Surfactants regulate the mixing state of organic-inorganic mixed aerosols undergoing liquid-liquid phase separation

Abstract. The mixing state of atmospheric aerosols undergoing liquid-liquid phase separation (LLPS) is crucial for regulating atmospheric chemistry and influencing global climate, often adding uncertainties to atmospheric and climate models. Despite its significance, understanding how coexisting species, such as surfactants, affect the mixing state of phase-separated aerosols remains limited. This study investigated the phase transition behaviors and resulting mixing states of aerosols composed of 1,2,6-hexanetriol and ammonium sulfate, with added surfactants. Contrary to the commonly assumed core-shell structure, we observed that at very low concentrations of hydrocarbon surfactants, the organic phase partially engulfed the aqueous inorganic phase, a configuration we termed partial organic-phase engulfing. Furthermore, we discovered the formation of partial inorganic-phase engulfing and inverse core-shell structures, where the inorganic phase partially or fully spread over the organic domain at higher surfactant levels. We identified a relationship between equilibrium particle morphology and spreading coefficients, primarily governed by surface tension reduction in the separated organic and inorganic phases. These distinctive mixing states may substantially alter the chemical, physical, and optical properties of organic-inorganic aerosols under real atmospheric conditions. Our findings bridge a critical knowledge gap regarding the role of surface tension evolution in the equilibrium particle morphology of internally mixed atmospheric particles and its potential impact on aerosol-chemistry-climate interactions. These insights emphasize the need to refine current aerosol models to incorporate the specific LLPS morphologies observed in this study.