Structural motifs of pre-nucleation clusters

Area of Science:

• Physical Chemistry
• Materials Science
• Aerosol Science

Background:

• Understanding the initial stages of crystallization is crucial for various chemical and physical processes.
• Supersaturated solutions and aerosol microparticles offer unique environments for studying nucleation phenomena.
• Calcium bromide (CaBr2) serves as a model system for investigating salt behavior in aqueous solutions.

Purpose of the Study:

• To investigate the structural motifs of pre-nucleation clusters in supersaturated CaBr2 aqueous aerosol microparticles.
• To correlate spectral shifts observed via X-ray absorption with the formation of these clusters.
• To elucidate the electronic structure changes associated with cluster formation and compare them to the solid crystalline phase.

Main Methods:

• Preparation of single, optically levitated supersaturated aqueous CaBr2 aerosol microparticles.
• Controlled variation of salt concentration by adjusting ambient humidity.
• X-ray absorption spectroscopy at the Br K-edge to probe electronic structure.
• First-principle density functional theory and molecular dynamics simulations for spectral analysis.

Main Results:

• Distinct red-shifts (up to 7.1 ± 0.4 eV) in Br K-edge position were observed with increasing salt concentration, transitioning from dilute to solid states.
• Spectral shifts in supersaturated solutions were found to be intermediate between dilute and solid states.
• Simulations confirmed that observed spectral changes are attributable to the formation of pre-nucleation clusters, specifically solvated CaBr2 moieties.

Conclusions:

• Pre-nucleation clusters in supersaturated CaBr2 aerosols exhibit distinct structural and electronic properties.
• These clusters are composed of solvated CaBr2 moieties, not the building blocks of the final calcium bromide hexahydrate crystal.
• The study provides insights into the early stages of salt crystallization in microdroplets.