Thermodynamic Control of Amorphous Precursor Phases for Calcium Carbonate via Additive Ions

Abstract

Calcium carbonate is an earth-abundant biomineral that exists in a variety of marine environments, including corals, shells of mollusks, sea urchins, and so forth. Particularly, amorphous calcium carbonate (ACC) phases have increasingly received scientific attention because their local order in the short range can affect the subsequent pathways for phase transition. In this regard, various types of additives have been employed to tailor the local structures and stability of ACC; however, their precise roles in controlling the phase transition pathways are still unclear. To address this ambiguity problem, the effects of additive ions on the structure and stability of amorphous precursor phases were theoretically traced using molecular dynamics simulation. Starting from the nucleation cluster in aqueous solution, the hydrated and anhydrous forms of ACC were systematically examined by varying the hydration levels and molar compositions of additive ions (e.g., Mg2+, Fe2+, Sr2+, and Ba2+). Our results revealed that each ion can exert promoting or inhibiting effect by tuning the local structures and stability of amorphous precursor phases depending on their hydrophilicity and ionic radii. Moreover, our findings suggested that the thermodynamic spontaneity of the overall phase transition process can be determined by the balance between two opposing factors—endothermic dehydration and exothermic crystallization.