Formation and Growth of Amorphous Colloidal CaCO₃ Precursor Particles as Detected by Time-Resolved SAXS

Abstract

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 CaCl2·2H2O 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/cm3, 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.

The role of sulfate groups in controlling CaCO3 polymorphism

“…The nucleation and growth of metastable CaCO 3 polymorphs is commonly related to the predominance of kinetic factors over thermodynamic properties (Ogino et al, 1987;Sawada, 1998). High supersaturation levels at nucleation and the initial stages of growth result in the formation of ACC, vaterite and aragonite (Sawada, 1998;Bolze et al, 2002;Pontoni et al, 2003). In agreement with Ostwald's step rule (Sö hnel and Garside, 1992), these three crystalline forms ultimately transform into the more stable calcite via dissolution-crystallization reactions of various degrees of complexity (Fernández-Díaz et al, 2009).…”

Amorphous dysprosium carbonate: characterization, stability, and crystallization pathways

“…These amorphous precursors are thermodynamically unstable and often transform into crystalline, but metastable intermediate phases before finally crystallizing into the thermodynamically stable solid endproducts (Meldrum and Cölfen 2008). Most of the research on such amorphous precursors has been focused on their compositional-dependent stabilities and on their formation and transformation conditions (i.e., amorphous calcium carbonates (Radha et al 2010;Goodwin et al 2010;Bolze et al 2002;Rodriguez-Blanco et al 2008) and amorphous calcium phosphates (Zyman et al 2010;Combes and Rey 2010). However, there are also salt systems where such amorphous precursors do either not crystallize even after very long time periods (Roncal-Herrero et al 2009;Tobler et al 2009) or where the formation of the thermodynamically stable end phase does not require an amorphous precursor as an initiators for the crystallization process (e.g., calcium sulfate, Van Driessche et al 2012;La and Nd-phosphates, Roncal-Herrero et al 2011).…”

“…For example, amorphous silica when precipitated from solution (Tobler et al 2009;Tobler and Benning 2011) is stable for extremely long times without crystallizing, while in the La and Nd-phosphate system ) no amorphous precursor has so far been observed. However, in some salt systems, the formation of an amorphous precursor may at times be missed because the crystallization rates may be very rapid, as shown for the calcium carbonate systems (Bolze et al 2002;Bots et al 2012).…”

Composition inversion to form calcium carbonate mixtures

“…This is a reasonable conclusion and in line with previously published results for calcium carbonate nucleation from Na 2 CO 3 and CaCl 2 solutions which revealed the formation of ACC as precursor particles before the appearance of crystalline polymorphs by time-resolved SAXS studies. 34,35 Obviously, the other reaction product halite is directly formed in its crystalline state. This result is in accordance with the fact that the production of amorphous sodium chloride powder needs very special conditions and has so far been observed after water evaporation from nanometer-sized droplets.…”