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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

In the work described here, the electronic structure of sulfuric acid in water is explored by liquid-jet photoelectron spectroscopy. From the S2p photoelectron spectra of H2SO4 (aq), measured over a large concentration range and aided by previously reported HSO4–/SO42- and HSO4–/H2SO4 concentration ratios in the bulk solution, we obtain detailed electronic structure information of each species. Comparing our results with previous studies on the dissociation of nitric acid, we argue that the solvation structure of H2SO4 (aq) changes around 5–7 M concentration, at which point a dramatic change in both the HSO4– photoelectron peak width and binding energy occurs.

Nitric acid is a prevalent component of atmospheric aerosols, and the extent of nitric acid dissociation at aqueous interfaces is relevant to its role in heterogeneous atmospheric chemistry. Several experimental and theoretical studies have suggested that the extent of dissociation of nitric acid near aqueous interfaces is less than that in bulk solution. Here dissociation of HNO3 at the surface of aqueous solution is quantified using X-ray photoelectron spectroscopy of the nitrogen local electronic structure. The relative amounts of undissociated HNO3(aq) and dissociated NO3–(aq) are identified by the distinguishable N1s core-level photoelectron spectra of the two species, and we determine the degree of dissociation, αint, in the interface (approximately the first three layers of solution) as a function of HNO3 concentration. Our measurements show that dissociation is decreased by ∼20% near the solution interface compared with bulk solution and furthermore that dissociation occurs in the topmost solution...

Molecular-level insight into the dissociation of nitric acid in water is obtained from X-ray photoelectron spectroscopy and first-principles molecular dynamics (MD) simulations. Our combined studies reveal surprisingly abrupt changes in solvation configurations of undissociated nitric acid at approximately 4 M concentration. Experimentally, this is inferred from shifts of the N1s binding energy of HNO(3)(aq) as a function of concentration and is associated with variations in the local electronic structure of the nitrogen atom. It also shows up as a discontinuity in the degree of dissociation as a function of concentration, determined here from the N1s photoelectron signal intensity, which can be separately quantified for undissociated HNO(3)(aq) and dissociated NO(3)(-)(aq). Intermolecular interactions within the nitric acid solution are discussed on the basis of MD simulations, which reveal that molecular HNO(3) interacts remarkably weakly with solvating water molecules at low concentration; around 4 M there is a turnover to a more structured solvation shell, accompanied by an increase in hydrogen bonding between HNO(3) and water. We suggest that the driving force behind the more structured solvent configuration of HNO(3) is the overlap of nitric acid solvent shells that sets in around 4 M concentration.