Is the neat water surface acidic or basic?
In plain words
Air bubbles and oil drops in pure water move in an electric field as if negatively charged, which suggests hydroxide ions gather at the surface. Spectroscopy and simulations instead point to hydronium ions there.
Precise statement
For the interface of neat water (no surfactant impurities, pH 7, 300 K) with air or a hydrophobic liquid, determine the surface excess of H3O+ and OH- and the origin of the measured negative zeta potential (about -$2 \times 10^{-4}$ statvolt at neutral pH for bubbles and oil drops), choosing between OH- adsorption, interfacial charge transfer between water molecules, and impurity effects. An answer is a quantitative account in which spectroscopic, electrokinetic and simulation results agree.
What would settle it
Electrokinetic measurements on rigorously purified systems combined with surface-specific spectroscopy and ab initio simulations of the ion distributions.
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See also
- Related Ab initio structure and capacitance of the metal-water double layer
- Related Solvation structure and infrared signature of the excess proton
- Related Which species carry charge between contacting insulators?
- Related Molecular mechanism of the Hofmeister series
- Related Spontaneous hydrogen peroxide formation in pure water microdroplets
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