Parity violation and parity conservation in unstirred crystallization: Effect of first crystals

Statistics of nucleation of chiral forms was studied to establish the effect of the number of first crystals and their handedness on distributions of enantiomers. Various bimodal, trimodal and unimodal distributions are obtained in unstirred crystallization, depending on the number of initial crystals and growth conditions. The binomial distribution satisfactorily describes experimental distributions of enantiomeric excess and may be used to predict distributions and probabilities of nucleation of enantiomers. The first nucleated crystals determine the handedness of secondary crystals, and number of initial crystals governs statistics of chiral nucleation. According to the binomial distribution if single crystals nucleate as the first, the bimodal distributions result with D and L peaks. If LD, LL, and DD pairs are nucleated as first, trimodal distributions with D, R, and L peaks are created, and if groups of crystals of various handedness nucleate as the first the unimodal distributions of enantiomeric excess with racemate R peaks are formed. Chiral nucleation experiments on sodium bromate were the basis for the theoretical considerations and verifications of predictions resulting from binomial distributions on probabilities of the creation of L and D crystals, and racemates, and the presence of D, L, and R peaks in the distributions. Growth conditions affect the number of the first crystals and effectiveness of cloning, and as a result, the distributions of enantiomers. Formation of pure enantiomers and/or racemates proves that the conservation of chiral symmetry, and the breakage of chiral symmetry can occur in unstirred crystallization. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chiral Symmetry Breaking in Sodium Chlorate Crystallization from Unstirred Solution

Statistics of nucleation of chiral forms of sodium chlorate from unstirred, achiral solutions was studied. The trimodal distribution of enantiomers was obtained in which apart from the peak for the same probability of the creation of L and D crystals, two other maxima for the large enantiomeric excesses of L and D forms were present. The first nucleated crystals govern the handedness of the secondary crystals and the formation of pure enantiomers and high enantiomeric excesses in crystallization from unstirred, unseeded solutions are caused by the process of chiral symmetry breaking rather than statistical fluctuations.     

氯酸钠晶体生长的手性对称破损研究

我们使用非搅拌的氯酸钠晶体水溶液,研究了晶体的手性对称破损问题.氯酸钠晶体溶液是通过溶解分析纯 NaClO3 粉末于去离子水中配制而成.采用 Mach-Zehnder 干涉方法和称重法测定晶体的溶解度.使用偏光显微镜鉴定了11545颗晶体的手性.这些晶体是从不同pH条件下的90个自发成核和生长实验中获得.实验结果表明:溶液的pH值可影响到氯酸钠晶体的手性分布.晶体手性分布的pH效应,可用随机动力学理论和胚晶凝聚二次成核理论来定性解释.     

Crystallization kinetics of MgSO4·12H2O from different scales of batch cooling scraped crystallizers

For reliably scaling up of crystallizers, a full kinetic model is required in addition to heat, mass and population balances. A method for extracting nucleation and growth kinetic parameters for scaling‐up seeded batch cooling crystallization was developed and demonstrated with a 15 L and in a 115 L scraped crystallizer using MgSO₄·12H₂O as the model system. The method includes fitting the time resolved measured solute concentration and the crystal size distributions with a dynamic population‐based model. The kinetic parameters extracted from the bench‐scale crystallizer agree with those obtained from the pilot scale, confirming that they can be employed for design purposes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New concept of the origin of life on earth

A model of coupled autocatalytic reactions with allowance for the crystallization of diastereomer is considered. It is shown that the differences in the physical properties of diastereomers can be a 100% enantioselective factor, which makes it possible to obtain a significant chiral polarization even at low autocatalysis enantioselectivity. In terms of a complicated model, more complex molecules should have a higher chiral polarization than simpler ones. The calculation of the dynamics of the model under consideration shows that the presence of binomial coefficients in the reaction of pair formation from two different enantiomers (provided that diastereomers have identical properties) leads to the occurrence of an additional 100% enantioselective factor. Estimation shows that the theoretical difference between the right- and left-handed molecules (which is due to weak interaction), described in the literature, is sufficient to explain the directed symmetry breaking and construction of biological molecules from L-amino acids and D-sugars at the origin of life on earth.     

Crystallization of inorganic salts from aqueous solutions in a microwave field

The crystallization of some inorganic salts (KH₂PO₄, NaCl, Sr(NO₃)₂, KNO₂, Ca(OH)₂) by the thermal-gradient (with decreasing temperature) and solvent-evaporation methods using microwave heating of solutions is investigated. It is established that the growth rates of single crystals in a microwave field are an order of magnitude higher than obtained in other known techniques at comparable crystallization temperatures and supersaturations. For example, the growth rate of prismatic faces {100} of KH₂PO₄ crystals is as high as 11 mm/day at supersaturations of ～1.2%. The results obtained are discussed in the context of the effect of microwave radiation on the adsorption surface layers of crystals. Fine-grained phases of the salts under study are obtained by evaporation of the solvent.     

Influence of drawing temperature on the strength of fluoride glass fibers

The tensile strength of fluorozirconate glass fibers was measured as a function of fiber drawing temperature, and strengths ranging from 60–220 MPa were observed. Statistical analysis of the strength data points to a bimodal behavior in the fibers drawn at high temperatures, while the mode of fracture is essentially unimodal for fibers drawn at lower temperatures. SEM examination of the fibers drawn at high temperatures revealed the presence of crystals in the fibers, and these crystals were associated with flaws which gave rise to the observed low strength distribution.     

Calculation of the Suspension Volumes of MSMPR Cascade Crystallizers

A new algorithm for the design of cascade crystallizers with and without pure solution removal for the individual crystallizers has been described. The algorithm enables the calculation of active suspension volumes and mean retention times, as well as mean size of product crystals and kinetic quantities for the individual stages for the both variants: — presumption of the mean product sizes in each stage, or — presumption of the mean retention times in each stage. Results have been shown and discussed for a two stage cascade of KCl-vacuum crystallizers as example.     

New Chiral Esters,Diastereomeric 2-(4-Carboxybenzylidene)-p-Menthane-3-One Derivatives,as Components of LC Systems with Induced Helical Structure

Abstract

Diastereomeric 1R, 4S- and 1R, 4R-2-carbomethoxybenzylidene-p-menthane-3-ones have been synthesized. The helical twisting power and the induced helical pitch temperature dependence have been measured for the liquid crystal mixtures based on the nematic 4-methoxybenzylidene-4′-butylaniline and 4-pentyl-4′-cyanobiphenyl containing the obtained diastereomeric chiral dopants. The distinction in the characteristics of the diastereomeric chiral dopants is discussed.     

Crystal size distribution in precipitates

After a short introduction on the principles of nucleation and growth – here especially considering defect crystals – particle size distribution in precipitates is discussed. At first batch crystallization is referred to briefly. The situation in stirred continuous tank crystallizers is investigated in detail. Size distribution is considered in dependence on steady state flow rates of the reacting solutions, residence time of crystals in the tank, resulting in an exponential equation for the number of crystals per unit volume: \documentclass{article}\pagestyle{empty}\begin{document}$ n\left(r \right) = \frac{{\dot n_0 }}{{f\left(s \right)}}e^ - \frac{{vr}}{{V \cdot f\left(s \right)}} $\end{document}. With two and more tanks in series size distribution additionally becomes dependent on the difference of the supersaturations, of the occurrence of nucleation, and of the residence time of crystals in the various tanks. Equations for the size distribution are given for special conditions with two and more tanks in series.     

Enantiomorphism in sodium chlorate crystals

Creation of enantiomorphous forms in sodium chlorate and enantiomorphous nature of surface micromorphology were studied by optical microscopy. It was shown that when the small number of crystals nucleates spontaneously in unstirred, unseeded solutions then exclusively left- (L) or right-handed (D) crystals can be formed. At the large number of nucleated crystals the probability of the creation of L and D form is nearly the same. L and D crystals can be distinguished by the shape of growth or dissolution centers.     

Synthesis and Some Properties of Low Melting Ferroelectric Liquid Crystals of Benzylidene Anilines

A series of chiral benzylidene-aniline compounds, 4-alkoxybenzylidene 4′-(2-methylbutyloxycarbonyl)anilines, have been synthesized and their mesomorphic properties are examined. They showed a monotropic chiral smectic C phase in conjunction with enantiotropic smectic A phase, and their chiral smectic C—smectic A phase transition temperatures are around room temperatures. Their spontaneous polarization values are moderately higher than those of DOBAMBC “4-decyloxybenzylidene 4′-amino-(2-methylbutyl)cinnamate”.     

Evaporite minerals succession

An explanation is given for the crystallization succession during sea-water evaporation, namely for the differences between the equilibrium solubility diagram of the marine-type system and the “solar diagram”: The magnesium ions are known to exist in sea-type solutions predominantly as hexaaqua complexes. Highest crystallization rates and lowest supersaturations required for nucleation should be exhibited by the magnesium salts, crystal structures of which incorporate unchanged hexaaqua magnesium complexes. For the salts with crystal structures in which the water molecules from the magnesium hexaaqua ions are partially or completely replaced by other ligands, the nucleations require high supersaturations and for this reason they do not appear in the “solar diagram”. This model is used to explain also some genetic pecularities of evaporite deposits: primary evaporite minerals are those which occur according to the “solar diagram” of crystallization. The evaporite minerals featuring in the equilibrium diagram and absent in the “solar diagram” occur in nature most frequently as secondary minerals.     

Growth morphology of potassium bichromate crystals from aqueous solutions

The results of a systematic study of growth morphology of potassium bichromate single crystals on seeds and by self-nucleation at different temperatures and supersaturations are presented. The observed growth morphology is analysed in the light of theoretical predictions. An increase in both growth temperature and supersaturation leads to a symmetrical morphology, and the transition from asymmetrical to symmetrical morphology is monotonous. The predicted sequence of morphological importance agrees only in the case of crystals grown at low temperatures and supersaturations.     

Occurrence and consequences of differences in the cation/anion ratios during classical resolution: D‐/L‐serine benzyl ester 2,3‐toluyl‐D‐tartrate

Differences in physical‐chemical properties of diaste‐reomeric salts allow the separation into the respective salts and subsequently into enantiomers by crystallization. Within this study unusual deviations in the cation‐/anion‐ratio of a diastereomeric salt pair were observed and characterized. While the n‐salt (L‐serine benzyl ester 2,3‐toluyl‐D‐tartrate) crystallizes in a ratio of two cations and one anion, the p‐salt (D‐serine benzyl ester 2,3‐toluyl‐D‐tartrate) consists of only one cation and one anion. Consequently the classical definition of a diastereomeric salt pair does not apply. In this contribution all differences in relevant thermodynamic properties of the unusual resulting diastereomeric salt pair are presented and discussed.     

基于光谱测试的纯净金刚石品质与缺陷研究

本文利用低温拉曼与光致发光光谱对纯净金刚石晶片的结晶质量、晶体应力分布进行表征分析,并结合电子辐照和快速退火对晶片的杂质缺陷结构开展研究。通过拉曼光谱对金刚石特征峰的表征分析发现,由于金刚石生长机制以及晶片的切割、抛光等因素影响,晶片的边缘与表面应力分布较高。光致发光光谱中的零声子线具有明显的温度依赖性,根据Jahn-Teller效应与电子-声子耦合理论阐明了测试温度变化引起中性单空位缺陷零声子线分裂与红移的机理。对晶片做电子辐照与退火调控处理后,晶片中氮-空位(NV)缺陷显著增多,表明在纯净晶片中氮主要以替代位氮杂质的形式存在。     

Epitaxial growth of silicon carbide layers by sublimation „sandwich method”︁ (I) growth kinetics in vacuum

The growth kinetics of SiC epitaxial layers has been investigated by sublimation sandwich-method in vacuum at temperature range from 1600 to 2100°C. The limiting stages of the crystallization process have been determined. Silicon deficit in the growth cell was shown to result in the great retarding of SiC epitaxial layers growth due to the decreasing of evaporation coefficient by a factor of 10¹–10² and more. The impurities introduced into the system at low supersaturations and temperatures, especially rare-earth elements, Al, B, Cr reduce as a rule evaporation and condensation coefficients and therefore the growth rate of epitaxial layers.     

Practical experimental design techniques for automatic and manual protein crystallization

When crystallizers are searching for the optimum crystallization conditions, they often carry out experiments that are confusing and difficult to interpret. This confusion arises because there are several important variables in any protein crystallization experiment (including protein concentration, precipitant concentration, pH and temperature) and these variables often interact–that is to say, changes in the level of one variable often change the optimum settings of the others. Confusion can be avoided by using appropriate experimental designs where all of the important variables are varied in each experimental run. Some well known and practical designs for automatic and manual crystallization are presented, and a simple practical example is given.     

Agglomeration of potash alum crystals

Crystals of potash alum, crystallized in suspension at higher supersaturations, are partially agglomerated. Two sets of experiments have been chosen: batch experiments with precipitation of potash alum from its components and continuous crystallization by cooling. Using three different ways described in the preceding paper the size distribution of potash alum crystals was analyzed and the effect of agglomeration on calculated kinetic data demonstrated.     

On the kinetic of crystallization of zinc oxalate (II). Determination of the specific surface energy at the crystal solution interface by kinetic studies

An apparatus based on turbidimetry was constructed for studying the kinetics of crystallization of sparingly soluble salts. The kinetics of crystallization of zinc oxalate was investigated at various supersaturations and pHs of the medium. The specific surface energy at the crystal-solution interface was determined from the induction periods of the S-shaped curves by means of the classical theory of nucleation. The calculations were made using the slope of the logarithmic dependence of I or τ, respectively, on supersaturation (equations 1 and 2) and the critical supersaturation which was also determined on the basis of kinetic data. Various values of σ were found for the different supersaturations. Different values for this quantity were also obtained when supersaturation was presented by the concentration ratio (S = C/C₀) or by the ratio of the product of the two ions concentrations in the supersaturated solution to that in the saturated one (S = ab/L_p). These σ values were lower than those obtained using the critical supersaturations at the two different solution pHs. For the present it is impossible to give a definite explanation of the results obtained. The experiments for determining the specific surface energy at the crystal-solution interface will be continued.