Vapor deposition of water on graphitic surfaces: Formation of amorphous ice, bilayer ice, ice I, and liquid water

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.

Partition Constant of Binary Mixtures for the Equilibrium between a Bulk and a Confined Phase

It is well-known that the thermodynamic, kinetic and structural properties of fluids, and in particular of water and its solutions, can be drastically affected in nanospaces. A possible consequence of nanoscale confinement of a solution is the partial segregation of its components. Thereby, confinement in nanoporous materials (NPM) has been proposed as a means for the separation of mixtures. In fact, separation science can take great advantage of NPM due to the tunability of their properties as a function of nanostructure, morphology, pore size, and surface chemistry. Alcohol-water mixtures are in this context among the most relevant systems. However, a quantitative thermodynamic description allowing for the prediction of the segregation capabilities as a function of the material-solution characteristics is missing. In the present study we attempt to fill this vacancy, by contributing a thermodynamic treatment for the calculation of the partition coefficient in confinement. Combining the multilayer adsorption model for binary mixtures with the Young equation, we conclude that the liquid-vapor surface tension and the contact angle of the pure substances can be used to predict the separation ability of a particular material for a given mixture to a semiquantitative extent. Moreover, we develop a Kelvin-type equation that relates the partition coefficient to the radius of the pore, the contact angle, and the liquid-vapor surface tensions of the constituents. To assess the validity of our thermodynamic formulation, coarse grained molecular dynamics simulations were performed on models of alcohol-water mixtures confined in cylindrical pores. To this end, a coarse-grained amphiphilic molecule was parametrized to be used in conjunction with the mW potential for water. This amphiphilic model reproduces some of the properties of methanol such as enthalpy of vaporization and liquid-vapor surface tension, and the minimum of the excess enthalpy for the aqueous solution. The partition coefficient turns out to be highly dependent on the molar fraction, on the interaction between the components and the confining matrix, and on the radius of the pore. A remarkable agreement between the theory and the simulations is found for pores of radius larger than 15 Å.

Robustness of Bilayer Hexagonal Ice against Surface Symmetry and Corrugation

Two-dimensional (2D) bilayer hexagonal ice (BHI) is regarded as the first intrinsic 2D ice crystal. However, the robustness of such a structure or its derivatives against surface symmetry and corrugation is still unclear. Here, we report the formation of 2D BHI on gold surfaces with 1D corrugation, using noncontact atomic force microscopy. The hexagonal arrangement of the first wetting layer was visualized on the Au(110)-1×2 surface. Upon depositing more water molecules, the first layer would rearrange and shrink, resulting in the formation of buckled BHI. Such a buckled BHI is hydrophobic despite the appearance of dangling OH, due to the strong interlayer bonding. Furthermore, the BHI is also stable on the Au(100)-5×28 surface. This work reveals the unexpected generality of the BHI on corrugated surfaces with nonhexagonal symmetry, thus shedding new light on the microscopic understandings of the low-dimensional ice formation on solid surfaces or under confinement.

Submolecular Insights into Interfacial Water by Hydrogen-Sensitive Scanning Probe Microscopy

ConspectusWater-solid interfaces have attracted extensive attention because of their crucial roles in a wide range of chemical and physical processes, such as ice nucleation and growth, dissolution, corrosion, heterogeneous catalysis, and electrochemistry. To understand these processes, enormous efforts have been made to obtain a molecular-level understanding of the structure and dynamics of water on various solid surfaces. By the use of scanning probe microscopy (SPM), many remarkable structures of H-bonding networks have been directly visualized, significantly advancing our understanding of the delicate competition between water-water and water-solid interactions. Moreover, the detailed dynamics of water molecules, such as diffusion, clustering, dissociation, and intermolecular and intramolecular proton transfer, have been investigated in a well-controlled manner by tip manipulation. However, resolving the submolecular structure of surface water has remained a great challenge for a long time because of the small size and light mass of protons. Discerning the position of hydrogen in water is not only crucial for the accurate determination of the structure of H-bonding networks but also indispensable in probing the proton transfer dynamics and the quantum nature of protons.In this Account, we focus on the recent advances in the H-sensitive SPM technique and its applications in probing the structures, dynamics, and nuclear quantum effects (NQEs) of surface water and ion hydrates at the submolecular level. First, we introduce the development of high-resolution scanning tunneling microscopy/spectroscopy (STM/S) and qPlus-based atomic force microscopy (qPlus-AFM), which allow access to the degrees of freedom of protons in both real and energy space. qPlus-AFM even allows imaging of interfacial water in a weakly perturbative manner by measuring the high-order electrostatic force between the CO-terminated tip and the polar water molecule, which enables the subtle difference of OH directionality to be discerned. Next we showcase the applications of H-sensitive STM/AFM in addressing several key issues related to water-solid interfaces. The surface wetting behavior and the H-bonding structure of low-dimensional ice on various hydrophilic and hydrophobic solid surfaces are characterized at the atomic scale. Then we discuss the quantitative assessment of NQEs of surface water, including proton tunneling and quantum delocalization. Moreover, the weakly perturbative and H-sensitive SPM technique can be also extended to investigations of water-ion interactions on solid surfaces, revealing the effect of hydration structure on the interfacial ion transport. Finally, we provide an outlook on the further directions and challenges for SPM studies of water-solid interfaces.

Patterning Configuration of Surface Hydrophilicity by Graphene Nanosheet towards the Inhibition of Ice Nucleation and Growth

Freezing of liquid water occurs in many natural phenomena and affects countless human activities. The freezing process mainly involves ice nucleation and continuous growth, which are determined by the energy and structure fluctuation in supercooled water. Herein, considering the surface hydrophilicity and crystal structure differences between metal and graphene, we proposed a kind of surface configuration design, which was realized by graphene nanosheets being alternately anchored on a metal substrate. Ice nucleation and growth were investigated by molecular dynamics simulations. The surface configuration could induce ice nucleation to occur preferentially on the metal substrate where the surface hydrophilicity was higher than the lateral graphene nanosheet. However, ice nucleation could be delayed to a certain extent under the hindering effect of the interfacial water layer formed by the high surface hydrophilicity of the metal substrate. Furthermore, the graphene nanosheets restricted lateral expansion of the ice nucleus at the clearance, leading to the formation of a curved surface of the ice nucleus as it grew. As a result, ice growth was suppressed effectively due to the Gibbs–Thomson effect, and the growth rate decreased by 71.08% compared to the pure metal surface. Meanwhile, boundary misorientation between ice crystals was an important issue, which also prejudiced the growth of the ice crystal. The present results reveal the microscopic details of ice nucleation and growth inhibition of the special surface configuration and provide guidelines for the rational design of an anti-icing surface.

Advances in Atomic Force Microscopy: Imaging of Two- and Three-Dimensional Interfacial Water

Interfacial water is closely related to many core scientific and technological issues, covering a broad range of fields, such as material science, geochemistry, electrochemistry and biology. The understanding of the structure and dynamics of interfacial water is the basis of dealing with a series of issues in science and technology. In recent years, atomic force microscopy (AFM) with ultrahigh resolution has become a very powerful option for the understanding of the complex structural and dynamic properties of interfacial water on solid surfaces. In this perspective, we provide an overview of the application of AFM in the study of two dimensional (2D) or three dimensional (3D) interfacial water, and present the prospect and challenges of the AFM-related techniques in experiments and simulations, in order to gain a better understanding of the physicochemical properties of interfacial water.

Effect of Graphene on Ice Polymorph

Recently, ice with stacking disorder structure, consisting of random sequences of cubic ice (Ic) and hexagonal ice (Ih) layers, was reported to be more stable than pure Ih/Ic. Due to a much lower free energy barrier of heterogeneous nucleation, in practice, the freezing process of water is controlled by heterogeneous nucleation triggered by an external medium. Therefore, we carry out molecular dynamic simulations to explore how ice polymorphism depends on the lattice structure of the crystalline substrates on which the ice is grown, focusing on the primary source of atmospheric aerosols, carbon materials. It turns out that, during the nucleation stage, the polymorph of ice nuclei is strongly affected by graphene substrates. For ice nucleation on graphene, we find Ih is the dominant polymorph. This can be attributed to structural similarities between graphene and basal face of Ih. Our results also suggest that the substrate only affects the polymorph of ice close to the graphene surface, with the preference for Ih diminishing as the ice layer grows.

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.

Advances in Atomic Force Microscopy: Weakly Perturbative Imaging of the Interfacial Water

The structure and dynamics of interfacial water, determined by the water-interface interactions, are important for a wide range of applied fields and natural processes, such as water diffusion (Kim et al., 2013), electrochemistry (Markovic, 2013), heterogeneous catalysis (Over et al., 2000), and lubrication (Zilibotti et al., 2013). The precise understanding of water-interface interactions largely relies on the development of atomic-scale experimental techniques (Guo et al., 2014) and computational methods (Hapala et al., 2014b). Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields (Ichii et al., 2012; Shiotari and Sugimoto, 2017; Peng et al., 2018a). In this perspective, we review the recent progress in the noncontact atomic force microscopy (nc-AFM) imaging and AFM simulation techniques and discuss how the newly developed techniques are applied to study the properties of interfacial water. The nc-AFM with the quadrupole-like CO-terminated tip can achieve ultrahigh-resolution imaging of the interfacial water on different surfaces, trace the reconstruction of H-bonding network and determine the intrinsic structures of the weakly bonded water clusters and even their metastable states. In the end, we present an outlook on the directions of future AFM studies of interfacial water as well as the challenges faced by this field.

In Situ Study of Molecular Structure of Water and Ice Entrapped in Graphene Nanovessels

Water is ubiquitous in natural systems, ranging from the vast oceans to the nanocapillaries in the earth crust or cellular organelles. In bulk or in intimate contact with solid surfaces, water molecules arrange themselves according to their hydrogen (H) bonding, which critically affects their short- and long-range molecular structures. Formation of H-bonds among water molecules designates the energy levels of certain nonbonding molecular orbitals of water, which are quantifiable by spectroscopic techniques. While the molecular architecture of water in nanoenclosures is of particular interest to both science and industry, it requires fine spectroscopic probes with nanometer spatial resolution and sub-eV energy sensitivity. Graphene liquid cells (GLCs), which feature opposing closely spaced sheets of hydrophobic graphene, facilitate high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) measurements of attoliter water volumes encapsulated tightly in the GLC nanovessels. We perform in situ TEM and EELS analysis of water encased in thin GLCs exposed to room and cryogenic temperatures to examine the nanoscale arrangement of the contained water molecules. Simultaneous quantification of GLC thickness leads to the conclusion that H-bonding strengthens under increased water confinement. The present results demonstrate the feasibility of nanoscale chemical characterization of aqueous fluids trapped in GLC nanovessels and offer insights on water molecule arrangement under high-confinement conditions.

Hexagonal Monolayer Ice without Shared Edges

When adsorbed on solids, water molecules are usually arranged into a honeycomb hydrogen-bond network. Here we report the discovery of a novel monolayer ice built exclusively from water hexamers but without shared edges, distinct from all conventional ice phases. Water grown on graphite crystalizes into a robust monolayer ice after annealing, attaining an exceedingly high density of 0.134  Å^{-2}. Unlike chemisorbed ice on metal surfaces, the ice monolayer can translate and rotate on graphite terraces and grow across steps, confirming its two-dimensional nature. First-principles calculations identify the monolayer ice structure as a robust self-assembly of closely packed water hexamers without edge sharing, whose stability is maintained by maximizing the number of intralayer hydrogen bonds on inert surfaces.

Dynamical insights into the mechanism of a droplet detachment from a fiber

Quantifying the detachment behavior of a droplet from a fiber is important in many applications such as fog harvesting, oil-water separation, or water management in fuel cells. When the droplets are forcibly removed from hydrophilic fibers, the ease of detachment strongly depends on droplet volume and the rate of the process controlled by the applied force. Experiments, conducted on a ferrofluid under magnetic force, as well as continuum level calculations from fluid mechanics have so far been unable to resolve the time-dependent dynamics of droplet detachment and, most importantly, to assess the role of the applied force as the key determinant of the volume of the droplet residue remaining on the fiber after detachment. In the present work, we study the mechanism of water droplet detachment and retention of residual water on smooth hydrophilic fibers using nonequilibrium molecular dynamics simulations. We investigate how the applied force affects the breakup of a droplet and how the minimal detaching force per unit mass decreases with droplet size. We extract scaling relations that allow extrapolation of our findings to larger length scales that are not directly accessible by molecular models. We find that the volume of the residue on a fiber varies nonmonotonically with the detaching force, reaching the maximal size at an intermediate force and associated detachment time. The strength of this force decreases with the size of the drop, while the maximal residue increases with the droplet volume, V, sub-linearly, in proportion to the V2/3.

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.

What Controls the Limit of Supercooling and Superheating of Pinned Ice Surfaces?

Cold-adapted organisms produce antifreeze proteins and glycoproteins to control the growth, melting and recrystallization of ice. It has been proposed that these molecules pin the crystal surface, creating a curvature that arrests the growth and melting of the crystal. Here we use thermodynamic modeling and molecular simulations to demonstrate that the curvature of the superheated or supercooled surface depends on the temperature and distances between ice-binding molecules, but not the details of their interactions with ice. We perform simulations of ice pinned with the antifreeze protein TmAFP, polyvinyl alcohol with different degrees of polymerization, and model ice-binding molecules to determine the thermal hystereses on melting and freezing, i.e. the maximum curvature that can be attained before, respectively, ice melts or grows irreversibly over the ice-binding molecules. We find that the thermal hysteresis is controlled by the bulkiness of the ice-binding molecules and their footprint at the ice surface. We elucidate the origin of the asymmetry between freezing and melting hysteresis found in experiments and propose guidelines to design synthetic antifreeze molecules with potent thermal hysteresis activity.

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

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

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.

Hydrogen-Bond Heterogeneity Boosts Hydrophobicity of Solid Interfaces

Experimental and theoretical studies suggest that the hydrophobicity of chemically heterogeneous surfaces may present important nonlinearities as a function of composition. In this article, this issue is systematically explored using molecular simulations. The hydrophobicity is characterized by computing the contact angle of water on flat interfaces and the desorption pressure of water from cylindrical nanopores. The studied interfaces are binary mixtures of hydrophilic and hydrophobic sites, with and without the ability to form hydrogen bonds with water, intercalated at different scales. Water is described with the mW coarse-grained potential, where hydrogen-bonds are modeled in the absence of explicit hydrogen atoms, via a three-body term that favors tetrahedral coordination. We found that the combination of particles exhibiting the same kind of coordination with water gives rise to a linear dependence of contact angle with respect to composition, in agreement with the Cassie model. However, when only the hydrophilic component can form hydrogen bonds, unprecedented deviations from linearity are observed, increasing the contact angle and the vapor pressure above their values in the purely hydrophobic interface. In particular, the maximum enhancement is seen when a 35% of hydrogen bonding molecules is randomly scattered on a hydrophobic background. This effect is very sensitive to the heterogeneity length-scale, being significantly attenuated when the hydrophilic domains reach a size of 2 nm. The observed behavior may be qualitatively rationalized via a simple modification of the Cassie model, by assuming a different microrugosity for hydrogen bonding and non-hydrogen bonding interfaces.

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.