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Crystal Habits (crystal + habit)
Selected AbstractsMechanisms Controlling Crystal Habits of Gold and Silver ColloidsADVANCED FUNCTIONAL MATERIALS, Issue 7 2005C. Lofton Abstract Examples of gold and silver anisotropic colloids, such as prisms and rods, have appeared in the literature for many years. In most cases, the morphologies of these thermodynamically unfavorable particles have been explained by the particular reaction environment in which they were synthesized. The mechanisms used to explain the growth generally fall into two categories, one in which chemically adsorbed molecules regulate the growth of specific crystal faces kinetically, and the other where micelle-forming surfactants physically direct the shape of the particle. This paper raises questions about the growth of anisotropic metal colloids that the current mechanisms cannot adequately address, specifically, the formation of multiple shapes in a single homogeneous reaction and the appearance of similar structures in very different synthesis schemes. These observations suggest that any growth mechanism should primarily take into consideration nucleation and kinetics, and not only thermodynamics or physical constrictions. The authors suggest an alternative mechanism where the presence and orientation of twin planes in these face-centered cubic (fcc) metals direct the shape of the growing particles. This explanation follows that used for silver halide crystals, and has the advantage of explaining particle growth in many synthesis methods. In this mechanism, twin planes generate reentrant grooves, favorable sites for the attachment of adatoms. Shape and structural data are presented for gold and silver particles synthesized using several different techniques to support this new model. Triangular prisms are suggested to contain a single twin plane which directs that growth of the initial seed in two dimensions, but limits the final size of the prism. Hexagonal platelets are suggested to contain two parallel twin planes that allow the fast growing edges to regenerate one another, allowing large sizes and aspect ratios to form. Rods and wires were found to have a fivefold symmetry, which may only allow growth in one dimension. It is expected that a superior mechanistic understanding will permit shape-selective synthesis schemes to be developed. [source] The influence of supersaturation on crystal morphology , experimental and theoretical studyCRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2005Jun Jun Lu Abstract The surface docking approach of molecular modeling for prediction of crystal morphology in the presence of additives is further developed in this work. It is modified in order to take into account the influence of supersaturation on the crystal habit. With the introduction of the BCF theory and the 2-D growth mechanism in the above mentioned approach predictions have been carried out for paracetamol and caprolactam crystals, respectively. The verifications by crystals grown from solutions as simulated by the computer model give good agreements. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Investigation of mechanism of Al(OH)3 crystal growthTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2009I. Nikoli Abstract Crystallization of Al(OH)3 that occurs during the decomposition of caustic soda solutions is an important part of Bayer process for alumina production. Several phenomena, which influence the physicochemical characteristics of precipitated Al(OH)3, occur simultaneously during this process. They are nucleation, agglomeration, and crystal growth of Al(OH)3. In this article, we have investigated the mechanism of Al(OH)3 crystal growth from pure caustic soda solutions and in the presence of oxalic acid. The results have shown that the growth of Al(OH)3 crystals from caustic soda solutions follow the B,+,S model (birth and spread). New Al(OH)3 particles, formed during the decomposition process of pure caustic soda solutions, are characterized by regular hexagonal shape. The nuclei have the same geometry as the contact face. However, microstructural investigations of Al(OH)3 samples, obtained by crystallization from caustic soda solutions in the presence of oxalic acid, have shown the presence of nuclei of irregular shape in addition to regular ones. So, the presence of oxalic acid in the caustic soda solutions leads to a change in crystal habit. Besides, the results obtained by kinetic investigation confirmed the mentioned mechanism of Al(OH)3 crystal growth. La cristallisation de l'Al(OH)3 qui accompagne la décomposition de solutions de soude caustique est une caractéristique importante du procédé Bayer de production d'alumine. Plusieurs phénomènes influant sur les caractéristiques physicochimiques de l'Al(OH)3 précipité, se produisent simultanément lors de ce procédé. Ce sont la nucléation, l'agglomération et la croissance des cristaux de l'Al(OH)3. Dans cet article, nous avons étudié le mécanisme de croissance des cristaux de l'Al(OH)3 à partir de solutions de soude caustique pures et en présence d'acide oxalique. Les résultats montrent que la croissance des cristaux d'Al(OH)3 à partir des solutions de soude caustique suit le modèle B,+,S (naissance (birth),+,dispersion (spread)). Les nouvelles particules d'Al(OH)3, formées lors du procédé de décomposition des solutions de soude caustique, se caractérisent par une forme hexagonale régulière. Les nuclei ont la même géométrie que la face de contact. Cependant, des recherches sur la microstructure des échantillons d'Al(OH)3 obtenus par cristallisation à partir des solutions de soude caustique en présence d'acide oxalique, montrent la présence de nuclei de forme irrégulière en plus de ceux de forme régulière. Ainsi, la présence d'acide oxalique dans les solutions de soude caustique mène à un changement dans le comportement des cristaux. En outre, les résultats obtenus dans l'étude cinétique confirment ce mécanisme de croissance des cristaux d'Al(OH)3. [source] An overview of the microphysical structure of cirrus clouds observed during EMERALD-1THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 607 2005M. W. Gallagher Abstract High-resolution ice microphysical, turbulence, heat and water vapour flux data in cirrus clouds were collected by the Airborne Research Australia's (ARA) Egret Grob 520T research aircraft during the first Egret Microphysics with Extended Radiation and Lidar experiment (EMERALD-1). The in situ cirrus measurements were guided by simultaneous airborne lidar measurements collected by the ARA Super King Air research aircraft which flew below the cirrus and whose horizontal position was synchronized with the Egret. This allowed the microphysics and turbulence measurements to be interpreted and evaluated within the context of large-scale cirrus structure and its evolution. A significant feature of the clouds observed was the presence on occasion of active convective columns. Large variations in the cirrus dynamics were observed, with significant variations in the ice crystal habit from cloud top to cloud base and within the evaporating fall-streaks of precipitation. However, on average the picture presented is consistent with that shown by Heymsfield and Miloshevich, and by Kajikawa and Heymsfield, with the upper supersaturated region of the cloud acting as an active particle-generation zone where homogeneous nucleation proceeds apace; ice crystals there are initially dominated by small irregular or spheroidally shaped particles, some of which can be identified as proto or ,germ' rosettes. These are then observed to grow into more open bullet rosette and columnar types as they fall into the less supersaturated middle and lower layers of the cloud. The mean recognisable ice particle size fell within a very narrow size band, 70,90 µm, but the actual size distribution is thought to increase in a continuous manner to smaller sizes. However, there are currently instrument limitations that make it difficult to confirm this unambiguously. Unlike most previous studies, however, the cirrus clouds observed here were mostly devoid of pristine plate-like crystals, as nucleation and growth within the planar growth regime was rarely encountered. During some cases bullet rosettes, once formed, did undergo transition to the plate growth regime with complex crystal shapes resulting. The mean size of pristine bullet rosettes was again confined to a relatively narrow range. The likely nucleation processes dominating in cirrus clouds are discussed in the light of the observations. Very high concentrations of small ice crystals were sometimes detected, concentrations reaching a maximum of 10 000 L,1. There is strong evidence supporting these high concentrations which are probably produced by the homogeneous freezing of aerosol. Copyright © 2005 Royal Meteorological Society [source] Moisture barrier and physical properties of acetylated derivatives with increasing acetylation degreeEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 5 2009Claire Bourlieu Abstract Four acetostearin products with increasing acetylation degree were synthesized by chemical interesterification followed by fractionation/blending stages. Their physical properties and functional barrier properties were studied and compared to the properties of technical tristearin. Increasing acetylation degree (AD) modified the triacylglycerols crystal habits and probably led to an increase in acyl chain fluidity, which induced, at macroscopic levels, a decrease in solid fat content (SFC), in melting point, in surface and bulk material hydrophobicity, and an increased moisture effective diffusivity. Water vapor permeability (WVP) coefficients of the materials were partially influenced by the AD factor, but also by the development of macroscopic cracks in lipids presenting high SFC. Acetylated stearin up to 47% (acetyl mol/mol of esterified chain) presented the lowest WVP at 20,°C resulting from an adequate balance between hydrophobicity and mechanical properties of the material. [source] Crystallization and preliminary crystallographic analysis of endonuclease VIII in its uncomplexed formACTA CRYSTALLOGRAPHICA SECTION D, Issue 8 2004Gali Golan The Escherichia coli DNA repair enzyme endonuclease VIII (EndoVIII or Nei) excises oxidized pyrimidines from damaged DNA substrates. It overlaps in substrate specificity with endonuclease III and may serve as a back-up for this enzyme in E. coli. The three-dimensional structure of Nei covalently complexed with DNA has been recently determined, revealing the critical amino-acid residues required for DNA binding and catalytic activity. Based on this information, several site-specific mutants of the enzyme have been tested for activity against various substrates. Although the crystal structure of the DNA-bound enzyme has been fully determined, the important structure of the free enzyme has not previously been analyzed. In this report, the crystallization and preliminary crystallographic characterization of DNA-free Nei are described. Four different crystal habits are reported for wild-type Nei and two of its catalytic mutants. Despite being crystallized under different conditions, all habits belong to the same crystal form, with the same space group (I222) and a similar crystallographic unit cell (average parameters a = 57.7, b = 80.2, c = 169.7,Å). Two of these crystal habits, I and IV, appear to be suitable for full crystallographic analysis. Crystal habit I was obtained by vapour diffusion using PEG 8000, glycerol and calcium acetate. Crystal habit IV was obtained by a similar method using PEG 400 and magnesium chloride. Both crystals are mechanically strong and stable in the X-ray beam once frozen under cold nitrogen gas. A full diffraction data set has recently been collected from a wild-type Nei crystal of habit I (2.6,Å resolution, 85.2% completeness, Rmerge = 9.8%). Additional diffraction data were collected from an Nei-R252A crystal of habit IV (2.05,Å resolution, 99.9% completeness, Rmerge = 6.0%) and an Nei-E2A crystal of habit IV (2.25,Å resolution, 91.7% completeness, Rmerge = 6.2%). These diffraction data were collected at 95,100 K using a synchrotron X-ray source and a CCD area detector. All three data sets are currently being used to obtain crystallographic phasing via molecular-replacement techniques. [source] An In Vitro Investigation of Pulmonary Alveolar Macrophage Cytotoxicity Introduced by Fibrous and Grainy Mineral DustsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2006DONG Faqin Abstract, In order to study the damage mechanism of mineral dusts on the pulmonary alveolar macrophage (AM), the changes in their death ratio, malandialdthyde (MDA) content and activities of lactate dehydrogenase (LDH) and superoxide dismutase (SOD) were measured, and the technique of cell culture in vitro was used to investigate the cytotoxicity of six mineral dusts (twelve crystal habits) from twelve mineral deposits. The results show that wollastonite and clinoptilolite have no AM cytotoxicity, while other fibrous and grainy mineral dusts damage pulmonary AM in various degrees. The cytotoxicity of fibrous mineral dusts was greater than that of the grainy ones, and the cytotoxicity of dusts was positively correlated with the active OH content in dusts, but not necessarily so with its SiO2 content. The high pH values produced by dust was unfavorable for the survival of cells and the dusts with low bio-resistance were safe for cells. The content of variable valence elements in dusts might influence their cytotoxicity and the surface charge of dusts was not a stable factor for their toxicity. It is demonstrated that the shape of mineral dusts was one of the factors affecting cytotoxicity, and that the cytotoxicity of mineral dusts depends mainly on their properties. [source] | |||||||||||||