# The Excess Proton at the Air-Water Interface: The Role of Instantaneous Liquid Interfaces

## TL;DR

This study uses ab-initio molecular dynamics to demonstrate that excess protons are more likely to reside at the air-water interface than previously thought, influenced by structured water and proton wires.

## Contribution

It reveals a stronger propensity for protons at the water surface due to water structuring and proton wires, emphasizing the role of instantaneous interfaces.

## Key findings

- Protons favor the water surface more than prior models suggested.
- Structured water layers facilitate proton confinement and transport.
- Proton wires run parallel to the surface, aiding proton mobility.

## Abstract

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.