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

Recent studies have reported spontaneous production of OH radicals and HO 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 HO formation compared to cations. The effect of the anions' identity on HO 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 HO 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 HO formation. This study shows that spontaneous formation of HO is driven by the solvation properties at the interface and not necessarily (or exclusively) by the presence of a strong electric field.