Photoelectron imaging of substituted benzenes in aqueous aerosol droplets.

Photochemical reactions can be orders of magnitude faster at the surface of water than in bulk solution, possibly due to changes in the stability of electronic ground and excited states. Yet, direct measurements of the interfacial electronic structure of aqueous reactants remain scarce, making it challenging to establish a clear connection between macroscopic photoreactivities and the underlying molecular-level electronic structure. Here, we employ surface-sensitive ultraviolet (UV) photoelectron velocity-map imaging to probe the valence electronic structure of 13 substituted benzenes at the interface of submicrometer-sized aqueous aerosol droplets. The droplet environment induces vertical binding energy (VBE) shifts of several electronvolts relative to the gas phase for aromatic anions, while neutral solutes show more modest gas-to-solution shifts. Increasing the solute concentration may shift the VBEs of some neutral, protic benzene derivatives, possibly due to increased solute-solute interactions such as hydrogen bonding or π-stacking. In contrast, their anionic conjugate bases show no such shift, likely due to electrostatic repulsion, preventing short-range solute-solute interactions. Changes in droplet surface tension and coverage were quantified through concentration-dependent photoelectron yields. The measured data reveal that 300-nm droplets require a 10 000-fold higher concentration of a proxy nonionic surfactant (Triton X-100) than macroscale solutions to achieve an equivalent surface tension. This observation exemplifies the altered surface partitioning behavior in submicron droplets. It underscores the necessity to account for significant solute depletion in the interior of droplets with considerable surface-to-volume ratios. Phenol-water clusters (170 water molecules) and dilute aqueous phenol droplets (50 mM) exhibit matching valence electronic structure, confirming surface selectivity in UV droplet photoelectron imaging and validating cluster studies as models for interfacial solvation.