Describing the Surface Tension of Aerosol Droplets Containing Mixtures of Organic Solutes

ALISON BAIN, Oregon State University

     Abstract Number: 79
     Working Group: Aerosol Physics

Abstract
The partitioning of surface-active molecules and surface tension of aerosol droplets play a role in their hygroscopic growth and the eventual formation of cloud droplets. Recent experimental work and modeling efforts have shown that, due to the high surface-area-to-volume ratio inherent to aerosol droplets, macroscopic surface tension measurements alone are insufficient to predict the surface tension in aerosol droplets. The small volume, and thus a finite reservoir of surfactant molecules, in addition to the relative increase in surface sites, can lead to bulk surfactant concentration depletion in droplets. This depletion means that at equilibrium, the fraction of surface sites occupied by surfactant molecules in a droplet may be lower than a macroscopic solution of the same total composition, resulting in the droplet having a higher surface tension than the macroscopic solution.

Aerosol droplets have high chemical complexity, and field measurements collecting aerosol samples find many surface-active molecules. Thus, it is necessary to understand surfactant partitioning and surface tension lowering in droplets containing two or more surface-active species. Here, I will describe surface tension measurements of picolitre volume droplets containing two strong surfactants and an organic cosolute. Three mixed surfactant partitioning schemes are used to make predictions of aerosol surface tension and compared to the experimental observations. Finally, we test an effective Langmuir isotherm and a surrogate surfactant approach to predict the surface tension of accumulation mode aerosol, where the individual properties of organics in a mixture are unknown.