Vapor Interface

Article pubs.acs.org/JPCC

Characterization of the Acetonitrile Aqueous Solution/Vapor Interface by Liquid-Jet X‑ray Photoelectron Spectroscopy Kathryn A. Perrine,† Marijke H. C. Van Spyk,† Alexandria M. Margarella,† Bernd Winter,‡ Manfred Faubel,§ Hendrik Bluhm,∥ and John C. Hemminger*,† †

Department of Chemistry, University of CaliforniaIrvine, Irvine, California 92697, United States Joint Laboratory for Ultrafast Dynamics in Solutions and at Interfaces, Helmholtz-Zentrum Berlin für Matrialien und Energie, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany § Max-Planck-Institut für Dynamik und Selbstorganisation, Bunsenstrasse 10, D-37073 Göttingen, Germany ∥ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

ABSTRACT: We report photoelectron spectroscopy measurements from binary acetonitrile−water solutions, for a wide range of acetonitrile mole fractions (xCH3CN = 0.011−0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH3CN from aqueous surface and bulk solution, we quantify CH3CN’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, xCH3CN, from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH3CN surface concentration, based on analysis of the relative amount of gas- versus liquid-phase CH3CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid- and gas-phase CH3CN, which is sensitive to the change in solution surface potential and thus sensitive to the surface concentration of CH3CN. Gibbs free energy of adsorption values are compared with previous literature estimates, and we consider the possibility of CH3CN 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 CH3CN surface monolayer, and discuss the implications for orientation of CH3CN molecules relative to the solution surface.

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To our knowledge, the present work reports the first PE spectroscopy study of a binary highly volatile solution studied over a wide range of concentrations. Here, we chose to investigate acetonitrile (H3C−CN)−water solutions because of their great importance in many areas of chemistry, including synthesis and electrochemistry. This particular system is also challenging because of the unique and not well understood vapor/solution interfacial structure. Various physical and chemical properties of CH3CN−water mixtures, with the two components being mutually soluble at any ratio, have been characterized for both the bulk solution24−33 and the solution interface,34−41 and yet, important questions regarding the exact solution structure, especially at the interface, remain unresolved. With PE spectroscopy, we apply a new technique for the spectroscopic characterization of the acetonitrile−water solution/vapor interface, which has previously been exclusively investigated by nonlinear optical spectroscopy (see below).

INTRODUCTION Experimental 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 vacuum1−3 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 © XXXX American Chemical Society

Special Issue: John C. Hemminger Festschrift Received: June 16, 2014 Revised: August 8, 2014

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dx.doi.org/10.1021/jp505947h | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. (Left) Cartoon of the molecular interactions and structure at and near the surface of acetonitrile−water solutions. Regions I−III illustrate interfacial structure characteristic as explained in the text. Main trends are the completion of the CH3CN surface monolayer and loss of hydrogen bonding between II and III, possible bilayer formation in II, and CH3CN-rich multilayer region of unknown structure in III. Gas molecules indicate the increasing CH3CN partial pressure when increasing the mole fraction, xCH3CN. (Right) Schematic of electron probing depth for short electron inelastic mean free path (dashed line) having 1−2 layers sensitivity, which probes the surface of solution. The electron probing depth for large electron inelastic mean free path (solid line) represents sensitivity for the bulk of solution.

reproduce the findings of most studies indicating that some structure changes occur near xCH3CN = 0.2 and 0.6, most likely associated with the completion of the CH3CN surface monolayer and buildup of a CH3CN-rich, possibly ordered multilayer solution interface, respectively. In the present work we explore and discuss the above described structural features of the vapor/CH3CN−water solution interface, over the entire range of xCH3CN, qualitatively summarized in Figure 1, from the perspective of combined gas and liquid phase core-level PE spectroscopy. Our main goal is the quantitative determination of the concentration of acetonitrile at the solution surface, in the following typically expressed by the surface mole fraction, xSCH3CN, that allows for an estimate of the adsorption free energy, ΔGCH3CN. Here we pursue different experimental approaches, based on (1) an evaluation of PE signal intensities, (2) measured PE core-level energies, and (3) a partial pressure measurement of CH3CN, each as a function of bulk mole fraction, xCH3CN. Using the energies of the PE core-level peaks we also determine the magnitude of the net surface dipole associated with the CH3CN monolayer, which can be used to estimate the average tilt angle of CH3CN molecules with respect to the solution surface normal. Furthermore, our measurements reveal PE signal and binding-energy variations coinciding with the regions I, II, and III of Figure 1 and will be discussed in the light of solution interfacial structure characteristics. The present study also provides the first comparative PE spectroscopy measurements from aqueous solutions in vacuum versus few-mbar (ambient) conditions, and can be considered a further experimental confirmation of (local) thermodynamic equilibrium under either condition, as discussed in the recent literature.4−6,45

The structure of the vapor/CH3CN−water solutions results from the interplay of multiple effects, including dipole−dipole interactions between CH3CN molecules, and the hydration of the cyano chromophore (CN). X-ray and neutron scattering experiments have provided evidence that the two solvents mix heterogeneously in the bulk liquid solution, forming clusters, in groups of 4−6 molecules. These clusters of molecules were suggested to be alternately aligned and oriented in an antiparallel configuration based on X-ray diffraction experiments, at concentrations between 0.3 and 0.6 mole fraction, xCH3CN.26,28,30,33,42 Whether analogous clustering also occurs at the vapor/CH3CN−water solution interface is rather unexplored. The only evidence comes from a theoretical study,39 reporting self-association of surface acetonitrile molecules, leading to cluster formation at the surface and from SFG studies.40 Nonlinear optical spectroscopy measurements,34−38,43 mostly using second harmonic (SHG) or vibrational sum frequency (SFG) generation and computational studies,24,39,44 reveal that CH3CN molecules are strongly adsorbed at the solution surface, thereby lowering the interface energy relative to the air/pure-water interface.34−37 How molecules are oriented at a given concentration,38 and this includes the preferred tilt angle that interfacial acetonitrile molecules may assume with respect to the solution surface normal, remains a matter of debate.24,34−40,44 Furthermore, at sufficiently low bulk-solution concentrations, xCH3CN < 0.07 (3.45 M CH3CN), interfacial CH3CN molecules are argued to be hydrogenbonded to water molecules through their nitrogen atom;36 CH3CN molecules are oriented with the methyl group pointing toward vacuum. Near xCH3CN = 0.2, approximately when the CH3CN surface monolayer is completed,37 the cyano groups of the topmost CH3CN molecules were suggested to break hydrogen bonds with water molecules.35,36 Loss of hydrogen bonding at higher concentration was argued to lead to less upright orientation of CH3CN molecules at the solution surface.34−37 At the same time, increasing dipole−dipole interactions among the interfacial CH3CN molecules lead to water molecules being pushed away from the interface, into the bulk solution.24,36,44 At xCH3CN ≈ 0.1 the interactions between acetonitrile and water in the solution interface were found to be largest.38 In Figure 1, we qualitatively depict the current view of the vapor/acetonitrile−water solution interface. Here we

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EXPERIMENTAL SECTION Nitrogen, carbon, and oxygen 1s photoemission (PE) measurements from acetonitrile aqueous solutions were performed utilizing a 20 μm sized liquid jet at the soft-X-ray U41 PGM undulator beamline of HZB BESSY II, Berlin, and a 24 μm liquid jet at the Molecular Environmental Sciences beamline (11.0.2) at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. In the Berlin experiments, the liquid microjet was operated at 10−5 mbar chamber background B



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proportional to e−z/IMFP, where z is the depth into solution at a constant attenuation density and IMFP is the electron inelastic mean free path.1,3,23 In previous works we have estimated probing depths of approximately 10−15 Å and 45−50 Å for the two situations.49 Throughout the text, we use the terms surface and bulk PE studies when referring to the 100/200 and 600 eV measurements, respectively. One important goal of the present work is to infer relative amounts of CH3CN (through N 1s and/or C 1s PE signal) and H2O molecules (through O 1s signal), both at the solution surface and deeper into the solution, the latter being sufficiently representative of bulk solution. We thus determine the ratios of the N 1s (aq, bulk)/O 1s (aq, bulk) PE signal intensities or the N 1s (aq, surf)/N 1s (aq, bulk) signal intensities as a function of concentration; the analogous analysis will be done for the C 1s PE signals. The assumption is that the ratio of the integrated peak area from two different species (or same species at different location) is a reasonably good measure of the species relative concentration in solution; however, a number of corrections are required. First, photoionization cross sections, σ(hν), must be taken into account. Since different photon energies must be applied to produce photoelectrons of the same KE from each atom, appropriate photon-energy dependent cross sections must be used. Since experimental ionization cross sections are unknown in aqueous solution, tabulated theoretical values for atomic ionization50 have been used in this work. In previous experimental studies we have shown that this procedure results in accurate relative concentrations for the bulk solution, for example, 1:1 anion-to-cation ratios for simple salt aqueous solutions such as KI(aq) or KBr(aq).7,12,49,51−53 In addition, scattering of photoelectrons in solution will inevitably broaden the angular distribution, the result being that electron angular distributions will be more isotropic as compared to the ones one would measure in the gas phase, leading to smaller anisotropy parameters, β(hν).23 However, as for the photoionization cross sections, β parameters are unknown for aqueous solutions (and also for the gas phase), and once again one has to rely on approximations. For the present work we argue that for KE > 100 eV, β can be expected to be sufficiently constant for ionization of 1s orbitals (compare Thurmer et al.23); the exact value of β should be of minor importance for the present work where we consider signal ratios of 1s PE peaks, obtained at the same detection angle, which are different though for our two experimental setups. Note that we make the assumption that 1s ionization of CH3CN(aq) and of water(aq) are characterized by the same β. Finally, photon flux (recorded through electron current in the synchrotron storage ring, and by a photodiode) and the transmission of the X-rays through gasphase water and acetonitrile must be taken into account. All effects, taken together then yield the particular ratio of interest, e.g., N 1s (aq, surf)/N 1s (aq, bulk), representing the relative amount of the CH3CN molecules at the surface versus bulk solution. Core-level 1s binding energies reported in this study were determined based on water O 1s and 1b1 (valence) binding energies reported in previous works.3,54 We use the ALS data also for the determination of absolute core-level energies for spectra measured at BESSY. Relative energy changes, occurring for instance when varying concentration, are equally accurate though at each experimental station, and these energy differences are of primary interest in the current work.

pressure. In contrast, in the ALS experiments the jet was operated at approximately 1.3 mbar. The multiple differential pumping stages of the electron-energy analyzer at the ALS6,45−47 enable us to extend measurements to considerably higher CH3CN concentrations than for the setup at BESSY. The vapor pressure for neat acetonitrile solution at 6 °C is 32 mbar, compared to 9.3 mbar for neat water.48 Note also that in previous liquid-jet PE measurements solute evaporation has typically been of no concern since only water evaporation was significant. The main effect of the backing pressure (translating to stronger pumping in the BESSY experiment) is the large amount of gas-phase signal in the PE spectra measured at ALS, and the overall lower liquid-phase signal due to increased number of scattering events for photoelectrons in the 1.3 mbar case. Other reasons for the larger gas-phase signal in the ALS experiment are the aforementioned larger diameter of the liquid jet and the larger focus of the X-ray beam. Details of the BESSY II experiment and setup have been described previously.3,20,21 Briefly, the liquid microjet was cooled to 6 °C and run at a constant flow rate of 0.50 mL/min. The jet position was oriented normal to both the synchrotronlight polarization vector and the detection axis of the hemispherical electron analyzer. The distance between liquid jet and analyzer was 0.3 mm. For the photon energies used in this work (390−1140 eV) the energy resolution of the beamline was approximately 200 meV for the lower photon energies, and approximately 450 meV for the higher ones; the energy resolution of the electron analyzer was 150 meV using a 20 eV pass energy and constant kinetic energy detection mode. Given the small focal size of the X-ray beam, 23 × 12 μm2, the gas-phase spectra (for nonvolatile solutes) typically is less than 5% of the solution signal-intensity. The PE-spectroscopy experiments at the ALS were operated in a 1.3 mbar H2O/CH3CN gas-phase environment, using a similar liquid-jet setup as at BESSY, and the jet temperature was also 6 °C. The liquid jet was oriented normal to the entrance of the hemispherical analyzer, but the angle between the linearly polarized X-ray beam and the electron detection axis was 70°. This geometry corresponds to 20° detection angle (as compared to 90° in the Berlin experiment). The distance between the microjet and the electron analyzer entranceaperture was 0.2 mm. Details of the experimental station at ALS have been reported elsewhere.4−6,45,46 The energy resolution of the ALS beamline varied between 150 and 540 meV for the range of photon energies used (493−1135 eV), and the focal size of the photon beam was 50 × 60 μm2. The resolution of the hemispherical energy analyzer was better than 560 meV for 20 eV pass energy, used for all measurements reported here. Aqueous solutions were prepared by diluting pure acetonitrile (>99.93%, Sigma-Aldrich) in highly demineralized water (conductivity ∼ 0.2 μS/cm, 18.2 MΩ, deionized water). Acetonitrile concentrations 0.58 to 10.74 M (xCH3CN = 0.011−0.30) were studied in experiments at BESSY, and concentrations 1.37 to 18.4 M (xCH3CN = 0.025−0.90) were studied at the ALS. In order to probe the solution interface, photon energies were chosen to yield photoelectrons with 100 or 200 eV kinetic energy (KE) which corresponds to a minimum in electron mean free path.1,22 For comparison, we also report measurements probing deeper into solution, and this is realized here by recording PE spectra for 600 eV photoelectrons (see, e.g., refs 1 and 23). The two situations are illustrated at the right side of Figure 1. Note that the measured PE signal intensity is C



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RESULTS AND DISCUSSION Photoelectron Spectra. Figure 2 shows representative surface N 1s PE spectra, at xCH3CN = 0.2, obtained at BESSY

which have similar C 1s binding energies. The BE difference for these two groups in the gas phase is only 0.3 eV.56 For our data we obtain good fits when using single Gaussians for the gas phase and the solution phase, for both the surface and the bulk PE peak. Surface Mole Fraction and Surface Adsorption Energy. In order to connect to the issues of the vapor/ CH3CN−water interface structure outlined in the Introduction (see also Figure 1), we first determine the ratios of the N 1s (aq)/O 1s (aq) and of C 1s (aq)/O 1s (aq) PE signal intensities as a function of acetonitrile concentration. Both ratios, when obtained for the KEs as described above, are an equal measure of the relative acetonitrile to water concentration. In Figure 3, N 1s surface measurements (N 1s (aq, surf)/O 1s

Figure 2. N 1s photoelectron spectra from aqueous acetonitrile solution, at 0.2 mole fraction. Top tier are data for the liquid jet probed in a 1.3 mbar water atmosphere (performed at ALS), and the bottom tier presents analogous measurements at 2 × 10−5 mbar (performed at BESSY). Photon energy was 598 eV (ALS) and 505 eV (BESSY), representative for surface probing. Dots are the experimental data, and the lines are Gaussian fits for solution and gas-phase CH3CN, respectively. The inset shows the full spectrum obtained at the ALS.

(using 503 eV photon energy; electron kinetic energies of 100 eV) and at the ALS (using 598 eV photon energy; electron kinetic energies of 190 eV), respectively. Each spectrum exhibits two peaks that identify CH3CN in solution and in the gas phase, respectively. Intensities are scaled to yield the same peak height of the low-energy (solution) peak. The larger signal-to-noise level as well as the smaller gas-phase contribution in the BESSY data is typical for the present work as was explained in the Experimental Section. Subtracting a Shirley background and fitting each peak by a Gaussian reproduces the experimental spectra well. The respective position of maxima of the Gaussian peaks provide the N 1s electron binding energies, BE = hν − KE. Using known values of liquid water O 1s and valence band 1b1 binding energies3,54,55 (see Experimental Section) we obtain N 1s BE of 404.8 eV for acetonitrile in water. The approximately 1 eV lower BE compared to the gas-phase56 results from solvation effects, including polarization screening of the surrounding water molecules and is characteristic of what we have observed for other organic solutes in aqueous solution.12 The peak width (full width at half-maximum, fwhm) for CH3CN(aq) is ≈1 eV, which is typical for the energy distribution due to the different solvation configurations occurring in the aqueous solution. The observed fwhm for gas-phase acetonitrile is consistent with values reported in the literature.56 Note that identical BEs and peak widths are obtained for the ALS and BESSY measurements. Measurements analogous to those presented in Figure 2 were also performed for carbon 1s ionization (not shown). We make qualitatively similar observations regarding the gas-to-solution PE signal ratios. However, the spectral analysis, in terms of an accurate determination of BEs, is complicated by the fact that the PE signal contains contributions from CN and CH3 groups,

Figure 3. N 1s (aq)/O 1s (aq) and C 1s (aq)/O 1s (aq) ratios obtained from the respective photoelectron signal intensities from acetonitrile−water solution as a function of bulk-solution mole fraction, xCH3CN. Ratios are shown for surface (using 100 and 200 eV photon energy, see text) and bulk-solution (using 600 eV photon energy) measurements. We also present results for both 1.3 mbar (ALS) and 2 × 10−5 mbar pressure (BESSY). Symbols: Red open squares (N 1s surface, ALS), red filled squares (N 1s surface, BESSY), black open diamonds (C 1s surface, ALS), black filled diamonds (C 1s surface, BESSY), blue open circles (C 1s bulk, ALS), and blue filled circles (C 1s bulk, BESSY). N 1s based ratios have been multiplied by 1.2, as explained in the text. Regions I, II, and III refer to the trends shown presented in Figure 1. The inset displays an enlargement of the main plot for low mole fractions.

(aq, surf)) are presented by red squares (open for ALS, and filled for BESSY), and the analogous C 1s surface data are given by the black diamonds (open for ALS, and filled for BESSY). The respective ratios for the bulk-solution measurements are presented by the blue circles (open for ALS and filled for BESSY) and are only shown for C 1s data since values for N 1s are very similar. Indicated experimental uncertainties result from multiple measurements under nominally identical conditions. The large error bars at very high concentrations are due to the small water signal, leading to low signal-to-noise O 1s spectra from liquid water. Ratios in Figure 3 were normalized such that the largest C 1s (aq)/O 1s (aq) ratio is unity. In addition, experimental N 1s (aq)/O 1s (aq) ratios were multiplied by 1.2 D



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to obtain the same experimental ratios. The fact that we need to scale the N 1s(aq)/O 1s(aq) ratio by 20% to match the C 1s (aq)/O 1s (aq) ratio is a reasonable measure of the inaccuracy of some of the assumptions required for the quantitative analysis of relative PE peak areas as explained in the Experimental Section. One likely explanation is that at the different detection angles in the ALS and BESSY experiments differences in β for water O 1s and N 1s (C 1s) ionization may not be the same. As can be seen from Figure 3, the ratios for the surface measurements exhibit considerable alterations, occurring at the concentrations separating regions II from I and III from II (compare Figure 1). The small and slowly increasing ratios for xCH3CN < 0.2 (region I), shown enlarged in the inset to Figure 3, are consistent with CH3CN molecules replacing water molecules at the interface. Larger ratios for the surface than for bulk measurement point to the propensity of CH3CN for accumulating at the solution surface, in agreement with previous works (see Introduction). Another important observation from Figure 3 is that BESSY and ALS data are reasonably consistent, and this is expected for thermodynamic equilibrium. At xCH3CN ≈ 0.2, the N 1s (C 1s) (aq, surf)/O 1s (aq, surf) ratios begin to increase with increasing xCH3CN and tend to level off at xCH3CN ≈ 0.4, and then remain constant up to xCH3CN ≈ 0.6−0.7. We argue that the initial increase reflects the development and eventual completion of the CH3CN surface monolayer, accompanied by the subsurface adsorption of CH3CN, possibly forming a CH3CN bilayer. It is perhaps interesting to point out that SFG studies from vapor/acetone−water, acetonitrile−water solutions have indeed provided evidence for the formation of crystal-like sub-surface layers with both acetone and acetonitrile molecules in the second layer being oriented antiparallel with respect to the first layer.40,57 Finally, the pronounced but gradual increase of the N 1s (aq, surf)/O 1s (aq, surf) signal ratio for xCH3CN > 0.6, near the transition from region II to III, is indicative that the interface now predominantly consists of CH3CN molecules. We cannot make any specific assignment here as to the actual nature of this phase and its size (region III), and we are not aware of any other experimental work that would provide structure details for these high CH3CN concentrations. Yet, we can speculate that the structure may be crystal-like, as was discussed for vapor/acetone−water57 and the vapor/acetonitrile−water,40 or there may exist CH3CN clusters also in the top solution layers, analogous to clustering in the bulk solution.25,27−30,33,58 Figure 3 can reveal additional information on the vapor/ CH3CN−water interfacial solution structure when presenting the data as surface mole fraction, xSCH3CN. Assuming that our experiment is selective to probing only the surface molecular layer of the solution, xS would correspond to the (C 1s/2)/ ((C 1s/2) + O 1s) or the N 1s/(N 1s + O 1s) surface PE signal ratios. Both ratios are shown in Figure 4 for both measurements, at the ALS and at BESSY. We can then determine the Gibbs free energy of adsorption to the surface, ΔGads, using the Langmuir isotherm model (see refs 59 and 60). x SCH3CN = xCH3CN/⎡⎣xCH3CN + x H2O × e−ΔG / RT ⎤⎦

Figure 4. N 1s (aq, surf)/[N 1s (aq, surf) + O 1s (aq, surf)] and C 1s (aq, surf)/[C 1s (aq, surf) + O 1s (aq, surf)] ratios obtained from the respective photoelectron signal intensities from acetonitrile−water solution as a function of bulk-solution mole fraction, xCH3CN. Here we use the same surface measurements considered in Figure 3. Symbols: red open squares (N 1s surface, ALS), red filled circles (N 1s surface, BESSY), blue open circles (C 1s surface, ALS), blue filled circles (C 1s surface, BESSY). The blue line is a Langmuir isotherm fit to acetonitrile C 1s (open blue markers) based ratios for measurements collected at the ALS, yielding ΔGCH3CN = −3.4 ± 0.23 kJ/mol. We only show one of the Langmuir fits to the figure to illustrate how the data fit. Averaging all the adsorption energies from all the fits yields ΔGCH3CN = −2.85 ± 0.17 kJ/mol, which represents the bilayer well.

signal ratios from the ALS and the BESSY data, and from multiple PE measurements under nominally identical experimental conditions. We attribute the discrepancy between ΔG values for the two atomic probes to the fact that PE signal intensities are interpreted based on approximate values for β and σ (see Experimental Section). No experimental value has been reported for ΔGCH3CN, but it is interesting to point out that our largest values for ΔG from Figure 3 is approximately half the value found for acetone adsorption at the acetone−water solution surface, ΔG = −7.95 ± 0.84 kJ/mol,57 and it is rather similar to ΔG = −3.77 ± 0.84 kJ/mol, which was estimated for the adsorption of the acetone bilayer.57 Aqueous acetonitrile mixtures were measured by the same group and were observed to have similar adsorption free energies of ΔG = −7.11 ± 0.84 kJ/mol for the monolayer and ΔG = −5.02 ± 1.26 kJ/mol for the bilayer.40 This seems to indicate that our experimental probing depths are larger than one layer, perhaps 2−3, which is very reasonable.1,3,23 It should be noted that here we use the term one layer to refer to the thickness of the first layer of acetonitrile molecules at the interface. Because acetonitrile is about 2−3 Å in length, this would correspond to an approximately 3−5 Å depth into solution and, for 2−3 layers, corresponds to a 6−10 Å probe depth into solution. Our measurement leading to Figure 4 thus averages over two different surface adsorption processes, monolayer and bilayer formation, yielding ΔG value characteristic of the average of that for the first two layers. In Figure 3 we have observed noticeable changes in the acetonitrile to water ratio that coincide with crossing between regions I, II, and III of Figure 1. However, in order to more clearly demonstrate the concentration at which the surface CH3CN monolayer is completed (marking the beginning of region I) we present in Figure 5 a plot of the N 1s (aq, surf)/ N 1s (aq, bulk) PE signal and C 1s (aq, surf)/C 1s (aq, bulk)

(1)

Here, we express the bulk-water mole fraction, xH2O, by 1 − xCH3CN. Fitting the carbon data by eq 1 yields the blue curve in Figure 4, and we find ΔGads = ΔGCH3CN = −3.4 ± 0.23 kJ/mol. For the nitrogen data, we obtain ΔGCH3CN = −2.3 ± 0.24 kJ/mol (the corresponding Langmuir fit is not shown in the figure). Indicated uncertainties in ΔG result from both averaging the PE E



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liquid−solution interface, connected with CH3CN density changes, can be picked up with great sensitivity. It is then interesting to replot the data of Figure 6A in an analogous way

PE signal ratios as a function of xCH3CN, which is a more sensitive measure of the CH3CN surface excess. In fact, the shape of the N 1s curve (drawn to guide the eye to the trend, red curve) is almost identical to the surface-excess curve reported in the literature,36,38,39,57 leveling off near xCH3CN ≈ 0.2 when the topsolution surface is saturated by CH3CN molecules. The curve initially increases with increasing (bulk) acetonitrile concentration, peaks at a concentration at which the surface may reorient,34,35,37 and near xCH3CN ≈ 0.4, the curve flattens. This

Figure 5. N 1s (aq, surf)/N 1s (aq, bulk) (red squares) and C 1s (aq, surf)/C 1s (aq, bulk) (blue circles) ratios obtained from the respective photoelectron signal intensities from acetonitrile−water solution as a function of bulk-solution mole fraction, xCH3CN. Surface spectra were taken at 598 and 493 eV photon energy for N 1s and C 1s, respectively, and the corresponding spectra from bulk aqueous solution were recorded at 998 and 893 eV for N 1s and C 1s photon energies, respectively. Figure 6. (A) N 1s (gas)/O 1s (gas) ratios obtained from the respective photoelectron signal intensities from acetonitrile−water solution as a function of bulk-solution mole fraction, xCH3CN. N 1s and O 1s photoelectron spectra were measured at 598 and 735 eV photon energies, respectively. Labels I, II, and III refer to regions of solution structure changes; compare Figure 1. (B) N 1s (gas)/[N 1s (gas) + O 1s (gas)] ratios, and otherwise, same as in (A). The red line is a Langmuir isotherm fit to the data, yielding ΔGCH3CN = −5.9 ± 0.29 kJ/mol. Combining the C 1s and the N 1s Langmuir isotherm fits, we obtain an average ΔGCH3CN = −6.7 ± 0.21 kJ/mol.

suggests that acetonitrile density increases at the interface at low concentrations. As the surface begins to saturate with acetonitrile, around xCH3CN ≈ 0.2, the sublayers also fill in, xCH3CN ≈ 0.4, and the surface/bulk ratio is observed to flatten. This indicates that the surface and bulk concentrations begin to become homogeneous as the sublayers or multilayers are filled in. The respective C 1s curve (blue) exhibits similar behavior but we do not observe the maximum near xCH3CN ≈ 0.1, possibly as a result of the large uncertainty (error bars) of the C 1s experimental data of Figure 5. As aforementioned, we also explored alternative routes attempting to measure the acetonitrile surface mole fraction. One way is to analyze the change in local CH3CN pressure as a function of xCH3CN. This measurement determines the CH3CN partial pressure above the solution using the PE signal intensity from gaseous CH3CN relative to gas-phase water. Here we make the simplified assumption that the increase in acetonitrile partial pressure is considerably larger than the decrease of water pressure. Results from the ALS measurements are shown in Figure 6, where we present N 1s (gas)/O 1s (gas) PE signal ratios obtained for the same lower photon energies (505 and 640 eV) used to determine the respective N 1s (aq, surf)/O 1s (aq, surf) PE signal ratios of Figure 3. As can be seen, the ratios of Figures 6A and 3 show very similar behavior at the borders between regions I and II, and II and III. Since the vapor pressure of CH3CN is proportional to the density of CH3CN molecules in the outermost molecular layer of the solution, this finding implies that those structural alterations in the

as was done for Figure 4, that is, presenting N 1s (gas)/ [N 1s (gas) + O 1s (gas)] as a function of xCH3CN. This yields the change of CH3CN partial pressure with respect to the change of total pressure, from acetonitrile plus water, near the solution surface. The resulting curve, displayed in Figure 6B, indeed exhibits the form characteristic of a Langmuir adsorption isotherm for concentrations below 0.2 mf acetonitrile, as expected. Above this concentration, the partial pressure saturates at values between 0.8 and 0.9, below a value of 1.0 that would be expected for the highest acetonitrile mole fractions (i.e. diminishing gas phase water arising from the liquid jet). This difference is due to small contributions to the gas phase water signal from background water in the chamber. Curve fitting using eq 1 yields ΔGCH3CN = −5.9 ± 0.29 kJ/mol; if we fit the C 1s- and N 1s-based data combined (not shown), we obtain ΔGCH3CN = −6.7 ± 0.21 kJ/mol. The just mentioned behavior at the highest mole fractions (due to background water in the chamber) has a minor impact on the value of the ΔGCH3CN F



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concentrations the two sets of data are consistent, and fitting the Langmuir isotherm yields ΔGCH3CN = −8.1 ± 0.42 and −8.9 ± 0.14 kJ/mol, for data from the present study at the ALS and from French,61 respectively. And as expected, ΔG is in very good agreement with values inferred from gas-phase PE spectroscopy measurements (Figure 6B). Overall, large error bars (Figure 7A) for the jet measurements are due to the small differences in the position of the liquid jet with respect to the skimmer of the electron analyzer; this is the same effect contributing to experimental uncertainties indicated in Figure 6B. Our data do not seem to exhibit the pressure decrease at large mole fraction observed in French61 and would seem consistent with an increase of the interfacial thickness with increasing CH3CN concentration, as discussed in the literature,24 and with the higher volatility of acetonitrile, which would rather lead to Δp increase. Below we will inspect the possibility to obtain a reliable estimate of ΔGCH3CN based on the core-level binding energies (Figure 7B). We next consider the changes of core-level peak positions in our PE spectra, and we discuss the implications for the vapor/ CH3CN−water interfacial solution structure. Typically, peak shifts can be associated with changes in the core-level binding energies since these are sensitive to the local chemical environment,1,3 revealing details of the solvation-shell configuration of a solute.18,62 For a surface-active solute such chemical shifts may be masked though by energy shifts in the PE peaks due to the surface dipole resulting from oriented molecules at the solution surface. We will discuss both aspects since an unequivocal distinction is not possible at present. The procedure for obtaining absolute BEs in this study has been described briefly in the Experimental Section. Here we provide more detail on the approach we use to set the absolute electron energy scale, as it has implications for the interpretation of the data over the very wide range of acetonitrile concentrations we have studied. For experiments carried out with high electron kinetic energies (600 eV), which we refer to as bulk solution data, we set the energy scale by setting the O 1s peak for water at a binding energy of 538.1 eV.3 However, for experiments carried out with low electron kinetic energies (150 or 200 eV, corresponding to “surface” sensitive experiments), the photon energy required to obtain C 1s and N 1s spectra at these low kinetic energies is below the O 1s absorption edge. As a result, for these spectra we set the energy scale by setting (charge correcting) the 1b1 peak of the water valence band to an energy of 11.16 eV.3 The result is that the surface sensitive spectra and the bulk solution spectra have different binding energy references. At low acetonitrile concentrations, this will make little or no difference as the O 1s and the 1b1 peak energies of liquid water are well-defined and we assume that the water valence and core energies vary little with acetonitrile concentration. Only N 1s BEs will be considered here because of aforementioned complications associated with the unresolved C 1s contributions from the methyl and the cyano group. Figure 8 shows experimental N 1s BEs as a function of CH3CN concentration; data from ALS are presented by the open symbols, and those from BESSY are presented by filled symbols. We find that N 1s energies for the surface measurements (red markers) decrease from ≈405.2 to ≈404.1 eV, when going from lowest mole fraction to approximately xCH3CN ≈ 0.6. At larger concentrations, energies rise, reaching a BE value of 404.5 eV at xCH3CN = 0.8. N 1s BEs for the bulk solutions (blue markers) are observed to rather stay constant up to xCH3CN ≈ 0.5,

obtained from the Langmuir fit. These free energies are in agreement with the free energies estimated from the SFG studies.40 Importantly though, ΔG inferred from the pressure analysis is considerably larger that than obtained from the analysis of the respective PE peaks of the liquid phase (−2.3 to −3.4 kJ/mol, compare Figure 4). Following the argument above, that the probe depth for low-energy photoelectrons is 2−3 layers, corresponding to ∼10 Å, the gas-phase measurements leading to Figure 6B appear to be the more suitable surface probe, yielding a more accurate determination of xSCH3CN and, hence, of ΔGCH3CN. Note that a similar value as obtained from the gas-phase PE signal measurements results when fitting a Langmuir isotherm to the actual pressure increase detected by an ion gauge, shown in Figure 7A.

Figure 7. (A) Change in vapor pressure above the acetonitrile−water solution as a function of xCH3CN. Black crosses are from French61 and red squares (ALS) are the changes of CH3CN partial pressure, as derived from the measured changes of total pressure (acetonitrile plus water) in the present experiments. Data from ALS (red squares) are fit to a Langmuir isotherm (red curve), yielding ΔGCH3CN = −8.1 ± 0.42 kJ/mol and for French61 ΔGCH3CN = −8.9 ± 0.14 kJ/mol. (B) Plot of the difference between N 1s (gas) and apparent N 1s (aq) binding energies, ΔEgas‑aq for the same solutions as in (A), yields ΔGCH3CN = −8.8 ± 0.31 kJ/mol (ALS).

Here we present data from French,61 performed for a solution temperature of 35 °C, and from our own measurements, where the total pressure change, Δp, with respect to the pressure for the neat water jet, was detected by an ion gauge mounted in the first pumping stage of the electron analyzer. Displayed normalized pressure in Figure 7A refers to Δp divided by the maximum pressure or the partial pressure. Except for the very high G



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that CH3CN molecules are sufficiently well ordered at the surface, leading to a molecular surface dipole. Before we quantify this dipole, and use it for an estimate of the average orientation of acetonitrile molecules with respect to the solution surface, we compare N 1s BEs from the liquid and from the gas-phase, and we show that this BE shift is also a sensitive measure of xSCH3CN. It is important to note that binding energy shifts such as this do not depend on the absolute calibration of the electron energy scale and as such are not affected by the issues raised in the previous paragraph. The difference in N 1s binding energy between CH3CN molecules in the gas phase, immediately adjacent to the solution, and those in solution at the liquid/vapor interface, ΔE(N 1s)gas‑aq, provides a measure of the change of the potential of the liquid-jet surface with respect to ground potential. As we will argue below, this change of potential is directly connected with the surface dipole of the solution, and varies with the number of CH3CN molecules adsorbed at the surface. Hence, ΔE(N 1s)gas‑aq is expected to reasonably well correlate with the relative changes in xSCH3CN and is confirmed experimentally by Figure 7B, showing ΔE(N 1s)gas‑aq as a function of concentration. Very similar results are obtained for the ALS and BESSY measurements (both using 598 eV photon energy), once again confirming that the identical solution surface (structure) is probed under 10−5 and few mbar conditions. For the ALS data that extends to high enough concentrations, and clearly showing saturation, we can assume that the ΔEgas‑aq is proportional to the surface coverage of acetonitrile. Using a Langmuir isotherm to fit this data, we find ΔGCH3CN = −8.8 ± 0.31 kJ/mol, which is the same within our experimental uncertainty to that obtained for our other gas-phase based methods aiming at quantifying xSCH3CN discussed above. We conclude that the measurements based on analysis of N 1s, C 1s, and O 1s PE signal intensities from solution phase are least suited for finding xSCH3CN because the experiment is likely to probe 2−3 layers into solution. Yet, even though the experiment does not have monolayer sensitivity, our results would support the occurrence of a fairly strongly adsorbed CH3CN bilayer. This is inferred from the average low adsorption energy obtained from the C 1s and N 1s fits, ΔGCH3CN = −2.85 ± 0.17 kJ/mol (compare Figure 4). The gasphase based measurements explored here yield rather consistent results, and values for ΔGCH3CN are considerably larger, more typical for strong adsorption, giving an average ΔGCH3CN = −8.16 ± 0.14 kJ/mol. This corroborates each method’s specific surface sensitivity, either through the CH3CN equilibrium vapor pressure (see Figures 6B and 7A) or through the surface potential associated the number of (oriented) CH3CN molecules at the solution surface (see Figures 7B). It would be difficult to conclude though which method provides the most accurate determination of xSCH3CN and ΔGCH3CN. In the absence of any previously reported experimental values, it is perhaps interesting to point out that our values are comparable with ΔGCH3CN = −7.11 ± 0.84 kJ/mol for the first layer of acetonitrile in water and ΔGCH3CN = −5.02 ± 1.26 kJ/mol for the bilayer, obtained from SFG experiments.40 Structure and Solution-Surface Dipole. We next address the magnitude of the CH3CN surface dipole in more detail which can be approximated from both Figures 8 and 7B. From the data in Figure 8 we obtain its value as the difference in the

Figure 8. N 1s binding energies (BE) from acetonitrile−water solution as a function of bulk-solution mole fraction, xCH3CN. Results are shown for surface (using 598 eV ALS and 503 eV BESSY photon energy) and bulk (using 998 eV ALS and 1005 eV BESSY photon energy) solution photoelectron-spectroscopy measurements, performed at both ALS and BESSY. Symbols: N 1s (surf, ALS) (red open squares); N 1s (surf, BESSY) (red filled squares); N 1s (bulk, ALS) (blue open circles); N 1s (bulk, BESSY) (blue filled circles). Labels I, II, and III refer to regions of solution structure changes (compare Figure 1).

at which point energies rise from approximately 404.9 to 406.8 eV (near xCH3CN = 0.8), being considerably higher than the energies at the surface for the same bulk-solution concentration. This 1.9 eV rise in N 1s BE could be a signature of bulk-solution phase change, involving clustering (as observed in those bulk studies highlighted in the Introduction), at higher concentrations. One possible interpretation of the gradual decrease of surface N 1s BEs in region I is that, with increasing concentration, and hence, with increase of CH3CN surface density (Figure 3), fewer CH3CN become hydrated, which leads to small changes of the electron density at the nitrogen-atom site. The slightly larger N 1s BE of the hydrated cyano group as compared to the nonhydrogen-bonded moiety may thus be due to some electron density being pulled from nitrogen toward H2O. There are, however, additional effects at play in this concentration regime, especially owing to the increased mutual interaction of the oriented CH3CN molecules in the solution surface. Quantification of these contributions is not possible based on the available experimental data. As mentioned above, our approach that uses the water core level and valence band energies as the energy scale reference assumes that these energies for water are constant. While this is reasonable for low acetonitrile concentrations, at some point the solutions we are studying should be considered as water solute molecules solvated in the acetonitrile solvent. At such high acetonitrile concentrations it is reasonable to assume that the absolute binding energy of the O 1s core level and the water 1b1 valence band peak may vary significantly. Importantly, at high concentrations the effect of the acetonitrile concentration on the water core level binding energy and the valence band energies may be different. Thus, the differences in the trends for the surface data and the bulk solution data, shown in Figure 8 at high acetonitrile concentrations, may be mostly a measure of the changes in the molecular water electronic structure as a function of solvation with acetonitrile. Resolving these differences at high acetonitrile concentrations may be possible with the help of high level electronic structure calculations, but that is beyond the scope of the present experimental study. The interpretation of the apparent BE shifts even at low acetonitrile concentrations is yet further complicated by the fact H



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apparent N 1s BEs (for surface measurement) obtained at lowest xCH3CN and at xCH3CN = 0.2, which corresponds to the just completed CH3CN top surface layer. From a linear fit of the surface N 1s(aq) binding energies as a function of xCH3CN up to xCH3CN = 0.2, and extrapolation to zero xCH3CN, we find ΔEdipole = 0.23 eV is a reasonable estimate of the maximum shift. Figure 7B yields a slightly larger value, ΔEdipole = 0.4 eV, which is probably more accurate because the measurement is not affected by intrinsic BE shifts (see below). ΔEdipole in the present study can be considered analogous to the change of work function, Δϕ, occurring for crystalline surfaces upon adsorption of molecules, giving rise to a net average surface dipole. Since for CH3CN the surface dipole is oriented toward the vacuum side, that is, the positive end, −CH3δ+, pointing to the vacuum, and the negative end, −CNδ−, pointing into the solution, an electron emitted within the interface or below will be accelerated by the dipole field acquiring larger kinetic energy and appear at lower BE in the PE spectra.63 Hence, in the absence of effects causing true changes in BE one can use the observed energy shift, ΔEdipole, to estimate one of the quantities in the Helmholtz equation,64 ΔEdipole = e·m⊥·n/εr·εo (eq 2), provided all other quantities are known. Here, n is the number of acetonitrile molecules per unit area, m⊥ is the projected molecular dipole (averaging over all tilt angles of CH3CN with respect to the surface normal), e is the charge of an electron, εr is the relative dielectric constant, for acetonitrile (37.5εo at 20 °C),48 and εo is the vacuum dielectric constant. Even though m⊥ and n are only approximately known, our experimental data would seem to support CH3CN molecules being preferentially oriented normal to the surface or perhaps somewhat inclined toward the solution surface. To be specific, with ΔEdipole = 0.36 eV, and assuming a typical surface coverage of 1 × 1015 molecules/cm2, eq 2 yields m⊥ = 3.6 D, which is right in between the values reported for the molecular dipole moment of CH3CN, 3.92 D,65 3.4 D66 to 4.0 D.44 For our largest value, ΔEdipole = 0.4 eV, we find m⊥ = 3.98 D, which is larger than the molecular dipole moment given by Steiner and Gordy.65 Even for the smaller ΔEdipole = 0.3 eV determined from experiment, we obtain m⊥ = 2.9 D, which corresponds to a change in angle of ≈40 degrees with respect to initial orientation of acetonitrile at low coverage. On average, our values thus appear to be in better agreement with a 25 degrees change reported by Kim et al.38 as compared to 70 degrees from Eisenthal and co-workers.34,35,37 However, the surface coverage of 4 × 1014 molecules/cm2 reported in the same study35 yields a far too large dipole moment, and hence, this surface coverage appears to be too low. XPS results cannot reveal the orientation of acetonitrile at the air/liquid interface, but MD simulations can aid in resolving the orientation and crystal-like sublayers that have been suggested by Wang and co-workers,40 and whether the interface behavior of acetonitrile influences the phenomena in the bulk.

estimate the adsorption free energy of CH3CN at the aqueous solution surface, which is found to be in the range of ΔGCH3CN = −8.16 ± 0.14 kJ/mol for the monolayer, measured by the partial pressures of solution and the dipole shift in BEs. Values vary somewhat for the different methods based on CH3CN partial pressure measurements (through N 1s and C 1s gas-phase PE signal) and based on change in the solution surface potential (through photoelectron kinetic energy shifts between gas and liquid phase CH3CN), indicating slightly different sensitivity to the surface and, hence, to probing the CH3CN surface concentration. Our N 1s and C 1s PE spectra from the solution reveal a much smaller value of ΔG which is attributed to probing slightly deeper into the solution, and the smaller ΔG of −2.85 ± 0.17 kJ/mol is thus suggested to represent an average value for the formation of the CH3CN mono- and bilayer. We have also characterized the solution surface dipole associated with adsorbed CH3CN molecules through the energy shift of photoelectron peak position which enabled an estimate of the surface molecules’ orientation. Our results suggest a rather upright orientation in agreement with recent SFG measurements.38,40 Surface structural changes, as a function of CH3CN bulk-solution concentrations, which have been addressed in previous works,24,34−37 are reflected qualitatively in our measurements. For instance, the concentration at which the surface monolayer is completed (near xCH3CN = 0.2) is reflected in both the relative photoelectron signal intensities (from gas phase and liquid phase) as well as in electron binding energies. Our data also supports an interfacial structure change occurring near xCH3CN = 0.6, that is, after completion of the bilayer. The exact nature of this CH3CN-rich interface, most likely extending beyond the second layer, cannot be resolved based on the present data. Hence, one important question that remains to be answered is whether molecular self-association and clustering, which leads to microinhomogeneity in bulk aqueous acetonitrile−water solutions, also influences the surface/interfacial structure. We are currently investigating new methods of molecular-dynamics simulations on these solutions to clarify the surface orientation. The present photoelectron spectroscopy experiment has proven to be highly complementary to nonlinear optical methods for probing the vapor/liquid interface of highly volatile solutions and will motivate similar studies on, for example, other binary liquid mixtures, such as acetone−water. Finally, our measurements, which were performed under different vacuum conditions, with background pressures of 1.3 and 2 × 10−5 mbar, are concluded to probe identical structures of the top few layers of the acetonitrile−water solution, demonstrating (local) thermodynamic equilibrium between liquid and gas phase for the liquid-jet parameters used here.

CONCLUSIONS Liquid-microjet photoelectron spectroscopy is demonstrated to provide insight into the interfacial behavior, both in terms of electronic and geometric structure, of high-vapor pressure solvents mutually miscible with water. The present study has focused on the vapor/acetonitrile−water interface, and through a consideration of CH3CN nitrogen and carbon 1s photoelectron signal (in comparison with water oxygen 1s signal) from both the solution and the gas phase, we are able to estimate the CH3CN surface concentration as a function of CH3CN bulksolution concentration (or mole fraction, xCH3CN). This is used to

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Grant No. CHE 0909227 and the AirUCI program at the University of California, Irvine. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Additional support from the Deutsche Forschungsgemeinschaft (Project WI 1327/3-1) I



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(18) Lewis, T.; Winter, B.; Stern, A. C.; Baer, M. D.; Mundy, C. J.; Tobias, D. J.; Hemminger, J. C. Dissociation of Strong Acid Revisited: X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations of HNO3 in Water. J. Phys. Chem. B 2011, 115 (30), 9445− 9451. (19) Cheng, M. H.; Callahan, K. M.; Margarella, A. M.; Tobias, D. J.; Hemminger, J. C.; Bluhm, H.; Krisch, M. J. Ambient Pressure X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulation Studies of Liquid/Vapor Interfaces of Aqueous NaCl, RbCl, and RbBr Solutions. J. Phys. Chem. C 2012, 116 (7), 4545−4555. (20) Margarella, A. M.; Perrine, K. A.; Lewis, T.; Faubel, M.; Winter, B.; Hemminger, J. C. Dissociation of Sulfuric Acid in Aqueous Solution: Determination of the Photoelectron Spectral Fingerprints of H2SO4, HSO4−, and SO42‑ in Water. J. Phys. Chem. C 2013, 117 (16), 8131−8137. (21) Lewis, T.; Winter, B.; Stern, A. C.; Baer, M. D.; Mundy, C. J.; Tobias, D. J.; Hemminger, J. C. Does Nitric Acid Dissociate at the Aqueous Solution Surface? J. Phys. Chem. C 2011, 115 (43), 21183− 21190. (22) Hufner, S. Photoelectron Spectroscopy: Principles and Applications, 3rd ed.; Springer: New York, 2010. (23) Thurmer, S.; Seidel, R.; Faubel, M.; Eberhardt, W.; Hemminger, J. C.; Bradforth, S. E.; Winter, B. Photoelectron Angular Distributions from Liquid Water: Effects of Electron Scattering. Phys. Rev. Lett. 2013, 111 (17), 173005−173005. (24) Paul, S.; Chandra, A. Liquid-Vapor Interfaces of WaterAcetonitrile Mixtures of Varying Composition. J. Chem. Phys. 2005, 123 (18), 184706. (25) Takamuku, T.; Tabata, M.; Yamaguchi, A.; Nishimoto, J.; Kumamoto, M.; Wakita, H.; Yamaguchi, T. Liquid Structure of Acetonitrile-water Mxtures by X-ray Diffraction and Infrared Spectroscopy. J. Phys. Chem. B 1998, 102 (44), 8880−8888. (26) Takamuku, T.; Noguchi, Y.; Yoshikawa, E.; Kawaguchi, T.; Matsugami, M.; Otomo, T. Alkali Chlorides-induced Phase Separation of Acetonitrile-Water Mixtures Studied by Small-Angle Neutron Scattering. J. Mol. Liq. 2007, 131, 131−138. (27) Takamuku, T.; Matsuo, D.; Yamaguchi, A.; Tabata, M.; Yoshida, K.; Yamaguchi, T.; Nagao, M.; Otomo, T.; Adachi, T. Small-angle Neutron Scattering Study on Aggregation in Acetonitrile-D2O and Acetonitrile-D2O-NaCl Mixtures. Chem. Lett. 2000, 8, 878−879. (28) Takamuku, T.; Noguchi, Y.; Nakano, M.; Matsugami, M.; Iwase, H.; Otomo, T. Microinhomogeneity for Aqueous Mixtures of WaterMiscible Organic Solvents. J. Ceram. Soc. Jpn. 2007, 115 (1348), 861− 866. (29) Bako, I.; Megyes, T.; Palinkas, G. Structural Investigation of Water-Acetonitrile Mixtures: An Ab Initio, Molecular Dynamics and X-ray Diffraction Study. Chem. Phys. 2005, 316 (1−3), 235−244. (30) Bako, I.; Megyes, T.; Grosz, T.; Palinkas, G.; Dore, J. Structural Investigation of Water-Acetonitrile Mixtures: Small-Angle and WideAngle Neutron Diffraction Study Compared to Molecular Dynamics Simulation. J. Mol. Liq. 2006, 125 (2−3), 174−180. (31) Mountain, R. D. Molecular Dynamics Study of WaterAcetonitrile Mixtures. J. Phys. Chem. A 1999, 103 (50), 10744−10748. (32) Mountain, R. D. Microstructure and Hydrogen Bonding in Water-Acetonitrile Mixtures. J. Phys. Chem. B 2010, 114 (49), 16460− 16464. (33) Takamuku, T.; Noguchi, Y.; Matsugami, M.; Iwase, H.; Otomo, T.; Nagao, M. Heterogeneity of Acetonitrile-Water Mixtures in the Temperature Range 279−307 K Studied by Small-Angle Neutron Scattering Technique. J. Mol. Liq. 2007, 136 (1−2), 147−155. (34) Zhang, D.; Gutow, J. H.; Eisenthal, K. B.; Heinz, T. F. Sudden Structural Change at an Air Binary−Liquid Interface - Sum Frequency Study of the Air Acetonitrile−Water Interface. J. Chem. Phys. 1993, 98 (6), 5099−5101. (35) Zhang, D.; Gutow, J. H.; Eisenthal, K. B. Structural Phase Transitions of Small Molecules at Air/Water Interfaces. J. Chem. Soc., Faraday Trans. 1996, 92 (4), 539−543.

is also gratefully acknowledged. We would like to thank Robert Seidel and Stephan Thürmer for technical help at the BESSY facility, Helmholtz Zentrum Berlin. We would also like to thank Andrey Shavorskiy for help with beam alignment at the ALS at LBNL.

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