The influence of xanthan on the crystallization of calcium carbonate

Calcium carbonate (CaCO₃) was crystallized in xanthan (XC) aqueous solutions. The CaCO₃ particles were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and thermogravimetry analysis (TGA) methods. The concentrations of XC, Ca²⁺ and CO₃^2? ions and the ratios [Ca²⁺]/[CO₃^2?] and [Mg²⁺]/[Ca²⁺] show evident influence on the aggregation and growth of CaCO₃ particles. The presence of Mg²⁺ ions influences not only the morphology, but also the polymorph of CaCO₃.     

Controllable mineralization of calcium carbonate on regenerated cellulose fibers

Gel–forming fibers (GF fibers) can serve as nucleation sites to prepare calcium carbonate (CaCO₃) because they can adsorb large amounts of Ca²⁺ due to their porous structure. In this paper, mineralization behavior of CaCO₃ on GF fibers in ethanol–water mixed solvents without any additives has been investigated. The results showed that some crystals covered the fibers, while others were embedded in fibers. Twin–sphere based vaterite, zonary and rodlike calcite with large aspect ratio could be prepared successfully. The effect of ethanol content inside GF fibers, concentration of Ca²⁺ and CO₃^2‐, mineralization time, miscibility between alcohol and water, and temperature were studied. Lastly, a possible mineralization mode was suggested. This work could provide a new method to prepare inorganic/polymer hybrid materials. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Conductometric study of calcium carbonate prenucleation stage: underlining the role of CaCO3o ion pairs

Calcium carbonate crystallization process, especially the prenucleation stage, has increasingly been the subject of several works. In the present work, a simple method based on electrical conductivity modeling applied to the FCP (Fast Controlled Precipitation) method data is used to highlight the role of CaCO₃^o ion pairs on calcium carbonate prenucleation stage. A good agreement was obtained between the resistivity vs pH curves estimated by the McCleskey model equation and obtained experimentally in a FCP test. Results showed that the nucleation process begins with the formation of CaCO₃^o ion pairs as pre‐nuclei as soon as the calcite‐equilibrium pH is reached. Additionally CaCO₃^o content increases with pH to form aggregates, which depend on the saturation state of the solution. Basing on our thermodynamic data, these aggregates do not form amorphous calcium carbonate ACC as an intermediate phase. They lead to the formation of stable calcium carbonate nuclei which will further evolve to crystallize. Furthermore we demonstrate that in addition to their inhibitory effect on the Ca²⁺ and CO₃²⁻ association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium polyacrylate (RPI) reduce ion pairs aggregation rate.     

Growth of hierarchical calcium carbonate crystals templated by biomolecules of lotus root

In this paper, crystal growth of calcium carbonate (CaCO₃) in the presence of biomolecules of lotus root was investigated. Scanning electron microscopy, Fourier transform infrared spectroscopy and X‐ray powder diffractometry were used to characterize the products. The results indicate that calcite spherical particles were constructed from small rhombohedral subunits. Similar CaCO₃ crystals were also gained when crystal growth of CaCO₃ in aqueous solution containing extracts of lotus root was performed, suggesting that the soluble biomolecules of lotus root play a crucial role in directing the formation of hierarchical calcite spherical particles. The possible formation mechanism of the CaCO₃ crystals by using lotus root is also discussed, which can be interpreted by particle‐aggregation based non‐classical crystallization laws. The biomolecules of lotus root might induce and control the nucleation and growth of calcium carbonate crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of various polymorphs of calcium carbonate on porous membrane by electrochemical approach

Calcium carbonate (CaCO₃) formation was observed without surface modification of the organic template and in the absence of chemical additives such as macromolecules and divalent cations. Our innovative electrochemical approach that involves the use of an alternating current facilitated the crystallization of CaCO₃ polymorphs on a porous polymer membrane. A solution of calcium chloride (CaCl₂) and sodium carbonate (Na₂CO₃) was filled in a glass cell, and the porous membrane was interposed in the cell. A sine waveform of 10 Hz was applied to the platinum electrodes using a high-speed bipolar power supply. An alternating current was generated for 60 min. The crystal morphology and crystal structure of the resulting hybrid membrane were studied. In this electrochemical approach, versatile polymorphs of vaterite, aragonite, and calcite were formed on the membrane, thereby showing that the alternating current induced the formation of various polymorphs of CaCO₃ on the porous membrane even in the absence of any additives.     

Investigation of calcium carbonate precipitated in the presence of alkanols

The influence of myristyl alcohol (CH₃(CH₂)₁₃OH), cetyl alcohol (CH₃(CH₂)₁₅OH) and behenyl alcohol (CH₃(CH₂)₂₁OH) on the structure, morphology, size and surface properties of calcium carbonate (CaCO₃) has been investigated. Changes in the nature of the washing solvent, in the C_nOH/Ca²⁺ and CO₃²⁻/Ca²⁺ molar ratios and in temperature have been also evaluated. The sole polymorph produced was rhombohedral calcite. At room temperature, while microspheres composed of submicrocubes were produced at a high molar ratio CO₃²⁻/Ca²⁺ and low CH₃(CH₂)₁₅OH concentration, a stoichiometric molar ratio CO₃²⁻/Ca²⁺ and high CH₃(CH₂)₁₅OH concentration induced the formation of microcubes and microboxes. In the presence of this alkanol (12 % molar) a significant enhancement of the water contact angle (ca. 40 °) resulted in a sample obtained with a stoichiometric CO₃²⁻/Ca²⁺ ratio. These results emphasize the key role played by the three non‐ionic surfactants in the formation of materials with variable crystal shape and wettability and thus technological interest for a range of applications.     

Nucleation-growth process of scale electrodeposition – influence of the magnesium ions

Electrodeposition of CaCO₃ was studied under the influence of magnesium ions present in a carbonically pure water. The investigation was performed using electrochemical, gravimetric and optical methods. The chronoamperometric measurements confirmed the inhibiting property of Mg²⁺ when its concentration is higher than 120 mg L⁻¹ in a solution containing 160 mg L⁻¹ of Ca²⁺. The optical technique led the nucleation-growth process to be accessed by means of an optical microscope positioned behind a transparent electrode. The increase of Mg²⁺ concentration changed drastically the calcite morphology. At 360 mg L⁻¹ of Mg²⁺, the calcite morphology was optically amorphous but the Raman spectrum confirmed its structure. The crystal growth was recorded in situ and the image analysis software characterised the nucleation process as well as the growth rate of the crystals. It allowed the influence of the Mg²⁺ ions on the crystallisation process of CaCO₃ to be quantified.     

Influence of clay suspensions on the precipitation of CaCO3 in seawater

The effect of montmorillonite and kaolinite, most common clay in marine water, on nucleation and growth of calcium carbonate in standard sea water was studied. Crystallization was induced by the degasification of the dissolved carbonic gas. It was shown by XRD and SEM analysis that CaCO₃ crystallize under its aragonite polymorph some either the clay concentration or type. It was also found that tested clays inhibited significantly the crystallization of calcium carbonate, especially for concentrations higher than 25 mgL^–1. From the fine analyses of the formed solid, it was suggested that the tested clays have an indirect effect on nucleation and growth of aragonite by increasing the Mg ions concentration, strong inhibitor of CaCO₃ formation, in the neighbourhood of clay particles where supersaturation is the higher and than crystallization can occur. In addition to its indirect role, kaolinite can interact with aragonite by adsorbing on their faces and blocking growth sites (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of stirring speed on the crystallization of calcium carbonate

Control over crystal morphology of calcium carbonate (CaCO₃) was investigated by simply changing the stirring speeds in the process of CaCO₃ formation. Scanning electron microscopy (SEM) and powder X‐ray diffraction (XRD) measurements explore the morphology evolution of CaCO₃ at varying stirring speeds. As the stirring speeds increase, rhombohedral calcite, spherical vaterite, and monoclinic crystal with coexistence of calcite phase and vaterite phase were formed, suggesting a facile control over calcium carbonate crystallization in constructing crystals with desired morphology. Moreover, almost pure vaterite spherical particles of narrow particle size distribution were formed at optimum stirring speed. Finally, also elucidated in this work is the mechanism investigation into the construction of various crystal forms via this simple route. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Speciation of Fe(II) and Fe(III) effect on CaCO3 crystallization

The present work was carried out to investigate separately the effect of Fe²⁺ and Fe³⁺ on the precipitation kinetics and the microstructure of CaCO₃. For this an experimental procedure was proposed. Precipitation tests were made by using the dissolved‐CO₂ degassing method. Both air and nitrogen were employed to strip the CO₂ from a Ca(HCO₃)₂ solution initially rich in this gas. At anoxic medium, it was shown that iron (II) prolongs the nucleation step and decelerates the crystalline growth rate. X‐ray diffraction analysis shows that its presence inhibits calcite and promotes aragonite variety. By using air, the reaction medium is rich in oxygen and iron (II) is rapidly oxidized. Seeing the higher solution pH (> 6.5), iron hydroxide forms before the onset of CaCO₃ precipitation and plays a role of seed permitting to initiate CaCO₃ nucleation. So, contrary to the observed effect of iron (II), the presence of iron (III) accelerates the precipitation rate of CaCO₃. As for iron (II), iron (III) inhibits calcite formation but favored the vaterite variety instead of the aragonite one.     

Influence of calcium binding proteins on the precipitation of calcium carbonate: A kinetic and morphologic study

In our experiments, the thermodynamic effect of calcium binding proteins (CBP) on the growth of calcium carbonate (CaCO₃) was studied in vitro. The CaCO₃ crystals obtained in systems with and without CBP were characterized by scanning electron microscope (SEM), Fourier Transform Infrared spectrograph (FT‐IR) and powder X‐ray diffractometer (XRD). The kinetic process was studied by monitoring the conductivity and pH value, which revealed the obvious inducement effect of CBP on the CaCO₃ crystals growth, and the possible formation mechanism of CaCO₃ in CBP solution was discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

CaCo3 crystallization control by poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer and O-(hydroxy isopropyl) chitosan

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) ((EO)₂₀-(PO)₇₂-(EO)₂₀) and O-(hydroxy isopropyl) chitosan (HPCHS) were employed as control agents of calcium carbonate crystal growth. The effect of the concentrations of polymers, [Ca²⁺] and [CO₃²⁻], the ratios of [Ca²⁺]-[CO₃²⁻] and the initial pH of the solutions were investigated. The obtained CaCO₃ particles were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The particles are mainly calcite with various morphologies; their size and morphologies are influenced by the polymer content. For (EO)₂₀-(PO)₇₂-(EO)₂₀ systems, the initial pH has a notable influence; but in the HPCHS solution pH shows little influence. The ratio of [Ca²⁺]-[CO₃²⁻] clearly affects the CaCO₃ particle size and aggregation degree. HPCHS showed more significant influence on CaCO₃ crystallization than (EO)₂₀-(PO)₇₂-(EO)₂₀. The mechanisms of the CaCO₃ crystallization as controlled by (EO)₂₀-(PO)₇₂-(EO)₂₀ and HPCHS are proposed and demonstrated by the molecular dynamics simulations.     

Control of calcium carbonate crystallization utilizing amphiphilic block copolypeptides

Control over the morphology of calcium carbonate (CaCO₃) crystals has been achieved through the use of anionic, amphiphilic block copolypeptides during crystallization. Microspheres of CaCO₃ can be synthesized by the introduction of preformed organic, amphiphilic block copolypeptide templates, poly(l-aspartate sodium salt)₁₀₀-block-[poly(l-phenylalanine)₂₅-random-(l-leucine)₂₅] (1) and poly(l-glutamate sodium salt)₁₀₀-block-[poly(l-phenylalanine)₂₅-random-(l-leucine)₂₅] (2). When cationic amphiphiles are used in lieu of the anionic amphiphiles, only CaCO₃ rhombohedra are produced. The self-assembling amphiphile controls the precipitation of the microspheres by acting as a template to form giant CaCO₃ microspheres. These microspheres are composed of nanocrystals ranging in size from 10 to 60 nm using 1 and 20 to 100 nm using 2.     

Preparation of calcium carbonate crystals in the presence of trisodium citrate

The dumbbell‐like calcium carbonate (CaCO₃) crystals were synthesized in the presence of trisodium citrate. Different morphologies were obtained by changing the reaction temperature and the trisodium citrate concentration. The obtained samples were characterized by means of X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the morphology of CaCO₃ crystals was markedly affected by the reaction temperature and citrate anion concentration. The possible growth mechanism of CaCO₃ crystals was proposed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of calcium carbonate via a bubbling method in the presence of Mg2+ and glucose

Abstract

The effect of Mg²⁺ on the crystallization of precipitated calcium carbonate (PCC) via a bubbling carbonation method and the mechanism of eliminating its influence by glucose were investigated. The polymorph and morphology of crystals were characterized by field emission-scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. These results demonstrated that Mg²⁺ and Mg/Ca molar ratio played significant roles on the morphology of PCC. When the Mg/Ca molar ratio was below 0.5, only spindle-like calcite formed. The rod-like aragonite started to form when the ratio was 0.6. As the ratio increased, the amount of aragonite increased and the length of rod-like aragonite became longer. Notably, the effect of Mg²⁺ could be eliminated efficiently when the 1.5?wt% glucose was added into the carbonation system, in which system, the PCC crystals were all spindle-like calcite. Furthermore, the mechanism of the glucose to eliminate the influence of Mg²⁺ on PCC crystallization was proposed.     

Influence of some foreign metal ions on crystal growth kinetics of brushite (CaHPO4·2H2O)

Brushite, CaHPO₄·2H₂O, has been precipitated at 25 °C in the presence of Mg²⁺, Ba²⁺ or Cu²⁺ at concentrations up to 0.5 mM. When initial pH is sufficiently low to exclude nanocrystalline apatite as the initial solid phase, overall crystal growth rate may be determined from simple mass crystallization by recording pH as function of time. A combination of surface nucleation (birth-and-spread) and spiral (BCF) growth was found. Edge free energy was determined from the former contribution and was found to be a linear function of chemical potential of the additive, indicating constant adsorption over a wide range of additive concentrations. Average distances between adsorbed additive ions as calculated from slopes of plots are compatible with lattice parameters of brushite: 0.54 nm for Mg²⁺, 0.43 nm for Ba²⁺ and 0.86 nm for Cu²⁺. With the latter a sharp decrease in growth rate occurred early in the crystallization process, followed by an equally sharp increase to the previous level. When interpreted in terms of the Cabrera–Vermilyea theory of crystal growth inhibition, the results are consistent with an average distance between Cu ions of 0.88 nm, in perfect agreement with the above value.     

Polymorphic composition and morphology of calcium carbonate as a function of ultrasonic irradiation

This paper reports on the precipitation of CaCO₃ polymorphs, having various crystal morphologies under different conditions. In particular, systems that were subject to ultrasonic irradiation were compared to the corresponding reference systems in the absence of such a treatment. The application of ultrasonic irradiation predominantly resulted in a change of particle size distribution and polymorphic composition of the precipitate, in comparison to the reference systems. Thus, it was found that the supersaturation and temperature influenced the size distribution, in both the reference and sonicated systems. A mixture of calcite, vaterite and aragonite was obtained in all reference systems, at 25 °C. At this temperature, the sonication caused the vaterite content to increase, while aragonite was not detected. In reference and sonicated systems at 80 °C, only aragonite precipitated. The results also indicate that the principle parameter responsible for the morphology of vaterite was the initial supersaturation: at higher supersaturation spherical vaterite particles precipitated, while at lower supersaturation hexagonal platelets were obtained. The morphological investigations also indicated different mechanisms of vaterite formation in the systems in which precipitation was initiated at higher supersaturation: spherulitic growth of vaterite was observed in sonicated systems, while the aggregation of primary particles was predominant in the reference systems. At lower supersaturation, the effect of c(Ca²⁺)/c(CO₃²⁻) on the morphology of hexagonal platelets of vaterite was observed as well. By varying the c(Ca²⁺)/c(CO₃²⁻), significant changes of the polymorphic composition were observed only in the sonicated systems, at 25 °C.     

Preparation and structure of calcium peroxide‐templated porous calcium carbonate crystals

Crystalline calcium carbonate with randomly dispersed porous structure was prepared through co‐ crystallization with calcium peroxide and the following template elimination by a post heating treatment and washing with water. The artificial CaCO₃ possess abundant macro‐mesopores structures and high surface area. This approach may open a new general route for the preparation of crystals with high porosity and structure specialty. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile preparation of diversified patterns of calcium carbonate in the presence of DTAB

Nano- and micro-sized calcium carbonate (CaCO₃) with various morphologies including multi-petal-flower-shaped, multi-step-cube-shaped, coral-shaped, dendrite-shaped and multi-antenna-shaped was successfully prepared using dodecyltrimethylammonium bromide (DTAB) micellar vevulsant. The effects of temperature, pH and the concentration of DTAB micellar solution on the morphology and crystalline form of CaCO₃ were systematically investigated. The prepared CaCO₃ was characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The concentration of DTAB micelle, pH and reaction temperature are found to play crucial roles in the morphology, size and crystalline form of the final products. On the base of the characterizations, a possible self-assembled mechanism was proposed. The novel multi-petal-flower-shaped and multi-antenna-shaped CaCO₃ may have some unique properties and potential applications in the future.     

Microwave-assisted synthesis of spheroidal vaterite CaCO3 in ethylene glycol–water mixed solvents without surfactants

Spheroidal vaterite CaCO₃ composed of irregular nanoparticals have been synthesized by a fast microwave-assisted method. The structures are fabricated by the reaction of Ca(CH₃COO)₂ with (NH₄)₂CO₃ at 90 °C in ethylene glycol–water mixed solvents without any surfactants. The diameters of the spheroidal vaterite CaCO₃ range from 1 to 2 μm, and the average size of the nanoparticals is about 70 nm. Bundle-shaped aragonite and rhombohedral calcite are also obtained by adjusting the experimental parameters. Our experiments show that the ratio of ethylene glycol to water, microwave power, reaction time, and two sources of ammonium ions and acetate anions are key parameters for the fabrication of spheroidal vaterite CaCO₃. A possible growth mechanism for the spheroidal structures has been proposed, which suggests that the spheroidal vaterite CaCO₃ is formed by an aggregation mechanism.