The growth of crystals in solution

The crystallization of sparingly soluble salts from their aqueous supersaturated solutions is discussed from the standpoint of two important applications; scale formation and biological mineralization. Theories of crystal growth are outlined and the importance of kinetic factors in determining the nature of the growing phases is discussed. The kinetic factors can be studied by using a highly reproducible seeded growth technique and under certain conditions secondary nucleation can also be induced on the surface of the inoculating seed crystals. The kinetics of crystallization of the alkaline earth surfaces and the calcium phosphates is discussed. In the latter systems, temperature, supersaturation, surface concentration, pH, ionic strength and the presence of foreign ions are important in determining the nature of the phase which grows on the added seed crystals. The mechanism of the retardation of crystal growth by added crystallization inhibitors is illustrated by the influence of organic phosphonate molecules upon the precipitation of calcium carbonate.     

Morphological control of calcium carbonate crystallized in reverse micelle system with anionic surfactants SDS and AOT

The influence of a surfactant over water on the polymorphism and crystal size of calcium carbonate produced by reaction crystallization in microemulsion systems was investigated in a mixing tank reactor. The crystallization was induced by the reaction between two aqueous micelle solutions (Na2CO3-CaCl2) stabilized by anionic surfactants, SDS (sodium dodecyl sulfate) or AOT (sodium bis(2-ethylhexyl) sulfosuccinate). With increasing surfactant ratio to water, the water-in-oil microemulsion was stably developed and the morphology of the calcium carbonate crystallized in the micelles sharply transformed from calcite to vaterite. The influence of SDS on the polymorphism and crystal size of calcium carbonate was much clearer than that of AOT. In addition, with AOT, certain step changes in the morphology and crystal size occurred around a surfactant ratio to water (R=[H2O]/[surfactant]) of 15 due to a two-phase separation of the microemulsion.     

Crystal growth of aragonite and calcite in presence of citric acid, DTPA, EDTA and pyromellitic acid

The influence of four calcium complexing substances, i.e., citric acid (CIT), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and pyromellitic acid (PMA), on the crystal growth rate of the calcium carbonate polymorphs aragonite and calcite has been studied. Using a seeded constant supersaturation method supersaturation was maintained at 4 by keeping a constant pH of 8.5 through addition of sodium carbonate and calcium chloride solutions. The unique composition of each solution was calculated using chemical speciation. The growth rate was interpreted in terms of an overall growth rate. For both calcite and aragonite, the crystal growth rate is significantly reduced in the presence of the calcium complexing substances. The growth retarding effect depends on both the concentration and the polymorph. The relative crystal growth rate was correlated to the total complexing agent concentration using a Langmuir adsorption approach. Aragonite appeared fully covered for lower total concentrations than calcite. Furthermore, CIT very efficiently blocked aragonite growth contrary to what was observed for calcite. This is thought to be related to certain distinct features of the dominant aragonite crystal faces compared to the dominant calcite faces.     

柠檬酸钠与碳酸钙晶型和形貌的控制

晶体的晶型和形貌是碳酸钙制备中的关键问题之一。 采用尿素水解均匀沉淀法来控制碳酸钙的晶型和形貌。 利用XRD和SEM等技术手段对CaCO₃晶体的结构和形貌进行了表征。 结果表明,高温条件下(90 ℃)水溶液中得到均匀的针状结构的文石晶体。 添加适量柠檬酸钠后,则得到的是特殊形貌的方解石晶体。 在乙醇/水混合溶剂中,柠檬酸钠对CaCO₃晶体的形貌也有重要影响,通过改变乙醇体积分数,得到了中空绒毛球状的球霰石。     

Effect of inorganic anions on the morphology and structure of magnesium calcite

Calcium carbonate was precipitated from calcium hydroxide and carbonic acid solutions at 25 degrees C, with and without addition of different magnesium (MgSO(4), Mg(NO(3))(2) and MgCl(2)) and sodium salts (Na(2)SO(4), NaNO(3) and NaCl) of identical anions, in order to study the mode of incorporation of magnesium and inorganic anions and their effect on the morphology of calcite crystals over a range of initial reactant concentrations and limited c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios. The morphology, crystal size distribution, composition, structure, and specific surface area of the precipitated crystals, as well as the mode of cation and anion incorporation into the calcite crystal lattice, were studied by a combination of optical and scanning electron microscopy (SEM), electronic counting, a multiple BET method, thermogravimetry, FT-IR spectroscopy, X-ray diffraction (XRD), and electron paramagnetic resonance (EPR) spectroscopy. In the systems of high initial relative supersaturation, precipitation of an amorphous precursor phase preceded the formation of calcite, whereas in those of lower supersaturation calcite was the first and only polymorphic modification of calcium carbonate that appeared in the system. The magnesium content in calcite increased with the magnesium concentration in solution and was correlated with the type of magnesium salt used. Mg incorporation caused the formation of crystals elongated along the calcite c axis and, in some cases, the appearance of new [011] faces. Polycrystalline aggregates were formed when the c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios in solution were increased. Addition of sulfate ions, alone, caused formation of spherical calcite polycrystalline aggregates.     

Crystallization of CaCO3 in the presence of sulfate and additives: experimental and molecular dynamics simulation studies

The effects of sulfate and BHTPMP (Bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) on the crystallization rate, phase composition and morphology of calcium carbonate have been studied. It was observed that sulfate reduces the nucleation rate and favors the formation of aragonite form in the calcium carbonate precipitate. Moreover, in the presence of sulfate the rhombohedral morphology of the calcite crystals is modified, and during the formation of calcite, the development of {104} faces are more significantly prohibited than {110} faces. In the presence of sulfate together with BHTPMP, the crystallization process is inhibited and the modified morphology and the dominant calcite form are observed in the solid. The results from molecular dynamics simulations show the more strong combination of sulfate with calcite surface, in particular the {104} face, in comparison with the aragonite surface. The strong interaction of BHTPMP with sulfate and the aragonite surface favors the formation of the dominant calcite phase in the precipitate.     

阴离子氨基酸表面活性剂调控碳酸钙的仿生合成

室温下, 在乙醇或乙醇-水混合体系中, 利用氨基酸表面活性剂N-酰基十二烷基肌氨酸钠(Sar)调控合成碳酸钙, 采用SEM, XRD和FTIR等技术表征了反应产物. 在乙醇体系中, 首先形成多面体形状的文石, 然后逐渐转变为圆球状的无定形碳酸钙. 在乙醇-水混合体系中, 合成了花簇状多级结构碳酸钙晶体. 增加N-酰基十二烷基肌氨酸钠的用量有助于形成球霰石结构, 当n(Ca2+)∶n(Sar)=1∶1 时, 得到的花状碳酸钙为球霰石和方解石的混合物, 当n(Ca2+)∶n(Sar)=1∶2 时, 得到纯净的球霰石, 其形貌为大小较均一的单分散的球, 直径约为7 μm; 另外, 当n(Ca2+)∶n(Sar)=1∶1时, 混合溶剂中水和乙醇的体积比由1.5∶1依次增加为7∶3和3∶1时, 碳酸钙晶体的形貌由花状逐渐向球形过渡, 晶体中球霰石和方解石的含量也随之变化, 其中, 当水和醇的体积比为7∶3时, 产物主要为球霰石型晶体.     

气相扩散法制备的碳酸钙的形貌控制和机理

采用不同质量比的柠檬酸作为晶型控制剂制备碳酸钙,研究了在气相扩散体系中柠檬酸对所制备的碳酸钙微晶的粒度、形貌的影响.结果表明,随着柠檬酸浓度的增大,碳酸钙在溶液中结晶成为纳米小单晶体的二次聚集体,其晶粒度逐步变小,其形态呈现从疏松结构的球体、密实球体、花生状颗粒、大长径比针状到密集针状球体有规律的转变,晶型从0.24%和0.72%的球霰石到更大浓度的方解石.通过对柠檬酸在溶液中的聚集状态和柠檬酸分子与碳酸钙微晶的相互作用初步解释了柠檬酸对碳酸钙微晶形貌控制作用的机理.     

溶液电导率法对碳酸钙结晶动力学的研究

溶液电导率法对碳酸钙结晶动力学的研究;结垢;电导率     

Formation of single-crystalline aragonite tablets/films via an amorphous precursor

Thin tablets and films of calcium carbonate have been grown at the air-water interface via an amorphous precursor route using soluble process-directing agents and a Langmuir monolayer based on resorcarene. By using appropriate concentrations of poly(acrylic acid-sodium salt) in combination with Mg2+ ion, an initially amorphous film is deposited on the monolayer template, which subsequently crystallizes into a mosaic film composed of a mixture of single-crystalline and spherulitic patches of calcite and aragonite. Of particular importance is the synthesis of single-crystalline "tablets" of aragonite (approximately 600 nm thick), because this phase generally forms needle-like polycrystalline aggregates when grown in vitro. To our knowledge, a tabular single-crystalline morphology of aragonite has only been observed in the nacreous layer of mollusk shells. Therefore, this in vitro system may serve as a useful model for examining mechanistic issues pertinent to biomineralization, such as the influence of organic templates on nucleation from an amorphous phase.     

The Application of Ammonium Hydroxide and Sodium Hydroxide for Reagent Softening of Water

The calcium carbonate crystallization from aqueous solutions in the presence of alkali additives such as sodium hydroxide and ammonium hydroxide has been researched. It is found CaCO₃ crystallizes predominantly in the modification of aragonite in the presence of ammonium hydroxide. The calcium carbonate formation rate in an alkaline medium and the gaseous reaction products due to sorption of gas bubbles on crystal surfaces, affect the aragonite structure formation. It is shown use of ammonium hydroxide for water treatment can solve two urgent tasks such as water softening and exclusion sediment of deposits on the equipment surfaces by a calcium carbonate crystallization in the form of aragonite.     

Interaction between biopolyelectrolytes and sparingly soluble mineral particles

We investigate the complex physicochemical behavior of dispersions containing calcium carbonate (CaCO(3)) particles, a sparingly soluble mineral salt; and carrageenans, negatively charged biopolyelectrolytes containing sulfate groups. We reveal that the carrageenans suspend and stabilize CaCO(3) particles in neutral systems by absorbing on the particle surface which provides electrosteric stabilization. In addition, carrageenans provide a weak apparent yield stress which keeps the particles suspended for several months. The absorption measurements of carrageenan on the CaCO(3) particle indicate that more carrageenan is removed from the solution than expected from the case of a simple monolayer adsorption. Confocal laser scanning microscopy observations confirm that polyelectrolyte-containing precipitate is formed in both CaCO(3)-carrageenan and CaCl(2)-carrageenan mixtures. On the basis of these results, we confirm that in the presence of carrageenan some CaCO(3) dissolves and the Ca(2+) ions interact with the sulfate groups leading to aggregation and formation of particle-like structures. These new insights are important for fundamental understanding of other mineral-polyelectrolyte systems and have important implications for various industrial applications where calcium carbonate is used.     

碳酸钙微米球的制备与表征

采用醋酸钙和碳酸钠为原料,在反应温度为5℃和柠檬酸三钠质量百分浓度为15%的条件下,采用沉淀法合成出了粒度为1～4μm、分散性好的球形碳酸钙粉体。用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、红外光谱仪(IR)、原子力扫描探针显微镜(ASPM)、光学显微镜、粒度分析仪等对样品进行了表征,并用光学显微镜跟踪考察了碳酸钙微米球的形成过程。结果表明,碳酸钙微米球是由大量纳米级颗粒组装而成。     

PVP为模板控制合成球形碳酸钙

Spherical calcium carbonate was prepared by the reaction of sodium carbonate with calcium chloride at the presence of a protein-like molecule， polyvinylpyrrolidone， as the template. The products were characterized by elemental analyses， XRD， SEM， and TG-DSC respectively. The effects of polyvinylpyrrolidone on the crystal form and morphology of the as-prepared CaCO₃ were investigated. It was found that the aggregative shape of the produced calcium carbonate crystalline could be well controlled by adjusting the concentration of the polyvinylpyrrolidone template. This may be of important meanings to the biomimetic synthesis of novel inorganic materials.     

Influence of PSSS additive and temperature on morphology and phase structures of calcium oxalate

Calcium oxalate (CaOx) particles with different morphologies and phase structures were prepared by a facile precipitation reaction of sodium oxalate with calcium chloride in the absence and presence of poly(sodium 4-styrene-sulfonate) (PSSS) at different temperatures. The as-prepared products were characterized with scanning electron microscopy and X-ray diffraction. The influence of experimental conditions including pH, temperature, and concentration of PSSS and CaC2O4 on the morphologies and phase structures of the prepared calcium oxalate particles were investigated. It was found that variations in the concentration of PSSS and CaC2O4, temperature, and pH significantly influenced the crystal structure, morphology, and particle size of the samples. Various crystal morphologies of calcium oxalate, such as plate, leaf-shaped, bipyramid, and cylinder could be fabricated, depending on the experimental conditions. Higher PSSS concentration and reaction temperature favored the formation of metastable calcium oxalate dihydrate (COD) crystals and stable calcium oxalate monohydrate (COM), respectively. Especially, cylinder-shaped CaC2O4 particles were obtained at 80 degrees C in the presence of PSSS for the first time. This research may provide new insight into understanding and potentially regulating the formation of kidney stones and the control of morphology and phase structures of calcium oxalate particles.     

Morphology and internal structure of barium sulfate--derivation of a new growth mechanism

Particle formation is the decisive step to control crystal morphology. Besides the classical primary processes, nucleation and molecular growth, the particle size can also increase by aggregation. The special case of self-assembled aggregation leads to the formation of highly ordered particles which often possess a porous internal structure. In the experiments of these studies the particle formation of barium sulfate has been investigated. SEM analysis shows a large variety of growth forms including plate-like, star-like, and spherical particles, whereas TEM exposures reveal the porous internal structure at all investigated supersaturation levels. The pore size and the volume fraction can be influenced by changing the supersaturation ratio. By means of a fast sampling technique in combination with cryo-TEM analysis it has been shown that the particles at the early stages of growth shortly after the beginning of nucleation consist of many small nanocrystallites which have aggregated in a highly ordered manner. The diffraction pattern indicates many small-angle grain boundaries, whereas the particles at the end of the precipitation process are monocrystalline. This leads to the conclusion that barium sulfate grows according to a self-assembled aggregation mechanism followed by a fast recrystallization process.     

Influence of the primary structure of enzymes on the formation of CaCO2 polymorphs: a comparison of plant (Canavalia ensiformis) and bacterial (Bacillus pasteurii) ureases

The influence of the primary structures of plant (Canavalia ensiformis) and bacterial (Bacillus pasteurii) ureases on the precipitation of calcium carbonate polymorphs in solutions of calcium salts and urea at room temperature was investigated. Despite a similar catalytic function in the decomposition of urea, these ureases exerted different influences on the crystal phase formation and on the development of unusual morphologies of calcium carbonate polymorphs. Spherical and uniform vaterite particles were precipitated rather than calcite in the presence of Bacillus urease, while the presence of Canavalia urease resulted in the precipitation of calcite only. Vaterite particles were shown to be built up of nanosized crystallites, proving the importance of nanoscale aggregation processes on the formation of colloidal carbonates. Reduction of the concentration of Bacillus urease in the reacting solution results in the formation of calcite crystals with a more complex surface morphology than the ones obtained by Canavalia urease. These differences may be explained by dissimilarities in the amino acid sequences of the two examined ureases and their different roles in nucleation and physicochemical interactions with the surface of the growing crystals, during the precipitation processes. This study exemplifies the diversity of proteins produced by different organisms for the same function, and the drastic effects of subtle differences in their primary structures on crystal phase formation and growth morphology of calcium carbonate precipitates, which occur as inorganic components in a large number of biogenic structures.     

Morphology of calcium carbonate prepared via homogeneous synthesis

The morphology of calcium carbonate prepared via homogeneous synthesis from carbonate–chloride solutions was studied. The precipitates were investigated by scanning electron microscopy (SEM) and Xray diffraction. The effects of the ratio between the chloride and carbonate components of a solution on the morphology, crystal structure, and particle size of precipitated calcium carbonate were illustrated.     

Continuous synthesis of dolutegravir sodium crystals using liquid-gas heterogeneous microreactor

In this work, a liquid-gas heterogeneous microreactor was developed for investigating continuous crystallization of dolutegravir sodium (DTG), as well as revealing reaction kinetics and mixing mechanism with 3-min data acquisition. The reaction kinetics models were established by visually recording the concentration variation of reactants over time in the microchannel via adding pH-sensitive fluorescent dye. The mixing intensification mechanism of liquid-gas flow was quantified through the fluorescent signal to indicate mixing process, demonstrating an outstanding mixing performance with a mixing time less than 0.1 s. Compared with batch crystallization, continuous synthesis of dolutegravir sodium using liquid-gas heterogenous microreactor optimizes crystal distribution size, and successfully modifies the crystal morphology in needle-like habit instead of rod-like habit. The microreactor continuous crystallization can run for 5 h without crystal blockage and achieve D90 of DTG less than 30 µm. This work provides a feasible approach for continuously synthesizing dolutegravir sodium, and can optimize the existing pharmaceutical crystallization.     

Controlled growth of calcium carbonate by poly(ethylenimine) at the air/water interface

Two metastable calcium carbonate polymorphs, hemispherical vaterite and needle-like aragonite, are selectively formed at the air/water interface by the mediation of poly(ethyleneimine)(with molecular weights of 25000 and 2000, respectively) dissolved in supersaturated calcium bicarbonate solution.