AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Quantification of the Partitioning Behaviour of Surfactants in Picolitre Droplets
BRYAN R. BZDEK, Jonathan P. Reid, University of Bristol
Abstract Number: 91 Working Group: Aerosol Chemistry
Abstract The surface tension of atmospheric particles helps determine the critical supersaturation required for a particle to grow into a cloud droplet and, therefore, the cloud droplet number concentration. However, constraining the surface tension of ambient aerosol is challenging due to the very small mass and complex chemical composition of atmospheric particles. Recent studies demonstrate that surfactants are important components of ambient aerosol and alter the pathway by which particles grow into cloud droplets, but the relationship between surfactant concentration and particle surface tension is not straightforward. Due to the very high surface area and limited volume of a particle, partitioning of surfactant molecules to the particle surface reduces their “bulk” concentration. The result is a very different relationship between surface tension and concentration in particles compared to bulk measurements, where the number of molecules available for partitioning is effectively infinite. In this contribution, holographic optical tweezers is used to measure the surface tensions of picolitre droplets containing representative surfactants. This measurement is accomplished through sub-microsecond time resolved monitoring of the periodic shape oscillations resulting from controlled coalescence of two optically trapped micron-sized droplets. Surface tensions are resolved to <1 mN/m. Comparison of the droplet measurements with bulk tensiometry approaches demonstrates that surface-bulk partitioning can result in a difference of >10 mN/m between the droplet measurement and the bulk measurement at the same nominal concentration. Moreover, the apparent critical micelle concentration in picolitre droplets is approximately a factor of two larger than the comparable bulk measurement. The droplet measurements are mapped onto bulk measurements to develop a relationship that correlates a bulk surface tension with a corresponding droplet surface tension. This work quantifies the partitioning behaviour of surfactants in confined volumes and provides valuable information for models considering bulk-surface partitioning to predict cloud droplet number concentrations.