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Title: Is the neat water surface acidic or basic
Plain: 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: 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 -2e-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.
Settled by: Electrokinetic measurements on rigorously purified systems combined with surface-specific spectroscopy and ab initio simulations of the ion distributions.
Kind: phenomenon
Answer type: classification
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: (empty)