Salting out, non-ideality and Synergism: how the Complex Chemical Composition of Atmospheric Aerosols Enhances Surfactant Efficiency

Abstract

Abstract In Earth’s atmosphere the efficiency of sub-micron aerosol particles to produce cloud droplets is expected to be affected by their surface tension. But this quantity cannot be measured directly and is inferred from the chemical compounds present in aerosols. Amphiphilic surfactants are present in aerosols but direct experimental information on the impacts of other aerosol components on their surface properties is lacking. This work explores systematically how NaCl, (NH4)2SO4, oxalic, and glutaric acid modify the surface properties of amphiphilic surfactants, SDS, Brij35, TritonX100, TritonX114, and CTAC, by determining the adsorption isotherms for these mixtures. The results reveal that all these water-soluble aerosol components enhance the efficiency of surfactants. The inorganic salts substantially lowered the surface tension (- Δσ > 10 mN m-1) and CMC of the surfactants by salting out while the surface tension of mixtures with organic acids deviated strongly from ideality (- Δσ = 10 - 30 mN m-1) and displayed some synergism. These results suggest that molecular interactions take place at the surface, even with non-ionic surfactants, that need to be further studied and included in models.