Counterintuitive Oxidation of Alcohols at Air-Water Interfaces.

This study shows that the oxidation of alcohols can rapidly occur at air-water interfaces. It was found that methanediols (HOCHOH) orient at air-water interfaces with a H atom of the -CH- group pointing toward the gaseous phase. Counterintuitively, gaseous hydroxyl radicals do not prefer to attack the exposed -CH- group but the -OH group that forms hydrogen bonds with water molecules at the surface via a water-promoted mechanism, leading to the formation of formic acids. Compared with gaseous oxidation, the water-promoted mechanism at the air-water interface significantly lowers free-energy barriers from ∼10.7 to ∼4.3 kcal·mol and therefore accelerates the formation of formic acids. The study unveils a previously overlooked source of environmental organic acids that are bound up with aerosol formation and water acidity.