Abnormal polymorph conversion of calcium carbonate from calcite to vaterite

Inspired by mineralization in biological organisms, the production of higher ordered CaCO₃ crystals modified by surfactants has received much attention. Normal micelles formed by anionic surfactant, sodium bis-2-ethylhexyl-sulfosuccinate (aerosol OT, AOT), was used as a template to direct the growth of CaCO₃ crystals in the present work. When the concentration of the AOT increased from 0.5 to 5.0 mM, the polymorph of CaCO₃ crystals changed from almost pure aragonite to almost pure vaterite at 150 °C. In the meantime, various morphologies of vaterite were obtained, such as astral and hexagonal structures. Based on the time-resolved experiments, we suggested that AOT micelles induced the formation of these structures. And the micelles were wrapped in the hexagonal structures, and dissolved in the aqueous solution with increase of the experimental time. Thus, hollow shells were produced. Abnormal polymorph transformation of CaCO₃ crystals from the calcite to the vaterite was found. These effects of AOT on polymorph and morphology of CaCO₃ crystals have not been reported as far as we know.