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Zusammenfassung:
Photoelectron angular distributions (PADs) from aqueous solution surfaces reveal details on 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)2S)O2, 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 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 little 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.
