Neutron diffraction study of water freezing on aircraft engine combustor soot

The study of the formation of condensation trails and cirrus clouds on aircraft emitted soot particles is important because of its possible effects on climate. In the present work we studied the freezing of water on aircraft engine combustor (AEC) soot particles under conditions of pressure and temperature similar to the upper troposphere. The microstructure of the AEC soot was found to be heterogeneous containing both primary particles of soot and metallic impurities (Fe, Cu, and Al). We also observed various surface functional groups such as oxygen-containing groups, including sulfate ions, that can act as active sites for water adsorption. Here we studied the formation of ice on the AEC soot particles by using neutron diffraction. We found that for low amount of adsorbed water, cooling even up to 215 K did not lead to the formation of hexagonal ice. Whereas, larger amount of adsorbed water led to the coexistence of liquid water (or amorphous ice) and hexagonal ice (I(h)); 60% of the adsorbed water was in the form of ice I(h) at 255 K. Annealing of the system led to the improvement of the crystal quality of hexagonal ice crystals as demonstrated from neutron diffraction.     

A Review on Graphene Oxide Two-dimensional Macromolecules: from Single Molecules to Macro-assembly

Graphene oxide(GO), which consists of two-dimensional(2 D) sp² carbon hexagonal networks and oxygen-contained functional groups, has laid the foundation of mass production and applications of graphene materials. Made by chemical oxidation of graphite, GO is highly dispersible or even solubilized in water and polar organic solvents, which resolves the hard problem of graphene processing and opens a door to wet-processing of graphene. Despite its defects, GO is easy to functionalize, dope, punch holes, cut into pieces, conduct chemical reduction, form lyotropic liquid crystal, and assemble into macroscopic materials with tunable structures and properties as a living building block. GO sheet has been viewed as a single molecule, a particle, as well as a soft polymer material. An overview on GO as a 2 D macromolecule is essential for studying its intrinsic properties and guiding the development of relevant subjects. This review mainly focuses on recent advances of GO sheets, from single macromolecular behavior to macro-assembled graphene material properties. The first part of this review offers a brief introduction to the synthesis of GO molecules. Then the chemical structure and physical properties of GO are presented, as well as its polarity in solvent and rheology behavior. Several key parameters governing the ultimate stability of GO colloidal behavior, including size, p H and the presence of cation in aqueous dispersions, are highlighted. Furthermore, the discovery of GO liquid crystal and functionalization of GO molecules have built solid new foundations of preparing highly ordered, architecture-tunable, macro-assembled graphene materials, including 1 D graphene fibers, 2 D graphene films, and 3 D graphene architectures. The GO-based composites are also viewed and the interactions between these target materials and GO are carefully discussed. Finally, an outlook is provided in this field, where GO is regarded as macromolecules, pointing out the challenges and opportunities that exist in the field. We hope that this review will be beneficial to the understanding of GO in terms of chemical structure,molecular properties, macro-assembly and potential applications, and encourage further development to extend its investigations from basic research to practical applications.     

An experimental study of crystallization and crystal growth of methane hydrates from melting ice

An experiment with well defined gas-water interfacial surface area was developed to study the crystallization and crystal growth of methane hydrates. Measurable formation rates were observed only when melting ice was involved. No hydrates nucleated from liquid water or from non-melting ice. It is concluded that melting ice, which like hydrate water is hydrogen-bonded, provides a template for hydrate nucleation as well as providing a heat sink for absorbing the heat of formation during hydrate growth. The experiment was conducted in the absence of mixing so that hydrate crystals grew under quiescent conditions.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

甘油对冰晶生长抑制作用的分子动力学模拟

采用分子模拟方法研究了正交晶系冰晶(020)生长面在不同浓度甘油水溶液中的生长情况. 通过统计分析氢键数、 密度分布函数、 均方根偏差和原子间径向分布函数研究了水分子和甘油分子的动态行为. 结果表明, 甘油分子在水溶液中可与水分子形成大量氢键, 这使水分子间的氢键作用受到抑制, 降低了水分子的扩散性, 致使冰晶不易成核和生长; 另外, 一些甘油分子可代替水分子吸附在晶面上, 甚至占据晶格位点, 这种行为打破了冰晶的对称性并且降低了冰晶的生长速率. 因此, 甘油可同时在晶面和液相中抑制冰晶的生长.     

Anisotropy in the crystal growth of hexagonal ice, I(h)

Growth of ice crystals has attracted attention because ice and water are ubiquitous in the environment and play critical roles in natural processes. Hexagonal ice, I(h), is the most common form of ice among 15 known crystalline phases of ice. In this work we report the results of an extensive and systematic molecular dynamics study of the temperature dependence of the crystal growth on the three primary crystal faces of hexagonal ice, the basal {0001} face, the prism {1010} face, and the secondary prism {1120} face, utilizing the TIP4P-2005 water model. New insights into the nature of its anisotropic growth are uncovered. It is demonstrated that the ice growth is indeed anisotropic; the growth and melting of the basal face are the slowest of the three faces, its maximum growth rates being 31% and 43% slower, respectively, than those of the prism and the secondary prism faces. It is also shown that application of periodic boundary conditions can lead to varying size effect for different orientations of an ice crystal caused by the anisotropic physical properties of the crystal, and results in measurably different thermodynamic melting temperatures in three systems of similar, yet moderate, size. Evidence obtained here provides the grounds on which to clarify the current understanding of ice growth on the secondary prism face of ice. We also revisit the effect of the integration time step on the crystal growth of ice in a more thorough and systematic way. Careful evaluation demonstrates that increasing the integration time step size measurably affects the free energy of the bulk phases and shifts the temperature dependence of the growth rate curve to lower temperatures by approximately 1 K when the step is changed from 1 fs to 2 fs, and by 3 K when 3 fs steps are used. A thorough investigation of the numerical aspects of the simulations exposes important consequences of the simulation parameter choices upon the delicate dynamic balance that is involved in ice crystal growth.     

Crystallization, melting, and structure of water nanoparticles at atmospherically relevant temperatures

Water nanoparticles play an important role in atmospheric processes, yet their equilibrium and nonequilibrium liquid-ice phase transitions and the structures they form on freezing are not yet fully elucidated. Here we use molecular dynamics simulations with the mW water model to investigate the nonequilibrium freezing and equilibrium melting of water nanoparticles with radii R between 1 and 4.7 nm and the structure of the ice formed by crystallization at temperatures between 150 and 200 K. The ice crystallized in the particles is a hybrid form of ice I with stacked layers of the cubic and hexagonal ice polymorphs in a ratio approximately 2:1. The ratio of cubic ice to hexagonal ice is insensitive to the radius of the water particle and is comparable to that found in simulations of bulk water around the same temperature. Heating frozen particles that contain multiple crystallites leads to Ostwald ripening and annealing of the ice structures, accompanied by an increase in the amount of ice at the expense of the liquid water, before the particles finally melt from the hybrid ice I to liquid, without a transition to hexagonal ice. The melting temperatures T(m) of the nanoparticles are not affected by the ratio of cubic to hexagonal layers in the crystal. T(m) of the ice particles decreases from 255 to 170 K with the particle size and is well described by the Gibbs-Thomson equation, T(m)(R) = T(m)(bulk) - K(GT)/(R - d), with constant K(GT) = 82 ± 5 K·nm and a premelted liquid of width d = 0.26 ± 0.05 nm, about one monolayer. The freezing temperatures also decrease with the particles' radii. These results are important for understanding the composition, freezing, and melting properties of ice and liquid water particles under atmospheric conditions.     

Poly(ionic liquid)s as phase-transporter for graphene oxide liquid crystals from aqueous to non-polar organic phase via noncovalent functionalization

We synthesise a novel poly(ionic liquid) (PIL) and develop a procedure that allows the phase transfer of graphene oxides (GO) from water to organic solvent, retaining graphene oxide liquid crystal (GOLC) phase in various organic solvents, especially non-polar organic solvents. PIL ([PEP-MIM]DBS) is exploited in this procedure as a noncovalent functional material that is capable of transporting GO from aqueous to non-polar organic phase. PILs can decorate GO noncovalently and keep GO nano-sheets exfoliated in solid-state PILs/GO hybrids, which can well redisperse in organic solvents without any agglomeration. This expands the number of known solvents which can support GOLC phase to dimethyl formamide, dimethyl sulfoxide, acetonitrile, tetrahydrofuran, and a number of other non-polar organic solvents, many of which were not known to afford GOLC phase prior to this report, such as dichloromethane, tetrachloromethane, dichloroethane and tetrachloroethane.     

(H2O)10 and (H2O)12 on a virtual metal surface: the growth of ice

(H2O)10 and (H2O)12 are used to investigate the growth of ice on metal surfaces with hexagonal symmetry. The model of the virtual metal surface was used to separate the electronic structure of the metal from that of the water cluster while maintaining the geometric constraints imposed by the metal surface on the water cluster. To complement the ab initio calculations on the water cluster, an additional multicenter analysis was done to analyze the hydrogen bonds within the clusters. These calculations suggested that the water bilayer structure adjacent to the virtual metal surface effectively shields the growing ice crystal from the metal surface.     

Shear-induced assembly of graphene oxide particles into stripes near surface

We report the shear-induced assembly of graphene oxide (GO) particles into periodic stripe-like patterns near the surface. These stripe-like patterns, which have an average periodic length of 100–250 μm, are aligned in a wavy manner along the normal to the flow direction. The self-assembled GO structures are investigated at different depths using three different analysis methods, namely, reflective microscopy observations of the photonic-crystalline GO dispersion, polarized optical microscopy, and fluorescence confocal laser scanning microscopy. The surface microstructures observed in reflection mode are different from the shear-induced band structures formed in bulk thermotropic liquid crystals and liquid crystal polymers, in terms of the shape and scale of the stripes. Further, there is also a difference in terms of the dependence of the stripe width on the shear rate. The observations suggest that the stripes are formed because of a competition between the stable surface-field-induced planar alignment of the GO particles near the surface and their relatively unstable shear-induced vertical alignment in the bulk. The findings of this study should advance our understanding of GO assembly under shear stress. Further, the proposed method is a novel one for inducing the assembly of GO particles into microstructures shaped as thread-like stripes.     

含有十二烷基酚聚氧乙烯(10)醚的溶致液晶体系的研究

本文以非离子表面活性剂十二烷基酚聚氧乙烯(10)醚(TX-10)/苯乙烯/水组成的三元体系为研究对象, 绘制了三元相图, 选取液晶区域作为研究对象, 配制系列样品, 摄制了纹理照片, 用小角X光衍射法测定了液晶中各种组分变化时间的层间距, 并结合^2H NMR谱图和纹理照片的对照以及互为补充的分析, 为精确区分液晶结构提供了新的途径。这不仅对于基础理论研究, 同时对于日用化工和帮次采油都具有一定指导意义。     

Thermodynamics of heterogeneous crystal nucleation in contact and immersion modes

One of the most intriguing problems of heterogeneous crystal nucleation in droplets is its strong enhancement in the contact mode (when the foreign particle is presumably in some kind of contact with the droplet surface) compared to the immersion mode (particle immersed in the droplet). Heterogeneous centers can have different nucleation thresholds when they act in contact or immersion modes. The underlying physical reasons for this enhancement have remained largely unclear. In this paper we present a model for the thermodynamic enhancement of heterogeneous crystal nucleation in the contact mode compared to the immersion one. To determine if and how the surface of a liquid droplet can thermodynamically stimulate its heterogeneous crystallization, we examine crystal nucleation in the immersion and contact modes by deriving and comparing with each other the reversible works of formation of crystal nuclei in these cases. The line tension of a three-phase contact gives rise to additional terms in the formation free energy of a crystal cluster and affects its Wulff (equilibrium) shape. As an illustration, the proposed model is applied to the heterogeneous nucleation of hexagonal ice crystals on generic macroscopic foreign particles in water droplets at T = 253 K. Our results show that the droplet surface does thermodynamically favor the contact mode over the immersion one. Surprisingly, the numerical evaluations suggest that the line tension contribution (from the contact of three water phases (vapor-liquid-crystal)) to this enhancement may be of the same order of magnitude as or even larger than the surface tension contribution.     

Shape change of calcite single crystals to accommodate interfacial curvature: Crystallization in presence of Mg2+ ions and agarose gel-networks

Synthetic calcite single crystals,due to their strong crystal habit,tend to grow into characteristic rhombohedra.In the nature,biogenic calcite crystals form composites together with biomacromolecular materials,spurring investigations of how the growing calcite single crystals change their habit to satisfy the curvature of the organic phase.In this work,we examine calcite crystallization on a flat surface of glass slide and a curved surface of polystyrene(PS)sphere.The crystals exhibit tiny contact area onto the glass substrate that is averagely only 15% of their projected area on the substrate.In sharp contrast,the contact area greatly increase to above 75% of the projected area,once magnesium ions or agarose gel networks are introduced into the crystallization media.Furthermore,the calcite crystals form rough and step-like interfaces with a curved surface.However,the interfaces become smooth and curved as the crystals grow in presence of magnesium ions or agarose gel networks.The discrepancy between the interfacial structures implies kinetic effects of the additives on the crystallization around the surfaces. This work may provide implications for understanding the formation mechanisms of single-crystal composite materials.     

Controlling wrinkles and assembly patterns in dried graphene oxide films using lyotropic graphene oxide liquid crystals

We fabricated graphene oxide (GO) films on glass substrates by blade coating a lyotropic GO liquid crystal dispersion. Substrate temperature and blading speed were precisely controlled to manipulate the surface morphologies of GO films. The temperature and blade speed influenced the drying rate of film and the amount GO dispersion supplied. By controlling these parameters, film-thickness modulation and three types of surface wrinkle patterns were selectively achieved. We also plotted the wrinkle patterns diagram as functions of the film fabrication conditions. The films exhibited different optical anisotropies depending on wrinkle patterns. GO films with controlled wrinkles can be used as electrodes for supercapacitor applications owing to the large surface areas.     

Synthesis of Poly(tartar amides) as Bio‐Inspired Antifreeze Additives

Summary: Multifunctional poly(tartar amides) have been synthesized and used as bio‐inspired antifreeze additives. It is shown that these polymers strongly interfere with the crystallization process of water in comparison to commercially available commodity polymers. While the addition of the poly(tartar amides) results in minor freezing point depression, as is shown by differential scanning calorimetry, a strong change in the ice crystal morphology is evident. Wide‐angle X‐ray scattering and optical microscopy indicate that the hexagonal structure of undisturbed ice‐crystals is oriented and partly deformed.

Light microscopy image of ice crystals at 223 K after a freezing assay with poly(tartar amides) shown at a polymer concentration of 2 wt.‐%.     

Growth of zeolite crystals with graphene oxide nanosheets

Growth of silicalite with graphene oxide (GO) nanosheets occurred via attachment of GO onto the silicalite surface, and entrapment of GO nanosheets inside single crystals. Electrically conductive composites were produced by calcination in nitrogen whereas silicalite crystals with slit-like mesopores of sizes 20-25 ? were obtained after GO burn-off.     

Formation of ice-like water structure on the surface of an antifreeze protein

Antifreeze proteins (AFPs) are found in different species from polar, alpine, and subarctic regions where they serve to inhibit ice crystal growth by adsorption to ice surfaces. Computational methods have the power to investigate the antifreeze mechanism in atomic detail. Molecular dynamics simulations of water under different conditions have been carried out to test our water model for simulations of biological macromolecules in extreme conditions: very low temperatures (200 K) and at the ice/liquid water interface. We show that the flexible F3C water model reproduces properties of water in the solid phase (ice I(h)), the supercooled liquid phase, and at the ice/liquid water interface. Additionally, the hydration of the type III AFP from ocean pout was studied as a function of temperature. Hydration waters on the ice-binding surface of the AFP were less distorted and more tetrahedral than elsewhere on the surface. More ice-like hydrating water structures formed on the ice-binding surface of the protein such that it created an ice-like structure in water within its first hydration layer but not beyond, suggesting that this portion of the protein has high affinity for ice surfaces.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

AEO-9体系溶致液晶性能及其组成方程

以非离子表面活性剂十二烷基醇聚氧乙烯醚(AEO-9)/正丁醇/正辛烷/水组成的四元体系为研究对象,绘制了拟三元相图。在液晶区选取样品点,拍摄纹理照片,并结合^2HNMR谱图确定了液晶类型其中主要是状液晶。利用小角X射线衍射测定了层状液晶的层间距d,得到层间距d和液晶含水量的关系。根据层状液晶结构的特点,推导出层状液晶组成方程,并对本体系进行了验证。结果表明,根据议程计算出的液晶区域的开头和位置与实验测得的相图中液晶的开头和位置基本相同,为实际应用提供了依据。     

Mechanical model for anisotropic curved interfaces with applications to surfactant-laden liquid-liquid crystal interfaces

A mechanical model for anisotropic curved interfaces, applicable to thermodynamically closed surfactant-laden liquid-liquid crystal interfaces is developed. The model takes into account the mechanical effects due to surface bending and surface tilting (anchoring) and incorporates liquid crystal anisotropy into classical fluid membrane mechanics. In the absence of the aligned liquid crystal, the model converges to the fluid membrane mechanical model, and in the absence of surfactant, it converges to the nematic interface mechanical model. Use of the well-known Helfrich-Rapini-Papoular surface energies leads to the Laplace equation for anisotropic curved interfaces, whose material limits are the vesicle shape equation and the liquid crystal Herring equation. Applications of the model to shape selection in liquid drops embedded in aligned nematic liquid crystals illustrates the competition between anchoring and bending and shows how anisotropic surface tension distorts the droplet and how bending tends to restore the spherical shape. This theory presented in this article shows that the interaction of interfacial anchoring and bending creates new regimes in classical fluid membrane mechanics.