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Spectroscopy and simulations instead point to hydronium ions there.","posed_since":"","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 -$2 \\times 10^{-4}$ 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.","problem_ref":"4e5ddefbc1816b05f8416558a07874f6ed91480a05c9cd70ee5912b3ee81805c","references":"","settled_by":"Electrokinetic measurements on rigorously purified systems combined with surface-specific spectroscopy and ab initio simulations of the ion distributions.","status_note":"","title":"Is the neat water surface acidic or 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