Hydronium and hydroxide at the air-water interface with a continuum solvent model

The distribution of hydronium and hydroxide ions at the air-water interface has been a problem of much interest in recent years. Here we explore what insights can be gained from a continuum solvent model. We extend our model of ionic solvation free energies and surface interaction free energies to include hydronium and hydroxide. The hydronium cation is attracted to the air-water interface, whereas the hydroxide anion is repelled. If the cavity size parameters required by the model are adjusted to reproduce solvation energies, quantitative agreement with experimental surface tensions is achieved. To the best of our knowledge, this is the most accurate theoretical estimation of this property so far. The results indicate that even if 'water structure' is important, its effects can be captured with a relatively simple model. They also contradict the inference from electrophoresis that there is strong hydroxide enhancement at the air-water interface.