Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to explore ion behavior at liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions. Interfacial depth profiles of ions were obtained from XPS spectra at a series of photoelectron kinetic energies. Depth profiles of the ratio of anion to cation show little difference among the solutions. Previously, these depth profiles were determined from the ratio of anion to cation signal-peak areas. However, using molecular dynamics simulations (MD), the individual anion and cation depth profiles are both observed to differ as a function of solution, but the differences are masked when only the anion-to-cation ratios are considered. Using the Cl À /O water ratio determined from the XPS measurements, surface-enhanced concentrations of Cl À are observed in the NaCl solution, but not in the RbCl solution, in agreement with predictions from MD simulations. We also report studies of aqueous solutions of RbBr. In contrast to an aqueous RbCl solution, our combination of AP-XPS experiments and MD simulations suggests that anion/cation ratios are enhanced at the surface for this system due to the separation of bromide and rubidium in the double layer near the surface, while the interfacial concentration of bromide does not differ considerably from the bulk.

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 H 2 SO 4 (aq), measured over a large concentration range and aided by previously reported HSO 4 − /SO 4 2and HSO 4 − /H 2 SO 4 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 H 2 SO 4 (aq) changes around 5−7 M concentration, at which point a dramatic change in both the HSO 4 − photoelectron peak width and binding energy occurs.

We report photoelectron spectroscopy measurements from binary acetonitrile−water solutions, for a wide range of acetonitrile mole fractions (x CH 3 CN = 0.011−0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH 3 CN from aqueous surface and bulk solution, we quantify CH 3 CN's larger propensity for the solution surface as compared to bulk solution. Quantification of the strong surface adsorption is through determination of the surface mole fraction as a function of bulk solution, x CH 3 CN , from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH 3 CN surface concentration, based on analysis of the relative amount of gas-versus liquid-phase CH 3 CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid-and gas-phase CH 3 CN, which is sensitive to the change in solution surface potential and thus sensitive to the surface concentration of CH 3 CN. Gibbs free energy of adsorption values are compared with previous literature estimates, and we consider the possibility of CH 3 CN bilayer formation. In addition, we use the observed changes in N 1s and C 1s peak positions to estimate the net molecular surface dipole associated with a complete CH 3 CN surface monolayer, and discuss the implications for orientation of CH 3 CN molecules relative to the solution surface. ■ INTRODUCTIONExperimental molecular-level investigations of the electronic structure of aqueous solutions have recently become possible by using photoelectron (PE) spectroscopy in combination with a liquid microjet either in vacuum 1−3 or at near ambient pressure conditions. 4−6 Studies reported to date are largely comprised of neat liquid water, aqueous solutions of common electrolytes, and low-concentration solutions containing common organic and inorganic solute molecules and ions. 7−21 Typically, PE spectroscopy accesses solute electron binding energies, both lowest ionization energies and core-level energies, the latter being most suited for interpreting differences in solvation configuration at the solution surface or in the bulk of solution. PE spectroscopy can also provide a quantitative measure of solute concentrations across the solution interface, or it can be used to characterize, for instance, chemical equilibria as a function of concentration or pH, both near the top surface region and more deeply into the solution. The possibility to make such a direct comparison between surface and bulk-solution properties is indeed a rather unique feature of PE spectroscopy. The method's variable information depth is due to the strongly energy-dependent electron mean free path, which can be adjusted experimentally by a suitable choice of applied ionization photon energies. 1,22,23 To our knowledge, the present work reports the first PE spectroscopy study of a binary highly volatile solution studied over a wide range of concent...