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

Interfacial Ice Density Fluctuations Inform Surface Ice-Philicity

The propensity of a surface to nucleate ice or bind to ice is governed by its ice-philicity─its relative preference for ice over liquid water. However, the relationship between the features of a surface and its ice-philicity is not well understood, and for surfaces with chemical or topographical heterogeneity, such as proteins, their ice-philicity is not even well-defined. In the analogous problem of surface hydrophobicity, it has been shown that hydrophobic surfaces display enhanced low water-density (vapor-like) fluctuations in their vicinity. To interrogate whether enhanced ice-like fluctuations are similarly observed near ice-philic surfaces, here we use molecular simulations and enhanced sampling techniques. Using a family of model surfaces for which the wetting coefficient, k, has previously been characterized, we show that the free energy of observing rare interfacial ice-density fluctuations decreases monotonically with increasing k. By utilizing this connection, we investigate a set of fcc systems and find that the (110) surface is more ice-philic than the (111) or (100) surfaces. By additionally analyzing the structure of interfacial ice, we find that all surfaces prefer to bind to the basal plane of ice, and the topographical complementarity of the (110) surface to the basal plane explains its higher ice-philicity. Using enhanced interfacial ice-like fluctuations as a measure of surface ice-philicity, we then characterize the ice-philicity of chemically heterogeneous and topologically complex systems. In particular, we study the spruce budworm antifreeze protein (sbwAFP), which binds to ice using a known ice-binding site (IBS) and resists engulfment using nonbinding sites of the protein (NBSs). We find that the IBS displays enhanced interfacial ice-density fluctuations and is therefore more ice-philic than the two NBSs studied. We also find the two NBSs are similarly ice-phobic. By establishing a connection between interfacial ice-like fluctuations and surface ice-philicity, our findings thus provide a way to characterize the ice-philicity of heterogeneous surfaces.

The Mechanism of Heterogeneous Ice Nucleation by Fatty Alcohol Monolayers

Organic ice nucleating substances (INSs) are thought to play an essential role in cloud formation and, hence, precipitation and climate. Organic INSs are an important but poorly understood class of INSs in the atmosphere. To study organic INSs with exposed hydroxylated surfaces, researchers have previously used fatty alcohol monolayers as model systems. For alcohol monolayers, ice nucleation temperatures increase with increasing alkyl chain length and show a high-low oscillation following the number (odd-even) of carbon atoms in the alkyl chains. We employ atomistic models, together with molecular dynamics simulations, to investigate ice nucleation by C20H41OH, C30H61OH, and C31H63OH monolayers. As expected, we find that ice nucleation by alcohol monolayers depends on the lattice match to ice, and a poorer lattice match can at least partially account for the reduced ice nucleation ability of C20H41OH monolayers compared to monolayers of the longer chain alcohols. More interestingly, our simulations identify a limited range of alcohol configurations that readily nucleate ice via the basal plane. For configurations outside this range, ice nucleation did not occur on the time scale of our simulations (i.e., 5000 ns). The configurational feature that crucially influences ice nucleation is the angle between the alcohol C-O bond and the interfacial plane. C-O bonds directed sharply toward or away from the water phase strongly inhibit ice nucleation. In contrast, ice nucleation is easily observed for a relatively narrow band of C-O bond orientations centered about the surface plane. For comparable surface configurations, the ice nucleating abilities of C30H61OH and C31H63OH monolayers are practically identical, but the existence of a narrow band of ice-compatible surface configurations can perhaps explain why odd-chain alcohol monolayers are better INSs than even-chain alcohol monolayers. Earlier simulations have shown that for alcohols differing by a single carbon atom, the odd-chain monolayer is less rigid than the even-chain monolayer. This suggests the possibility that for odd-chain alcohol monolayers, the orientation of the C-O bonds can more easily adjust into the ice-compatible range than their even-chain counterparts, accounting for their enhanced ice nucleating ability.

Molecular Dynamics Simulation Investigation of Freezing Point Depression in NaClO4 Electrolyte Solution by CaCl2

The development of inorganic antifreeze electrolytes is of paramount importance for the application of sodium-ion batteries under low-temperature conditions. However, there is little reported about their molecular mechanism for lowering the freezing point of electrolytes. Therefore, this study explores the mechanism by which CaCl2 lowers the freezing point of the NaClO4 electrolyte. Hexagonal ice (ice Ih) was used as the ice seed, and CaCl2 was selected as the antifreeze agent. The coexistence system of ice and solution was constructed to simulate the freezing process. It was found that there is ion rejection at the ice layer, with ions predominantly distributed in the solution. Over time, ions form an ion adsorption layer at the ice-solution interface. The radial distribution function (RDF) and spatial distribution function (SDF) of Na+, ClO4-, Ca2+, and Cl- revealed that ions form the first solvation shells with water molecules. The interaction energy between ions and water molecules is greater than that between ice nuclei and water. Therefore, ions are better able to maintain the stability of their solvation shells and inhibit the growth of ice Ih through a mechanism of competition for water molecules. Furthermore, the dissolution free energy of Na+ and Ca2+ in the aqueous phase was studied. The results indicated that Ca2+ has a stronger affinity for water molecules than Na+, making it more competitive in competing for water with ice Ih. Therefore, CaCl2 in NaClO4 solution can reduce the freezing point. This work provides a molecular-level understanding of how CaCl2 reduces the freezing point of NaClO4 solution, which is beneficial for designing strategies for low-temperature electrolytes.

Electric Field-Enhanced Ion Rejection Rate in Freeze Desalination

Freeze desalination is an appealing method for seawater desalination through freezing seawater. The percentage of ions in the liquid phase, which is termed ion rejection rate, is a critical factor affecting the performance of freeze desalination. Improving the ion rejection rate is an important topic for freeze desalination. In this work, we investigate the effects of electric fields on the ion rejection rate during the freezing of seawater through molecular dynamics simulations. It is found that the ion rejection rate increases with increasing electric field strength. The enhanced ion rejection rate is due to the reduction of the energy barrier at the ice-water interface caused by the electric field, which affects the orientation of water molecules and ion-water interactions. However, the electric field hinders the ice growth rate, which affects the productivity of freeze desalination. Nevertheless, the finding in this work offers a new idea to improve the ion rejection rate. Practically, a trade-off needs to be found to optimize the overall performance of freeze desalination.

Shear-Enhanced Ion Rejection during Seawater Freezing

Ion rejection during seawater freezing is the basis for freeze desalination. A high ion rejection rate is desired for improving the performance of freeze desalination. In this work, we propose a method to enhance the ion rejection rate through external shear, which is demonstrated through molecular dynamics (MD) simulations and experiments. MD simulations show that the ion rejection rate increases with an increasing shear rate. This is attributed to the disruption of the hydration bonds between ions and water molecules in the hydration shell caused by the shear. Consequently, the mobility of ions is increased, and the energy barrier is reduced at the ice-water interface such that ions have a greater chance of diffusing into the aqueous solution, leading to an enhanced ion rejection rate. The MD results in this work are qualitatively confirmed by experiments and provide insights into the enhancement of the ion rejection rate through external parameters.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Fully periodic, computationally efficient constant potential molecular dynamics simulations of ionic liquid supercapacitors

Molecular dynamics (MD) simulations of complex electrochemical systems, such as ionic liquid supercapacitors, are increasingly including the constant potential method (CPM) to model conductive electrodes at a specified potential difference, but the inclusion of CPM can be computationally expensive. We demonstrate the computational savings available in CPM MD simulations of ionic liquid supercapacitors when the usual non-periodic slab geometry is replaced with fully periodic boundary conditions. We show how a doubled cell approach, previously used in non-CPM MD simulations of charged interfaces, can be used to enable fully periodic CPM MD simulations. Using either a doubled cell approach or a finite field approach previously reported by others, fully periodic CPM MD simulations produce comparable results to the traditional slab geometry simulations with a nearly double speedup in computational time. Indeed, these savings can offset the additional cost of the CPM algorithm, resulting in periodic CPM MD simulations that are computationally competitive with the non-periodic, fixed charge equivalent simulations for the ionic liquid supercapacitors studied here.

Correction: Concentrations and properties of ice nucleating substances in exudates from Antarctic sea-ice diatoms

Correction for 'Concentrations and properties of ice nucleating substances in exudates from Antarctic sea-ice diatoms' by Yu Xi et al., Environ. Sci.: Processes Impacts, 2021, 23, 323-334, DOI: 10.1039/D0EM00398K.

Molecular understanding of ion rejection in the freezing of aqueous solutions

In this work, we investigate the microscopic mechanism of ion rejection phenomena during the freezing of aqueous NaCl solutions through molecular dynamics simulations. It is found that the hydration energy for the ion-water interaction is stronger than that between ions and ice, which is the fundamental reason giving rise to the phenomenon of ion rejection. The probability of ions being rejected by ice is determined by the competition between the energy barrier at the ice-water interface and the thermal effect. The ion rejection rate increases with increasing temperature. Furthermore, it is found that the rejection rate of Na+ is higher than that of Cl- because of the relatively large hydration energy difference between Na+-water and Na+-ice interactions. The role of temperature in the applications of ion rejection in freeze desalination is also discussed.

A review of clay based photocatalysts: Role of phyllosilicate mineral in interfacial assembly, microstructure control and performance regulation

Over the past decades, inspired by the outstanding properties of clay minerals such as abundance, low-cost, environmental benignity, high stability, and regularly arranged silica-alumina framework, researchers put much efforts on the interface assembly and surface modification of natural minerals with bare photocatalysts, i.e. TiO₂, g-C₃N₄, ZnO, MoS₂, etc. The clay-based hybrid photocatalysts have resulted in a rich database for their tailor-designed microstructures, characterizations, and environmental-related applications. Therefore, in this study, we took a brief introduction of three representative minerals, i.e. kaolinite, montmorillonite and rectorite, and discussed their basic merits in photocatalysis applications. After that, we summarized the recent advances in construction of stable visible-light driven photocatalysts based on these minerals. The structure-activity relationships between the properties of clay types, pore structure, distribution/dispersion and light absorption, carrier separation efficiency as well as redox performance were illustrated in detail. Such representative information would provide theoretical basis and scientific support for the application of clay based photocatalysts. Finally, we pointed out the major challenges and future directions at the end of this review. Undoubtedly, control and preparation of novel photocatalysts based on clays will continue to witness many breakthroughs in the arena of solar-driven technologies.

Concentrations and properties of ice nucleating substances in exudates from Antarctic sea-ice diatoms

The ocean contains ice nucleating substances (INSs), some of which can be emitted to the atmosphere where they can influence the formation and properties of clouds. A possible source of INSs in the ocean is exudates from sea-ice diatoms. Here we examine the concentrations and properties of INSs in supernatant samples from dense sea-ice diatom communities collected from Ross Sea and McMurdo Sound in the Antarctic. The median freezing temperatures of the samples ranged from approximately -17 to -22 °C. Based on our results and a comparison with results reported in the literature, the ice nucleating ability of exudates from sea-ice diatoms is likely not drastically different from the ice nucleating ability of exudates from temperate diatoms. The number of INSs per mass of DOC for the supernatant samples were lower than those reported previously for the sea surface microlayer and bulk sea water collected in the Arctic and Atlantic. The INSs in the supernatant sample collected from Ross Sea were not sensitive to temperatures up to 100 °C, were larger than 300 kDa, and were different from ice shaping and recrystallization inhibiting molecules present in the same sample. Possible candidates for these INSs include polysaccharide containing nanogels. The INSs in the supernatant sample collected from McMurdo Sound were sensitive to temperatures of 80 and 100 °C and were larger than 1000 kDa. Possible candidates for these INSs include protein containing nanogels.

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.