Why Proton Grotthuss Diffusion Slows down at the Air-Water Interface while Water Diffusion Accelerates

Excess proton diffusion at aqueous interfaces is crucial for applications including electrocatalysis, aerosol chemistry, and biological energy conversion. While interfaces have been proposed as pathways for channeling protons, proton diffusion at interfaces remains far less understood than in the bulk. Here we focus on the air-water interface and use density functional theory-based deep potential molecular dynamics simulations to reveal the contrasting interface's impacts: excess proton diffusion slows down compared to the bulk, while water diffusion accelerates. This contrast stems from reduced hydrogen-bond coordination at the interface, which facilitates water diffusion and transient unstable proton rattling but impedes the stable proton hops central to Grotthuss diffusion. As a result, at the interface, excess protons and water molecules diffuse at comparable rates, in stark departure from bulk behavior. This mechanistic insight delineates distinct limiting regimes for bulk-enhanced interfacial proton diffusion, with important implications for interfacial chemistry.