Prenucleation Cluster Pathway is Inconsistent with CaCO₃ Kinetics

Area of Science:

• Geochemistry
• Materials Science
• Chemical Physics

Background:

• Classical nucleation theory posits a critical cluster size for nucleation.
• Nonclassical nucleation involves prenucleation clusters (PNCs) that restructure.
• The exact nucleation pathway for calcium carbonate (CaCO3) remains debated.

Purpose of the Study:

• To investigate the nucleation mechanism of CaCO3.
• To determine the size of the transition state during CaCO3 nucleation.
• To assess the role of prenucleation clusters (PNCs) in CaCO3 nucleation.

Main Methods:

• Analysis of CaCO3 nucleation kinetics.
• Experimental characterization of prenucleation cluster (PNC) size.
• Thermodynamic and kinetic modeling of nucleation pathways.

Main Results:

• The transition state for CaCO3 nucleation comprises approximately 10 formula units.
• This transition state size is independent of supersaturation.
• The transition state is significantly smaller than experimentally observed PNCs.

Conclusions:

• Prenucleation clusters (PNCs) are likely not involved in the nucleation of CaCO3.
• The nucleation kinetics suggest that if CaCO3 nucleates classically, the transition state is an unusually unstable cluster.
• This finding challenges existing models of CaCO3 formation.