Multi-step Nucleation in Supersaturated Salt Solutions Using Soft Matter Electrostatic Levitator with In-situ Scattering Techniques

For more than a century, nucleation has been viewed as a single-step process as described by classical nucleation theory. With advancements in characterization techniques, complex nucleation mechanisms via intermediate species larger than simple monomeric units have been observed in proteins, colloids, electrolytes, and biominerals. However, the stochastic nature of nucleation at the (sub) nanoscale level makes it challenging to characterize the intermediate phases and interpret their role in complex nucleation. This work aims at understanding the mechanism of complex nucleation in aqueous sodium and magnesium sulfate solutions using a Soft matter Electrostatic Levitator (SEL). 2-3 mm sized droplets were levitated in ambient air in the SEL and supersaturation was achieved by evaporation of water. Changes in solution structure at the molecular level were monitored via in-situ Raman spectroscopy from undersaturation up to nucleation. OH-stretch spectra of water showed the presence of large clusters in supersaturated sodium sulfate solution. Multi-step and concentration-dependent crystallization pathways were observed in both the salts revealing that the solution structure influences the nucleation phase. Crystal microstructure varied depending on the crystallization pathway. The SEL will be integrated with synchrotron X-ray at Advanced Photon Source in Argonne National Laboratory and neutron scattering at Oak Ridge National Laboratory to identify short-range order in the solution and role of solution structure on crystallization pathway.