, we evaluate the behavior of 11 common rigid three-point water models over a wide range of temperatures. From liquid-vapor coexistence simulations, we estimated the critical points and studied the spontaneous evaporation of these water models. Moreover, we investigated the influence of the system size, choice of the pressure-coupling algorithm, and rate of heating on the process and compared them with the experimental data. We found that modern rigid three-point water models reproduce the critical point surprisingly well. Furthermore, we discovered that the critical temperature correlates with the quadrupole moment of the respective water model. This indicates that the spatial arrangement of the partial charges is important for reproducing the liquid-vapor phase transition. Our findings may guide the selection of water models for simulations conducted at high temperatures.

Long-range, water-mediated interaction between a moderately active antifreeze protein molecule and the surface of ice

Using molecular dynamics simulations, we show that a molecule of moderately active antifreeze protein (type III AFP, QAE HPLC-12 isoform) is able to interact with ice in an indirect manner. This interaction occurs between the ice binding site (IBS) of the AFP III molecule and the surface of ice, and it is mediated by liquid water, which separates these surfaces. As a result, the AFP III molecule positions itself at a specific orientation and distance relative to the surface of ice, which enables the effective binding (via hydrogen bonds) of the molecule with the nascent ice surface. Our results show that the final adsorption of the AFP III molecule on the surface of ice is not achieved by chaotic diffusion movements, but it is preceded by a remote, water-mediated interaction between the IBS and the surface of ice. The key factor that determines the existence of this interaction is the ability of water molecules to spontaneously form large, high-volume aggregates that can be anchored to both the IBS of the AFP molecule and the surface of ice. The results presented in this work for AFP III are in full agreement with the ones obtained by us previously for hyperactive CfAFP, which indicates that the mechanism of the remote interaction of these molecules with ice remains unchanged despite significant differences in the molecular structure of their ice binding sites. For that reason, we can expect that also other types of AFPs interact with the ice surface according to an analogous mechanism.

Identification of local structures in water from supercooled to ambient conditions

Studies of water thermodynamics have long been tied to the identification of two distinct families of local structures, whose competition could explain the origin of the many thermodynamic anomalies and the hypothesized liquid-liquid critical point in water. Despite the many successes and insights gained, the structural indicators proposed throughout the years were not able to unequivocally identify these two families over a wide range of conditions. We show that a recently introduced indicator, Ψ, which exploits information on the hydrogen bond network connectivity, can reliably identify these two distinct local environments over a wide range of thermodynamic conditions (188-300 K and 0-13 kbar) and that close to the liquid-liquid critical point, the spatial correlations of density fluctuations are identical to those of the Ψ indicator. Our results strongly support the idea that water thermodynamic properties arise from the competition between two distinct and identifiable local environments.

The molecular scale mechanism of deposition ice nucleation on silver iodide

Heterogeneous ice nucleation is a ubiquitous process in the natural and built environment. Deposition ice nucleation, i.e. heterogeneous ice nucleation that - according to the traditional view - occurs in a subsaturated water vapor environment and in the absence of supercooled water on the solid, ice-forming surface, is among the most important ice formation processes in high-altitude cirrus and mixed-phase clouds. Despite its importance, very little is known about the mechanism of deposition ice nucleation at the microscopic level. This study puts forward an adsorption-based mechanism for deposition ice nucleation through results from a combination of atomistic simulations, experiments and theoretical modelling. One of the most potent laboratory surrogates of ice nucleating particles, silver iodide, is used as a substrate for the simulations. We find that water initially adsorbs in clusters which merge and grow over time to form layers of supercooled water. Ice nucleation on silver iodide requires at minimum the adsorption of 4 molecular layers of water. Guided by the simulations we propose the following fundamental freezing steps: (1) Water molecules adsorb on the surface, forming nanodroplets. (2) The supercooled water nanodroplets merge into a continuous multilayer when they grow to about 3 molecular layers thick. (3) The layer continues to grow until the critical thickness for freezing is reached. (4) The critical ice cluster continues to grow.

Ice Recrystallization Unveils the Binding Mechanism Operating at a Diffused Interface

Recrystallization of ice is a natural phenomenon that causes adverse effects in cryopreservation, agriculture, and in frozen food industry. It has long been recognized that ice recrystallization occurs through the Ostwald ripening and accretion processes. However, neither of these processes has been explored in microscopic detail by state-of-the-art experimental techniques. We carried out atomistic molecular dynamics (MD) simulations to explore ice recrystallization through the accretion process. Attempts have been made to elucidate the binding mechanism that is operating at the diffused ice-water interface. It is demonstrated that two ice crystals spontaneously recognize each other and bind together to form a large crystal in liquid water, resulting in ice recrystallization by accretion. Interestingly, the study reveals that the binding occurs due to the freezing of the interfacial water layer present between the two ice planes, even at a temperature above the melting point of the ice crystal. The synergistically enhanced ordering effect of two ice surfaces on the interfacial water leads to such freezing occurring during the binding process. However, proper crystallographic alignment is not necessarily required for the binding of the two crystals. Simulations have also been carried out to study the binding between an ice crystal and the model ice-binding surface (IBS) of an antifreeze protein above the melting point of the ice crystal. It is found that such binding at the IBS is accompanied by freezing of the interfacial water. This establishes that the synergetic ordering-driven freezing of interfacial water is a common binding mechanism at the diffused surfaces of ice crystals. We believe that this mechanism will provide a microscopic understanding of the process of recrystallization inhibition and thus help in designing suitable materials for potent applications in recrystallization inhibition.

Confinement enhanced viscosity vs shear thinning in lubricated ice friction

The ice surface is known for presenting a very small kinetic friction coefficient, but the origin of this property remains highly controversial to date. In this work, we revisit recent computer simulations of ice sliding on atomically smooth substrates, using newly calculated bulk viscosities for the TIP4P/ice water model. The results show that spontaneously formed premelting films in static conditions exhibit an effective viscosity that is about twice the bulk viscosity. However, upon approaching sliding speeds in the order of m/s, the shear rate becomes very large, and the viscosities decrease by several orders of magnitude. This shows that premelting films can act as an efficient lubrication layer despite their small thickness and illustrates an interesting interplay between confinement enhanced viscosities and shear thinning. Our results suggest that the strongly thinned viscosities that operate under the high speed skating regime could largely reduce the amount of frictional heating.

Understanding the impact of ammonium ion substitutions on heterogeneous ice nucleation

Understanding the mechanisms underpinning heterogeneous ice nucleation in the presence of ionic inclusions is important for fields such as cryopreservation and for improved models of climate and weather prediction. Feldspar and ammonium are both present in significant quantities in the atmosphere, and experimental evidence has shown that feldspar can nucleate ice from ammonium-containing solutions at temperatures warmer than water alone. In recent work, Whale hypothesised that this increase in nucleation temperature is due to an increase in configurational entropy when an ammonium ion is included in the ice hydrogen bond network (T. F. Whale, J. Chem. Phys., 2022, 156, 144503). In this work, we investigate the impact of the inclusion of an ammonium ion on the hydrogen bond network by direct enumeration of the number of structures found using Rick's algorithm. We also determine the energy of these systems and thus compare the effects of enthalpy and entropy to test Whale's hypothesis. We find that the inclusion of an ammonium ion increases the total number of configurations under conditions consistent with a realistic surface charge. We also find that the enthalpic contribution is dominant in determining the location of the ammonium ion within the structure, although we note that this neglects other practicalities of ice nucleation.

The limit of macroscopic homogeneous ice nucleation at the nanoscale

Nucleation in small volumes of water has garnered renewed interest due to the relevance of pore condensation and freezing under conditions of low partial pressures of water, such as in the upper troposphere. Molecular simulations can in principle provide insight on this process at the molecular scale that is challenging to achieve experimentally. However, there are discrepancies in the literature as to whether the rate in confined systems is enhanced or suppressed relative to bulk water at the same temperature and pressure. In this study, we investigate the extent to which the size of the critical nucleus and the rate at which it grows in thin films of water are affected by the thickness of the film. Our results suggest that nucleation remains bulk-like in films that are barely large enough accommodate a critical nucleus. This conclusion seems robust to the presence of physical confining boundaries. We also discuss the difficulties in unambiguously determining homogeneous nucleation rates in nanoscale systems, owing to the challenges in defining the volume. Our results suggest any impact on a film's thickness on the rate is largely inconsequential for present day experiments.

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.

Observing growth and interfacial dynamics of nanocrystalline ice in thin amorphous ice films

Ice crystals at low temperatures exhibit structural polymorphs including hexagonal ice, cubic ice, or a hetero-crystalline mixture of the two phases. Despite the significant implications of structure-dependent roles of ice, mechanisms behind the growths of each polymorph have been difficult to access quantitatively. Using in-situ cryo-electron microscopy and computational ice-dynamics simulations, we directly observe crystalline ice growth in an amorphous ice film of nanoscale thickness, which exhibits three-dimensional ice nucleation and subsequent two-dimensional ice growth. We reveal that nanoscale ice crystals exhibit polymorph-dependent growth kinetics, while hetero-crystalline ice exhibits anisotropic growth, with accelerated growth occurring at the prismatic planes. Fast-growing facets are associated with low-density interfaces that possess higher surface energy, driving tetrahedral ordering of interfacial H2O molecules and accelerating ice growth. These findings, based on nanoscale observations, improve our understanding on early stages of ice formation and mechanistic roles of the ice interface.

Graph-Neural-Network-Based Unsupervised Learning of the Temporal Similarity of Structural Features Observed in Molecular Dynamics Simulations

Classification of molecular structures is a crucial step in molecular dynamics (MD) simulations to detect various structures and phases within systems. Molecular structures, which are commonly identified using order parameters, were recently identified using machine learning (ML), that is, the ML models acquire structural features using labeled crystals or phases via supervised learning. However, these approaches may not identify unlabeled or unknown structures, such as the imperfect crystal structures observed in nonequilibrium systems and interfaces. In this study, we proposed the use of a novel unsupervised learning framework, denoted temporal self-supervised learning (TSSL), to learn structural features and design their parameters. In TSSL, the ML models learn that the structural similarity is learned via contrastive learning based on minor short-term variations caused by perturbations in MD simulations. This learning framework is applied to a sophisticated architecture of graph neural network models that use bond angle and length data of the neighboring atoms. TSSL successfully classifies water and ice crystals based on high local ordering, and furthermore, it detects imperfect structures typical of interfaces such as the water-ice and ice-vapor interfaces.

Water Electric Field Induced Modulation of the Wetting of Hexagonal Boron Nitride: Insights from Multiscale Modeling of Many-Body Polarization

Understanding the behavior of water contacting two-dimensional materials, such as hexagonal boron nitride (hBN), is important in practical applications, including seawater desalination and energy harvesting. Water, being a polar solvent, can strongly polarize the hBN surface via the electric fields that it generates. However, there is a lack of molecular-level understanding about the role of polarization effects at the hBN/water interface, including its effect on the wetting properties of water. In this study, we develop a theoretical framework that introduces an all-atomistic polarizable force field to accurately model the interactions of water molecules with hBN surfaces. The force field is then utilized to self-consistently describe the water-induced polarization of hBN using the classical Drude oscillator model, including predicting the hBN-water binding energies which are found to be in excellent agreement with diffusion Monte Carlo (DMC) predictions. By carrying out molecular dynamics (MD) simulations, we demonstrate that the polarizable force field yields a water contact angle on multilayered hBN which is in close agreement with the recent experimentally reported values. Conversely, an implicit modeling of the hBN-water polarization energy utilizing a Lennard-Jones (LJ) potential, a commonly utilized approximation in previous MD simulation studies, leads to a considerably lower water contact angle. This difference in the predicted contact angles is attributed to the significant energy-entropy compensation resulting from the incorporation of polarization effects at the hBN-water interface. Our work highlights the importance of self-consistently modeling the hBN-water polarization energy and offers insights into the wetting-related interfacial phenomena of water on polarizable materials.

System Size Dependence of the Diffusion Coefficients in MD Simulations: A Simple Correction Formula for Pure Dense Fluids

A practical correction formula relating the self-diffusion coefficient of dense liquids from molecular dynamics (MD) simulations with periodic boundary conditions to the self-diffusion coefficient in the thermodynamic limit is discussed. This formula applies to pure dense fluids and has a very simple form D = D0(1 - γN-1/3), where D0 is the self-diffusion coefficient in the thermodynamic limit and N is the number of particles in the simulation. The numerical factor γ is dependent on the geometry of the simulation cell. Remarkably, γ ≃ 1.0 is the most popular cubic geometry. The success of this formula is supported by results from MD simulations, including very recent simulations with a "magic" simulation geometry.

Multiscale Molecular Dynamics Simulations of Ice-Binding Proteins

Ice-binding proteins (IBPs) are a diverse class of proteins that are essential for the survival of organisms in cold conditions. IBPs are diverse in their function and can prevent or promote ice growth and selectively bind to specific crystallographic planes of the growing ice lattice. Moreover, IBPs are widely utilized to modulate ice crystal growth and recrystallization in the food industry and as cryoprotectants to preserve biological matter. A key unresolved aspect of the mode of action is how the ice-binding sites of these proteins distinguish between ice and water and interact with multiple crystal facets of the ice. The use of molecular dynamics (MD) simulation allows us to thoroughly investigate the binding mechanism and energetics of ice-binding proteins, to complement and expand on the mechanistic understandings gained from experiments. In this chapter, we describe a series of molecular dynamics simulation methodologies to investigate the mechanism of action of ice-binding proteins. Specifically, we provide detailed instructions to set up MD simulations to study the binding and interaction of ice-binding proteins using atomistic and coarse-grained simulations.

Estimation of the ice melting point in molecular dynamics simulations based on the finite-size effects

Predicting the ice melting point using molecular dynamics (MD) simulations is nontrivial due to uncertainty associated with the stochastic nature of the simulation and effect of finite domain sizes on the simulated ice-water phase transition. We developed a method based on the percolation theory to make use of the finite size effects to allow determination of a unique critical phase transition temperature as the melting point. The method involves construction of melting/freezing probability curves from multiple simulations with varying temperatures for different domain sizes. While the domain sizes affect the apparent melting/freezing probability and hence generate different curves with a wider probability distribution for a smaller size, the intersection of these curves is unique and locates the melting point. Based on MD simulations using the Tip4p/Ice water model, we tested and demonstrated the effectiveness of this method in locating the critical ice-water phase transition at a melting temperature of 268.78 K. Our analysis also showed that the apparent melting probability at this critical point is ∼0.69, not 0.5 assumed in the ad hoc method used previously. Our method, making no assumption about the system size, may provide a generic framework for analyzing phase transitions influenced by the finite size effects in MD simulations.

Lessons Learned from Multiobjective Automatic Optimizations of Classical Three-Site Rigid Water Models Using Microscopic and Macroscopic Target Experimental Observables

The development of accurate water models is of primary importance for molecular simulations. Despite their intrinsic approximations, three-site rigid water models are still ubiquitously used to simulate a variety of molecular systems. Automatic optimization approaches have been recently used to iteratively refine three-site water models to fit macroscopic (average) thermodynamic properties, providing state-of-the-art three-site models that still present some deviations from the liquid water properties. Here, we show the results obtained by automatically optimizing three-site rigid water models to fit a combination of microscopic and macroscopic experimental observables. We use Swarm-CG, a multiobjective particle-swarm-optimization algorithm, for training the models to reproduce the experimental radial distribution functions of liquid water at various temperatures (rich in microscopic-level information on, e.g., the local orientation and interactions of the water molecules). We systematically analyze the agreement of these models with experimental observables and the effect of adding macroscopic information to the training set. Our results demonstrate how adding microscopic-rich information in the training of water models allows one to achieve state-of-the-art accuracy in an efficient way. Limitations in the approach and in the approximated description of water in these three-site models are also discussed, providing a demonstrative case useful for the optimization of approximated molecular models, in general.

Competitive Adsorption of Trace Gases on Ice at Tropospheric Temperatures: A Grand Canonical Monte Carlo Simulation Study

The coadsorption of two atmospheric trace gases on ice is characterized by using, for the first time, grand canonical Monte Carlo (GCMC) simulations performed in conditions similar to those of the corresponding experiments. Adsorption isotherms are simulated at tropospheric temperatures by considering two different gas mixtures of 1-butanol and acetic acid molecules, and selectivity of the ice surface with respect to these species is interpreted at the molecular scale as resulting from a competition process between these molecules for being adsorbed at the ice surface. It is thus shown that the trapping of acetic acid molecules on ice is always favored with respect to that of 1-butanol at low pressures, corresponding to low coverage of the surface, whereas the adsorption of the acid species is significantly modified by the presence of the alcohol molecules in the saturated portion of the adsorption isotherm, in accordance with the experimental observations. The present GCMC simulations thus confirm that competitive adsorption effects have to be taken into consideration in real situations when gas mixtures present in the troposphere interact with the surface of ice particles.

Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F-, Br-, I-, and Cl- ions were close, while SO42- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I- and SO42- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO42- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl2 solution has the best hydrate promotion under an electric field environment.

Molecular simulation of imperfect structure I CO2 hydrate growth in brine

In order to investigate the viability of carbon dioxide (CO2) storage in seawater, molecular dynamics techniques were employed to study the dynamic evolution of CO2 hydrate in saline water. The simulation was conducted under specific conditions: a temperature of 275 K, a pressure of 10 MPa and a simulated marine environment achieved using a 3.4 wt% sodium chloride (NaCl) solution. The total simulation time was 1000 ns. The results of the simulation indicate that the pre-existence of CO2 hydrate crystals as seeds leads to rapid growth of CO2 hydrate. However, analysis of the F3 and F4 order parameters reveals that the hydrate does not meet the standard values of the perfect structure I (sI) type, confirming the existence of an imperfect structure during the simulation. Additionally, the changes in the number of different phase states of water molecules during the hydrate growth process shows that there are always some liquid water molecules, which means some water molecules fail to form solid water cages. Further investigation suggests that the presence of Na+ and Cl- hampers the hydrogen bonds between water molecules, resulting in incomplete cage structures. By analyzing the density variations in the system, it is observed that CO2 hydrate, with a density of around 1.133 g cm-3, forms rapidly, surpassing the average density of seawater. This density increase facilitates the efficient and swift containment of CO2 on the seabed, thereby supporting the feasibility of the CO2 storage theory.

A Multifaceted Computational Approach to Understanding the MERS-CoV Main Protease and Brown Algae Compounds' Interaction

Middle East Respiratory Syndrome (MERS) is a viral respiratory disease caused b a special type of coronavirus called MERS-CoV. In the search for effective substances against the MERS-CoV main protease, we looked into compounds from brown algae, known for their medicinal benefits. From a set of 1212 such compounds, our computer-based screening highlighted four-CMNPD27819, CMNPD1843, CMNPD4184, and CMNPD3156. These showed good potential in how they might attach to the MERS-CoV protease, comparable to a known inhibitor. We confirmed these results with multiple computer tests. Studies on the dynamics and steadiness of these compounds with the MERS-CoV protease were performed using molecular dynamics (MD) simulations. Metrics like RMSD and RMSF showed their stability. We also studied how these compounds and the protease interact in detail. An analysis technique, PCA, showed changes in atomic positions over time. Overall, our computer studies suggest brown algae compounds could be valuable in fighting MERS. However, experimental validation is needed to prove their real-world effectiveness.

Understanding why constant energy or constant temperature may affect nucleation behavior in MD simulations: A study of gas hydrate nucleation

Molecular dynamics simulations have been widely used in exploring the nucleation behavior of many systems, including gas hydrates. Gas hydrates are ice-like solids in which gas molecules are trapped in water cages. During hydrate formation, a considerable amount of heat is released, and previous work has reported that the choice of temperature control scheme may affect the behavior of hydrate formation. The origins of this effect have remained an open question. To address this question, extensive NVE simulations and thermostatted (NPT and NVT) simulations with different temperature coupling strengths have been performed and compared for systems where a water nanodroplet is immersed in a H2S liquid. Detailed analysis of the hydrate structures and their mechanisms of formation has been carried out. Slower nucleation rates in NVE simulations in comparison to NPT simulations have been observed in agreement with previous studies. Probability distributions for various temperature measures along with their spatial distributions have been examined. Interestingly, a comparison of these temperature distributions reveals a small yet noticeable difference in the widths of the distributions for water. The somewhat reduced fluctuations in the temperature for the water species in the NVE simulations appear to be responsible for reducing the hydrate nucleation rate. We further conjecture that the NVE-impeded nucleation rate may be the result of the finite size of the surroundings (here the liquid H2S portion of the system). Additionally, a local spatial temperature gradient arising from the heat released during hydrate formation could not be detected.

Atomic-Level Insights into Hollow Silica-Based Materials for Drug Delivery: Effects of Wettability and Porosity

Experimental evidence has demonstrated that the drug carrier capacity can be significantly enhanced through the use of hollow silica particles. Nevertheless, the effects of varying functional drug carrier surfaces and porous structures remain ambiguous. This study employs molecular dynamics simulations to examine the effects of varying the surface wettability, pore size, and flow velocity on the transfer process. The different levels of wettability of the silica surface with the coarse-grained water model is illustrated by adjusted interaction parameters. The effect of wettability is investigated. With weak interactions, the flow molecules form a nanodroplet to transfer through the porous structure. A strong interaction will lead to molecules flowing as a liquid film to transfer through the structure. Interestingly, the "contradiction effect" is observed when the flow molecules fail to penetrate the porous structure with weak interactions, during which surface tension dominates their flow behavior. Moreover, different porous structures are considered. The flow behaviors are divided into three processes: (1) fast flowing, (2) transient point, and (3) penetration flowing. Furthermore, the concept of surface molecules is defined to quantitatively measure the effect of porosity. A recommended contact angle is proposed. The results will pave the way for more carrier structures in medical engineering.

Resolving Temperature-Dependent Hydrate Nucleation Pathway: The Role of "Transition Layer"

Understanding the nucleation of natural gas hydrate (NGH) at different conditions has important implications to NGH recovery and other industrial applications, such as gas storage and separation. Herein, vast numbers of hydrate nucleation events are traced via molecular dynamics (MD) simulations at different degrees of supercooling (or driving forces). Specifically, to precisely characterize a hydrate nucleus from an aqueous system during the MD simulation, we develop an evolutionary order parameter (OP) to recognize the nucleus size and shape. Subsequently, the free energy landscapes of hydrate during nucleation are explored by using the newly developed OP. The results suggest that at 270 K (or 0.92 Tm supercooling, where Tm is the melting point), the near-rounded nucleus prevails during the nucleation, as described from the classical nucleation theory. In contrast, at relatively strong driving forces of 0.85 and 0.88 Tm, nonclassical nucleation events arise. Specifically, the pathway toward an elongated nucleus becomes as important as the pathway toward a near-rounded nucleus. To explain the distinct nucleation phenomena at different supercoolings, a notion of a "transition layer" (or liquid-blob-like layer) is proposed. Here, the transition layer is to describe the interfacial region between the nucleus and aqueous solution, and this layer entails two functionalities: (1) it tends to retain CH4 depending on the degrees of supercooling and (2) it facilitates collision among CH4, which thus promote the incorporation of CH4 into nucleus. Our simulation indicates that compared to the near-rounded nucleus, the transition layer surrounding the elongated nucleus is more evident with the higher collision rate among CH4 molecules. As such, the transition layer tends to promote the elongated nucleus pathway, while offsetting the cost of larger surface free energy associated with the elongated nucleus. At 0.92 Tm, however, the transition layer gradually disappears, and classical nucleation events dominate. Overall, the notion of "transition layer" offers deeper insight into the NGH nucleation at different degrees of supercooling and could be extended to describe other types of hydrate nucleation.

Appraisal of TIP4P-type models at water surface

In view of the current situation in which non-polarizable rigid water models have been scarcely examined against surface-specific properties, we appraise TIP4P-type models at the liquid water surface on the basis of heterodyne-detected sum frequency generation (HD-SFG) spectroscopy. We find in the HD-SFG spectrum of the water surface that the peak frequency of the hydrogen-bonded OH band, the half width at half maximum of the hydrogen-bonded OH band, and the full width at half maximum of the free OH band are best reproduced by TIP4P, TIP4P/Ew, and TIP4P/Ice, respectively, whereas it is already well known that TIP4P/2005 best reproduces the surface tension. These TIP4P-type models perform better at the water surface in terms of the present appraisal items than some polarizable models in the literature.

Neural Network Water Model Based on the MB-Pol Many-Body Potential

The MB-pol many-body potential accurately predicts many properties of water, including cluster, liquid phase, and vapor-liquid equilibrium properties, but its high computational cost can make applying it in large-scale simulations quite challenging. In order to address this limitation, we developed a "deep potential" neural network (DPMD) model based on the MB-pol potential for water. We find that a DPMD model trained on mostly liquid configurations yields a good description of the bulk liquid phase but severely underpredicts vapor-liquid coexistence densities. By contrast, adding cluster configurations to the neural network training set leads to a good agreement for the vapor coexistence densities. Liquid phase densities under supercooled conditions are also represented well, even though they were not included in the training set. These results confirm that neural network models can combine accuracy and transferability if sufficient attention is given to the construction of a representative training set for the target system.

Self-Assembled Nanostructures of Homo-Oligopeptide as a Potent Ice Growth Inhibitor

This study reports the formation of self-assembled nanostructures with homo-oligopeptides consisting of amino acids (i.e., alanine, threonine, valine, and tyrosine), the resulting morphologies (i.e., spherical shape, layered structure, and wire structure) in aqueous solution, and their potential as ice growth inhibitors. Among the homo-oligopeptides investigated, an alanine homo-oligopeptide (n = 5) with a spherical nanostructure showed the highest ice recrystallization inhibition (IRI) activity without showing a burst ice growth property and with low ice nucleation activity. The presence of nanoscale self-assembled structures in the solution showed superior IRI activity compared to an amino acid monomer because of the higher binding affinity of structures on the growing ice crystal plane. Simulation results revealed that the presence of nanostructures induced a significant inhibition of ice growth and increased lifetime of hydrogen bonding compared with unassembled homo-oligopeptide. These results envision extraordinary performance for self-assembled nanostructures as a desirable and potent ice growth inhibitor.

Molecular Dynamics Simulation of the Three-Phase Equilibrium Line of CO2 Hydrate with OPC Water Model

The three-phase coexistence line of the CO2 hydrate was determined using molecular dynamics (MD) simulations. By using the classical and modified Lorentz-Berthelot (LB) parameters, the simulations were carried out at 10 different pressures from 3 to 500 MPa. For the OPC water model, simulations with the classic and the modified LB parameters both showed negative deviations from the experimental values. For the TIP4P/Ice water model, good agreement with experimental equilibrium data can be achieved when the LB parameter is adjusted based on the solubility of CO2 in water. Our results also show that the influence of the water model on the equilibrium prediction is much larger than the CO2 model. Current simulations indicated that the H2O-H2O and H2O-CO2 cross-interactions' parameters might contribute equally to the accurate prediction of T3. According to our simulations, the prediction of T3 values showed relatively higher accuracy while using the combination of TIP4P/Ice water and EPM2 CO2 with modified LB parameter. Furthermore, varied χ values are recommended for accurate T3 estimation over a wide pressure range. The knowledge obtained in this study will be helpful for further accurate MD simulation of the process of CO2/CH4 replacement.

Molecular simulation of the confined crystallization of ice in cement nanopore

Freezing of water under nanoconfinement exhibits physical peculiarities with respect to the bulk water. However, experimental observations are extremely challenging at this scale, which limits our understanding of the effect of confinement on water properties upon freezing. In this study, we use molecular dynamic simulations to investigate how confinement affects the kinetics of growth of ice and the thermodynamic equilibrium of ice-liquid coexistence. TIP4P/Ice water model and CSH-FF model were applied to simulate ice crystallization in a confined cement system at temperatures down to 220 K. We adapted an interface detection algorithm and reparameterized the CHILL/CHILL+ algorithm to capture ice growth. The confinement leads to a shift of the maximum growth rate of ice to a higher temperature than for bulk water. Both the confinement and surface impurities contribute to slowing down the ice growth. For the ice-liquid coexistence at equilibrium, we derive a formulation of Thomson's equation adapted to statistical physics quantities accessible by molecular simulation, and we show that this adapted equation predicts accurately the melting line of bulk and confined ice Ih as a function of pressure. The confinement decreases systematically the melting temperature of ice of about 5 K compared with bulk ice Ih. A premelted water film about 1 nm thick is observed between the solid wall and ice, and its thickness is found to decrease continuously as temperature is lowered. We note that the surface impurities are key to the formation of the premelted water nanofilm when the temperature is lower than 250 K.

Glassy dynamics of water in TIP4P/Ice aqueous solutions of trehalose in comparison with the bulk phase

We perform molecular dynamics simulations of TIP4P/Ice water in solution with trehalose for 3.65 and 18.57 wt. % concentrations and of bulk TIP4P/Ice water at ambient pressure, to characterize the structure and dynamics of water in a sugar aqueous solution in the supercooled region. We find here that TIP4P/Ice water in solution with trehalose molecules follows the Mode Coupling Theory and undergoes a fragile to strong transition up to the highest concentration investigated, similar to the bulk. Moreover, we perform a Mode Coupling Theory test, showing that the Time Temperature Superposition principle holds for both bulk TIP4P/Ice water and for TIP4P/Ice water in the solutions and we calculate the exponents of the theory. The direct comparison of the dynamical results for bulk water and water in the solutions shows upon cooling along the isobar a fastening of water dynamics for lower temperatures, T < 240 K. We found that the counter-intuitive behavior for the low temperature solutions can be explained with the diffusion anomaly of water leading us to the conclusion that the fastening observed below T = 240 K in water dynamics is only fictitious, due to the fact that the density of water molecules in the solutions is higher than the density of the bulk at the same temperature and pressure. This result should be taken into account in experimental investigations which are often carried out at constant pressure.