Increased Propensity of I_aq⁻ for the Water Surface in Non-neutral Solutions: Implications for the Interfacial Behavior of H₃O_aq⁺ and OH_aq⁻

By a combination of surface-sensitive photoelectron spectroscopy and molecular dynamics simulations, we characterize the surface propensity of the iodide anion in aqueous solutions at acidic, neutral, and basic conditions (pH = 1, 6.8 and 13). In both experiments and simulations, an increased surface concentration of I⁻ is found in non-neutral solutions. We interpret these findings in terms of I⁻ surface coadsorption with hydronium at low pH and “salting out” of iodide by hydroxide from the bulk solution to the surface at high pH. The present study is thus in accord with the picture of a weak surface adsorption of hydronium and surface depletion of hydroxide, being in sharp contradiction with the interpretation of electrophoretic and titration measurements in terms of strong surface enrichment of OH⁻