Is the liquid water surface basic or acidic? Macroscopic vs. molecular-scale investigations

Many physical phenomena are affected by the intrinsic acidity/basicity of the free liquid water surface, yet it remains an active and controversial subject. Macroscopic bubble and droplet experiments have been interpreted to indicate an air–water interface covered with hydroxide, whereas recent molecular-scale studies produce the opposite conclusion, viz. that hydroxide is repelled from the interface while hydronium is strongly adsorbed. Here we report results from resonant UV second harmonic generation (SHG) experiments that are best modeled by surface depletion of hydroxide and establish at most a weak surface adsorption. This finding is consistent with our earlier SHG measurements indicating surface enhancement of hydrated protons, as well as with other molecular-scale experiments and simulations, but stands in stark contrast to the results from macroscopic studies. The acidity, or basicity, of aqueous surfaces could strongly influence heterogeneous atmospheric chemical processes, such as aerosol reactions and gas uptake.