Great challenges remain in our understanding of biomineralisation which impede the design and production of a new class of materials. Recent studies have suggested that calcium carbonate clusters are stable in solution before nucleation, and that these participate in the formation of amorphous calcium carbonate (ACC). The structure and stability of these clusters has not been fully determined. Furthermore, the dehydration of stable ACC before crystallisation remains ambiguous. Exhaustive computational searches have been carried out to provide a sample of clusters up to the sizes suggested for particles persisting before nucleation. A large sample of clusters were simulated at high concentration in water using molecular dynamics (MD). The results suggest that cluster stability is a balance between ionic coordination and ion hydration. At high concentration clusters are generally dynamic in the lower limit of stability, forming chains to which ions frequently aggregate and dissolve. Free energy calculations showed a transition in the favoured coordination levels with cluster size. One dimensional Umbrella Sampling calculations showed that at small sizes a collection of clusters with low average cation–anion coordination were thermodynamically stable. For systems containing sixty ions and above, more compact clusters with internal water, close to the stoichiometries identified for stable hydrated ACC in vivo, were lower in free energy. From MD simulations at experimental concentrations and pH, while dynamic ordering was found, ion pairs dominated in solution and the largest clusters observed contained no more than four ions. These findings, combined with the data at high concentration, allow for a re-evaluation of the proposed nucleation mechanisms for calcium carbonate from solution. ACC simulations identified water–filled channels within the ionic framework. Percolating clusters were found when H2O/CaCO3 was greater than 0.75–0.8. The ACC system fitted well with the percolation theory on a simple cubic site lattice for water, and critical exponents showed a good fit to the theoretical values. Non–standard diffusion was found for water, with a “jump” mechanism of diffusion observed and a cascade of molecule displacements within channels. This original result allows new light to be shed on the dehydration mechanism of ACC.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:640993 |
| Date | January 2014 |
| Creators | Finney, Aaron R. |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/66884/ |
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