Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
Variation in Average Molecular Orientation of an Organic Anion at the Air−Aqueous Interface: Influence of Halide Ions Daniel Headley, Ryan S. Young, Margaret Reece, and Mahamud Subir* Department of Chemistry, Ball State University, Muncie, Indiana 47306, United States S Supporting Information *
ABSTRACT: Molecular adsorption and orientation of an organic anion, p-nitrophenolate (p-NP−), at the air−aqueous interface have been studied using second harmonic generation techniques and surface tensiometry. It is shown that p-NP− not only adsorbs to the neat air−aqueous interface but also exhibits an orientational rearrangement at the surface. The average p-NP− orientation with respect to the surface normal is found to increase from ∼36 to ∼52° with increasing p-NP− surface coverage. Dipole−dipole and electrostatic interactions between p-NP− adsorbates are not the source of this orientation change. Exploration of this intriguing behavior in the presence of electrolytes revealed that interaction between inorganic ions and p-NP− augments its binding affinity and the observed orientation fluctuation. This study provides a critical understanding of the role halide, and plausibly hydroxide ions, play in influencing adsorbate orientation at an interface and thus paves the way to better elucidate chemical reactivity in sea salt aerosols and related environmental surfaces.
■
INTRODUCTION Surfaces or more precisely, interfaces, have been the subject of immense fundamental interest for decades,1−3 if not centuries.4 Aside from the most rudimentary and philosophical interests, applied chemistry and technological innovations also form the basis for a thorough understanding of surfaces. For instance, solid surfaces play a major role in a diverse set of applications, including catalysis and nanotechnology,1−3,5−8 whereas soft interfaces, such as liquid−liquid and liquid−vapor, reveal their prominence in biological membranes,9,10 petroleum weathering processes,11,12 and atmospheric aerosol chemistry.13−16 Even though the fundamental and practical motivations are copious, it has often been a formidable task to probe the molecularly thin region defined as the interface. This is particularly true for the air−aqueous interface, which is the focus of this study. Progress in surface sciences is indelibly intertwined with the development of new experimental tools. A spectroscopic tool that has added considerably to our understanding of the air− aqueous interface in the last few decades is based on the nonlinear interaction of light with matter.17−19 Nonlinear spectroscopy is a laser-based technique, which includes both second harmonic (SHG) and sum frequency generation (SFG). In general, SHG and SFG signals originate from an anisotropic medium and because centrosymmetry is broken at an interface, these techniques tend to be surface selective.15,20−23 Throughout the past decades, SHG and vibrational SFG (VSFG) have been applied to elucidate information pertaining to the molecular structure, energetics of adsorption, and orientation of molecules at various interfaces. Amid the vast number of © XXXX American Chemical Society
physical insights these techniques revealed, one that is most recent and relevant to this work is their application in providing direct experimental evidence of ions adsorbed at the air− aqueous interface.24−28 This is significant because ions have been classically deemed to be void at the water surface.27,29 Likewise, both SHG and VSFG have been utilized to shed light at the tantalizing question involving the proclivity of hydronium (H3O+) and hydroxide (OH−) ions at the air−water interface.30−32 In this work, we have used SHG to study the adsorption and orientational behavior of an organic anion, p-nitrophenolate (pNP−), at various air−aqueous interfaces. There are a number of SHG investigations involving p-nitrophenol (p-NP),33−38 the neutral form of the conjugate acid−base pair; however, knowledge of the interfacial behavior of p-NP− is limited. Nitrophenols belong to the class of phenolic compounds that are generated by many industrial processes and weathering of petroleum products. For example, p-NP is used in the production of parathion and acetaminophen and has been identified as the photodegradation product of the herbicide nitrofen.39,40 Because of their water solubility and toxicity, both p-NP and p-NP− are also qualified as priority pollutants by the Environmental Protection Agency.41 Their widespread usage in agrochemical, pharmaceutical, and other industries results in these compounds making their way into the water system.42 Received: December 21, 2017 Revised: February 8, 2018 Published: February 12, 2018 A
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
SHG field is represented as Γ. The corresponding SHG intensities, ISHG,S and ISHG,P, are expressed as follows
Thus, it is also of practical importance to understand the transport and fate of these compounds in the aquatic environment. Using SHG in conjunction with surface tension measurements, we report that p-NP− not only exhibits an affinity for the air−aqueous interface but shows an intriguing variation in its average orientation as a function of its surface coverage. This significant change in orientation is only observed for the ionic p-NP− but not for the neutral p-NP. To better understand the source of this molecular rearrangement, which could not be explained using simple dipole−dipole interaction between pNP− molecules, we have explored the influence of modifying the air−aqueous interface with varying electrolyte solutions on the adsorption and interfacial orientation of p-NP−. Our findings suggest substantial interaction between the adsorbed pNP− and halide ions, not observed hitherto. The overall results also appear to give credence to the plausibility of the presence of hydroxide ions at the air−aqueous interface. The molecular level understanding, presented herein, which includes knowledge of binding affinity and alignment of ionic species at a surface, is paramount to the prediction of fate and chemical reactivity43 of organic compounds at various environmental and atmospheric liquid−vapor interfaces. Polarization-Dependent SHG: Molecular Orientation Determination. At this juncture, a brief description on the use of SHG methodology to determine the average molecular orientation is desirable. SHG is a frequency doubling process, in which the fundamental frequency, ω, is converted to 2ω when light interacts with the material. The basic principle dictates that the induced second-order polarization, P(2), which acts as the source term in the Maxwell equation for the generated second harmonic field, E⃗ SHG, can be expressed as (2) ⃗ ⃗ . Here, χ⃡ (2) represents the second-order P ⃗ = χ ⃡ (2) EE susceptibility of the molecule and E⃗ is the fundamental laser field. Neglecting local field effects, the second-order suscept(2) ibility tensor can be expressed as χ(2) IJK = Ns∑⟨RIiRJjRKk⟩βijk , denote the number density of the surfacewhere Ns and β(2) ijk bound molecules and the second-order hyperpolarizability tensor elements, respectively.19 The RΛl terms designate the elements of Euler coordinate transformation, with I, J, and K representing the unit vectors in the laboratory frame, and i, j, and k, the molecular frame. The brackets refer to an average over molecular orientations with respect to the laboratory frame. The key point on the basis of these relationships is that E⃗ SHG is directly related to the surface population and the orientation of the adsorbed species, which can therefore be obtained experimentally. Specifically, the average molecular orientation is determined from the distinct susceptibility terms measured by using different polarization combinations of the fundamental and the SHG fields. The formalism we applied to determine the average orientation of p-NP− and p-NP has been described previously.35,36 In brief, C2v symmetry is invoked, which is true for p-NP− and assumed in the case of p-NP. Moreover, isotropic molecular orientation distribution within the surface plane has been considered. Accordingly, there are three (2) (2) independent susceptibility elements, χ(2) XXZ, χZXX, and χZZZ, that contribute to the SHG signal. These terms are obtained by measuring the intensity of the S- (Γ = 0°) and the P- (Γ = 90°) SHG-polarized light separately as a function of the fundamental beam (or the input) polarization, γ. The polarization of the
(2) ISHG,S ∝ |a1χXXZ sin(2γ )|2
(1)
ISHG,P ∝ (2) (2) (2) (2) |(a 2χXXZ + a3χZXX + a4χZZZ )cos2(γ ) + a5χZXX sin(2γ )|2
(2)
In these equations, ai, which are related to the Fresnel coefficients relating fraction of light reflected and transmitted from the monolayer at ω and 2ω, are calculated on the basis of the nonlinear polarization sheet model of the surface. By means of fitting the experimental data, the susceptibility values are obtained, from which the orientation parameter, D, is calculated. Equation 3 shows that D, which is a ratio independent of Ns, is related to the angle, θ, between the molecular z axis and the surface normal. D=
(2) (2) (2) − χZXX + χXXZ χZZZ (2) (2) (2) + 3χXXZ − χZXX χZZZ
=
⟨cos3(θ )⟩ ⟨cos(θ)⟩
(3)
This approach of determining the average molecular orientation is valid for molecules that are single axial in β(2) ZZZ. It has been applied to determine the average orientation of p-NP at various solid−liquid and liquid−vapor interfaces,35,36 which allow for a comparative analysis with the results obtained in this study. Optical SHG is an effective tool for probing molecular orientation at various planar interfaces.44−48 Further discussion on the use of eqs 1−3 in determining the average orientation with a narrow distribution is provided in the SHG Data Analysis section. Additionally, determination of adsorption isotherms by utilizing the orientation-insensitive polarization combination is discussed therein.
■
EXPERIMENTAL METHODS Chemicals and Sample Preparation. Unless stated otherwise, all compounds were purchased from Sigma-Aldrich and used as received. Stock solutions of pH 2 and 13 were prepared by dissolving concentrated HCl (ACS reagent, 37%, 320331-500ML) and solid NaOH (ACS reagent, ≥97.0%, 221465-500G) in deionized water (Millipore, Milli-Q, 18.2 MΩ/cm), respectively. The pH was measured using a pH meter (AB150, Fischer Scientific) that was calibrated immediately prior to use. Solid p-NP (reagent grade, ≥99%, 241326-50G) was dissolved in the pH 2 or 13 stock solutions to yield 500 mL of 70 mM p-NP and 130 mM p-NP−, respectively. The p-NP and p-NP− solutions were then diluted using the appropriate stock pH 2 or 13 solution to obtain a range of concentrations. The pH of the resulting solutions was tested and showed little deviation from the pH of the stock solutions. For each sample, 50 mL of solution was added to a Teflon dish (diameter 6.5 cm) and given 20 min for the surface to equilibrate. All glassware and the Teflon dish were soaked in aqua regia for at least 15 min and then rinsed with ample deionized and then Millipore water prior to use. Investigation of p-NP− at the air−aqueous interface of 1.0 and 2.5 M NaCl (ACS reagent, ≥99%, 74398-500G) and NaBr (ACS reagent, 3588-01, J.T. Baker) solutions were also performed. First, the electrolyte solutions were prepared using the stock pH 13 solution, which was in turn used to prepare the different concentrations of p-NP− solutions. Likewise, the pH of the B
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
du Noü y Ring Tensiometry. Surface tension was measured using a du Noüy Tensiometer (CSC Scientific Company, Inc.), which uses a fine torsion wire to apply the necessary force required to withdraw a platinum-iridium ring (Cat. No. 70545, circumference = 6.116 cm, R/r = 57.183) from the surface of the sample under investigation. The tensiometer was periodically calibrated, and the neat water surface tension was measured as a reference check between different measurements. Between each measurement, the ring was washed using Millipore water followed by ethanol and then placed under a flame to remove organic contaminants. The sample (15 mL) was placed in a clean Teflon dish (diameter 4.3 cm) and given 20 min for the surface to equilibrate. The surface tension with a resolution of 0.1 dyne/cm was measured at the breaking point of the film by adjusting the torsion of the wire. The data presented throughout the manuscript represents an average of three independent trials. To determine the Gibbs surface excess, Γ, the surface tension, γ, for a given sample, was plotted as a function of solute concentration. The data was then fit using an exponential function from which the slope, dγ , was determined. The surface dc excess in mol/m2 was then calculated using the following c dγ equation: − RT dc .8 In this equation, R is the gas constant, c is the concentration, and T is the temperature in kelvin. An adsorption isotherm for each of the samples studied was generated by plotting the surface excess versus the initial solution concentration, c. We assume this to be comparable to the equilibrium concentration because depletion of the solute in the bulk solution is negligible. SHG Data Analysis. For the purpose of determining the average molecular orientations of the surface-bound molecule, the SH intensity generated at the air−aqueous interface was measured by rotating the input polarization of light, γ, at a constant output polarization, Γ, of either S- or P-polarized light. This procedure has been carried out for both p-NP and p-NP− solutions of a wide range of concentrations. The collected polarization anisotropy data; i.e., ISHG versus γ data, were then fit using eqs 1 and 2 for S- and P-output polarizations, respectively. The values of the fitting parameters, ai, for the various interfaces studied are provided in the Supporting Information (Table S.1). First, ISHG,S versus γ was fit with eq 1 and χ(2) XXZ, the only susceptibility term the SHG intensity depends upon for the S-out polarization, was determined. Subsequently, the plot of the P-out polarization data was fit using eq 2 with the value of χ(2) XXZ held constant. The values of (2) χ(2) ZXX and χZZZ terms were obtained from this fit. The D parameter and the orientation angle are then calculated on the basis of eq 3. For the electrolyte experiments, the data presented are an average two independent trials. The individual trials are shown in the Supporting Information (Figure S.2). It is important to note that both ISHG,S and ISHG,P were analyzed with and without subtracting the nonresonant contribution from the subphase. It was found that the average orientation angle calculated using these methodologies differed only by few degrees. However, both returned the same trend in the orientation angle as a function of the solute concentration. In certain cases, for the lower p-NP− concentrations, where the SHG intensities were comparable to those of the neat pH 13 air−aqueous interface, subtraction led to large uncertainties in the fitting parameters and/or resulted in calculation of undefined orientation values. To preserve all of the data points, orientation angle calculated without subtracting the subphase
resulting solutions was measured to show no significant difference from that of the original pH 13 solution. Surface Second Harmonic Generation. The SHG experiments were carried out using a Ti:sapphire tsunami oscillator (Spectra-Physics, 3941X1BB), generating 70 fs pulses at 80 MHz. The oscillator was pumped by a solid state Nd:YVO4 laser (Spectra-Physics, Millennia PRO 15sJ). In general, the fundamental beam was tuned to an 800 nm ca. wavelength, which was directed toward a Teflon dish containing the sample to be tested. Prior to impinging upon the surface, the beam passed through various optical components. These optics and other mechanical modules were purchased from Thorlabs, Inc. and Newport Corporation. Neutral density filters were used to reduce and adjust the incident power. The power before the sample, immediately prior to the focusing lens (LA4148, f = 5 cm), was adjusted to approximately 550 mW at the onset of each experiment and was monitored to be within 2% throughout the duration of the experiment. This power was selected to ensure that sufficient SHG intensity, showing a quadratic relationship with respect to the fundamental beam power (see Figure S.1, Supporting Information), was obtained. The beam was focused onto the surface at an incidence angle of 70°. To prevent any thermal effects, the sample dish was placed on a motorized translation stage (MTS50-Z8), which oscillated over a distance of 7 mm in a direction perpendicular to both the surface normal and the direction of the laser propagation vector. The stage oscillated at a velocity no greater than 0.2 mm/s to prevent any perturbation of the surface. A Glan−Thompson polarizer (GTH10M) and a half-wave plate (WPH10M-808) were included to select and then modulate the linear polarization of light. Both the polarizer and half-wave plate were calibrated in the experimental geometry and installed with respect to the surface normal such that P-polarized light (γ = 0°) is parallel to the surface normal whereas S-polarized light (γ = 90°) is perpendicular. The half-wave plate was mounted on a motorized rotation stage (PRM1Z8) that was driven using a T-Cube DC Servo Motor (TDC001) controller. In a typical polarization-dependent experiment, at least three data points for every 4.5°, ranging from 0 to 180°, were collected. This corresponds to a modulation of the light such that polarization data was collected every 9° over 360° of rotation. In certain cases, five data points were taken for every 3° of rotation of the half-wave plate. Before focusing on the sample, the light was also passed through a long-pass filter to remove any light in the vicinity of SHG wavelength (ca. 400 nm) that could be present in this geometry. The reflected light from the sample was collimated and passed through a blue band-pass filter to block any residual fundamental beam. A polarizer (GLB10) was then used to select the polarization of light to be analyzed. The desired SH polarization, P-out (Γ = 0°) or S-out (Γ = 90°), was selected by manually rotating the polarizer. The generated SH signal, ISHG, was focused into a monochromator (Acton SP2500, Princeton Instruments) for detection by a photomultiplier tube (PMT, H11461P-01, Hamamatsu). The photocurrent from the PMT was delivered into a 350 MHz preamplifier (SR445A, Stanford Research Systems, Inc.), with three 50 Ω channels cascaded for a gain of 125×. The amplified signal was then sent to a single photon counter (SR400, Stanford Research Systems, Inc.) to be analyzed and then processed by LabView program (National Instruments). C
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a) UV−vis spectra of p-NP (red markers, 91 μM in pH 2 solution) and p-NP− (blue markers, 96 μM in pH 13 solution). (b) Average orientation angle of p-NP and p-NP− plotted as a function of their respective bulk solution concentration. The dashed line corresponds to an average angle of 48° determined for p-NP at the air−aqueous interface in an earlier study.36
contribution is presented in this article. The values obtained after subtracting the subphase SHG contribution are provided in the Supporting Information (Figure S.3). Analysis of SHG-based adsorption isotherms, which are plots of ESHG versus bulk solute concentration, depended on the molecules studied. For p-NP, which is nonresonant with the SHG frequency, the SHG field was calculated in the following manner: ESHG,pNP = ISHG,raw − ISHG,neat , where ISHG,raw and ISHG,neat correspond to the SHG intensities from the air− aqueous interfaces of solutions with and without p-NP, respectively. In the case of resonant p-NP−, ESHG,pNP− = ISHG,raw − ISHG,neat , with ISHG,neat representing the SHG intensity from neat air−aqueous (pH = 13) interface. As noted earlier, ESHG contains both orientational and surface number density information. Thus, to generate adsorption isotherms in scenarios where average molecular orientation varied with surface population, a polarization combination that showed little or no sensitivity to orientation change was used. The orientation-insensitive input polarization angle, γ*, for the P-output configuration has been calculated using eq 4,48 where ai represents the same coefficients used in orientation calculations (eqs 1 and 2). ⎛ ⎞ a5 γ*=cos−1⎜ ⎟ ⎝ 3a4 + a5 − a 2 − a3 ⎠
measurements, θ =
N , Nmax
where Nmax is the maximum surface
excess. c
θ=
■
Keq 55.5 c
Keq 55.5 + 1
(5)
RESULTS AND DISCUSSION We begin our discussion with a comparison of the average orientation of p-NP and p-NP−, as determined by SHG. The pKa of p-NP is 7.15;50 therefore, the use of pH 2 and 13 ensured that p-NP and p-NP− were the dominant species in the respective solutions. Figure 1a shows the UV−vis spectra of the two species. Given the fundamental wavelength in our experiment is ∼800 nm and the SHG is at ∼400 nm, it is clear that p-NP− is in resonance with 2ω, whereas p-NP is nonresonant. Figure 1b shows the average molecular orientation angle, p-NP in red and p-NP− in blue, as a function of bulk solute concentration. The data points extend to a concentration corresponding to the solubility limit, which is ∼70 and ∼130 mM for p-NP in pH 2 and p-NP− in pH 13 solutions, respectively. It is evident that the orientation of p-NP remains constant with increasing p-NP solution concentration. The p-NP orientation is in the range of 47−50° with respect to the surface normal. This magnitude obtained is in excellent agreement with the value of 48° determined previously for pNP at a concentration of 50 mM.36 Previous studies also measured the absolute orientation of p-NP, which showed that the −OH functional group is to be immersed into the bulk liquid phase. We assume that is also true for p-NP−, as it would be energetically favorable for the negative charge on the oxygen to be hydrated. Figure 1b also shows the orientational behavior of p-NP− at the air−aqueous interface. However, before delving into the discussion about its orientation it is important to highlight that p-NP− does exhibit a proclivity for the air−aqueous interface. We have observed SHG intensity to increase with increasing pNP− concentration, corresponding to the increased number density at the surface. A detailed discussion on the thermodynamics of its adsorption will follow its orientation analysis. It is revealed (Figure 1b) that p-NP− exhibits a
(4)
For the different air−aqueous interfaces studied, the γ* is 61.5°. The ai coefficients were assumed to be the same for the different electrolyte solutions. This is a reasonable assumption because there is a negligible deviation in the refractive indices of electrolyte solutions. At the highest electrolyte concentration studied, there is approximately 1.9% increase in the refractive index compared to that of the neat water at 20 °C.49 Adsorption Isotherm Analysis. The adsorption isotherms were fit using a Langmuir model (eq 5) to obtain adsorption equilibrium constants, Keq. In this equation, c is the concentration of the solute and 55.5 M corresponds to the molarity of neat water. The fraction of solute adsorbed is represented by θ. In the case of SHG adsorption isotherms, E θ = E SHG . For the isotherms based on surface tension SHG,max
D
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. (a) Area occupied per molecule of p-NP (red markers, pH 2 solution) and p-NP− (blue markers, pH 13 solution) vs initial solute concentration, as determined by surface tension measurements. (b) Simulation of potential energy of different types of molecular interactions, as a function of intermolecular distance. The dotted black line represents thermal energy, kBT, at the experimental temperature of 295 K. The equations used to model the various molecular interactions are presented in the Supporting Information.
(see the Supporting Information) are compared as a function of intermolecular separation in angstroms. We see that at these minimum separation lengths, none of these interactions are stronger than the thermal energy of 4.1 × 10−21 J calculated for the experimental temperature of 22 °C. It appears that even the electrostatic repulsion, if we treat the two negatively charged pNP− molecules as point charges, is not sufficient enough to cause the orientational adjustment observed. In contrast, the constant average orientation angle observed for p-NP can be understood from the fact that dipole−dipole interaction is significantly weak even at its maximum surface coverage. To shed light on the source of the orientation change in the case of p-NP−, we have further probed its behavior by varying the solution ionic strength. The motivation for this experiment has been twofold: (1) Given that p-NP− is negatively charged, it was hypothesized that in the presence of an electrolyte, there would be an accumulation of the counter ion, in our case, Na+ cation, at the surface, which would in turn disrupt the electrostatic repulsion, if any. Because the simulation of potential energies based on molecular separation (Figure 2b) suggests that electrostatic repulsion between p-NP− molecules is miniscule, we anticipate electrolytes to either diminish the extent of p-NP− orientation variation slightly or have no effect. (2) It is known that p-NP is subject to a salting-out effect;37,38 that is, addition of an electrolyte, such as LiCl, lowers the solubility of the organic solute. It was found that the decrease in solubility led to an increase in the SHG signal, indicating an enhancement of the surface population of p-NP at higher electrolyte concentrations. To the best of our knowledge, the effect of electrolytes on the surface activity of p-NP− has not been reported. If we assume that p-NP− also exhibits salting-out effect, a higher surface population at lower bulk solution concentration can be anticipated. The hypothesis here was that with enhanced surface population, the intermolecular separation will decrease, which may then manifest into an observable effect on the p-NP− orientation. The influence of two different concentrations, 1 and 2.5 M, of NaCl on the molecular area and the average orientation of p-NP− are shown in Figure 3.
significant variation in its average orientation as its bulk concentration is increased. In the most dilute solution, the molecule is more upright at an angle of ∼37° with respect to the surface normal and at the highest concentration it is oriented at an angle of ∼52°, slightly more parallel to the surface. With increasing concentration, a gradual increase in the angle is observed, which is not indicative of a sharp phase transition. Cursory inspection suggests that the transition from a vertical position to a horizontal configuration is perhaps due to electrostatic repulsion. That is, it would be energetically favorable for p-NP− molecules to align such that the positive end of one molecule interacts with the negative end of another. Vertical alignment with up-down orientation of the dipoles would require the negatively charged oxygen to be in the gaseous phase and not hydrated. This would be energetically costly and yield a sharp phase transition, which is not observed in the experimental result. p-NP is neutral and p-NP− is negatively charged, thus subject to long-range interaction. To elucidate this further, it is necessary to have knowledge of surface population. Because the orientation is changing and the number density based on ESHG is coupled to the orientational information, we have used surface tensiometry to obtain surface population values. Surface tension is a macroscopic property of the solvent, which is predominantly dependent upon the surface excess and not the orientation of the solute species at a fixed temperature and area. The surface tension data are provided in the Supporting Information (Figure S.4). On the basis of these measurements, the area occupied per molecule for a given bulk solution concentration has been calculated and is shown in Figure 2a. It is clear that the surface coverage of pNP is much greater than that of p-NP− at all concentrations explored. For these molecules, the respective surface number densities, Nmax, at the highest bulk solution concentration studied are ∼110 and ∼340 Å2/molecule. Assuming radial coverage, this translates to an intermolecular separation of ∼11.8 Å for p-NP and ∼20.8 Å for p-NP−. These values are highlighted in Figure 2b, where the potential energies of dipole−dipole, fixed dipole−charge, and Coulomb interactions E
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
electrostatic repulsion, is the source of orientation change in pNP− as a function of its surface coverage, it is not due to repulsion between two or more p-NP− molecules. Interaction between p-NP− and other interfacial species is perhaps the source of this orientation change. However, it is plausible that because of the salting-out effect, p-NP− surface population is enhanced and therefore p-NP−−pNP− distance is reduced. On the basis of the molecules per area (left axis) shown in Figure 3, we confirm that this is not the case. For 1 M NaCl, there is a slight increase in p-NP− surface coverage (red solid line with square markers) for a given bulk pNP− concentration but the increase is not sufficient to lead to a significant Coulomb interaction. In fact, for 2.5 M NaCl, in which p-NP− exhibits pronounced variation in orientation, we find its surface coverage (green solid line with square markers) to be less than that of the neat solution with no additional NaCl. Here, we digress to discuss the influence of electrolytes on the binding affinity of p-NP− for the air−aqueous interface. Figure 4a,b shows the adsorption isotherms obtained on the basis of SHG and surface tension measurements, respectively. The adsorption Gibbs free-energy values of p-NP− for the air− aqueous interfaces, obtained by fitting the experimental data using Langmuir model, are reported in Table 1. It is clear that salting-out effect increases (larger ΔGads in magnitude) the pNP− surface affinity but not necessarily the surface population, Nmax. A comparison between the ΔGads values suggests that in 2.5 M NaCl solution, p-NP− exhibits greater affinity for the air−aqueous interface than p-NP in neat aqueous solution (Table 1). The key information obtained from the surface tension measurements is that p-NP− surface population is not sufficiently enhanced to account for the more pronounced change in the orientation angle in the presence of NaCl. This is yet another evidence that adsorbate−adsorbate, i.e., p-NP−−pNP−, interaction is not responsible for the p-NP− orientation and its variations with surface coverage. On the basis of the electrolyte experiments, we deduce that the interaction between the surface-bound p-NP− and other interfacial species is at play. In the case of solutions containing NaCl, we hypothesize that the electrostatic interaction between p-NP− and chloride ion (Cl−) is the source of the augmented orientation variation. It is well established that polarizable halide ions have a strong propensity for the air−aqueous interface.52,53 For instance, at 1.2 M sodium chloride solution, the relative concentration of Cl− ions at the air−aqueous interface with respect to the bulk
Figure 3. Molecular area (left axis) and average orientation angle (right axis) of p-NP− in different electrolyte concentrations (0, 1.0, and 2.5 M NaCl) plotted as a function of bulk p-NP− concentration.
First, we found the solubility of p-NP− to decrease with increasing salt concentration. The approximate solubility of pNP− at 1.0 and 2.5 M NaCl were 70 and 30 mM, respectively. For a given p-NP− concentration, the SHG intensity is also greater at a higher electrolyte concentration. This can be observed in Figure 4a, which shows ESHG versus bulk p-NP− concentration at different electrolyte concentrations. The ESHG plotted to generate these adsorption isotherms has been collected at an orientation-insensitive polarization angle (see SHG Data Analysis section). These observations indicate that p-NP− is indeed subject to the salting-out effect. Second, it is apparent from Figure 3 that the degree of variation in p-NP− orientation is enhanced in the presence of electrolyte. In the 1 M NaCl solution, the difference in the orientation angle from low to high p-NP− concentration is approximately 18° and it is about 23° in the case of 2.5 M NaCl solution. In the neat pH 13 solution, the extent of orientation change over the entire concentration range is roughly 15°. Certainly, the higher concentration of electrolyte appears to favor an average molecular orientation of p-NP− that is more parallel to the surface than upright. Because we do not see a diminished variation in orientation, we conclude that p-NP−−p-NP− electrostatic repulsion does not play a role. This is consistent with the scenario explored on the basis of Coulomb interaction (Figure 2b). That is to say, if long-range interaction, such as the
Figure 4. Adsorption isotherm of p-NP− in various electrolyte solutions based on (a) SHG and (b) surface tension measurements. The adsorption isotherms of p-NP in pH 2 solution are shown in the Supporting Information (Figure S.5). F
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Table 1. Thermodynamics Values Pertaining to Adsorption of p-NP− and p-NP at Various Air−Aqueous Interfaces electrolyte concentration (M)
ΔGads (kJ/mol) (SHG)
ΔGads (kJ/mol) (tensiometer) −
0 (neat) [NaCl] = [NaCl] = [NaBr] = [NaBr] = 0 (neat)
a
1.0 2.5 1.0 2.5
Nmax (×10−6 mol/m2) (tensiometer)
a
p-NP (pH 13 Solution) −14.1 ± 0.1 −15.5 ± 0.1 −19.8 ± 0.1 −16.2 ± 0.1 −19.9 ± 0.3 p-NP −21.3 ± 0.8 (pH = 2)51 −16.4 ± 0.2 (pH = 7)36 35 −19 (pH not reported) −18.2 ± 0.1 (pH = 2)a −19.4 ± 1.2 (pH = 2)a −15.9 ± 0.5 −16.4 ± 0.7 −20.8 ± 0.9 −17.1 ± 0.6 −19 ± 1
1.15 1.42 0.61 1.10 0.46
± ± ± ± ±
0.03 0.04 0.02 0.03 0.02
1.51 (pH = 7)36 2.55 ± 0.05 (pH = 2)a
Experimental values obtained in this work.
Figure 5. Molecular area (left axis) and average orientation angle (right axis) of p-NP− as a function of bulk p-NP− concentration in (a) 1.0 M and (b) 2.5 M NaCl and NaBr solutions.
concentration is 0.71 and that of Na+ ion is 0.16.53 This means the surface concentration of Cl− ions is comparable to that of the bulk Cl− ion concentration. Assuming a homogeneous distribution of ions, these values suggest that the separation between two Cl− ions is less than 15 Å and the separation between a Cl− ion and p-NP− is plausibly even smaller. This is sufficient for charge−charge or charge−dipole interaction to take place between the surface-bound p-NP− and the Cl− ions near the surface. Thus, it is reasonable to conclude that p-NP− undergoes a rearrangement in its orientation to minimize the repulsive interaction energy. However, it is also possible that other types of interactions such as ion−π and hydrogen bonding play a role in the observed orientation change. It has been shown that anion−π interaction can occur in both electron-rich and electron-deficient aromatic rings.54,55 Tilting of p-NP− from vertical to horizontal configuration, as observed using SHG, would conceivably maximize the probability of complex formation between the halide ion and the aromatic ring. However, additional investigation is needed to pinpoint the type of interaction that is dominant for the orientation change in p-NP−. To further confirm that halide ion is indeed playing a role, we have probed the effect of bromide (Br−) ion on the orientational variation in p-NP− adsorbed at the air−aqueous interface. The relative surface-to-bulk concentration of Br− ions at 1.2 M solution is 2.10 and that of Na+ ion is 0.81.53 Thus, compared with Cl− ion, the surface population of Br− ion is greatly enhanced. Interestingly, interfacial concentration of Na+ ion is also greater in NaBr solution compared to that in NaCl solution. We see the influence of these factors on the orientation of p-NP− in Figure 5. The effects of 1.0 M NaCl
and NaBr solutions are compared in Figure 5a. It is clear that the variation in p-NP− orientation (right axis) in the presence of Br− ion is more pronounced. We attribute this to the fact that there are more Br− ions at the surface and therefore the greater degree of interaction with p-NP−. The surface affinity of p-NP− appears to be slightly higher (refer to ΔGads values in Table 1) in the presence of NaBr compared to that in the presence of NaCl. However, the maximum p-NP− surface population is diminished in the case of NaBr (Table 1). This is most likely due to the fact that as there are more halide ions at the surface, as is true with Br− ion relative to Cl− ion, the number of surface sites for p-NP− adsorption is reduced. The pNP− area per molecule (left axis) for the concentration range studied remains comparable for the NaCl and NaBr solutions, suggesting the surface population of p-NP− is not the determining factor for the observed orientation change. In the case of 2.5 M NaBr solution, we observe orientation variation (Figure 5b) that is not as pronounced as in the case of NaCl. As in the case of 1.0 M electrolyte solutions, the surface population of p-NP− remains equivalent for NaCl and NaBr solutions. The lack of marked enhancement in the orientation change is possibly due to the presence of Na+ ions at the subsurface in the case of 2.5 M NaBr solution. On the basis of the relative ion concentrations,53 and assuming this is constant for the entire concentration range, there are approximately 2.6 Br− ions and 4.4 Cl− ions for every Na+ ion at or near the interface. Noting that at 2.5 M, the overall Na+−Br− ion pair population near the interface would be greater than that of Na+−Cl− ion pair, the intermolecular separation between Na+ and Br− ions can be assumed to be smaller compared to the distance between Na+ and Cl− ions. This could render Br− ion G
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Understanding the behavior of ions at the air−aqueous interface is of paramount importance in elucidating chemical processes involving atmospheric aerosols.53,63−65 These airborne particles serve as cloud condensation nuclei, impact climate change, and are a key factor in the transport and fate of organic contaminants. Sea salt aerosols are chiefly composed of NaCl and can include a substantial amount of organic compounds. The surface halide concentration in these particles is significantly enhanced.28,66,67 The fundamental insights presented in this study highlight the importance of deciphering binding affinity and orientational behavior of organic compounds at the gas−aerosol interface. This is because surface population and orientation of a molecule is directly related to its chemical reactivity, which in turn impacts the aerosol properties and thereby climate and fate of atmospheric pollutants. Knowledge of interfacial photochemical reactions in aerosolic environment is thus essential and is the subject of our ongoing investigations.
to exhibit a diminished net negative charge compared to that of Cl− ion at the air−aqueous interface at high salt concentration. This in turn can disrupt the potential charge−charge or charge−dipole interaction between the halide ion and p-NP−, leading to a less pronounced change in its orientation at a higher concentration of NaBr, as observed in Figure 5b. At 1.0 M salt concentration, the Na+−Br− ion pair separation distance is possibly large enough such that it is not sufficient to reduce the effective repulsive interaction experienced by the Br− and pNP−. As discussed earlier, the possibility of other factors, such as anion−π interaction and hydrogen bonding, as the cause of the observed trend, cannot be excluded without further theoretical and experimental investigation.
■
CONCLUSIONS Using surface SHG and surface tension measurements, we have shown p-NP−, an organic anion, to not only adsorb at the air− aqueous interface but to exhibit a surface coverage-dependent variation in its average interfacial orientation. Orientational fluctuation, as observed using SHG, has been previously reported for certain surface-bound small nitrile molecules.56 For these molecules, the orientation change observed was abrupt and dipole−dipole repulsion between the adsorbates was determined to be the source of the orientational phase transition. In the present study, we have demonstrated that adsorbate−adsorbate interaction, such as dipole−dipole or Coulomb interaction is not the cause of the gradual orientation variation in p-NP−. The surface number density of p-NP− is insufficient for p-NP−−p-NP− interaction to be the source for the orientation change with increasing p-NP− surface coverage. Further investigation on the influence of electrolytes on the adsorption and orientation of p-NP− at the air−aqueous interface provided new insights on the origin of the orientational variation. It was found that p-NP− does experience a salting-out effect. Moreover, it is also revealed for the first time that adsorption of halide ions at the air−aqueous interface can have a significant impact on the average orientation of a surface-bound organic anion. In this work, a complex interaction between halide and p-NP− ions that leads to an orientational rearrangement in p-NP− has been demonstrated. Although it is evident that Cl− and Br− ions interact with pNP−, the source of p-NP− orientation change in neat (pH 13) air−aqueous interface is, however, not clear. It is interesting to note that similar observations in basic solution have been made for phenolic surfactants. Insoluble long-chain aniline and phenolate monolayers have been shown to exhibit orientational fluctuation.57,58 In contrast, long-chain molecules with anilinium and phenol head group did not show this behavior. Only the species in the basic solutions were reported to exhibit an “unexpected” orientational fluctuation. In light of the findings with respect to halide ions presented in this work, it is reasonable to surmise that OH− ion is the species that is interacting with p-NP− and phenolic monolayers57,58 observed in the earlier study. This of course assumes that OH− ions are adsorbed at the air−aqueous interface, which is a subject of vibrant debate.59 Recent findings do provide strong experimental and theoretical evidence to suggest that surface of aqueous solutions of pH 2 or greater is basic.60−62 Certainly, higher surface population of OH− ion is likely in pH 13 solutions. However, direct evidence, perhaps using surface vibrational spectroscopy, is warranted in elucidating the exact interaction between the OH− ion and the adsorbed organic species.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12583. SHG orientational fitting coefficients, power dependent SHG data, individual SHG orientation measurements, comparison of orientation angle calculations, p-NP and p-NP− surface tension data, and p-NP adsorption isotherms (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mahamud Subir: 0000-0003-0971-6673 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (Grant No. 53906-UNI5).
■
REFERENCES
(1) Somorjai, G. A. The Evolution of Surface Chemistry. A Personal View of Building the Future on Past and Present Accomplishments. J. Phys. Chem. B 2002, 106, 9201−9213. (2) Yates, J. T., Jr.; Campbell, C. T. Surface Chemistry: Key to Control and Advance Myriad Technologies. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 911−916. (3) Duke, C. B. The Birth and Evolution of Surface Science: Child of the Union of Science and Technology. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3858−3864. (4) Paparazzo, E. How Old Is Surface Science? J. Electron Spectrosc. Relat. Phenom. 2004, 134, 9−24. (5) Somorjai, G. A.; Li, Y. Impact of Surface Chemistry. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 917−924. (6) Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The Surface Science of Nanocrystals. Nat. Mater. 2016, 15, 141−153. (7) Kolasinski, K. W. Surface Science: Foundation of Catalysis and Nanoscience; John Wiley & Sons Ltd.: Chichester, 2008. (8) Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces, 2nd ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006.
H
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (9) Baszkin, A.; Norde, W. Physical Chemistry of Biological Interfaces; Marcel Dekker, Inc.: New York, 2000. (10) Yan, E. C. Y.; Fu, L.; Wang, Z.; Liu, W. Biological Macromolecules at Interfaces Probed by Chiral Vibrational Sum Frequency Generation Spectroscopy. Chem. Rev. 2014, 114, 8471− 8498. (11) Boehm, P. D.; Fiest, D. L.; Mackay, D.; Paterson, S. Physical− Chemical Weathering of Petroleum Hydrocarbons from the IXTOC I Blowout: Chemical Measurements and a Weathering Model. Environ. Sci. Technol. 1982, 16, 498−505. (12) Feldman, M. H. Petroleum Weathering: Some Pathways, Fate, and Disposition on Marine Waters; National Environmental Research Center, Office of Research and Development, U.S. Environmental Protection Agency: Corvallis, 1973. (13) Finlayson-Pitts, B. J. Reactions at Surfaces in the Atmosphere: Integration of Experiments and Theory as Necessary (but Not Necessarily Sufficient) for Predicting the Physical Chemistry of Aerosols. Phys. Chem. Chem. Phys. 2009, 11, 7760−7779. (14) Tobias, D. J.; Stern, A. C.; Baer, M. D.; Levin, Y.; Mundy, C. J. Simulation and Theory of Ions at Atmospherically Relevant Aqueous Liquid−Air Interfaces. Annu. Rev. Phys. Chem. 2013, 64, 339−359. (15) Jubb, A. M.; Hua, W.; Allen, H. C. Environmental Chemistry at Vapor/Water Interfaces: Insights from Vibrational Sum Frequency Generation Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 107−130. (16) Al-Abadleh, H. A. Review of the Bulk and Surface Chemistry of Iron in Atmospherically Relevant Systems Containing Humic-Like Substances. RSC Adv. 2015, 5, 45785−45811. (17) Franken, P. A.; Hill, A. E.; Peters, C. W.; Weinreich, G. Generation of Optical Harmonics. Phys. Rev. Lett. 1961, 7, 118−119. (18) Boyd, R. W. Nonlinear Optics, 2nd ed.; Academic Press: San Diego, 2003. (19) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons, Inc.: Hoboken, 2003. (20) Eisenthal, K. B. Liquid Interfaces. Acc. Chem. Res. 1993, 26, 636−643. (21) Eisenthal, K. B. Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy. Chem. Rev. 1996, 96, 1343−1360. (22) Moore, F. G.; Richmond, G. L. Integration or Segregation: How Do Molecules Behave at Oil/Water Interfaces? Acc. Chem. Res. 2008, 41, 739−748. (23) Geiger, F. M. Second Harmonic Generation, Sum Frequency Generation, and χ(3): Dissecting Environmental Interfaces with a Nonlinear Optical Swiss Army Knife. Annu. Rev. Phys. Chem. 2009, 60, 61−83. (24) Brandes, E.; Karageorgiev, P.; Viswanath, P.; Motschmann, H. Breaking the Symmetry of Ions at the Air−Water Interface. J. Phys. Chem. C. 2014, 118, 26629−26633. (25) 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. (26) Onorato, R. M.; Otten, D. E.; Saykally, R. J. Adsorption of Thiocyanate Ions to the Dodecanol/Water Interface Characterized by UV Second Harmonic Generation. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15176−15180. (27) Garrett, B. C. Ions at the Air/Water Interface. Science 2004, 303, 1146−1147. (28) Petersen, P. B.; Saykally, R. J. On the Nature of Ions at the Liquid Water Surface. Annu. Rev. Phys. Chem. 2006, 57, 333−364. (29) Onsager, L.; Samaras, N. N. T. The Surface Tension of Debye Hückel Electrolytes. J. Chem. Phys. 1934, 2, 528−536. (30) Petersen, P. B.; Saykally, R. J. Is the Liquid Water Surface Basic or Acidic? Macroscopic Vs. Molecular-Scale Investigations. Chem. Phys. Lett. 2008, 458, 255−261. (31) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P. Unified Molecular Picture of the Surfaces of Aqueous Acid, Base, and Salt Solutions. J. Phys. Chem. B 2005, 109, 7617−7623.
(32) 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. (33) Beildeck, C. L.; Liu, M. J.; Brindza, M. R.; Walker, R. A. Solvation of P-Nitrophenol at a Water/Alkane Interface: The Role of Ionic Strength and Salt Identity. J. Phys. Chem. B 2005, 109, 14604− 14610. (34) Woods, B. L.; Walker, R. A. pH Effects on Molecular Adsorption and Solvation of P-Nitrophenol at Silica/Aqueous Interfaces. J. Phys. Chem. A 2013, 117, 6224−6233. (35) Higgins, D. A.; Abrams, M. B.; Byerly, S. K.; Corn, R. M. Resonant Second Harmonic Generation Studies of P-Nitrophenol Adsorption at Condensed-Phase Interfaces. Langmuir 1992, 8, 1994− 2000. (36) Tamburello-Luca, A. A.; Hébert, P.; Brevet, P. F.; Girault, H. H. Resonant-Surface Second-Harmonic Generation Studies of Phenol Derivatives at Air/Water and Hexane/Water Interfaces. J. Chem. Soc., Faraday Trans. 1996, 92, 3079−3085. (37) Das, K.; Sarkar, N.; Das, S.; Datta, A.; Nath, D.; Bhattacharyya, K. Salt Effect on P-Nitrophenol at the Water Surface: A Surface Second Harmonic Generation Study. J. Chem. Soc., Faraday Trans. 1996, 92, 4993−4996. (38) Sarkar, N.; Das, K.; Das, S.; Nath, D.; Bhattacharyya, K. Effect of Urea and Surfactant on P-Nitrophenol at the Water Surface: A Surface Second-Harmonic Generation Study. J. Chem. Soc., Faraday Trans. 1995, 91, 1769−1773. (39) Nakagawa, M.; Crosby, D. G. Photodecomposition of Nitrofen. J. Agric. Food Chem. 1974, 22, 849−853. (40) Samuel, M. S.; Sivaramakrishna, A.; Mehta, A. Bioremediation of P-Nitrophenol by Pseudomonas putida 1274 Strain. J. Environ. Health Sci. Eng. 2014, 12, 53. (41) United States Environmental Protection Agency. Quality Criteria for Water; United States Environmental Protection Agency: Washington DC, 1986. (42) Dsikowitzky, L.; Schwarzbauer, J. Organic Contaminants from Industrial Wastewaters: Identification, Toxicity and Fate in the Environment. In Pollutant Diseases, Remediation and Recycling; Lichtfouse, E., Schwarzbauer, J., Robert, D., Eds.; Springer International Publishing: Cham, 2013; pp 45−101. (43) Taylor, H. S. Chemical Reactions at Surfaces. Chem. Rev. 1931, 9, 1−46. (44) Brevet, P. F. Surface Second Harmonic Generation; Presses polytechniques et universitaires romandes, 1997. (45) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Determination of Molecular Orientation of Monolayer Adsorbates by Optical SecondHarmonic Generation. Phys. Rev. A 1983, 28, No. 1883. (46) Rao, Y.; Tao, Y.-s.; Wang, H.-f. Quantitative Analysis of Orientational Order in the Molecular Monolayer by Surface Second Harmonic Generation. J. Chem. Phys. 2003, 119, 5226−5236. (47) Simpson, G. J.; Rowlen, K. L. Orientation-Insensitive Methodology for Second Harmonic Generation. 1. Theory. Anal. Chem. 2000, 72, 3399−3406. (48) Simpson, G. J. New Tools for Surface Second-Harmonic Generation. Appl. Spectrosc. 2001, 55, 16A−32A. (49) Tan, C.-Y.; Huang, Y.-X. Dependence of Refractive Index on Concentration and Temperature in Electrolyte Solution, Polar Solution, Nonpolar Solution, and Protein Solution. J. Chem. Eng. Data 2015, 60, 2827−2833. (50) Lide, D. R. CRC Handbook of Chemistry and Physics: A ReadyReference Book of Chemical and Physical Data; CRC Press: Boca Raton, 2003. (51) Castro, A.; Ong, S.; Eisenthal, K. B. Studies of Molecular Properties at the Surface of a Liquid Jet by Second Harmonic Generation. Chem. Phys. Lett. 1989, 163, 412−416. (52) Piatkowski, L.; Zhang, Z.; Backus, E. H. G.; Bakker, H. J.; Bonn, M. Extreme Surface Propensity of Halide Ions in Water. Nat. Commun. 2014, 5, No. 4083. (53) Jungwirth, P.; Tobias, D. J. Molecular Structure of Salt Solutions: A New View of the Interface with Implications for I
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B 2001, 105, 10468−10472. (54) Wang, D.-X.; Wang, M.-X. Anion−π Interactions: Generality, Binding Strength, and Structure. J. Am. Chem. Soc. 2013, 135, 892− 897. (55) Berryman, O. B.; Hof, F.; Hynes, M. J.; Johnson, D. W. Anion−π Interaction Augments Halide Binding in Solution. Chem. Commun. 2006, 506−508. (56) Zhang, D.; Gutow, J. H.; Eisenthal, K. B. Structural Phase Transitions of Small Molecules at Air/Water Interfaces. J. Chem. Soc., Faraday Trans. 1996, 92, 539−543. (57) Zhao, X.; Eisenthal, K. B. Monolayer Orientational Fluctuations and a New Phase Transition at the Air Water Interface Detected by Second Harmonic Generation. J. Chem. Phys. 1995, 102, 5818−5826. (58) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Orientational Fluctuations and Phase Transitions of Long Chain Molecules at the Air/Water Interface. Phys. Rev. Lett. 1991, 67, 2025−2028. (59) Saykally, R. J. Air/Water Interface: Two Sides of the Acid−Base Story. Nat. Chem. 2013, 5, 82−84. (60) 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. U.S.A. 2012, 109, 18679−18683. (61) Bai, C.; Herzfeld, J. Surface Propensities of the Self-Ions of Water. ACS Cent. Sci. 2016, 2, 225−231. (62) Beattie, J. K.; Djerdjev, A. M.; Warr, G. G. The Surface of Neat Water Is Basic. Faraday Discuss. 2009, 141, 31−39. (63) Enami, S.; Fujii, T.; Sakamoto, Y.; Hama, T.; Kajii, Y. Carboxylate Ion Availability at the Air−Water Interface. J. Phys. Chem. A 2016, 120, 9224−9234. (64) Kumar, M.; Francisco, J. S. Ion Pair Particles at the Air−Water Interface. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 12401−12406. (65) George, C.; Ammann, M.; D’Anna, B.; Donaldson, D. J.; Nizkorodov, S. A. Heterogeneous Photochemistry in the Atmosphere. Chem. Rev. 2015, 115, 4218−4258. (66) Knipping, E. M.; Lakin, M. J.; Foster, K. L.; Jungwirth, P.; Tobias, D. J.; Gerber, R. B.; Dabdub, D.; Finlayson-Pitts, B. J. Experiments and Simulations of Ion-Enhanced Interfacial Chemistry on Aqueous NaCl Aerosols. Science 2000, 288, 301−306. (67) Kolb, C. E.; Worsnop, D. R. Chemistry and Composition of Atmospheric Aerosol Particles. Annu. Rev. Phys. Chem. 2012, 63, 471− 491.
J
DOI: 10.1021/acs.jpcc.7b12583 J. Phys. Chem. C XXXX, XXX, XXX−XXX
