Molecular Arrangement of a Mixture of Organosulfur Surfactants at the Aqueous Solution–Vapor Interface Studied by Photoelectron Intensity and Angular Distribution Measurements and Molecular Dynamics Simulations

Photoelectron angular distributions PADs from aqueous solution surfaces reveal details of the spatial arrangement of solute molecules at the solution gas phase interface. This is demonstrated here for mixed equimolar aqueous solutions of dimethyl sulfoxide dimethyl sulfone CH3 2SO CH3 2SO2 and dimethyl sulfoxide dimethyl sulfite CH3 2SO CH3 2SO3 , all molecules having a propensity to reside near the solution surface. Although the surface active molecules coexist at the surface, CH3 2SO2 yields a more intense sulfur 2p surface photoelectron signal than CH3 2SO, and for CH3 2SO3, the effect is even larger. To understand this behavior, we have for one of the solutions mixtures, CH3 2SO CH3 2SO2, performed PAD measurements. Surprisingly, both molecules exhibit almost identical PADs, implying that the emitted photoelectrons have experienced a similar limited amount of scattering interactions. Hence, the molecules reside at the same distance with respect to the solution vacuum interface rather than CH3 2SO2 being closer to the surface than CH3 2SO, as one may have assumed based on the relative photoelectron signal intensities. Instead, the relative surface and bulk concentrations of the two compounds differ. We also report S 2p photoelectron spectra from single component dimethyl sulfide, CH3 2S, aqueous solutions measured at a single detection angle. The exceptionally large surface propensity of CH3 2S is recognized by a narrow, gas phase like photoelectron spectrum, revealing that CH3 2S experiences very few hydration interactions. Experimentally observed trends in surface activity for the different molecules, which are complemented here by molecular dynamics simulations, agree with findings obtained with other surface sensitive techniques. New information on the surface structure of mixed solutions is uniquely obtained from the anisotropic angular distributions of the photoelectrons