Probing the structure and stability of calcium carbonate pre-nucleation clusters

Recent advances in our understanding of the emergence of biomineral phases from solution has provoked new and challenging questions. A consensus is beginning to form, attesting to the existence of pre-nucleation clusters of calcium carbonate in solution, which subsequently aggregate to initiate solid growth. The structure and stability of these clusters has not yet been fully determined; this needs to be addressed if biomineralisation mechanisms are to be exploited. Here, we present the results of an exhaustive computational study in the search for possible candidate pre-nucleation clusters that might arise in clusters up to 80 ions in size. Both anhydrous and hydrated clusters have been studied. A significant sample of the clusters were simulated, using molecular dynamics, to elucidate the metastability of these nanoclusters in water. An analogous study was conducted for hydrated clusters containing aspartate, to observe the effects of this amino acid on the structure and stability of candidate pre-nucleation clusters. Our results suggest that pre-nucleation cluster stability is a balance between ionic coordination and ion hydration. We find that clusters are generally dynamic in the lower limit of stability, forming chains to which ions frequently aggregate or dissolve. Larger calcium carbonate clusters retain a higher level of coordination in solution but swell to maximise hydration. The effect of additives on the structure and stability of clusters as a function of cluster size is intriguing, with trends in our data suggesting that aspartate can limit ion dissolution, but still allow for dynamic ordering and increasing ion hydration. Finally we find a bias in the ionic charge distributions for relatively dense clusters, indicating that these clusters sustain a negatively charged surface.