Evaporation Kinetics and Final Particle Morphology of Multicomponent Salt Solution Droplets

Area of Science:

• Atmospheric Chemistry
• Materials Science
• Chemical Engineering

Background:

• Aerosol droplets containing multiple inorganic solutes are common in nature and industry.
• Understanding their drying kinetics and resulting particle morphology is crucial for applications like drug delivery, disease transmission mitigation, and climate modeling.
• Previous studies have primarily focused on single-component aerosols, leaving a gap in knowledge for mixed salt systems.

Purpose of the Study:

• To investigate the drying kinetics of mixed salt aqueous aerosol droplets (NaCl-(NH4)2SO4, NaCl-NH4NO3, NaCl-CaCl2).
• To determine the impact of drying kinetics on the morphology and composition of the resulting dried microparticles.
• To establish the relationship between mixed salt composition, drying behavior, and final particle characteristics.

Main Methods:

• Utilized a comparative kinetics electrodynamic balance to measure evaporation profiles of mixed salt aerosols across a range of relative humidities (0-50% RH).
• Employed a falling droplet column to dry populations of mixed salt droplets under controlled RH conditions.
• Analyzed particle morphology using scanning electron microscopy (SEM) and composition using energy dispersive spectroscopy (EDS) for atomic mapping.

Main Results:

• Evaporation kinetics measurements were consistent with the "Single Aerosol Drying Kinetics and Trajectories" model.
• The relationship between drying kinetics and particle morphology in mixed salt droplets was found to be compositionally dependent.
• Particle morphology and microstructure homogeneity were determined by the predominant crystallizing salts (e.g., (NH4)2SO4, Na2SO4, NaCl) and influenced by the drying rate.

Conclusions:

• The drying kinetics and resultant particle morphology of mixed salt aerosols are strongly dictated by their specific salt composition.
• The crystallization behavior of dominant salts governs the final particle structure.
• Drying rate significantly impacts the compositional homogeneity within the microstructure of the dried particles.