Sprayed water microdroplets containing dissolved pyridine spontaneously generate pyridyl anions

Significance Water microdroplets can accelerate chemical reactions by orders of magnitude compared to the same reactions in bulk water and/or trigger spontaneous reactions that do not occur in bulk solution. Among the properties of water microdroplets, the unique redox ability resulting from the spontaneous dissociation of OH − into a released electron and •OH at the air–water interfaces is especially intriguing. At the air–water interface, OH − exhibits a strong reducing potential, and the resulting •OH is highly oxidative, making water microdroplets a unity of opposites. We report the reduction of pyridine into pyridyl anions (C 5 H 5 N − ) and the oxidation of pyridine into hydroxypyridine, which extends what we know about the redox power of water microdroplets.