Properties of amorphous calcium carbonate and the template action of vaterite spheres

The fast mixing of aqueous solutions of calcium chloride and sodium carbonate could immediately result in amorphous calcium carbonate (ACC). Under vigorous stirring, the formed ACC in the precipitation system will dissolve first and, then, transform within minutes to produce crystalline forms of vaterite and calcite. After that, the solution-mediated mechanism dominates the transformation of the thermodynamically unstable vaterite into the thermodynamically stable calcite. Although ACC is the least stable form of the six anhydrous phases of calcium carbonate (CaCO(3)), it could be, however, produced and stabilized by a variety of organisms. To better understand the formation-transformation mechanism of ACC and vaterite into calcite, ex-situ methods (i.e., scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectroscopy) were used to characterize the formation-transformation process of ACC and vaterite in aqueous systems without organic additives, showing that ACC sampled at different conditions has different properties (i.e., lifetime, morphology, and spectrum characterization). It is also very interesting to capture the obviously polycrystalline particles of CaCO(3) during the transformation process from vaterite to calcite, which suggests the formation mechanism for the calcite superstructure with multidimensional morphology.     

Role of Polyelectrolytes in Crystallogenesis of Calcium Carbonate

The effects anionic polyelectrolytes, having various molecular weights and repeating unit structures, on the crystallization of calcium carbonate in supersaturated solutions are studied. The induction times of the crystals grown in the presence of the polymers were optically evaluated; X-ray diffraction and Scanning Electronic Microscopy (SEM) analyses were performed to determine, respectively, their crystalline structures and morphologies. The polyelectrolyte is found to lengthen the induction time and to reduce the size of CaCO₃ nanocrystallites, to an extent depending on the interaction efficiency between the polymer anionic repeating units and the calcium ions. Further, depending on their sizes and their crystalline structures (calcite, vaterite) the nanocrystallites aggregate and yield final calcium carbonate particles having various sizes and morphologies. The data indicate that nanocrystals having vaterite structure, as determined by X-ray analysis, give spherical CaCO₃ final particles, while nanocrystals having calcite structure lead to either acicular or flower shapes of CaCO₃ final particles.     

A quasi-time-resolved CryoTEM study of the nucleation of CaCO3 under langmuir monolayers

Calcium carbonate biomineralization uses complex assemblies of macromolecules that control the nucleation, growth, and positioning of the mineral with great detail. To investigate the mechanisms involved in these processes, for many years Langmuir monolayers have been used as model systems. Here, we descibe the use of cryogenic transmission electron microscopy in combination with selected area electron diffraction as a quasi-time-resolved technique to study the very early stages of this process. In this way, we assess the evolution of morphology, polymorphic type, and crystallographic orientation of the calcium carbonate formed. For this, we used a self-assembled Langmuir monolayer of a valine-based bisureido surfactant (1) spread on a CaCl2-containing subphase and deposited on a holey carbon TEM grid. In a controlled environment, the grid is exposed to an atmosphere containing NH3 and CO2 (the (NH4)2CO3 diffusion method) for precisely determined periods of time (reaction times 30-1800 s) before it was plunged into melting ethane. This procedure allows us to observe amorphous calcium carbonate (ACC) particles growing from a few tens of nanometers to hundreds of nanometers and then crystallizing to form [00.1] oriented vaterite. The vaterite in turn transforms to yield [10.0] oriented calcite. We also performed the reaction in the absence of monolayer or in the presence of a nondirective monolayer of surfactant containing an oligo(ethylene oxide) 2 head group. Both experiments also showed the formation of a transient amorphous phase followed by a direct conversion into randomly oriented calcite crystals. These results imply the specific though temporary stabilization of the (00.1) vaterite by the monolayer. However, experiments performed at higher CaCl2 concentrations show the direct conversion of ACC into [10.0] oriented calcite. Moreover, prolonged exposure to the electron beam shows that this transformation can take place as a topotactic process. The formation of the (100) calcite as final product under different conditions shows that the surfactant is very effective in directing the formation of this crystal plane. In addition, we present evidence that more than one type of ACC is involved in the processes described.     

Homogeneous Precipitation of Calcium Carbonates by Enzyme Catalyzed Reaction

Catalytic decomposition of urea by urease in aqueous calcium chloride solutions was used to rapidly prepare calcium carbonate polymorphs at room temperature. The nature of the resulting particles depended on the concentration of the enzyme and, in a strong manner, on the agitation of the reacting solutions. In an undisturbed system an amorphous precipitate is formed first, which readily crystallized to vaterite and upon aging changed to calcite. Under the influence of magnetic stirring, the amorphous phase could be not observed; instead smaller particles were initially obtained, which aggregated to vaterite and calcite. Similarly, the application of ultrasonic energy produced small vaterite particles at the early stages. It is apparent that enzyme macromolecules are important in the development of calcite faces and, as such, they exert significant influence on calcite morphology, without being present in detectable amounts in the resulting solids. Copyright 2001 Academic Press.     

Thermally induced transformations of calcium carbonate polymorphs precipitated selectively in ethanol/water solutions

For the precipitation of calcium carbonate polymorphs in ethanol/water solutions of calcium chloride by the diffusion of the gases produced by sublimation–decomposition of solid ammonium carbonate, polymorph selection and morphology control of the precipitates were demonstrated by the effect of ethanol/water ratio in the mother liquor. The precipitated phases change systematically from gel-like aggregates of hydrated amorphous calcium carbonate in the absolute ethanol solution to well-shaped rhombohedral particles of calcite in the absolute aqueous solution via almost pure phase of vaterite with dendrite structure in 75%-ethanol/25%-aqueous and 50%-ethanol/50%-aqueous solutions. On heating the precipitated sample in flowing dry nitrogen, all the samples transformed to calcite before the thermal decomposition, where the thermal decomposition temperature shifts to higher temperatures with increasing the water content in the mother liquor due to the systematic increase in the particle size of calcite. Accordingly, the present method of controlled precipitation of calcium carbonate polymorphs is also useful to control the particle size and reactivity of calcite produced by heating the precipitates. Selecting vaterite with dendrite structure from the present series of precipitated samples, the structural phase transition to calcite was characterized as the three-dimensional growth of rhombohedral particles of calcite with the enthalpy change ΔH = ? 2.8 ± 0.1 kJ mol^?1 and the apparent activation energy E_a = 289.9 ± 5.8 kJ mol^?1.     

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.     

Synthesis of vaterite and aragonite crystals using biomolecules of tomato and capsicum

In this paper, biomimetic synthesis of calcium carbonate (CaCO₃) in the presence of biomolecules of two vegetables-tomato and capsicum is investigated. Scanning electron microscopy and X-ray powder diffractometry were used to characterize the CaCO₃ obtained. The biomolecules in the extracts of two vegetables are determined by UV-vis or FTIR. The results indicate that a mixture of calcite and vaterite spheres constructed from small particles is produced with the extract of tomato, while aragonite rods or ellipsoids are formed in the presence of extract of capsicum. The possible formation mechanism of the CaCO₃ crystals with tomato biomolecules can be interpreted by particle-aggregation based non-classical crystallization laws. The proteins and/or other biomolecules in tomato and capsicum may control the formation of vaterite and aragonite crystals by adsorbing onto facets of them.     

Influence of sodium dodecyl sulfate and static magnetic field on the properties of freshly precipitated calcium carbonate

Properties of calcium carbonate precipitated from aqueous solutions of CaCl(2) and Na(2)CO(3) in the presence of sodium dodecyl sulfate (SDS) and S-S 0.1 T magnetic field (MF) were studied. The nucleation and precipitation processes of CaCO(3) were investigated by pH and zeta potential measurements at 20 +/- 1 degrees C up to 2 h after mixing the solutions. Also the amounts of calcium carbonate deposited on the glass surfaces and its structure were examined. It was found that SDS influences the kinetics of precipitation, crystallographic forms, and crystal size of CaCO(3). The SDS effects are more pronounced in MF presence. A small amount of SDS accelerates transformation of vaterite into calcite, whereas increasing surfactant concentration moderates such a transformation. On the other hand, in all the systems, MF in the presence of SDS causes a slower transformation of vaterite into calcite. These effects are reflected in pH and zeta potential changes, although there is no clear dependence between the SDS amount present during the precipitation and changes of the parameters investigated. It seems that MF effect is most significant at a defined optimal SDS concentration. The results, however, do not allow suggestion of any detailed mechanism of the field interaction.     

Stabilization of amorphous calcium carbonate in inorganic silica-rich environments

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution.     

Crystallization of calcium carbonate controlled by Pluronic P123 in room-temperature ionic liquid

The crystallization of calcium carbonate (CaCO₃) controlled by Pluronic P123 in a room-temperature ionic liquid, ethylamine nitrate (EAN), was investigated. The CaCO₃ aggregates were obtained by rapid mixing of ammonium carbonate ((NH₄)₂CO₃) and calcium chloride (CaCl₂). Cubic calcite, spherical vaterite, and bagel-like vaterite were obtained easily by changing P123 concentration and reaction temperature. The morphologies of the as-prepared CaCO₃ aggregates were investigated by transmission electron microscopy and scanning electronic microscopy. The phase change of the obtained crystals was confirmed by X-ray diffraction and Fourier transform infrared spectroscopy. It was shown that higher P123 concentration and higher reaction temperature favor the formation of vaterite in EAN. Unusual bagel-like vaterite was first obtained at 60 °C in the presence of 5 g/L P123 in EAN. Mineralization of CaCO₃ regulated by P123 in EAN is a simple, novel, and environment-friendly strategy for vaterite synthesis.     

Crystallization and aggregation behaviors of calcium carbonate in the presence of poly(vinylpyrrolidone) and sodium dodecyl sulfate

An anionic surfactant interacts strongly with a polymer molecule to form a self-assembled structure, due to the attractive force of the hydrophobic association and electrostatic repulsion. In this crystallization medium, the surfactant-stabilized inorganic particles adsorbed on the polymer chains, as well as the bridging effect of polymer molecules, controlled the aggregation behavior of colloidal particles. In this presentation, the spontaneous precipitation of calcium carbonate (CaCO3) was conducted from the aqueous systems containing a water-soluble polymer (poly(vinylpyrrolidone), PVP) and an anionic surfactant (sodium dodecyl sulfate, SDS). When the SDS concentrations were lower than the onset of interaction between PVP and SDS, the precipitated CaCO3 crystals were typically hexahedron-shaped calcite; the increasing SDS concentration caused the morphologies of CaCO3 aggregates to change from the flower-shaped calcite to hollow spherical calcite, then to solid spherical vaterite. These results indicate that the self-organized configurations of the polymer/surfactant supramolecules dominate the morphologies of CaCO3 aggregates, implying that this simple and versatile method expands the morphological investigation of the mineralization process.     

On the structure of amorphous calcium carbonate--a detailed study by solid-state NMR spectroscopy

The calcium carbonate phases calcite, aragonite, vaterite, monohydrocalcite (calcium carbonate monohydrate), and ikaite (calcium carbonate hexahydrate) were studied by solid-state NMR spectroscopy ( (1)H and (13)C). Further model compounds were sodium hydrogencarbonate, potassium hydrogencarbonate, and calcium hydroxide. With the help of these data, the structure of synthetically prepared additive-free amorphous calcium carbonate (ACC) was analyzed. ACC contains molecular water (as H 2O), a small amount of mobile hydroxide, and no hydrogencarbonate. This supports the concept of ACC as a transient precursor in the formation of calcium carbonate biominerals.     

Water as the Key to Proto‐Aragonite Amorphous CaCO3

Temperature and pH value can affect the short‐range order of proto‐structured and additive‐free amorphous calcium carbonates (ACCs). Whereas a distinct change occurs in proto‐vaterite (pv) ACC above 45 °C at pH 9.80, proto‐calcite (pc) ACC (pH 8.75) is unaffected within the investigated range of temperatures (7–65 °C). IR and NMR spectroscopic studies together with EXAFS analysis showed that the temperature‐induced change is related to the formation of proto‐aragonite (pa) ACC. The data strongly suggest that the binding of water molecules induces dipole moments across the carbonate ions in pa‐ACC as in aragonite, where the dipole moments are due to the symmetry of the crystal structure. Altogether, a (pseudo‐)phase diagram of the CaCO₃ polyamorphism in which water plays a key role can be formulated based on variables of state, such as the temperature, and solution parameters, such as the pH value.     

Calcium carbonate phase transformations during the carbonation reaction of calcium heavy alkylbenzene sulfonate overbased nanodetergents preparation

The preparation and application of overbased nanodetergents with excess alkaline calcium carbonate is a good example of nanotechnology in practice. The phase transformation of calcium carbonate is of extensive concern since CaCO(3) serves both as an important industrial filling material and as the most abundant biomineral in nature. Industrially valuable overbased nanodetergents have been prepared based on calcium salts of heavy alkylbenzene sulfonate by a one-step process under ambient pressure, the carbonation reaction has been monitored by the instantaneous temperature changes and total base number (TBN). A number of analytical techniques such as TGA, DLS, SLS, TEM, FTIR, and XRD have been utilized to explore the carbonation reaction process and phase transformation mechanism of calcium carbonate. An enhanced understanding on the phase transformation of calcium carbonate involved in calcium sulfonate nanodetergents has been achieved and it has been unambiguously demonstrated that amorphous calcium carbonate (ACC) transforms into the vaterite polymorph rather than calcite, which would be of crucial importance for the preparation and quality control of lubricant additives and greases. Our results also show that a certain amount of residual Ca(OH)(2) prevents the phase transformation from ACC to crystalline polymorphs. Moreover, a vaterite nanodetergent has been prepared for the first time with low viscosity, high base number, and uniform particle size, nevertheless a notable improvement on its thermal stability is required for potential applications.     

双亲水性嵌段共聚物对CaCO_3结晶的影响

以合成的带有磺酸基为端基的线型-超支化二嵌段共聚物PEG-b-PEI-SO3H为模板,探讨了其对CaCO3结晶的影响,并用FTIR、XRD、SEM、TEM等对产物进行了表征.结果表明,带有—SO3H端基的线型-超支化双亲水性嵌段共聚物PEG-b-PEI-SO3H对CaCO3晶体形貌和晶型表现出较强的调控能力.培养1天时得到空心环状方解石型CaCO3晶体,但当培养时间为3天和5天时,得到的CaCO3晶体形貌既有河蚌状也有类球状,同时其晶型既有方解石也有球霰石,而当培养时间达到7天后,得到的就只有球状球霰石CaCO3晶体.     

利用室温固态反应制备球霰石型CaCO3粒子

合成形态、大小及结构可人为调控的无机材料是现代材料科学的重要研究方向[1]. 借助于各类有机添加剂及模板剂的调控作用, 可利用溶液合成方法制备出形貌与结构受到有效调控的无机粒子[2,3]. 室温固态化学反应已被成功地应用于多种无机纳米粒子[4]及纳米线[5]的合成, 并显示出高效、节能、无污染和操作简便等优点, 因而在材料合成领域具有应用前景[6].     

Formation of rod-shaped calcite crystals by microemulsion-based synthesis

Novel rod-shaped calcite crystals are formed by precipitation from cetyltrimethylammonium bromide (CTAB)/1-pentanol/cyclohexane microemulsions containing calcium chloride and ammonium carbonate. The calcium carbonate initially precipitates as hexagon-shaped vaterite crystals. The vaterite crystals transform to unusual rod-shaped calcite crystals over several days. The rod-shaped calcite crystals are prismatic, with the longest crystal axis displaying (110) crystal faces. A possible mechanism of crystal growth is discussed. The elongated shape of the crystals facilitates the assembly into hierarchical structures and can allow the crystals to be used as templates for fabricating advanced materials.     

Morphology and Kinetics of Growth of CaCO<Subscript>3</Subscript> Precipitates Formed in Saline Water at 30°C

The crystallization kinetics and morphology of CaCO₃ crystals precipitated from the high salinity oilfield water were studied. The crystallization kinetics measurements show that nucleation and nuclei growth obey the first order reaction kinetics. The induction period of precipitation is extended in the high salinity solutions. Morphological studies show that impurity ions remain mostly in the solution phase instead of filling the CaCO₃ crystal lattice. The morphology of CaCO₃ precipitates can be changed from a smooth surface (calcite) to rough spheres (vaterite), and spindle rod bundles, or spherical, ellipsoid, flowers, plates and other shapes (aragonite).     

Mineralization of CaCO3 in the presence of egg white lysozyme

The influence of egg white lysozyme on the size, shape, crystallography, and chemical composition of amorphous calcium carbonate (ACC) particles obtained from aqueous CaCl2-dimethyl carbonate (DMC)-NaOH solutions was studied. At the onset of precipitation, the presence of lysozyme led to much smaller particles (50-400 nm spherical amorphous lysozyme-calcium carbonate particles (Ly-ACC)) than those obtained from lysozyme-free solution. The nanospheres were in some cases aggregated and in addition embedded in a faint network. Their size and interconnection depended on the concentration of egg white lysozyme. When the Ly-ACC particles were left in contact with the mother liquor (CaCl2/DMC/NaOH/lysozyme solution) for 24 h, they transformed directly and exclusively into crystalline calcite. The observed results may be of relevance for a better understanding of the role of lysozyme in the process of eggshell mineralization.     

PREPARATION OF CaCO3 PARTICLES IN WATER POOL IN NONAQUEOUS NONIONIC SURFACTANT SOLUTIONS

ABSTRACT

CaCO₃ particles were prepared by bubbling of CO₂ into the systems consisting of polyoxyethylene(6) nonylphenyl ether/aqueous Ca(OH) ₂/cyclohexane. Spherical particles were formed in any systems, but those size distribution depended on the solubility behavior of aqueous Ca(OH) ₂ in surfactant solutions, i.e., monodisperse particles were formed in colorless solution, whereas in blue translucent solution they became bimodal. Such change was also observed for the size of reversed micelles. This suggested that the formation of CaCO₃ particles were related with that of micelles. On the other hand, the particles obtained were consisted of calcite, vaterite and aragonite. Those fraction differed also from the solubility behavior of aqueous Ca(OH) ₂. Both the vaterite and aragonite were transformed into calcite and those rate constants were order of 10^?6 sec^?1.