Salting out, non‑ideality and synergism enhance surfactant efficiency in atmospheric aerosols

The surface tension of sub-micron aerosol particles is expected to affect their efficiency in becoming cloud droplets. Over the last years the role of surfactants in the activation of atmospheric aerosols into cloud droplets has received a growing interest. However, most of the investigations have focused on mixtures containing only one surfactant, while the composition of atmospheric aerosol is complex. Until now, there was little experimental information on the surface properties of mixtures of surfactants with other aerosol components. In this work pendant droplet tensiometry was used to determine the adsorption isotherms and cmc of aqueous mixtures of amphiphilic surfactants (SDS, Brij35, TritonX100, TritonX114, and CTAC) with inorganic salts (NaCl, (NH₄)₂SO₄) and soluble organic acids (oxalic and glutaric acid). Interestingly, inorganic salts and organic acids systematically enhanced the efficiency of the surfactants by further lowering the surface tension and, in some cases, the CMC. Furthermore, all the mixtures studied were strongly non-ideal, some even displaying some synergism, thus demonstrating that the common assumption of ideality for aerosol mixtures is not valid. The molecular interactions between the mixture components were either in the bulk (salting out), in the mixed surface monolayer (synergy on the surface tension) or in the micelles (synergy on the CMC) and need to be included when describing such aerosol mixtures.

Figure 1: Evolution of the minimal surface tension for mixtures of amphiphilic surfactants and organic acids (left) and two amphiphilic surfactants (right).