Ice Nanocrystals Research Articles

Multitwinned Ice Nanocrystals.

Multitwinned nanocrystals are commonly found in substances that preferentially adopt tetrahedral local arrangements, but not yet in water crystals. Ice nanocrystals are pivotal in cloud microphysics, and their surfaces become increasingly prominent in determining structure as crystal size decreases. Nevertheless, discussions on nanocrystal structures have predominantly centered on ice polymorphs observed in bulk: hexagonal (Ih), cubic (Ic), and stacking-disordered (Isd) ices. Here, we demonstrate, through molecular dynamics (MD) simulations, that decahedral and icosahedral nanocrystals form from liquid water droplets of a few nanometers in size without violating the ice rule. The brute force spontaneous crystallization is conducted using the mW model, and the thermodynamic stability is examined using the TIP4P/Ice model. During the crystallization process, the formation of twin boundaries precedes the emergence of centers exhibiting 5-fold and icosahedral symmetry. The free energy calculation suggests the icosahedron has comparable stability with ice Ih nanocrystal. The frequent occurrence of these unreported ice nanocrystals aligns with the fact that natural polycrystalline snow crystals predominantly display a 70.5-degree angle between the Ih c-axes of adjacent branches. Moreover, we show that the formation of multitwinned ice nanocrystals is enhanced within a fullerene, providing a potential avenue for experimental observations.

About the structure of water

It is shown that during 10–10–10–11s, only clusters consisting of 4 or 3 water molecules can statistically form in water. It is shown that Brownian motion in water occurs as a result of collision of ice nanocrystals with Brownian particles. Brownian motion is an experimental confirmation of the nanocrystalline structure of water. It is shown that water consists of 13 % molecules and 87 % ice nanocrystals. Such a two‑phase structure provides water with structural properties, high fluidity and elasticity of steam.

Features of water crystallization

Water is mainly composed of ice nanocrystals. The peculiarities of water crystallization can be explained on the basis of its nanostructural structure and nanostructural crystallization. Atmospheric air molecules dissolve well in water and are adsorbed by its nanocrystals. It is shown that the amount of expansion of ice, when water solidifies, is proportional to the concentration of air dissolved in it. The concentration of air dissolved in water decreases significantly with an increase in its temperature. Hot water solidifies faster than cold water because there is less air concentration in hot water. Its bubbles, released on ice crystals, reduce the rate of crystallization of water. A large supercooling of water occurs as a result of the blocking action of adsorbed air, which prevents the unification of ice nanocrystals into crystallization centers. Shaking a bottle of supercooled water leads to desorption of air and accelerated crystallization of water. Air bubbles released on dendritic ice crystals reduce the degree of branching of these crystals. It has been shown that music increases the intensity of removal of gas bubbles and is able to influence the shape of dendritic ice crystals during water crystallization. It has been shown that an increase in the volume of sound and (or) a decrease in its frequency increase the intensity of removal of air bubbles from dendritic ice crystals and increase the branching of these crystals.

Tofukasu-derived biochar with interconnected and hierarchical pores for high efficient removal of Cr (VI)

Herein, we report the synthesis of interconnected hierarchical pore biochar (HTB) via an ice-templating strategy using bio-waste (tofukasu). The abundance of N- and O-containing functional groups in tofukasu makes it easy to form hydrogen bonds with water molecules and water clusters, resulting in nano-micro structures like ice clusters and snow crystals during freezing process. More importantly, tofukasu will be squeezed by micron-scale snow crystals to form coiled sheet-like structures, and its surface and interior will be affected by needle-like ice nanocrystals from several nanometers to tens of nanometers to form transverse groove needles and mesopores. The ice crystals are then removed by sublimation with tofukasu, leaving the interconnected pore structure intact. Therefore, the ice template synthesis strategy endowed the interconnected hierarchical pore structure of HTB with a large specific surface area (SBET, 733 m2⋅g−1) and hierarchical porosity (30.30% for mesopores/total pore volume ratio), which is significantly higher than the normal dry treated tofukasu biochar (TB), which had a SBET of 436 m2⋅g−1 and contained 1.53% mesopores. In addition, the sheet-like structure with interconnected pores of HTB favors high exposure of active sites (N- and O-containing functional groups), and a fast electron transport rate. As a result, HTB had an excellent adsorption capacity of 159.65 mg⋅g−1, which is 4.7 times that of typical block biochar of TB (33.89 mg⋅g−1) according to Langmuir model. Electrochemical characterization, FTIR and XPS analysis showed that the mechanism of Cr(VI) removal by HTB included electrostatic attraction, pore filling, reduction and surface complexation.Graphical

Surface premelting of ice far below the triple point

Premelting of ice, a quasi-liquid layer (QLL) at the surface below the melting temperature, was first postulated by Michael Faraday 160 y ago. Since then, it has been extensively studied theoretically and experimentally through many techniques. Existing work has been performed predominantly on hexagonal ice, at conditions close to the triple point. Whether the same phenomenon can persist at much lower pressure and temperature, where stacking disordered ice sublimates directly into water vapor, remains unclear. Herein, we report direct observations of surface premelting on ice nanocrystals below the sublimation temperature using transmission electron microscopy (TEM). Similar to what has been reported on hexagonal ice, a QLL is found at the solid-vapor interface. It preferentially decorates certain facets, and its thickness increases as the phase transition temperature is approached. In situ TEM reveals strong diffusion of the QLL, while electron energy loss spectroscopy confirms its amorphous nature. More significantly, the premelting observed in this work is thought to be related to the metastable low-density ultraviscous water, instead of ambient liquid water as in the case of hexagonal ice. This opens a route to understand premelting and grassy liquid state, far away from the normal water triple point.

Crystallization and vitrification of solutions with weak intermolecular interaction of the components

An experimental study of the phenomenon of cluster crystallization and vitrification of solutions with weak intermolecular interaction of the components was performed with use of a purposely developed method of microvolumetric scanning tensodilatometry. Volumetric effects accompanying cooling and subsequent heating of aqueous solutions of glycerol and dimethyl sulfoxide were recorded within temperature range 300…135 K, and they are completely in line with patterns of cluster crystallization. The same phenomenon was used to analyze the structure of amorphous fractions of such solutions. Formation inside of them of ice nanocrystals without breakage of hydrogen bonds between water molecules and molecules of cryoprotectant dramatically reduces stability of amorphous fractions. Based on the resulted tensodilatograms, state diagrams for solutions, which include cluster phase, were for the first time plotted.

Freezing of water and melting of ice: theoretical modeling at the nanoscale.

Freezing of water and melting of ice at the nanoscale play critical roles in science and technology fields, including aviation systems, infrastructures, and other broad spectrum of technologies. To cope with the icing challenge, nanoscale anti-icing surface technology has been developed. The freezing and melting temperatures can be tailored by manipulating the size (the radius of water or ice); however, it lacks systemic research. In this work, the size effect on the melting temperature of ice nanocrystals was first established, which considered the variation of bond energy and equivalent heat energy from the perspective of the force-heat equivalence energy density principle. Based on the heterogeneous nucleation mode and by further considering the size and temperature effects on the interface energy involved solid-liquid energy and liquid-vapor energy as well as the above developed melting temperature model, another model is established to accurately predict the freezing temperature of water nanodroplets. The parameters required by the two models established in this paper have a clear physical meaning and establish the quantitative relationships among freezing temperature, melting temperature, surface stress, interface energy, and other thermodynamic parameters. The agreement between model prediction and experimental simulation data confirms the validity and universality of the established models. The higher prediction accuracy of this work compared to the other theoretical models, due to the more detailed consideration and the reference point, captures the errors introduced by the experiment or simulation. This study contributes to a deeper understanding of the underlying mechanism of freezing of water and melting of ice nanocrystals and provides theoretical guidance for the design of cryopreservation systems and anti-icing systems for aviation.

Nanostructural mechanisms of physicochemical reactions

It is generally accepted that physicochemical reactions are mainly molecular atomic. It was shown that liquid water consists of 12 % molecules and 88 % of ice nanocrystals. When melting, microcrystals of substances break down into nanocrystals and molecules or into nanocrystals and atoms. During the crystallization of liquids, the reverse process occurs. Based on the above physicochemical reactions, it is shown that nanostructural mechanisms prevail in their basis.

About brownian movement in liquids

It has been shown that Brownian movement in water occurs as a result of elastic collisions of ice nanocrystals with Brownian particles. Water consists of 87 % of ice nanocrystals and 13 % of water molecules. At 300 K, the ice nanocrystal in water on average consists of 24 water molecules. Brownian movement is an experimental confirmation of the nanocrystalline structure of liquids. This concept of liquids is of great importance for the theory of crystallization and modification of alloys. In metallic liquids, Brownian motion refers to microscopic non-metallic particles and intermetallides that have densities comparable to melt densities. In liquid aluminum alloys, Brownian particles are microscopic alumina particles that remain in the castings when they solidify.

Study of polyvinyl alcohols (9 and 31 kDa) aggregation in aqueous solutions by fluorescent probing

Background: When developing low-temperature cell storage methods, a serious problem is recrystallization, which leads to cell damage during thawing. Previous studies have shown the promising use of polyvinyl alcohol (PVA) as an inhibitor of recrystallization. But the mechanisms of protective action of PVA are not finally clarified. So, it is not known what structural features contribute to implementation of PVA antirecrystallization properties in the cryoprotective concentration range. Objectives: Establishing the peculiarities of structuring PVA molecules in aqueous solutions using the fluorescent probe. Materials and Methods: Aqueous solutions of 0.1–5% (wt.%) PVA with molecular mass (m.m.) of 9 and 31 kDa) were studied. Fluorescence probe method, photometry, stalagmometry, and molecular modeling were used. Results: Using the 3-hydroxy-4¢-(N,N-dimethylamino)flavones (FME) fluorescent probe it was found that in 0.1–5% of PVA (m.m. 9 and 31 kDa) aqueous solutions the structural organization of polymers changes with formation of different in size and structure of local hydrophobic regions. In PVA solutions, m.m. 9 kDa micelles with smaller cavities are formed in which FME is densely surrounded by polymer segments. In the case of PVA m.m. 31 kDa, it forms micelles with smaller cavities surrounded by polymer segments. PVA m.m. 31 kDa forms micelles with larger in size and more hydrophilic cavities. If the content is more than 3%, PVA m.m. 31 kDa aggregates are partially destroyed, which may be the result of increased water content. Under these conditions, PVA m.m. 9 kDa micelles are enlarged. as a result of aggregation. According to molecular modeling data, PVA is able to form strong hydrogen-linked complexes with the surface of ice nanocrystals. Such complex, having a hydrophobic surface, can depolarize water molecules, thus slowing down further growth of ice crystals. Conclusions: Changes in the structural organization, which may affect the recrystallization properties, have been found in water solutions of PVA. The mechanism of implementation of polymer anticrystallization activity has been suggested. The possible role of structure and supramolecular organization of PVA in aqueous solutions in understanding the mechanisms of depressing recrystallization during freeze-thawing of cells is discussed.

Latent heat induced rotation limited aggregation in 2D ice nanocrystals.

The basic science responsible for the fascinating shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional (2D) system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provide compelling evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20 °C and without significant thermal contact to the ambient. The growth is studied in situ, in real time and space at the nanoscale. We find that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to Diffusion Limited Aggregation (DLA). However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be key ingredients for understanding the evolution of the snow (ice) flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the "perennial" snow (ice) flake enigma.

Size- and Dimensionality-Dependent Thermodynamic Properties of Ice Nanocrystals

Although the melting of ice is the most ubiquitous phase transition, (pre)melting and the quasi-liquid layer remain a matter of debate, and little is known about the relationship between the thermodynamic properties of ice nanocrystals and their size and dimensionality. Here, we model analytically the size- and dimensionality-dependent melting temperature, premelting temperature, and melting enthalpy of hydrogen-bonded ice nanocrystals. These three thermodynamic parameters are found to increase with increasing size and dimensionality where the size effect is principle while the dimensionality effect is secondary, and the size dependence of premelting temperature almost follows the same trend as that of melting temperature. The model predictions correspond to the available molecular dynamic simulation and experimental results of ice nanoparticles and nanowires. These agreements enable us to determine theoretically the thickness of the quasi-liquid layer for the first time, which is found to be not constant but slightly increase with increasing size and thus accounts for the occurrence of different reported thicknesses of the quasi-liquid layer.

Comment on “HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study” by P. Parent, J. Lasne, G. Marcotte and C. Laffon, Phys. Chem. Chem. Phys., 2011,13, 7142

On the basis of NEXAFS, photoemission and FTIR spectra of ice films with low doses of adsorbed HCl, the authors of the PCCP paper "HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study", Phys. Chem. Chem. Phys., 2011, 13, 7142, have come to conclusions regarding the behavior of submonolayer amounts of HCl at 50 K that contradict published results of the authors of this Comment. Our purpose is to argue that the conclusion, attributed going forward to PLML (authors' initials), that nearly 100% of HCl ionizes for dosage levels near to 0.16 monolayer (ML) or 0.3 Langmuir (L) at 50 K is questionable. Rather, we reaffirm our conclusions of much lower levels of ionization for similar temperatures and HCl dosages based on reactive ion scattering (RIS) and low energy sputtering (LES) data for ice films and FTIR spectra of ice nanocrystals. A second current paper by Ayotte et al., J. Phys. Chem. A, 2011, 115, 6002, that largely parallels in method and results the RAIR spectroscopy of PLML, is also given special notice.

Melting the Ice: On the Relation between Melting Temperature and Size for Nanoscale Ice Crystals

Although the melting of ice is an everyday process, important issues remain unclear particularly on the nanoscale. Indeed despite extensive studies into ice melting and premelting, little is known about the relationship between (pre)melting and crystal size and morphology, with, for example, the melting temperature of ice nanocrystals being unclear. Here we report extensive long-time force-field-based molecular dynamics studies of the melting of hexagonal ice nanocrystals in the ca. 2 to 8 nm size range. We show that premelting is initiated at the corners of the crystals, then the edges between facets, and then the flat surfaces; that is, the melting temperature is related to the degree of coordination. A strong size dependence of the melting temperature is observed, with the combination of small particle size and premelting leading nanosized ice crystals to have liquid-like surfaces as low as about 130 K below the bulk ice melting temperature. These results will be of relevance in understanding the size dependence of ice crystal morphology and the surface reactivity of ice particles under atmospheric conditions.

Peculiarities of state diagrams of aqueous solutions of cryoprotective agents

The phase transitions in aqueous solutions of glycerol and PEO-1500 within the temperature range of +30 to −150 °C have been studied using the methods of thermoplastic analysis and volumetric scanning tensodilatometry. We present the revealed phenomenon of cluster cyrystallization of these solutions as well as principles of describing this phenomenon using state diagrams, containing the intervals of concentration corresponding to the existence of amorphous and cryocolloid fractions. We note that for the cryocolloid fraction, a low temperature association of molecules of cryoprotective agents leads the formation of ice nanocrystals either close to or directly inside the aggregations. These fractions exist in cooled cryoprotective solutions near the vitrification temperatures of the liquid phase and may contribute to the initiation of damaging events in cryopreserved biological systems. Our data may be helpful in explaining the peculiarities observed during crystallization of cryoprotective solutions and may further contribute to a broader understanding of the principles of protection and protocol optimization of biological materials at temperatures approaching vitrification.

A thermoporometry and small-angle x-ray scattering study of wet silica sonogels as thepore volume fraction is varied

Silica sonogels with different porosities were prepared by acid sono-hydrolysis oftetraethoxysilane. Wet sonogels were studied using small-angle x-ray scattering(SAXS) and differential scanning calorimetry (DSC). The DSC shows a broadthermal peak below the normal water melting point associated with the melting ofconfined ice nanocrystals, or nanoporosity. The nanopore size distribution wasdetermined from the Gibbs–Thomson equation. As the porosity is increased, asecond sharp DSC thermal peak with onset temperature at the water meltingpoint is apparent, which was associated with the melting of ice macrocrystals, ormacroporosity. The DSC result could be causing misinterpretation of the macroporositybecause water may not be exactly confined in very feeble silica network regions insonogels with high porosity. The structure of the wet gels can be described fairlywell as mutually self-similar mass fractal structures with characteristic lengthξ increasingfrom ∼1.8 to∼5.4 nm and mass fractaldimension D diminishingdiscretely from ∼2.6 to ∼2.3 as the porosity increases in the range studied. More specifically, such astructure could be described using a two-parameter correlation functionγ(r)∼rD−3exp(−r/ξ), which is limited at larger scale by the cut-off distanceξ but without a well-defined small scale cut-off distance, at least up to the maximum angulardomain probed using SAXS in the present study.

Evidence for the surface origin of point defects in ice: Control of interior proton activity by adsorbates

Spectroscopic studies are presented of H-D isotopic exchange in the interior of ice nanocrystals. The exchange process is dominated by ionic and orientational defects long viewed as governing the electrical properties of ice. A new finding that interior exchange rates can be controlled by acidic and basic adsorbates is evidence that the defects originate at the ice surface. In particular, it is argued that interior isotopic exchange is a reflection of proton concentrations equilibrated at the ice surface.

Ice nanocrystals, graphene spin valves, nanotube reactors, DNA templates and more

Ice nanocrystals, graphene spin valves, nanotube reactors, DNA templates and more

Water surface is acidic.

Water autoionization reaction 2H2O --> H3O- + OH- is a textbook process of basic importance, resulting in pH = 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H3O+ and OH- ions, density functional studies of (H2O)(48)H+ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O+ does, but OH- does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.

Slow and fast dynamics in glycerol–water mixtures

We discuss the relaxation dynamics of glycerol–water mixtures as studied by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). BDS studies were performed in a frequency range of 1Hz to 250MHz at temperatures ranging from 173 to 323K and DSC measurements were made between 138 and 313K. The experimental results obtained for the glycerol-rich mixtures in terms of dielectric loss ‘master plots’ suggest that the main dielectric relaxation process, as well as the high frequency ‘excess wing’ and dc-conductivity, follow the same temperature dependence. This result indicates that all of these processes are based on the same physical origin. A new phenomenological relationship for the complex dielectric permittivity is proposed. The structural and dynamic properties of glycerol–water mixtures are discussed. The experimental data for water-rich mixtures show the existence of a critical concentration of 40mol% of glycerol that relates to the numbers of hydrogen bonds of glycerol and of water molecules. Below this concentration, water cooperative domains appear, coexisting with the glycerol cooperative domains. Below 20mol% of glycerol, ice nanocrystals appeared that led to three relaxation processes; result from ice, mesoscopic glycerol–water domains, and water on the interface between former two.