Structural Transformation of Hydrated Excess Protons at the Water Surface

The microscopic hydration structure of excess protons (H⁺) at the air/water interface remains elusive despite extensive surface-specific studies. In bulk water, hydrated protons exhibit a broad vibrational "proton continuum" arising from dynamically delocalized hydration structures. However, proton hydration at aqueous interfaces has not been resolved. Here, we combine interface-selective heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy with ab initio molecular dynamics (AIMD) simulations to investigate aqueous HCl surfaces over a wide concentration range (0-9 M). HD-VSFG spectroscopy reveals pronounced changes in the interfacial vibrational spectra with increasing HCl concentration: (1) the amplitude of the free OH band decreases monotonically, reflecting depletion of normal interfacial water molecules; (2) the amplitude of the hydrogen-bonded OH band markedly increases upon addition of 1 M HCl, which is attributable to the enhanced orientation of water molecules in the electric double layer formed by the excess protons and Cl^-; and (3) the amplitude of a low-frequency feature gradually increases while extending its spectrum to develop into the proton continuum at high HCl concentrations (>5 M). AIMD simulations indicate a significant asymmetry of the location of the excess proton localized at the interface at low HCl concentration, whereas this asymmetry is substantially reduced at high concentrations. This suggests that interfacial protons preferentially adopt localized, Eigen-like hydration structure under weakly acidic conditions, while more delocalized Eigen-Zundel-Eigen-like hydration structure becomes accessible at highly acidic conditions. This study indicates that the preferred hydration structure of excess protons at the water surface depends on the HCl concentration, providing crucial insights into proton transfer at aqueous interfaces.