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

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