  The magnitude of the pH of the surface of water continues to be a contentious
topic in the physical chemistry of aqueous interfaces. Recent theoretical
studies have shown little or no preference for the proton to be at the surface
compared to the bulk\cite{baer2014toward}. Using ab-initio molecular dynamics
simulations, we revisit the propensity of the excess proton for the air-water
interface with a particular focus on the role of instantaneous liquid
interfaces. We find a much a stronger propensity of the proton for the surface
of water. The enhanced water structuring around the proton results in the
presence of proton wires that run parallel to the surface as well as a
hydrophobic environment made up of under-coordinated topological defect water
molecules, both of which create favorable conditions for proton confinement at
the surface. The Grotthuss mechanism within the structured water layer involves
a mixture of both concerted and closely spaced stepwise proton hops. The proton
makes excursions within the first solvation layer either in proximity to or
along the instantaneous interface.
