Dehydration-Induced Amorphous Phases of Calcium Carbonate

Abstract

Amorphous calcium carbonate (ACC) is a critical transient phase in the inorganic precipitation of CaCO3 and in biomineralization. The calcium carbonate crystallization pathway is thought to involve dehydration of more hydrated ACC to less hydrated ACC followed by the formation of anhydrous ACC. We present here computational studies of the transition of a hydrated ACC with a H2O/CaCO3 ratio of 1.0 to anhydrous ACC. During dehydration, ACC undergoes reorganization to a more ordered structure with a significant increase in density. The computed density of anhydrous ACC is similar to that of calcite, the stable crystalline phase. Compared to the crystalline CaCO3 phases, calcite, vaterite, and aragonite, the computed local structure of anhydrous ACC is most-similar to those of calcite and vaterite, but the overall structure is not well described by either. The strong hydrogen bond interaction between the carbonate ions and water molecules plays a crucial role in stabilizing the less hydrated ACC compositions compared to the more hydrated ones, leading to a progressively increasing hydration energy with decreasing water content.

Direct Experimental Measurement of Water Interaction Energetics in Amorphous Carbonates MCO3 (M = Ca, Mn, and Mg) and Implications for Carbonate Crystal Growth

“…The loss of the final fraction of hydroxyl/trapped water above 300°C is found to be crucial as it triggers the crystallization. 44,46,49 Similarly, Ihli et al 44 have identified four stages in the mechanism of dehydration of ACC leading to crystallization as stage-I: loss of surface-bound water at ∼40°C, stage-II: loss of water from the interior of the ACC with shrinkage of ACC particles around 140°C, stage-III: expulsion of the most deeply located water around 290°C, and stage-IV: crystallization to calcite at 315°C. In this work, to avoid any structural changes upon heating or crystallization, the first set of water adsorption calorimetric experiments was carried out on amorphous samples degassed 6−8 h under vacuum at 25°C.…”

“…6,44,46,49 The water adsorption enthalpy profiles of ACC in Figure 3a shows the existence of two different energetic regions for both 25 and 100°C degassed samples. The first region corresponding to the adsorption enthalpies for the initial doses of water (coverage up to 3 H 2 O/nm 2 ) is more exothermic and shows a sharp decrease in magnitude from −100 to −60 kJ/mol.…”

“…Furthermore, the different energetic regions in water adsorption enthalpy curves of ACC can be correlated to the TGA dehydration steps and consequently to different types of water reported in the earlier structural studies. 44,46,49 The energetics of water adsorption in the first step probably correspond to the strongly bound restrictedly mobile and rigid H 2 O components (dehydration step-II in TGA of ACC). ACC degassed at 100°C appear to have a higher number of these sites and hence has slightly more exothermic adsorption enthalpy than the 25°C degassed ACC sample.…”

Uncovering the True Atomic Structure of Disordered Materials: The Structure of a Hydrated Amorphous Magnesium Carbonate (MgCO3·3D2O)

“…20 Hence, using experimentally driven structural refinement techniques, such as RMC, without taking into account the energetic penalties that may exist for certain bonding environment can result in fictitious structural representations for highly disordered materials. Other investigations on the structure of hydrated AMC/ACC samples have not reported the existence of water-containing 'pores' or pockets, 8,9,12,17,18,57 although Tribello et al did report the occurrence of water trapping in calcium carbonate clusters (using MD and umbrella sampling) in explanation of the existence of hydrated ACC, 20 and therefore our DFT-PDF iterative methodology has revealed new insight into the local structural environment present in this hydrated AMC structure (MgCO 3 ·3D 2 O) and indicates that similar structural arrangements may also exist in hydrated ACC.…”

Hydration Effects on the Stability of Calcium Carbonate Pre-Nucleation Species

“…Ions in solution are able to modify water structure dynamics in their local environment as a result of effects associated with their hydration shells [26], which immobilize and electrostrict water [27]. The dehydration kinetics of ions (or clusters) in solution will be a competition between ion (or clusters)-water and water-water interactions [28,29], which can be significantly modified by the presence of background ions in solution [30]. At low ionic strength, the effect of background electrolytes on ion (or clusters)-water electrostatic interactions will be dominant [26].…”