Double-layer distribution of hydronium and hydroxide ions in the air-water interface

The acid-base nature of the aqueous interface has long been controversial. Most macroscopic experiments suggest that the air-water interface is basic based on the detection of negative charges at the interface that indicates the enrichment of hydroxides (OH–), whereas microscopic studies mostly support the acidic air-water interface with the observation of the hydronium (H3O+) accumulation in the top layer of the interface. It is crucial to clarify the interfacial preference of OH– and H3O+ ions for rationalizing the debate. In this work, we perform deep potential molecular dynamics simulations to investigate the preferential distribution of OH– and H3O+ ions at aqueous interfaces. The neural network potential energy surface is trained based on density functional theory calculations with the SCAN functional, which can accurately describe the diffusion of these two ions both in the interface and in the bulk water. In contrast to the previously reported single ion enrichment, we show that both OH– and H3O+ surprisingly prefer to accumulate in interfaces, but at different interfacial depths, rendering a double-layer ionic distribution within ~1 nm below the Gibbs dividing surface. The H3O+ is preferentially adsorbed in the topmost layer of the interface, but the OH–, which is enriched in the deeper interfacial layer, has a higher equilibrium concentration due to the more negative free energy of interfacial stabilization (–0.90 (OH–) vs. –0.56 (H3O+) kcal/mol). The air-water interface is therefore negatively charged, in agreement with the macroscopic charge detection and not in contradiction with the microscopic studies. The present finding of the ionic double-layer distribution qualitatively offers a self-consistent explanation for the long-term controversy about the acid-base nature of the air-water interface.