Liquid–Liquid Phase Separation in Supermicrometer and Submicrometer Aerosol Particles

Abstract

The interactions of aerosol particles with light and clouds are among the most uncertain aspects of anthropogenic climate forcings. The effects of aerosol particles on climate depend on their optical properties, heterogeneous chemistry, water uptake behavior, and ice nucleation activity. These properties in turn depend on aerosol physics and chemistry including composition, size, shape, internal structure (morphology), and phase state. The greatest numbers of particles are found at small, submicron sizes, and the properties of aerosol particles can differ on the nanoscale compared with measurements of bulk materials. As a result, our focus has been on characterizing the phase transitions of aerosol particles both in supermicron and submicron particles. The phase transition of particular interest for us has been liquid-liquid phase separation (LLPS), which occurs when components of a solution phase separate due to a difference in solubilities. For example, organic compounds can have limited solubility in salt solutions especially as the water content decreases, increasing the concentration of the salt solution, and causing phase separation between organic-rich and inorganic-rich phases. To characterize the systems of interest, we primarily use optical microscopy for supermicron particles and cryogenictransmission microscopy for submicron particles.

An organic crystalline state in ageing atmospheric aerosol proxies: spatially resolved structural changes in levitated fatty acid particles

“…within the particlethe two phases being the crystalline acid-soap complex and an inverse micellar phase. Previous work has focussed on liquid-liquid phase separations at high RH in organic aerosols (Freedman, 2017(Freedman, , 2020Liu et al, 2018). In this study, a water-containing inverse micellar phase forms initially on the outside, creating a clear phase gradient during humidification.…”

The persistence of a proxy for cooking emissions in megacities: a kinetic study of the ozonolysis of self-assembled films by simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and Raman microscopy

“…[11][12][13][14][15][16][17][18][19][20] Solid and semi-solid phases have been observed for secondary organic aerosols. 21 In addition to the effects of organic-aqueous phase separation, 22 viscous phases are identified as a key factor in influencing fundamental aerosol processes such as uptake of water and reactive gasses. 23 In particular, glassy and semi-solid phases have been studied and show drastic differences in their physical properties.…”

Effects of liquid–liquid phase separation and relative humidity on the heterogeneous OH oxidation of inorganic–organic aerosols: insights from methylglutaric acid and ammonium sulfate particles

“…Additional uncertainties in heterogeneous reactivity of organic compounds arise when these droplets undergo phase separation. Recent studies suggest that these organic-inorganic droplets can undergo liquid-liquid phase separation (LLPS) depending on environmental conditions (e.g., RH and temperature) and aerosol composition (including different types of inorganic salts, the average oxygento-carbon (O : C) elemental ratio of organic compounds and OIR) (Bertram et al, 2011;Zuend and Seinfeld, 2013;You et al, 2014;Qiu and Molinero, 2015;Charnawskas et al, 2017;Losey et al, 2018;Freedman, 2020). These phaseseparated droplets exhibit two distinct liquid phases: typically, an inorganic-rich inner phase and an organic-rich outer phase as depicted in Fig.…”