The impact of stoichiometry on the initial steps of crystal formation: Stability and lifetime of charged triple-ion complexes

JA Koskamp and SYMH Seepma and VFD Peters and D Toroz and D Di Tommaso and M Wolthers, CHEMISTRY-A EUROPEAN JOURNAL, 30, e202303860 (2024).

DOI: 10.1002/chem.202303860

Minerals form in natural systems from solutions with varying ratios of their lattice ions, yet non-stoichiometric conditions have generally been overlooked in investigations of new formation (nucleation) of ionic crystals. Here, we investigated the influence of cation:anion ratio in the solution on the initial steps of nucleation by studying positively and negatively charged triple ion complexes and subsequent particle size evolution. Our model systems are carbonates and sulfates of calcium and barium, as it was recently shown that solution stoichiometry affects the timing and rate of their nucleation. Molecular dynamics (MD) simulations and dynamic light scattering (DLS) flow experiments show that nucleation correlates with the stability and lifetime of the initial complexes, which were significantly impacted by the cation:anion stoichiometry and ion type. Specifically, BaSO422- $\rm B\rm a\left(\rm S\rm O_4\right)_2<^>2-$ was found to have higher association constants and its lifetime was twofold longer than Ba2SO42+ $\rm B\rm a_2\rm S\rm O_4<^>2+$ . Similar trends were observed for BaCO3 $\rm B\rm a\rm C\rm O_3$ and CaSO4 $\rm C\rm a\rm S\rm O_4$ . Contrastingly, for CaCO3 $\rm C\rm a\rm C\rm O_3$ , CaCO322- $\rm C\rm a\left(\rm C\rm O_3\right)_2<^>2-$ was found to have lower association constants and its lifetime was shorter than Ca2CO32+ $\rm C\rm a_2\rm C\rm O_3<^>2+$ . These trends in stability and lifetime follow the same asymmetrical behaviour as observed experimentally for particle formation using techniques like DLS. This suggests a causal relationship between the stability and lifetime of the initial charged complexes and the nucleation under non- stoichiometric conditions. Charged Triple-Ion Clusters (CTICs) form by addition of an anion or cation to an ion pair and are intermediate species on ionic-crystal nucleation pathways. Computations show that the energies and lifetimes for positive and negative CTICs vary differently for BaSO4 and CaCO3. This difference can be related to asymmetrical nucleation behavior observed in solutions with cation- versus with anion excess. image

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