posted on 2021-01-28, 17:06authored bySergěj Y. M. H. Seepma, Sergio E. Ruiz-Hernandez, Gernot Nehrke, Karline Soetaert, Albert P. Philipse, Bonny W. M. Kuipers, Mariette Wolthers
The effect of stoichiometry on the
new formation and subsequent
growth of CaCO3 was investigated over a large range of
solution stoichiometries (10–4 < raq < 104, where raq = {Ca2+}:{CO32–}) at various,
initially constant degrees of supersaturation (30 < Ωcal < 200, where Ωcal = {Ca2+}{CO32–}/Ksp), pH of 10.5 ± 0.27, and ambient temperature and pressure.
At raq = 1 and Ωcal <
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 raq ≠
1 (Ωcal = 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 raq. Complementary molecular dynamics (MD) simulations confirm that raq affects CaCO3 nanoparticle formation
substantially. At raq = 1 and Ωcal ≫ 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
Ωcal, 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 CaCO3 in geological, industrial, and geo-engineering
settings.