Letter pubs.acs.org/JPCL
Enhanced Surface Partitioning of Nitrate Anion in Aqueous Bromide Solutions Angela C. Hong,† Sumi N. Wren,† and D. J. Donaldson*,†,‡ †
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario Canada M5S 3H6 Department of Physical and Environmental Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario Canada M1C 1A4
‡
S Supporting Information *
ABSTRACT: The proximity of nitrate anions to the air−water interface is thought to strongly influence their photodissociation quantum yield, due to a reduced solvent cage effect at the water surface. Although nitrate in aqueous solution exhibits little or no surface affinity, the release of gas phase NO2 (nitrate’s primary photodissociation product) has been reported to be enhanced when halides, in particular bromide, are also present in solution. Here, we use glancing-angle Raman spectroscopy to investigate whether solutions containing both nitrate and halides show different propensities for nitrate at the air−water interface. We find that bromide enhances, and chloride has little effect on (or perhaps suppresses) the surface partitioning of nitrate anions.
SECTION: Environmental and Atmospheric Chemistry, Aerosol Processes, Geochemistry, and Astrochemistry he nitrate ion (NO3−) is an important inorganic species found in many environmentally relevant media such as atmospheric aerosols1,2 and the quasi-liquid layer of snow and ice.3−5 Aqueous NO3− absorbs light in the actinic range and can photodissociate to form (primarily) NO2 and OH, with low quantum yield.6−8 The low quantum yield in bulk solution is associated with a solvent cage effect;9−11 the quantum yields of surface adsorbed nitrate photolysis have been reported to be higher by several orders of magnitude.9,12,13 This has motivated considerable interest in NO3− photolysis at the surface of aqueous solutions (in particular, aerosols and snow), as this process may influence the oxidative chemistry of both the immediate aqueous environment and the overlying atmosphere. There is at this time a general consensus that NO3− is present at the air-aqueous interface but is neither in deficit nor excess compared to the bulk.14−18 This is in contrast to the more polarizable halide ions, especially bromide and iodide, which have relatively high intrinsic surface propensities.19−22 Although significant advances have been made in elucidating the propensity of inorganic ions to exist at or near the airaqueous interface in single-salt solutions, much less is known about their behavior in multicomponent solutions. Recent experimental results have demonstrated enhanced NO2(g) production from illuminated sodium nitrate solutions in the presence of added sodium chloride and/or bromide.23−25 These were interpreted using molecular dynamics (MD) simulations as being a consequence of halides’ known surface activity, whereby excess halide at the water surface causes an increase in the sodium cation concentration immediately
below; in turn, the sodium ions “draw up” nitrate to the interface region, reducing the solvent cage effect upon photolysis. In effect, the presence of halides enhances the partitioning of nitrate to the air−water interface. An alternative hypothesis26 is that under these conditions, bromide anions may efficiently scavenge OH, reducing the recombination of nitrate photolysis products, and thus enhancing NO 2(g) production. To date, there have been no experimental studies that directly interrogate the air−aqueous interface of multicomponent nitrate-halide solutions to support the hypothesis of an enhanced NO3− surface affinity mediated by halide ions (X−). In the present study, we use glancing-angle Raman spectroscopy to directly measure any enhancement in concentration of NO3− in the interfacial region (operationally defined as ca. 10 to 100 monolayers thick27) of multicomponent NaNO3(aq)−NaX(aq) solutions by monitoring the intensity of the nitrate symmetric stretch mode, near 1040 cm−1. We have used this technique recently to investigate nitrate partitioning to the interface of nitric acid solutions, as well as the exclusion of nitrate to the air−ice interface during the freezing of nitrate solutions.18 Because the absolute intensity of the nitrate symmetric stretch band observed using glancing-angle Raman spectroscopy is dependent on variables such as laser intensity, alignment,
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© 2013 American Chemical Society
Received: July 24, 2013 Accepted: August 21, 2013 Published: August 21, 2013 2994
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Figure 1. Glancing-angle Raman spectrum of a 0.5 M nitrate solution showing the nitrate symmetric stretch (at 1040 cm−1) and the water bending mode (at 1650 cm−1). The intensity scale has been normalized to the water feature as described in the text.
etc., which may change between acquisitions, we normalize the spectra to the water bending feature, seen near 1650 cm−1. Figure 1 illustrates a sample spectrum normalized in this manner. In the present experiments, this normalization is complicated by the fact that halide ions disrupt the hydrogen bonding network in water, giving rise to an increase in water Raman intensities.28,29 That is, the addition of NaX results in an increase in the water bend intensity, I(ν-H2O), compared to neat water. In the Supporting Information we illustrate this effect for 0.5 M NaNO3 solutions containing 2.0 M Cl− or Br− in the bulk phase measured using a commercial FT-Raman instrument. Over the range of halide concentrations investigated here (0.1 − 2.0 M X−), the enhancement in I(ν-H2O) is more pronounced for Br− than Cl− (Figure S1). Although it is possible for NO3− and halides to interact, leading to an altered nitrate Raman spectrum, we did not observe any discernible effect for up to 2.0 M Cl− or Br− on I(ν-NO3−). Without an independent method to measure I(ν-NO3−) at the interface, we assume this observation in the bulk can be extended to the interface despite possible differences in solvation environment. Importantly, the presence of 0.5 M nitrate does not affect I(νH2O), and the absolute intensity and band shape of the nitrate feature at ∼1040 cm−1 are also independent of halide concentrations over the concentration range studied here. Because I(ν-H2O) increases with increasing halide concentrations but the nitrate intensity, I(ν-NO3¯), is not affected, it follows that at constant nitrate concentration the waternormalized intensity of the nitrate symmetric stretch will decrease with increasing [X−]. Indeed this is true in bulk solution, as illustrated by the solid symbols in Figure 2a. Note that in this figure, the normalized nitrate intensities, I(νNO3¯)/I(ν−H2O), have been plotted relative to the [X¯] = 0 result and are denoted as Irel. The solid symbols in Figure 2a show that at constant [NaNO3], increasing the concentration of NaX gives rise to decreasing values of Irel, simply due to the increase in I(ν-H2O). This trend is more pronounced for Br− than Cl− for all concentrations of halide salt. The corresponding surface region results are presented in Figure 2a as hollow symbols. Again, the bulk nitrate concentration in solution is held constant, but the Irel shows little dependence on halide concentration. It is clear, though, that for all [X−], the normalized nitrate intensities measured at the surface are greater than their bulk counterparts. The difference from the bulk is weak for the more dilute chloride solutions, but is significant for all concentrations of bromide. The results shown in the figure indicate that both the identity and the concentration of the halide ion present govern the difference between surface and bulk nitrate behavior. The
Figure 2. (a) The Irel values, as defined in the text, of the nitrate Raman intensity measured in 0.5 M NaNO3(aq) solutions are plotted against the concentration of dissolved sodium halide salt. The surface region (hollow symbols) and the bulk phase (solid symbols) results are shown for Br− (red circles) and Cl− (blue triangles). Each marker represents the mean of at least three composite spectra. The error bars symbolize the error propagated from the standard deviation of the mean. (b) The data shown in Figure 2a are displayed following correction for the halide-dependent water intensities as described in the text. Again, solid symbols represent bulk phase results, and hollow symbols show the surface region results.
different dependences of the nitrate intensity on halide concentration are compelling and strongly suggest that the concentrations of NO3¯ in the surface and bulk domains are differently influenced by polarizable halide ions. In order to relate the Raman intensity of nitrate measured at the surface to its concentration there, we must eliminate the artifact which arises from normalizing I(ν-NO3¯) by I(ν-H2O). To do this, the surface Irel values are corrected by dividing these by the corresponding bulk phase values (i.e., the hollow symbol values in Figure 2a are divided by the solid symbol values). Since the nitrate concentration in the bulk is independent of halide concentration, this procedure eliminates the apparent dependence of [NO3¯] on [X−] in the bulk phase. Assuming similar effects of halide anions on the water bend intensity in the bulk and the surface regions, this procedure should also account for the influence of halides on the water bend intensity in the glancing-angle spectra. Application of this correction to the data shown in Figure 2a gives the results displayed in Figure 2b, again with bulk 2995
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intensities shown as solid symbols and glancing-angle results as hollow symbols. For bromide, the surface results all lie above their bulk counterparts, and the difference between the two domains increases with increasing [Br−]. These data support the hypothesis that addition of Br− increases the propensity for NO3− to partition to the surface. By contrast, the corrected nitrate amounts in the surface region are essentially unaffected by the presence of chloride, indicating that this anion exerts only a very weak (or no) enhancement on nitrate surface propensity, at least for 0.5 M NO3−. Liu et al.21 monitored the water OH-stretching vibration in sodium halide solutions, both at the air−aqueous interface, using vibrational sum frequency generation, and in the bulk phase, using Raman and attenuated total reflection FTIR spectroscopies. They demonstrated that Br− perturbs the structure of interfacial water more than it does that of the bulk solution, but the effect of Cl− on the OH-stretching vibration at the interface was not detectable. The water bending mode that we monitor is also diagnostic of the hydrogenbonding environment.30 By analogy to the water stretch results, we posit that the effect of X− on I(ν-H2O) at the surface will be at least as strong as in the bulk, and hence the application of the correction described above to the glancing-angle measurements should give at least a lower limit to the true nitrate surface region concentrations. The effect of sodium halides on nitrate surface concentrations is further explored in Figure 3, which displays nitrate
concentration near the air-aqueous interface in bromidecontaining solutions.24 By contrast, the presence of chloride has little effect on the surface-region concentration of nitrate, at least at lower concentrations of both anions.23 It is likely that the relatively weaker ionic pairing among Cl−−Na+−NO3− in combination with chloride’s lower surface affinity results in a reduced overall tendency for nitrate to be drawn to the surface in chloride-containing solutions. The apparent suppression of surface region nitrate at higher bulk NO3− concentrations in solutions containing 0.5 M Cl− may be a real effect, or an artifact of the normalization procedure we employ. Because we derive correction factors for the water Raman intensity only at 0.5 M NO3−, we cannot rule out a possible effect of high nitrate concentration on the water bend intensity, and in particular, different effects in the presence of Cl− or Br−on this intensity change. However, we do note that Figure 3 shows that the nitrate surface concentrations in the presence of 0.5 M Br− are always greater than or equal to those in the presence of 0.5 M Cl−. The enhancement of nitrate anions at the water surface in the presence of Br− may have important atmospheric consequences, as discussed in ref 2. It is of particular importance to heterogeneous reactions in aqueous aerosols and at the quasiliquid layer of snow and ice, where nitrate acts as an important source of atmospheric oxidants. The diminished solvent cage present in the surface region should increase the production of species such as NOx and OH upon nitrate photolysis. An increase in OH production would increase the oxidation of near-surface halide anions, thereby producing the strong oxidants Cl and Br, and influencing the local oxidative strength in the atmosphere.
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EXPERIMENTAL METHODS Bulk phase spectra of aqueous halide containing nitrate solutions were measured using a commercial FT-Raman spectrometer with 1064 nm excitation, a quartz beamsplitter, and a Ge detector. Calibration was performed with neat naphthalene. All spectra acquired with this system were the result of 128 coadded scans over the 4000 to 1 cm−1 spectral range at 5 cm−1 resolution and room temperature. The FTRaman spectra were not further processed. Most experiments employed the setup described in Wren and Donaldson.18 Raman scattering was induced with the 355 nm output of a frequency-tripled pulsed Nd:YAG laser (pulse energy ∼1 mJ, pulse repetition 10 Hz). For glancing-angle Raman surface studies, 10 mL of solution was placed in a Teflon sample holder and the unfocused laser beam impinged the surface at >87° to the surface normal. Scattered light was collected by a liquid light guide (LLG) suspended ∼0.5 cm above the sample surface and imaged onto the entrance slit of a 1/4 m monochromator, where it passed through a 355 nm laser-line long pass filter before detection by a photomultiplier tube. The photomultiplier output was sent to an oscilloscope, which averaged an intensity versus time of 64 laser shots. A LABVIEW program sampled a 50 ns window centered on the maximum intensity of the Raman scattering signal. Spectra were acquired by manually scanning the monochromator in 0.3 nm (approximately 30 cm−1) steps from 360 to 380 nm. For bulkphase studies, the incident light entered the side of a 1.0 cm quartz cuvette horizontal to the surface and normal to the LLG detector. All other conditions were the same as for the glancingangle work.
Figure 3. The glancing-angle Raman data are shown as the corrected Irel as a function of bulk nitrate concentration for three conditions NaNO3(aq) without sodium halide salt (0 M X−(aq) black filled circles), NaNO3(aq) with 0.5 M Br−(aq) (red hollow circles), and NaNO3(aq) with 0.5 M Cl−(aq) (blue hollow triangles). The dashed trace is a linear regression fitting the NaNO3(aq) data. Each marker represents the mean of at least 3 composite spectra and the error bars symbolize the error propagated from the standard deviation of the mean response and blank.
adsorption isotherms for NaNO3(aq) and mixed NaNO3(aq)−0.5 M NaBr(aq) and NaNO3(aq)−0.5 M NaCl(aq) solutions. The results for mixed nitrate-halide solutions show markedly different trends than that observed from the single-component results (black filled circles). With added 0.5 M NaBr(aq) (red hollow circles), there is a slight enhancement of NO3− in the interfacial region despite the variability of the data (as suggested by the error bars). By contrast, the isotherm obtained with 0.5 M chloride added (blue hollow triangles) suggests that nitrate is suppressed at the interface under these conditions. The spectroscopic results presented above support the MDbased prediction of a significant enhancement of the NO3− 2996
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(9) Thomas, J. L.; Roeselová, M.; Dang, L. X.; Tobias, D. J. Molecular Dynamics Simulations of the Solution−Air Interface of Aqueous Sodium Nitrate. J. Phys. Chem. A 2007, 111, 3091−3098. (10) Nissenson, P.; Dabdub, D.; Das, R.; Maurino, V.; Minero, C.; Vione, D. Evidence of the Water-Cage Effect on the Photolysis of NO3− and FeOH2+. Implications of this Effect and of H2O2 Surface Accumulation on Photochemistry at the Air−Water Interface of Atmospheric Droplets. Atmos. Environ. 2010, 44, 4859−4866. (11) Thøgersen, J.; Réhault, J.; Odelius, M.; Ogden, T.; Jena, N. K.; Jensen, S. J. K.; Keiding, S. R.; Helbing, J. Hydration Dynamics of Aqueous Nitrate. J. Phys. Chem. B 2013, 117, 3376−3388. (12) Dubowski, Y.; Colussi, A. J.; Boxe, C.; Hoffmann, M. R. Monotonic Increase of Nitrite Yields in the Photolysis of Nitrate in Ice and Water between 238 and 294 K. J. Phys. Chem. A 2002, 106, 6967− 6971. (13) Bartels-Rausch, T.; Donaldson, D. J. HONO and NO 2 Evolution from Irradiated Nitrate-Doped Ice and Frozen Nitrate Solutions. Atmos. Chem. Phys. Discuss. 2006, 6, 10713−10731. (14) Minofar, B.; Vácha, R.; Wahab, A.; Mahiuddin, S.; Kunz, W.; Jungwirth, P. Propensity for the Air/Water Interface and Ion Pairing in Magnesium Acetate vs Magnesium Nitrate Solutions: Molecular Dynamics Simulations and Surface Tension Measurements. J. Phys. Chem. B 2006, 110, 15939−15944. (15) Dang, L. X.; Chang, T.-M.; Roeselova, M.; Garrett, B. C.; Tobias, D. J. On NO3−−H2O Interactions in Aqueous Solutions and at Interfaces. J. Chem. Phys. 2006, 124, 66101. (16) 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. (17) Xu, M.; Tang, C. Y.; Jubb, A. M.; Chen, X.; Allen, H. C. Nitrate Anions and Ion Pairing at the Air−Aqueous Interface. J. Phys. Chem. C 2009, 113, 2082−2087. (18) Wren, S. N.; Donaldson, D. J. Glancing-Angle Raman Study of Nitrate and Nitric Acid at the Air−Aqueous Interface. Chem. Phys. Lett. 2012, 522, 1−10. (19) 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. (20) Jungwirth, P.; Tobias, D. J. Ions at the Air/Water Interface. J. Phys. Chem. B 2002, 106, 6361−6373. (21) Liu, D.; Ma, G.; Levering, L. M.; Allen, H. C. Vibrational Spectroscopy of Aqueous Sodium Halide Solutions and Air−Liquid Interfaces: Observation of Increased Interfacial Depth. J. Phys. Chem. B 2004, 108, 2252−2260. (22) Ghosal, S.; Hemminger, J. C.; Bluhm, H.; Mun, B. S.; Hebenstreit, L. D.; Ketteler, G.; Ogletree, D. F.; Requejo, F. G.; Salmeron, M.; Mun, B. S.; Hebenstreit, E. L. D. Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides. Science 2005, 307, 563−566. (23) Wingen, L. M.; Moskun, A. C.; Johnson, S. N.; Thomas, J. L.; Roeselová, M.; Tobias, D. J.; Kleinman, M. T.; Finlayson-Pitts, B. J. Enhanced Surface Photochemistry in Chloride-Nitrate Ion Mixtures. Phys. Chem. Chem. Phys. 2008, 10, 5668−5677. (24) Richards, N. K.; Wingen, L. M.; Callahan, K. M.; Nishino, N.; Kleinman, M. T.; Tobias, D. J.; Finlayson-Pitts, B. J. Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions. J. Phys. Chem. A 2011, 115, 5810−5821. (25) Richards, N. K.; Finlayson-Pitts, B. J. Production of Gas Phase NO2 and Halogens from the Photochemical Oxidation of Aqueous Mixtures of Sea Salt and Nitrate Ions at Room Temperature. Environ. Sci. Technol. 2012, 46, 10447−10454. (26) Das, R.; Dutta, B. K.; Maurino, V.; Vione, D.; Minero, C. Suppression of Inhibition of Substrate Photodegradation by Scavengers of Hydroxyl Radicals: The Solvent-Cage Effect of Bromide on Nitrate Photolysis. Environ. Chem. Lett. 2009, 7, 337−342. (27) Kahan, T. F.; Reid, J. P.; Donaldson, D. J. Spectroscopic Probes of the Quasi-Liquid Layer on Ice. J. Phys. Chem. A 2007, 111, 11006− 11012.
For each experimental condition (i.e., nitrate and halide concentration), approximately 10 individual spectra were averaged. Prior to averaging the individual spectra, each raw spectrum was baseline-corrected by subtracting its minimum intensity over the range in Stokes shifts (400 − 1890 cm−1) and then internally normalized to the maximum intensity of the water bend, I(ν-H2O), as described by Wren and Donaldson.18 Sodium nitrate solutions were prepared by dissolving NaNO3(s) (Sigma-Aldrich, reagent grade, ≥ 99.9%) in deionized water (ELGA, 18.0 MΩ). Multicomponent sodium nitrate−sodium halide solutions were prepared by gravimetric addition of the appropriate mass of NaBr(s) (ACP Chemicals Inc., ACS reagent) or NaCl(s) (ACP Chemicals Inc., ACS reagent) to NaNO3(aq) solutions.
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1, showing the effect of halides on the Raman intensity of the water bend. This material is available free of charge via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by NSERC. As well, S.N.W. thanks NSERC for the award of a PGS-D Scholarship, and A.C.H. thanks OGS for a Scholarship. The authors are indebted to Professor Heather Allen for helpful discussions on the effect of halides on water Raman spectra and to Professor Barbara Finlayson-Pitts for providing unpublished data.
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REFERENCES
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