The interaction of water, 1,4 dioxane, and gaseous nitrogen dioxide, has been studied as a function of distance measured through the liquid-vapour interface by Raman spectroscopy with a narrow (&lt;0.1 mm) laser beam directed parallel to the interface. The Raman spectra show that water is present at the surface of a dioxane-water mixture when gaseous NO2 is absent, but is virtually absent from the surface of a dioxane-water mixture when gaseous NO2 is present. This is consistent with recent theoretical calculations that show NO2 to be mildly hydrophobic

Gerber, R Benny

Murdachaew, Garold

Phillips, Leon F

van der Veer, Wytze E

Varner, Mychel E

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleRaman spectroscopy of solutions and interfaces containing nitrogen dioxide, water, and 1,4 dioxane: Evidence for repulsion of surface water by NO2 gasPermalinkhttps://escholarship.org/uc/item/7t6098n9JournalThe Journal of Chemical Physics, 140(18)ISSN0021-9606AuthorsMurdachaew, GaroldVarner, Mychel Evan der Veer, Wytze Eet al.Publication Date2014-05-14DOI10.1063/1.4874640Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaTHE JOURNAL OF CHEMICAL PHYSICS 140, 184702 (2014)Raman spectroscopy of solutions and interfaces containing nitrogendioxide, water, and 1,4 dioxane: Evidence for repulsion of surface waterby NO2 gasGarold Murdachaew,1 Mychel E. Varner,2 Wytze E. van der Veer,2 R. Benny Gerber,1,2and Leon F. Phillips2,3,a)1Institute of Chemistry and the Fritz Haber Research Center for Molecular Dynamics, Hebrew University,Jerusalem 91904, Israel2Department of Chemistry, University of California, Irvine, California 92697, USA3Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand(Received 26 November 2013; accepted 21 April 2014; published online 8 May 2014)The interaction of water, 1,4 dioxane, and gaseous nitrogen dioxide, has been studied as a func-tion of distance measured through the liquid-vapour interface by Raman spectroscopy with a narrow(<0.1 mm) laser beam directed parallel to the interface. The Raman spectra show that water is presentat the surface of a dioxane-water mixture when gaseous NO2 is absent, but is virtually absent fromthe surface of a dioxane-water mixture when gaseous NO2 is present. This is consistent with re-cent theoretical calculations that show NO2 to be mildly hydrophobic. © 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4874640]I. INTRODUCTIONKolb et al.1 have presented a thorough review, up to2010, of outstanding theoretical and experimental problemsrelating to the reactions of atmospheric trace gases, includingNO2, with a variety of wet and dry surfaces, and it is clearthat there are still many interesting problems that need to beinvestigated.A recent theoretical study2 of the interaction of nitro-gen dioxide with the surface of water led to the conclusionthat, even though the binding energy of the O2N–OH2 com-plex was found to be 9.5 kJ mol−1, a factor of 2 larger thanpreviously calculated, nitrogen dioxide is at least “mildly”hydrophobic. Instead of taking part in hydrogen bonds, theoxygen atoms of NO2 were found to be repelled by watermolecules, which led to an unexpected preferential orienta-tion of NO2 at the surface, with the oxygen atoms pointing outof the interface, and suggested that, in a system with mixedsolvents, the surface might actually be depleted of water. Thiscast new light on the processes by which nitrous acid (HONO)is produced in the troposphere, indicated the need for an ex-perimental test of the theoretical results, and provided the mo-tivation for the present study.Donaldson and co-workers3–8 have shown that glancing-angle Raman spectroscopy is a very useful surface-selectivetechnique for probing a surface region with a thickness ofthe order of 10–100 molecular layers, being able to measureboth surface concentrations of solutes and surface depletionof solvents. Thus, for example, Wren and Donaldson7 usedglancing-angle Raman spectroscopy to study the reaction ofgas-phase ozone with NaBr and NaI at the surfaces of aque-ous solutions.a)Author to whom correspondence should be addressed. Electronic mail:leon.phillips@canterbury.ac.nzThe experiments reported here do not require surface se-lectivity of the kind employed in the experiments of Donald-son et al.,3–8 but simply exploit the ability of Raman spec-troscopy with a narrow (<0.1 mm) laser beam to obtainspectra that arise from species close to and within the in-terface. This limited selectivity, perhaps somewhat unexpect-edly, proves to be sufficient to detect the mutual repulsion be-tween water molecules and NO2 molecules at the gas-liquidinterface. To make the experiment work, it was necessary touse a mixed solvent comprised of water and 1,4 dioxane, thepurpose of the dioxane being to provide a reference surfacerelative to which the displacement of water could be observed.Surface tension measurements9 indicate that 1,4 dioxane be-haves as a weak surfactant in water, but this surfactant effectplays no role in our experiments, which could not resolve asurface monolayer. Dioxane was chosen as the reference sub-stance because it is both unreactive and totally miscible withwater, properties that have often been exploited by electro-chemists and spectroscopists.As in the earlier study of NO2 gas interacting with ni-tric acid,10 Raman spectra of H2O.NO2 species were soughtbut, consistent with the theoretical prediction of very weakbinding between H2O and NO2 and the small calculated fre-quency differences between the vibrations of unbound andbound species (Table I), no such spectra were found. Theabsence of large red shifts in the IR upon formation of theNO2–H2O complex is another indication that normal hydro-gen bonds are not formed, but rather the relatively weaker andlonger bonds discussed in Ref. 2.Low-resolution fluorescence spectra of gaseous NO2were observed with comparable intensity to the Raman spec-tra. The long (100 μs) radiative lifetime11 of NO2 at the ex-citation wavelength (523 nm) used in our experiments causedthe fluorescence spectrum of NO2 gas excited close to the in-terface to be distinctly different from that of gas excited only0021-9606/2014/140(18)/184702/7/$30.00 © 2014 AIP Publishing LLC140, 184702-1184702-2 Murdachaew et al. J. Chem. Phys. 140, 184702 (2014)TABLE I. Calculated vibrational frequencies (harmonic, unscaled, usingBLYP-D2/aug-cc-pVDZ, see Ref. 2 for details) for NO2 and H2O and fortwo isomers of the H2O.NO2 complex. Experimental frequencies for NO2and H2O are also shown.NO2 calc. H2O calc.753 1393 1704 1616 3791 3903Cs H2O.NO2 calc. Cs H2O.NO2 calc.754 1398 1712 1610 3792 3906C2v H2O.NO2 calc. C2v H2O.NO2 calc.754 1396 1707 1610 3789 3903NO2 expt. H2O expt.750 1318 1618 1595 3657 3756a few millimeters higher in the cell (Figs. 1(a) and 1(b)). Thelow-wavenumber Raman spectrum of the bulk solution sev-eral millimeters below the interface (Fig. 1(c)) showed peaksthat could be assigned to HNO3, HNO2, N2O4, N2O3, NO2,NO, N2O, nitrite and nitrate ions, as well as dioxane. The oddmembers of this collection are NO, formed by decompositionof N2O3, N2O3, which is responsible for the blue colour ofthe solution, and N2O (not shown in Fig. 1(c)), for which aplausible production mechanism12 isH2O + N2O4 ⇀↽ HONO + HNO3, (1)HONO + HONO ⇀↽ H2O + N2O3, (2)HONO + HONO ⇀↽ HNO + HNO3, (3)HNO + HNO ⇀↽ H2O + N2O, (4)where everyone of these reversible processes involves someinteresting dynamics. In our system, the resonance Ramanspectrum of NO2 is about 150 times more intense than thenon-resonance Raman spectrum of N2O. The N2O peak at2223.5 cm−1 is overlapped by a fluorescence band of NO2but could be observed when the NO2 concentration, as indi-cated by the intensity of the blue colour of the frozen sample,was low.II. EXPERIMENTALThe Raman spectrometer and sample cell were the sameas before10 except that, on the viewing window of the cell,the curved parts of the meniscus where it met the windowsat right angles to the viewing window were obscured by nar-row, vertical strips of metal foil glued onto the outside of thecell, and a Dove prism was inserted into the optical systemfor collecting scattered light, in order to rotate the horizon-tal green image of the laser-irradiated solution to match thevertical spectrometer slit. Otherwise the laser diode, notch fil-ter, spectrometer, and CCD detector were as previously de-scribed. All of the experiments reported here used a spec-FIG. 1. (a) Fluorescence spectrum of NO2 measured close to the liquidsurface (<0.5 mm away). The wave-number scale indicates displacementfrom the 523 nm exciting line. (b) Fluorescence spectrum of NO2 measured∼2 mm above the liquid surface. The wave-number scale indicates displace-ment from the 523 nm exciting line. (c) Raman spectrum of a blue solutionof NO2 in a 20:80 (by volume) mixture of water and 1,4 dioxane, showingthe detectable components below 2200 cm−1.trometer slit width of 0.25 mm. Exposure times varied be-tween 15 and 180 s, and Raman intensities, plotted in arbitraryunits in the graphs that follow, were scaled inversely with184702-3 Murdachaew et al. J. Chem. Phys. 140, 184702 (2014)exposure time, so that the intensity values indicated along thevertical axes of the graphs are significant. The sample cellwas mounted on a micrometer-driven platform that enabledthe vertical separation of the laser beam from the liquid sur-face to be adjusted reproducibly to within about 0.01 mm.The calculated beam waist within the cell was less than 0.1mm in diameter but, because the beam intensity profile is ex-pected to be Gaussian, the effective spatial resolution of theRaman scattering was somewhat greater than this. As a result,spectra taken at 0.1 mm intervals were slightly contaminatedwith spectra from adjacent 0.1 mm steps. This contaminationwas not fatal to our objective, which was to compare the sur-face distributions of water and 1,4 dioxane in the presenceand absence of NO2. Many of the spectra, especially thosewith long exposure times, were also affected by “cosmic-rayspikes.” The spikes, which were almost invariably positive-going and seldom more than 3 pixels wide, were removedby hand, using a computer program written in ChipmunkBasic.13A mixture of spectroscopic grade 1,4 dioxane with milli-Q water, of known composition by volume, was degassedin the side-arm of the cell by a series of freeze-pump-thawcycles. For most of the experiments, the mixture comprisedequal volumes of the two liquids. An equilibrium mixtureof nitrogen dioxide and dinitrogen tetroxide was preparedby adding a large excess of oxygen to nitric oxide that hadbeen purified by trap-to-trap distillation. The mixture was leftovernight before trapping the NO2/N2O4 as a white solid atliquid nitrogen temperature and pumping off the excess oxy-gen. The resulting bulb of gas at a pressure of a few hundredtorr was used to introduce NO2/N2O4 into the cooled side-armof the cell until the frozen solid was a more-or-less intensedark blue. Most of the spectra shown here were obtained withdark blue samples. The side-arm was then shut off from thevacuum system, the solid was allowed to thaw, and the cellwas shaken well before being transferred to the micrometer-driven platform, where it remained for up to a week whilespectra were being taken. Measurements made at intervalsof several days were in excellent agreement, which impliesthat any observed depletion of water in the surface layer wasnot merely the result of slow dissolving of NO2 from the gasphase.III. RESULTS AND DISCUSSIONThe aim of these experiments was to determine whetherthe presence of NO2 gas above the interface affected the con-centration of H2O at the interface. As a first step, we neededto obtain Raman spectra of pure 1,4 dioxane and of a dioxane-water mixture in our system, as shown in Figs. 2(a) and 2(b),respectively, and of dioxane vapour very close to the surface,as shown in Fig. 2(c). The water peak at 3400 cm−1 is a dis-tinctive feature of Fig. 2(b).The next step was to obtain Raman spectra in the3400 cm−1 region for a 1:1 water-dioxane mixture, withoutNO2, at a series of heights that included the interface. Typicalspectra are shown in Figs. 3(a)–3(f), with the beam height inmm indicated on each graph, the height increasing as the laserbeam rose through the interface. The dioxane peak intensityFIG. 2. (a) Part of the Raman spectrum of pure 1,4 dioxane. (b) Raman spec-trum of a 1:1 mixture of water and 1,4 dioxane. (c) Raman spectrum of diox-ane vapour near the liquid surface, with no NO2 present.decreased by a factor of ten as the beam height rose from 6.0to 6.6 mm, and the water peak was still present at 6.6 mm.At 6.7 mm and above, a comparison with Fig. 2(c) revealsthat much of the remaining dioxane peak was due to dioxane184702-4 Murdachaew et al. J. Chem. Phys. 140, 184702 (2014)FIG. 3. (a)–(f) Raman spectra of a 1:1 by volume mixture of water and 1,4 dioxane, with no NO2, as a function of the height of the laser beam. The height ofthe surface is between 6.4 and 6.6 mm and water is present all the way to the surface.vapour, so we can conclude that, in the absence of NO2, thereis no evidence for a marked decrease in the water concen-tration relative to dioxane as the beam moves up through theinterface. This shows incidentally that, with a height resolu-tion of the order of 0.05 mm, the surfactant nature of dioxanein dioxane-water mixtures did not result in significant exclu-sion of water from the surface of the mixed solvent. This isexpected, because such behaviour should result in the forma-tion of a monolayer, which would not be detectable in ourexperiments.Next, we examined the effect of moving the laser beamup through the interface for a cell containing water, dioxane,184702-5 Murdachaew et al. J. Chem. Phys. 140, 184702 (2014)FIG. 4. (a)–(d) Low wave-number Raman spectrum of a 1:1 (by volume) mixture of water and 1,4 dioxane in the presence of NO2, as a function of theheight of the laser beam. As the beam height increases the dioxane peaks diminish and the NO2 peaks and fluorescence background increase. The heightof the liquid surface is about 8.6 mm. Peak assignments for Fig. 4(a) are the same as in Fig. 1(c), with minor differences due to the differing solventcomposition.and NO2, looking first at the 1300 cm−1 region. Typical spec-tra are shown in Figs. 4(a)–4(c). The height scale does notcorrespond precisely to that in Fig. 3 because the cell had tobe moved to the vacuum system to introduce the NO2 andthen back again, but the height scales in Figs. 4 and 5 do cor-respond exactly. The spectrum in Fig. 4(a) is similar to that inFig. 1(c), showing mainly dioxane peaks with almost noNO2 gas. Small, gas-like NO2 and NO peaks are to beexpected from the liquid because, in the 1:1 solvent mix-ture, some of the NO2 and NO molecules will be sur-rounded by solvent shells composed entirely of dioxane.The peaks of NO2 gradually increase and those of diox-ane decrease as the beam moves upward, and it appearsthat the interface is located at a beam height of about8.6 mm.The plots in Figs. 5(a)–5(c), for the 3400 cm−1 re-gion show, in addition to the composite dioxane peak, achange from a small water peak with superimposed NO2bands at a height of 8.5 mm, to the spectrum of NO2 gasclose to the liquid surface at a height of about 8.7 mm.The plot of Fig. 5(b), at the level of 8.6 mm that wenow assign to the interface region, appears to consist of adioxane peak adjoining the NO2 fluorescence of Fig. 1(a),with no sign of the water peak at 3400 cm−1. The plot inFig. 5(d) is intended to make this point more obvious. Theportion of the plot that is identical with Fig. 5(b) is over-laid quite neatly by the scaled NO2 fluorescence spectrumof Fig. 1(a) in the 3400 cm−1 region. The plot of Fig. 5(a)has been scaled to fit the dioxane peak of Fig. 2(a) between2800 and 3000 cm−1, where it too fits well. However, the3400 cm−1 water peak of Fig. 2(b) is not able to be accom-modated anywhere. Hence it appears that, in the presenceof NO2, the surface layer of liquid is strongly depleted ofwater.184702-6 Murdachaew et al. J. Chem. Phys. 140, 184702 (2014)FIG. 5. (a)–(c) As in Fig. 4, but observing in the region of the water peak at 3400 cm−1. The spectrum in Fig. 5(a) shows the dioxane peak and the water peakwith weak NO2 fluorescence superimposed on the water peak. The spectrum obtained at a beam height of 8.6 mm shows the dioxane peak between 2800 and3000 cm−1 and the NO2 fluorescence close to the liquid surface, but the water peak at 3400 cm−1 is not evident above 8.5 mm. (d) The Raman spectrum ofFig. 5(b), analysed into the dioxane spectrum, plus NO2 fluorescence. The water peak of Fig. 3(a) (shown by the relatively noise-free solid line) clearly is notpresent in Fig. 5(b).IV. CONCLUSIONSThe mutual repulsion of water and NO2, which occurs atthe molecular level, has been found to have significant con-sequences on a macroscopic scale at the surface of a blue so-lution of 1,4 dioxane and water in the presence of gaseousNO2. The thickness of the relevant surface layer is of the or-der of 0.05 mm, which is much greater than the thickness ofthe region probed by grazing-incidence Raman spectroscopy.This possibly surprising observation depends critically on thetotal miscibility and lack of mutual reactivity of the dissimi-lar solvents 1,4 dioxane and water, which permit the surfacelayer to become depleted of water in the presence of NO2 gas.Thus, the present paper supports the theoretical conclusion1that direct formation of HONO from water and NO2 underatmospheric conditions is not a facile process, because waterand gas-phase NO2 tend to repel one another in the surface re-gion. It might also be regarding as providing indirect supportfor the suggestion14 that the formation of atmospheric HONOcould involve NO2− ions formed by electron capture by NO2from electrically charged surfaces.ACKNOWLEDGMENTSWe are grateful to Professor Barbara Finlayson-Pitts forproviding the environment and stimulus for this work, whichwas supported by Grant No. 0909227 from the United StatesNational Science Foundation (NSF).1C. E. Kolb, R. A. Cox, J. P. D. Abbatt, M. Ammann, E. J. Davis, D. J.Donaldson, B. C. Garrett, C. George, P. T. Griffiths, D. R. Hanson, M.Kulmala, G. McFiggans, U. Pöschl, I. Riipinen, M. J. Rossi, Y. Rudich, P.E. Wagner, P. M. Winkler, D. R. Worsnop, and C. D. O’ Dowd, Atmos.Chem. Phys. 10, 10561–10605 (2010).2G. Murdachaew, M. E. Varner, L. F. Phillips, B. J. Finlayson-Pitts, and R.B. Gerber, Phys. Chem. Chem. Phys. 15, 204–212 (2013).3T. F. Kahan, J. P. Reid, and D. J. Donaldson, J. Phys. Chem. 111, 11006–11012 (2007).4D. I. Reeser, A. Jammoul, D. Clifford, M. Brigante, B. D’Anna, C. George,and D. J. Donaldson, J. Phys. Chem. C 113, 2071–2077 (2009).184702-7 Murdachaew et al. J. Chem. Phys. 140, 184702 (2014)5D. Clifford and D. J. Donaldson, J. Phys. Chem. A 111, 9809–9814 (2007).6T. F. Kahan and D. J. Donaldson, J. Phys. Chem. A 111, 1277–1285 (2007).7T. F. Kahan, N. O. A. Kwamena, and D. J. Donaldson, Atmos. Environ. 40,3448–3459 (2006).8S. N. Wren and D. J. Donaldson, Phys. Chem. Chem. Phys. 12, 2648–2664(2010).9F. Hovorka, R. A. Schaefer, and D. Dreisbach, J. Am. Chem. Soc. 58, 2264–2267 (1936).10M. A. Kamboures, W. E. van der Veer, R. B. Gerber, and L. F. Phillips,Phys. Chem. Chem. Phys. 10, 4748–4753 (2008).11V. M. Donnelly and F. Kaufman, J. Chem. Phys. 69, 1456–1460(1978).12B. J. Finlayson-Pitts, L. M. Wingen, A. L. Syomin, and K. Ramazan, Phys.Chem. Chem. Phys. 5, 223–242 (2003).13See www.nicholson.com/rhn/basic/ for Chipmunk Basic source files.14L. F. Phillips, Phys. Chem. Chem. Phys. 15, 10749–10752 (2013).

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Raman spectroscopy of solutions and interfaces containing nitrogen dioxide, water, and 1,4 dioxane: Evidence for repulsion of surface water by NO2 gas

Raman spectroscopy of solutions and interfaces containing nitrogen dioxide, water, and 1,4 dioxane: evidence for repulsion of surface water by NO2 gas.

Raman spectroscopy of solutions and interfaces containing nitrogen dioxide, water, and 1,4 dioxane: evidence for repulsion of surface water by NO2 gas.

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