The surface charge distribution affects the ice nucleating efficiency of silver iodide

Effect of substrate mismatch, orientation, and flexibility on heterogeneous ice nucleation

Heterogeneous nucleation is the main path to ice formation on Earth. The ice nucleating ability of a certain substrate is mainly determined by both molecular interactions and the structural mismatch between the ice and the substrate lattices. We focus on the latter factor using molecular simulations of the mW model. Quantifying the effect of structural mismatch alone is challenging due to its coupling with molecular interactions. To disentangle both the factors, we use a substrate composed of water molecules in such a way that any variation on the nucleation temperature can be exclusively ascribed to the structural mismatch. We find that a 1% increase in structural mismatch leads to a decrease of ∼4 K in the nucleation temperature. We also analyze the effect of orientation of the substrate with respect to the liquid. The three main ice orientations (basal, primary prism, and secondary prism) have a similar ice nucleating ability. We finally assess the effect of lattice flexibility by comparing substrates where molecules are immobile to others where a certain freedom to fluctuate around the lattice positions is allowed. Interestingly, we find that the latter type of substrate is more efficient in nucleating ice because it can adapt its structure to that of ice.

Using machine learning with atomistic surface and local water features to predict heterogeneous ice nucleation

Heterogeneous ice nucleation (HIN) has applications in climate science, nanotechnology, and cryopreservation. Ice nucleation on the earth's surface or in the atmosphere usually occurs heterogeneously involving foreign substrates, known as ice nucleating particles (INPs). Experiments identify good INPs but lack sufficient microscopic resolution to answer the basic question: What makes a good INP? We employ molecular dynamics (MD) simulations in combination with machine learning (ML) to address this question. Often, the large amount of computational cost required to cross the nucleation barrier and observe HIN in MD simulations is a practical limitation. We use information obtained from short MD simulations of atomistic surface and water models to predict the likelihood of HIN. We consider 153 atomistic substrates with some surfaces differing in elemental composition and others only in terms of lattice parameters, surface morphology, or surface charges. A range of water features near the surface (local) are extracted from short MD simulations over a time interval (≤300 ns) where ice nucleation has not initiated. Three ML classification models, Random Forest (RF), support vector machine, and Gaussian process classification are considered, and the accuracies achieved by all three approaches lie within their statistical uncertainties. Including local water features is essential for accurate prediction. The accuracy of our best RF classification model obtained including both surface and local water features is 0.89 ± 0.05. A similar accuracy can be achieved including only local water features, suggesting that the important surface properties are largely captured by the local water features. Some important features identified by ML analysis are local icelike structures, water density and polarization profiles perpendicular to the surface, and the two-dimensional lattice match to ice. We expect that this work, with its strong focus on realistic surface models, will serve as a guide to the identification or design of substrates that can promote or discourage ice nucleation.

Properties of aqueous electrolyte solutions at carbon electrodes: effects of concentration and surface charge on solution structure, ion clustering and thermodynamics in the electric double layer

Surfaces are able to control physical-chemical processes in multi-component solution systems and, as such, find application in a wide range of technological devices. Understanding the structure, dynamics and thermodynamics of non-ideal solutions at surfaces, however, is particularly challenging. Here, we use Constant Chemical Potential Molecular Dynamics (CμMD) simulations to gain insight into aqueous NaCl solutions in contact with graphite surfaces at high concentrations and under the effect of applied surface charges: conditions where mean-field theories describing interfaces cannot (typically) be reliably applied. We discover an asymmetric effect of surface charge on the electric double layer structure and resulting thermodynamic properties, which can be explained by considering the affinity of the surface for cations and anions and the cooperative adsorption of ions that occurs at higher concentrations. We characterise how the sign of the surface charge affects ion densities and water structure in the double layer and how the capacitance of the interface-a function of the electric potential drop across the double layer-is largely insensitive to the bulk solution concentration. Notably, we find that negatively charged graphite surfaces induce an increase in the size and concentration of extended liquid-like ion clusters confined to the double layer. Finally, we discuss how concentration and surface charge affect the activity coefficients of ions and water at the interface, demonstrating how electric fields in this region should be explicitly considered when characterising the thermodynamics of both solute and solvent at the solid/liquid interface.

Characterizing Surface Ice-Philicity Using Molecular Simulations and Enhanced Sampling

The formation of ice, which plays an important role in diverse contexts ranging from cryopreservation to atmospheric science, is often mediated by solid surfaces. Although surfaces that interact favorably with ice (relative to liquid water) can facilitate ice formation by lowering nucleation barriers, the molecular characteristics that confer icephilicity to a surface are complex and incompletely understood. To address this challenge, here we introduce a robust and computationally efficient method for characterizing surface ice-philicity that combines molecular simulations and enhanced sampling techniques to quantify the free energetic cost of increasing surface-ice contact at the expense of surface-water contact. Using this method to characterize the ice-philicity of a family of model surfaces that are lattice matched with ice but vary in their polarity, we find that the nonpolar surfaces are moderately ice-phobic, whereas the polar surfaces are highly ice-philic. In contrast, for surfaces that display no complementarity to the ice lattice, we find that ice-philicity is independent of surface polarity and that both nonpolar and polar surfaces are moderately ice-phobic. Our work thus provides a prescription for quantitatively characterizing surface ice-philicity and sheds light on how ice-philicity is influenced by lattice matching and polarity.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

RSeeds: Rigid Seeding Method for Studying Heterogeneous Crystal Nucleation

Heterogeneous nucleation is the dominant form of liquid-to-solid transition in nature. Although molecular simulations are most uniquely suited to studying nucleation, the waiting time to observe even a single nucleation event can easily exceed the current computational capabilities. Therefore, there exists an imminent need for methods that enable computationally fast and feasible studies of heterogeneous nucleation. Seeding is a technique that has proven to be successful at dramatically expanding the range of computationally accessible nucleation rates in simulation studies of homogeneous crystal nucleation. In this article, we introduce a new seeding method for heterogeneous nucleation called Rigid Seeding (RSeeds). Crystalline seeds are treated as pseudorigid bodies and simulated on a surface with metastable liquid above its melting temperature. This allows the seeds to adapt to the surface and identify favorable seed-surface configurations, which is necessary for reliable predictions of crystal polymorphs that form and the corresponding heterogeneous nucleation rates. We demonstrate and validate RSeeds for heterogeneous ice nucleation on a flexible self-assembled monolayer surface, a mineral surface based on kaolinite, and two model surfaces. RSeeds predicts the correct ice polymorph, exposed crystal plane, and rotation on the surface. RSeeds is semiquantitative and can be used to estimate the critical nucleus size and nucleation rate when combined with classical nucleation theory. We demonstrate that RSeeds can be used to evaluate nucleation rates spanning many orders of magnitude.

Heterogeneous Ice Nucleation Studied with Single-Layer Graphene

Control of heterogeneous ice nucleation (HIN) is critical for applications that range from iceophobic surfaces to ice-templated materials. HIN on 2D materials is a particular interesting topic that still lacks extensive experimental investigations. Here, we focus on the HIN on single-layer graphene (SLG) transferred onto different substrates, including silicon, silica, and thermal oxide on silicon. Complemented by other samples without SLG, we obtain a large range of wetting contact angles (WCAs) from 2° to 95°. All pristine SLG samples exhibit a large contact angle of ∼95°, which is close to the theoretical value of 96° for free-standing SLG, irrespective of the substrate and even in the presence of nanoscale wrinkles on SLG, which are due to the transfer process, indicating that the topographical features have little impact on the wetting behavior. Interestingly, SLG displays changes in hydrophobicity upon repeated water droplet freezing-melting-drying cycles due to a shift in Fermi level and/or enhanced water-substrate polar molecular interactions, likely induced by residual adsorption of H₂O molecules. We found that a 0.04 eV decrease in SLG Fermi level reduces the SLG/water interface energy by ∼6 mJ/m², thereby making SLG less hydrophobic. Counterintuitively, the reduction in SLG/water interface energy and the enhanced hydrophilicity after repeated freezing-melting-evaporation cycles actually decreases the freezing temperature by ∼3-4 °C, thereby slightly retarding rather than enhancing HIN. We also found that the water droplet freezing temperature differed by only ∼1 °C on different substrates with WCAs from 2° to 95°, an intriguing and yet reasonable result that confirms that wettability alone is not a good indicator of HIN capability. The HIN rate is rather determined by the difference between substrate/water and substrate/ice interface energies, which was found to stay almost constant for substrates weakly interacting with water/ice via van der Waals or hydrogen bonds, irrespective of hydrophilicity.

Influences of 1-Propanol and Methanol Additives on Crystallization of Thin Amorphous Solid Water Films

In general, randomly oriented ice crystallites are formed by heating amorphous solid water (ASW) films at ∼160 K via homogeneous nucleation. Here, we demonstrate that monolayers of methanol and 1-propanol additives incorporated in the multilayer ASW film lead to heterogeneous nucleation at the substrate interface of Pt(111), as evidenced by the occurrence of epitaxial ice growth. The mobility of water in direct contact with the Pt(111) substrate is decreased relative to that in the bulk, but it can be increased via interactions with hydrophobic moieties of alcohols that are segregated to the interfacial region. As a result, heterogeneous nucleation occurs at ca. 160 K along with homogeneous nucleation in the film interior. However, the template effect is quenched when the alcohols are in direct contact with the substrate. The methanol adspecies deposited onto the ASW film surface induces heterogeneous nucleation at a temperature as low as 145 K, but the 1-propanol adspecies has no such an effect. Their different ability of heterogeneous nucleation at the free ASW film surface, as well as their uptake behaviors in the near surface region, is associated with the hydrophobic hydration of the alcohols resulting from different lengths of the aliphatic moiety.

Silica as a Model Ice-Nucleating Particle to Study the Effects of Crystallinity, Porosity, and Low-Density Surface Functional Groups on Immersion Freezing

Aerosol particles can facilitate heterogeneous ice formation in the troposphere and stratosphere by acting as ice-nucleating particles, modulating cloud formation/dissipation, precipitation, and their microphysical properties. Heterogeneous ice nucleation is driven by ice embryo formation on the particle surface, which can be influenced by features of the surface such as crystallinity, surface structure, lattice structure, defects, and functional groups. To characterize the effect of crystallinity, pores, and surface functional groups toward ice nucleation, samples of comparable silica systems, specifically, quartz, ordered and nonordered porous amorphous silica samples with a range of pore sizes (2-11 nm), and nonporous functionalized silica spheres, were used as models for mineral dust aerosol particles. The ice nucleation activity of these samples was investigated by using an immersion freezing chamber. The results suggest that crystallinity has a larger effect than porosity on ice nucleation activity, as all of the porous silica samples investigated had lower onset freezing temperatures and lower ice nucleation activities than quartz. Our findings also suggest that pores alone are not sufficient to serve as effective active sites and need some additional chemical or physical property, like crystallinity, to nucleate ice in immersion mode freezing. The addition of a low density of organic functional groups to nonporous samples showed little enhancement compared to the inherent nucleation activity of silica with native surface hydroxyl groups. The density of functional groups investigated in this work suggests that a different arrangement of surface groups may be needed for enhanced immersion mode ice nucleation activity. In summary, crystallinity dictates the ice nucleation activity of silica samples rather than porosity or low-density surface functional groups. This work has broader implications regarding the climate impacts resulting from ice cloud formation.

Chemical Nature of Heterogeneous Electrofreezing of Supercooled Water Revealed on Polar (Pyroelectric) Surfaces

The ability to control the icing temperature of supercooled water (SCW) is of supreme importance in subfields of pure and applied sciences. The ice freezing of SCW can be influenced heterogeneously by electric effects, a process known as electrofreezing. This effect was first discovered during the 19th century; however, its mechanism is still under debate. In this Account we demonstrate, by capitalizing on the properties of polar crystals, that heterogeneous electrofreezing of SCW is a chemical process influenced by an electric field and specific ions. Polar crystals possess a net dipole moment. In addition, they are pyroelectric, displaying short-lived surface charges at their hemihedral faces at the two poles of the crystals as a result of temperature changes. Accordingly, during cooling or heating, an electric field is created, which is negated by the attraction of compensating charges from the environment. This process had an impact in the following experiments. The icing temperatures of SCW within crevices of polar crystals are higher in comparison to icing temperatures within crevices of nonpolar analogs. The role played by the electric effect was extricated from other effects by the performance of icing experiments on the surfaces of pyroelectric quasi-amorphous SrTiO₃. During those studies it was found that on positively charged surfaces the icing temperature of SCW is elevated, whereas on negatively charged surfaces it is reduced. Following investigations discovered that the icing temperature of SCW is impacted by an ionic current created within a hydrated layer on top of hydrophilic faces residing parallel to the polar axes of the crystals. In the absence of such current on analogous hydrophobic surfaces, the pyroelectric effect does not influence the icing temperature of SCW. Those results implied that electrofreezing of SCW is a process influenced by specific compensating ions attracted by the pyroelectric field from the aqueous solution. When freezing experiments are performed in an open atmosphere, bicarbonate and hydronium ions, created by the dissolution of atmospheric CO₂ in water, influence the icing temperature. The bicarbonate ions, when attracted by positively charged pyroelectric surfaces, elevate the icing temperature, whereas their counterparts, hydronium ions, when attracted by the negatively charged surfaces reduce the icing temperature. Molecular dynamic simulations suggested that bicarbonate ions, concentrated within the near positively charged interfacial layer, self-assemble with water molecules to create stabilized slightly distorted “ice-like” hexagonal assemblies which mimic the hexagons of the crystals of ice. This occurs by replacing, within those ice-like hexagons, two hydrogen bonds of water by C–O bonds of the HCO₃^– ion. On the basis of these simulations, it was predicted and experimentally confirmed that other trigonal planar ions such as NO₃^–, guanidinium⁺, and the quasi-hexagonal biguanidinium⁺ ion elevate the icing temperature. These ions were coined as “ice makers”. Other ions including hydronium, Cl^–, and SO₄^–2 interfere with the formation of ice-like assemblies and operate as “ice breakers”. The higher icing temperatures induced within the crevices of the hydrophobic polar crystals in comparison to the nonpolar analogs can be attributed to the proton ordering of the water molecules. In contrast, the icing temperatures on related hydrophilic surfaces are influenced both by compensating charges and by proton ordering.

Impact of surface nanostructure and wettability on interfacial ice physics

Ice accumulation on solid surfaces is a severe problem for safety and functioning of a large variety of engineering systems, and its control is an enormous challenge that influences the safety and reliability of many technological applications. The use of molecular dynamics (MD) simulations is popular, but as ice nucleation is a rare event when compared to simulation timescales, the simulations need to be accelerated to force ice to form on a surface, which affects the accuracy and/or applicability of the results obtained. Here, we present an alternative seeded MD simulation approach, which reduces the computational cost while still ensuring accurate simulations of ice growth on surfaces. In addition, this approach enables, for the first time, brute-force all-atom water simulations of ice growth on surfaces unfavorable for nucleation within MD timescales. Using this approach, we investigate the effect of surface wettability and structure on ice growth in the crucial surface-ice interfacial region. Our main findings are that the surface structure can induce a flat or buckled overlayer to form within the liquid, and this transition is mediated by surface wettability. The first overlayer and the bulk ice compete to structure the intermediate water layers between them, the relative influence of which is traced using density heat maps and diffusivity measurements. This work provides new understanding on the role of the surface properties on the structure and dynamics of ice growth, and we also present a useful framework for future research on surface icing simulations.

Effect of interfacial dipole on heterogeneous ice nucleation

In this work, we performed molecular dynamics simulations of ice nucleation on a rigid surface model of cubic zinc blende structure with different surface dipole strength and orientation. Our results show that, although substrates are excellently lattice-matched to cubic ice, ice nucleation merely occurred as the interfacial water molecules (IWs) show identical or similar orientations to that of water molecules in cubic ice. Free energy landscapes revealed that, as substrates have non-suitable dipole strength/orientation, there exist large free energy barriers for rotating dipole IWs to the right orientation to trigger ice formation. This study stresses that, beyond the traditional view of lattice match and the similarity of lattice length between the substrate and new-formed crystal, the similarity between molecular orientations of interfacial component and component in the specific new-formed crystalline face is also critical for promoting ice nucleation.

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

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

Role of Nanoscale Interfacial Proximity in Contact Freezing in Water

Contact freezing is a mode of atmospheric ice nucleation in which a collision between a dry ice nucleating particle (INP) and a water droplet results in considerably faster heterogeneous nucleation. The molecular mechanism of such an enhancement is, however, still a mystery. While earlier studies had attributed it to collision-induced transient perturbations, recent experiments point to the pivotal role of nanoscale proximity of the INP and the free interface. By simulating the heterogeneous nucleation of ice within INP-supported nanofilms of two model water-like tetrahedral liquids, we demonstrate that such nanoscale proximity is sufficient for inducing rate increases commensurate with those observed in contact freezing experiments, but only if the free interface has a tendency to enhance homogeneous nucleation. Water is suspected of possessing this latter property, known as surface freezing propensity. Our findings therefore establish a connection between the surface freezing propensity and kinetic enhancement during contact nucleation. We also observe that faster nucleation proceeds through a mechanism markedly distinct from classical heterogeneous nucleation, involving the formation of hourglass-shaped crystalline nuclei that conceive at either interface and that have a lower free energy of formation due to the nanoscale proximity of the interfaces and the modulation of the free interfacial structure by the INP. In addition to providing valuable insights into the physics of contact nucleation, our findings can assist in improving the accuracy of heterogeneous nucleation rate measurements in experiments and in advancing our understanding of ice nucleation on nonuniform surfaces such as organic, polymeric, and biological materials.

Macroscopic surface charges from microscopic simulations

Attaining accurate average structural properties in a molecular simulation should be considered a prerequisite if one aims to elicit meaningful insights into a system's behavior. For charged surfaces in contact with an electrolyte solution, an obvious example is the density profile of ions along the direction normal to the surface. Here, we demonstrate that, in the slab geometry typically used in simulations, imposing an electric displacement field D determines the integrated surface charge density of adsorbed ions at charged interfaces. This allows us to obtain macroscopic surface charge densities irrespective of the slab thickness used in our simulations. We also show that the commonly used Yeh-Berkowitz method and the "mirrored slab" geometry both impose vanishing integrated surface charge densities. We present results both for relatively simple rocksalt (1 1 1) interfaces and the more complex case of kaolinite's basal faces in contact with an aqueous electrolyte solution.

Multivalent Surface Cations Enhance Heterogeneous Freezing of Water on Muscovite Mica

Heterogeneous ice nucleation is a crucial phenomenon in various fields of fundamental and applied science. We investigate the effect of surface cations on freezing of water on muscovite mica. Mica is unique in that the exposed ion on its surface can be readily and easily exchanged without affecting other properties such as surface roughness. We investigate freezing on natural (K⁺) mica and mica in which we have exchanged K⁺ for Al³⁺, Mg²⁺, Ca²⁺, and Sr²⁺. We find that liquid water freezes at higher temperatures when ions of higher valency are present on the surface, thus exposing more of the underlying silica layer. Our data also show that the size of the ion affects the characteristic freezing temperature. Using molecular dynamics simulations, we investigate the effects that the ion valency and exposed silica layer have on the behavior of water on the surface. The results indicate that multivalent cations enhance the probability of forming large clusters of hydrogen bonded water molecules that are anchored by the hydration shells of the cations. These clusters also have a large fraction of free water that can reorient to take ice-like configurations, which are promoted by the regions on mica devoid of the ions. Thus, these clusters could serve as seedbeds for ice nuclei. The combined experimental and simulation studies shed new light on the influence of surface ions on heterogeneous ice nucleation.

Use of Ion Exchange To Regulate the Heterogeneous Ice Nucleation Efficiency of Mica

Heterogeneous ice nucleation (HIN) triggered by mineral surfaces typically exposed to various ions can have a significant impact on the regional atmosphere and climate. However, the dependence of HIN on the nature of the mineral surface ions is still largely unexplored due to the complexity of mineral surfaces. Because K⁺ on the atomically flat (001) surface of mica can be readily replaced by different cations through ion exchange, muscovite mica was selected; its simple nature provides a very straightforward system that can serve as the model for investigating the effects of mineral surface ions on HIN. Our experiments show that the surface (001) of H⁺-exchanged mica displays markedly higher HIN efficiencies than that of Na-/K-mica. Vibrational sum-frequency generation spectroscopy reveals that H-mica induces substantially less orientation ordering than Na-/K-mica within the contact water layer at the interface. Molecular dynamics simulations suggest that the HIN efficiency of mica depends on the positional arrangement and orientation of the interfacial water. The formation of the hexagonal ice Ih basal-type structure in the first water layer atop the mica surface facilitates HIN, which is determined by the size of the protruding ions atop the mica surface and by the surface adsorption energy. The orientational distribution is optimal for HIN when 25% of the water molecules in the first water layer atop the mica surface have one OH group pointing up and 25% have one OH group pointing down, which, in turn, is determined by the surface charge distribution.

Nucleation and growth of water ice on oxide surfaces: the influence of a precursor to water dissociation

We have investigated how nucleation and growth processes of ice are influenced by interfacial molecular interactions on some oxide surfaces, such as rutile TiO2(110), TiO2(100), MgO(100), and Al2O3(0001), based on the diffraction patterns of electrons transmitted through ice crystallites under the experimental configuration of reflection high energy electron diffraction (RHEED). The cubic ice Ic grows on the TiO2(110) surface with the epitaxial relationship of (110)Ic//(110)TiO2 and [001]Ic//[11[combining macron]0]TiO2. The epitaxial ice growth tends to be disturbed on the TiO2(110) surface under the presence of oxygen vacancies and adatoms. The result is not simply ascribable to small misfit values between TiO2 and ice Ic lattices (∼2%) because ice grains are formed randomly on TiO2(100). No template effects are identified during ice nucleation on the pristine MgO(100) and Al2O3(0001) surfaces either. The water molecules are chemisorbed weakly on these surfaces as a precursor to dissociation via the acid-base interaction. Such anchored water species act as an inhibitor of epitaxial ice growth because the orientation flexibility of physisorbed water during nucleation is hampered at the interface by the preferential formation of hydrogen bonds.

Predicting heterogeneous ice nucleation with a data-driven approach

Water in nature predominantly freezes with the help of foreign materials through a process known as heterogeneous ice nucleation. Although this effect was exploited more than seven decades ago in Vonnegut's pioneering cloud seeding experiments, it remains unclear what makes a material a good ice former. Here, we show through a machine learning analysis of nucleation simulations on a database of diverse model substrates that a set of physical descriptors for heterogeneous ice nucleation can be identified. Our results reveal that, beyond Vonnegut's connection with the lattice match to ice, three new microscopic factors help to predict the ice nucleating ability. These are: local ordering induced in liquid water, density reduction of liquid water near the surface and corrugation of the adsorption energy landscape felt by water. With this we take a step towards quantitative understanding of heterogeneous ice nucleation and the in silico design of materials to control ice formation.

Heterogeneous Electrofreezing Triggered by CO2 on Pyroelectric Crystals: Qualitatively Different Icing on Hydrophilic and Hydrophobic Surfaces

By performing icing experiments on hydrophilic and hydrophobic surfaces of pyroelectric amino acids and on the x-cut faces of LiTaO₃ , we discovered that the effect of electrofreezing of super cooled water is triggered by ions of carbonic acid. During the cooling of the hydrophilic pyroelectric crystals, a continuous water layer is created between the charged hemihedral faces, as confirmed by impedance measurements. As a result, a current of carbonic acid ions, produced by dissolved environmental CO₂ , flows through the wetted layer towards the hemihedral faces and elevates the icing temperature. This proposed mechanism is based on the following: (i) on hydrophilic surfaces, water with dissolved CO₂ (pH 4) freezes at higher temperatures than pure water of pH 7. (ii) In the absence of the ionic current, achieved by linking the two hemihedral faces of hydrophilic crystals by a conductive paint, water of the two pH levels freeze at the same temperature. (iii) On hydrophobic crystals with similar pyroelectric coefficients, where there is no continuous wetted layer, no electrofreezing effect is observed.

Heterogeneous Electrofreezing of Super-Cooled Water on Surfaces of Pyroelectric Crystals is Triggered by Trigonal Planar Ions

Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO₃ and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO₃ ^- ) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H₃ O⁺ ) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO₃ ^- ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO₃ ^- and guanidinium (Gdm⁺ ), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl^- and SO₄ ^2- ions of different configurations reduce the icing temperature.

Effects of Inorganic Ions on Ice Nucleation by the Al Surface of Kaolinite Immersed in Water

Molecular dynamics simulations are employed to investigate the influence of inorganic salts on ice nucleation by the Al surface of kaolinite, terminated with hydroxyl groups. Seven salt solutions (LiI(Cl), NaI(Cl), KI(Cl), and NH₄I) are considered. Simulations were performed at 300 K to obtain equilibrium surface-ion and surface-water density profiles. These simulations show no specific ion adsorption at the kaolinite surface. There are weak surface-ion correlations, with cations preferring to be closer to the surface than the anions. At a supercooling of 26 K (taking account of freezing point depression), 1 M salt solutions slowed ice nucleation by a factor of 2-3 compared with pure water and significantly reduced the rate of ice growth after nucleation. All salt solutions had similar influences on ice nucleation, and no specific ion effects were identified. Ice nucleation simulations for 1 M NaI(Cl), KI(Cl), and LiI solutions were performed for a range of temperatures. In all cases, the supercooling required for ice nucleation was larger by ∼1-6 K, after accounting for freezing point depression, than that required for pure water. For 1 M LiI solution an earlier laboratory study using kaolin as ice nucleating particles (INP) reported that the supercooling required for ice nucleation was ∼11 K smaller than that required for pure water. Our simulation results are not consistent with this finding. In this paper, we report new laboratory results for 1 M LiI solution employing kaolinite as INP. In our experiments ice nucleation in the LiI solution required the same supercooling as pure water, which is more consistent with our simulations.

A generalized deep learning approach for local structure identification in molecular simulations

Identifying local structure in molecular simulations is of utmost importance. The most common existing approach to identify local structure is to calculate some geometrical quantity referred to as an order parameter. In simple cases order parameters are physically intuitive and trivial to develop (e.g., ion-pair distance), however in most cases, order parameter development becomes a much more difficult endeavor (e.g., crystal structure identification). Using ideas from computer vision, we adapt a specific type of neural network called a PointNet to identify local structural environments in molecular simulations. A primary challenge in applying machine learning techniques to simulation is selecting the appropriate input features. This challenge is system-specific and requires significant human input and intuition. In contrast, our approach is a generic framework that requires no system-specific feature engineering and operates on the raw output of the simulations, i.e., atomic positions. We demonstrate the method on crystal structure identification in Lennard-Jones (four different phases), water (eight different phases), and mesophase (six different phases) systems. The method achieves as high as 99.5% accuracy in crystal structure identification. The method is applicable to heterogeneous nucleation and it can even predict the crystal phases of atoms near external interfaces. We demonstrate the versatility of our approach by using our method to identify surface hydrophobicity based solely upon positions and orientations of surrounding water molecules. Our results suggest the approach will be broadly applicable to many types of local structure in simulations.

Stabilization of AgI's polar surfaces by the aqueous environment, and its implications for ice formation

AgI is a potent inorganic ice nucleating particle, a feature often attributed to the lattice match between its {0001} surfaces and ice. Dissolved ions are found to be essential to the stability of these polar surfaces, and crucial to ice formation.

Heterogeneous Ice Nucleation: Interplay of Surface Properties and Their Impact on Water Orientations

Ice is ubiquitous in nature, and heterogeneous ice nucleation is the most common pathway of ice formation. How surface properties affect the propensity to observe ice nucleation on that surface remains an open question. We present results of molecular dynamics studies of heterogeneous ice nucleation on model surfaces. The models surfaces considered emulate the chemistry of kaolinite, an abundant component of mineral dust. We investigate the interplay of surface lattice and hydrogen bonding properties in affecting ice nucleation. We find that lattice matching and hydrogen bonding are necessary but not sufficient conditions for observing ice nucleation at these surfaces. We correlate this behavior to the orientations sampled by the metastable supercooled water in contact with the surfaces. We find that ice is observed in cases where water molecules not only sample orientations favorable for bilayer formation but also do not sample unfavorable orientations. This distribution depends on both surface-water and water-water interactions and can change with subtle modifications to the surface properties. Our results provide insights into the diverse behavior of ice nucleation observed at different surfaces and highlight the complexity in elucidating heterogeneous ice nucleation.

Fundamental interfacial mechanisms underlying electrofreezing

This article reviews the fundamental interfacial mechanisms underlying electrofreezing (promotion of ice nucleation via the application of an electric field). Electrofreezing has been an active research topic for many decades, with applications in food preservation, cryopreservation, cryogenics and ice formation. There is substantial literature detailing experimental and simulations-based studies, which aim to understand the complex mechanisms underlying accelerated ice nucleation in the presence of electric fields and electrical charge. This work provides a critical review of all such studies. It is noted that application-focused studies of electrofreezing are excluded from this review; such studies have been previously reviewed in literature. This review focuses only on fundamental studies, which analyze the physical mechanisms underlying electrofreezing. Topics reviewed include experimental studies on electrofreezing (DC and AC electric fields), pyroelectricity-based control of freezing, molecular dynamics simulations of electrofreezing, and thermodynamics-based explanations of electrofreezing. Overall, it is seen that electrofreezing can enable disruptive advancements in the control of liquid-to-solid phase change, and that our current understanding of the underlying mechanisms can be significantly improved through further studies of various interfacial effects coming into play.

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.