Impact of nonionic surfactants on the water activity of binary and ternary aqueous solutions

Atmospheric aerosol droplets have large surface-area-to-volume ratios, leading to bulk-to-surface partitioning. This partitioning affects both the surface tension and water activity of aerosol when strong surfactants are present. The Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model predicts the water activity of solutions containing mixtures of inorganic ions and organic species using a group contribution approach. However, AIOMFAC predictions for the water activity of solutions containing strong surfactants have not been validated against experimental measurements. Here, the water activities of solutions containing strong, nonionic surfactants and their mixtures with NaCl are compared against AIOMFAC model predictions. For molecules with a polyethylene glycol (PEG)-like tail, using the oxyethylene subgroup outperforms an alternative ether and alkyl subgroup approach for representing the repeating molecular substructure. This model-measurement comparison shows the importance of selecting appropriate subgroup descriptions to provide the most accurate predictions of water activity, which could improve predictions of cloud droplet activation.