Comment on ‘Behaviour of hydroxide at the water/vapor interface’ [Chem. Phys. Lett. 474 (2009) 241]

Application of the Gibbs adsorption isotherm to the available data for the pH-dependence of the surface tension of water leads to two conclusions. First, a strong hydroxide surface affinity implies the surface tension nearly pH-independent near pH 7. Second, surface tension data at high bulk hydroxide concentration are consistent with a large surface excess of hydroxide at low bulk concentration. These in part contradict the claims made by Winter et al., but also allow a reinterpretation of their spectra, revealing the essential importance of their conclusion that hydroxide is absent from the top few molecular layers near the water–vapour interface.