Multi-step nucleation in supersaturated salt solutions using soft matter electrostatic levitator with in-situ Raman and synchrotron X-ray scattering techniques
Katamreddy, Sai Charan
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Recent developments in research related to crystallization have revealed that nucleation occurs via multiple stages that involve entities larger than atoms, ions, or molecules as intermediates. Pre-nucleation clusters (PNCs), which are clusters of several ions that form via ion pairing, were observed in biominerals and electrolyte solutions and have gained interest in the recent years, due to their role in determining the crystallization pathways and polymorphic form of the final crystal. So, in order to understand crystallization and to engineer ways to predict and control the process, it is important to characterize the different stages involved in nucleation. However, due to the stochastic nature of nucleation at the (sub) nanoscale level, and their very small volume fraction, in situ observation of PNCs has been extremely challenging. In this work, a containerless technique – the soft matter electrostatic levitator (SEL), was developed to investigate multi-step nucleation in supersaturated salt solutions. The materials considered for this research are aqueous solutions of Na2SO4, MgSO4, and NaCl. The SEL was integrated with in situ Raman scattering and wide-angle X-ray scattering (WAXS) to probe the evolution of evolution of structure in the solution and detect any cluster formation. The difference in ion pairing behaviors of Na2SO4 and MgSO4 solutions was seen in their Raman spectra. MgSO4 solutions showed evidence of ion pairing and clustering at very dilute conditions, with this tendency increasing up to deep supersaturation. In contrast, clusters in Na2SO4 were detected at concentrations slightly above the solubility limit. Results from WAXS experiments revealed the development of a medium-range order in Na2SO4 solutions around the supersaturation levels predicted by Raman data. In the case of MgSO4, development of a medium-range order was observed from an undersaturated state up to the point of nucleation. Attempts to unveil the structural evolution in NaCl solutions were not successful, as crystallization occurred right above the solubility limit due to its short metastable zone width. Concentration-dependent multi-pathway crystallization was observed in both the solution systems. Crystalline polymorphs of unidentified phases were noticed in both the systems. Microstructures of Na2SO4 crystals obtained from two different pathways showed distinct differences in crystal morphology, despite transformation into the same crystal polymorph in both the pathways. Concentration-dependent crystallization pathways could mean that the structure of the PNCs is closely related to the structure of emerging crystalline polymorph, and these results strengthen the argument that PNCs influence nucleation mechanisms and phase selection.