Molecular Dynamics Analysis of NaOH Aqueous Solution Surface and

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Molecular Dynamics Analysis of NaOH Aqueous Solution Surface and the Sum Frequency Generation Spectra: Is Surface OH Detected by SFG Spectroscopy? -

Takako Imamura, Tatsuya Ishiyama, and Akihiro Morita J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

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TheJournalofPhysicalChemistry

Molecular Dynamics Analysis of NaOH Aqueous Solution Surface and the Sum Frequency Generation Spectra: Is Surface OH Detected by SFG Spectroscopy? Takako Imamura a,† Tatsuya Ishiyama b,† and Akihiro Morita c,†,‡ Department of Chemistry, Graduate School of Science, Tohoku University Aoba-ku, Sendai 980-8578, Japan , and Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan E-mail: [email protected]

a Current

address: WPI-AIMR, Tohoku University, Sendai 980-8577, Japan. Current address: Department of Environmental Applied Chemistry, University of Toyama, Toyama 930-8555, Japan. c Phone number: +81-22-795-7717 b

Abstract We elucidated the phase-sensitive Sum Frequency Generation spectrum of NaOH aqueous solution by molecular dynamics simulation. The MD analysis reproduced the effects of NaOH on the imaginary χ 2 spectrum, characterized with a positive (upward) shift in the amplitude in mid-frequency region (3300–3600 cm 1 ) and a negative (downward) shift in low-frequency (3000–3200 cm 1 ). We found that the positive shift is a consequence of electric double layers, while the negative shift is attributed to the first solvation shell of OH– . The latter mechanism To

whom correspondence should be addressed University ‡ Kyoto University † Tohoku

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ACSParagonPlusEnvironment


generally holds for electrolyte solutions, though it becomes conspicuous for the NaOH solution 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in the low-frequency tail region. These perturbation mechanism evidences that OH– at the surface is fully hydrated and not preferentially exposed to the surface. Keywords: electric double layer, first solvation shell, χ2 phase, SFG simulation

1 Introduction Aqueous solutions contain H 3 O+ and OH– ions as a consequence of autoionization of water, 2 H2 O

ªH O 3

+

+ OH– , and their relative amounts vary with pH of the solution. The pH value is

an indicator of proton activity in solution, which has profound influences on a diverse range of fields in aqueous chemistry and biology. While the relative amounts of hydronium and hydroxide ions in bulk aqueous solutions with varying pH are well known, their relative amounts at the water surface are still in controversy. 1–3 In particular, the surface of basic solution in high pH condition is less understood than that of acidic solution, since a number of previous investigations have not reached a consistent molecular picture about the basic solution surface, as described below. The apparent discrepancies among previous experimental and theoretical studies imply either uncertainty in their experimental interpretation, unless the experimental measurement itself is problematic, or insufficient accuracy of the computational simulations. It may be hard to prove the reliability of an experimental or theoretical study by its method alone. Thus complementary collaboration of experiment and theory is a quite powerful route toward establishing a unified picture about the basic surface. Therefore, we perform the molecular dynamics (MD) analysis of the sum frequency generation (SFG) spectroscopy for the basic solution surfaces. The present MD simulation provides reliable microscopic interpretation for the SFG spectra of the basic solution surfaces, while the SFG spectroscopy provides a critical opportunity to examine the accuracy of the MD simulation. Acidic water surface in low pH conditions has come to be relatively well understood by various investigations, such as surface tension, 4 MD simulations, 5–7 and surface-specific nonlinear spec-

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troscopy. 6,8,9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The surface tension of the acidic water decreases with its concentration, 4 which is

interpreted via the thermodynamics as positive surface excess of hydronium ions and their counter anions. This surface tension measurement implies a picture that relatively abundant hydronium and counter ions are present at the water surface in low pH conditions. Theoretical simulation studies to date have supported the above picture. Using the multistate empirical valence bond (MS-EVB) model, Voth and co-workers predicted that the protons are preferentially distributed on the water surface. 5 Classical MD studies with polarizable models also reached the similar conclusion, 6,7 and the stabilization of protonated water at the surface is further supported by QM/MM calculation. 10 The surface structure of acid solutions was also studied by surface-specific nonlinear spectroscopy. The vibrational SFG spectroscopy, which can selectively provide vibrational spectra of surface species, showed enhanced intensity in hydrogen-bonded OH stretching region by the acid. 6,8,9

The perturbed vibrational spectra implied a perturbed surface structure of the acid solution,

though detailed interpretation was not straightforward. Ishiyama and Morita subsequently analyzed the SFG spectra of acid solutions with the MD simulation, 7 and succeeded in reproducing the spectral features by the MD simulation. Their analysis corroborated the microscopic surface structure of acid solution calculated by MD simulation in relation to the observed SFG spectra. The mechanism of the spectral perturbation by the acid is briefly summarized in the following manner. The surface-active hydronium cation and the counter anion residing at slight interior formed an electric double layer at the water-vapor interface, which orients the surface water in between to direct their OH moieties to the liquid phase. The enhanced orientational order is reflected in the observed enhancement of the SFG intensity. To summarize the studies on acidic surface, the enhanced distribution of the hydronium and counter ions is deduced from the techniques with molecular resolution, such as the surface-sensitive SFG spectroscopy and the MD simulation. The microscopic picture is consistent to the thermodynamic argument for the positive excess on the basis of the surface tension measurement. 11 Basic solution surface, on the other hand, is less understood, since the surface propensity of OH– is a controversial issue among a variety of experimental observations, including surface ten-

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sion, 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

zeta potential, 12

mass spectrometry using aqueous jet, 13

ice surface, 14

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and surface-sensitive

nonlinear spectroscopies. 3,6,8,9 The surface tension of basic solution increases with concentration, 4

indicating negative surface excess of OH– . Electrophoresis measurement shows that bubbles or

oil droplets in basic solutions are attracted to the positive electrode. Such a negative zeta potential on the bubbles is observed in a wide range of pH, from basic to relatively weak acidic condition. 12 The excess negative charge on the surfaces of bubbles or oil droplets in the neat water is thought to be originated from OH– via the autoionization of water. 2 Surface-sensitive titration experiment using aqueous microjet and electrospray mass spectrometry by Mishra et al. argued that the water surface is more basic than bulk. 13 In a related system of ice surface, Kang and co-workers showed that OH– migrates from interior to the surface, 14 because OH– behaves as a defect in the hydrogen bond network of crystalline water. A number of surface-selective spectroscopies were also applied to the basic surfaces to probe their structure. SHG measurement by Petersen and Saykally excluded the OH– enhancement at the water-vapor interface. 3 Photoelectron spectroscopy by Winter et al. also denied the surface enhancement of OH– . 15 The vibrational SFG spectroscopy reported decreased intensity of hydrogen-bonding OH stretching band by the addition of NaOH at pH 6,8,9

13.

However, the interpretation of the perturbed spectra in relation to the surface structure is not

so straightforward. Previous theoretical studies on the basic surfaces have not formed a consensus yet. Ab initio MD simulation of Mundy et al. showed that a single OH – has a small stabilization at the air-water interface compared to the bulk. 16 MS-EVB MD simulation of Wick and Dang showed that the free energy curve of a single OH– is flat near the surface, implying neither stabilization nor repulsion, 17 whereas the OH– is repelled from the air-water interface in the presence of counter sodium cation. 18

Classical MD simulation with a polarizable force field of Mucha et al. showed that OH – is

weakly repelled from the air-water interface in NaOH aqueous solution with finite concentration. 6

Such apparent discrepancy among different models should be resolved via critically comparing

these calculations with experimental data. Therefore, we performed the MD analysis of the SFG spectra of basic solutions. Because the SFG spectroscopy provides detailed information on the sur-

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face structure with a comparable molecular resolution to the MD simulation, the close cooperation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

is quite useful to assess the reliability of the surface structure calculated by the MD simulations. We have demonstrated the usefulness of the MD analysis of SFG in electrolyte aqueous surfaces, such as some acid and salt solutions. 19,20 The perturbation mechanism of the solute ions on the SFG spectra has been mainly explained in terms of the electric double layer formation. 7,21,22 The electric field generated by the charge separation in the double layers may influence the orientation of water molecules near the surface and thereby their SFG spectra. In the present study, the MD analysis is applied to the surface of aqueous NaOH solution. We found that another perturbation mechanism of the solute ions is important in addition to the electric double layer formation, in relation to the first solvation shell of the ions. The analysis renders clear interpretation to the observed SFG spectra of the basic solution as well as crucial examination to the MD simulation of the basic solution surfaces. Thereby we argue a strong evidence that OH – detected by the SFG spectroscopy is not exposed to the surface, as shown below. The remainder of this paper is constructed in the following. Theoretical aspects and computational settings are described in Section 2. The calculated surface structure of basic solution is presented in Section 3, and the SFG spectra are thoroughly analyzed in Section 4. Concluding remarks follow in Section 5.

2 Simulation Methods In the present simulation, we treat the surface of 1.2 M NaOH solution. This concentration was chosen in common with that of previous experimental SFG spectra 8,9 to facilitate comparison. We also examined for the surfaces of pure water, NaCl and NaI solutions for control systems, as described below. The MD simulations were conducted with our program developed for the SFG calculations. The numerical computations were carried out using the supercomputers at Research Center for Computational Science, Okazaki, Japan.

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2.1 MD Conditions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

System dimension:

The MD system for the 1.2 M NaOH solution consists of 958 water molecules,

21 Na and 21 OH ions, while the pure water system consists of 1000 water molecules. Those molecules are put in a rectangular unit cell of 30 Å

30 Å

150 Å with three-dimensional pe-

riodic boundary condition. The unit cell is elongated along the z axis so that the system takes a slab geometry with the interfaces normal to the z axis. The liquid-gas interfaces are formed at both sides of the slab.

Molecular models:

All the constituent species are described with flexible and polarizable mod-

els to allow for the SFG calculation. Each species has interaction sites which carry the LennardJones parameters and partial charges for the non-bonding Lennard-Jones and Coulombic interactions, respectively. Besides these non-bonding parameters, each molecule has a polarizability at the center of mass to represent the induced dipole. The molecular models for water and Na were taken from our previous work. 23 The polarizable model of OH was adopted from the model 2 of Ref. [ 24 ] by Vácha et al., with intramolecular force field implemented from the water model. 23 The present model of OH– does not take account of charge transfer or covalent character, which is beyond the classical description of intermolecular interaction. The present model is still useful enough to describe the perturbed water structure at the surface, since the observed SFG spectrum of the basic solution is found to be governed by the perturbed water structure rather than the OH – itself, as discussed in Section 4.2. The short-range electrostatic interactions were properly attenuated using the damping functions 23 during the MD trajectory calculations. The Ewald summation method 25 was utilized to correct long-range Coulombic and dipole interactions, where the Ewald separation parameter was set to be 0.242 Å 1 . 23

Equilibration and sampling: The MD trajectories were generated from independent 256 initial configurations in parallel using parallel computers. The Newton’s equation of motion was integrated by the velocity Verlet algorithm 25 with a few kinds of time steps appropriate for different

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equilibration and sampling phases as shown below. Initially, the constituent molecules were ran1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

domly enclosed within a confined volume of the cell in 15 Å z 15 Å, and the steepest descent relaxation was performed to remove the overlap between molecules. Then the equilibration stage prior to the sampling simulation is constructed with the following three phases. The first equilibration phase was used mainly to facilitate the translational relaxation of the system including the ions, where the intramolecular deformations were prohibited by the RATTLE procedure. 26 The hydrogen mass of water and OH– was tripled in order to slow the librational motions and thereby to allow for a relatively long time step, while it does not affect the equilibrium structure in the classical mechanics. This equilibration phase was performed for 1 ns with a time step of 5 fs, while the temperature was maintained at 298.15 K by velocity scaling. In this phase, the centers of mass of the molecules were confined in z thick 2 z zthick 2 to prevent vaporization, where the thicknesses zthick was determined from the bulk densities of the liquids; i.e. 31.7 and 33.2 Å for the NaOH solution and the pure water, respectively. Subsequently, the second equilibration phase was carried out by lifting the aforementioned constraints, i.e. the RATTLE procedure (allowing the intramolecular vibrations) and the confinement on the centers of mass. In this phase, the time step was gradually reduced toward 0.61 fs, and a total of 4 ps simulation was performed for each trajectory. Then the third equilibrium run was carried out for 3 ps in the same conditions with those in the sampling simulation. Accordingly, the hydrogen mass of water and OH– was changed to the normal one, and the time step of integration was 0.61 fs. The temperature control at 298.15 K was done using the Berendsen thermostat with a damping time constant of 0.4 ps. 23 The statistical sampling of surface structure and spectra was subsequently taken for 860 ps for each trajectory generated from different initial configuration. During the sampling run, we monitored the density profiles and confirmed that they are stable during the sampling period. The 256 trajectories = 220 ns for each system, 1.2M

total sampling time amounts to 860 ps/trajectory NaOH solution or pure water.

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2.2 SFG Calculation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The computation of the SFG spectra was performed using the time-dependent representation of the non-linear susceptibility. 27,28 Here we briefly summarize the SFG calculation procedure. The SFG spectrum is essentially determined by the second-order nonlinear susceptibility tensor element,

2

χ pqr , where p q r denote the spatial coordinates, x z. The conventional SFG intensity spectrum

2

2

2

is proportional to the absolute square of χ pqr , ISFG ∝ χ pqr , whereas the recent phase-sensitive

2

SFG spectroscopy 9,29,30 allows us to experimentally detect the real and imaginary parts of χ pqr itself. The relevant suffixes of p q r are determined by the combination of experimental light polarizations. In the following discussion, we deal with ssp combination of light polarization, where the SFG, visible and infrared (IR) are in s, s, and p polarizations, respectively, as the most intensively used combination in previous experiments for aqueous solutions. 6,9 In the vibrational SFG spectroscopy, χ 2 is decomposed into the resonant and non-resonant parts to the IR frequency,

2

2 res χ 2 nonres

χ pqr

χ pqr

(1)

pqr

2 nonres , is considered as real and constant over the IR frequency range.

The non-resonant part, χ pqr

This constant value was taken from our previous work of water 28 so as to be consistent to the experimental spectra. The value of the non-resonant part gives a constant background to the real part of χ 2 (Reχ 2 ), while it does not affect on the imaginary part of χ 2 (Imχ 2 ). The

subsequent discussion in Section 3 will mainly deal with Imχ 2 , where the non-resonant part does not come into play.

2 res , is calculated by the following time correlation

On the other hand, the resonant part, χ pqr formula, 28

2 res

χ pqr

iωIR kB T

∞ 0

dt exp iωIRt A pq 0Mr t

(2)

where ωIR , kB , and T are the angular frequency of the infrared light, Boltzmann constant, and temperature, respectively. A pq and Mr stand for the components of polarizability tensor and dipole moment vector, respectively, of the interface system. In the ssp combination, the relevant compo8


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nents in Eq. (2) are Ayy (or Axx ) and Mz . The A and M are calculated at every four time step of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the sampling MD simulation using the flexible and polarizable models, where the self-consistent condition of polarization is realized. 28 In the self-consistent calculation of A and M, the bare electrostatic interactions among point charges and dipoles were employed with no damping function, to effectively account for the augmented coupling in the short-range interactions in the hydrogenbonding systems. 28,31 In applying the time correlation calculations of Eq. (2) to the slab systems of liquid, the amplitude of χ 2 originates from both sides of the slab. Therefore, the statistical sampling of the interface signal was taken from the two interfaces separately. Since the interfaces at the two sides of the slab are reversed along the z axis, we need to reverse the z axis to take the statistical average over the two sides. Accordingly, we introduced the local coordinate Zˆ common to both sides, where Zˆ

0 is set at the Gibbs dividing surface and Zˆ

0 (Zˆ 0) corresponds to the vapor (liq-

uid) side. The local coordinate Zˆ determines the relative position from the interface along the depth direction.

2.3 Decomposition of SFG spectra The time correlation function of χ 2 in Eq. (2) allows for various ways of decomposition analysis, which could provide further insight into the SFG spectra beyond the experimental observation. We employed the following decomposition schemes in the present work.

Decomposition by species:

The χ 2 signal of Eq. (2) can be decomposed into contributions of

chemical species, i.e. H2 O, Na+ and OH– , in the case of the aqueous NaOH solution. To analyze the effects of the solute species (Na+ and OH– ) on the SFG spectra, we perform the decomposition of χ 2 by the species in the following manner. The time correlation function in Eq. (2) is expressed

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as 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A pq 0Mr t

molecules

α pq k 0

∑ k

species

∑

molecules

∑

μr l t

l

∑ α pq k0

species

∑

k¾i

i

j

∑

l ¾ j

μr l t

(3)

where k, l denote the constituent molecules and ions, and i, j the chemical species. In Eq. (3), α pq k is effective polarizability including the local field correction factors. 27 The last form of Eq. (3) can be decomposed into 9 terms as a product of the three species i (=H 2 O, Na+ , OH– ) and j (=H2 O, Na+ , OH– ). Among the 9 terms, the diagonal ones (i species, while the off-diagonal ones (i

j) are attributed to the contribution to each

j) to the cross contributions. Such decomposition based

on Eq. (3) is used to discuss the spectral perturbation in terms of the constituent chemical species.

Decomposition by depth:

The χ 2 signal should arise from the vicinity of interface ( Zˆ 0), but

not from the interior of the slab (Zˆ 0) where molecular orientation is isotropic. We can identify

the spatial region from which the SFG signal originates, by decomposing χ 2 into the depth region

near the surface. In the practical calculations of A and M in Eq. (2), we employed the following filtering function, 22

gZˆ

Zˆ tanh

Zˆ ¼ Zˆ w

0

Zˆ ¼

(4)

Zˆ Zˆ ¼

where Zˆ ¼ is the lower threshold depth and w is a tapering width. g Zˆ in Eq. (4) acts as a high-pass filter to eliminate the contribution from the spatially deep region of Zˆ

Zˆ ¼.

The tapering width

w was set to be 1 Bohr (= 0.53 Å). When constructing A and M from molecular contributions of various depth, each molecular contribution is multiplied with this filtering function so that only the contributions above Zˆ ¼ were taken into account in Eq. (2). The threshold Zˆ ¼ was varied from Zˆ ¼

3 Å to 14 Å to see the convergence behavior. We found that the calculated χ 2 amplitude

in Eq. (2) is converged at Zˆ ¼

9 Å for both the 1.2M NaOH solution and water, indicating that 10


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the SFG signal actually comes from the surface region of Zˆ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

9 Å. Therefore, we calculated A

9 Å in the following simulation unless otherwise noted.

and M of Eq. (2) using Zˆ ¼

2.4 Effect of first solvation shell The mechanism of spectral perturbation by the solute ions is further discussed by investigating the contribution of the first solvation shell (FSS) around the ions, since the water molecules in the vicinity of the solute ions should be most perturbed by the ions. The FSS of OH – was defined within the center-of-mass distance from OH– smaller than 3.5 Å, while that of Na+ within the distance smaller than 3.0 Å. These threshold distances correspond to the first minimum of the respective calculated radial distribution functions. The number of water molecules in FSS and their orientational structure were examined as a function of the depth from the surface. We calculated the contribution of the FSS water to the time correlation function by A pq 0Mr t

FSS

ions

∑ m

∑

k¾FSSm

α pq k 0

∑

l ¾FSSm

μr l t

(5)

where m denotes the ions, and k l denotes the water molecules in the FSS of the ion m. The suffix m may refer to the ions of either species of Na+ or OH– . During the MD time development to calculate Eq. (5), water molecules may migrate from the FSS to the outer region or vice versa, which blurs the definition of the FSS contribution in Eq. (5). To circumvent the problem associated to the finite duration time in FSS, we updated the assignment of FSS every 5 ps during the calculation of Eq. (5). The update interval was chosen so as to be sufficiently short compared to the duration time of a water molecule in the FSS of OH– . Comparison with Cl– and I– : The FSS of OH– at the surface and its influence on the SFG spectra are further investigated in comparison with those of other anions, such as Cl – and I– . The surface structure and the SFG spectra of NaCl and NaI solutions have been previously studied in our group, 22,23 though the effect of FSS on the spectra has not been examined. Therefore,

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we calculated the 1.2 M NaCl and NaI aqueous solutions in the same conditions with the NaOH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

solution, except that OH– was replaced with either Cl– or I– . The molecular models for Cl– and I– were adopted from Ref. [ 23 ]. We prepared the NaCl and NaI solutions by replacing OH– of the NaOH solution with Cl– and I– , respectively, and sufficient equilibration was performed to reach the equilibrium surface structure for both solutions. After we confirmed that the calculated density profiles of the NaCl and NaI solutions reproduced those in Ref.[ 23 ], the analysis of the FSS for these solutions was performed in the same way as that for the NaOH solution described above.

3 Surface Structure This section presents the MD results of surface structure of 1.2 M NaOH solution, including the density profiles of constituent species, orientational structure of water molecules and OH – ions. The influence of the solute on the surface structure is discussed in comparison with the surface structure of pure water.

Density profiles:

Figure 1 displays the density profiles of H2 O, Na+ and OH– across the surface

of 1.2 M NaOH solution, where the density of each species is normalized with that in the bulk liquid. For comparison, the density profile of H 2 O at the pure water surface is also given in the pale blue bashed line. We see that the profile at pure water (pale blue dashed) nearly coincides with that at the NaOH solution (black), indicating that the ions little influence on the density profile of H2 O. The 10-90 thicknesses for the H2 O profiles are estimated to be 298 002 and 309 002 Å for the NaOH solution and for pure water, respectively. The density profiles of Na+ and OH– in the NaOH solution show two following features. First, the relative concentration of either ion species is obviously smaller near the Gibbs dividing surface (Zˆ

0 Å) than in the bulk (Zˆ 0 Å), and the density profile of either ion is a monotonic curve

with no maximum point. This feature indicates that these ion species have little surface affinity. Second, the OH– profile penetrates to the surface slightly more than Na+ . Consequently, the OH– density is slightly higher than Na + in the vapor side from the crossover point ( Zˆ 12


5 Å), while

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vice versa in the liquid side. Such charge separation of the OH– and Na+ in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

13 Å º Zˆ º 0 Å

implies formation of electric double layers. This behavior is common with the previous results. 6,15

Water orientation:

The ion distribution mentioned above may have significant influences on

the water orientation near the surface. The water orientation is presented with the tilt angle, θ , as illustrated in the inset of Figure 2. θ is defined as an angle between the surface normal and the bisector of the H-O-H angle of the water molecule. Consequently, cos θ orientation that the water dipole points to the vapor phase while cos θ

0 designates an

0 to the liquid phase.

Figure 2 shows the average cos θ of water molecules as a function of their center-of-mass

ˆ Comparing the cos θ profiles of pure water (pale blue) and location at the depth coordinate Z. the NaOH solution (pink dashed), one can see the following two features. First, around the topmost layer (Zˆ

² 3 Å), the two profiles are quite similar and commonly have a minimum at Zˆ 1 Å.

This indicates that the water orientation at the topmost layer is little perturbed by the solute ions. The intact orientational structure at the topmost layer of the NaOH solution is consistent to the ion distribution discussed above in Figure 1, since the topmost layer includes few ions of Na + or OH– . Second, in the pure water, the negative cos θ characteristic to the topmost layer quickly decays to cos θ 0 below Zˆ of the bulk liquid.

5 Å, which is indicative of random orientation in the interior In the NaOH solution, however, a slight positive feature of cos θ arises in

10 Å º Zˆ º 3 Å.

This orientational order is generated in the depth region where the charge

separation of Na+ and OH– is observed in Figure 1. The positive orientation of water molecules is arguably induced by the electric field within the electric double layers of Na + and OH– in the subsurface region. OH– orientation:

The tilt angle θ of the dipole vector from the surface normal is also defined for

the orientation of OH– near the surface. cos θ

1 designates the upright orientation of OH – that

the hydrogen site of OH– points to the vapor, while cos θ

1 to the bulk liquid. We investigated

ˆ Figure 3 displays the twothe orientational distribution of OH – with varying depth coordinate Z. ˆ The 2-dimensional plot shows a dimensional density profile of OH – as a function of cos θ and Z. 13



ˆ = (1015 Å). In the topmost layer near the Gibbs conspicuous density maximum at (cos θ , Z)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dividing surface (Zˆ 0 Å), the upright orientation of OH– (cos θ 1) is dominant. The preferential orientation of OH– at the topmost layer quickly decays inside the bulk, and the orientation of OH – along the cos θ coordinate approaches constant at about Zˆ

5 Å.

We see from this figure that

the OH– orientation becomes almost randomized just below the topmost layer. This feature will have implications to the influence of OH– ion on the SFG spectrum, as discussed in Section 4.3.

4 Analysis of SFG Spectra The preceding section has reported the calculated structure of NaOH solution surface by the MD simulation. Here we analyze the experimental SFG spectra of the basic solution with the calculated surface structure. The analysis aims at critically examining the reliability of the calculated structure of the basic aqueous surface.

4.1 SFG spectra Comparison with experiment:

The SFG spectra in the OH vibrational frequency region for

the 1.2 M NaOH solution were calculated and compared to the ones for pure water. Figure 4 summarizes the computational SFG spectra as well as the experimental ones for comparison. The computational spectra are presented in two ways: the intensity spectra ( χ 2 2 ) in Panel A1 and

the imaginary χ 2 spectra (Imχ 2 ) in B1. The corresponding experimental counterparts 9 are presented in Panels A2 and B2, respectively. Regarding the intensity spectra in Panels A1 and A2, the calculated spectrum for the NaOH solution well reproduces the experimental features, including the entire decrease in intensity in

the hydrogen-bonding frequency region (3000 3600 cm 1 ) for the NaOH solution. 8,9 The sharp band at around 3700 cm 1 , assigned to the dangling OH at the topmost layer, is relatively little perturbed in both panels. This is understood since the NaOH solute has a minor influence on the structure at the topmost layer, as discussed in Section 3. 32 Regarding the imaginary χ 2 spectra 14


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in Panels B1 and B2, the present MD calculation excellently captures the experimental features 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

about the perturbation of NaOH on the Imχ 2 spectrum, in particular the negative (downward)

shift of Imχ 2 in the low-frequency region (3000 3200 cm 1 ) and the positive (upward) shift in the middle region (3300 3600 cm 1 ). These agreements of the computational spectrum with the experimental one strongly corroborate the reliability of the present MD simulation.

Influence of Fresnel factors:

Before discussing the perturbed SFG spectra in relation to the

interface structure of the NaOH solution, we briefly argue that the possible change in the Fresnel factors by NaOH is not likely to be a significant cause of the above spectral perturbation. First, the experimental spectra in Figure 4 (Panels A2 and B2) are shown with the Fresnel factor removed, 9 and thus the observed perturbation of the SFG spectra is not attributed to the varied Fresnel factors. Second, the Fresnel factors are determined by the refractive indexes of the bulk and interface besides the optical geometry of the measurement. 33 Although the interfacial refractive index in the relevant frequency range is hard to be known exactly, the change in the Fresnel factors by the addition of NaOH should be negligibly small to account for the spectral perturbation. The IR absorbance of the 1.2 M NaOH solution in the OH stretching region, which is pertinent to the imaginary refractive index of the bulk solution, agrees with that of pure water within 1 %. 34 This strongly implies that the real refractive index of the NaOH solution should be quite similar as well in the frequency region, since the dispersion of the real refractive index is correlated to the IR absorbance lineshape in general. The optical refractive index is also nearly identical, 1.33 in pure water and 1.34 in 1.2 M NaOH solution. 35 Therefore, the spectral perturbation of the NaOH solution should be attributed to perturbed surface structure rather than the Fresnel factors.

Implication to surface structure — picture of electric double layer and beyond:

The pertur-

bation on the Imχ 2 spectrum provides a valuable clue to the surface structure. In most previous literature, the perturbations of ions were discussed in terms of the electric double layer formation at the aqueous surfaces. The charge separation of the cation and anion layers in the vicinity of the solution surfaces influences the orientation of water molecules via the net electric fields generated 15



Page16of38

between the double layers. The electric double layers are formed in the present NaOH solution 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

surface as well. As we argued in Section 3, the slight charge separation of the Na+ and OH– layers (see Figure 1) induces the orientation of water molecules with their dipole points to the vapor (cos θ

0 in Figure 2). Such induced orientation should result in a positive shift of Imχ 2 ,

since the sign of Imχ 2 reflects the orientation of transition dipole in the ssp polarization. 36 How-

ever, both Panels B1 and B2 of Figure 4 indicate the Imχ 2 spectrum of NaOH solution with the

opposite shifts in the low (3000 3200 cm 1 ) and middle (3300 3600 cm 1 ) frequency bands.

While the positive shift of Imχ 2 is in accord with the electric double layers, the negative shift

in the low frequency region is at variance with this scenario. The opposite shifts of the Imχ 2 spectrum are beyond the aforementioned simple picture of ion perturbation based on the electric double layers. Elucidating this behavior is an intriguing issue, and will be the main issue in the following.

4.2 Decomposition analysis of SFG spectra To understand the perturbation mechanism of the NaOH solution surface, we present the results of the decomposition analyses that were described in Section 2.3.

Contributions of species:

First we discuss whether the spectral perturbation is attributed to the

signal from the solute ions or to the perturbed water structure. The χ 2 spectrum based on the time correlation function of Eq. (3) is decomposed into 9 terms as a product of the polarizability (A) and the dipole moment (M) of three constituent species, namely H2 O (W), Na+ (N) and OH– (I). Accordingly, the 9 terms are denoted by A(W)M(W), ,

A(I)M(I). Figure 5 displays Imχ 2 spectrum for the NaOH solution as well as the water contri-

bution, A(W)M(W). This figure clearly shows that the Imχ 2 spectrum of the NaOH solution is

dominated by the A(W)M(W) contribution, except for a minor deviation in the low-frequency region. This figure also displays the Imχ 2 spectrum of pure water for comparison, which includes only the water contribution. By comparing the A(W)M(W) term of the NaOH solution with that

16


Page17of38


of pure water, we see that the opposite shifts of Imχ 2 in the different frequency regions men-

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

tioned in Section 4.1 are manifested in the A(W)M(W) term. The present decomposition analysis indicates that the mechanism of the opposite shifts of Imχ 2 should be mainly attributed to the perturbed water structure at the surface of NaOH solution, rather than direct contribution of ions.

Depth dependence: region in Zˆ

The calculated Imχ 2 spectra that originate from the restricted surface

Zˆ ¼ are displayed in Figure 6 with varying threshold from Zˆ ¼

0 Å to Zˆ ¼

10 Å.

By comparing the convergence behavior of the NaOH solution (pink dashed) and pure water (pale blue), the difference of the two systems is mostly originated from Zˆ ¼

4 Å to Zˆ ¼ 8 Å. This

analysis indicates that the perturbed SFG spectrum of the NaOH solution reflects the surface water in the depth region of

4 Å ² Zˆ ² 8 Å.

This depth region nearly coincides with the region

where the electric double layers of Na+ and OH– are formed in Figure 1. We also notice that the negative shift of Imχ 2 in the low-frequency region (3000 – 3200 cm 1 ) and the positive shift in

the middle-frequency region (3300 – 3600 cm 1 ) arise in the similar depth region.

Figure 6 also displays the contribution of water in the first solvation shell (FSS) of ions (dark blue lines), where the filtering function gZˆ of Eq. (4) was applied to the center of mass of a water molecule in FSS. We note that this FSS contribution with the dark blue line is included in the Imχ 2 spectrum with the pink dashed line in each panel of Figure 6. The signal of the FSS

water arises in relatively low frequency region (3000 – 3400 cm 1 ) and has negative contribution to Imχ 2 . Therefore, the observed negative shift of Imχ 2 in the low-frequency region is largely attributed to the FSS water molecules. The depth profile in Figure 6 indicates that the signal from the FSS water comes from the depth region from 2 Å to 6 Å. The effects of the FSS water are further analyzed in the next subsection.

4.3 Effect of first solvation shell In this subsection we investigate the FSS water molecules around ions and their effect on the SFG spectra. The argument will be given by comparing three electrolyte solutions, NaOH, NaCl,

17



Page18of38

and NaI, of the same concentration described in Section 2.4. These solutions have the common 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cation, Na+ , and thus differences are entirely attributed to the different anions, OH – , Cl– and I– . Comparing the three anions, OH– is non-spherical while Cl– and I– are spherical. I– has strong propensity to expose itself at the topmost layer of the aqueous solution, while Cl – has weak or little propensity to come to the topmost surface. 23,37 The comparison with Cl– and I– will be utilized to elucidate the feature of the FSS water around OH– and its perturbation on Imχ 2 . Number of FSS water:

First we discuss the structure of FSS water in the three solutions. Fig-

ure 7 shows the average number of FSS water around the ions. The average number of FSS water obviously decreases near the surface at Zˆ

² 5 Å for all the ion species. In particular, Panel A

shows that the decrease of the solvation number at the surface is most pronounced for I – . We note that previous MD studies showed that the local concentration of I – at the surface is significantly enhanced than that in the bulk concentration, 23,37 implying that the enhanced I– ions at the surface tends to have incomplete first solvation shell of water. On the other hand, the reduction of the average number at the surface is less significant for OH– and Cl– . These ions do not preferentially come to the surface region of Zˆ

Orientation of FSS water:

² 5 Å (see Figure 1 in the OH

–

case).

Figure 8 displays the orientational structure of the FSS water molecules

near the surface. We see a clear tendency that ρ cos θ around OH– or Cl– is negative in Panel A while ρ cos θ of the FSS water around Na+ is positive in Panel B at the surface. These opposite orientations are understood with an intuitive picture that the FSS water molecule tends to orient its dipole in favor of the solvating ion, Na + , OH– or Cl– . As illustrated in Figure 9, the dipole orientation of the FSS water molecules tends to be reversed between the anion (OH– , Cl– ) and the cation (Na+ ). The contribution from the surface top layer emerges in the ρ cos θ profiles in Figure 8. Figure 10 A illustrates the dipole distribution of the FSS water around anions. Suppose we average the dipole of the FSS water molecules that are located at an arbitrary Zˆ coordinate. Then their dipole orientations cancel each other at any Zˆ coordinate, when the ions are uniformly distributed, except for the surface top layer. Therefore, the ρ cos θ profile of the FSS water has 18


Page19of38


a finite contribution only at the surface region, as schematically exhibited with the green line in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10 B. This green profile of ρ cos θ is equivalent to that of Panel A of Figure 8 for OH– (blue) or Cl– (green) anion. The above mechanism of the FSS contribution arising from the surface top layer may not be obvious, because an ion (OH– or Cl– ) in the vicinity of the surface is surrounded by the FSS water as illustrated in Figure 9, where the dipole of FSS water above the ion should cancel with that below the ion. We can nevertheless argue that the FSS contribution to the dipole orientation is significant in the surface region in the following, by integrating the dipole orientation of FSS water over the surface region. The orientational distribution of FSS water could be considered in an alternative way if we plot the ρ cos θ profile of FSS at the Zˆ coordinate of the central anion instead of individual water molecules of FSS. The profile as a function of the anion coordinate is schematically illustrated with the blue line in Figure 10 B. The blue line shows no net dipole of FSS water in the surface region, where each ion is surrounded by a full FSS water as illustrated in Figure 10 A. However, if we consider the dipole orientation of the FSS water up to a certain depth Zˆ moment emerges when the anion comes close to the threshold, Zˆ

Zˆthres , a net dipole

Zˆthres , and its FSS crosses

the threshold (see Figure 10 A and the blue line of Figure 10 B). This is because the upper part of the FSS of the anions could have a finite contribution to the integral when the FSS lies across the threshold Zˆ thres . We note that this result of integration does not depend on the choice of the threshold Zˆ thres if it is sufficiently deep, and that the integrals of the two distributions (green and blue lines in Figure 10 B) are identical. The above mechanism is common to a certain type of the quadrupole contribution, called χ IQB , in the general SFG theory of the quadrupole. 38 We note that the essentially analogous mechanism also plays a crucial role to determine the surface potential. 39 This mechanism of dipole orientation is quite general and commonly valid for OH – and Cl– anions, no matter whether the ions are spherical or not. Possible polar orientation of OH – is insignificant to the above orientational mechanism of FSS water, as the orientation of OH – quickly decays and is randomized just below the surface, as we argued in Figure 3. On the other hand, the

19



ρ cos θ profile of the FSS water around 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Zˆ

I–

shows a large positive region near the surface around

4 Å, in contrast to the above anions.

This is because a significant amount of I – ions are

exposed to the surface and thus have incomplete FSS, as also illustrated in Figure 9. Contribution to Imχ 2 :

Then we consider the influence of the FSS water on the Imχ 2

spectra. We calculated the contribution of FSS water on Imχ 2 using Eq. (5), and the results are

displayed in Figure 11. The effect of Na+ is quite similar among the three electrolyte solutions in Panel B, irrespective of the different counter anions. On the other hand, the FSS water around the three anions, OH– , Cl– , and I– , has distinct influences on the Imχ 2 spectra in Panel A. The

FSS water around OH– or Cl– shows a negative contribution to Imχ 2 , while that around I–

has a positive contribution in the frequency region around 3200 cm 1 . The opposite tendency is

consistent to the orientational structure of the FSS water molecules discussed above in Figure 8 and Figure 9. The FSS water around I– also shows a noticeable contribution to the dangling frequency region about 3700 cm 1 , since the FSS water around I– is arguably more exposed to the surface than those around other ions. By comparing OH– and Cl– , we find that the negative contribution of the FSS water around OH– appears in the lower frequency region about 3200 cm 1 than that around Cl– about 3450 cm 1 . This difference reflects the fact 40,41 that OH– tends to form stronger hydrogen bonds with surrounding water molecules than Cl– . In the NaOH solution, the negative FSS contribution around OH– (see Figure 11 A) partly cancels the positive FSS contribution around Na+ (Figure 11 B), though the negative FSS contribution around OH – remains in the low frequency region. Thus the FSS of OH– has considerable influence on the calculated Imχ 2 spectrum of NaOH solution shown in Figure 6. In the NaCl solution, on the other hand, the FSS contribution of Cl – largely overlaps that of Na+ in the frequency range, and consequently the overall contribution of FSS water around these solute ions is not substantial in the Imχ 2 spectrum of NaCl solution. Summary:

To summarize the above argument, the FSS water molecules around OH– give notice-

able negative contribution to the Imχ 2 spectrum of water surface. This contribution elucidates 20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the observed negative perturbation of the Imχ 2 amplitude in the relatively low frequency region

of the NaOH solution below 3200 cm 1 . This mechanism of FSS contribution is quite general, while it is remarkably observed in the NaOH solution as it appears in particularly low frequency

region. The negative contribution of the FSS water strongly supports the picture that OH – does not preferentially come to the topmost surface, in contrast to I – .

5 Concluding Remarks While the surface propensity of H3 O+ has been supported by most of the experimental and theoretical studies of molecular science, the behavior of OH– at aqueous solution surface is still in controversy. The phase-sensitive experimental SFG spectroscopy was applied to NaOH solution by Shen and co-workers, 9 and they reported two major characteristics in the ssp-polarized spectrum in the OH stretching region, i.e. positive (upward) shift of the Imχ 2 amplitude in the middle

frequency region (3300 – 3600 cm 1 ) and negative (downward) shift in the lower frequency (3000 – 3200 cm 1 ). This non-uniform shift induced by the solute NaOH was thought to be an evidence of accumulated OH– at the water surface, though it is hard to be elucidated in relation to the surface structure. We successfully reproduced the perturbed SFG spectrum of NaOH solution by MD calculation, and thereby elucidated the perturbation mechanism of NaOH on the aqueous surface. The present MD simulation resulted that neither Na + nor OH– shows surface propensity, though Na+ is slightly more buried than OH – . The slight charge separation induces the water orientation in the sub-surface layers and thereby results in the positive perturbation of Imχ 2 amplitude in the middle frequency range of the hydrogen-bonded OH stretching region. On the other hand, we found that the water molecules in the first solvation shell (FSS) around OH – are largely responsible to the negative perturbation of Imχ 2 in the low frequency range. This contribution is particularly significant in the NaOH solution, since the water molecules around OH – give rise to large red shift of the OH stretching, and hence it is distinguished from the whole negative band of Imχ 2 in the mid-frequency region of surface water. This mechanism indicates that OH – at the surface is fully

21



Page2of38

–

hydrated, implying that OH does not have surface preference. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In previous literature, the influence of electrolyte on the water surface and its SFG spectra has been widely explained with the picture of electric double layer formation. The electric field between the double layers changes the orientation of surface water and thereby the SFG spectra. In the present work we found another general perturbation mechanism of electrolyte that is attributed to the FSS water around the ions. Though the contribution of FSS is often embedded in the whole SFG spectra of electrolyte solutions, the FSS contribution of OH – becomes significant in the low frequency region of the NaOH spectrum.

Acknowledgement We are grateful to Prof. Pavel Jungwirth for valuable discussion. This work was supported by the Grants-in-Aid for Scientific Research, MEXT, Japan.

References (1) Buch, V.; Milet, A.; Vácha, R.; Jungwirth, P.; Devlin, J. P. Water Surface is Acidic. Proc. Natl. Acad. Sci. USA 2007, 104, 7342–7347. (2) Beattie, J. K.; Djerdjev, A. M.; Warr, G. G. The Surface of Neat Water is Basic. Faraday Discuss. 2009, 141, 31–39. (3) Petersen, P. B.; Saykally, R. J. Chem. Phys. Lett. Is the Liquid Water Surface Basic or Acidic? Macroscopic vs. Molecular-Scale Investigations. 2008, 458, 255–261. (4) Randles, J. E. B. Structure at the Free Surface of Water and Aqueous Electrolyte Solutions. Phys. Chem. Liq. 1977, 7, 107–179. (5) Petersen, M. K.; Iyengar, S. S.; Day, T. J. F.; Voth, G. A. The Hydrated Proton at the Water Liquid/Vapor Interface. J. Phys. Chem. B 2004, 108, 14804–14806.

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(6) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Unified Molecular Picture of the Surface of Aqueous Acid, Base, and Salt Solutions. J. Phys. Chem. B 2005, 109, 7617–7623. (7) Ishiyama, T.; Morita, A. Molecular Dynamics Analysis of Interfacial Structures and Sum Frequency Generation Spectra of Aqueous Hydrogen Halide Solutions. J. Phys. Chem. A 2007, 111, 9277–9285. (8) Tarbuck, T. L.; Ota, S. T.; Richmond, G. L. Spectroscopic Studies of Solvated Hydrogen and Hydroxide Ions at Aqueous Surfaces. J. Am. Chem. Soc. 2006, 128, 14519–14527. (9) Tian, C.; Ji, N.; Waychunas, G. A.; Shen, Y. R. Interfacial Structures of Acidic and Basic Aqueous Solutions. J. Am. Chem. Soc. 2008, 130, 13033–13039. (10) Takahashi, H.; Maruyama, K.; Karino, Y.; Morita, A.; Nakano, M.; Jungwirth, P.; Matubayasi, N. Energetic Origin of Proton Affinity to the Air/Water Interface. J. Phys. Chem. B 2011, 115, 4745–4751. (11) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, sixth ed.; A Wiley-Interscience Publication, 1997; Chapter III-6. (12) Takahashi, M. ζ Potential of Microbubbles in Aqueous Solutions: Electrical Properties of the Gas-Water Interface. J. Phys. Chem. B 2005, 109, 21858–21846. (13) Mishra, H.; Enami, S.; Nielsen, R. J.; Stewart, L. A.; Hoffmann, M. R.; Goddard, W. A.; Colussi, A. J. Brønsted Basicity of the Air-Water Interface. Proc. Natl. Acad. Sci. USA 2012, 109, 18679–18683. (14) Kim, S.; Park, E.; Kang, H. Segregation of Hydroxide Ions to an Ice Surface. J. Chem. Phys. 2011, 135, 074703 (9 pages). (15) Winter, B.; Faubel, M.; Vachá, R.; Jungwirth, P. Behavior of Hydroxide at the Water/Vapor Interface. Chem. Phys. Lett. 2009, 474, 241–247. 23



(16) Mundy, C. J.; Kuo, I.-F. W.; Tuckerman, M. E.; Lee, H.-S.; Tobias, D. J. Hydroxide Anion at 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the Air-Water Interface. Chem. Phys. Lett. 2009, 481, 2–8. (17) Wick, C. D.; Dang, L. X. Investigating Hydroxide Anion Interfacial Activity by Classical and Multistate Empirical Valence Bond Molecular Dynamics Simulations. J. Phys. Chem. A 2009, 113, 6356–6364. (18) Wick, C. D.; Dang, L. X. The Behavior of NaOH at the Air-Water Interface: A Computational Study. J. Chem. Phys. 2010, 133, 024705 (8 pages). (19) Morita, A.; Ishiyama, T. Recent Progress in Theoretical Analysis of Vibrational Sum Frequency Generation Spectroscopy. Phys. Chem. Chem. Phys. 2008, 10, 5801–5816. (20) Ishiyama, T.; Imamura, T.; Morita, A. Theoretical Studies of Structures and Vibrational Sum Frequency Generation Spectra at Aqueous Interfaces. Chem. Rev. 2014, in press, DOI: 10.1021/cr4004133. (21) Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Sum Frequency Generation Spectroscopy of the Aqueous Interface: Ionic and Soluble Molecular Solutions. Int. Rev. Phys. Chem. 2000, 19, 123–153. (22) Ishiyama, T.; Morita, A. Molecular Dynamics Study of Gas-Liquid Aqueous Sodium Halide Interface. II. Analysis of Vibrational Sum Frequency Generation Spectra. J. Phys. Chem. C 2007, 111, 738–748. (23) Ishiyama, T.; Morita, A. Molecular Dynamics Study of Gas-Liquid Aqueous Sodium Halide Interface. I. Flexible and Polarizable Molecular Modeling and Interfacial Properties. J. Phys. Chem. C 2007, 111, 721–737. (24) Vácha, R.; Megyes, T.; Bakó, I.; Pusztai, L.; Jungwirth, P. Benchmarking Polarizable Molecular Dynamics Simulations of Aqueous Sodium Hydroxide by Diffraction Measurements. J. Phys. Chem. A 2009, 113, 4022–4027. 24


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(25) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, UK, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1987. (26) Andersen, H. C. Rattle: A "Velocity" Version of the Shake Algorithm for Molecular Dynamics Calculations. J. Comput. Phys 1983, 52, 24–34. (27) Morita, A.; Hynes, J. T. A Theoretical Analysis of the Sum Frequency Generation Spectrum of the Water Surface. II. Time-Dependent Approach. J. Phys. Chem. B 2002, 106, 673–685. (28) Ishiyama, T.; Morita, A. Analysis of Anisotropic Local Field in Sum Frequency Generation Spectroscopy with the Charge Response Kernel Water Model. J. Chem. Phys. 2009, 131, 244714 (17 pages). (29) Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Direct Evidence for Orientational Flip-Flop of Water Molecules at Charged Interfaces: a Heterodyne-Detected Vibrational Sum Frequency Generation Study. J. Chem. Phys. 2009, 130, 204704 (5 pages). (30) Chen, X.; Hua, W.; Huang, Z.; Allen, H. C. Interfacial Water Structure Associated with Phospholipid Membranes Studied by Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy. J. Am. Chem. Soc. 2010, 132, 11336–11342. (31) Ishiyama, T.; Morita, A. Vibrational Spectroscopic Response of Intermolecular Orientational Correlation at the Water Surface. J. Phys. Chem. C 2009, 113, 16299–16302. (32) The computational intensity spectrum in Panel A1 apparently reproduces the slightly enhanced intensity of the dangling band at about 3700 cm 1 , though this feature might be affected by the assumed non-resonant part χ 2 nonres in Eq. (1). In any event, the uncertainty of the assumed non-resonant part is not relevant to the Imχ 2 spectra, as χ 2 nonres is a real constant. (33) Born, M.; Wolf, E. Principles of Optics, 7th ed.; Cambridge Univ. Press: Cambridge, 1999.

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(34) Levering, L. M. A Vibrational Spectroscopic Study of Aqueous Hydrogen Halide Solutions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Applicaion to Atmospheric Aerosol Chemistry. M.Sc. thesis, The Ohio State University, 2005; Figure 3.3. (35) Lide, D. R., Ed. CRC Handook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, 2000. (36) Morita, A.; Hynes, J. T. A Theoretical Analysis of the Sum Frequency Generation Spectrum of the Water Surface. Chem. Phys. 2000, 258, 371–390. (37) 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. (38) Shiratori, K.; Morita, A. Theory of Quadrupole Contributions from Interface and Bulk in Second-Order Optical Processes. Bull. Chem. Soc. Jpn. 2012, 85, 1061–1174. (39) Wilson, M. A.; Pohorille, A.; Pratt, L. R. Comment on “Study on the Liquid-Vapor Interface of Water. I. Simulation Results of Thermodynamic Properties and Orientational Structure”. J. Chem. Phys. 1989, 90, 5211–5213. (40) Xantheas, S. S. Theoretical Study of Hydroxide Ion-Water Clusters. J. Am. Chem. Soc. 1995, 117, 10373–10380. (41) Busing, W. R.; Hornig, D. F. The Effect of Dissolved KBr, KOH or HCl on the Raman Spectrum of Water. J. Phys. Chem. 1961, 65, 284–292.

26


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Page27of38

1.0 ρ(z)/ρbulk

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5

H2O Na+ − OH (pure water)

0.0 −15

−10

−5 ^ z (Å)

0

5

Figure 1: Normalized density profiles of constituent species for 1.2 M NaOH solution surface as a ˆ The normalization is performed for each species with the bulk function of the depth coordinate Z. density of the respective species. The density profile of H 2 O in pure water is also shown by the pale blue dashed line for comparison.

27



pure water 1.2M NaOH 0.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page28of38

−0.1 −15

−10

−5 ^ z (Å)

0

5

ˆ Pale blue Figure 2: Average cos θ of water orientation as a function of the depth coordinate Z. line: pure water, pink dashed line: 1.2 M NaOH solution. The definition of θ is illustrated in the inset.

28


Page29of38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.5 0.0

1.0

−15 0.5 cosθ

−10

0.0 −0.5 −1.0

0

−5 ^ Z (Å)

Figure 3: Normalized density distribution of OH – as a function of the depth Zˆ and cosine of the tilt angle cos θ . The density of OH – is normalized with that in the isotropic bulk solution. Several isodepth curves by every 2.5 Angstroms were highlighted in white.

29


SFG intensity (a.u.)

A1. CALC.

Page30of38

NaOH WATER

A2. EXPR.

3000

3200

3400 3600 -1 IR wavenumber (cm )

3800

3400 3600 -1 IR wavenumber (cm )

3800

Imag (a.u.)

B1. CALC.

B2. EXPR. Imag (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SFG intensity (a.u.)


3000

3200

Figure 4: ssp-polarized SFG spectra of 1.2 M NaOH solution (pink) and pure water (pale blue). (A1) calculated SFG intensity ( χ 2 2 ) spectra, (A2) experimental intensity, 9 (B1) calculated Imχ 2 , (B2) experimental Imχ 2 . 9 (The experimental results in Panels A2 and B2 were reproduced from ref. 9 with permission. Copyright 2008, American Chemical Society)

30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Imag (arb. unit)

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5 4 3 2 1 0 -1 -2 -3 -4

AM(total) A(W)M(W) Pure water

3000

3200

3400

3600

3800

IR wavenumber (cm-1) Figure 5: Decomposed Imχ 2 spectra into species contributions for 1.2 M NaOH solution. The purple dashed line denotes the whole Imχ 2 spectrum, and the back solid line the contribution of water, A(W)M(W), described in Section 2.3. The spectrum of pure water is also shown in pale blue for comparison.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Imag (a.u.) 3.0

z’= −0 Å

0.0 −3.0 3.0

−2 Å

0.0 −3.0 3.0

−4 Å

0.0 −3.0 3.0

−6 Å

0.0 −3.0 3.0

−8 Å

0.0 −3.0 3.0

−10 Å

0.0 −3.0 3000

3400 3800 −1 IR wavenumber (cm )

Figure 6: Calculated Imχ 2 spectra from the restricted surface region in Zˆ Zˆ ¼ , where the threshold Zˆ ¼ is varied from Zˆ ¼ 0 Å (top panel) to Zˆ ¼ 10 Å (bottom). The pink dashed and pale blue lines denote 1.2 M NaOH solution and pure water, respectively. The dark blue lines denote the contribution of water in first solvation shell of ions in the NaOH solution.

32


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Page3of38

Number of FSS water / ion

8.0

A 7.0

6.0

5.0 −15

Number of FSS water / ion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


around OH− Cl− − I −10

−5

6.0

0

B

5.0

4.0

3.0 −15

around Na+(NaOH) Na+(NaCl) Na+(NaI) −10

−5

0

^ Z (Å) Figure 7: Average number of FSS water molecules around the ions in three electrolyte solutions: NaOH, NaCl, and NaI. Panel A denotes the average number of FSS water per anion (OH– , Cl– , I– ), while Panel B per cation (Na+ ).

33



ρ (10−3 Å−3)

0.5

OH−− Cl− I

A

0

−0.5 −15

ρ (10−3 Å−3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page34of38

−10

−5

0

B

0.5

0 Na++(NaOH) Na+(NaCl) Na (NaI)

−0.5

−15

−10

−5

0

^ Z (Å) Figure 8: Product of the number density ρ and the average orientation cos θ of the FSS water around the ions. Panel A denotes the ρ cos θ profiles of the FSS water around the anions (OH– , Cl– , I– ) in three electrolyte solutions: NaOH, NaCl and NaI; Panel B around the cations (Na + ) in the three solutions.

34


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vapor

Surface

Bulk

Figure 9: Schematic illustration of the FSS water and its orientation around the ions. The mechanism of the ρ cos θ band in Figure 8 is illustrated with red arrows.

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

^ Z

Vapor

0

Surface

Page36of38

B

A

UcosT

0

^Z thres Bulk Figure 10: Schematic mechanism of FSS dipoles around anions. In Panel A, blue particles and surrouding green spheres denote the anions and their FSS, respectively. The red arrows illustrate the dipole orientation of FSS water around the anions. In Panel B, green line shows the ρ cos θ profile of FSS water as a function of the Zˆ coordinate of individual water molecules of FSS, whereas blue line plots the same profile as a function of the Zˆ coordinate of the central anions. In the former (green) plot a finite contribution appears at the topmost layer, whereas in the latter (blue) it emerges near the lower threshold Zˆthres , as discussed in Section 4.3.

36


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Imag (arb. unit)

1 0.5

−

OH (NaOH) − Cl (NaCl) − I (NaI)

A

0 −0.5 −1 2800

3000

3200

3400

3600

3800

3200 3400 3600 −1 IR wavenumber (cm )

3800

1

B Imag (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 0 +

−0.5 −1 2800

Na (NaOH) Na++(NaCl) Na (NaI) 3000

Figure 11: Contribution of the FSS water to the Imχ 2 spectra in three solutions: NaOH, NaCl, and NaI. Panel A denotes the contribution of FSS around the anions (OH– , Cl– , I– ), while Panel B around the cations (Na+ ). The scale of ordinates is common with those of Figure 5 and Figure 6.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Im[]

TOC Graphic. 2 inch

2 inch

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Molecular Dynamics Analysis of NaOH Aqueous Solution Surface and

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