Melting and Crystallization of Ice in Partially Filled Nanopores

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.

Water Self-Dissociation is Insensitive to Nanoscale Environments

Nanoconfinement effects on water dissociation and reactivity remain controversial, despite their importance to understand the aqueous chemistry at interfaces, pores, or aerosols. The pKw in confined environments has been assessed from experiments and simulations in a few specific cases, leading to dissimilar conclusions. Here, with the use of carefully designed ab initio simulations, we demonstrate that the energetics of bulk water dissociation is conserved intact to unexpectedly small length-scales, down to aggregates of only a dozen molecules or pores of widths below 2 nm. The reason is that most of the free-energy involved in water autoionization comes from breaking the O-H covalent bond, which has a comparable barrier in the bulk liquid, in a small droplet of nanometer size, or in a nanopore in the absence of strong interfacial interactions. Thus, dissociation free-energy profiles in nanoscopic aggregates or in 2D slabs of 1 nm width reproduce the behavior corresponding to the bulk liquid, regardless of whether the corresponding nanophase is delimited by a solid or a gas interface. The present work provides a definite and fundamental description of the mechanism and thermodynamics of water dissociation at different scales with broader implications on reactivity and self-ionization at the air-liquid interface.

Icephobic/anti-icing properties of superhydrophobic surfaces

In the winter, icing on solid surfaces is a typical occurrence that may create a slew of hassles and even tragedies. Anti-icing surfaces are one of the effective solutions for this kind of problem. The roughness of a superhydrophobic surface traps air and weakens the contact between the solid surface and liquid water, allowing water droplets to be removed before freezing. At present, the conventional anti-icing methods including mechanical or thermal technology are not only surface structure unfriendly but also have the obsessions of low efficiency, high energy consumption and high manufacturing costs. Hence, developing a way to remove ice by just modifying the surface shape or chemical composition with a low surface energy is extremely desirable. Numerous attempts have been made to investigate the evolution of ice nucleation and icing on superhydrophobic surfaces under the direction of the ice nucleation hypothesis. In this paper, the research progress of ice nucleation in recent years is reviewed from theoretical and application. The icephobic surfaces are described using the wettability and classical nucleation theories. The benefits and drawbacks of anti-icing superhydrophobic surface are summarized, as well as deicing methods. Finally, several applications of ice phobic materials are illustrated, and some problems and challenges in the research field are discussed. We believed that this review will be useful in guiding future water freezing initiatives.

Ice in biomolecular cryocrystallography

Diffraction data acquired from cryocooled protein crystals often include diffraction from ice. Analysis of ice diffraction from crystals of three proteins shows that the ice formed within solvent cavities during rapid cooling is comprised of a stacking-disordered mixture of hexagonal and cubic planes, with the cubic plane fraction increasing with increasing cryoprotectant concentration and increasing cooling rate. Building on the work of Thorn and coworkers [Thorn et al. (2017), Acta Cryst. D73, 729-727], a revised metric is defined for detecting ice from deposited protein structure-factor data, and this metric is validated using full-frame diffraction data from the Integrated Resource for Reproducibility in Macromolecular Crystallography. Using this revised metric and improved algorithms, an analysis of structure-factor data from a random sample of 89 827 PDB entries collected at cryogenic temperatures indicates that roughly 16% show evidence of ice contamination, and that this fraction increases with increasing solvent content and maximum solvent-cavity size. By examining the ice diffraction-peak positions at which structure-factor perturbations are observed, it is found that roughly 25% of crystals exhibit ice with primarily hexagonal character, indicating that inadequate cooling rates and/or cryoprotectant concentrations were used, while the remaining 75% show ice with a stacking-disordered or cubic character.

Machine Learning Based Prediction of Nanoscale Ice Adhesion on Rough Surfaces

It is widely recognized that surface roughness plays an important role in ice adhesion strength, although the correlation between the two is far from understood. In this paper, two approaches, molecular dynamics (MD) simulations and machine learning (ML), were utilized to study the nanoscale intrinsic ice adhesion strength on rough surfaces. A systematic algorithm for making random rough surfaces was developed and the surfaces were tested for their ice adhesion strength, with varying interatomic potentials. Using MD simulations, the intrinsic ice adhesion strength was found to be significantly lower on rougher surfaces, which was attributed to the lubricating effect of a thin quasi-liquid layer. An increase in the substrate–ice interatomic potential increased the thickness of the quasi-liquid layer on rough surfaces. Two different ML algorithms, regression and classification, were trained using the results from the MD simulations, with support vector machines (SVM) emerging as the best for classifying. The ML approach showed an encouraging prediction accuracy, and for the first time shed light on using ML for anti-icing surface design. The findings provide a better understanding of the role of nanoscale roughness in intrinsic ice adhesion and suggest that ML can be a powerful tool in finding materials with a low ice adhesion strength.

Nanofibrillar Ionic Polymer Composites Enable High-Modulus Ion-Conducting Membranes

Polymer electrolyte membranes (PEMs) with high volume fractions of ionic liquids (IL) and high modulus show promise for enabling next-generation gas separations, and electrochemical energy storage and conversion applications. Herein, we present a conductive polymer-IL composite based on a sulfonated all-aromatic polyamide (sulfo-aramid, PBDT) and a model IL, which we term a PBDT-IL composite. The polymer forms glassy and high-aspect-ratio hierarchical nanofibrils, which enable fabrication of PEMs with both high volume fractions of IL and high elastic modulus. We report direct evidence for nanofibrillar networks that serve as matrices for dispersed ILs using atomic force microscopy and small- and wide-angle X-ray scattering. These supramolecular nanofibrils form through myriad noncovalent interactions to produce a physically cross-linked glassy network, which boasts the best combination of room-temperature modulus (0.1-2 GPa) and ionic conductivity (8-4 mS cm^-1) of any polymer-IL electrolyte reported to date. The ultrahigh thermomechanical properties of our PBDT-IL composites provide high moduli (∼1 GPa) at temperatures up to 200 °C, enabling a wide device operation window with stable mechanical properties. Together, the high-performance nature of sulfo-aramids in concert with the inherent properties of ILs imparts PBDT-IL composites with nonflammability and thermal stability up to 350 °C. Thus, nanofibrillar ionic networks based on sulfo-aramids and ILs represent a new design paradigm affording PEMs with exceptionally high moduli at exceedingly low polymer concentrations. This new design strategy will drive the development of new high-performance conductive membranes that can be used for the design of gas separation membranes and in electrochemical applications, such as fuel cells and Li-metal batteries.

Water in Mesoporous Confinement: Glass-To-Liquid Transition or Freezing of Molecular Reorientation Dynamics?

The first mechanical relaxation measurements (f = 400 Hz) of water confined in micro-porous silica were performed more than 40 years ago. The authors reported a so called “capillary transition” (here denoted as P3) of water in the core of the pores and a second one at a lower temperature, which they called the “adsorbate transition” (P1 in present work) related to water near the surface of the pores. The capillary transition was identified with the freezing of water in the centre of the pores. However, even 40 years later, the origin of the adsorbate transition is not yet clear. One study relates it to the liquid-to-glass transition of the supercooled water in the pores, and another study to the freezing of the proton reorientations at the lattice defects. The present work shows the data from extensive dynamic mechanical analysis (DMA) measurements (f = 0.1 Hz–70 Hz) of water confined in mesoporous silica (d = 2.5, 5 and 10 nm), which are in favour of a liquid-to-glass scenario.

Ice formation and solvent nanoconfinement in protein crystals

Ice formation within protein crystals is a major obstacle to the cryocrystallographic study of protein structure, and has limited studies of how the structural ensemble of a protein evolves with temperature in the biophysically interesting range from ∼260 K to the protein-solvent glass transition near 200 K. Using protein crystals with solvent cavities as large as ∼70 Å, time-resolved X-ray diffraction was used to study the response of protein and internal solvent during rapid cooling. Solvent nanoconfinement suppresses freezing temperatures and ice-nucleation rates so that ice-free, low-mosaicity diffraction data can be reliably collected down to 200 K without the use of cryoprotectants. Hexagonal ice (Ih) forms in external solvent, but internal crystal solvent forms stacking-disordered ice (Isd) with a near-random stacking of cubic and hexagonal planes. Analysis of powder diffraction from internal ice and single-crystal diffraction from the host protein structure shows that the maximum crystallizable solvent fraction decreases with decreasing crystal solvent-cavity size, and that an ∼6 Å thick layer of solvent adjacent to the protein surface cannot crystallize. These results establish protein crystals as excellent model systems for the study of nanoconfined solvent. By combining fast cooling, intense X-ray beams and fast X-ray detectors, complete structural data sets for high-value targets, including membrane proteins and large complexes, may be collected at ∼220-240 K that have much lower mosaicities and comparable B factors, and that may allow more confident identification of ligand binding than in current cryocrystallographic practice.

Pore condensation and freezing is responsible for ice formation below water saturation for porous particles

The formation of ice at relative humidity below 100% is assumed to proceed without the presence of liquid water. However, it has been shown that liquid water can exist well below water saturation in narrow cracks and pores. Here we show that the barrier for deposition nucleation of ice directly from the vapor is insurmountable in experiments; liquid water is involved in ice formation on porous particles, regardless of the ambient humidity. Thus, our results render deposition nucleation unlikely for the formation of ice clouds in the atmosphere.

Ice nucleation in the atmosphere influences cloud properties, altering precipitation and the radiative balance, ultimately regulating Earth’s climate. An accepted ice nucleation pathway, known as deposition nucleation, assumes a direct transition of water from the vapor to the ice phase, without an intermediate liquid phase. However, studies have shown that nucleation occurs through a liquid phase in porous particles with narrow cracks or surface imperfections where the condensation of liquid below water saturation can occur, questioning the validity of deposition nucleation. We show that deposition nucleation cannot explain the strongly enhanced ice nucleation efficiency of porous compared with nonporous particles at temperatures below −40 °C and the absence of ice nucleation below water saturation at −35 °C. Using classical nucleation theory (CNT) and molecular dynamics simulations (MDS), we show that a network of closely spaced pores is necessary to overcome the barrier for macroscopic ice-crystal growth from narrow cylindrical pores. In the absence of pores, CNT predicts that the nucleation barrier is insurmountable, consistent with the absence of ice formation in MDS. Our results confirm that pore condensation and freezing (PCF), i.e., a mechanism of ice formation that proceeds via liquid water condensation in pores, is a dominant pathway for atmospheric ice nucleation below water saturation. We conclude that the ice nucleation activity of particles in the cirrus regime is determined by the porosity and wettability of pores. PCF represents a mechanism by which porous particles like dust could impact cloud radiative forcing and, thus, the climate via ice cloud formation.

Thermodynamics of sustaining liquid water within rough icephobic surfaces to achieve ultra-low ice adhesion

The build-up of ice on aircraft, bridges, oil rigs, wind turbines, electrical lines, and other surfaces exposed to cold environments diminishes their safe and effective operation. To engineer robust surfaces that reduce ice adhesion, it is necessary to understand the physics of what makes a surface icephobic (“ice-hating”) as well as the relationship between icephobicity and ice adhesion. Here we elucidate the molecular origin of icephobicity based on ice-surface interactions and characterize the correlation between material icephobicity and liquid wettability. This fundamental understanding of icephobic characteristics enables us to propose a robust design for topologically textured, icephobic surfaces. The design identifies the critical confinement length scale to sustain liquid water (as opposed to ice) in between roughness features and can reduce the strength of ice adhesion by over a factor of twenty-seven compared to traditional hydrophobic surfaces. The reduction in ice adhesion is due to the metastability of liquid water; as ambient ice cleaves from the textured surface, liquid water leaves confinement and freezes – a process which takes the system from a local energy minimum to a global energy minimum. This phase transition generates a detachment force that actively propels ambient ice from the surface.

Impact of surface wettability on dynamics of supercooled water confined in nitrogen-doped ordered mesoporous carbon

Confinement of water to nanoscale dimensions enables substantial supercooling through disruption of the hydrogen bonding network. However, there remain questions associated with the importance of the nature of the water-surface interactions relative to physical confinement defined by the pore geometry on the dynamics of supercooled water. Here, a simple route to tune the surface wetting properties through nitrogen doping of carbon is reported. This method leads to nearly indistinguishable mesopore sizes to enable separation of surface wettability and pore size effects. Quasielastic neutron scattering (QENS) is used to probe the proton dynamics of water confined within the mesopores with an average diameter of 4.85 ± 0.05 nm as a function of temperature from 267 K to 189 K. The motions of water in the mesopores follow jump-diffusion. For the temperatures examined, the diffusivity of water in the mesopores decreases with increasing nitrogen doping of the carbon framework. The activation energy associated with proton dynamics increases by approximately 30% with N-doping when compared to the undoped carbon surface, which is attributed to the enhanced surface wettability (favorable interactions between water and pore surface). This acts to provide an energy barrier for the water motions. This work suggests that the influence of surface chemistry on the dynamics of supercooled water confined in mesopores is less than the influence of nanopore size.

Ice-Liquid Oscillations in Nanoconfined Water

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

Phase transition-like behavior of the water monolayer close to the polarized surface of a nanotube

By molecular dynamics simulations, we have investigated effects of temperature on the dynamical behavior of water layers at the charged surface of a nanotube. The behavior of the first water monolayer at the charged surface is very different from that of bulk water. There are three different temperature regions for the axial diffusion coefficient and they increase in different ways (linearly or exponentially) with temperature. The dipole distribution of water molecules was chosen as the order parameter to analyze the phase transition-like behavior. The simulation results indicate that the transition from ordered water to disordered water is continuous, which has not been found in the bulk counterpart. The mechanism behind the unexpected phenomenon was also investigated.

Ice-Nucleating and Antifreeze Proteins Recognize Ice through a Diversity of Anchored Clathrate and Ice-like Motifs

Cold-adapted organisms produce antifreeze and ice-nucleating proteins to prevent and promote ice formation. The crystal structure of hyperactive bacterial antifreeze protein (AFP) MpAFP suggests that this protein binds ice through an anchored clathrate motif. It is not known whether other hyperactive AFPs and ice-nucleating proteins (INPs) use the same motif to recognize or nucleate ice. Here we use molecular simulations to elucidate the ice-binding motifs of hyperactive insect AFPs and a model INP of Pseudomonas syringae. We find that insect AFPs recognize ice through anchored clathrate motifs distinct from that of MpAFP. By performing simulations of ice nucleation by PsINP, we identify two distinct ice-binding sites on opposite sides of the β-helix. The ice-nucleating sequences identified in the simulations agree with those previously proposed for the closely related INP of Pseudomonas borealis based on the structure of the protein. The simulations indicate that these sites have comparable ice-nucleating efficiency, but distinct binding motifs, controlled by the amino acid sequence: one is an anchored clathrate and the other ice-like. We conclude that anchored clathrate and ice-like motifs can be equally effective for binding proteins to ice and promoting ice nucleation.

Controlling drug release from mesoporous silica through an amorphous, nanoconfined 1-tetradecanol layer

Mesoporous silica materials are promising nano-carriers for drug delivery systems. Even though there are many strategies for controlling the drug release kinetics, these must be adapted through trial and error on a case-by-case basis. Here we explore the possibility of tailoring the release kinetics of hydrophilic, water soluble therapeutic agents from mesoporous silica through addition of a hydrophobic excipient, 1-tetradecanol. In vitro drug release experiments performed at 37 °C, in phosphate buffer solution (pH 7.4) show that the addition of tetradecanol yields slower drug release kinetics, which was correlated with the presence of a liquid fatty alcohol interfacial layer. The layer mass is 11-23 wt.% of the metoprolol-loaded silica sample, and it causes up to 1.6 times decrease of initial release rate with respect to materials without the fatty alcohol. This effect does not depend of carrier pore arrangement, being noticed for both hexagonal MCM-41 and cubic KIT-5 mesoporous silica. The toxicity of tetradecanol-containing materials was evaluated by formazan-based viability assay on Opossum kidney epithelial cell line, and no significant toxicity was observed.

Control of ice nucleation: freezing and antifreeze strategies

Water freezing remains a perennial topic of great relevance to many important aspects of our lives; from the climate to human society and from economics to medicine, frozen water profoundly influences our living environment and life activities.

Ice Nucleation Efficiency of Hydroxylated Organic Surfaces Is Controlled by Their Structural Fluctuations and Mismatch to Ice

Heterogeneous nucleation of ice induced by organic materials is of fundamental importance for climate, biology, and industry. Among organic ice-nucleating surfaces, monolayers of long chain alcohols are particularly effective, while monolayers of fatty acids are significantly less so. As these monolayers expose to water hydroxyl groups with an order that resembles the one in the basal plane of ice, it was proposed that lattice matching between ice and the surface controls their ice-nucleating efficiency. Organic monolayers are soft materials and display significant fluctuations. It has been conjectured that these fluctuations assist in the nucleation of ice. Here we use molecular dynamic simulations and laboratory experiments to investigate the relationship between the structure and fluctuations of hydroxylated organic surfaces and the temperature at which they nucleate ice. We find that these surfaces order interfacial water to form domains with ice-like order that are the birthplace of ice. Both mismatch and fluctuations decrease the size of the preordered domains and monotonously decrease the ice freezing temperature. The simulations indicate that fluctuations depress the freezing efficiency of monolayers of alcohols or acids to half the value predicted from lattice mismatch alone. The model captures the experimental trend in freezing efficiencies as a function of chain length and predicts that alcohols have higher freezing efficiency than acids of the same chain length. These trends are mostly controlled by the modulation of the structural mismatch to ice. We use classical nucleation theory to show that the freezing efficiencies of the monolayers are directly related to their free energy of binding to ice. This study provides a general framework to relate the equilibrium thermodynamics of ice binding to a surface and the nonequilibrium ice freezing temperature and suggests that these could be predicted from the structure of interfacial water.

What Determines the Ice Polymorph in Clouds?

Ice crystals in the atmosphere nucleate from supercooled liquid water and grow by vapor uptake. The structure of the ice polymorph grown has strong impact on the morphology and light scattering of the ice crystals, modulates the amount of water vapor in ice clouds, and can impact the molecular uptake and reactivity of atmospheric aerosols. Experiments and molecular simulations indicate that ice nucleated and grown from deeply supercooled liquid water is metastable stacking disordered ice. The ice polymorph grown from vapor has not yet been determined. Here we use large-scale molecular simulations to determine the structure of ice that grows as a result of uptake of water vapor in the temperature range relevant to cirrus and mixed-phase clouds, elucidate the molecular mechanism of the formation of ice at the vapor interface, and compute the free energy difference between cubic and hexagonal ice interfaces with vapor. We find that vapor deposition results in growth of stacking disordered ice only under conditions of extreme supersaturation, for which a nonequilibrium liquid layer completely wets the surface of ice. Such extreme conditions have been used to produce stacking disordered frost ice in experiments and may be plausible in the summer polar mesosphere. Growth of ice from vapor at moderate supersaturations in the temperature range relevant to cirrus and mixed-phase clouds, from 200 to 260 K, produces exclusively the stable hexagonal ice polymorph. Cubic ice is disfavored with respect to hexagonal ice not only by a small penalty in the bulk free energy (3.6 ± 1.5 J mol(-1) at 260 K) but also by a large free energy penalty at the ice-vapor interface (89.7 ± 12.8 J mol(-1) at 260 K). The latter originates in higher vibrational entropy of the hexagonal-terminated ice-vapor interface. We predict that the free energy penalty against the cubic ice interface should decrease strongly with temperature, resulting in some degree of stacking disorder in ice grown from vapor in the tropical tropopause layer, and in polar stratospheric and noctilucent clouds. Our findings support and explain the evolution of the morphology of ice crystals from hexagonal to trigonal symmetry with decreasing temperature, as reported by experiments and in situ measurements in clouds. We conclude that selective growth of the elusive cubic ice polymorph by manipulation of the interfacial properties can likely be achieved at the ice-liquid interface but not at the ice-vapor interface.

Free energy contributions and structural characterization of stacking disordered ices

Crystallization of ice from deeply supercooled water and amorphous ices - a process of fundamental importance in the atmosphere, interstellar space, and cryobiology - results in stacking disordered ices with a wide range of metastabilities with respect to hexagonal ice. The structural origin of this high variability, however, has not yet been elucidated. Here we use molecular dynamics simulations with the mW water model to characterize the structure of ice freshly grown from supercooled water at temperatures from 210 to 270 K, the thermodynamics of stacking faults, line defects, and interfaces, and to elucidate the interplay between kinetics and thermodynamics in determining the structure of ice. In agreement with experiments, the ice grown in the simulations is stacking disordered with a random distribution of cubic and hexagonal layers, and a cubicity that decreases with growth temperature. The former implies that the cubicity of ice is determined by processes at the ice/liquid interface, without memory of the structure of buried ice layers. The latter indicates that the probability of building a cubic layer at the interface decreases upon approaching the melting point of ice, which we attribute to a more efficient structural equilibration of ice at the liquid interface as the driving force for growth wanes. The free energy cost for creating a pair of cubic layers in ice is 8.0 J mol(-1) in experiments, and 9.7 ± 1.9 J mol(-1) for the mW water model. This not only validates the simulations, but also indicates that dispersion in cubicity is not sufficient to explain the large energetic variability of stacking disordered ices. We compute the free energy cost of stacking disorder, line defects, and interfaces in ice and conclude that a characterization of the density of these defects is required to predict the degree of metastability and vapor pressure of atmospheric ices.

Stacking disorder in ice I

Traditionally, ice I was considered to exist in two well-defined crystalline forms at ambient pressure: stable hexagonal ice (ice Ih) and metastable cubic ice (ice Ic). However, it is becoming increasingly evident that what has been called cubic ice in the past does not have a structure consistent with the cubic crystal system. Instead, it is a stacking-disordered material containing cubic sequences interlaced with hexagonal sequences, which is termed stacking-disordered ice (ice Isd). In this article, we summarise previous work on ice with stacking disorder including ice that was called cubic ice in the past. We also present new experimental data which shows that ice which crystallises after heterogeneous nucleation in water droplets containing solid inclusions also contains stacking disorder even at freezing temperatures of around -15 °C. This supports the results from molecular simulations, that the structure of ice that crystallises initially from supercooled water is always stacking-disordered and that this metastable ice can transform to the stable hexagonal phase subject to the kinetics of recrystallization. We also show that stacking disorder in ice which forms from water droplets is quantitatively distinct from ice made via other routes. The emerging picture of ice I is that of a very complex material which frequently contains stacking disorder and this stacking disorder can vary in complexity depending on the route of formation and thermal history.

Low-density liquid water is the mother of ice: on the relation between mesostructure, thermodynamics and ice crystallization in solutions

Predicting the temperature and extent of ice freezing in aqueous solutions is crucial for areas as diverse as cryobiology and materials design. It has long been recognized that the thermodynamics of liquid water controls the temperature and kinetics of ice crystallization. Parameterizations of the freezing temperatures in terms of the water activity of the solution have been successfully established, but the fundamental origin of the thermodynamic control of the non-equilibrium crystallization of ice has remained elusive. Here we use large-scale molecular simulations to elucidate the relationship between the structure, thermodynamics, and ice crystallization temperatures for solutions of mW water and a strongly hydrophilic solute that mimics LiCI ions. Fast cooling of solutions with up to 20 mol% ions results in the formation of nanosegregated glasses with domains of low-density amorphous ice and an ion-rich vitrified solution. Slow cooling of the mixtures results in nucleation and growth of ice within the domains of four-coordinated liquid water. The temperature of crystallization Tx coincides with the temperature of appearance of nanoscopic domains of four-coordinated liquid water in the mixture, T(L). We use the insight provided by the simulations to derive a thermodynamic expression for the crystallization temperature as a function of the water activity, T(X)(a(W)), analogous to the dependence of the melting temperature, T(m)(a(W)). The simple expression derived in this work provides a good account of the experimental freezing temperatures of water and the well-known steepest dependence of Tx on solute concentration compared to that of T(m).

The microscopic features of heterogeneous ice nucleation may affect the macroscopic morphology of atmospheric ice crystals

It is surprisingly difficult to freeze water. Almost all ice that forms under "mild" conditions (temperatures > -40 degrees C) requires the presence of a nucleating agent--a solid particle that facilitates the freezing process--such as clay mineral dust, soot or bacteria. In a computer simulation, the presence of such ice nucleating agents does not necessarily alleviate the difficulties associated with forming ice on accessible timescales. Nevertheless, in this work we present results from molecular dynamics simulations in which we systematically compare homogeneous and heterogeneous ice nucleation, using the atmospherically important clay mineral kaolinite as our model ice nucleating agent. From our simulations, we do indeed find that kaolinite is an excellent ice nucleating agent but that contrary to conventional thought, non-basal faces of ice can nucleate at the basal face of kaolinite. We see that in the liquid phase, the kaolinite surface has a drastic effect on the density profile of water, with water forming a dense, tightly bound first contact layer. Monitoring the time evolution of the water density reveals that changes away from the interface may play an important role in the nucleation mechanism. The findings from this work suggest that heterogeneous ice nucleating agents may not only enhance the ice nucleation rate, but also alter the macroscopic structure of the ice crystals that form.

Heterogeneous nucleation of ice on carbon surfaces

Atmospheric aerosols can promote the heterogeneous nucleation of ice, impacting the radiative properties of clouds and Earth's climate. The experimental investigation of heterogeneous freezing of water droplets by carbonaceous particles reveals widespread ice freezing temperatures. It is not known which structural and chemical characteristics of soot account for the variability in ice nucleation efficiency. Here we use molecular dynamics simulations to investigate the nucleation of ice from liquid water in contact with graphitic surfaces. We find that atomically flat carbon surfaces promote heterogeneous nucleation of ice, while molecularly rough surfaces with the same hydrophobicity do not. Graphitic surfaces and other surfaces that promote ice nucleation induce layering in the interfacial water, suggesting that the order imposed by the surface on liquid water may play an important role in the heterogeneous nucleation mechanism. We investigate a large set of graphitic surfaces of various dimensions and radii of curvature and find that variations in nanostructures alone could account for the spread in the freezing temperatures of ice on soot in experiments. We conclude that a characterization of the nanostructure of soot is needed to predict its ice nucleation efficiency.

Model of waterlike fluid under confinement for hydrophobic and hydrophilic particle-plate interaction potentials

Molecular dynamic simulations were employed to study a waterlike model confined between hydrophobic and hydrophilic plates. The phase behavior of this system is obtained for different distances between the plates and particle-plate potentials. For both hydrophobic and hydrophilic walls, there are the formation of layers. Crystallization occurs at lower temperature at the contact layer than at the middle layer. In addition, the melting temperature decreases as the plates become more hydrophobic. Similarly, the temperatures of maximum density and extremum diffusivity decrease with hydrophobicity.

Elastic properties of liquid and solid argon in nanopores

We have measured sorption isotherms and determined the intrinsic longitudinal elastic modulus β(Ar,ads) of nanoconfined material via ultrasonic measurements combined with a special effective medium analysis. In the liquid regime the adsorbate only contributes to the measured effective properties when the pores are completely filled and the modulus is bulklike. At partial fillings its contribution is cancelled out by the high compressibility of the vapour phase. In contrast, at lower temperatures frozen argon as well as underlying liquid surface layers cause a linear increase of the effective longitudinal modulus upon filling. During sorption the contribution of the liquid surface layers near the pore wall β(Ar,surf) increases with the thickness of the solid layers reaching the bulk value β(Ar,liquid) only in the limit of complete pore filling. We interpret this effect as due to the gradual stiffening of the solid argon membrane. The measurements and their analysis show that longitudinal ultrasonic waves are well suited to the study of the elastic properties and liquid-solid phase transitions in porous systems. This method should also help to detect the influence of nanoconfinement on elastic properties in further research.

Continuous freezing of argon in completely filled mesopores

We have studied the phase transition of argon in completely filled mesopores. Our effective medium analysis of ultrasonic measurements clearly indicates a continuous phase transition of argon in completely filled pores over a broad temperature range of about 45 K. With decreasing temperature, the amount of frozen argon increases and below about 30 K all adsorbed argon (including the first few layers near the pore wall) is frozen with a shear modulus about equal to the bulk shear modulus. It is remarkable, that in a system showing such a pronounced confinement effect--a continuous phase transition over 45 K--the bulk properties are preserved. A comparison with temperature cycles with one and two adsorbed layers shows, that due to the presence of solid argon in the center of the pores the first few layers are already frozen at a higher temperature (30 K) compared to single layers (20 K). The transfer of our technique from the simple model system that we present in this Letter to other more complex adsorbates and different porous samples should help to enlighten the phase behavior under confinement in further studies.