The Surface Activity of the Hydrated Proton Is Substantially Higher than That of the Hydroxide Ion

Abstract

Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.

Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets

“…47 The implication is that SFG spectroscopy may not provide information about the acidity of the OTL probed by our experiments. 18,19 The absence of H 3 O + proton donors at the OTL of pH > 4 water is consistent with the pH dependence of the electrophoretic mobilities of air bubbles, which display an isoelectric point in the pH 3−4 range. 15,48 Bubbles and droplets are negatively charged above pH ∼ 4 and positively below pH ∼ 4 arguably due to the presence of excess OH − and H 3 O + ions on their surfaces.…”

“…Recent DFT–MD simulations of the structure of an air–water interface containing one hydronium ion concluded that surface-specific sum frequency generation (SFG) spectroscopy is blind to the effects induced by H 3 O + ions in the structure of OTL water [the binding interfacial layer (BIL) in the original report] . The implication is that SFG spectroscopy may not provide information about the acidity of the OTL probed by our experiments. , …”

“…Nor it is the acidity reported by surface specific spectroscopies that probe deeper interfacial layers of aqueous solutions. 18 We emphasize that the affinity of H 3 O + ions for the surface of undissociated water clusters predicted by model simulations it is not by itself evidence that the surface of water is "acidic", 16,19,20 because acidity refers to the extent of proton transfer between conjugate acid−base pairs, in this case between OTL H 3 O + ions and colliding gas molecules.…”

Experimental Data Contributing to the Elusive Surface Charge of Inert Materials in Contact with Aqueous Media

“…In view of the contradictory conclusions, it is not unlikely that the four above-mentioned causes (and maybe others) are all simultaneously at play. The literature on the various topics is exhaustive and extending year by year [16][17][18], and it would be futile to even try and discuss all the information that is being accumulated.…”

Affinity of Hydrated Protons at Intrinsic Water/Vapor Interface Revealed by Ion-Induced Water Alignment

“…Protons at water/vapor interfaces play a key role in atmospheric chemistry and environmental science. , They have been the topics of extensive theoretical and experimental studies for decades, − but good understanding of the surface affinity of protons on the microscopic and quantitative level is still lacking. Molecular dynamics (MD) simulations generally predict that protons (H + ) could appear at the interface in the form of hydronium (H 3 O + ), while the quantitative details, such as the adsorption free energy and the depth profile of ion concentrations, depend very much on the molecular model and interaction potentials assumed. − Several experimental approaches have been used to verify the qualitative conclusion from the simulations, − but extension of these efforts to quantitative characterizations of the proton adsorption remains challenging . This largely limits our knowledge of the underlying mechanisms and the role of protons in many relevant interfacial processes.…”