CONTROLS ON CALCITE NUCLEATION BY THE FREE ENERGY OF THE ORGANIC-INORGANIC INTERFACE

Calcium carbonate mineralization in natural environments is often associated with organic surfaces presented by macromolecules or microbial biofilms. Classical nucleation theory predicts that these surfaces should significantly affect nucleation rate by modulating interfacial free energy, which determines the thermodynamic barrier to nucleation. While previous studies have qualitatively demonstrated that the chemical functionalities present at macromolecular and microbial surfaces influence the nucleating mineral phase and morphology, a mechanistic understanding of their control over nucleation kinetics and thermodynamics is not well established.

We test the hypothesis that specific chemistries regulate the kinetics and thermodynamics of CaCO₃ formation by using self-assembled monolayers (SAMs) as model substrates for calcite nucleation. We measured nucleation rates on SAMs with different termini (-COOH, -PO₄, and -SH) and chain lengths (16-C and 11-C) for chemical driving forces above and below the solubility of amorphous calcium carbonate (ACC). There was no evidence of ACC formation. Using classical nucleation theory to interpret the kinetic data, we calculated interfacial energies and estimated effects on the thermodynamic barrier to nucleation. Calcite – substrate interaction forces were quantified with independent dynamic force spectroscopy (DFS) measurements.

The kinetic measurements show calcite nucleation rate depends on SAM chemistry and chain length. The rate data is accurately described with a classical model of nucleation at all saturations, and calcite-substrate interaction force is correlated with interfacial free energy by a systematic relationship general to chemistry and chain length. Organic surfaces decrease the interfacial free energy associated with the nucleating phase by up to 25% compared to the value reported for calcite nucleation from solution. The findings demonstrate a physical basis for how organic surfaces reduce the thermodynamic barrier to nucleation through decreases in interfacial free energy and demonstrate that template-directed calcite nucleation proceeds without an amorphous intermediate. These observations suggest that emerging views of calcite mineralization as a widespread non-classical process need to be carefully examined.