Acidity-Driven Spontaneous Degradation of Trifluoroacetic Acid at Droplet Air–Water Interfaces

Trifluoroacetic acid (TFA), an ultrashort-chain per- and polyfluoroalkyl substance, is increasingly detected in the environment and has raised widespread concern because of its negative effects on health and ecosystems. However, whether TFA can degrade naturally in the environment remains unresolved. Here, we report the spontaneous degradation of TFA at the air–water interface of droplets, yielding F– and fluorine-containing transformation products, including CHF3O, CF3O2H, and CH2F2O2. The extent of degradation is strongly dependent on droplet acidity, with the F– yield at pH 10 being 12-fold higher than at pH 5. Density functional theory calculations reveal a mechanism favored by alkaline conditions and oxygen in which interfacial decarboxylation triggers sequential oxidation and defluorination. These findings identify the air–water interface of pH-regulated droplets as a reactive microenvironment for spontaneous TFA degradation, offering new insights into the environmental transformation of ultrashort-chain PFAS.