Intrinsic pH of water/vapor interface revealed by ion-induced water alignment

Protons at the water/vapor interface are relevant for atmospheric and environmental processes, yet to characterize their surface affinity on the quantitative level is still challenging. Here we utilize phase-sensitive sum-frequency vibrational spectroscopy to quantify the surface density of protons (or their hydronium form) at the intrinsic water/vapor interface, through inspecting the surface-field-induced alignment of water molecules in the electrical double layer of ions. With hydrogen halides in water, the surface adsorption of protons is found to be independent of specific proton-halide anion interactions and to follow a constant adsorption free energy, G about -3.74 (+/-0.56) kJ/mol, corresponding to a reduction of the surface pH with respect to the bulk value by 0.66 (+/-0.10), for bulk ion concentrations up to 0.3 M. Our spectroscopic study is not only of importance in atmospheric chemistry, but also offers a microscopic-level basis to develop advanced quantum-mechanical models for molecular simulations.