Classical nucleation theory predicts a barrier on the order of ~100 kT for calcite formation, which excludes direct nucleation from a solution of Ca2+ and CO32- ions. Alternate theories for the mechanism of early mineral formation have been put forward, such as the formation of prenucleation clusters and liquid-liquid phase separation. In both theories, liquid species similar to amorphous calcium carbonate are proposed to form before calcite. In this contribution, we present a quantitative phase diagram for such amorphous phases, as liquid-liquid phase separation, from supersaturated CaCO3 solutions, predicted using density functional theory and the COSMO-RS implicit solvent model. In the limit of high pH, where only Ca2+ and CO32- ions are included in the simulations we find that the critical point lies below -170 °C, i.e. liquid-liquid phase separation occurs at all temperatures. We also find that the barrier for liquid-liquid phase separation is ~kT for ion concentration product at least one order of magnitude higher than the binodal line. The spinodal line lies at signficantly higher concentration than the binodal line and therefore, no spinodal decomposition occurs &lt; 0.01 M CaCO3. Additional simulations including HCO3- ions allowed us to define pH via the CO32-/HCO3- ratio, (Figure 1). Liquid-liquid phase separation as a mechanism for nucleation of amorhpous calcium carbonate (ACC) is predicted to occur mainly for pH above ~10. Nucleation events occurring below pH 9 are likely not to involve liquid-liquid phase separation as a mechanism.

Insight into CaCO3 nucleation from the liquid-liquid phase diagram predicted by COSMO-RS

253rd National Meeting (2017)

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