Toward an Understanding of the Specific Ion Effect Using Density

LETTER pubs.acs.org/JPCL

Toward an Understanding of the Specific Ion Effect Using Density Functional Theory Marcel D. Baer and Christopher J. Mundy* Chemical and Materials Science Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ABSTRACT: Although it is now accepted that some anions adsorb at the air
water interface following a reverse Hofmeister series [Kunz, W.; Henle, J.; Ninham, B. W. Curr. Opinion Colloid Interface Sci. 2004, 9, 19
37; Tobias, D. J.; Hemminger, J. C. Science 2008, 319, 1197
1198], the nature of the microscopic interactions driving ions to interfaces is currently the subject of active research. We use extensive density functional theory (DFT)based interaction potentials to study the free energy of transfer of iodide from the interior to the surface in both a cluster and the extended air
water interface. Our research supports a picture that empirical polarizable interaction potentials may overestimate surface adsorption for iodide. These results, in conjunction with previous theoretical and experimental studies on iodide solvation, have implications toward the necessary interactions that give rise to the specific ion effect at the air
water interface. SECTION: Statistical Mechanics, Thermodynamics, Medium Effects

T

he nature of the air
water interface of salt solutions has been a topic that has received attention both experimentally2
9 and theoretically.1,10,11 The reason for this, in large part, is due to a now-accepted notion that adsorption of halide anions to the air
water interface follows a reverse Hofmeister series where iodide has a propensity for the interface and fluoride does not. This is in contrast to our understanding before the new millenium (∼2000) where the accepted picture was that electrolyte anions (such as halides) were repelled from the interface in accordance with the theory of Onsagar and Samaras (OS)12 and the Gibbs adsorption isotherm (GAI). These aforementioned theoretical results were corroborated by molecular simulation where nonpolarizable interaction potentials were utilized to study sodium chloride in water.13 The traditional picture of the ion depletion layer was put into question when molecular dynamics (MD) simulations of concentrated electrolytes utilizing empirical polarizable interaction potentials were performed.10,11 The resulting mass density profiles provided a picture where the most polarizable halide anions (iodide and bromide) were significantly adsorbed at the air
water interface. A subsequent simulation emerged where the potential of mean force (PMF) for the isolated halide anions was computed as a function of the interface coordinate using similar empirical polarizable interaction potentials.14,15 The resulting PMF for iodide produced a distinct minimum of ∼2 kcal/mol (see Figure 1) relative to the bulk in the vicinity of the Gibbs dividing surface (GDS), supporting the picture put forth by the finite concentration studies. Only recently, the connection between the OS theory and the molecular simulations with empirical polarizable interaction potentials has been established. An extension of a dielectric continuum theory (DCT) to treat both the finite size of the ion in the vicinity of the GDS and the polarizability was put forth.16,17 This DCT was able to reconcile the adsorption of ions at the r 2011 American Chemical Society

interface within the thermodynamic constraints imposed by the GAI and reproduce the well-established surface tension measurements for electrolytes.16 The salient picture that emerged from this theory suggests that polarizable anions can be adsorbed at the interface, but in much lower concentration than was previously predicted by the MD studies using empirical polarizable potentials. A manifestation of the DCT prediction of lower concentrations of anions at the interface is also seen in the predicted single-anion PMF for iodide adsorption yielding an extremely shallow local minimum of ∼0.5 kcal/mol in the vicinity of the GDS (see Figure 1). Simultaneously, researchers have been reinvestigating the performance of empirical polarizable interaction potentials.18,19 The results of these theoretical studies suggest that empirical polarizable potentials may overestimate the induced dipole.18
20 Specifically, when more realistic screening is adopted, the adsorption of all halide anions is significantly less, in line with DCT.16,18 Other DCT studies are providing a picture where one can explain the specific ion effect at the air
water interface by including only the size of the anion.21 Researchers are also trying to incorporate the role of the fluctuating interface into a successful theory of the specific ion effect.22 A comprehensive density functional theory (DFT) MD approach to this problem is thus sorely needed. Although DFT MD is plagued with many uncertainties such as finite sampling and the accuracy of the exchange
correlation (XC) functionals, the exact treatment of the electrostatics could allow one to better understand the role of the ion
water interaction in the vicinity of the air
water interface. Another serious shortcoming of DFT as Received: March 10, 2011 Accepted: April 13, 2011 Published: April 19, 2011 1088

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Figure 1. (bottom) The PMF along the reaction coordinate ξ for transferring the I
relative to the GDS (ξ = 0) at 300 K, using the empirical polarizable model as given in ref 14 (green), DFT (red), and the results from DCT taken from ref 16 (black line). (top) The resulting PMF of the iodide anion from the center of mass (COM) of the cluster I
(H2O)27 to its surface at 100 K.

applied to the aqueous liquid
vapor interface is that traditional gradient-corrected (GGA) XC functionals significantly underestimate the density of liquid water by 15
20%.23
26 Given that we are interested in relative free energetics going from bulk to interfacial solvation, having a wrong bulk reference density could add a systematic bias to the results. One empirical solution to correct the bulk density of water is to add the empirical dispersion corrections due to Grimme27 referred to here as DFT þ dispersion (DFT-D). It was shown in a recent DFT-D MD study in the isothermal
isobaric (NpT) ensemble28 that the addition of this simple and efficient correction significantly improved the density of water to within a few percent of the experimental value. These results have been corroborated by studies of aqueous systems with open boundary conditions,29,30 where it was crucial to obtain the correct bulk density of water. In order to further support the use of DFT-D for the study of the free energy of transfer of halide anions at the air
water interface, benchmark calculations were compared against extended X-ray absorption spectroscopy (EXAFS).31 This study demonstrated that DFT-D was indeed able to fit the experimentally determined EXAFS spectra better than the empirical polarizable interaction potentials used by Dang and co-workers.14 Differences in the two models can be traced to the induced dipole moments. DFT-D predicts a dipole moment of ∼1.2 D, whereas the polarizable interaction potentials predict a dipole moment of ∼2.5 D.20,31 The larger dipole produces a more ordered solvation shell that does not support the experimental data. Furthermore, there is mounting experimental evidence that the specific ion effect may be a local phenomenon rather than being attributed to an ion’s effect on the long-range structure of liquid water (see ref 1 and references therein). Thus, we believe that the EXAFS studies, which provide a detailed local molecular picture of the solvation structure of iodide, may have implications for how

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much of the anion is adsorbed at the air
water interface. Given that we have established a precedent for DFT-D to yield a first solvation shell structure of iodide that is consistent with experiment, we aim to establish a benchmark calculation of the free energy of transfer of iodide from the bulk to the air
water interface utilizing a first-principles statistical mechanics approach. Results presented herein utilize the CP2K simulation suite containing the QuickStep module for the DFT calculations.32 The first-principle molecular dynamics simulations utilized a density cutoff of 280 Ry in conjunction with a double-ζ basis set that has been optimized for the condensed phase33 in conjunction with GTH pseudopotentials.34 A Nos
e
Hoover thermostat was attached to every degree of freedom to ensure equilibration.35 The Becke exchange36 and correlation due to Lee, Yang, and Parr (LYP)37 was utilized in addition to the dispersion correction put forth by Grimme27 with a 40 Å cutoff. The water cluster consists of 1 iodide anion and 27 water molecules. The distance r of the iodide anion with respect to the center of mass of the water molecules was restrained to different distances r0 in the range from 0.0 to 4.0 Å in 0.25 Å increments. The restraint potential was harmonic V (r) = 0.5k(r
r0)2 with a force constant k of 444.8168 kcal/mol/Å2. Each window was equilibrated for 4 ps, and an additional 12 ps were used to estimate the underlying free-energy using the weighted histogram analysis method (WHAM).38,39 The probability is given by P(r) = 4πr2 exp(
βA(r)). Here, β = 1/kBT, where kB is Boltzmann’s constant, T is temperature, and A is the PMF.40 For the DFT-D simulations, our extended interfacial system contained 215 water molecules and a single iodide ion within a supercell of 15  15  71.44 Å. This choice of system size has been shown to produce a stable bulk liquid in the center of the slab.25,26,41 To estimate the Gibbs dividing surface (zGDS), the width of the interfacial region (δ) and bulk density (Fl), the density along the z-axis (F(z)) was fitted to a hyperbolic tangent function F(z) = (1/2)(Fl
Fv)[tanh((z
zGDS)/δ)]. The bulk density in the interior is 0.99 g/cm3, and the GDS is estimated to be at 14.59 Å and of 1.83 Å width. An identical simulation size was used for the empirical polarizable interaction potential (DC) using parameters defined in ref 14. For both extended interfacial systems, the distance z of the iodide anion with respect to the center of mass of the water molecules projected onto the surface normal was restrained to values z0 using a harmonic potential V (z) = 0.5k(z
z0)2. For the DFT-D simulations, a series of 52 independent simulations was performed in the range from 9.511 to 16.266 Å in 0.13225 Å increments with a force constant k of 444.8168 kcal/mol/Å2. Each window was equilibrated for 10 ps, and an additional 10
15 ps was used to estimate the underlying free energy using the WHAM method. For the DC simulations, a series of 60 independent simulations was performed in the range from 0.0 to 15.875 Å in 0.944 Å increments with a force constant k of 44.48168 kcal/mol/Å2. Each window was equilibrated for 20 ps, and an additional 160 ps was used to estimate the underlying free energy using the WHAM method. The results are presented using the reaction coordinate ξ, which is the restrained distance r and z shifted by the GDS for the cluster and extended interface simulations, respectively. For the DFT-D simulations, molecular dipole moments are calculated using maximally localized Wannier centers.42
44 In Figure 1, we establish our main result for the PMF of an iodide anion from bulk to interfacial solvation using DFT-D as described above. Additionally, we are in good agreement with the 1089

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The Journal of Physical Chemistry Letters established result of the PMF for iodide from the bulk to interface obtained with a empirical polarizable potential14 using identical system size as that for the DFT-D calculations. Thus, we are confident that our chosen system size will not create any obvious artifacts pertaining to the PMF. It is also apparent that the PMF from the DFT-D calculations is quantitatively different than the one obtained by using the empirical polarizable potential. This is not too surprising given our previous studies on the bulk solvation of iodide.31 In the top panel of Figure 1, we show the PMF of transferring an iodide anion from bulk to interfacial solvation in a cluster I
(H2O)27. For a cluster of this size, our findings are that iodide sheds its solvation shell and resides at the surface of the cluster. Our results for the iodide cluster are consistent with previous studies performed with empirical polarizable potentials using a similar size for the cluster.40 An intriguing result is that our PMF for iodide at the extended interface using DFT-D displays a local minimum in good agreement with the PMF obtained from a recent DCT that only contains cavitation and polarization.16 It would appear that we have established a correspondence between a molecular picture, where both the interface can fluctuate and explicit hydrogenbonding is present, and a DCT picture where the results are determined by parameters such as ion polarizabilites and ionic size. In fact, we can estimate the value for the ionic radius used in the DCT by the position of the first maximum of the oxygen
iodide radial distribution function (3.65 Å)31 minus the radius of a water molecule as estimated as the position of the first maximum in the oxygen
oxygen radial distribution (2.76 Å, rO = 1.38 Å) function.29 The resulting value of 2.27 Å for the ionic radius of iodide agrees well with those accepted values used in the literature and those used in the DCT.16,45 Another objective of this study is to establish a molecular picture of the important interactions that give rise to interfacial anion adsorption. Because we have access to the electronic structure, we can investigate whether the PMF for both the extended interface and cluster simulations can be rationalized in terms of the progression of the local fields along the reaction coordinate. Figure 2 depicts the dipole moments for both the water molecules and iodide as one approaches the extended air
water interface and the surface of the cluster. The first observation is that for both the extended interface and cluster simulations, in the region of bulk solvation (e.g., ξ <
2 Å for the extended interface and in the center of the cluster), the iodide dipoles are in good agreement with their bulk solvated values.31 This is extremely interesting given that in this region of bulk solvation, the dipole moments for the water molecules in the extended interface and cluster simulations differ, namely, ∼3 and ∼2.8 D, respectively. Thus, our calculations suggest that although the fields of the solvating water molecules differ between the extended interface and cluster geometries, the local field experienced by the anion is nearly identical. This supports a picture that the static dipole moment of water, as determined by molecular geometry, is largely responsible for the iodide response. Overall, the iodide dipole moment increased by a factor of ∼1.5 as the anion approached the interface, a trend that is also found for the empirical polarizable model.46 If one examines the local dipoles of both the waters and the iodide in the extended interface simulations, one can see that once the water molecules start to deviate from their bulk dipole value, there is a significant change in the response of the iodide dipoles. Specifically, our data in Figure 2 points to a small

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Figure 2. Averaged dipole moment Æμæ for water molecules (open circles), iodide (open squares), and the projected dipole moment of the anion along the reaction coordinate (filled circles); dashed lines represent the average bulk dipole moment for water molecules. The top panel is for the cluster geometry, and the bottom panel is for the extended interface. The gray scaled bar represents the average dipole moment (including the standard deviation) for iodide under bulk solvation.31 Values for the dipole moments of both the solvating waters and iodide for the extended interface are in good agreement with bulk studies in the interior region of the slab.20,31,49

decrease in the iodide dipole from its bulk value as the iodide responds to the broken symmetry of the interface, namely, when the water dipoles start to deviate from their bulk value. Interestingly, it is in this region of decrease of the iodide dipole moment that we see a concomitant change in the PMF as it starts to experience a small rise before being stabilized. Clearly, whether this small deviation in the iodide dipole and PMF are statistically significant cannot be determined by the limited sampling in the DFT-D calculations. Nevertheless, a statistically significant small rise in the PMF before the local minimum has been observed in MD simulations of iodide at the extended air
water interface.21 Moreover, this small rise in the PMF has real physical significance in the DCT (see Figure 1), namely, it is the point where the polarizable anion starts to see a repulsion from its image charge before it is stabilized by polarization/cavitation. Whether this feature in the PMF and dipole moments has any statistical significance in the present calculations will be addressed in future research. The last point to discuss in Figure 2 is the evolution of the dipole projected along the reaction coordinate for both the cluster and extended interface geometries. Figure 2 shows that the iodide in both the cluster and extended interface geometries experiences a similar trajectory to their respective surfaces. However, differences in the projected dipole moments tell a different story. For the extended interface, it is already evident that the iodide is experiencing the interface well before the GDS.47 This can be gleaned by examining the solid circles in the bottom panel of Figure 2, where we see that the value of the projected dipole almost equals the dipole moment indicating that 1090

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Scheme 1. Hydrogen Atom Radial Angular Distribution Functions about Iodide Approaching the Interface, in Which the Angle Is Defined between the Iodide
Hydrogen Vector and the Surface Normala

a

Each panel is a different location along the reaction coordinate as described in the text. The purple circle represents the size of the anion based on our estimated size. For reference, the size of a water molecule is given in the blue circle in the lower right-hand corner of the figure. The black arrows reveal the direction and magnitude of the projected dipole moment onto the reaction coordinate (scale shown in fourth panel). The dipole moment is directed from the center of the negative charge to the ion position.

it is nearly aligned with the interface normal. Therefore, even though the value of the iodide dipole is consistent with the bulk solvated value, it is already experiencing the broken symmetry. For the case of the cluster, we see a different scenario. Here, the projected dipole moment of iodide in the interior of the cluster does not feel the effects of the surface. However, there is an abrupt transition in the cluster going from spherical solvation to a more ordered solvation shell. At this point, the dipole moment of the iodide increases to maximize its interaction energy. In the extended interface, this transition is much more subtle and gradual. This can be rationalized in terms of the finite curvature of a cluster surface where water molecules at the surface have fewer options to maximize their water
water interaction,48 whereas in the extended interface, there are significantly more microstates available to satisfy water’s hydrogen bonding. Last, in Scheme 1, we show a progression of the solvation shell of iodide as a function of the reaction coordinate in the extended DFT-D interface geometry. A similar analysis for empirical potentials with and without polarization are discussed in the literature.46,50 In the first panel of Scheme 1, we find that in the bulk regions of the slab, we establish a spherically symmetric solvation of iodide. When the first solvation shell of the anion starts to experience the GDS (panel 2 of Scheme 1), there is a change in the directionality of the solvation of the iodide. Specifically, we see a transition from the spherically symmetric solvation to a structured one immediately before the GDS where the solvation shell distorts. Slightly before the GDS (third panel in Scheme 1), we see the solvation pattern evolve into a halfsolvation shell, where the anion is still uniformly solvated in the half-shell. Here, the dipole moment starts to grow, but the first solvation shell water molecules do not display significant ordering. This flexibility of the half-solvation shell, in conjunction with the ordering of the dipole moment, is where the anion experiences the local minimum in the PMF in Figure 1. At the GDS (forth panel in Scheme 1), the dipole of the iodide remains strongly aligned with the interfacial coordinate, but we begin to see a more structured half-solvation shell of water molecules. It is in this region where the induced water structure around the anion likely contributes to the repulsive region in the PMF. A similar analysis can be performed in Scheme 2 for the cluster simulations. Panels 1 and 2 in Scheme 2 are the solvation of iodide in the cluster at
2.5 and
1.5 Å, respectively, as defined in the top

Scheme 2. Same analysis as that in Scheme 1 for Iodide in the Cluster Geometrya

a

The dashed line is the estimated cluster surface. All other quantities are similarly defined in Scheme 1.

panel of Figure 2. Here, we see that the jump in the directionality of the dipole moment in Figure 2 is accompanied by the development of a highly directed solvation structure. We have performed extensive simulations of iodide adsorption at both the surface of clusters and the extended air
water interface using DFT-D. Our results suggest that the single-anion PMF from bulk to the air
water interface is quantitatively different from those obtained by earlier empirical polarizable interaction potentials.14 Our previous findings support a picture where empirical polarizable potentials produce an overstructured first solvation shell for iodide that seems to manifest itself in a larger free energy of transfer from the bulk to the air
water interface (see Figure 1).31 The PMF of iodide from bulk to the extended air
water interface obtained from the DFT-D calculations is in good agreement with a recent DCT that contains only cavitation and polarization.16 Understanding the precise role of interfacial fluctuations22 that are contained in the present molecular picture to the stability of the anion at the air
water interface is an important question that is the subject of future research. With the analysis put forth in the present study, we rationalize the evolution of the PMF by examining both the local solvation structure and polarization response of iodide as it evolves along the reaction coordinate. Although it is hard to directly compare the driving force of the free energetics between clusters (100 K) and the extended interface (300 K), we see subtle differences in the iodide response due to the flexibility of the local solvating water molecules. A systematic study of the PMF of halides in 1091

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The Journal of Physical Chemistry Letters water clusters of increasing size using the aforementioned protocol is currently underway to attempt to make a more sound connection between the cluster and extended interface results. In closing, our simulations point to a picture where both the first solvation shell of the anion and its polarization response are crucial ingredients for anion adsorption. It is clear that more thorough studies are needed, utilizing interaction potentials based in quantum mechanics, with more extensive sampling, to understand the precise balance of interactions that give rise to adsorption of ions at interfaces.22 Recent attempts have been successful to lower the free energies of adsorption of iodide at the extended air
water interface by modifying either the polarization response of iodide18
20 or the empirical potential for iodide.21,22 These studies have brought into full view the subtle nature of the balance between polarization and cavitation, which are necessary for anion adsorption. However, we believe that a sophisticated treatment of the electrostatics is imperative in order to capture the correct molecular structure of the first solvation shell of iodide.31 In this study, we have established the role of the first solvation shell and its effect on the PMF, although future research will be needed to investigate whether DFT-D can reproduce other important thermodynamic properties of ions.51

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy (DOE) Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific Northwest National Laboratory (PNNL) is operated for the Department of Energy by Battelle. The extended slab calculations were performed under the auspices of our INCITE 2008-2011 award using the CRAY XT5 at Oak Ridge National Laboratory (ORNL) that is supported by the Office of Science of the U.S. DOE under Contract No. DE-AC0500OR22725. Cluster calculations were performed using the CRAY XT5 at NERSC at Lawrence Berkeley National Laboratory. Bulk studies were performed using NWICE at the Environmental Molecular Sciences Laboratory at PNNL. M.D.B. is grateful for the support of the Linus Pauling Distinguished Postdoctoral Fellowship Program at PNNL. We also acknowledge insightful conversations with Profs. Doug Tobias and Yan Levin and useful discussions with our PNNL colleagues, Dr. Greg Schenter, Dr. Shawn Kathmann, and Dr. Liem Dang. ’ REFERENCES (1) Tobias, D. J.; Hemminger, J. C. Chemistry — Getting Specific about Specific Ion Effects. Science 2008, 319, 1197–1198. (2) Petersen, P. B.; Johnson, J. C.; Knutsen, K. P.; Saykally, R. J. Direct Experimental Validation of the Jones
Ray Effect. Chem. Phys. Lett. 2004, 397, 46–50. (3) Winter, B.; Weber, R.; Schmidt, P. M.; Hertel, I. V.; Faubel, M.; Vrbka, L.; Jungwirth, P. Molecular Structure of Surface-Active Salt Solutions: Photoelectron Spectroscopy and Molecular Dynamics Simulations of Aqueous Tetrabutylammonium Iodide. J. Phys. Chem. B 2004, 108, 14558–14564. (4) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, E. L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M.

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Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563–566. (5) Petersen, P. B.; Saykally, R. J.; Mucha, M.; Jungwirth, P. Enhanced Concentration of Polarizable Anions at the Liquid Water Surface: SHG Spectroscopy and MD Simulations of Sodium Thiocyanide. J. Phys. Chem. B 2005, 109, 10915–10921. (6) Petersen, P. B.; Saykally, R. J. Adsorption of Ions to the Surface of Dilute Electroyte Solutions: The Jones
Ray Effect Revisited. J. Am. Chem. Soc. 2005, 127, 15446–15452. (7) Petersen, P. B.; Saykally, R. J. Probing the Interfacial Structure of Aqueous Electrolytes with Femtosecond Second Harmonic Generation Spectroscopy. J. Phys. Chem. B 2006, 110, 14060–14073. (8) Otten, D. E.; Petersen, P. B.; Saykally, R. J. Observation of Nitrate Ions at the Air/Water Interface by UV-Second Harmonic Generation. Chem. Phys. Lett. 2007, 449, 261–265. (9) Brown, M. A.; D’Auria, R.; Kuo, I.-F. W.; Krisch, M. J.; Starr, D. E.; Bluhm, H.; Tobias, D. J.; Hemminger, J. C. Ion Spatial Distributions at the Liquid
Vapor Interface of Aqueous Potassium Fluoride Solutions. Phys. Chem. Chem. Phys. 2008, 10, 4778–4784. (10) Jungwirth, P.; Tobias, D. J. Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B 2001, 105, 10468–10472. (11) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259–1281. (12) Onsager, L.; Samaras, N. N. T. The Surface Tension of Debye
H€uckel Electrolytes. J. Chem. Phys. 1934, 2, 528–537. (13) Benjamin, I. Theoretical-Study of Ion Solvation at theWater Liquid
Vapor Interface. J. Chem. Phys. 1991, 95, 3698–3709. (14) Dang, L. X. Computational Study of Ion Binding to the Liquid Interface of Water. J. Phys. Chem. B 2002, 106, 10388–10394. (15) Dang, L. X.; Chang, T.-M. Molecular Mechanism of Ion Binding to the Liquid/Vapor Interface of Water. J. Phys. Chem. B 2002, 106, 235–238. (16) Levin, Y.; dos Santos, A. P.; Diehl, A. Ions at the Air
Water Interface: An End to a Hundred-Year-Old Mystery?. Phys. Rev. Lett. 2009, 103, 257802. (17) Levin, Y. Polarizable Ions at Interfaces. Phys. Rev. Lett. 2009, 102, 084715. (18) Wick, C. D. Electrostatic Dampening Dampens the Anion Propensity for the Air
Water Interface. J. Chem. Phys. 2009, 131, 084715. (19) Zhao, Z.; Rogers, D. M.; Beck, T. L. Polarization and Charge Transfer in the Hydration of Chloride Ions. J. Chem. Phys. 2010, 132, 014502. (20) Guardia, E.; Skarmoutsos, I.; Masia, M. On Ion and Molecular Polarization of Halides in Water. J. Chem. Theory Comput. 2009, 5, 1449–1453. (21) Horinek, D.; Herz, A.; Vrbka, L.; Sedlmeier, F.; Mamatkulov, S. I.; Netz, R. R. Specific Ion Adsorption at the Air/Water Interface: The Role of Hydrophobic Solvation. Chem. Phys. Lett. 2009, 479, 173–183. (22) Noah-Vanhoucke, J.; Geissler, P. L. On the Fluctuations that Drive Small Ions toward, and away from, Interfaces between Polar Liquids and Their Vapors. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15125–15130. (23) Kuo, I.-F. W.; Mundy, C. J. An Ab Initio Molecular Dynamics Study of the Aqueous Liquid
Vapor Interface. Science 2004, 303, 658–660. (24) McGrath, M. J.; Siepmann, J. I.; Kuo, I.-F. W.; Mundy, C. J. Vapor
Liquid Equilibria of Water from First Principles: Comparison of Density Functionals and Basis Sets. Mol. Phys. 2006, 104, 3619–3626. (25) Baer, M. D.; Kuo, I.-F. W.; Bluhm, H.; Ghosal, S. Interfacial Behavior of Perchlorate versus Chloride Ions in Aqueous Solutions. J. Phys. Chem. B 2009, 113, 15843–15850. (26) Ho, M.-H.; Klein, M. L.; Kuo, I.-F. W. Bulk and Interfacial Aqueous Fluoride: An Investigation via First Principles Molecular Dynamics. J. Phys. Chem. A 2009, 113, 2070–2074. (27) Grimme, S. Accurate Description of Van der Waals Complexes by Density Functional Theory Including Empirical Corrections. J. Comput. Chem. 2004, 25, 1463–1473. 1092

dx.doi.org/10.1021/jz200333b |J. Phys. Chem. Lett. 2011, 2, 1088–1093

The Journal of Physical Chemistry Letters (28) Schmidt, J.; VandeVondele, J.; Kuo, I.-F. W.; Sebastiani, D.; Siepmann, J. I.; Hutter, J.; Mundy, C. J. Isobaric-Isothermal Molecular Dynamics Simulations Utilizing Density Functional Theory: An Assessment of the Structure and Density of Water at Near-Ambient Conditions. J. Phys. Chem. B 2009, 113, 11959–11964. (29) Mundy, C. J.; Kuo, I.-F. W.; Tuckerman, M. E.; Lee, H.-S.; Tobias, D. J. Hydroxide Anion at the Air
Water Interface. Chem. Phys. Lett. 2009, 481, 2–8. (30) K€uhne, T. D.; Pascal, E. K.; Jung, Y. New Insights into the Structure of the Vapor/Water Interface from Large-Scale First-Principles Simulations. J. Phys. Chem. Lett. 2011, 2, 105–113. (31) Fulton, J. L.; Schenter, G. K.; Baer, M. D.; Mundy, C. J.; Dang, L. X.; Balasubramanian, M. Probing the Hydration Structure of Polarizable Halides: A Multiedge XAFS and Molecular Dynamics Study of the Iodide Anion. J. Phys. Chem. B 2010, 114, 12926–12937. (32) VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. QUICKSTEP: Fast and Accurate Density Functional Calculations using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103–128. (33) VandeVondele, J.; Hutter, J. Gaussian Basis Sets for Accurate Calculations on Molecular Systems in Gas and Condensed Phases. J. Chem. Phys. 2007, 127, 114105. (34) Goedecker, S.; Teter, M.; Hutter, J. Separable Dual-Space Gaussian Pseudopotentials. Phys. Rev. B 1996, 54, 1703–1710. (35) Martyna, G. J.; Klein, M. L.; Tuckerman, M. E. Nose
Hoover Chains — The Canonical Ensemble via Continous Dynamics. J. Chem. Phys. 1992, 97, 2635–2643. (36) Becke, A. D. Density-Functional Exchange-Energy Approximation with Dorrect Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098–3100. (37) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle
Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785–789. (38) Kumar, S.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A.; Rosenberg, J. M. The Weighted Histogram Analysis Method for FreeEnergy Calculations on Biomolecules. 1. the Method. J. Comput. Chem. 1992, 13, 1011–1021. (39) Roux, B. The Calculation of the Potential of Mean Force using Computer Simulations. Comput. Phys. Commun. 1995, 91, 275–282. (40) Koch, D. M.; Peslherbe, G. H. On the Transition from Surface to Interior Solvation in Iodide
Water Clusters. Chem. Phys. Lett. 2002, 359, 381–389. (41) Kuo, I.-F. W.; Mundy, C. J.; Eggimann, B. L.; McGrath, M. J.; Siepmann, J. I.; Chen, B.; Vieceli, J.; Tobias, D. J. Structure and Dynamics of the Aqueous Liquid
Vapor Interface: A Comprehensive Particle-Based Simulation Study. J. Phys. Chem. B 2006, 110, 3738–3746. (42) Marzari, N.; Vanderbilt, D. Maximally Localized Generalized Wannier Functions for Composite Energy Bands. Phys. Rev. B 1997, 56, 12847–12865. (43) Silvestrelli, P. L.; Parrinello, M. Water Molecule Dipole in the Gas and in the Liquid Phase. Phys. Rev. Lett. 1999, 82, 3308–3311. (44) Silvestrelli, P. L.; Parrinello, M. Erratum: Water Molecule Dipole in the Gas and in the Liquid Phase [Phys. Rev. Lett. 82, 3308, (1999)]. Phys. Rev. Lett. 1999, 82, 5415–5415. (45) Latimer, W. M.; Pitzer, K. S.; Slansky, C. M. The Free Energy of Hydration of Gaseous Ions, and the Absolute Potential of the Normal Calomel Electrode. J. Chem. Phys. 1939, 7, 108–111. (46) Wick, C. A.; Xantheas, S. S. Computational Investigation of the First Solvation Shell Structure of Interfacial and Bulk Aqueous Chloride and Iodide Ions. J. Phys. Chem. B 2009, 113, 4141–4146. (47) Archontis, G.; Leontidis, E. Dissecting the Stabilization of Iodide at the Air
Water Interface into Components: A Free Energy Analysis. Chem. Phys. Lett. 2006, 420, 199–203. (48) Stuart, S. J.; Berne, B. J. Surface Curvature Effects in the Aqueous Ionic Solvation of the Chloride Ion. J. Phys. Chem. A 1999, 103, 10300–10307. (49) Heuft, J. M.; Meijer, E. J. Density Functional Theory Based Molecular-Dynamics Study of Aqueous Iodide Solvation. J. Chem. Phys. 2005, 123, 094506.

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(50) Yagasaki, T.; Saito, S.; Ohmine, I. Effects of Nonadditive Interactions on Ion Solvation at the Water/Vapor Interface: A Molecular Dynamics Study. J. Phys. Chem. A 2010, 114, 12573–12584. (51) Rogers, D. M.; Beck, T. L. Quasichemical and Structural Analysis of Polarizable Anion Hydration. J. Chem. Phys. 2010, 132, 014505.

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