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(+)/ODhyd- 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 ODhyd- 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.