Propensity of water self-ions at air(oil)-water interfaces revealed by deep potential molecular dynamics with enhanced sampling

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.