Measurements and Model Predictions for the Surface Tension of Aerosol Droplets Containing Mixtures of Strong Nonionic Surfactants and Organic Solutes.

Ambient aerosols are complex mixtures containing thousands of unique organic molecules. However, due to the large surface-area-to-volume ratio of aerosol droplets, surface-active molecules can become depleted in the droplet bulk, resulting in a size-dependent surface tension. Size-dependent partitioning models have been extensively tested for droplets containing a single surfactant and cosolute. However, these simple systems do not mimic the complexity of the ambient aerosols. Here, we compare surface tension predictions from mixed-surfactant partitioning schemes to experimentally measured droplet surface tensions for nonionic surfactants octyl-β-D-thioglucopyranoside and Tween20 with soluble organic glutaric acid. We first compare model predictions from two macroscopic mixed-surfactant partitioning schemes, the multicomponent Eberhart model and the ideal mixture of homologues model. We then pair these models with size-dependent partitioning models, which take into account the droplet's surface-area-to-volume ratio. Additionally, we tested the applicability of treating a complex mixture with a surrogate surfactant or an effective Langmuir isotherm for the prediction of ambient aerosol surface tension. Our results suggest that an effective Langmuir isotherm, determined from macroscopic measurements of aerosol extracts, paired with a simple kinetic partitioning model, may be sufficient to describe the surface tension of the accumulation mode aerosol.