Double-Layer Distribution of Hydronium and Hydroxide Ions in the Air–Water Interface

Abstract

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 hydronium (H 3 O + ) accumulation in the top layer of the interface. It is crucial to clarify the interfacial preference of OH − and H 3 O + ions for rationalizing the debate. In this work, we perform deep potential molecular dynamics simulations to investigate the preferential distribution of OH − and H 3 O + ions at the 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 H 3 O + surprisingly prefer to accumulate in interfaces but at different interfacial depths, rendering a double-layer ionic distribution within ∼1 nm near the Gibbs dividing surface. The H 3 O + preferentially resides 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 (H 3 O + ) kcal/mol]. The present finding of the ionic double-layer distribution may qualitatively offer a self-consistent explanation for the long-term controversy about the acid−base nature of the air−water interface.

Hydroxide and Hydronium Ions Modulate the Dynamic Evolution of Nitrogen Nanobubbles in Water

“…In the DFT labeling stage, the exchange–correlation interaction was treated by using a meta-generalized-gradient-approximation (meta-GGA) known as the strongly constrained and appropriately normed (SCAN) semilocal density functional, which has been demonstrated to be able to well describe the electrical, structural, and dynamic properties of water molecules and water self-ions in previous tests. ,− A plane wave basis set with a cutoff energy of 600 eV was employed. The Brillouin zone was sampled by using a single k-point.…”

Intramolecular and Water Mediated Tautomerism of Solvated Glycine

“…More recently, many variants of this approach have been proposed, such as using Gaussian approximation potential, deep potential molecular dynamics (DP), , SchNet, and other equivariant approaches . Here, we perform a deep potential molecular dynamics simulation, which has proven its usefulness in many applications, including studies of water and reactions in water. − …”

“…Specifically, the OH – can separate from [C] at a larger distance (>4.5 Å), while H 3 O + and [A] prefer to stay close together (Figure d). This phenomenon is justified by the lower ability of [C] to form hydrogen bonds (Figure e), which builds up a hydrophobic environment discouraging proximity to the hydrophilic OH – ion. , In contrast, [A] establishes a larger number of hydrogen bonds with the surrounding water molecules, favoring the formation of the contact [A]–H 3 O + ion pair.…”

“…In the H 3 O + –PT pathway, the carboxylic proton is first transferred to a nearby water molecule forming H 3 O + and [A]. Then, the proton jumps via the Grotthuss mechanism ,, forming a Zundel intermediate with another water molecule close to the −NH 2 group. A final proton transfer from H 3 O + to the −NH 2 group forms the −NH 3 + of the [Z] state.…”

“…In contrast, the first step in the OH – –PT pathway is the proton transfer from a nearby water molecule to the −NH 2 group to form [C] and OH – . After this step, the OH – diffuses away from the amino side group of [C] via the Grotthuss mechanism. ,, The subsequent approach of OH – to the carboxylic group of [C] allows its deprotonation and formation of the [Z] state. This represents a novel pathway not documented in prior studies.…”