Role of CaCO3° Neutral Pair in Calcium Carbonate Crystallization

Role of Mass Transport in the Deposition, Growth, and Transformation of Calcium Carbonate on Surfaces at High Supersaturation

We demonstrate how combined in-situ measurements and finite element method modeling can provide new insight into the relative contribution of mass transport to the growth of calcium carbonate on two model surfaces, glass and gold, under high-supersaturation conditions relevant to surface scaling. An impinging jet-radial flow system is used to create a high-supersaturated solution at the inlet of different cells: an optical microscope cell presenting a glass surface for deposition and quartz crystal microbalance (QCM) and in-situ IR spectroscopy cells, both presenting a gold surface. The approach described is quantitative due to the well-defined mass transport, and both time-lapse optical microscopy images and QCM data are analyzed to provide information on the growth kinetics of the calcite crystals. Initially, amorphous calcium carbonate (ACC), formed in solution, dominates the deposition process. At longer times, the growth of calcite is more significant and, on glass, is observed to consume ACC from the surface, leading to surface regions depleted of ACC developing around calcite microcrystals. On Au, the mass increase becomes linear with time in this region. Taken together, these microscopic and macroscopic measurements demonstrate that calcite growth has a significant component of mass transport control at high supersaturation. Finite element method (FEM) simulations of mass-transport-limited crystal growth support the strong mass transport contribution to the growth kinetics and further suggest that the observed growth must be sustained by more than just the Ca²⁺ and CO₃ ^2- in solution, with dissolution/direct attachment of ACC and/or ion pairs also contributing to the growth process.

Thermodynamic and Kinetic Studies on the Conversion of Solvent-Shared to Contact Ion Pairs in Sparingly Soluble MF2 (M = Mg2+ and Ca2+) Aqueous Solutions: Implications for Understanding Supersaturated Behavior and Association Constant Determination

The thermodynamic and kinetic behaviors of Mg²⁺-F^- ion pairing in aqueous solution are investigated theoretically and experimentally and are contrasted to those of Ca²⁺-F^-. Thermodynamically, similar to CaF_x(H₂O)₁₄^2-x (x = 1 and 2), MgF(H₂O)_y⁺ (y = 14-20) contact ion pairs (CIPs) are more stable than their solvent-shared ion pairs (SSIPs), whereas the CIPs and SSIPs of MF₂(H₂O)_y are almost isoenergetic. However, in kinetics, the conversion of SSIPs to CIPs for M²⁺-F^- (M = Mg²⁺ and Ca²⁺) ion pairing must overcome a high energy barrier due to the strong hydration of Mg²⁺ and F^-. The kinetics dominate after the thermodynamics and kinetics are balanced, which hinders the formation of M²⁺-F^- CIPs in practical MF₂ aqueous solutions (less than or equal to saturated concentrations). This result is also supported by the ¹⁹F nuclear magnetic resonance spectra of saturated MF₂ solutions. Although the interaction between Mg²⁺ and F^- is slightly stronger than that between Ca²⁺ and F^- due to the smaller radius of Mg²⁺, the formation of Mg²⁺-F^- CIPs needs to go through two rate-limiting steps, the dehydration and entrance of F^- (i.e., via exchange mode) with a higher energy barrier, due to the ability of strongly bound water molecules and rigorous octahedral coordinated configuration of Mg²⁺, while the formation of Ca²⁺-F^- CIPs only goes through a single rate-limiting step, the entrance of F^- (i.e., via swinging mode) with a lower energy barrier, due to the flexible coordination configuration of Ca²⁺. This is responsible for precipitation in MgF₂ aqueous solution requiring a larger supersaturation degree and a lower precipitation rate than in CaF₂. These kinetic factors lead to the association constants previously reported for MF⁺ determined by a fluoride ion-selective electrode (ISE) combined with the titration method, where the MF₂ solutions were always unsaturated at the titration end point, which actually corresponds to those of the ligand process going from completely free M²⁺ and F^- to their SSIPs. A possible strategy to accurately determine the association constants of MF⁺ and MF₂(aq) CIPs by fluoride ISEs is proposed. The present results suggest that judging the formation of M²⁺-F^- CIPs in practical solutions from a theoretical calculation perspective requires significant consideration of the kinetic factors, except for the thermodynamic factors.

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.

Controlling CaCO3 Particle Size with {Ca2+}:{CO3 2-} Ratios in Aqueous Environments

The effect of stoichiometry on the new formation and subsequent growth of CaCO₃ was investigated over a large range of solution stoichiometries (10^-4 < r _aq < 10⁴, where r _aq = {Ca²⁺}:{CO₃ ^2-}) at various, initially constant degrees of supersaturation (30 < Ω_cal < 200, where Ω_cal = {Ca²⁺}{CO₃ ^2-}/K _sp), pH of 10.5 ± 0.27, and ambient temperature and pressure. At r _aq = 1 and Ω_cal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1-10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At r _aq ≠ 1 (Ω_cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on r _aq. Complementary molecular dynamics (MD) simulations confirm that r _aq affects CaCO₃ nanoparticle formation substantially. At r _aq = 1 and Ω_cal ≫ 1000, the largest nanoparticle in the system had a 21-68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ω_cal, stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO₃ in geological, industrial, and geo-engineering settings.

Ikaite nucleation at 35 °C challenges the use of glendonite as a paleotemperature indicator

Glendonites have been found worldwide in marine sediments from the Neoproterozoic Era to the Quaternary Period. The precursor of glendonite, ikaite (CaCO₃ · 6H₂O), is metastable and has only been observed in nature at temperatures <7 °C. Therefore, glendonites in the sedimentary record are commonly used as paleotemperature indicators. However, several laboratory experiments have shown that the mineral can nucleate at temperatures>7 °C. Here we investigate the nucleation range for ikaite as a function of temperature and pH. We found that ikaite precipitated at temperatures of at least 35 °C at pH 9.3 −10.3 from a mixture of natural seawater and sodium carbonate rich solution. At pH 9.3, we observed pseudomorphic replacement of ikaite by porous calcite during the duration of the experiment (c. 5 hours). These results imply that ikaite can form at relatively high temperatures but will then be rapidly replaced by a calcite pseudomorph. This finding challenges the use of glendonites as paleotemperature indicators.

Ion Pairing and Multiple Ion Binding in Calcium Carbonate Solutions Based on a Polarizable AMOEBA Force Field and Ab Initio Molecular Dynamics

The speciation of calcium carbonate in water is important to the geochemistry of the world's oceans and has ignited significant debate regarding the mechanism by which nucleation occurs. Here, it is vital to be able to quantify the thermodynamics of ion pairing versus higher order association processes in order to distinguish between possible pathways. Given that it is experimentally challenging to quantify such species, here we determine the thermodynamics for ion pairing and multiple binding of calcium carbonate species using bias-enhanced molecular dynamics. In order to examine the uncertainties underlying these results, we derived a new polarizable force field for both calcium carbonate and bicarbonate in water based on the AMOEBA model to compare against our earlier rigid ion model, both of which are further benchmarked against ab initio molecular dynamics for the ion pair. Both force fields consistently indicate that the association of calcium carbonate ion pairs to form larger species is stable, though with an equilibrium constant that is lower than for ion pairing itself.

Reconsidering Calcium Dehydration as the Rate-Determining Step in Calcium Mineral Growth

The dehydration of cations is generally accepted as the rate-limiting step in many processes. Molecular dynamics (MD) can be used to investigate the dynamics of water molecules around cations, and two different methods exist to obtain trajectory-based water dehydration frequencies. Here, these two different post-processing methods (direct method versus survival function) have been implemented to obtain calcium dehydration frequencies from a series of trajectories obtained using a range of accepted force fields. None of the method combinations reproduced the commonly accepted experimental water exchange frequency of 10-8.2 s-1. Instead, our results suggest much faster water dynamics, comparable with more accurate ab initio MD simulations and with experimental values obtained using neutron scattering techniques. We obtained the best agreement using the survival function method to characterize the water dynamics, and we show that different method combinations significantly affect the outcome. Our work strongly suggests that the fast water exchange kinetics around the calcium ions is not rate-limiting for reactions involving dissolved/solvated calcium. Our results further suggest that, for alkali and most of the earth alkali metals, mechanistic rate laws for growth, dissolution, and adsorption, which are based on the principle of rate-limiting cation dehydration, need careful reconsideration.

Countercations Control Local Specific Bonding Interactions and Nucleation Mechanisms in Concentrated Water-in-Salt Solutions

One of the continuing challenges presented in salt solutions is understanding ion association reactions driving dynamic demixing from solvation, complexation, and solute clustering. The problems understanding this phenomenon are exacerbated in the highly concentrated water-in-salt solutions, where the deficiency of water leads to a dramatic retardation of water solvent and formation of extended solvent-solute clustering networks. By probing microscopic dynamics of water and prenucleation clusters using quasi-elastic neutron scattering and proton nuclear magnetic resonance spectroscopy, we observed contrasting mechanistic specifics of ion-water mobilities in highly concentrated Na⁺- versus K⁺-based aluminate solutions (diffusion coefficients of 0.2 vs 2.6 × 10^-10 m² s^-1 at 293 K, respectively). The magnitude of the differences is far beyond countercations acting as simple innocent charge-balancing species or water solvents functioning as a simple medium for ion diffusion. The distinct crystallization mechanisms observed further imply that different prenucleation cluster dynamics can either frustrate or promote crystallization, as described by nonclassical nucleation theory.

Insights into water-mediated ion clustering in aqueous CaSO4 solutions: pre-nucleation cluster characteristics studied by ab initio calculations and molecular dynamics simulations

The molecular structure of growth units building crystals is a fundamental issue in the crystallization processes from aqueous solutions. In this work, a systematic investigation of pre-nucleation clusters and their hydration characteristics in aqueous CaSO₄ solutions was performed using ab initio calculations and molecular dynamics (MD) simulations. The results of ab initio calculations and MD simulations indicate that the dominant species in aqueous CaSO₄ solutions are monodentate ion-associated structures. Compared with charged ion clusters, neutral clusters are more likely to be present in an aqueous CaSO₄ solution. Neutral (CaSO₄)_m clusters are probably the growth units involved in the pre-nucleation or crystallization processes. Meanwhile, hydration behavior around ion associated species in aqueous CaSO₄ solutions plays an important role in related phase/polymorphism selections. Upon ion clustering, the residence of some water molecules around Ca²⁺ in ion-associated species is weakened while that of some bridging waters is enhanced due to dual interaction by Ca²⁺ and SO₄^2-. Some phase/polymorphism selections can be achieved in aqueous CaSO₄ solutions by controlling the hydration around pre-nucleation clusters. Moreover, the association trend between calcium and sulfate is found to be relatively strong, which hints at the low solubility of calcium sulfate in water.

Supersaturated calcium carbonate solutions are classical

Mechanisms of CaCO₃ nucleation from solutions that depend on multistage pathways and the existence of species far more complex than simple ions or ion pairs have recently been proposed. Herein, we provide a tightly coupled theoretical and experimental study on the pathways that precede the initial stages of CaCO₃ nucleation. Starting from molecular simulations, we succeed in correctly predicting bulk thermodynamic quantities and experimental data, including equilibrium constants, titration curves, and detailed x-ray absorption spectra taken from the supersaturated CaCO₃ solutions. The picture that emerges is in complete agreement with classical views of cluster populations in which ions and ion pairs dominate, with the concomitant free energy landscapes following classical nucleation theory.