Additive-mediated crystallization of complex calcium carbonate superstructures in reverse microemulsions

Morphologically complex forms of calcium carbonate were produced within 24 h in unstirred mixtures of Ca(AOT)2 (AOT = bis(2-ethylhexyl)sulfosuccinate) reverse micelles and carbonate-containing NaAOT microemulsions (w = 10). The structures form by controlled aggregation of surfactant-coated amorphous calcium carbonate primary particles, which resulted in micrometer-sized doughnut-shaped structures consisting of densely packed layers of platelike aragonite crystals. These superstructures were progressively in-filled to produce polycrystalline multilayered spindle-shaped particles several micrometers in length. Addition of the crystallization additive sodium polyphosphate at concentrations exceeding 1 g L^-1 to the carbonate-containing NaAOT microemulsions inhibited aragonite crystallization, with the consequence that single crystals of calcite with bundlelike filamentous texture along with platelike vaterite single crystals were produced. Energy-dispersive X-ray analysis indicated that AOT and polyphosphate ions were strongly associated with the crystals even after extensive washing. Unlike the aragonite superstructures, which became decorated within a few days with epitaxially oriented rodlike outgrowths of calcite, the platelike vaterite and filamentous calcite crystals remained unchanged even after 7 days. The results demonstrate that encapsulation of crystallization additives such as polyphosphate in the water droplets of reverse microemulsions can be used to influence nucleation and growth processes over a range of length scales and suggest that this novel approach could be of interest in crystal science in general.