Transient Calcium Carbonate Hexahydrate (Ikaite) Nucleated and Stabilized in Confined Nano- and Picovolumes

Abstract

Calcium carbonate precipitation at different values of the nominal ionic activity product (IAP) is studied in nanoliter and picoliter droplets at (20 ± 2 °C). Experiments are carried out through direct mixing of equimolar reactant solutions using two different setups: first, droplet-based microfluidics using Teflon capillaries (nanoliter experiments) and second, the microinjection technique under oil (picoliter droplets). Instantaneous precipitation of a metastable CaCO3 phase is initially observed. This phase is stabilized in time by reducing the initial volume of the experiments from the nano- to picoliters range and when the CaCl2/Na2CO3 ratio approaches 1. Further analysis by X-ray diffraction, transmission electron microscopy, and selected area electron diffraction confirms the first nucleated phase is CaCO3·6H2O (ikaite) and in few droplets ikaite plus CaCO3·H2O (monohydrocalcite). No evidence of amorphous calcium carbonate (ACC) is found even in conditions where the IAP exceeds the solubility product of this phase. The in vitro finding of ikaite formation and stabilization due to volume confinement is an unexpected result since it is the first time that this hydrous phase is stabilized at room temperature (it is normally found at near 0 °C) in the absence of additives. This result can be of interest for those biomineralization processes occurring in the confined volumes of intracellular vesicles and for biomimetic materials science in general.