Solvation of CaCl2 at the Water–Vapor Interface Revealed by Ambient Pressure X-ray Photoelectron Spectroscopy

The determination of ion concentrations at aqueous interfaces is of paramount importance for the precise modeling of atmospheric reactions. Despite their significant concentrations observed in bulk sea spray aerosol studies, the concentration of Ca2+ ions at the liquid–vapor interface remains unclear. Ambient pressure X-ray photoelectron spectroscopy measurements performed on a liquid flat sheet combined with molecular dynamics simulations reveal the solvation structure and density profiles of Ca2+ and Cl– in the first nanometers of the liquid at the liquid–vapor interface. We report first photoelectron angular distribution measurements on a liquid flat sheet and use them to calculate the photoionization anisotropy parameter of the O 1s, Cl 2p, and Ca 2p core levels. The measurements show the influence of the elastic electron scattering on the anisotropy parameter at low electron kinetic energies, and we include this effect in our Ca2+ and Cl– photoelectron depth profiles. Our measurements and molecular simulations reveal a depletion of the Ca2+ ion concentration and an increased Cl– over Ca2+ ion ratio at the liquid–vapor interface compared to the bulk of the solution.