Liquid microjet photoelectron spectroscopy is an increasingly common technique to measure vertical ionization energies (VIEs) of aqueous solutes, and such data are available for a catalogue of common inorganic anions. Interpretation of the experiments is subject to questions regarding sensitivity to bulk versus interfacial solvation environments, which is especially relevant for ions that may partition preferentially at the air/water interface. We compute VIEs for a set of aqueous anions, using a combination of molecular dynamics simulations and electronic structure calculations with continuum boundary conditions. The results are in good agreement with experiment, regardless of whether the simulation data are restricted to ions at the air/water interface or to those in isotropic (bulk) liquid water. Although VIEs for these ions are quite sensitive to the presence of short-range hydrogen bonds, we find that the nature of anion--water hydrogen bonding is sufficiently similar in the bulk and interfacial environments so that it proves impossible to discriminate between the two on the basis of the VIE.  This has important implications for the interpretation of liquid-phase photoelectron spectroscopy, suggesting that the issue of surface sensitivity in these experiments is largely moot. More generally, analysis of the bulk and interfacial simulation data suggest that partitioning of soft anions at the air/water interface is largely a second (or third) solvation shell effect arising from disruption of water--water hydrogen bonds, rather than from any significant changes in anion−water hydrogen bonds.

Interfacial effects on ionization energies and the suprising banality of anion−water hydrogen bonding at the air/water interface

ACS Spring Meeting 2021

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