Ion association behaviors in the initial stage of calcium carbonate formation: An ab initio study.

This article focuses on the investigation of ion association mechanisms and dynamic structures of calcium carbonate (CaCO₃) precursor species in aqueous solutions across varying pH levels using static density functional theory (DFT) calculations and ab initio metadynamics simulations. It reveals that at near-neutral pH, Ca²⁺ ions predominantly associate with bicarbonate (HCO₃⁻) ions via an associative ligand substitution mechanism, favoring monodentate coordination, while at higher pH, carbonate (CO₃²⁻) ions dominate and form more stable bidentate complexes with Ca²⁺, promoting CaCO₃ nucleation. In alkaline to hyperalkaline conditions, calcium hydroxide species (Ca(OH)⁺ and Ca(OH)₂) form, which reduce Ca²⁺ coordination numbers and favor a dissociative ligand substitution mechanism; the presence of OH⁻ ions inhibits CaCO₃ monomer formation by neutralizing Ca²⁺ charges and decreasing its capacity to coordinate carbonate ions. These findings provide atomic-level insights into how solution pH influences CaCO₃ formation, with implications for understanding biomineralization affected by ocean acidification and for managing industrial scaling issues.