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 D2 O-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 D2 O 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 D2 O surface. The hydrated proton is thus orders of magnitude more surface-active than hydroxide at the water-air interface.