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 D(2)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 D(2)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 D(2)O surface. The hydrated proton is thus orders of magnitude more surface‐active than hydroxide at the water–air interface.