Hydroxide and hydronium, which represent the ionic products of water autolysis,
exhibit a peculiar surface behavior. While consensus has been established that the
concentration of hydronium cations is enhanced at the surface with respect to the bulk, the
affinity of hydroxide anions for the water/vapor interface has been a subject of an ongoing
controversy. On the one hand, electrophoretic and titration measurements of air bubbles or oil
droplets in water have been interpreted in terms of a dramatic interfacial accumulation of OH-.
On the other hand, surface-selective non-linear spectroscopies, surface tension measurements,
and molecular simulations show no or at most a weak surface affinity of hydroxide ions. Here,
we summarize the current situation and provide new evidence for the lack of appreciable
surface enhancement of OH-, based on photoelectron spectroscopy from a liquid jet and on
molecular dynamics simulations with polarizable potentials at varying hydroxide
concentrations.