Influence of Surfactants on Morphology and Phase State of Internally Mixed Atmospheric Aerosol Particles

REENA JAGLAN, Miriam Freedman, The Pennsylvania State University

     Abstract Number: 326
     Working Group: Aerosol Physics

Abstract
Liquid-liquid phase separation (LLPS) and the mixing state of atmospheric aerosols influence particle morphology, phase behavior, and physicochemical properties, thereby modulating their role in critical environmental processes such as cloud formation, radiative forcing and climate dynamics. However, while surfactants are common in aerosol particles, the influence of surfactants on aerosol physical and chemical properties remains uncertain, introducing significant challenges in understanding aerosol behavior and its broader environmental impacts. Herein, we investigate the effects of various hydrocarbon surfactants on the phase transitions and morphology of model secondary organic aerosol (SOA) particles. To form these aerosol particles, surfactants were mixed with model SOA compounds (2-methylglutaric acid or 1,2,6-hexanetriol) and ammonium sulfate to produce supermicron droplets for study with optical microscopy. Optical microscopy showed a range of morphologies, including core-shell, partially engulfed-like structures, depending on surfactant type, concentration and relative humidity (RH). At higher concentration of surfactants, an anionic surfactant exhibits partially engulfed-like structures, while neutral surfactants favor a core-shell morphology. Cationic surfactants displayed varied morphology based on the organic SOA compound. These morphological transitions are primarily controlled by interfacial energy, with spreading coefficients responding to differences between surface/interfacial tension between the air, aqueous, organic, and inorganic phases. Furthermore, in aerosol particles containing neutral surfactants, we observed a primary LLPS transition forming an organic-rich outer shell and a mixed organic-inorganic phase core, followed by a secondary LLPS transition at lower RH, redistributing ammonium sulfate to the core. Also at the submicron scale, transmission electron microscopy (TEM) images of surfactant-containing aerosol particles agree at sufficiently larger sizes with our results for supermicron droplets, enhancing our understanding of surfactant effects on aerosol particles morphology. Our findings provide a detailed understanding of how different types of surfactants drive phase transitions and morphological evolution in internally mixed aerosols, offering key insights into their role in atmospheric processes and aerosol-climate interactions.