  The preference of water self-ions (hydronium and hydroxide) near
air/oil-water interfaces is one of the hottest topics in water research due to
its importance for understanding properties, phenomena, and reactions of
interfaces. In this work, we performed enhanced-sampling molecular dynamics
based on state-of-the-art neural network potentials with M06-2X accuracy to
investigate the propensity of hydronium and hydroxide ions at air/oil-water
interfaces, which can simultaneously describe well the water autoionization
process forming these ions, recombination of ions, and ionic distribution along
the normal distance to the interface by employing a set of appropriate Voronoi
collective variables. The results support a stable ionic double-layer
distribution near the interface for both air-water and oil-water interface
systems. Hydronium tends to reside in the topmost layer of the interface, while
hydroxide with a slightly stronger interfacial stabilization free energy is
enriched in the deeper interfacial layer. This double-layer distribution may
help to understand the longstanding controversy about the interfacial acid-base
nature.
