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...

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K + and Li + ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li + adsorbs to the aqueous solution−vapor interface, while K + does not. Thus, we provide experimental validation of the "surfactant-like" behavior of Li + predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K + and Li + ions to a difference in the resilience of their hydration shells.ion adsorption | air−water interface | specific ion effects | Hofmeister series | aqueous ionic solvation M yriad chemical and biochemical processes that occur in aqueous salt solutions exhibit trends that depend systematically on the identities of the salt ions. These trends, which are commonly referred to as specific ion effects, generally follow the Hofmeister series, a ranking of the ability of salt ions to precipitate proteins that was developed by Franz Hofmeister (1) in the late 1800s. The Hofmeister series applies, however, to a wide range of other seemingly unrelated phenomena, such as colloidal stability, critical micelle concentrations, chromatographic selectivity, protein denaturation temperatures, and the interfacial properties of aqueous salt solutions (2, 3). Early attempts to explain the Hofmeister series relied on the notion that salt ions have a long-range effect on the structure of water, with ions on one side of the series acting as "structure makers" and ions on the other side as "structure breakers" (2, 4). However, more recently, several experimental and computational studies have questioned the role of long-range ordering/disordering effects (4-9), and have provided compelling evidence that ion-specific behavior at aqueous interfaces must be taken into consideration when attempting to explain Hofmeister effects (7, 10-13).Specific anion effects on the interfacial properties of aqueous salt solutions, such as surface tensions and surface potentials, closely follow the Hofmeister series for anions (14). For example, surface tension increments (STIs; differences between the surface tension of a salt solution and that of neat water) of sodium salts at the same concentration decrease in the order: SO 4 2− > Cl − > Br − > NO 3 − > I − (15, 16). Molecular dynamics (MD) simulations have predicted that the propensity of anions to adsorb to the solution-vapor interface follows the Hofmeister series in reverse (7,14,17), and this prediction has largely been confirmed experimentally (14,(18)(19)(20)(21)(22). Moreover, MD simulations have shown that, with few exceptions [e.g., SO 4 2− in (NH 4 ) 2...