The Surface Potential at the Air-water Interface Computed Using Density Functional Theory

TL;DR

This study uses Density Functional Theory to accurately compute the surface potential at the air-water interface, highlighting the importance of molecular quadrupoles and the influence of computational details on results.

Contribution

It provides a DFT-based prediction of the air-water surface potential and analyzes the molecular origins and computational sensitivities of this property.

Findings

Surface potential at 0.92 g/cc water density is +3.63 V.

Molecular quadrupoles dominate the surface potential.

Standard DFT codes may not directly match experimental values.

Abstract

An accurate prediction of the surface potential at the air-water interface is critical to calculating ion hydration free energies and electrochemical half-cell potentials. Using Density Functional Theory (DFT), model interfacial configurations, and a theoretical definition of the surface potential, we report a value of +3.63~volt at 0.92~g/cc water density. A maximally localized Wannier function analysis confirms that the surface potential is dominated by molecular quadrupole (or "spherical second moment") contributions. We find that the predicted surface potential depends on computational details, and conclude that standard DFT codes may not yield surface potentials directly comparable to existing experiments.