Zusammenfassung
Calcium carbonate pptn. proceeds via a complex multistage scenario involving neutral ion clusters as precursors and amorphous phases as intermediates, which finally transform to crystals. Although the existence of stable clusters in soln. prior to nucleation has been demonstrated, the mol. mechanisms by which they ppt. are still obscure. Here, direct insight into the processes that drive the ...
Zusammenfassung
Calcium carbonate pptn. proceeds via a complex multistage scenario involving neutral ion clusters as precursors and amorphous phases as intermediates, which finally transform to crystals. Although the existence of stable clusters in soln. prior to nucleation has been demonstrated, the mol. mechanisms by which they ppt. are still obscure. Here, direct insight into the processes that drive the transformation of individual clusters into amorphous nanoparticles is provided by progressive colloidal stabilization of different transient states in silica-contg. environments. Nucleation of calcium carbonate in the presence of silica can only take place via cluster aggregation at low pH values. At higher pH, pre-nucleation clusters become colloidally stabilized and cannot aggregate. Nucleation through structural reorganization within the clusters is not obsd. under these conditions, indicating that this pathway is blocked by kinetic and/or thermodn. means. The degree of stabilization against nucleation is found to be sufficient to allow for a dramatic enrichment of solns. with pre-nucleation clusters and enable their isolation into the dry state. This approach renders direct analyses of the clusters by conventional techniques possible and is thus likely to facilitate deeper insight into the chem. and structure of these elusive species in the future.
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