Formation and stability of cubic ice in water droplets

There is growing evidence that a metastable phase of ice, cubic ice, plays an important role in the Earth's troposphere and stratosphere. Cubic ice may also be important in diverse fields such as cryobiology and planetary sciences. Using X-ray diffraction, we studied the formation of cubic ice in pure water droplets suspended in an oil matrix as a function of droplet size. The results show that droplets of volume median diameter 5.6 microm froze dominantly to cubic ice with stacking faults. These results support previous suggestions that cubic ice is the crystalline phase that nucleates when pure water droplets freeze homogeneously at approximately 235 K. It is also shown that as the size of the water droplets increased from 5.6 to 17.0 microm, the formation of the stable phase of ice, hexagonal ice, was favoured. This size dependence can be rationalised with heat transfer calculations. We also investigated the stability of cubic ice that forms in water droplets suspended in an oil matrix. We observe cubic ice up to 243 K, much higher in temperature than observed in many previous studies. This result adds to the existing literature that shows bulk ice I(c) can persist up to approximately 240 K. The transformation of cubic ice to hexagonal ice also showed a complex time and temperature dependence, proceeding rapidly at first and then slowing down and coming to a halt. These combined results help explain why cubic ice forms in some experiments described in the literature and not others.     

Novel ice structures in carbon nanopores: pressure enhancement effect of confinement

We report experimental results on the structure and melting behavior of ice confined in multi-walled carbon nanotubes and ordered mesoporous carbon CMK-3, which is the carbon replica of a SBA-15 silica template. The silica template has cylindrical mesopores with micropores connecting the walls of neighboring mesopores. The structure of the carbon replica material CMK-3 consists of carbon rods connected by smaller side-branches, with quasi-cylindrical mesopores of average pore size 4.9 nm and micropores of 0.6 nm. Neutron diffraction and differential scanning calorimetry have been used to determine the structure of the confined ice and the solid-liquid transition temperature. The results are compared with the behavior of water in multi-walled carbon nanotubes of inner diameters of 2.4 nm and 4 nm studied by the same methods. For D(2)O in CMK-3 we find evidence of the existence of nanocrystals of cubic ice and ice IX; the diffraction results also suggest the presence of ice VIII, although this is less conclusive. We find evidence of cubic ice in the case of the carbon nanotubes. For bulk water these crystal forms only occur at temperatures below 170 K in the case of cubic ice, and at pressures of hundreds or thousands of MPa in the case of ice VIII and IX. These phases appear to be stabilized by the confinement.     

Is it cubic? Ice crystallization from deeply supercooled water

Ice crystallized below 200 K has the diffraction pattern of a faulty cubic ice, and not of the most stable hexagonal ice polymorph. The origin and structure of this faulty cubic ice, presumed to form in the atmosphere, has long been a puzzle. Here we use large-scale molecular dynamics simulations with the mW water model to investigate the crystallization of water at 180 K and elucidate the development of cubic and hexagonal features in ice as it nucleates, grows and consolidates into crystallites with characteristic dimensions of a few nanometres. The simulations indicate that the ice crystallized at 180 K contains layers of cubic ice and hexagonal ice in a ratio of approximately 2 to 1. The stacks of hexagonal ice are very short, mostly one and two layers, and their frequency does not seem to follow a regular pattern. In spite of the high fraction of hexagonal layers, the diffraction pattern of the crystals is, as in the experiments, almost identical to that of cubic ice. Stacking of cubic and hexagonal layers is observed for ice nuclei with as little as 200 water molecules, but a preference for cubic ice is already well developed in ice nuclei one order of magnitude smaller: the critical ice nuclei at 180 K contain approximately ten water molecules in their core and are already rich in cubic ice. The energies of the cubic-rich and hexagonal-rich nuclei are indistinguishable, suggesting that the enrichment in cubic ice does not have a thermodynamic origin.     

The proper structure of cubic ice confined in mesopores

To examine the reason for the formation and the structure of cubic ice in a restricted space, we measured the powder x-ray diffraction patterns of cubic ice formed within the mesopores of porous silicas as a function of pore size (4-70 nm). The results strongly suggest that cubic ice formed in the mesopores does not take a cubic structure as envisaged by Konig. It may be actually composed of very small crystallites of hexagonal ice that contains a large amount of growth faults depending on the crystallite size, that is, ice with disordered stacking sequence. Suppression of crystal growth of ice in the mesopores seems to be a vital factor for the formation and the stability of cubic ice.     

Simultaneous XRD-DSC measurements of water-2-propanol at sub-zero temperatures.

A simultaneous low-temperature X-ray powder-diffractometric (XRD)-DSC technique was applied to the solid state and melting process of frozen aqueous solutions of 2-propanol. 1H NMR spectra were also obtained at low temperatures. The chemical shifts of the CH3 proton and the CH proton can be classified into four temperature regions: higher than -20 degrees C, around -20 degrees C, -50 to -20 degrees C, and lower than -50 degrees C. In the XRD data, five small diffraction peaks for 2 theta at 21.0 degrees, 25.2 degrees, 27.8 degrees, 31.1 degrees and 32.1 degrees can be attributed to the peritectic, while five diffraction peaks at 22.5 degrees, 24 degrees, 25.6 degrees, 33.4 degrees and 39.8 degrees can be attributed to ice; these peaks are due to the hexagonal form of ice, which disappears upon melting. However, the diffraction peak at 33.4 degrees showed a different pattern than the other peaks due to hexagonal ice. These results indicate that the temperature dependence of the diffraction peak at 33.4 degrees for 2 theta is related to the formation of hydrogen bonds between 2-propanol and water. The simultaneous XRD-DSC technique was effective for investigating this water-alcohol mixture at low temperatures.     

密堆积六方结构Ni纳米颗粒的制备与表征

采用溶胶-凝胶方法制备前驱体, 将前驱体进行热处理制得密堆积六方结构(hcp)的Ni纳米粉末. 利用TG-DTA, XRD和TEM等测试手段对材料的合成条件、结构、形貌以及结构演变过程进行了分析. 结果表明, 于300 ℃进行热处理所合成的样品为球形的具有密堆六方结构的Ni纳米颗粒, 晶胞参数a=0.2652 nm, c=0.4334 nm, 平均晶粒尺寸约为12 nm. 随着热处理温度的升高, 样品结构发生由密堆六方结构向面心立方结构的转变.     

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.     

预处理条件对La_2O_3上过氧物种光诱导生成的影响

采用原位显微Raman光谱技术详细考察了焙烧温度和焙烧时间对La(OH)3分解制备的La2O3结构以及过氧物种光诱导生成性能的影响,结果表明,经700℃焙烧所得La2O3样品较经800℃以上长时间焙烧的样品更有利于过氧物种的生成.对La(OH)3热分解过程的原位XRD测试结果表明,焙烧温度需达到近700℃才可使La(OH)3完全转化为La2O3.在700℃焙烧的样品上,除了六方相的La2O3外,还可检出介稳态的立方相La2O3.经800℃以上长时间(≥5 h)焙烧后,介稳态的立方相La2O3将转化为稳定的六方相La2O3物种.在相同的实验条件下,立方相的稀土倍半氧化物较六方相更有利于过氧物种的光诱导生成,其原因可能源于前者含更多氧空位,因而更有利于对分子氧的吸附和活化.     

Inhibition of solute crystallisation in aqueous H(+)-NH(4)(+)-SO4(2-)-H2O droplets

Ice clouds in the Earth's upper troposphere can form via homogeneous nucleation of ice in aqueous droplets. In this study we investigate the crystallisation, or lack of crystallisation, of the solute phase and ice in aqueous (NH(4))(3)H(SO(4))(2)/H(2)O and NH(4)HSO(4)/H(2)O droplets. This is done using in situ X-ray diffraction of emulsified solution droplets mounted on a cold stage. From the diffraction patterns we are able to identify the phases of crystalline solute and ice that form after homogeneous freezing in micrometer sized droplets. An important finding from this study is that crystallisation of the solute does not always occur, even when crystallisation is strongly thermodynamically favoured. The nucleation and growth of solute phase crystals becomes inhibited since the viscosity of the aqueous brine most likely increases dramatically as the brine concentration increases and temperature decreases. If ice nucleates below a threshold freezing temperature, the brine appears to rapidly become so viscous that solute crystallisation is inhibited. This threshold temperature is 192 K and 180 K, in (NH(4))(3)H(SO(4))(2) and NH(4)HSO(4), respectively. We also speculate that the formation of cubic ice within a highly viscous brine blocks the solvent mediated cubic to hexagonal phase transformation, thus stabilising the metastable cubic ice in the most concentrated solution droplets.     

Water's size-dependent freezing to cubic ice

Water has been occasionally found to freeze to cubic ice. To investigate this occurrence thermodynamically, we use the known enthalpy and interfacial energy of hexagonal and cubic ices and calculate a critical radius r(c) of appromximately 15 nm for a water droplet and a critical thickness delta(c) of approximately 10 nm for water's flat film. Accordingly, water droplets smaller than 15 nm radius and films thinner than 10 nm would freeze to cubic ice in the 160-220 K range and bigger droplets and thicker films would freeze to hexagonal ice. This provides a thermodynamic basis for the occasionally found presence of cubic ice in the atmosphere, and explains why water's nanometer-sized clusters and water confined to nanometer-sized pores freeze to cubic ice. Conditions for cubic ice-hexagonal ice phase inversion have been discussed. Impurities in water and different extents of proton ordering in the crystallites of cubic and hexagonal ices would have a significant effect on r(c) and delta(c).     

Effect of high-pressure high-temperature treatment on the structure of a hexagonal modification of fullerite C<Subscript>60</Subscript>

Powder X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and molecular dynamics have been employed to investigate structural transformations in hexagonal and cubic modifications of fullerite C₆₀ after the action of high pressure (4 GPa) within the temperature range 20–1450°C. It has been found that fullerene molecules polymerize to afford polymer structures only in the case of face-centered cubic samples. Under the effect of high pressure and temperature, fullerite C₆₀ with a hexagonal close-packed structure is initially transformed into the cubic modification and, then, forms polymerized structures, which, during an increase in the treatment temperature, become less stable and ordered than the same polymerized structures obtained directly from cubic fullerite C₆₀. X-ray photoelectron spectroscopy measurements suggest deformation of the cages of fullerene molecules in the polymerized structures.     

Structures of hydrocarbon hydrates during formation with and without inhibitors

The formation of hydrates from a methane-ethane-propane mixture is more complex than with single gases. Using nuclear magnetic resonance (NMR) and high-pressure powder X-ray diffraction (PXRD), we have investigated the structural properties of natural gas hydrates crystallized in the presence of kinetic hydrate inhibitors (KHIs), two commercial inhibitors and two biological ice inhibitors, or antifreeze proteins (AFPs). NMR analyses indicated that hydrate cage occupancy was at near saturation for controls and most inhibitor types. Some exceptions were found in systems containing a new commercial KHI (HIW85281) and a recombinant plant AFP, suggesting that these two inhibitors could impact the kinetics of cavity formation. NMR analysis confirmed that the hydrate composition varies during crystal growth by kinetic effects. Strikingly, the coexistence of both structures I (sI) and II (sII) were observed in NMR spectra and PXRD profiles. It is suggested that sI phases may form more readily from liquid water. Real time PXRD monitoring showed that sI hydrates were less stable than sII crystals, and there was a conversion to the stable phase over time. Both commercial KHIs and AFPs had an impact on hydrate metastability, but transient sI PXRD intensity profiles indicated significantly different modes of interaction with the various inhibitors and the natural gas hydrate system.     

Thermal expansion of structure-H clathrate hydrates

The temperature dependence of the unit cell parameters of two newly identified hexagonal structure clathrate hydrates of hexamethylethane (HME) and 2,2-dimethylbutane (DMB) have been measured by X-ray powder diffraction. The thermal expansion of the two distinct crystallographic axes was found to be inequivalent. However, the coefficients of cubic expansion are comparable to that in the cubic structure I and II hydrates. The larger thermal expansivity in the clathrate hydrates relative to ice is attributed to the weakening of the host lattice due to the internal pressure generated by the rattling motions of the encaged guests.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Interactions of surfactants in nonionic/anionic reverse hexagonal mesophases and solubilization of α-chymotrypsinogen A

In an attempt to form H_II mesophases at room temperature we prepared lyotropic liquid crystals with two surfactants of the same lipophilic tails (glycerol monooleate, GMO, and oleyl lactate, OL) but differing in the size and charge of the headgroups.Increasing OL concentration significantly affected the hydration of the headgroups and subsequently the lipids packing. At low OL content the cubic mesophase was formed, while at higher OL contents the formation of hexagonal mesophase was favored. It was assumed that OL competed on the water binding, tuning the headgroups’ curvature and the packing parameter inducing the formation of reverse hexagonal mesophase. It was detected that cubic mesophase transformed upon heating to hexagonal structures. The hexagonal mesophases, which were formed both immediately after preparation and after aging, remained stable at elevated temperatures.α-Chymotrypsinogen was solubilized into the obtained LLCs at relatively high concentration (up to 1 wt%). The lattice parameter of the host LLCs exhibited a decrease as a function of the protein content. This process was assigned to partial dehydration of the GMO polar moieties in favor to CTA hydration.Generally speaking, the present study indicated that adding anionic to nonionic lipid is highly beneficial to gain additional compositional and structural characteristics of LLCs.     

蛋清蛋白辅助水热法制备CeO_2纳米棒及其发光性能

本文以氯化铈和氢氧化钠为反应原料,以新鲜蛋清为络合剂,探索在水热体系中制备CeO2纳米棒。通过XRD、FESEM等测试手段对其进行了结构和形貌分析;应用荧光光谱法对样品的发光性能进行了测试。XRD结果表明,实验所得产物为立方萤石结构CeO2。FESEM分析可知,所得CeO2纳米棒为规则六方柱结构,其长度约为200 nm、直径约为几十纳米。荧光光谱分析可知其在紫蓝色发光区具有较强的发光性能。     

Phase- and size-controlled synthesis of hexagonal and cubic CoO nanocrystals

Highly crystalline, phase- and size-controlled CoO nanocrystals of hexagonal and cubic phases have been prepared by thermal decomposition of Co(acac)3 in oleylamine under an inert atmosphere. Kinetic and thermodynamic control for the precursor formation leads to two different seeds of hexagonal and cubic phases at higher temperatures. The crystal size of both CoO phases can be easily manipulated by changing the precursor concentration and reaction temperature.     

纳米管状结构立方萤石型CeO2的合成(英)

低温水热条件下，Ce⁴⁺离子经尿素水解均匀沉淀，在十八胺分子模板导向下形成了多层有序排列的纳米管状结构的立方萤石型CeO₂，采用粉末X射线衍射(PXRD)、透射电镜(TEM)、高分辨透射电镜(HRTEM)以及选区电子衍射(SEAD)等手段对产物进行了表征。结果表明，该结构是由单根外、内径分别为10～20 nm、5～6 nm左右的纳米管有序排列形成的纳米管束构成，且产物具有结晶性良好的立方萤石型CeO₂晶体结构。在管束的生长过程中，反应体系首先形成单根纳米管，继而管与管之间又相互缔合生长成多层有序排列的纳米管束。控制尿素的水解，获得了不同pH值下的反应产物，发现反应体系pH值在5～6之间时产物为立方萤石型CeO₂、6～8时为立方萤石型CeO₂与六方型Ce(OH)CO₃的混合物、大于8则为六方型Ce(OH)CO₃。     

Temperature effect on the structure and characteristics of ZnS-based quantum dots

The synthesis of zinc sulfide (ZnS) quantum dots (QDs) by microwave heating in a water-ethanol medium is proposed. The effect of the synthesis temperature (80 °C, 100 °C, 120 °C, and 150 °C) on the QD characteristics is examined. Based on the analysis of X-ray diffraction profiles the conclusion is drawn that the hexagonal ZnS phase of wurtzite type with an average nanocrystal size of 2.6-3.7 nm forms in the synthesized QDs. The nanocrystallite size is found to increase with the QD synthesis heating temperature. The analysis of X-ray absorption spectra (XANES) at the zinc K-edge indicates a higher crystallinity of the QD samples prepared at higher synthesis temperatures. The combined analysis of X-ray diffraction profiles, optical diffuse reflectance spectra, and X-ray absorption spectra implies the following possible QD structure: the pure hexagonal ZnS phase of wurtzite type in the bulk of nanoparticles and the amorphous ZnO phase in the surface layer of nanoparticles.     

An ice phase of lowest thermal conductivity

On pressurizing at temperatures near 130 K, hexagonal and cubic ices transform implosively at 0.8-1 GPa. The phase produced on transformation has the lowest thermal conductivity among the known crystalline ices and its value decreases on increase in temperature. An ice phase of similar thermal conductivity is produced also when high-density amorphous ice kept at 1 GPa transforms on slow heating when the temperature reaches approximately 155 K. These unusual formation conditions, the density and its distinguished thermal conductivity, all indicate that a distinct crystal phase of ice has been produced.     

紫外光辐照法制备金纳米盘及其反应机理

用紫外光辐照氯金酸、聚乙烯吡咯烷酮(PVP)和纳米金种子的混合溶液, 在室温下用30 min制备出尺度小于100 nm的截角三角形或六边形金纳米盘. X射线能谱和衍射分析表明粒子是以{111}面为盘状面的高纯面心立方金单晶, 红外透射光谱表明金粒子与PVP之间存在作用. 产物的可见吸收光谱表现出纳米盘的各向异性表面等离子体共振吸收峰. 不同实验条件下产物的吸收光谱分析表明: PVP起还原剂和包覆剂的作用; 高强度紫外光加速了反应进行; 种子对反应具有催化作用; 种子的加入量有最佳值, 在该值下纳米盘平均尺度最大(达80 nm), 吸收谱上的面内偶极共振峰位于950 nm处; 种子的加入量超过该值时, 纳米盘尺度变小, 面内偶极共振峰发生蓝移.