Controlling CaCO₃ particle size with {Ca²⁺}:{CO₃^2-} ratios in aqueous environments

The impact of stoichiometry (r_aq = {Ca²⁺}:{CO₃^2-}) on the new formation and subsequent growth of CaCO₃ is important, as most natural waters and industrial crystallization processes proceed nonstoichiometrically. Therefore, we investigated in a broad range (10 ^-4 < r_aq < 10⁴) the effect of solution stoichiometry 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 (Seepma et al., 2021). 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 dominant growth mechanism. At r_aq ≠ 1 (Ω_cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 hours. 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 non-stoichiometric systems. Our results imply that, besides Ω_cal, stoichiometry affects particle size and persistence, growth and ripening time towards μm-sized crystals. Our results may help to improve understanding, prediction and formation of CaCO₃ in geological, industrial and geo-engineering settings.