Quantifying the Surface Tension of Aerosol through Single Droplet Measurements and Modeling Surfactant Partitioning

ALISON BAIN, Kunal Gosh, Nønne L. Prisle, Bryan R. Bzdek, University of Bristol

     Abstract Number: 11
     Working Group: Aerosols, Clouds and Climate

Abstract
The surface tension of aerosol is a necessary parameter in understanding the morphology of spray dried particles, droplet spreading on surfaces, new particle formation and cloud droplet activation. In atmospheric science, the Köhler equation is used to predict the number of cloud droplets that will activate at a given level of supersaturation. A common assumption to these predictions is that the surface tension of a droplet is equal to water. However, using this assumption, field studies can greatly underpredict the number of cloud condensation nuclei (CCN). In some cases, reducing the surface tension leads to better predictions of CCN concentrations.

Field studies have also found large concentrations of surfactants in aerosol samples. Surfactants, organic molecules that preferentially adsorb to interfaces including aerosol droplet surfaces, reduce the surface tension. A reduction in aerosol surface tension due to surfactant adsorption has been suggested to explain the discrepancies between CCN observations and predictions. However, surface tension is typically measured with macroscopic samples. In high surface-area-to-volume ratio droplets, surfactant partitioning to the surface can leave the bulk concentration depleted, thus requiring a larger total surfactant concentration to reduce the surface tension. This size-dependent surface tension can be described with a partitioning model.

Here, we investigate the partitioning behaviour for a variety of surfactants having surface activities similar to the range of surfactants found in atmospheric samples. Comparing surface tensions of droplets to macroscopic solutions shows depletion occurs in most systems. Additionally, we compare a thermodynamic partitioning model and a simple kinetic partitioning scheme to predict the surface tension and critical activation point of aerosol droplets. These results highlight the importance of considering bulk-to-surface partitioning in surfactant containing aerosol droplets and that it is crucial to choose a partitioning scheme that accurately describes droplet surface tension in order to make better predictions of cloud formation.