The Surface Activity of the Hydrated Proton Is Substantially Higher than That of the Hydroxide Ion

The behavior of hydroxide and hydrated protons, the auto-ionization products of water, at surfaces is important for a wide range of applications and disciplines. However, it is unknown at which bulk concentration these ions start to become surface active at the water–air interface. Here, we report changes in the D₂O–air interface in the presence of excess D⁺_hyd/OD⁻_hyd determined using surface-sensitive vibrational sum-frequency generation (SFG) spectroscopy. The onset of the perturbation of the D₂O surface occurs at a bulk concentration as low as 2.7±0.2 mm D⁺_hyd. In contrast, a concentration of several hundred mm OD⁻_hyd is required to change the D₂O surface. The hydrated proton is thus orders of magnitude more surface-active than hydroxide at the water–air interface.