Hydration Effects on the Stability of Calcium Carbonate Pre-Nucleation Species

Recent experimental evidence and computer modeling have shown that the crystallization of a range of minerals does not necessarily follow classical models and theories. In several systems, liquid precursors, stable pre-nucleation clusters and amorphous phases precede the nucleation and growth of stable mineral phases. However, little is known on the effect of background ionic species on the formation and stability of pre-nucleation species formed in aqueous solutions. Here, we present a systematic study on the effect of a range of background ions on the crystallization of solid phases in the CaCO3-H2O system, which has been thoroughly studied due to its technical and mineralogical importance, and is known to undergo non-classical crystallization pathways. The induction time for the onset of calcium carbonate nucleation and effective critical supersaturation are systematically higher in the presence of background ions with decreasing ionic radii. We propose that the stabilization of water molecules in the pre-nucleation clusters by background ions can explain these results. The stabilization of solvation water hinders cluster dehydration, which is an essential step for precipitation. This hypothesis is corroborated by the observed correlation between parameters such as the macroscopic equilibrium constant for the formation of calcium/carbonate ion associates, the induction time, and the ionic radius of the background ions in the solution. Overall, these results provide new evidence supporting the hypothesis that pre-nucleation cluster dehydration is the rate-controlling step for calcium carbonate precipitation.This research was done within the grants MAT2012-37584, CGL2015-70642-R and P11-RNM-7550, funded by the Spanish Government, European Commission (ERDF funds) and the Junta de Andalucía. Additional funding was provided by the research group RNM-179 of the Junta de Andalucía and the Unidad Científica de Excelencia UCE-PP2016-05 of the University of Granada. Encarnacion Ruiz-Agudo acknowledges the receipt of a Ramón y Cajal grant from the Spanish Government (Ministerio de Economía y Competitividad) and CVP acknowledges funding from the EU Marie Curie initial training networks: Minsc, CO2 React and Flowtrans as well as an Australian Research Council (ARC) grant awarded to Julian Gale at Curtin University, Perth Australia