6533b86dfe1ef96bd12ca80e

RESEARCH PRODUCT

Snapshots of calcium carbonate formation – a step by step analysis

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Abstract Recent advances in our understanding of CaCO3 nucleation from solution have provoked new and challenging questions. We have studied CaCO3 formation using precipitation by carbonate ester hydrolysis which ensures precipitation from a strictly homogeneous solution state and allows “titrating” carbonate to a solution with a given Ca2+ concentration on a timescale suited for kinetic studies. Nucleation and crystallization were traced by combining dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS served as in situ technique to identify the nucleation time, to monitor particle size evolution, to discriminate different precipitation mechanisms and to validate reproducibility. TEM snapshots taken during different stages of the precipitation process identified different phases and morphologies. At a high level of supersaturation homogeneous nucleation in solution led to the formation of amorphous CaCO3 particles (Ø≈30 nm), which transformed via vaterite to calcite. Nucleation occurred uniformly in solution which appears to be unique for the CaCO3 system. In the presence of Na-polymethacrylate (Na-PMA), heterogeneous nucleation was suppressed and Ca-polymer aggregates were formed in the prenucleation stage. Beyond a critical threshold supersaturation CaCO3 particles formed in solution outside of these aggregates. The nucleation process resembled that without additive, indicating that Na-PMA exerts only a minor effect on the CaCO3 nucleation. In the postnucleation stage, the polymer led to the formation of extended liquid-like networks, which served as a precursor phase for solid ACC particles that formed alongside the network.