Confinement Leads to Control over Calcium Sulfate Polymorph

Although many processes of great biological, technological, and environmental importance, such as the formation of biominerals, the templating of nanostructures, and salt weathering, occur in confinement rather than in bulk solution, little is known about the influence of confinement over the precipitation of inorganic crystals. The effects of confinement on the precipitation of calcium sulfate are investigated using a crossed-cylinder apparatus which offers confinement that varies continuously from zero to tens of micro meters. While the thermodynamically stable form of calcium sulfate (gypsum) is always observed at large surface separations, a remarkable stabilization of the metastable phases amorphous calcium sulfate (ACS) and calcium sulfate hemihydrate (bassanite, plaster of Paris) is observed even at micrometer-scale separations. For the first time, the approach is extended to study the combined effects of soluble additives and confinement, which are often present in natural or synthetic systems, and it is shown that this considerably extends the lifetimes of ACS and hemihydrate. These confinement effects are attributed to hindered aggregation of precursor particles at small surface separations, which limits polymorph conversion. While these results have immediate relevance to salt weathering and biomineralization processes, they are also important to the many crystallization and aggregation-driven processes occurring in small volumes.