In Situ Liquid SAXS Studies on the Early Stage of Calcium Carbonate Formation

Abstract

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and nonclassical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different scientific areas, little is known about the early stage of its formation. Therefore, contactless devices are designed that are capable of providing informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) techniques. In particular, SAXS is exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level is controlled and kept constant by mixing four diluted solutions (i.e., NaOH, CaCl2, NaHCO3, H2O) at constant T and pH. The scattering data are collected on a liquid jet generated about 75 s after the mixing point. The data are modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.

Water: How Does It Influence the CaCO3 Formation?

“…Prenucleation clusters have been observed in undersaturated and supersaturated solutions in titration experiments, as exemplified in Figure A . In addition, clusters with a well‐defined diameter of around 2 nm have been observed by analytical ultracentrifugation (AUC), cryogenic transmission electron microscopy (cryo‐TEM), and small‐angle X‐ray scattering (SAXS) . These experimental findings are supported by molecular dynamics (MD) simulations that indicate that PNCs are dynamically ordered liquid‐like oxyanion polymers (DOLLOPs) composed of alternating Ca 2+ and CO 3 2− ions, as shown in Figure B …”

“…[3] C) Schematic illustration of the composition and structure of the platelets made of nanocrystalline aragonite grains that are separated by organic additives. [40,41] These experimental findings are supported by molecular dynamics (MD) simulations that indicate that PNCs are dynamically ordered liquid-like oxyanion polymers (DOLLOPs) composed of alternating Ca 2+ and CO 3 2À ions,a sshown in Figure 3B. [3] been reported to precede the ACCformation, as summarized in Figure 2.…”

Wasser: Wie beeinflusst es die CaCO3‐Bildung?

“…[49] In addition, clusters with a well-defined diameter of around 2 nm have been observed with analytical ultracentrifugation (AUC), [49] cryogenic transmission electron microscopy (cryo-TEM), [36] and small angle X-ray scattering (SAXS). [52,53] These experimental findings are supported by molecular dynamics (MD) simulations that indicate that PNCs are dynamically ordered liquid-like oxyanion polymers (DOLLOPs) composed of alternating Ca 2+ and CO3 2ions, as shown in Figure 3B. [54] Figure 2.…”

Formation and Evolution of Nanoscale Calcium Phosphate Precursors Under Biomimetic Conditions

“…23,24 These PNS are highly dynamic nanometric ionic clusters that spontaneously form in solution preceding the precipitation of solid crystalline phases. 25 Today, PNS have been documented for calcium phosphates (CaP) 21,23,[26][27][28][29] and carbonates (CaC) [30][31][32][33] as well as for iron oxides. 34 Interestingly, their involvement in the formation of biominerals challenges the long-standing nucleation-and-growth paradigm, 35,36 which has stimulated an ongoing update of the existing crystallization theories.…”