Behavior of the eigen form of hydronium at the air/water interface

Surface affinity of hydronium was explored using umbrella sampling molecular dynamics simulations with a refined polarizable potential. The polarizable interaction potential of H₃O⁺ was reparametrized against accurate ab initio calculations for geometries including a water molecule approaching the Eigen cation from its oxygen side. Although there is no true hydrogenbonding with H₃O⁺ acting as an acceptor, respecting in the force field the very shallow ab initio minimum corresponding to this interaction leads to a decrease in surface propensity of hydronium compared to previous results. Qualitatively, the mild surface affinity and strong surface orientation of hydronium is, nevertheless, robustly predicted by various computational approaches, as well as by spectroscopic experiments.