CaSO4 mineralization in carboxy- and amino-functionalized reverse micelles unravels shape-dependent transformations and long-term stabilization pathways for poorly hydrated nanophases (bassanite)

Mineralization studies on calcium sulfates when compared to those on phosphates and carbonates are relatively rare despite the economic relevance of these sulfates. A series of recent findings, however, have renewed the interest in calcium sulfate mineralization. These studies, similar to those on calcium carbonates and phosphates, have shown a richer than previously thought scenario especially at the nanoscale. Here, we intentionally use confinement reactors with fast exchange rates (organically/inorganically functionalized reverse micelles) to rapidly establish pathways relevant to calcium sulfate mineralization in solution (including long-term stabilization). Furthermore, we rationalize all the results on the basis of an adsorption model with solubility inputs for inorganic additives. Thus, the typical bassanite nanofibers involving oriented attachment are observed either at weak-adsorption conditions or in the presence of less soluble inorganic dopants. At strong adsorption conditions, nanofibers are no longer observed, and long-term stabilization of nanobassanites is chemically possible if calcium/carboxylate bridging bonds are established during the first stages of nucleation and growth (aliquots after 30 min are similar to 24 h and remain preserved after 5 months). The results here presented are thus important, as they suggest specific conditions for the stabilization of biogenic bassanites. Furthermore, our results suggest that when oriented attachment is no longer energetically favored at intermediate stages (presence of rounded nanoparticles instead of nanorods), gypsum crystallization still can occur from small nanobassanites, however, through a classical dissolution-recrystallization mechanism