The seeded growth of calcium phosphates. Surface characterization and the effect of seed material

The kinetics of growth of calcium phosphate on seed materials hydroxyapatite, “amorphous” calcium phosphate, enamel, and dentine has been studied at 25°C and at constant physiological pH in stable supersaturated solutions of calcium phosphate. The grown material was characterized chemically, by x-ray powder diffraction and specific surface area measurements and by scanning electron microscopy. In addition, dissolution kinetic studies have been made of the material grown at different times in the crystal growth experiments and of “amorphous” calcium phosphate. The crystallization experiments were made over a range of initial supersaturation to obtain information about the nature of the calcium phosphate phases formed. The specific surface area of the grown phases show marked changes during the growth experiments. The results of the dissolution experiments indicate that the solid material formed in the early stages of crystallization has a stoichiometry corresponding to OCP.     

Determination of phosphate based on inhibition of crystal growth of calcite

The growth of calcium carbonate seed crystals (calcite) is strongly inhibited by the presence of phosphate ions. The kinetics of crystal growth were followed potentiometrically using a calcium ion-selective electrode or through the use of a pH electrode. The study of different variables on such a process was carried out with the aim of developing kinetic methods to determine phosphate ions (50–400 ng ml^?1). The selectivity and sensitivity of these processes allow the application of crystallization reactions to the determination of phosphate in human urine.     

Heterogeneous nucleation and growth of calcium carbonate on calcite and quartz

The precipitation of calcium carbonate as a binding salt for the consolidation of loose sand formations is a promising approach. The heterogeneous nucleation and growth of calcite were investigated in supersaturated solutions. The ionic activities in the solutions tested were selected so that they included both supersaturations in which crystal growth took place only following the introduction of seed particles and supersaturations in which precipitation occurred spontaneously past the lapse of induction times. In the latter case the supersaturation conditions were sufficiently low to allow the measurement of induction times preceding the onset of precipitation. The stability domain of the calcium carbonate system was established at pH 8.50, 25 degrees C, measuring the induction times in the range between 30 min and 2 h. The rates of precipitation following the destabilization of the solutions were measured from the pH and/or concentration-time profiles. The induction times were inversely proportional and rates proportional to the solution supersaturation as expected. The high-order dependence of the rates of precipitation on the solution supersaturation suggested a polynuclear growth mechanism. Fitting of the induction time-supersaturation data according to this model yielded a value of 64 mJ/m2 for the surface energy of the calcite nucleus. In the concentration domain corresponding to stable supersaturated solutions, seeded growth experiments at constant supersaturation showed a second-order dependence on the rates of crystal growth of calcite seed crystals. Inoculation of the stable supersaturated solutions with quartz seed crystals failed to induce nucleation. Raising supersaturation to reach the unstable domain showed interesting features: calcite seed crystals yielded crystal growth kinetics compatible with the polynuclear growth model, without any induction time. The presence of quartz seed crystals reduced the induction times and resulted in nucleation in the bulk solution. The kinetic data in the latter case were consistent with the polynuclear growth model and the surface energy for the newly forming embryo was calculated equal to 31.1 mJ/m2, because of the dominantly heterogeneous nature of the process.     

The role of vinyl sulfonic acid homopolymer in calcium oxalate crystallization

In this study, the effect of water soluble homopolymer of vinylsulfonic acid on spontaneous crystallization of calcium oxalate (CaOx) was investigated. CaOx crystals exhibiting different shapes and phase structures were produced in the presence of polymer. While the crystal growth of calcium oxalate was inhibited by homopolymer, the morphology of calcium oxalate transformed from monohydrate to dihydrate. Inhibition of calcium oxalate crystallization was provided by adsorption of homopolymer onto the active growth sites of crystals on account of the charge and hydrophilic effects. Polyelectrolyte effects were interpreted in terms of the adsorption of inhibitors onto the active growth sites on the crystal surface.     

The separation of racemic crystals into enantiomers by chiral block copolymers

A series of chiral double hydrophilic block copolymers (DHBCs) was synthesized and employed as additives in the crystallization of calcium tartrate tetrahydrate (CaT). We found that appropriate polymers can slow down the formation of the thermodynamically most stable racemic crystals as well as the formation of one of the pure enantiomeric crystals so that chiral separation by crystallization occurs even when racemic crystals can be formed. In addition, the presence of DHBCs results in major modifications of crystal morphology, creating unusual morphologies of higher complexity. Our study demonstrates the potential application of chiral DHBCs in the control of chirality throughout crystallization, in particular for racemic crystal systems, and also shows that enantiomeric excess of one enantiomer can be maximized by the kinetic control of crystallization.     

A kinetic model to simulate protein crystal growth in an evaporation-based crystallization platform

The quality, size, and number of protein crystals grown under conditions of continuous solvent extraction are dependent on the rate of solvent extraction and the initial protein and salt concentration. An increase in the rate of solvent extraction leads to a larger number of crystals. The number of crystals decreases, however, when the experiment is started with an initial protein concentration that is closer to the solubility boundary. Here we develop a kinetic model capable of predicting changes in the number and size of protein crystals as a function of time under continuous evaporation. Moreover, this model successfully predicts the initial condition of drops that will result in gel formation. We test this model with experimental crystal growth data of hen egg white lysozyme for which crystal nucleation and growth rate parameters are known from other studies. The predicted and observed rates of crystal growth are in excellent agreement, which suggests that kinetic constants for nucleation and crystal growth for different proteins can be extracted by applying a kinetic model in combination with observations from a few evaporation-based crystallization experiments.     

Growth rates of protein crystals

Protein crystallization is important for structural biology. The rate at which a protein crystallizes is often the bottleneck in determining the protein's structure. Here, we give a physical model for the growth rates of protein crystals. Most materials crystallize faster under stronger growth conditions; however, protein crystallization slows down under the strongest conditions. Proteins require a crystallization slot of 'just right' conditions. Our model provides an explanation. Unlike simpler materials, proteins are orientationally asymmetrical. Under strong conditions, protein molecules attempt to crystallize too quickly, in wrong orientations, blocking surface sites for more productive crystal growth. The model explains the observation that increasing the net charge on a protein increases the crystal growth rate. The model predictions are in good agreement with experiments on the growth rates of tetragonal lysozyme crystals as a function of pH, salt concentration, temperature, and protein concentration.     

Investigating the kinetics of liquid-free,OSDA-free ZSM-5 zeolite synthesis from iron ore tailings

In this study, ZSM-5, which is a Mobil-type five-type zeolite with well-defined crystal morphology, is successfully synthesized via a seed-assisted, liquid-free method that uses iron ore tailings as a silica source. The ZSM-5 crystallization kinetics at 423, 433, and 443 K and different synthesis times are investigated to identify the nucleation and crystallization mechanisms of the synthesized ZSM-5 zeolites, and results suggest that the crystallization kinetics follow a Kolmogorov-Johnson-Mehl-Avrami-type behavior. The activation energies for the induction and transition periods are 112.38 and 58.35 kJ mol⁻¹, respectively. Furthermore, the Avrami exponent indicates three-dimensional crystal growth from both sporadic and instantaneous nucleation mechanisms. A comparison of our results with previous reports of the ZSM-5 crystallization mechanism demonstrates that the seed crystals play a significant role in nucleation and crystal growth. Finally, seed surface crystallization and new nuclei crystallization dual mechanism has been proposed to describe the crystallization process of ZSM-5.     

Assisted desolvation as a key kinetic step for crystal growth

The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.     

Simulation of calcium oxalate stone in vitro

Urolithiasis constitutes a serious health problem that affects a significant section of mankind. Between 3% and 14% of the population, depending on the geographical region, suffer from this illness[1]. For example, the incidence of urolithiasis in Florida in the United States of America was 15.7 in 100000 people and increased to 20.8 in 1996. Urolithiasis remains a major medical prob-lem in China, especially in Guangdong Province. A survey in 1997 in Shenzhen City, the most southern city i…     

A potentiometric technique for kinetic determination of citrate, based on inhibition of crystalline growth of lead carbonate seed crystals

The growth of lead carbonate seed crystals is strongly inhibited by the presence of citric acid. The kinetics of crystal growth were followed potentiometrically with a lead ion-selective electrode. The study of different variables on such a process was carried out with the aim of developing kinetic procedures to determine citrate (0.5–2.0 μg/ml). The selectivity and sensitivity of these processes allow the application of his crystallization reaction to direct determination of citrate in drinks and in a pharmaceutical product.     

Secondary nucleation in the formation of methane crystal hydrate

The kinetic data on crystallization and a morphological analysis of a layer of CH₄ · 6H₂O hydrate crystals formed on the surface of water as a result of methane absorption showed that secondary nucleation occurred during hydrate crystallization. The mutual arrangement of crystals in the layer revealed photographically in situ was evidence that part of nuclei produced on the surface of previously formed crystals went away from the surface into solution and grew there independently of “mother” crystals, although the probability of such transfer into an immobile solution remained low. In view of this, a model of crystal growth generating secondary crystals was developed.     

Determination of phytic acid based on inhibition of crystalline growth of calcium oxalate monohydrate

It was found that the crystalline growth of calcium oxalate monohydrate seed crystals is strongly inhibited by the presence of phytic acid. The kinetics of crystal growth were followed potentiometrically using a calcium ion-selective electrode. Investigation of the process led to the development of a kinetic method for the determination of phytic acid (30–600 ng ml^?1). The procedure was applied to the direct determination of phytic acid in urine and in a pharmaceutical product.     

Rough-surface crystallization: Some applications to poly(ethylene oxide)

A model incorporating a rough (disordered) crystal growth surface is capable of treating many of the observations and measurements on the crystal growth of short chains of poly(ethylene oxide) from the melt. A simple preliminary treatment if presented which aims primarily to analyze the growth rate data within one growth “branch,” i.e., for a regime in which crystal thickness is approximately constant. Under these conditions the growth rate is approximately linear with crystallization temperature, as expected for rough growth surfaces, but not for smooth (faceted) ones. Simulation results are included which are in agreement with a simple equation and with experiment. The analysis enables the growth rates for different branches to be compared in a systematic way. A very steep decrease in growth rate with increasing crystal thickness is clearly illustrated, together with some influence of molecular weight. The general trend for chain folding can be seen as a consequence of this steep decrease. Parallel work on systems in which crystal thicknesses vary continuously with crystallization temperature led to the realization that “rounding off” will occur at the crystal perimeter and that this will give rise to an entropic barrier for the crystal advance. This argument is presented in the context of extended-chain and once-folded crystallization, from which it is clear that growth rates should be much lower as crystal thicknesses increase, and for folded-chain as compared with extended-chain crystals. Different morphologies are interpreted in terms of changes in surface structure which are probably due to kinetic as well as equilibrium effects.     

Induction and inhibition of preferential enrichment by controlling the mode of the polymorphic transition with seed crystals

Both induction and inhibition of "preferential enrichment", an unusual symmetry-breaking enantiomeric-resolution phenomenon observed upon simple recrystallization of a certain kind of racemic crystals from organic solvents, have been successfully achieved by controlling the mode of the polymorphic transition during crystallization with appropriate seed crystals. Such control of the polymorphic transition can be interpreted in terms of a novel phenomenon consisting of 1) the adsorption of prenucleation aggregates, 2) the heterogeneous nucleation and crystal growth of a metastable crystalline form, and 3) the subsequent polymorphic transition into the more stable form; these three processes occur on the same surface of a seed crystal. We refer to this phenomenon as an "epitaxial transition", which has been confirmed by means of in situ attenuated total reflection (ATR) FTIR spectroscopy in solution and the solid state, differential scanning calorimetry (DSC) measurements of the deposited crystals, and X-ray crystallographic analysis of the single crystals or the direct-space approach employing the Monte Carlo method with the Rietveld refinement for the structure solution from the powder X-ray diffraction data.     

片晶折叠表面成核机制与结晶温度的相关性研究

利用自晶种方法,在邻二氯苯稀溶液中培养聚丁二酸丁二醇酯(PBS)晶体,系统研究了结晶温度对其晶体形貌的影响.使用PBS单晶作为研究对象,有效避免小尺寸观察不具有统计意义的缺点.在结晶过程中,通过改变结晶温度和自晶种温度,可有效调控稀溶液中生长的PBS晶体尺寸大小和晶体中缺陷的数量,得到了单层无缺陷的单晶、双层晶体和多层晶体等一系列PBS片晶.基于对不同实验条件下得到片晶的形貌和表面粗糙度的统计结果,提出晶体中可容忍的缺陷数量与结晶温度和晶种温度密切相关这一结论,通过建立热力学模型,定性分析了晶体中缺陷数量和结晶温度的依赖关系,从片晶表面粗糙度统计结果出发,提出高分子片晶折叠表面成核机制,较好地解释了实验中观察到的不同PBS晶体的形貌.     

Monte Carlo simulations of single crystals from polymer solutions

A novel "anisotropic aggregation" model is proposed to simulate nucleation and growth of polymer single crystals as functions of temperature and polymer concentration in dilute solutions. Prefolded chains in a dilute solution are assumed to aggregate at a seed nucleus with an anisotropic interaction by a reversible adsorption/desorption mechanism, with temperature, concentration, and seed size being the control variables. The Monte Carlo results of this model resolve the long-standing dilemma regarding the kinetic and thermal roughenings, by producing a rough-flat-rough transition in the crystal morphology with increasing temperature. It is found that the crystal growth rate varies nonlinearly with temperature and concentration without any marked transitions among any regimes of polymer crystallization kinetics. The induction time increases with decreasing the seed nucleus size, increasing temperature, or decreasing concentration. The apparent critical nucleus size is found to increase exponentially with increasing temperature or decreasing concentration, leading to a critical nucleus diagram composed in the temperature-concentration plane with three regions of different nucleation barriers: no growth, nucleation and growth, and spontaneous growth. Melting temperatures as functions of the crystal size, heating rate, and concentration are also reported. The present model, falling in the same category of small molecular crystallization with anisotropic interactions, captures most of the phenomenology of polymer crystallization in dilute solutions.     

乌梅提取液对草酸钙晶体生长的抑制作用研究

本文研究了水体系中加入乌梅提取液对草酸钙晶体生长的抑制作用,通过FTIR、SEM及XRD等测试方法对所得晶体进行表征。结果表明,不加乌梅提取液的体系中形成的晶体为一水合草酸钙(COM)晶体,加入乌梅提取液后,形成的是二水合草酸钙(COD)晶体,而且COD晶体的尺寸随着乌梅提取液浓度的增大而减小,直至消失,这说明乌梅提取液具有抑制草酸钙晶体生长的作用,且这种抑制作用随乌梅浓度的增大而增大。本文还通过电导率法研究了草酸钙晶体生长的动力学过程,发现乌梅提取液主要能抑制草酸钙晶体的成核过程。     

Simulation of calcium oxalate stone <Emphasis Type="Italic">in vitro</Emphasis>

Crystallization of calcium oxalate is studied mainly in the diluted healthy urine using scanning electron microscopy (SEM), and is compared with the crystallization in the diluted pathological urine. It suggests that the average sizes of calcium oxalate crystals are not in direct proportion to the concentrations of Ca²⁺ and Ox^2- ions. Only in the concentration range of 0.60-0.90 mmol/L can larger size of CaOx crystals appear. When the concentrations of Ca²⁺ and Ox^2- ions are 1.20, 0.80, 0.60, 0.30 and 0.15 mmol/L in the healthy urine, the average sizes of calcium oxalate crystallites are 9.5 X 6.5, 20.0 X 13.5 and 15.0 jj,m X 10.0 jj,m, respectively, for the former three samples after 6 d crystallization. No crystal appears even after 30 d crystallization for the samples of concentrations of 0.30 and 0.15 mmol/L due to their low supersaturations. The results theoretically explain why the probability of stone forming is clinically not in direct proportion to the concentrations of Ca²⁺ and Ox^2- ions. Laser scattering technology also confirms this point. The reason why healthy human has no risk of urinary stone but stone-formers have is that there are more urinary macromolecules in healthy human urines than that in stone-forming urines. These macromolecules may control the transformation in CaOx crystal structure from monohydrate cal-cium oxalate (COM) to dihydrate calcium oxalate (COD). COD has a weaker affinity for renal tubule cell membranes than COM. No remarkable effect of the crystallization time is observed on the crystal morphology of CaOx. All the crystals are obtuse hexagon. However, the sizes and the number of CaOx crystals can be affected by the crystallization time. In the early stage of crystalli-zation (1-6 d), the sizes of CaOx crystals increase and the number of crystal particles changes little as increasing the crystallization time due to growth control. In the middle and late stages (6-30 d), the number of crystals increases markedly while the growth rate changes little due to the nucleation control.