Chemistry of Evaporating Charged Alcohol Microdroplets in Humid Environments

There has been recent interest in the physics of evaporating alcohol droplets under humid conditions. These studies focus on delineating time scales for different physical processes, but what about the chemistry? Given the novel reactivity of aqueous microdroplets, we surmise that there will also be interesting chemistry. Here we focus on the oxidation chemistry of single evaporating charged alcohol microdroplets, e.g., 1-nonanol, and find the formation of the corresponding aldehyde, carboxylic acid, and ester. Oxidation is observed only under humidified conditions, suggesting that water is the oxidant. This hypothesis is supported by isotope experiments using H218O. Furthermore, oxidation is charge-dependent with no oxidation products observed for alcohol microdroplets with minimal charge. We therefore propose a mechanism involving charge accumulation at the interface during evaporation followed by water adsorption and activation to yield hydroxyl radicals that initiate oxidation. Our findings reveal a critical role for water and interfacial charge in driving oxidation at the alcohol microdroplet interface, with broader implications for microdroplet reactivity and interfacial chemistry of nonaqueous microdroplets.