Affinity of Hydrated Protons at Intrinsic Water/Vapor Interface Revealed by Ion-Induced Water Alignment.

Protons at the water/vapor interface are relevant for atmospheric and environmental processes, yet characterizing 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, Delta G approximate to -3.76 (+/- 0.79) kJ/mol, corresponding to a partitioning coefficient of the surface with respect to bulk water by 3.3 similar to 6.2, for bulk ion concentrations up to 0.3 M. Our spectroscopic study not only is of importance in atmospheric chemistry but also offers a microscopic-level basis to develop advanced quantum-mechanical models for molecular simulations.