Nature Communications (Jan 2024)

Colloidal pathways of amorphous calcium carbonate formation lead to distinct water environments and conductivity

  • Maxim B. Gindele,
  • Sanjay Vinod-Kumar,
  • Johannes Rochau,
  • Daniel Boemke,
  • Eduard Groß,
  • Venkata SubbaRao Redrouthu,
  • Denis Gebauer,
  • Guinevere Mathies

DOI
https://doi.org/10.1038/s41467-023-44381-x
Journal volume & issue
Vol. 15, no. 1
pp. 1 – 14

Abstract

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Abstract CaCO3 is the most abundant biomineral and a major constituent of incrustations arising from water hardness. Polycarboxylates play key roles in controlling mineralization. Herein, we present an analytical and spectroscopic study of polycarboxylate-stabilized amorphous CaCO3 (ACC) and its formation via a dense liquid precursor phase (DLP). Polycarboxylates facilitate pronounced, kinetic bicarbonate entrapment in the DLP. Since bicarbonate is destabilized in the solid state, DLP dehydration towards solid ACC necessitates the formation of locally calcium deficient sites, thereby inhibiting nucleation. Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy of poly-aspartate-stabilized ACC reveals the presence of two distinct environments. The first contains immobile calcium and carbonate ions and structural water molecules, undergoing restricted, anisotropic motion. In the second environment, water molecules undergo slow, but isotropic motion. Indeed, conductive atomic force microscopy (C-AFM) reveals that ACC conducts electrical current, strongly suggesting that the mobile environment pervades the bulk of ACC, with dissolved hydroxide ions constituting the charge carriers. We propose that the distinct environments arise from colloidally stabilized interfaces of DLP nanodroplets, consistent with the pre-nucleation cluster (PNC) pathway.