Using Confinement To Study the Crystallization Pathway of Calcium Carbonate

Studies of crystallization in small volumes have shown that constraining the space in which precipitation occurs can provide an effective means of studying the pathways by which crystallization occurs. This article describes an investigation into the precipitation of calcium carbonate in a crossed cylinder apparatus, where the confinement of low solution concentrations between unfunctionalized glass surfaces delivers a dramatic increase in the lifetime of metastable phases. This phenomenon allows the direct observation of amorphous calcium carbonate (ACC) in very dilute systems where it is virtually undetectable in bulk solution, thus confirming the predictions of a recent titration study which showed that the solubility product of ACC is an order of magnitude lower than previously believed. The aggregation of ACC particles is also seen to significantly decrease with increasing confinement, suggesting that the stability of ACC in confinement is related to its aggregation state and restricted mobility. Finally, we observe that ACC transforms to calcite via the metastable phase vaterite in constrained volumes at solution concentrations where direct transformation to calcite occurs in bulk solution. This work underscores the utility of using confinement to detect and preserve transient metastable phases in bulk solution, where this may have applications in materials syntheses utilizing amorphous precursor phases.