Surface and Interfacial Tensions Affect Size-dependent Phase Separation in Droplets

ANDREAS ZUEND, Ryan Schmedding, McGill University

     Abstract Number: 479
     Working Group: Aerosol Physics

Abstract
Ultrafine aerosol particles are sensitive to the chemical composition and physical properties of surfaces and interior interfaces. For these particles, the thermodynamic equilibrium state depends on size, composition, and temperature with a tight coupling of bulk and surface properties. Liquid–liquid phase separation (LLPS) has impacts on size-dependent particle hygroscopicity, heterogeneous reactions, and phase viscosities. In turn, the presence of LLPS introduces additional interfacial tension contributions to a particle’s overall Gibbs energy. Numerous laboratory experiments had characterized the onset relative humidity of LLPS in larger aerosol particles and macroscopic bulk systems. However, in sufficiently small particles, the interfacial tension between two liquid phases constitutes an energetic barrier that may prevent the formation of an additional liquid phase. Determining this small-size limit is a key question, one that is difficult to explore experimentally. Moreover, in models the small-size limit hinges on the method used to estimate interfacial tensions and on proper accounting for feedbacks on the air–liquid surface tension of an aqueous aerosol. We will introduce a predictive equilibrium droplet model based on the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model. This model enables size-dependent computations of surface and interfacial tension effects on bulk–surface partitioning within phase-separated and single-phase particles. We evaluate a selection of methods for computing interfacial tension in multicomponent aerosol. We found that Antonov’s rule often provides good agreement with observed liquid–liquid interfacial tensions in the case of highly immiscible mixtures, while a different method is more adequate for partially miscible systems. Among the options studied, we show that two interfacial tension approaches substantially lower the onset relative humidity of LLPS as particle diameter decreases in the ultrafine regime. Our methods can also be adopted to model interfacial tension effects in dynamic, time-resolved simulations.