Ambient Pressure X-ray Photoelectron Spectroscopy and Molecular

ARTICLE pubs.acs.org/JPCC

Ambient Pressure X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulation Studies of Liquid/Vapor Interfaces of Aqueous NaCl, RbCl, and RbBr Solutions Ming Hsin Cheng,† Karen M. Callahan,† Alexandria M. Margarella, Douglas J. Tobias,* and John C. Hemminger* Department of Chemistry and AirUCI, University of California at Irvine, Irvine, California 92697, United States

Hendrik Bluhm Lawrence Berkeley National Laboratory, Mail Stop 6R2100, One Cyclotron Road, Berkeley, California 94720, United States

Maria J. Krisch Department of Chemistry, Trinity College, Hartford, Connecticut 06106, United States ABSTRACT: 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
/Owater ratio determined from the XPS measurements, surface-enhanced concentrations of Cl
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.

’ INTRODUCTION Specific ion effects at the liquid/vapor interface play an important role in biochemical processes and atmospheric chemistry.1 For example, a proposed mechanism for the formation of molecular chlorine in the atmosphere involves heterogeneous reactions of OH radicals and chloride anions at the interface of aqueous NaCl aerosols.2 The behavior of ions near the surface of salt solutions has been studied for several decades, and the conventional wisdom held that ions are repelled from the liquid/vapor interface due to electrostatic image forces.3 However, Jungwirth and Tobias4,5 studied the behavior of ions at the interface with classic molecular dynamics (MD) simulations employing polarizable force fields and predicted that the larger, more polarizable halide anions are present at the liquid/vapor interface. The degree of halide anion surface enhancement in the salt solutions increases with increasing anion size and polarizability: I
> Br
> Cl
> F
. The investigation of the mechanism causing halide anion surface enhancement is still a topic of broad r 2011 American Chemical Society

interest.6 Halide anion surface propensity at the liquid/vapor interface not only depends on properties of the halide but also can be affected by other environmental factors, such as the solvent. For example, molecular dynamics simulations predict that the large iodide surface enhancement present in aqueous NaI solutions does not occur in methanol solution.7 Owing to the requirement of electroneutrality, which is expected to be achieved within a few molecular diameters of the aqueous surface, positively charged species, such as cations, must be present in the vicinity of an enhanced population of anions, such as near the liquid/vapor interface. The effect of cation size on the stability of halide anions at the aqueous liquid/ vapor interface has not previously been studied in detail. Cations are generally far less polarizable than anions and tend to be Received: June 12, 2011 Revised: December 23, 2011 Published: December 27, 2011 4545

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The Journal of Physical Chemistry C repelled from the surface of water; however, ion size may still play a role in determining the preferred location of cations near the liquid/vapor interface, which may, in turn, affect the behavior of their counteranions. Although the role of cations on the interfacial structure of aqueous solutions has not been studied directly, insight can be gained from the current understanding of the effect of ions on the organization of water molecules. Ions can be divided into two classes based on their binding strength with water molecules: kosmotropes and chaotropes. Collins et al.8 pointed out that water molecules bind tightly to kosmotropes (small ions) as a consequence of the high charge density of small ions, whereas water molecules bind weakly to chaotropes (large ions) due to the low charge density of large ions. Consequently, the structure of water molecules is dominated by either ion
water or water
water interactions. Hribar et al.9 performed Monte Carlo simulations to study the arrangement of water molecules around different sized cations. The results indicated that large cations would accommodate the formation of water
water hydrogen bonds due to low charge densities whereas the strong electrostatic forces resulting from small cations would disrupt the formation of water
water hydrogen bonds. For instance, they calculated that the average number of hydrogen bonds per water molecule formed around rubidium ions (chaotropes) and sodium ions (kosmotropes) are 2.1 and 1.8, respectively. Kollman et al.10 carried out ab initio calculations to determine cation hydration energies, and they reported that smaller cations interact more strongly with water molecules. Lee et al.11 used MD simulations to explore the cation
water systems and showed that the coordination numbers of water molecules in the first solvent shell of cations increase with increasing cation size. The aforementioned studies have revealed that the size of a cation affects the interaction between cations and water molecules. Moreover, cation size might be expected to affect anion behavior at the liquid/ vapor interface. In previous experimental studies, the enhanced anion concentration near the surface of salt solutions has been observed by several techniques. For example, Liu et al.12 used vibrational sum frequency generation spectroscopy (SFG) to examine the liquid/ vapor interface of sodium halide solutions, and their results indicated that water structure at the interface of sodium bromide or sodium iodide is distorted more strongly than that of sodium chloride or sodium fluoride, suggesting that the role of halide anions at the interface increases with respect to increasing size and polarizability. Petersen et al.13,14 used femtosecond second harmonic generation (SHG) spectroscopy to verify an enhanced azide and thiocyanate concentration at the aqueous liquid/vapor interface. Furthermore, previous work in our group has examined the ion compositions at the interface of KI, KBr, and KF solutions using ambient pressure X-ray photoelectron spectroscopy (APXPS) in conjunction with an energy-tunable synchrotron X-ray source. This technique allows us to study ion compositions at the liquid/vapor interface as a function of photoelectron kinetic energy, which is a measure of probe depth, in a salt solution. Not only was an anion surface enhancement observed at the liquid/ vapor interface of the heavier halide salt solutions, but the results also confirmed that salt solutions with larger, more polarizable anions exhibited more anion surface enhancement.15 Previous AP-XPS studies of an aqueous NaCl solution doped with bromide, which showed that Br
is significantly enhanced relative to Cl
at the interface of such solutions, have been reported by Ghosal et al.16 We also used this technique to explore an aqueous KF solution,

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showing that there is no anion surface enhancement for an aqueous KF solution.17 Thus far, we have focused on specific anion effects on halide anion distributions at the liquid/vapor interface of salt solutions containing a given cation. In this study, neat NaCl and RbCl aqueous solutions are examined using AP-XPS in order to study specific cation effects on the interfacial distributions of chloride at the liquid/vapor interface of salt solutions. Chloride species are the major component of seawater18 and play a significant role in the chemistry of sea salt aerosols in the marine boundary layer, such as the reaction products of the photolysis of ozone and chloride anions.19,20 Recent measurements have shown that chloride also plays a role in inland atmospheric chemistry.21 Investigating the chloride ion distributions at the interface of alkali chloride aqueous solutions will contribute to a better understanding of heterogeneous chemical reactions occurring on atmospheric aerosols. In addition to AP-XPS measurements, we report classical MD simulations employing polarizable force fields to elucidate specific cation effects on the interfacial composition of alkali chloride solutions. Ion distributions in aqueous RbBr solutions were also investigated to gain additional insight into the effects of cations on anion surface propensity. Previous AP-XPS experiments and simulations with NaBr and KBr solutions have shown that the bromide concentration is enhanced at the liquid/vapor interface relative to that in the bulk.15,16 On the other hand, X-ray reflectivity (XRR) measurements on concentrated salt solutions, including RbBr, were interpreted by Sloutskin et al.22 in terms of a depletion of Br
near the surface of aqueous RbBr solution. In light of the disparity between previous XPS experiments and MD simulations on NaBr and KBr and the XRR experiments on RbBr, we also report here an investigation of the interfacial composition of aqueous RbBr solutions using AP-XPS and MD simulations employing polarizable force fields.

’ EXPERIMENT Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS). The experiments were performed at the Molecular En-

vironmental Sciences beamline (11.0.2) of the Advanced Light Source at Lawrence Berkeley National Laboratory.23 The energytunable nature of synchrotron X-rays at the Advanced Light Source has allowed us to obtain AP-XPS spectra for each element in the sample over a wide range of photoelectron kinetic energies. The kinetic energies of the emitted photoelectrons depend upon the energies of the incident X-rays and identities of the atoms and orbitals from which they are emitted. As the emitted photoelectrons pass through the solution, they can be scattered elastically and inelastically by bound electrons. Elastic scattering is thought to be very weak.24 The inelastic mean free path (IMFP), which is the distance that photoelectrons travel before they lose energy from inelastic collisions, depends upon the kinetic energy of the photoelectron: photoelectrons with higher kinetic energy can reach the detector from deeper in solution than photoelectrons with lower kinetic energy. Therefore, photoelectrons with lower kinetic energy have a greater surface bias, and this is the origin of the “depth profiling” aspect of our experiment. However, the relation between the absolute IMFP values and photoelectron kinetic energies in aqueous solutions is still a subject of ongoing current research.24 The details of the AP-XPS experimental apparatus have been described elsewhere.25,26 Briefly, AP-XPS experiments are 4546

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The Journal of Physical Chemistry C generally performed with the sample at pressures up to a few Torr in the sample chamber. The apparatus utilizes a differentially pumped electrostatic lens, which is set between a hemispherical electron energy analyzer and the sample chamber. Photoelectrons are focused using the electrostatic lens system in order to reduce the loss of photoelectron signal intensity. Additionally, the collection aperture, which is the entry point for photoelectrons, is brought close to the sample surface (to a distance of about 0.5 mm). The small distance between the collection aperture and the sample surface reduces the scattering of photoelectrons with gas molecules in the high-pressure region. In this study, single-crystal RbCl (100) and RbBr (100) samples were purchased from MaTeck GmbH and single-crystal NaCl (100) samples were purchased from Hilger Crystals. A single-crystal sample was cleaved in air before mounting it on a sample holder. In a typical experiment, a freshly cleaved single crystal sample was transferred to the sample chamber at ultrahigh vacuum (10
9 Torr), and water vapor was subsequently added to a pressure of 1.6 Torr in the sample chamber, resulting in the relative humidity (RH) of about 7%. Before deliquescence, XP spectra of Cl (2p), Rb (3d), Na (2s), O (1s), and C (1s) were acquired for the RbCl and NaCl samples from lower kinetic energy (KE) of 200 eV to higher kinetic energy (KE) of 600 eV. XP spectra of Br (3d), Rb (3d), O (1s), and C (1s) were acquired for RbBr samples from lower KE of 125 eV to higher KE of 650 eV. Halide spectra were acquired first because there is beam damage from the incident X-ray for halogen elements. We minimized the effect of beam damage on the halide spectra by taking each set of spectra at a new spot of the sample. After collecting a series of spectra from different photoelectron kinetic energies, the integrated areas of photoelectron peaks for a cation (e.g., Na or Rb) and an anion (e.g., Cl or Br) at the same photoelectron kinetic energy were obtained. Then, the atomic composition ratio of an anion to a cation at the same photoelectron kinetic energy can be calculated. The reason for the same photoelectron kinetic energy is to ensure that the elemental signals were acquired from the same analyzing depth. Since the atomic composition ratio at a given KE for “dry” alkali halides at RH of 7% should be 1, the atomic composition ratio calculated from each given KE is defined as a “intrinsic sensitivity factor” at this given KE. Following the collection of data at a RH of 7%, a salt-saturated solution was formed on the sample surface by lowering the sample temperature until the sample reached the deliquescence point.27 The salt solution on the sample surface could be visually observed. Lowering the temperature of the sample was achieved by using a circulating chiller and a Peltier block attached to the sample holder. The sample temperature was read by a thermocouple attached to the bottom of the salt crystal. Because the sample surface is the lowest temperature point in the sample chamber, water vapor will condense on the salt first. The forming temperatures of all three salt-saturated solutions are about
12 °C, which corresponds to about 80% RH. The concentration of saturated NaCl and RbCl solutions are 6.2 and 7.8 M at 25 °C, and the concentration of a saturated RbBr solution is 7.0 M at 25 °C, but we do not know the exact concentration of the three salt-saturated solutions prepared at
12 °C. After the formation of a salt solution on the sample surface, XP spectra of Cl (2p), Rb (3d), Na (2s), O (1s), and C (1s) were acquired for aqueous RbCl and NaCl solutions from 200 to 600 eV KE. XP spectra of Br (3d), Rb (3d), O (1s), and C (1s) were acquired for an aqueous RbBr solution from 125 to 650 eV KE.

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Table 1. Simulation Force Field Parameters atom

parameter set

q (e)a

α (Å)b

ratom (Å)c

ε (kcal/mol)d 0.1000

Dange

+1

1.400

1.9800

+

Na

PBf

+1

0.240

1.3190

0.1300

Cl


PBf


1

3.250

2.4192

0.1000

Br


PBf


1

4.530

2.6380

0.1000

O

POL3g


0.730

0.528

1.798

0.156

H1, H2

POL3g

0.365

0.170

0.0

0.0

Rb+

a

Charge or partial charge of atom or ion. b Atomic polarizability. c Position of the minimum in the Lennard-Jones potential. d LennardJones well depth. e References 32 and 33. f References 30 and 31. g Reference 29.

After collecting a series of spectra at different photoelectron kinetic energies, the integrated areas of photoelectron peaks for a cation (e.g., Na or Rb) and an anion (e.g., Cl or Br) at the same photoelectron kinetic energy were obtained. Then, the atomic composition ratio of an anion to a cation was calculated at the same photoelectron kinetic energy. This calculated atomic composition ratio was normalized by the intrinsic sensitivity factor obtained from the “dry” sample at the same photoelectron kinetic energy. This normalized atomic composition ratio at a given KE corresponds to the ion concentration ratio of an anion to a cation at a corresponding analyzing depth. Since a series of atomic composition ratios can be obtained from a series of different photoelectron kinetic energies, ion distribution depth profiles at the liquid/vapor interface of salt solutions can be obtained. Note that the atomic composition ratio at a given KE was calculated in previous studies by normalizing X-ray flux and corresponding subshell photoionization cross section.16,17,28 In this study, the intrinsic sensitivity factor method is used to obtain anion/cation atomic composition ratios in RbCl, NaCl, and RbBr solutions. However, chloride concentration depth profiles, which are Cl/O atomic composition ratios as a function of photoelectron kinetic energy, in RbCl and NaCl solutions are obtained by normalizing X-ray flux and corresponding subshell photoionization cross section because the intrinsic sensitivity factor of the oxygen element in the solutions could not be acquired. Note that oxygen is the normalized O (1s) peak area from the condensed phase of adsorbed water so the Cl/O atomic composition ratio represents the chloride concentration in the solutions. MD Simulations. Liquid
vapor interfaces of sodium chloride, rubidium chloride, and rubidium bromide were simulated using a slab geometry in which unit cells of 30 Å  30 Å  100 Å were replicated using three-dimensional periodic boundary conditions. The z dimension of the cell is elongated orthogonal to the liquid
vapor interface, so that a vacuum separates the periodic images in the vertical direction. All slab simulations contained 864 water molecules and 96 chloride or bromide ions, corresponding to a concentration of x = 0.1 in the absence of interfacial partitioning (x = mole fraction). In addition, 96 sodium or rubidium ions were included. Force field parameters are provided in Table 1. All of the simulations incorporated the polarizable POL3 water model29 and the chloride model of Perera and Berkowitz.30 The bromide polarizability of Sremaniak, Perera, and Berkowitz was employed.30,31 The sodium force field is attributed to the work by Petersen et al.,14 and the rubidium force field is that of Dang.32,33 The average Rb
O distance predicted in our simulations is 3.01
3.09 Å depending 4547

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Figure 1. Ambient pressure XP spectra of an aqueous RbCl solution taken at lower photoelectron kinetic energy (200 eV; more surface sensitive) and higher photoelectron kinetic energy (600 eV; less surface sensitive). In each set of spectra, the chloride spectrum was taken first in order to avoid beam damage. Experimental data points are denoted by circles, and fits to the data are given by solid lines.

Figure 2. Ambient pressure XP spectra of an aqueous NaCl solution taken at lower photoelectron kinetic energy (200 eV) and higher photoelectron kinetic energy (600 eV). In each set of spectra, the chloride spectrum was taken first in order to avoid beam damage. Experimental data points are circles, and fits to the data are shown by solid lines.

on concentration, and the coordination number for rubidium (waters + ions) is ∼8, in agreement with the varied and somewhat limited available experimental data.34
36 The molecular dynamics program employed was Sander in the AMBER 8 suite of programs.37 Specifically, we used a version of Sander that has a modified calculation of the induced dipoles to avoid polarization catastrophe.14 Particle mesh Ewald summation was used to treat the long-range electrostatic interactions. The real-space part of the Ewald sum and the Lennard-Jones interactions were truncated at 12 Å.38,39 The time step was 1 fs, and trajectory data were recorded every picosecond. Water bond lengths and angles were constrained using the SHAKE algorithm.40 The initial coordinates for the NaCl simulation were taken from a previously converged NaCl simulation. For RbCl and RbBr, small clusters of RbCl (or RbBr) + 9 water molecules were placed on a grid and allowed to condense into a bulk solution under a constant pressure of 1 atm while the system was heated to 300 K and then allowed to equilibrate for a short period of time before the length of the simulation cell in the z direction was extended to create the slab configuration. The cluster coordinates were chosen so that there were no ion pairs in the initial conditions. Harmonic constraints were employed to maintain a solvation shell of water around rubidium until the clusters had condensed, and then the restraints were removed and spontaneous ion pairing occurred. All of the production runs were carried out at a constant temperature of 300 K.

were investigated in order to understand specific cation effects on halide ion distributions at the liquid/vapor interface. We took XP spectra of Cl (2p), Rb (3d), Na (2s), O (1s), and C (1s) from aqueous RbCl and NaCl solutions at a series of photoelectron kinetic energies (KEs): 200, 250, 350, 450, and 600 eV. Figure 1 shows XP spectra of an aqueous RbCl solution from photoelectron KE of 200 and 600 eV, which are more sensitive to the surface and bulk region of the solution, respectively. For the doublet Cl (2p) spectra, there is an additional peak appearing at the higher binding energy (BE). We use two chloride components with identical fitting parameters to fit this Cl (2p) spectrum. On the basis of the kinetic energy dependence, it appears that the higher BE chloride component resides at the surface region of the solution, whereas the lower BE chloride component is located deeper into the solution. The chloride component with the higher BE might be attributed to asymmetric solvation by water molecules at the surface region, whereas the chloride component with lower BE might be attributed to more symmetrical solvation of water molecules deeper in the solution. For the doublet Rb (3d) spectra, we only need to use one rubidium component to fit the doublet Rb (3d) spectra. The small shifts of binding energy on all spectra are not indicative of changes in chemical states. These small shifts are attributed to sample charging. We collected XP spectra of an aqueous NaCl solution with photoelectron KE varying from 200 to 600 eV, comparable to those of the aqueous RbCl solution, examples of which were shown in Figure 1. Figure 2 shows XP spectra of an aqueous NaCl solution from photoelectron KE of 200 and 600 eV, respectively. As in the case of the RbCl solution, two doublet chloride components were necessary to fit the Cl (2p) spectra. The mechanism giving rise to two chloride components is assumed to be the same as in the aqueous RbCl solution. Only one sodium component was necessary to fit Na (2s) spectra. The integrated areas of the Cl (2p), Rb (3d), and Na (2s) photoelectron signals were obtained from spectra taken over a

’ RESULTS AND DISCUSSION Ion Distribution Depth Profiles at the Liquid/Vapor Interface for Aqueous NaCl and RbCl Solutions Using AP-XPS and MD Simulations. Experimental and theoretical studies4,15,16

have shown that the anion surface enhancement in aqueous alkali halide solutions increases with increasing anion size and polarizibility. In this study, aqueous RbCl and NaCl solutions

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Figure 3. Anion/cation atomic composition ratios as a function of photoelectron kinetic energy for aqueous RbCl and NaCl solutions. The squares are Cl/Rb data from aqueous RbCl solutions, and the circles are Cl/Na data from aqueous NaCl solutions. Error bars are the standard deviation from different sample spots and samples.

range of photoelectron kinetic energies from 200 to 600 eV to provide the anion/cation atomic composition ratios as a function of photoelectron kinetic energy, shown in Figure 3. The anion/ cation ratios obtained from lower KE spectra probe the surface region of the solution and those obtained from higher KE spectra probe deeper into the solution. The anion/cation atomic composition ratio for both NaCl and RbCl solutions is about 1.5 from spectra obtained at the lowest KE (200 eV), indicating that both RbCl and NaCl solutions exhibit a small anion surface enhancement (surface enhancement factor is 1.5). This slightly enhanced chloride concentration relative to the cation at the liquid/vapor interface of aqueous alkali chloride solution is consistent with classical MD simulations by Jungwirth and Tobias.4 However, the anion/cation atomic composition ratios for both solutions are almost identical when we probe from surface to deeper regions of the solution. The AP-XPS experimental results presented thus far do not reveal any cation dependence on chloride surface enhancement of aqueous alkali chloride solutions when the chloride surface enhancement is determined from anion/ cation atomic composition ratios as a function of KE. Integrated density profiles comparable to those obtained from AP-XPS measurements can be calculated from MD simulations by integrating the atomic density profiles. Brown et al.41 describe an approach wherein electron attenuation in the solution was modeled by a simple exponential decay with respect to the depth into solution, as described in the equation Si ðKEÞ ¼

Z þ∞ 0

þ

Fi ðz0 Þ dz0

Z 0
∞

0

Fi ðz0 Þe
z =IMFPðKEÞ dz0

ð1Þ

Here Si(KE) is the XPS signal area of atom type i, obtained from electrons with a given kinetic energy, KE, Fi(z) is the density profile of atom type i, z0 is distance from the Gibbs dividing surface (GDS), and IMFP(KE), in angstroms, is the kineticenergy-dependent inelastic mean free path, which gives the depth of the solution probed by escaping photoelectrons with kinetic energy KE. The first term reflects an assumed lack of photoelectron attenuation above the GDS of water (z0 = 0). In this region the density of water, which is ∼10 times greater than the density of the salt, is at most 50% of its bulk value and there are very few molecules in the vapor phase, and hence inelastic scattering of photoelectrons headed out of solution and toward the detector has relatively low probability. The second term

describes exponential attenuation of photoelectrons with a rate of decay governed by IMFP(KE). Equation 1 is based on the approximation that the density of scatterers is constant below the GDS. This would be a reasonable assumption for low-concentration solutions where the inelastic scattering of the photoelectrons would be due primarily to water. This formula is comparable to the straight-line approximation in the case where the detector is located perpendicular to the surface of interest.42 As photoelectrons are inelastically scattered by bound electrons in solution, we have determined that the effect of the ion concentrations in solution on the electron attenuation can be quite considerable if heavy ions and/or high concentrations are present. Therefore, the exponential attenuation of the photoelectrons depends not only on the kinetic energy of the ejected photoelectrons, but also can be related, approximately, to the electron density of the solution, which has a spatial dependence if there is surface enhancement of the solute. A variation of eq 1 that explicitly takes into account differences between the electron density of neat water and that of the salt solution calculated from the MD simulations is presented below. We define Fe‑,water(z) as the electron density profile of neat water, and Fe‑,soln(z) as the electron density profile of the ionic solution under consideration. While the atomic densities in eq 1 are based on the position of atom centers, the electron densities are approximated by Gaussian distributions about the atom centers with widths chosen as the Lennard-Jones radii used in the force fields. In eq 2, the electron density-dependence on the attenuation of the photoelectron signal is separated out from the IMFP: Si ðKEÞ ¼ þ

Z 0
∞

Z þ∞ 0

Fi ðz0 Þ dz0

Fi ðz0 Þfe½
1=IMFPðKEÞ

R0


z0

½Fe
, soln ðz00 Þ=½Fe
, water ðz00 Þ dz00

g dz0

ð2Þ Thus, the IMFP so-defined is no longer strictly the true experimental IMFP of the photoelectrons, which contains all dependencies on photoelectron kinetic energy and solution properties. Several assumptions remain in this new formula for Si(KE). We assume that elastic scattering is minimal, the mechanism for inelastic photoelectron scattering is dependent on the electron density alone, and all bound electrons are equally capable of scattering photoelectrons. Additionally, we assume that the electron density at any given distance, z, from the GDS is uniform in the xy-plane. We will use the second formula to calculate integrated density profiles from MD simulations throughout this paper. In the work presented throughout this article, density profiles from the MD simulations are convoluted (as described previously using eq 2) with the inelastic scattering function to produce plots of predicted photoelectron signal strengths as a function of the inelastic mean free path (IMFP) used in the convolution integral. Since, for salt solutions, a quantitative relationship between the IMFP value and the photoelectron kinetic energy remains a subject of discussion,24 we have refrained from plotting the MD signal predictions as a function of photoelectron kinetic energy. However, the IMFP is a monotonic function of the photoelectron kinetic energy over the range used in our experiments (100
650 eV). As a result, the plots that we generate from the convolution of the MD density profiles (e.g., Figure 4) can be directly compared to plots of the 4549

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experimental data in the form of integrated photoelectron peak signals (or ratios) plotted versus photoelectron kinetic energy (e.g., Figure 3). Figure 4 shows that the anion/cation signal ratios computed (using eq 2) from MD simulations of NaCl and RbCl solutions are essentially identical; this is in agreement with experimental observations of the XPS depth profiles shown in Figure 3. At first glance this suggests a lack of cation effects between sodium and rubidium in chloride solutions. However, the snapshots and atomic density profiles (which were used in making the integrated density profiles) presented in Figure 5 tell a different story. While some chloride is at the surface of RbCl, its concentration is depleted relative to the bulk chloride concentration. On the other hand, the concentration of chloride at the surface of the NaCl solution exceeds that of the bulk, although there is a depletion of chloride concentration in the region just below the surface. The cation density profiles for the two solutions also differ greatly. While sodium shows a large enhancement in concentration beneath the solution surface, so that a pronounced double layer is formed near the surface of the NaCl solution, the rubidium concentration is nearly constant from the bulk throughout most of the slab and

tapers off just beneath the solution surface. Additionally, Rb+ appears to reside more deeply below the surface than Na+; this difference is greater than the 0.6 Å difference in ionic radii. The integrated density profile ratios look quite similar for the two solutions because the anion/cation ratios are similar, despite the differences of the atomic distributions in the two solutions. We conclude that the appearance of an enhancement in the halide/ cation ratios in integrated density profiles can result not only from anion enhancement but also from cation depletion. We have demonstrated that the anion/cation atomic composition ratios might be insensitive to cation size effects on chloride surface enhancement of aqueous alkali chloride solutions because different cation behavior in RbCl and NaCl solutions might cancel differences in chloride behavior in the ratios. In order to investigate whether or not there is a cation size effect on chloride surface enhancement, chloride concentration depth profiles in RbCl and NaCl solutions were determined in reference to water O concentration profiles. Figure 6 shows the integrated density profiles of the Cl/O ratios as a function of IMFP calculated from MD simulations using eq 2. The simulated Cl/O ratio profiles

Figure 4. Cl/cation atomic composition ratio as a function of inelastic mean free path (IMFP) calculated from MD simulations using eq 2. The black squares are Cl/Rb data from aqueous RbCl solution, and the red circles are Cl/Na data from aqueous NaCl solution.

Figure 6. Cl/O atomic composition ratios as a function of inelastic mean free path (IMFP) calculated from MD simulations using eq 2. The black squares are Cl/Rb data from aqueous RbCl solution, and the red circles are Cl/Na data from aqueous NaCl solution.

Figure 5. Left: snapshots of the surface of simulation slabs of RbCl (top) and NaCl (bottom) solutions. Center: snapshots of cross-sectional views of the top half of the simulation slabs. Right: atomic density profiles, normalized so that the area of all components of the density profiles is equal to 0.5. The Gibbs dividing surface is set to Z = 0 Å. The colors in the density profiles correspond to the atom colors in the snapshots. 4550

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Figure 7. Cl/O atomic composition ratios as a function of photoelectron kinetic energy for aqueous RbCl and NaCl solutions. Oxygen is the normalized O (1s) peak area from the condensed phase of adsorbed water so the Cl/O ratio represents the chloride concentration for aqueous RbCl and NaCl solutions, respectively. The squares are Cl/O data from an aqueous RbCl solution, and the circles are Cl/O data from an aqueous NaCl solution. Error bars are the standard deviation from different sample spots.

display a clear interfacial chloride anion enhancement in the NaCl solution, but no such enhancement in the RbCl solution. The results from AP-XPS also show different chloride anion enhancements for the two solutions. Since the oxygen signal we observe is a direct measure of the water, by calculating Cl/O atomic composition ratios from the Cl and O spectra as a function of photoelectron kinetic energy (Figure 7), we obtain a direct measure of the Cl concentration, in contrast to the anion/cation ratio discussed previously. From depth profiles of Cl/O ratios in XPS measurement, we observe that there is a large chloride surface enhancement in the NaCl solution. In addition, we observe a flat chloride concentration depth profile for the RbCl solution, indicating that there is no chloride surface enhancement in this system. The trend of decreasing surface chloride concentration with increasing counterion size was observed in both the experimental XPS results and the MD simulations, although a small depletion of chloride at the surface of the RbCl solution predicted by the MD simulation (Figure 6) was not observed experimentally (Figure 7). Thus, using chloride concentration depth profiles determined directly from the XPS Cl/O ratios, we have explicitly demonstrated the existence of a specific cation size effect on chloride surface enhancement in aqueous alkali chloride solutions, which is in qualitative agreement with MD simulations results. Ion Distribution Depth Profiles at the Liquid/Vapor Interface for the Aqueous RbBr Solution Using AP-XPS and MD Simulations. We obtained XP spectra of Br (3d), Rb (3d), O (1s), and C (1s) from an aqueous RbBr solution at a series of photoelectron kinetic energies: 125, 150, 200, 300, 500, and 650 eV (Figure 8). Because photoelectrons with lower energies experience more inelastic scattering, detected photoelectrons with kinetic energies of 125 or 150 eV are more likely to have originated near the liquid/vapor interface than photoelectrons with kinetic energy of 650 eV. We used two bromide components with identical fitting parameters to fit the doublet Br (3d) spectra. The origin of two bromide components is likely to be the same as that of chloride in aqueous alkali chloride solutions. As in the case of the RbCl solution, the doublet Rb (3d) spectra from KE of 125 and 650 eV consisted of one rubidium component.

Figure 8. Ambient pressure XPS spectra of aqueous RbBr solutions taken at lower photoelectron kinetic energy (125 eV) and higher photoelectron kinetic energy (650 eV). In each set of spectra, the bromide spectrum was taken first in order to avoid beam damage. Experimental data points are given by circles, and fits to the data are shown by solid lines.

Figure 9. Br/Rb atomic composition ratios as a function of photoelectron kinetic energy for aqueous RbBr solution. Error bars are the standard deviation from different sample spots and samples.

After integrating Br (3d) and Rb (3d) photoelectron signals at several photoelectron KEs (125
650 eV), the Br/Rb atomic composition ratios as a function of photoelectron KE were obtained, as shown in Figure 9. The Br/Rb atomic composition ratio for an aqueous RbBr solution is about 2.5 at the lowest KE of 125 eV, indicating aqueous RbBr solution exhibits anion surface enhancement relative to the cation. In addition, a slightly enhanced bromide concentration was observed even at photoelectron signals with a kinetic energy of 500 eV. At first glance, the bromide ion surface enhancement for the aqueous RbBr solution in Figure 9 appears contradictory to the results from recent XRR experiments, which suggested a depletion of bromide ions near the air/solution interface.22 An MD simulation of an aqueous RbBr solution was performed, both to provide an atomic-scale perspective of the solution from which AP-XPS and XRR results can be discussed and to provide another example for the discussion of specific cation effects on halide anion adsorption. We begin by showing 4551

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The Journal of Physical Chemistry C in Figure 10 an integrated density profile calculated from MD simulation using eq 2. We have included a comparison showing the Br
/Rb+ ratio with attenuation of the photoelectrons only from the electron density of water in the solution (i.e., using eq 1). We show that the observed anion to cation ratio is increased for IMFP < 10 Å, although when only the water electron density is explicitly included in the photoelectron attenuation, the Br
enhancement appears to diminish more quickly. While the shape of the integrated density profile differs somewhat from the experimental ones, we point out that the actual relationship between photoelectron kinetic energy and the depth probed is only qualitatively understood at best.24 The presence of bromide at the surface of RbBr is observed in both XPS experiments and MD simulations, though we would argue that the persistence of Br
enhancement at moderate KE (300
500 eV) in the XPS experiments is not necessarily indicative of a deeply permeating anion enhancement. It may be explained alternatively by increased attenuation of the signal by the electron density from Rb+ and Br
in concentrated RbBr solutions, relative to the attenuation experienced in other systems. We have only approximately accounted for this by including the electron density of the solution relative to that of neat water, and the actual degree of photoelectron attenuation in RbBr solutions may be greater than that accounted for by our approximation (eq 2). However, as mentioned above, XRR experiments reported previously by Sloutskin et al.22 have suggested that there is a depletion of Br
near the air
water interface relative to the

Figure 10. Br/Rb atomic composition ratios as a function of inelastic mean free path (IMFP) calculated from MD simulations of an aqueous RbBr solution using eq 2. The black squares correspond to exponential attenuation of photoelectron signal from all electrons in the solution. The red circles correspond to attenuation based upon the electron density of the water in the solution.

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bulk concentration, which is in sharp contrast to the picture inferred from the anion/cation ratios obtained from our XPS data and MD simulations. If we look to the atomic density profiles computed from the MD simulations and plotted in Figure 11, we see that Br
is present at the interface, in agreement with the XPS results, but there is also a significant depletion of Br
a few angstroms beneath the interface, in the region where the Rb+ concentration is enhanced. The surface bias of the XPS experiment results in large measure from attenuation of the photoelectrons by inelastic scattering as they exit the solution. The inelastic scattering is dominated by interactions of the photoelectrons with the electron density of the solution. The number of photoelectrons reaching the detector from any depth within the solution is governed by an exponential decay function involving photoelectron kinetic energy, and the ability of the solution to scatter photoelectrons of that given energy. XRR is a complementary technique that probes inhomogeneities in the electron density profile normal to the interface. In the Born approximation, the theoretical wavevector dependence of the specular reflectivity is proportional to an integral along the direction normal to the interface of the gradient of the electron density profile, weighted by the reciprocal space density operator. The experimentally measured reflectivity is the theoretical reflectivity convoluted with a function that accounts for the surface roughness. Under the assumption of an analytical function (usually assumed to be Gaussian) representing the smoothing of the electron density profile due to thermal fluctuations, a model electron density profile can be extracted by inversion of XRR data. The particular model applied to recent XRR measurements on RbBr solutions suggested a depletion of Br
anions near the air
solution interface of a concentrated RbBr solution.22 Such a depletion is incompatible with our XPS data and MD simulations of similar RbBr solutions. A recent MD simulation study33 showed that reasonable, but not quantitative, agreement with XRR data could be obtained from MD simulations that predicted a substantial enhancement of Br
anions in a concentrated RbBr solution. The extent of Br
enhancement at the air
solution interface in that study33 was significantly greater than that observed in our simulation (Figure 11), which employed a different polarizable force field to model the water molecules. Our simulation of RbBr solution exhibits a depletion in Br
below the air
solution interface (Figure 11), with concomitant enhancement at the interface, which together lead to decent agreement with the XPS data presented herein (Figure 9) after accounting for the depth-dependent attenuation of photoelectrons based on the electron density profiles (Figure 10). The enhancement of Br
at the interface is not compatible with

Figure 11. Left: snapshot of the surface of a simulation slab of RbBr solution. Center: snapshot of cross-sectional view of the top half of the simulation slab. Right: atomic density profiles, normalized so that the area of all components of the density profiles are equal to 0.5. The Gibbs dividing surface is set to Z = 0 Å. The colors in the density profiles correspond to the atom colors in the snapshots. 4552

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Figure 12. Plot of the average number of contact ion pairs formed in alkali halide solutions as a function of depth in the slab (Z).

the model used to fit the XRR data.22 The reconciliation of XRR measurements with simulation and XPS results is a subject of ongoing research that will require a better understanding of the role of surface roughness and model dependence in the XRR measurements, the nature of the depth-dependent photoelectron attenuation of the XPS signal, and the force field dependence of the extent of ion adsorption to the air
water interface predicted by MD simulations. Contact-Ion Pairs and Induced Dipoles of Halide in Aqueous NaCl, RbCl, and RbBr Solutions. Using MD simulations, we have shown that halide anion surface propensity is slightly dependent upon the identity of the countercation. While cation size has an effect on the strength of its interactions with water,9,10,43 at high concentrations a large amount of direct interaction (i.e., ion pairing) between alkali cations and halide anions in solution can be expected. However, there has been considerable disagreement on the extent of contact ion pairing both in MD simulations and as determined by experiment. Recently, several reports of MD simulations employing AMBER force fields have noted unusual crystallization of NaCl and KCl with some combinations of water and ion parameters, even at low concentrations and in contrast with expected behavior.44,45 This resulted from using cation and anion force fields that were not optimized to be used together. More recently, new ion parameters have been developed that take into account not only the interactions of individual ions with water but also the lattice constants and lattice energies of the salt crystal in an effort to prevent undue crystallization.31 Other groups have explored the relationship between ion and water parameters and the resulting extent of contact ion pairing.45
48 Below we discuss the current knowledge of ion pairing in alkali halide solutions and show that, based upon the limited available information, our results are reasonable. When sufficient water is available for solvation, ions appear to follow Collins’ law of matching water affinities: ions that are of similar size and charge density are likely to ion pair, whereas cations and anions that have greatly differing size and water affinity tend to remain dissociated.8,45,49 At low concentrations, conductivity measurements suggest that NaCl forms more ion pairs than RbCl and RbBr.50 Similar trends are observed in MD simulations.47 The picture is not necessarily the same at high concentrations, where there is simply not enough water to completely solvate all the ions. Knowledge of the behavior of ions at high concentrations, and moreover an understanding of the ability of force fields to model this behavior accurately, is limited. While ion pairing can be observed through reduced conductivity of a solution, these measurements are only sensitive at very low concentrations

where it can be assumed that ions are either fully solvated or in Bejerrum pairs. Conductivity measurements cannot distinguish between Bejerrum pairs and larger aggregates of ions. In addition to conductivity, colligative properties, or solution properties which depend on the number of dissolved solutes rather than their identity, can be used to study ion pairing. Luo and Roux have explicitly shown the relationship between the degree of contact ion pairing in NaCl and KCl solutions and osmotic pressure. They have used osmotic pressure as an explicit criterion to optimize force fields with respect to ion pairing, even at high concentrations.48 In 5 M NaCl simulations with coordination of 0.7
0.8 Na+ around Cl
, and vice versa, Luo and Roux reproduced the experimental value for osmotic pressure. Figure 12 shows the average coordination of the halides around the alkali cations, and vice versa, for each system simulated in the present study. We point out the average number of counterions coordinated around the ion type of interest in each system appears to follow the density profile of its counterion; i.e., there are more halides around alkali metals in the regions where the halide ion concentration is enhanced. The degree of ion pairing seen in our NaCl solution is similar to that of previous studies,51,52 including that of Luo and Roux.48 A greater amount of ion pairing is present in rubidium chloride than in sodium chloride. This is in contradiction with experimental and computational trends at low concentrations.47,50 However, the current study was performed near saturation where the same trend is not necessarily followed. Rb+ has a larger solvation shell than Na+; therefore, because of very limited availability of water molecules (9 per salt), it is reasonable to expect more ion pairing in RbCl and RbBr solutions than in NaCl solutions. Additionally, a recent neutron diffraction study by Mancinelli et al.52 suggests that at high concentrations (1 NaCl: 10 waters and 1 KCl: 13 waters) K+ may form more ion pairs than Na+ (1.3 ( 1.1 and 0.9 ( 1.0, respectively). As Rb+ is larger than K+, and higher concentrations were used in the present study, a considerable increase of ion pairing in RbCl over NaCl is not unreasonable. While it has been previously shown that halide ion stabilization is correlated with ion size and polarizability, a more generally applicable explanation of the interfacial affinity of ions in different salt solutions is less well understood.53 For example, in this work we have characterized the effect of a counterion on halide surface propensity. MD simulations provide an atomic-scale view of molecular systems, in which interactions between individual atoms and molecules are described by a force field. In our simulations a nonadditive, polarizable potential based on atom-centered induced dipoles has been included. Although polarization may not be the most important driving force for ion adsorption, the 4553

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Figure 13. (a) Plot of the average magnitude of induced dipole of the halide in RbCl, NaCl, and RbBr solutions as a function of Z. (b) Plot of the average direction of the induced dipole of chloride in solutions of RbCl and NaCl as a function of Z. “Cl
” refers to a system with a single Cl
and 864 waters.

induced dipole is partly responsible for stabilizing the halide while it experiences incomplete solvation in an interfacial setting.54 In NaCl simulations in which there is no explicit modeling of polarizability of either chloride, water, or both, chloride concentrations were depleted at the air
water interface.55,56 Just as proximity of ions to the interface between two media with different dielectric constants or incomplete solvation may result in an induced dipole in halide ions, ion pairing will also induce a dipole in a halide ion. The magnitude of this induced dipole depends on the cation
anion distance as well as the anion polarizability. Figure 13a shows the average magnitude of the induced dipole as a function of Z for halides in concentrated solutions. We see that, in the bulk phase (Z <
10 Å), Cl
in NaCl has the greatest induced dipole, followed by Br
in RbBr and Cl
in RbCl. This ordering persists in the alkali halide solutions, though the induced dipoles gain magnitude near the interface. However, in the region where cation concentrations are enhanced the induced dipoles of Cl
in NaCl and of Br
in RbBr are similar. Overall, the average magnitude of the induced dipole of the halides in the alkali halide solutions correlates with their surface propensity. In Figure 13b, the Z component of the unit vector along the average induced dipole shows random orientation for Z <
7 Å in all systems. Although there are noticeable differences in the surface propensity of the halides and the magnitude of their induced dipoles in the different solutions, we see that the orientation of each is similar as they approach the interface. The average orientation of the induced dipole of the chloride in RbCl solution and at infinite dilution is slightly less vertical than that of the halides in RbBr and NaCl, possibly because of wider orientational fluctuations stabilized by a slightly greater degree of solvation.

’ CONCLUSIONS Ion behavior at liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions was examined by AP-XPS and MD simulations. This was the first time that the anion to cation ratios demonstrating surface enhancement of neat NaCl, RbCl, and RbBr aqueous solutions measured by AP-XPS have been reported. Our AP-XPS depth profiles do not reveal any effect of cation size on the anion/cation ratio surface enhancement in the RbCl and NaCl solutions. However, direct measurements of the chloride concentration profiles obtained from the Cl/O ratios show an enhancement of chloride at the surface of the NaCl solution and a lack of chloride surface enhancement for the RbCl solution. MD simulations employing polarizable force fields indicate an enhancement of chloride at the interface for the NaCl solutions and a slight depletion of chloride at the RbCl

solution surface. The MD simulations also show that the sensitivity of anion/cation ratio depth profiles to cation distributions could hide differences in the chloride depth profiles, reinforcing that the Cl/O depth profiles are a more informative measure of the chloride surface enhancements. Additionally, MD simulations were able to provide insight into the role of the cations in the surface partitioning of chloride for the two solutions. Specifically, it was shown that cations can induce larger dipoles in chloride anions than from asymmetric water solvation alone. In studies of an aqueous RbBr solution, bromide surface enhancement was evident in the anion/cation ratio depth profiles using both APXPS and MD simulations. This enhancement was visible for an unusually broad range of photoelectron kinetic energies in the AP-XPS experiment (125
500 eV), which would generally be interpreted as a very deeply penetrating bromide anion surface enhancement. However, using MD simulations as a guide, it was proposed that these results could also be obtained from a small separation of Br
and Rb+ near the interface if there was much stronger photoelectron attenuation in concentrated, aqueous RbBr than in other solutions such as NaCl.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected], Ph 949-824-6022, Fax 949-824-2261 (J.C.H.). E-mail [email protected], Ph 949-824-4295, Fax 949-8248571 (D.J.T.). Author Contributions †

M.H.C. and K.M.C. contributed equally to this work.

’ ACKNOWLEDGMENT This work is supported by the AirUCI Environmental Molecular Sciences Institute under Grant CHE 0431312 from National Science Foundation. 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 DE-AC0205CH11231. The MD simulations were carried out on the AirUCI cluster, supported by the UCI Physical Sciences Computing Support Group, and the UCI Medium Performance Computing cluster, administered by Joseph Farran. ’ REFERENCES (1) Tobias, D. J.; Hemminger, J. C. Science 2008, 319, 1197. (2) Knipping, E. M.; Lakin, M. J.; Foster, K. L.; Jungwirth, P.; Tobias, D. J.; Gerber, R. B.; Dabdub, D.; Finlayson-Pitts, B. J. Science 2000, 288, 301. 4554

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