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

Abstract

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 CaCO 3 -H 2 O 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.

Nanoscale kinetics of amorphous calcium carbonate precipitation in H2O and D2O

“…Previously it was suggested that an increase in induction times, observed when CO 3 2À salts with smaller counter cations were used, was due to stabilisation of pre-nucleation clusters. 49 This conclusion contrasts with our observation, if we assume that D 2 O would also stabilise ion clusters. The longer induction times at [CO 3 2À ] = 48 mM in D 2 O may be indicative of the role of mass transport in .…”

How Can Additives Control the Early Stages of Mineralisation?

“…Additives can influence ion association even without direct binding by modulating the water structure and introducing kosmotropic or chaotropic effects [36,37]. Due to the direct impact on the entropic balance of ion association through the release of hydration waters [38], these effects can increase or weaken ion-ion and/or ion-additive interactions [39], providing additional means to tune the strength of corresponding intermolecular interactions.…”

From Solute, Fluidic and Particulate Precursors to Complex Organizations of Matter

“…External modulators of solvation also regulate the formation of PNCs from nanoscopic solute precursors. For instance, PNC‐associated ions and organic molecules presenting higher hydration contents hinder the dehydration of the ionic species, thereby enhancing the supersaturation levels required for particle nucleation ,. Thus, the nucleation process cannot be deconstructed to a simple aggregation‐based size evolution from individual ions to particles, but also is ingrained by the configurational and molecular nature of solute precursors in relation to their environment, (Figure a–c).…”