How the Acidity of Water Droplets and Films Is Controlled by the Air–Water Interface

Abstract

Acidity is a key determinant of chemical reactivity in atmospheric aqueous aerosols and water microdroplets used for catalysis. However, many fundamental questions about these systems have remained elusive, including how their acidity differs from that of bulk solutions, the degree of heterogeneity between their core and surface, and how the acid−base properties are affected by their size. Here, we perform hybrid density functional theory (DFT)-quality neural network-based molecular simulations with explicit nuclear quantum effects and combine them with an analytic model to describe the pH and self-ion concentrations of droplets and films for sizes ranging from nm to μm. We determine how the acidity of water droplets and thin films is controlled by the properties of the air−water interface and by their surface-tovolume ratio. We show that while the pH is uniform in each system, hydronium and hydroxide ions exhibit concentration gradients that span the two outermost molecular layers, enriching the interface with hydronium cations and depleting it with hydroxide anions. Acidity depends strongly on the surface-to-volume ratio for system sizes below a few tens of nanometers, where the core becomes enriched in hydroxide ions and the pH increases as a result of hydronium stabilization at the interface. These results obtained for pure water systems have important implications for our understanding of chemical reactivity in atmospheric aerosols and for catalysis in aqueous microdroplets.

My Vision of Electric-Field-Aided Chemistry in 2050

“…The combination of a continuous-flow setup and employing molecular entities as the sources of OEEFs ,− is another promising approach. Thus, for example, Zare et al , use water droplets, in which the O–H bonds on the surface of the droplet, as well as ions in the interface between droplets, deliver powerful OEEFs ,, that catalyze unusual reactions, with efficiency that increases with decrease in the diameter of the droplets .…”

“…Zare et al , showed that interfaces of water–air, water–solid, and water–oil generate water droplets which give rise to H 2 O 2 and other unusual products. Recently, Laage et al used molecular dynamic simulations and demonstrated that the droplet surface is acidic and contains excess H 3 O + ions. As such, the droplet surface generates a unique source of molecular electric field, which accelerates nucleophilic-cleavage reactions .…”

Neural Network-Based Sum-Frequency Generation Spectra of Pure and Acidified Water Interfaces with Air

“…With acidity recognized as a key determinant of reactivity in aqueous phases, considerable experimental − and simulation − efforts have been devoted to characterizing the acid–base properties of the air–water interface in recent decades. Although no definitive answer to this question has been obtained yet, there is accumulating evidence of substantial surface activity of hydronium (H 3 O + ) cations. − ,− ,,, Among the experimental techniques that have supported this acid-enriched picture of the air–water interface, vibrational sum-frequency generation (SFG) spectroscopy has been instrumental because of its surface specificity. SFG measurements have revealed that the vibrational spectrum of the neat water surface changes markedly when the solution is acidified, − ,− which has been suggested to arise from the presence of hydronium ions at the air–water interface.…”

“…A series of 16 independent 1 ns long reactive molecular dynamics simulations of a pure water slab and of a slab of water with one excess proton (at an effective concentration of approximately 0.3 mol/L; see Supporting Information) are propagated using a neural network potential (NNP). This NNP was constructed as part of our recent study of water self-dissociation using the DeePMD-kit software , to reproduce the hybrid functional revPBE0-D3 level of theory. Because this network was trained to describe proton exchanges, our trajectory is reactive.…”

“…Water interfaces exhibit a unique chemical reactivity that differs from both the bulk liquid and the gas phase. , Understanding this specific reactivity is critical in numerous fields, ranging from atmospheric chemistry to microdroplet catalysis . With acidity recognized as a key determinant of reactivity in aqueous phases, considerable experimental − and simulation − efforts have been devoted to characterizing the acid–base properties of the air–water interface in recent decades. Although no definitive answer to this question has been obtained yet, there is accumulating evidence of substantial surface activity of hydronium (H 3 O + ) cations. − ,− ,,, Among the experimental techniques that have supported this acid-enriched picture of the air–water interface, vibrational sum-frequency generation (SFG) spectroscopy has been instrumental because of its surface specificity.…”