Formation and growth of amorphous colloidal CaCO3 precursor particles as detected by time-resolved SAXS

Time-resolved synchrotron small-angle X-ray scattering (SAXS) studies were performed to investigate the unseeded formation and growth of colloidal calcium carbonate particles. Equimolar aqueous solutions of CaCl2.2H(2)O and Na2CO3 were rapidly mixed in a stopped-flow apparatus, and SAXS data were recorded using an image-intensified CCD detector. It is shown that SAXS allows studying those processes in situ, with a very good time resolution. It can provide unsurpassed real-time information about the particle size, shape, polydispersity, inner structure, and density. In these studies, well-defined, spherical CaCO3 particles with colloidal dimensions up to ca. 270 nm and a remarkable uniformity in size could be observed. After a short nucleation period, the number density of the growing spheres remains constant. From the evaluation of the absolute scattering intensities, the particle mass density could be determined to be ca. 1.62 g/cm(3), which is considerably lower than the density of the crystalline modifications. Our data thus point to the formation of colloidal, amorphous particles that are a precursor modification of the thermodynamically stable calcite. It was found that these particles are isolated and do not form larger aggregates. Upon lowering the concentration of the educts, particle formation and growth are considerably slowed and smaller particles are being formed.