Recent advances in high-resolution experimental and computational studies have revealed complex pathways of crystal nucleation and growth that do not conform to classical monomer-by-monomer growth (MMG). In particular, the non-classical growth mechanism of particle-mediated growth (PMG) is of great interest for systems that do not form metastable intermediate phases en route to the most stable crystalline product. For a material that can grow via both MMG and PMG, for example, very low supersaturation conditions yield low particle production and monomer addition rates, but MMG dominates because the relative difference in chemical potential between small and large particles is high. In contrast, at high supersaturation, all particles above the critical size see a net flux regardless of their relative differences in chemical potential and PMG rates outcompete MMG. Conversely, because particle growth rates scale with solute concentration while particle diffusion rates are independent of it, MMG should naturally outpace PMG when solute concentrations are high. As the combination of very high supersaturation and low solute concentration can only be achieved for low solubility systems, the solubility of a given mineral is likely a master parameter controlling the molecular-scale mechanism of crystal growth. To understand how rates of particle nucleation, growth, attachment, and coarsening compete with one another as a function of supersaturation, we use metal sulfates as our model system. The equilibrium cation concentration for sulfate solutions ranges from less than 10−6 in the case of Ba2+ to more than 1 for Cu2+, a difference in solubility of approximately 6 orders of magnitude. Using DLS, AFM, and TEM, we measured and compare the growth rates of bulk crystals to those of synthetic nanoparticles of BaSO4, CaSO4, ZnSO4, and CuSO4 at several different supersaturation values. X-ray diffraction (XRD) and TGA confirmed a consistent structural and chemical composition during the growth process. Results from this study hold great promise in understanding the impact of intrinsic and extrinsic factors on predicting the crystal growth mechanism for any given system in a wide range of chemical environments.

Growth of sulfate minerals: Impact of solubility on molecular-scale mechanism

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.