Revisiting the Effect of the Air-Water Interface of Ultrasonically Atomized Water Microdroplets on HO Formation.

Studying chemical processes at the air-water interface is always challenging. A recent report claimed that HO was formed spontaneously on the surface of condensed water microdroplets. However, a newer report concluded that the detected HO in the previous report could originate in part from the water vapor source that involved ultrasonic atomization of liquid water. Here, this phenomenon is reinvestigated regarding the influence of ultrasonic cavitation, surface modification of droplets, and solutes in the bulk liquid on HO production. When the droplet surfaces were modified by surfactants, HO production did not change, whereas adding gases or inorganic compounds to the bulk solution caused significant changes in HO production. These results confirm that HO formation originates from cavitation in bulk solutions. It is concluded that the air-water interface of water microdroplets itself does not generate HO.