Gas-Liquid Interface of Aqueous Solutions of Surface Active Aromatic Molecules Studied Using Extreme Ultraviolet Laser Photoelectron Spectroscopy and Molecular Dynamics Simulation

We investigated the gas-liquid interface of aqueous solutions containing phenol and related aromatic compounds using extreme ultraviolet laser photoelectron spectroscopy and molecular dynamics simulations. The interfacial densities of protonated and deprotonated forms of phenol, aniline, and 4-nitrophenol were found to be primarily determined by their surface affinities and exhibit similar concentration dependences to their respective bulk densities. Despite the distinct interfacial orientations of their permanent dipole moments, these compounds monotonically decreased the surface potential at higher concentrations. The exception was sodium phenolate, whose surface potential first increased and then decreased with increasing concentration. This behavior is attributed to the opposing effects of the electric double layer formed by Na⁺ and phenolate ions at the gas-liquid interface and the induced electronic polarization of phenolate.