10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Measurements and Modelling of Surfactant Coated Aerosol Particles
BRYAN R. BZDEK, Jussi Malila, Nonne Prisle, Jonathan P. Reid, University of Bristol
Abstract Number: 112 Working Group: Aerosol Chemistry
Abstract The surface tension of atmospheric particles is key to determining the critical supersaturation required for a particle to activate into a cloud condensation nucleus. Most models generally assume the surface tension of activating particles is equivalent to that of pure water. However, recent experiments have shown that the surface tension of particles can be much less than that of pure water, for instance due to the presence of condensed organic films, core-shell phase separations, and/or surfactants. Indeed, recent field studies have indicated that surfactants are important components of organic aerosol composition. For surfactants to be relevant to particle activation into cloud droplets, at least two parameters must be considered. First, the concentration of surfactant in the initial particle must be sufficiently large that surface tension depression is maintained over the course of activation, despite the dilution that occurs as water condenses onto the particle. Second, the high surface to volume ratio of micron and submicron particles necessitates partitioning of more surfactant molecules to the particle surface than in a typical solution, resulting in a depletion of the bulk concentration and an increase in the surface tension relative to a bulk sample.
This presentation will provide a comprehensive account of the size dependent partitioning behaviour of surfactant containing particles through a combination of measurements and modelling. The experimental approach utilises holographic optical tweezers, which allows quantification of the surface tension of picolitre volume surfactant-containing droplets. The measurement is accomplished by sub-microsecond resolved monitoring of the periodic shape oscillations resulting from controlled coalescence of two optically trapped droplets. Surface tensions are resolved to <1 mN/m. These experiments, when compared to bulk tensiometry measurements, demonstrate the surface-bulk partitioning effects in micron-sized droplets. Indeed, 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. The experimental data are compared to a surface monolayer model that is constrained only by bulk surface tension measurements. This model agrees well with experimental observations, providing confidence in its treatment of particle surface-bulk partitioning. As a result, particle surface tension can be modelled to determine the sensitivity of particle surface tension to particle size and surface tension. This approach therefore will provide key information about the significance of the surface tension assumptions inherent in climate models.