Controlling CaCO Particle Size with {Ca}:{CO } Ratios in Aqueous Environments.

The effect of stoichiometry on the new formation and subsequent growth of CaCO was investigated over a large range of solution stoichiometries (10 < < 10, where = {Ca}:{CO }) at various, initially constant degrees of supersaturation (30 < Ω < 200, where Ω = {Ca}{CO }/ ), pH of 10.5 ± 0.27, and ambient temperature and pressure. At = 1 and Ω < 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 the dominant growth mechanism. At ≠ 1 (Ω = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. 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 . Complementary molecular dynamics (MD) simulations confirm that affects CaCO nanoparticle formation substantially. At = 1 and Ω ≫ 1000, the largest nanoparticle in the system had a 21-68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ω, stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO in geological, industrial, and geo-engineering settings.