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

Abstract. Seaspray aerosol 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 and surface-active organics, and field studies commonly identify ionic surfactants in aerosol samples. In macroscopic solutions, divalent cations present in seaspray have been found to 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 dodecylsulfate, anionic, and cert tetrammonium bromide, cationic) 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 which account 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. No difference in droplet surface tension was observed between NaCl and sea spray mimic cosolutes. The total concentration of ionic surfactant required to reach the minimum surface tension in these droplets (water activity ~0.99) was 2.0 ± 0.5 mM when the macroscopic critical micelle concentration is < 2 mM. These results are consistent with previous observations in droplets containing nonionic surfactants. The partitioning of ionic surfactants in salt-containing droplets has implications for cloud droplet activation and chemistry occurring at the interface of sea spray aerosol.