Specific Ion Properties Induce Spontaneous H₂O₂ Production at the Air-Water Interface

Recent studies have reported spontaneous production of OH radicals and H₂O₂ at the air-water interface of water droplets. However, the mechanism(s) behind this chemistry remain elusive, with the presence of a strong electric field at the interface being considered one reason for this spontaneous chemistry. Here, we provide evidence that in salt-containing aqueous droplets, the amount of oxidant formation is strongly related to the identity and concentration of the ions present in the solution. Anions have a significantly stronger effect on H₂O₂ formation compared to cations. The effect of the anions' identity on H₂O₂ formation follows the Hofmeister series, which describes changes in the solvation properties of a solution due to the presence of ions. We present a quantitative relationship between two Hofmeister parameters and peroxide concentration derived from our experimental results. This link between H₂O₂ formation and the Hofmeister series suggests that anions disrupt the water structure at the interface, reducing the solvation of OH^- anions and promoting their dissociation into OH radicals and free electrons, leading to an increase in H₂O₂ formation. This study shows that spontaneous formation of H₂O₂ is driven by the solvation properties at the interface and not necessarily (or exclusively) by the presence of a strong electric field.