Using simulation to understand the structure and properties of hydrated amorphous calcium carbonate

Abstract

We report results from studies using four different protocols to prepare hydrated amorphous calcium carbonate, ranging from random initial structures to melting hydrated mineral structures.All protocols give good agreement with experimental X-ray structure factors. However, the thermodynamic properties, ion coordination environments, and distribution of water for the structures produced by the protocols show statistically significant variation depending on the protocols used. We discuss the diffusivity of water through the various structures and its relation to experiments. We show that one protocol (based on melting ikaite) gives a structure where the water is mobile, due to the presence of porosity in the amorphous structure. We conclude that our models of hydrated amorphous calcium carbonate do give a range of behaviour that resembles that observed experimentally, although the variation is less marked in the simulations than in experiments.3

Water: How Does It Influence the CaCO3 Formation?

“…[83] Thes urrounding matrix includes H 2 Om olecules that strongly interact with Ca 2+ ions such that these molecules have alow mobility. [83,89,91] This result is consistent with experimental 1 Hs olid-state NMR measurements that reveal two types of water molecules possessing distinctly different mobilities. [83,89,91] This result is consistent with experimental 1 Hs olid-state NMR measurements that reveal two types of water molecules possessing distinctly different mobilities.…”

“…[47,[55][56][57]61,68,79,[84][85][86] Molecular dynamics simulations and experiments reveal that the vast majority of water molecules is incorporated into the structure of ACC. [87,89] Similarly, water molecules form hydrogen bonds with oxygen atoms contained in carbonate ions.Indeed, the formation of hydrogen bonds between water and carbonate ions is more favorable than that between two water molecules. [87,89] Similarly, water molecules form hydrogen bonds with oxygen atoms contained in carbonate ions.Indeed, the formation of hydrogen bonds between water and carbonate ions is more favorable than that between two water molecules.…”

“…[90] To obtain ab etter understanding of the distribution of water within ACC, at hree-dimensional (3D) atomic ACC model has been developed on the basis of pair distribution function (PDF) analysis.T his model indicates that ACC consists of an anoporous Ca-rich matrix encompassing ap ercolating network of channels that are filled with CO 3 2À ions and water, as shown in Figure 5A. [83,89,91] By contrast, the nanochannels contain water molecules that form weaker hydrogen bonds with CO 3 2À such that their mobility is much higher. [83,89,91] By contrast, the nanochannels contain water molecules that form weaker hydrogen bonds with CO 3 2À such that their mobility is much higher.…”

“…[73] C, D) Average Ca···Cacorrelations in C) as-synthesized ACC particles and D) those exposedt oapressure of 11.9 GPa obtained from reverse Monte Carlo (RMC) modeling of their PDFs. [73,74,89,91] During dehydration, H 2 Om olecules present in the first Ca-O coordination shell are replaced by CO 3 2À groups.Asaresult of this exchange,the values of CN in the first Ca-O coordination shell and the average distance between Oand Ca atoms, R,remain nearly constant, as shown in Figure 5B. [96] E) OK-edge and F) Ca L-edge XANES spectra of ACC with different degrees of hydration (n)a nd different CaCO 3 crystalline polymorphs including vaterite, aragonite, calcite, monohydrocalcite (MHC), and calcium carbonate hemihydrate (CCHH).…”

“…[55,66,73,74,85,87] This rearrangement is kinetically hindered by the interactions between water and ions contained in ACC, which decrease the mobility of water and ions. [56,73,89,91] With progressing dehydration, the interactions between water and ions contained in ACCb ecome stronger such that the energy barrier for further water removal increases. [56,73,89,91] With progressing dehydration, the interactions between water and ions contained in ACCb ecome stronger such that the energy barrier for further water removal increases.…”