Amorphous CaCO3: Influence of the Formation Time on Its Degree of Hydration and Stability

Interaction and Stability of Nanobubbles and Prenucleation Calcium Clusters during Ultrasonic Treatment of Hard Water

To investigate the stability of nanobubbles in natural hard water, a series of eight samples ranging in hardness from 0 to 332 mg/L CaCO3 were sonicated for periods of 5-45 min with an ultrasonic horn. Conductivity, temperature, ζ-potential, composition, and pH of the water were analyzed, together with the crystal structure of any calcium carbonate precipitate. Quasi-stable populations of bulk nanobubbles in Millipore and soft water are characterized by a ζ-potential of -35 to -20 mV, decaying over 60 h or more. After sonicating the hardest waters for about 10 min, they turn cloudy due to precipitation of amorphous calcium carbonate when the water temperature reaches 40 °C; the ζ-potential then jumps from -10 to +20 mV and remains positive for several days. From an analysis of the change of conductivity of the hard water before and after sonication, it is estimated that 37 ± 5% of calcium was not originally in solution but existed in nanoscale prenucleation clusters, which decorate the nanobubbles formed in the early stages of sonication. Heating and charge screening in the nanobubble colloid cause the decorated bubbles to collapse or disperse, leaving an amorphous precursor of aragonite. Sonicating the soft supernatant increases its conductivity and pH and restores the negative ζ-potential associated with bulk nanobubbles, but there is no further precipitation. Our study of the correlation between nanobubble production and calcium agglomeration spanning the hardness and composition ranges of natural waters shows that the sonication method for introducing nanobubbles is viable only for hard water if it is kept cold; the stability of the nanobubble colloid will be reduced in any case by the presence of dissolved calcium and magnesium.

Localized Phase Transformation Triggering Lattice Matching of Metal Oxide and Carbonate Hydroxide for Efficient CO2 Photoreduction

Orderly heterostructured catalysts, which integrate nanomaterials of complementary structures and dimensions into single-entity structures, have hold great promise for sustainability applications. In this work, it is showcased that air as green reagent can trigger in situ localized phase transformation and transform the metal carbonate hydroxide nanowires into ordered heterostructured catalyst. In single-crystal nanowire heterostructure, the in situ generated and nanosized Co3 O4 will be anchored in single-crystal Co6 (CO3 )2 (OH)8 nanowires spontaneously, triggered by the lattice matching between the (220) plane of Co3 O4 and the (001) plane of Co6 (CO3 )2 (OH)8 . The lattice matching allows intimate contact at heterointerface with well-defined orientation and strong interfacial coupling, and thus significantly expedites the transfer of photogenerated electrons from tiny Co3 O4 to catalytically active Co6 (CO3 )2 (OH)8 in single-crystal nanowire, which elevates the catalytic efficiency of metal carbonate catalyst in the CO2 reduction reaction (VCO = 19.46 mmol g-1 h-1 and VH2 = 11.53 mmol g-1 h-1 ). The present findings add to the growing body of knowledge on exploiting Earth-abundant metal-carbonate catalysts, and demonstrate the utility of localized phase transformation in constructing advanced catalysts for energy and environmental sustainability applications.

Controlling the crystal structure of succinic acid via microfluidic spray-drying

Many properties of materials, including their dissolution kinetics, hardness, and optical appearance, depend on their structure. Unfortunately, it is often difficult to control the structure of low molecular weight organic compounds that have a high propensity to crystallize if they are formulated from solutions wherein they have a high mobility. This limitation can be overcome by formulating these compounds within small airborne drops that rapidly dry, thereby limiting the time molecules have to arrange into the thermodynamically most stable phase. Such drops can be formed with a surface acoustic wave (SAW)-based spray-drier. In this paper, we demonstrate that the structure of a model low molecular weight compound relevant to applications in pharmacology and food, succinic acid, can be readily controlled with the supersaturation rate. Succinic acid particles preserve the metastable structure over at least 3 months if the initial succinic acid concentration is below 2% of its saturation concentration such that the supersaturation rate is high. We demonstrate that also the stability of the metastable phases against their transformation into the most stable phase increases with decreasing initial solute concentration and hence with increasing supersaturation rate of the spray-dried solution. These insights open up new opportunities to control the crystal structure and therefore properties of low molecular weight compounds that have a high propensity to crystallize.

Structural water in amorphous carbonate minerals: ab initio molecular dynamics simulations of X-ray pair distribution experiments

Water is known to play a controlling role in directing mineralization pathways and stabilizing metastable amorphous intermediates in hydrous carbonate mineral MCO₃·nH₂O systems, where M²⁺ is a divalent metal cation. Despite this recognition, the nature of the controls on crystallization is poorly understood, largely owing to the difficulty in characterizing the dynamically disordered structures of amorphous intermediates at the atomic scale. Here, we present a series of atomistic models, derived from ab initio molecular dynamics simulation, across a range of experimentally relevant cations (M = Ca, Mg, Sr) and hydration levels (0 ≤ n ≤ 2). Theoretical simulations of the dependence of the X-ray pair distribution function on the hydration level n show good agreement with available experimental data and thus provide further evidence for a lack of significant nanoscale structure in amorphous carbonates. Upon dehydration, the metal coordination number does not change significantly, but the relative extent of water dissociation increases, indicating that a thermodynamic driving force exists for water dissociation to accompany dehydration. Mg strongly favors monodentate conformation of carbonate ligands and shows a marked preference to exchange monodentate carbonate O for water O upon hydration, whereas Ca and Sr exchange mono- and bidentate carbonate ligands with comparable frequency. Water forms an extensive hydrogen bond network among both water and carbonate groups that exhibits frequent proton transfers for all three cations considered suggesting that proton mobility is likely predominantly due to water dissociation and proton transfer reactions rather than molecular water diffusion.

High drug loading nanosized amorphous solid dispersion (NASD) with enhanced in vitro solubility and permeability: Benchmarking conventional ASD

Through liquid-liquid phase separation (LLPS), it is possible to generate drug-rich nanoparticles during the dissolution of conventional amorphous solid dispersions (ASDs). These self-generated nanoparticles may improve the oral absorption of poorly water-soluble drugs by enhancing the drug's apparent solubility and effective membrane permeability. However, due to the high concentration threshold required for LLPS, conventional ASDs that can consistently generate drug-rich nanoparticles during dissolution are rare. More importantly, the quality of these meta-stable drug-rich nanoparticles is hard to control during dissolution, leading to inconsistency in formulation performances. This work has described a continuous twin-screw extrusion process capable of producing nanosized ASD (NASD) formulations that can offer better solubility and permeability enhancements over conventional ASD formulations. Two polymeric carriers, polyvinylpyrrolidone-co-vinyl acetate (PVPVA) and hydroxypropyl methylcellulose acetate succinate (HPMCAS), with a model hydrophobic drug celecoxib (BCS II), were formulated into both ASD and NASD formulations. Compared to the conventional ASD formulation, the prefabricated NASD (sizes ranging between 40 and 200 nm) embedded within a polyol matrix can be rapidly dispersed into a nanoparticle suspension in the presence of aqueous media. The resulting NASDs achieved drug loadings up to 80 % w/w and a maximum of 98 % encapsulation efficiency. Because of the TSE platform's high drug-loading capacity and high scalability, the developed method may be useful for continuously producing personalized nanomedicines.

Highly hydrated paramagnetic amorphous calcium carbonate nanoclusters as an MRI contrast agent

Amorphous calcium carbonate plays a key role as transient precursor in the early stages of biogenic calcium carbonate formation in nature. However, due to its instability in aqueous solution, there is still rare success to utilize amorphous calcium carbonate in biomedicine. Here, we report the mutual effect between paramagnetic gadolinium ions and amorphous calcium carbonate, resulting in ultrafine paramagnetic amorphous carbonate nanoclusters in the presence of both gadolinium occluded highly hydrated carbonate-like environment and poly(acrylic acid). Gadolinium is confirmed to enhance the water content in amorphous calcium carbonate, and the high water content of amorphous carbonate nanoclusters contributes to the much enhanced magnetic resonance imaging contrast efficiency compared with commercially available gadolinium-based contrast agents. Furthermore, the enhanced T₁ weighted magnetic resonance imaging performance and biocompatibility of amorphous carbonate nanoclusters are further evaluated in various animals including rat, rabbit and beagle dog, in combination with promising safety in vivo. Overall, exceptionally facile mass-productive amorphous carbonate nanoclusters exhibit superb imaging performance and impressive stability, which provides a promising strategy to design magnetic resonance contrast agent.

Sensitive, biocompatible and stable contrast agents for MRI are in demand. Here, the authors combine gadolinium ions with amorphous calcium carbonate to make stable paramagnetic amorphous carbonate nanoclusters with high MRI contrast and significantly improved biocompatibility over commercial gadolinium-based agents.

Microfluidic Synthesis and Analysis of Bioinspired Structures Based on CaCO3 for Potential Applications as Drug Delivery Carriers

Naturally inspired biomaterials such as calcium carbonate, produced in biological systems under specific conditions, exhibit superior properties that are difficult to reproduce in a laboratory. The emergence of microfluidic technologies provides an effective approach for the synthesis of such materials, which increases the interest of researchers in the creation and investigation of crystallization processes. Besides accurate tuning of the synthesis parameters, microfluidic technologies also enable an analysis of the process in situ with a range of methods. Understanding the mechanisms behind the microfluidic biomineralization processes could open a venue for new strategies in the development of advanced materials. In this review, we summarize recent advances in microfluidic synthesis and analysis of CaCO₃-based bioinspired nano- and microparticles as well as core-shell structures on its basis. Particular attention is given to the application of calcium carbonate particles for drug delivery.

Effect of poly(acrylic acid) on crystallization of calcium carbonate in a hydrogel

As carbonate ions are diffused into an agarose hydrogel containing calcium ions and poly(acrylic acid), elliptical and spherical calcites are controllably formed depending on the concentration of poly(acrylic acid) and the position of the hydrogel.

Pyrophosphate-stabilised amorphous calcium carbonate for bone substitution: toward a doping-dependent cluster-based model

Multiscale and multitool advanced characterisation of pyrophosphate-stabilised amorphous calcium carbonates allowed building a cluster-based model paving the way for tunable biomaterials.

Possible embryo and precursor of crystalline nuclei of calcium carbonate observed by LC-TEM

Several different building blocks or precursors play an important role in the early stages of crystallization of calcium carbonate (CaCO3). Substantial number of studies have been conducted to understand the...

In vitro crystallization of calcium carbonate mediated by proteins extracted from P. placenta shells

The ASM extracted from the shells of P. placenta can stabilize ACC and inhibit secondary nucleation for 10 hours, and an explosive secondary nucleation and quick crystal growth from 50 nm to 10 μm can be finished on the shell surface in one hour.

Pressure-induced crystallization and densification of amorphized calcium carbonate hexahydrate controlled by interfacial water

Amorphous calcium carbonate (ACC) is widely known as a metastable precursor in the formation of crystalline calcium carbonate biominerals. However, the exact role of water during the crystallization of ACC remains elusive. Here, a novel ACC with high specific surface area and nanopores is synthesized by solvent-induced dehydration and amorphization of crystalline calcium carbonate hexahydrate (ikaite), denoted as I-ACC. Comparing I-ACC and typical spherical ACC (S-ACC) nanoparticles, it reveals that the crystallization pathways of ACC under heating or pressure are not dictated by the total amount of water in ACC as reported, but rather the interfacial water that is released from ACC bulk and adsorbed on the surface of the particles. We show that the crystallization pathways of I-ACC to calcite single crystal with high specific surface area or vaterite can be easily controlled by tuning the release of water during heating. In addition, densely packed pure vaterite can be obtained via pressured-induced transformation of I-ACC at room temperature, which is otherwise difficult to form using S-ACC. These insights contribute to the understanding of the biological control of mineral formation via amorphous precursors and offer new opportunities to bioprocess inspired fabrication of strong bulk material at room temperature.

Pressure-driven fusion of amorphous particles into integrated monoliths

Amorphous calcium carbonate is a hard material that is difficult to make into large, clear blocks. It is also sensitive to heating, and compacting the starting nanoparticles too much tends to lead to crystallization. Mu et al. determined the optimal amount of water in amorphous calcium carbonate to create clear, solid monoliths through compression. The key is to regulate the amount of diffusion in the system so that particle boundaries fuse without triggering sample-wide crystallization. This fusion strategy may also work for similar amorphous inorganic ionic compounds.

Science , abg1915, this issue p. 1466

Generality of liquid precursor phases in gas diffusion-based calcium carbonate synthesis

We confirm the presence of liquid calcium carbonate precursor species in absence of additives in gas diffusion systems.

A Multifaceted Kinetic Model for the Thermal Decomposition of Calcium Carbonate

The existing kinetic models often consider the influence of a single factor alone on the chemical reaction and this is insufficient to completely describe the decomposition reaction of solids. Therefore, the existing kinetic models were improved using the pore structure model. The proposed model was verified using the thermal decomposition experiment on calcium carbonate. The equation has been modified as fα=n1−α1−1n−ln1−α−1m1−ψln1−α12. This led to the conclusion that the pore structure, generated during the thermal decomposition of calcite, has an important influence on the decomposition kinetics. The existing experimental data show that the improved model, with random pores as the main body, reasonably describes the thermal decomposition process of calcite.

The Crystallization Process of Vaterite Microdisc Mesocrystals via Proto-Vaterite Amorphous Calcium Carbonate Characterized by Cryo-X-ray Absorption Spectroscopy

Investigation on the formation mechanism of crystals via amorphous precursors has attracted a lot of interests in the last years. The formation mechanism of thermodynamically meta-stable vaterite in pure alcohols in the absence of any additive is less known. Herein, the crystallization process of vaterite microdisc mesocrystals via proto-vaterite amorphous calcium carbonate (ACC) in isopropanol was tracked by using Ca K-edge X-ray absorption spectroscopy (XAS) characterization under cryo-condition. Ca K-edge X-ray absorption near edge structure (XANES) spectra show that the absorption edges of the Ca ions of the vaterite samples with different crystallization times shift to lower photoelectron energy while increasing the crystallization times from 0.5 to 20 d, indicating the increase of crystallinity degree of calcium carbonate. Ca K-edge extended X-ray absorption fine structure (EXAFS) spectra exhibit that the coordination number of the nearest neighbor atom O around Ca increases slowly with the increase of crystallization time and tends to be stable as 4.3 (±1.4). Crystallization time dependent XANES and EXAFS analyses indicate that short-range ordered structure in proto-vaterite ACC gradually transform to long-range ordered structure in vaterite microdisc mesocrystals via a non-classical crystallization mechanism.

Solid-State Phase Transformation and Self-Assembly of Amorphous Nanoparticles into Higher-Order Mineral Structures

Materials science has been informed by nonclassical pathways to crystallization, based on biological processes, about the fabrication of damage-tolerant composite materials. Various biomineralizing taxa, such as stony corals, deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further assemble and transform into higher-order mineral structures. Here, we examine a similar process in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of high-resolution imaging and in situ solid-state nuclear magnetic resonance spectroscopy, we reveal the underlying mechanism of the solid-state phase transformation of these amorphous nanoparticles into crystals under aqueous conditions. These amorphous nanoparticles are covered by a hydration shell of bound water molecules. Fast chemical exchanges occur: the hydrogens present within the nanoparticles exchange with the hydrogens from the surface-bound H₂O molecules which, in turn, exchange with the hydrogens of the free H₂O molecule of the surrounding aqueous medium. This cascade of chemical exchanges is associated with an enhanced mobility of the ions/molecules that compose the nanoparticles which, in turn, allow for their rearrangement into crystalline domains via solid-state transformation. Concurrently, the starting amorphous nanoparticles aggregate and form ordered mineral structures through crystal growth by particle attachment. Sphere-like aggregates and spindle-shaped structures were, respectively, formed from relatively high or low weights per volume of the same starting amorphous nanoparticles. These results offer promising prospects for exerting control over such a nonclassical pathway to crystallization to design mineral structures that could not be achieved through classical ion-by-ion growth.

Stable Prenucleation Calcium Carbonate Clusters Define Liquid-Liquid Phase Separation

Liquid-liquid phase separation (LLPS) is an intermediate step during the precipitation of calcium carbonate, and is assumed to play a key role in biomineralization processes. Here, we have developed a model where ion association thermodynamics in homogeneous phases determine the liquid-liquid miscibility gap of the aqueous calcium carbonate system, verified experimentally using potentiometric titrations, and kinetic studies based on stopped-flow ATR-FTIR spectroscopy. The proposed mechanism explains the variable solubilities of solid amorphous calcium carbonates, reconciling previously inconsistent literature values. Accounting for liquid-liquid amorphous polymorphism, the model also provides clues to the mechanism of polymorph selection. It is general and should be tested for systems other than calcium carbonate to provide a new perspective on the physical chemistry of LLPS mechanisms based on stable prenucleation clusters rather than un-/metastable fluctuations in biomineralization, and beyond.

Molecular Understanding of Humic Acid-Limited Phosphate Precipitation and Transformation

Phosphorus (P) availability is widely assumed to be limited by the formation of metal (Ca, Fe, or Al) phosphate precipitates that are modulated by soil organic matter (SOM), but the SOM-precipitate interactions remain uncertain because of their environmental complexities. Here, we present a model system by quantifying the in situ nanoscale nucleation kinetics of calcium phosphates (Ca-Ps) on mica in environmentally relevant aqueous solutions by liquid-cell atomic force microscopy. We find that Ca-P precipitate formation is slower when humic acid (HA) concentration is higher. High-resolution transmission electron microscopy observations demonstrate that HA strongly stabilizes amorphous calcium phosphate (ACP), delaying its subsequent transformation to thermodynamically more stable phases. Consistent with the formation of molecular organo-mineral bonding, dynamic force spectroscopy measurements display larger binding energies of organic ligands with certain chemical functionalities on HA to the initially formed ACP than to mica that are responsible for stabilization of ACP through stronger HA-ACP interactions. Our results provide direct evidence for the proposed importance of SOM in inhibiting Ca-P precipitation/transformation. We suggest that similar studies of binding strength in SOM-Fe/Al-P may reveal how both organic matter and metal ions control P availability and fate, and thus the eventual P management for agronomical and environmental sustainability.

Recent experimental explorations of non-classical nucleation

This work provides a clearer picture for non-classical nucleation by revealing the presence of various intermediates using advanced characterization techniques.

Water: How Does It Influence the CaCO3 Formation?

Nature produces biomineral-based materials with a fascinating set of properties using only a limited number of elements. This set of properties is obtained by closely controlling the structure and local composition of the biominerals. We are far from achieving the same degree of control over the properties of synthetic biomineral-based composites. One reason for this inferior control is our incomplete understanding of the influence of the synthesis conditions and additives on the structure and composition of the forming biominerals. In this Review, we provide an overview of the current understanding of the influence of synthesis conditions and additives during different formation stages of CaCO₃ , one of the most abundant biominerals, on the structure, composition, and properties of the resulting CaCO₃ crystals. In addition, we summarize currently known means to tune these parameters. Throughout the Review, we put special emphasis on the role of water in mediating the formation of CaCO₃ and thereby influencing its structure and properties, an often overlooked aspect that is of high relevance.

Aerosol-based synthesis of pure and stable amorphous calcium carbonate

A facile aerosol-based method for the synthesis of pure and stable amorphous calcium carbonate (ACC) is presented. The method relies on the instantaneous carbonation of calcium hydroxide aerosols with carbon dioxide followed by rapid drying of the freshly formed ACC. The ACC display extended stability against humidity induced crystallization.

The Investigation of Flory-Huggins Interaction Parameters for Amorphous Solid Dispersion Across the Entire Temperature and Composition Range

Amorphous solid dispersion (ASD) is one of the most promising enabling formulations featuring significant water solubility and bioavailability enhancements for biopharmaceutical classification system (BCS) class II and IV drugs. An accurate thermodynamic understanding of the ASD should be established for the ease of development of stable formulation with desired product performances. In this study, we report a first experimental approach combined with classic Flory–Huggins (F–H) modelling to understand the performances of ASD across the entire temperature and drug composition range. At low temperature and drug loading, water (moisture) was induced into the system to increase the mobility and accelerate the amorphous drug-amorphous polymer phase separation (AAPS). The binodal line indicating the boundary between one phase and AAPS of felodipine, PVPK15 and water ternary system was successfully measured, and the corresponding F–H interaction parameters (χ) for FD-PVPK15 binary system were derived. By combining dissolution/melting depression with AAPS approach, the relationship between temperature and drug loading with χ (Φ, T) for FD-PVPK15 system was modelled across the entire range as χ = 1.72 − 852/T + 5.17·Φ − 7.85·Φ². This empirical equation can provide better understanding and prediction for the miscibility and stability of drug-polymer ASD at all conditions.

Anhydrous amorphous calcium carbonate (ACC) is structurally different from the transient phase of biogenic ACC

An in situ ambient pressure soft X-ray spectroscopic study of the phase transformation of ACC exposed to water vapor in the mbar pressure range in conjunction with heat treatment.

The multiple roles of carbonic anhydrase in calcium carbonate mineralization

Carbonic anhydrase (CA) accelerates, templates and arrests calcium carbonate mineralization by playing both enzymatic and structural protein roles.