Partitioning of ionic surfactants in aerosol droplets containing glutaric acid, sodium chloride, or sea salts

Abstract

Sea spray is the largest contributor to atmospheric aerosol by mass and contains mixtures of inorganic salts and organics. The chemically complex organic fraction can contain soluble, highly surface-active organics, and field studies commonly identify ionic surfactants in aerosol samples. In macroscopic solutions, divalent cations present in sea spray can alter the partitioning of ionic surfactants. Furthermore, the high surface area-to-volume (SA / V) ratio of aerosol droplets may lead to depletion of surfactant from the bulk, requiring more surfactant, relative to its volume, to lower the surface tension of a droplet compared to a macroscopic solution. Here, we investigate the partitioning of model ionic surfactants (sodium dodecyl sulfate, an anionic surfactant, and cetrimonium bromide, a cationic surfactant) in 6–10 µm radius droplets containing glutaric acid, NaCl, or sea spray mimic cosolutes. Surface tension measurements are compared to two independent partitioning models accounting for the SA / V ratio of the droplets. Salting out of the ionic surfactants leads to strong bulk depletion in 6–10 µm radius droplets, with no observable difference in droplet surface tension between NaCl and sea spray mimic cosolutes. The total ionic surfactant concentration required to reach the minimum surface tension for these droplets was 2.0 ± 0.5 mM, consistent with previous observations in droplets containing strong surfactants. Modeling results suggest that surfactant concentrations on the order of tens to hundreds of millimolar are required to significantly reduce surface tension in 100 nm droplets. These results have implications for cloud droplet activation and chemistry occurring at the interface of sea spray aerosol.