Quantitative Time-Resolved Vibrational Sum Frequency Generation

Article pubs.acs.org/JPCA

Quantitative Time-Resolved Vibrational Sum Frequency Generation Spectroscopy as a Tool for Thin Film Kinetic Studies: New Insights into Oleic Acid Monolayer Oxidation Joscha Kleber,† Kristian Laß,† and Gernot Friedrichs*,†,† †

Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Straße 1, D-24118 Kiel, Germany KMS Kiel Marine Science-Centre for Interdisciplinary Marine Science, Kiel University, Olshausenstraße 40, D-24098 Kiel, Germany

‡

ABSTRACT: Environmental air−water interfaces are often covered by thin films of surface-active organic substances that play an important role for air−sea gas exchange and aerosol aging. Surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy has been widely used to study the static structure of organic monolayers serving as simple model systems of such films. Probably due to the difficulties to correlate the SFG signal intensity with the surface concentration, corresponding time-resolved studies of surface reactions are scarce. In this study, quantitative timeresolved measurements have been performed on the oleic acid monolayer ozonolysis, which is considered a benchmark system for investigating the reactivity and fate of unsaturated natural organics. Surface concentration calibration data have been obtained by combining the pressure−area isotherm and VSFG spectra acquisition such that the 2D phase behavior of the oleic acid film could be properly taken into account. In contrast to literature reports, surface-active oxidation products were found to be negligible and do not interfere with the VSFG measurements. A pseudo-first-order kinetic analysis of the time-resolved data yielded a bimolecular rate constant of k2(oleic acid + O3 → products) = (1.65 ± 0.64) × 10−16 cm3 molecules−1 s−1, corresponding to an uptake coefficient of γ = (4.7 ± 1.8) × 10−6. This result is in very good agreement with most recent monolayer measurements based on alternative methods and underlines the reliability of the time-resolved VSFG approach.

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INTRODUCTION Natural air−water interfaces including the ocean surface and the surface of aqueous aerosols play important roles for cycling and exchange of matter, chemical transformations, and hence for atmospheric chemistry and the global climate. Coupled ocean−atmosphere climate models need to take into account air−sea gas exchange as well as the primary production and atmospheric fate of primary aqueous aerosols and their ability to act as cloud condensation nuclei. In particular, the physicochemical properties of aerosols are affected by the presence of organics, often surface-active substances. For example, it is known that thin films of surfactants, next to changing surface tension, diminish gas exchange by direct (i.e., by acting as an additional diffusion barrier) and indirect effects (i.e., by changing the viscoelastic properties of the surface, resulting in wave damping and thus less turbulent gas transfer).1 Moreover, the presence of organics affects the hydrophilicity, the scattering, and the absorption properties of the particle.2 Owing to advanced instrumental approaches such as high-resolution mass spectrometry, nuclear magnetic resonance spectroscopy, improved colorimetric and chromatographic protocols, and nonlinear laser spectroscopic methods, significant progress has been made to characterize the composition and regional and seasonal trends of dissolved organic matter (DOM), of the marine microlayer, and of primary and secondary organic aerosols.3−6 Although these methods provide quite comprehensive organic content analysis, © 2013 American Chemical Society

a thorough characterization must also consider the ubiquitous reactive transformation of the organic substance pool. Interactions with oxidizing species present in the atmosphere and photoinduced reactions take place both as bulk liquid phase and heterogeneous processes and are difficult to assess. In this work, a calibration-based method to quantitatively measure heterogeneous reactions at monolayer organic films will be introduced and demonstrated on the oleic acid (OA)− ozone oxidation system. Oleic Acid−Ozone Reaction System. The OA−ozone reaction system has been chosen by many authors as a simple model system to identify key issues of oxidative processing of organic aerosols. An excellent review of the current knowledge of the OA−O3 reaction system and its implication for tropospheric chemistry has been quite recently given by Zahardis and Petrucci.7 It focuses on sources of fatty acids, their roles as surfactants in the troposphere, reaction pathways of OA ozonolysis including secondary chemistry, and reactive uptake of ozone by OA films and particles including internally mixed and coated aerosols. Only some key aspects of the OA system as required for the data interpretation as well as a short overview of applied experimental methods and their limitations in terms of OA monolayer reactivity will be briefly outlined Received: April 25, 2013 Revised: June 26, 2013 Published: June 28, 2013 7863

dx.doi.org/10.1021/jp404087s | J. Phys. Chem. A 2013, 117, 7863−7875

The Journal of Physical Chemistry A

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here. OA, cis-9-octadecenoic acid, is found in atmospheric aerosols8 as well as in natural water samples.9 It is nonsoluble in water and forms well-structured monolayer films at the air− water interface. Ozone reacts with the olefinic double bond of OA and forms the primary ozonide, which decomposes to nonanal or 9-oxononanoic acid and the corresponding carbonyl oxides generally named as Criegee intermediates10

atmospheric lifetime of merely 30 s would be expected. This discrepancy has been widely attributed to the multicomponent character and phase state of natural liquid and solid particles,24 which have been experimentally verified to react at considerably lower rates. γ = 6.1 × 10−4 and 0.6 × 10−4 have been determined for pure OA and OA/hexadecanoic acid particles, respectively.25 For air−water interfaces and aqueous aerosols, it may be anticipated that the surfactant OA resides at the interface are easily accessible to the ozone and hence should react fast. However, studies on organic monolayers that serve as suitable proxies to separate surface reactions from interfering bulk and secondary chemistry challenge this simplistic view. Due to the need for surface-sensitive and time-resolved detection tools as well as the well-known experimental challenges to quantitatively investigate the kinetics of organic monolayer films, so far such studies are scarce and have led to partly contradictory conclusions. Details of these experiments will be highlighted and compared with the finding of this study in the discussion. Briefly, reactive uptake coefficients in the range of 2.7 × 10−7 ≤ γ ≤ 1.3 × 10−5 have been obtained. These values are consistently much lower than the above-mentioned reported values for pure OA samples, 5 × 10−5 (solid films) < γ < 7 × 10−3 (pure OA aerosols). This difference again stresses the importance of a separate treatment of surface chemistry, diffusion of ozone into the bulk, and secondary bulk-phase chemistry. VSFG Spectra of Organic Monolayers. An optical method holding the potential to quantitatively measure the decay of organic monolayers on liquid surfaces is vibrational sum frequency generation (VSFG) spectroscopy. This nonlinear laser spectroscopic method offers inherent surface specificity and, as a method yielding surface vibrational spectra, chemical specificity as well. VSFG spectroscopy provides information on the frequencies of molecular vibrations (surface composition), the surface density of the VSFG-active molecules (surface coverage), as well as the conformation of the molecules at the surface (surface structure). The method has found numerous applications in various areas such as surface dynamics and catalysis, 26 spectroscopy of air−aqueous interfaces,27 and for the examination of environmental and biomolecular surfaces.28,29 Recently, we were able to show its applicability for investigating the composition, chemical reactivity, and structure of natural marine nanolayers.6,30,31 Kinetic analyses on conformational changes of polypeptides and a pH-induced isomerization of an azo compound at water interfaces have been reported by Yan and co-workers32,33 and Wei and Ye,34 respectively. Finally, first VSFG spectra measurements performed during the ozonolysis of an OA monolayer were presented by Voss et al.35 The theory of VSFG spectroscopy is well developed,36 and only a brief overview of the method is presented here. Two laser pulses, usually a visible or near-infrared upconversion pulse with a frequency ωvis and a tunable infrared pulse with a frequency ωIR, are spatially and temporally overlapped at the air−water interface. The efficiency of the SFG signal at frequency ωSFG = ωvis + ωIR depends on the second-order susceptibility χ(2) of the sample, which vanishes in centrosymmetric media. Therefore, gaseous species and species dissolved in the bulk liquid are SFG-inactive. At phase boundaries, the centrosymmetry is inherently broken, and a resonantly enhanced SFG light beam can be observed at a well-defined direction. Scanning the IR laser frequency while detecting the

H3C−(CH 2)7 −CHCH−(CH 2)7 −COOH + O3 → H3C−(CH 2)7 −CH−(−O3−)−CH−(CH 2)7 −COOH ⊕

→ H3C−(CH 2)7 −CHO + ⊖O−O − CH−(CH 2)7 −COOH or

⊕

OHC−(CH 2)7 −COOH + ⊖O−O − CH−(CH 2)7 −CH3

For a monolayer at an air−water interface, the reaction takes place in a protic surrounding. Hence, the Criegee intermediates directly react with water instead of forming the secondary ozonide. The resulting hydroxyhydroperoxides are unstable and decompose to form aldehydes according to11 ⊕

⊖

O−O − CH−R1,2 + H 2O → HOO(HO−)CH−R1,2 → OHC−(CH 2)7 −COOH + H 2O2 or

H3C−(CH 2)7 −CHO + H 2O2

Here, R1,2 corresponds to the alkyl or carboxyalkyl moiety, respectively. In agreement with experiments, nonanal and 9oxononanoic acid can be assumed to constitute the main products of the overall OA monolayer ozonolysis, but the carboxylic acids HOOC−(CH2)7−COOH (azelaic acid) and H3C−(CH2)7−COOH (nonanoic acid, NA) and some higher molecular weight products formed through polymerization have been reported as side products as well.7,12−14 Whereas the volatile nonanal escapes to the gas phase, all of the oxygenated products, due to their shorter alkyl chain length and their additional carbonyl or carboxyl groups, are more hydrophilic than OA and can be expected to be removed from the surface by dissolution into the subsurface water. Overall, the transformation of the OA monolayer into volatile and dissolved organics changes the hydrophilicity of the interface.15 Quite different experimental approaches have been reported in the literature to determine the reactive uptake coefficient γ of OA ozonolysis. Coated flow tube reactors coupled to mass spectrometers have been used to monitor gas-phase ozone loss and yielded reactive uptake coefficients of γ = 8 × 10−4 for liquid and γ = 5 × 10−5 for frozen OA films.16 Significantly higher values of 7.5 × 10−4 ≤ γ ≤ 7.3 × 10−3 were obtained from aerosol flow tube experiments with mass spectrometric analysis of the particle loss and composition.12,17−20 Both pure and mixed aerosols as well as size-selected particles with diameters ranging from 0.68 to 2.4 μm have been analyzed. Similar results have been obtained by photoelectron resonance capture ionization aerosol mass spectrometry, thermal desorption particle beam mass spectrometry, and field-induced droplet ionization mass spectrometry.7 The systematically higher reactive uptake coefficients obtained from particle analysis compared to those of the gas-phase ozone loss measurements have been attributed to secondary chemistry. Therefore, modeling approaches have been applied to separate surface and bulk reactions.18,19,21,22 In fact, the reported high γ values are inconsistent with the observed high OA concentrations in atmospheric particles that imply OA lifetimes on the order of several days.23 For example, assuming a typical atmospheric O3 mole fraction of 50 ppb and γ ≈ 10−3, an 7864



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even small impurities may seriously affect the outcome of the reactant monolayer experiments. The ozonolysis of OA serves as a well-studied test case. Unfortunately, as has been already indicated above, even for the well-studied OA−ozone reaction system, inconsistencies regarding the overall reactive uptake coefficients, the oxidation products, and their presence or absence directly at the interface remain. They have direct implications for the data interpretation and therefore have been readdressed where required in this work as well. In this spirit, next to the presented methodological aspects of VSFG spectroscopy as a quantitative tool for studying heterogeneous kinetics, this study also provides new insights into the OA− ozone reaction system.

spatially separated SFG signal beam gives rise to the vibrational spectrum of the molecules that are directly located at the interface. In addition, the direction of the transition dipole moment vector of the probed vibration has to match the appropriate component of the second-order nonlinear susceptibility tensor, hence imposing characteristic restrictions on the SFG signal response. For example, when using the linear polarization combination ssp (s-polarized SFG, s-polarized visible, and p-polarized IR light), primarily vibrations with a transition dipole moment oriented perpendicular to the surface yield SFG signal. In general, χ(2) is determined by the average of the molecular hyperpolarizabilities β of the molecules at the surface. So far, the use of time-dependent VSFG spectroscopy to measure reactive uptake coefficients of surface reactions is mainly limited to functionalized solid surfaces. For example, Stokes et al.37,38 studied the ozone oxidation of cyclic and noncyclic alkenes on glass and were able to show that the accessibility of the CC double bond at the surface correlates with its reactivity.37,38 In these experiments, it has been assumed that the average of β does not change during the reaction such that the SFG signal intensity is given by ISFG ∝ (β × N)2. Here, N represents the number of surface oscillators. This simple N2 dependence does not hold for monolayers at gas−liquid interfaces. Depending on the surface concentration changes, the average molecular orientation of the laterally mobile surfactant molecules adapts, and β has to be regarded as a function of N itself

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EXPERIMENTAL SECTION Combined VSFG spectroscopy and Langmuir trough experiments have been used as surface-specific detection tools to investigate OA monolayers at the air−water interface. Both the surface concentration and monolayer structure have been investigated by monitoring CH stretch vibration SFG signal intensities and surface pressure−area isotherms, respectively. Following exposure to controlled levels of ozone, time-resolved OA surface concentrations were determined from SFG signal intensities measured for constant surface area samples placed in a small chemical reactor. In supplemental control experiments, OA monolayer oxidation was followed by simultaneous VSFG and surface pressure measurements as well. The monolayers were prepared from a 5 mM solution of OA (∼ 99%, Sigma Aldrich) in chloroform (≥99%, Sigma Aldrich). Different volumes of this solution were spread on the water surface by means of pipettes (Eppendorf reference). The chloroform was given 5−10 min to evaporate to finally form the OA monolayer. Similarly, the surface properties of the reaction product NA (≥99.5%, Fluke Analytics) were characterized by spreading 5 mM chloroform solutions. Subphase water was either high-purity water (18.2 MΩ cm) from a lab water purification system (Elga Purelab Ultra) or commercial bidistillated water (>0.5 MΩ cm, Carl Roth GmbH). All bulk phases were unbuffered and, due to dissolved CO2, were slightly acidic. VSFG Spectrometer. All VSFG spectra were measured using a commercial picosecond scanning-type VSFG spectrometer (EKSPLA), which has been described in more detail in a previous publication.30 Briefly, the second harmonic of a Nd:YAG laser system at 532 nm with pulse energies of about 600 μJ served as the up-conversion light. It was spatially and temporally overlapped with tunable IR light with pulse energies up to 200 μJ and a spectral bandwidth of 3.5 cm−1. The IR light was generated by an optical parametric and difference frequency generator in the wavelength range from 2300 to 10000 nm (OPG and DFG, Ekspla PG401/DFG2-10P). Test experiments showed that SFG signals were linear with respect to the used vis and IR laser pulse energies. All spectra were measured with incident angles of 59 (vis) and 53° (IR) in the ssp polarization combination. In order to ensure repeatability of the spectrometer performance, monolayer spectra of the phospholipid DPPC (dipalmitoyl- sn-glycero-3-phosphocholine) with known surface concentrations have been recorded on a daily basis. VSFG spectra of clean and covered air−water interfaces were recorded using a small Teflon dish (diameter 5.1 cm) filled with 10 mL of high-purity water. Before the preparation of a monolayer, the absence of the SFG signal intensity in the CH stretching region ensured that the air−

ISFG ∝ (β(N ) × N )2

Whereas this distinct surface coverage dependence of the SFG spectral intensities has often been used to deduce detailed structural information on the orientational alignment of alkyl chains,39 from the perspective of a kinetics experiments, it complicates the data evaluation that relies on the conversion of signal intensity to surface concentrations in the first place. This is probably the main reason why VSFG spectroscopy has only been used in a qualitative way to investigate the overall time scales of monolayer reactivity at air−liquid interfaces so far. Scope of This Work. Starting from the first successful but qualitative VSFG experiments of Voss et al.,35 the present work aims to establish time-resolved VSFG spectroscopy as a means for quantitative kinetic measurements on surfactant monolayers on liquids. To achieve this goal, several issues need to be addressed. First, solid procedures need to be worked out that allow one to convert SFG signal intensity to surface concentration. Here, calibration polynomials based on simultaneous measurements of the SFG signal intensity and surface pressure in a Langmuir trough experiment have been utilized. Second, as VSFG is a nonlinear spectroscopic method, the reproducibility of the absolute signal intensities and the applicability of calibration polynomials are not necessarily straightforward. This problem has been addressed by exploiting the relative intensities of the CH3 and CH2 symmetric stretching bands measured before the actual oxidation experiment as an internal standard for surface concentration. Third, it has to be assured that the 2D phase dynamics of the monolayer film is fast compared to the overall reaction and that possible reaction products do not interfere with the measurements. Both issues would complicate or even preclude the use of calibration polynomials that have been obtained for pure and equilibrated monolayers of the reactant. Finally, the reproducibility of the kinetic experiments needs to be analyzed as it is known that 7865



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675 ppb) were achieved by diluting ozone-rich gas mixtures generated with an ozone generator (SANOZON 300SE Model 2009, Gemke Technik GmbH). The undiluted ozone concentrations of 5.6 × 1014 to 1.8 × 1015 molecules cm−3 (22−73 ppm) were determined with a simple light absorption setup (O3 Monitor) using a UV-LED (Roithner Lasertechnik GmbH, T9H25C, approximately 100 μW) with an emission maximum at λ = 258 ± 7 nm as the light source. Beforehand, the effective absorption cross section σ of ozone had been determined based on I3 titration. The obtained σ = (1.08 ± 0.11) × 10−17 cm2 molecule−1 is in quantitative agreement with an estimated value of σ = 1.11 × 10−17 cm2 molecule−1, which was obtained by convoluting high-resolution literature data40 with the emission profile of the used UV-LED. For an oxidation experiment, a dense OA monolayer was spread at the air−water interface, and the Teflon sample dish was immediately inserted into the reactor. After slight realignment of the laser beams, the signal intensity was continuously recorded at one or two preset IR frequencies. Pure air was allowed to flow through the reactor over a minimum time interval of 5 min to ensure the evaporation of the spreading solvent. After the detected SFG signals became stable, the oxidation reaction was started by adding a controlled amount of ozone to the pure air flow. The course of the reaction was monitored until a change of VSFG signal intensity was no longer observed.

water interface was free of significant amounts of organic surface-active impurities. Langmuir Trough. A thin 100 mL Langmuir trough (Riegler & Kirstein GmbH) equipped with two movable Teflon barriers was integrated into the SFG beam path by a heightadjustable stage unit. Starting from the open position, the barriers symmetrically compressed the surface area of the films from 190 to 27 cm2. The baseplate of the trough was temperature-controlled and was set close to room temperature, typically T = 22 °C. The surface pressure was measured with a precision better than 0.5 mN m−1 using a Wilhelmy balance equipped with a replaceable piece of filter paper. In order to avoid surface-active impurities, each day, the trough was cleaned with high-purity water and some drops of a special detergent (Tickopur R 33, Dr. H. Stamm GmbH) at a temperature above 40 °C for 10 min and was thoroughly flushed with high-purity water afterward. For each measurement, the subphase was renewed, and the purity of the air− water interface was checked by detecting the surface pressure changes during a fast compression experiment. The interface was specified as sufficiently clean when the increase of the surface pressure was lower than 0.8 mN m−1. Starting at surface concentrations corresponding to areas well above 65 Å2 molecule−1, compression isotherms were recorded with barrier speeds ranging from 0.31 to 3.17 Å2 molecule−1 min−1. Oxidation Setup. The OA monolayer oxidation experiments were performed in a small, hemispherical reactor built from polished stainless steel UHV vacuum chamber parts. A schematic setup of the experiment is shown in Figure 1. Two

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RESULTS Following a brief interpretation of the VSFG spectrum of an OA monolayer, a justification for the selected detection peaks used in the kinetic experiments is given. The surface concentration dependence of the SFG intensity along the 2D phase behavior of OA monolayers and the feasibility of using calibration polynomials to convert SFG signal intensity into OA surface concentration is outlined. Finally, the results of the time-resolved ozone oxidation experiments yielding the surface oxidation rate constant and reactive uptake coefficient are presented. VSFG OA Monolayer Spectrum. The VSFG spectrum of an OA monolayer on water together with a surfactant-free clean water reference spectrum is illustrated in Figure 2. The black curve marked with symbols represents the 5-times-averaged spectrum of a dense OA monolayer with a mean molecular area

Figure 1. Schematic experimental setup for OA monolayer oxidation. MFC: calibrated mass flow controller.

CaF2 windows were used for the inlet and outlet of the vis, IR, and SFG beams. The reactor with a volume of 200 cm3 was operated under ambient pressure conditions by setting the flow rate of the reaction gases using calibrated mass flow controllers (FC-7700, Aera Advanced Energy Industries, 1000 sccm for compressed air and 10 sccm for air−ozone mixtures). In order to remove any potential condensable impurities, the compressed air passed through a dry ice/isopropanol cooling trap first. Typically, the total gas flow through the reactor was set to 650 cm3 min−1, resulting in a replacement of the reactor volume in less than 19 s. Compared to the time scale of a typical oxidation experiment (approximately 10 min), this replacement rate was high enough to ensure (i) a quick adjustment of the ozone concentration and (ii) a negligible loss and hence constant ozone gas-phase concentration during the ensuing surface oxidation reaction. Ozone concentrations ranging from 1.9 × 1012 to 1.7 × 1013 molecules cm−3 (75−

Figure 2. VSFG spectra of a surfactant-free clean water surface (gray, average of 47 measurements6) and an OA monolayer at a mean molecular area of 35 Å2 molecule−1 (black, average of 5 measurements). CH and OH vibration range and carbonyl vibration range (inset) measured in the ssp polarization combination. 7866



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of 35 Å2 molecule−1, and the gray curve corresponds to the 47times-averaged clean air−water interface spectrum adopted from our previous publication.6 The plot spans the range of CH (2750−3050 cm−1) and OH (3000−3850 cm−1) stretch vibrations. The inset illustrates the carbonyl wavenumber region at around 1720 cm−1. For the purpose of this study, the clean water spectrum simply served as a background signal. Its detailed spectral shape and interpretation are still subject to ongoing scientific discussion.30,41,42 Qualitatively, the spectrum is composed of a narrow peak at around 3710 cm−1, which can be assigned to the OH dangling bond of water molecules spanning across the air−water interface, and a broad structureless spectral feature observed at 3000−3600 cm−1. The latter has been assigned to hydrogen bonds in different coordination geometries. Some SFG intensity is also found below 3000 cm−1, hence overlapping with the CH stretch vibration range. This lowfrequency signal component most probably arises from the superposition of a broad OH stretching mode centered at around 3117 cm−1 and nonresonant SFG signal contributions.41 The overall shape of the VSFG spectrum of the dense OA monolayer is in very good agreement with previous broad-band VSFG measurements reported by Voss et al.35 The peak assignment of the CH stretch vibrations of OA was based on comprehensive assignment rules worked out by Wang et al.39 and is listed in Table 1. The two strongest, well-separated peaks

the presence of the organic monolayer. On the other hand, a spectral enhancement of the broad hydrogen bond network signal is observed, which is due to alignment effects of the water molecules with respect to the polar or even charged carboxy head group of OA.43,44 Time-resolved kinetic experiments were performed by setting the IR laser to a fixed detection frequency. In order to enable quantitative OA surface concentration measurements during ozone oxidation, the detection frequency had to be chosen carefully with regard to an attainable signal-to-noise ratio and OA selectivity. In terms of the highest OA selectivity, monitoring the SFG signal decay of the olefinic C−H bond, which is cleaved and disappears during ozonolysis, seemed to suggest itself. Unfortunately, the intensity of the corresponding peak at 3012 cm−1 was low, and it was strongly influenced by the interfering (out-of-phase) asymmetric CH3 stretch vibration as well as the broad OH hydrogen bond signals. In fact, the absolute peak height of the olefinic peak was found to be less reproducible. Moreover, both interfering signal contributions are expected to change during the reaction, hence making the C−H bond signal a second choice as a reaction progress indicator. Much better signal-to-noise ratios and reproducibility have been observed for intensities of the d+ and r+ symmetric stretch vibrations of CH2 and CH3. A potential disadvantage of alkyl-chain-related signals, however, is that they may arise from the reaction products as well. Nevertheless, as will be outlined below, these signals turned out to be a suitable choice for the kinetic measurements. Langmuir Isotherm and VSFG Calibration Curve. In order to relate the SFG spectral intensity to OA surface concentration, pressure−area isotherms and VSFG spectroscopy were run simultaneously. In principle, the SFG signal intensity can be well-described by the superposition of nonresonant and resonant signal contributions. A single resonance (e.g., an OA-related CH2 or CH3 vibration) superimposed on a nonresonant background (e.g., a spectrally broad signal contribution stemming from the water surface) yields SFG intensity according to45

Table 1. Assignment of the OA VSFG Spectruma ν̃/cm−1

functional group

vib mode

1715 2805 2850 2880 2895 2930 2945 2978 3012

CO CH2 CH2 CH3 CH2 CH2 CH3 CH3 C−H

stretch stretch d+ r+ d− d+FR r+FR r− stretch

(2) (2) ISFG ∝ (χnonresonant + χresonant )2

a

Reported peak wavenumbers are based on a detailed simulation of the observed spectral band shape.

⎛ ⎞2 A iϕ = ⎜Abackground × e + ⎟ ωIR − ω − iΓ ⎠ ⎝

belong to the symmetric stretch vibrations of CH2 at 2850 cm−1, denoted d+, and CH3 at 2880 cm−1, denoted r+. Using the ssp polarization combination, the two corresponding antisymmetric stretch vibrations d− and r− are expected to exhibit much lower SFG intensity and are out-of-phase relative to the symmetric stretch and water background signal. Spectral simulations showed that the breakdown of SFG intensity at around 2900 cm−1 and the low signal intensity at around 2990 cm−1, which is even lower than that of the water background, can be consistently explained by allowing for d− intensity centered at 2895 cm−1 and r− intensity centered at 2978 cm−1. The olefinic C−H stretch vibration is visible as a small and narrow peak centered at 3012 cm−1. Other peaks stemming from Fermi resonances are denoted d+FR and r+FR, and the CO stretch vibration of the carboxylic acid group shows up at 1715 cm−1. The OH stretch vibrational range observed at 3000−3600 cm−1 reveals the typical features of a water surface covered by a dense film of fatty acids. On the one hand, the disappearance of the narrow OH dangling bond intensity at 3710 cm−1 indicates

The vibrational resonance is described by its resonance frequency ω, line width Γ, and amplitude A. Abackground refers to the amplitude of the nonresonant susceptibility, with ϕ constituting the phase of the background relative to the phase of the vibrational contribution. Assuming an in-phase situation as it is valid for the r+ and d+ signal contributions of OA ISFG −

(2) IBackground ∝ χresonant

holds, and background-corrected and normalized SFG intensity curves can be calculated according to ISFG − 2

2

> 65 Å /molecule IBackground

< 30 Å /molecule − ISFG

2

∝ β (N ) × N

> 65 Å /molecule IBackground

In Figure 3, the measured surface pressure (right axis) and the corresponding corrected and normalized SFG intensities (left axis) measured at 2850 (d+) and 2880 cm−1 (r+) are plotted 7867



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measurement, and the dashed curves correspond to an averaged curve of five measurements. For background correction, the low SFG intensity values measured at surface concentrations corresponding to mean molecular areas > 65 Å2 molecule−1 were taken as the baseline. At low surface concentration, the SFG intensity is almost zero due to the random orientation of the alkyl chains in the gaseous 2D phase of the OA monolayer. In order to minimize the influence of the day-to-day variations of absolute SFG intensity, typically ±15%,6 all signals were normalized with respect to the maximum CH3 signal intensity observed at nominal surface concentrations above the collapse point corresponding to 100 higher than the highest measured initial reactive loss of ozone (about 1 × 1012 molecules s−1). k1 values were extracted from the experiment by fitting a monoexponential function and are plotted as function of the ozone concentration for the three separate measurement series

Figure 5. (a) SFG signal intensity of the CH3 mode during the oxidation of OA monolayers with 223 (circles) and 507 ppb (squares) ozone. Dashed curves are shown to guide the eye. (b) Corresponding OA surface concentration−time profiles fitted with monoexponential decay functions. 7869



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in Figure 6. The slopes of the straight lines through the data yield the bimolecular rate constants k2 = (1.71 ± 0.28) × 10−16

The number of reacting molecules is determined by the bimolecular rate equation and is divided by the number of total collisions with the surface from the gas kinetic theory γ=

1 ·[O3]· c ̅ 4

=

4·k 2·[OA] c̅

Here, [OA] was set to a representative OA surface concentration of 2.55 × 1014 molecules cm−2, corresponding to the middle of our dynamic range. With the mean velocity of ozone molecules, c ̅ = 360 m/s, the reactive uptake coefficient becomes γ = (4.7 ± 1.8) × 10−6. Finally, k2 can be expressed in surface units by taking into account the solubility constant of ozone, k2,surface = k2/(HRTδ). With Henry’s constant H = 4.8 × 10−4 mol cm−3 atm−1,48 RT = 2.412 × 104 atm cm3 mol−1 at T = 294 K, and by assuming a surface layer thickness48 of δ = 2 × 10−7 cm, the bimolecular surface rate constant becomes k2,surface= (7.1 ± 2.8) × 10−11 cm2 molecules−1 s−1.

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Figure 6. Plot of the pseudo-first-order rate constant k1 as a function of the ozone concentration for three measurement series. The slopes of the regression lines correspond to k2; see the text.

DISCUSSION The experimental findings need to be discussed in two ways. On the one hand, methodological aspects are of interest to assess the potential of VSFG spectroscopy as a quantitative tool for studying heterogeneous kinetics. On the other hand new insights into the OA oxidation system are highlighted and critically analyzed in comparison with existing literature data. Both aspects are closely linked to each other. For example, whether surface-active oxidation products reside at the air− water interface is of interest from the viewpoint of product analysis and is an important basic assumption for analyzing the time-resolved VSFG spectra as well. Although the OA oxidation has been thoroughly studied by several groups, major inconsistencies regarding the reaction products and the value of the reactive uptake coefficient remain. In comparison with OA monolayer systems, up to 3 orders of magnitude higher uptake coefficients were found for pure OA films and aerosol samples. It has been shown by modeling work22 that these high uptake coefficients arise from diffusion into and reaction of ozone in the bulk as well as secondary chemistry. Therefore, we limit our discussion to the more comparable measurements on OA monolayers. Available literature data are summarized and compared with our results in Table 2. Although the study of monolayer oxidation should allow one to separate the initial surface oxidation step more easily, reported uptake coefficients scatter by a factor of 50 and range from γ = 2.7 × 10−7 to 1.3 × 10−5. Moreover, whereas most experiments showed that no or minor direct oxidation

(series 1), (2.07 ± 0.14) × 10−16 (series 2), and (1.09 ± 0.16) × 10−16 cm3 molecules−1s−1 (series 3). The stated uncertainties corresponds to the 2σ error of the fitted slopes. Allowance was made for minor ozone concentration errors resulting from oxidative or reductive gas contaminations in the used compressed air and/or gas supply lines by forcing the linear fits through zero. Appropriate offsets of the ozone scale were −1.61 × 1012 (series 1), +0.51 × 1012 (series 2), and −1.75 × 1012 molecules cm−3 (series 3). To avoid this systematic correction in future measurements, the apparatus will be provided with a more sensitive ozone analyzer soon that is capable of quantitative measurements of ppb levels of ozone in the outflow of the reactor. Overall, the intraday reproducibility within a single series was good. However, in particular, the significantly lower value obtained for series 3 reveals some longterm variability that will be further discussed below. Combining all three measurements, an error-weighted average bimolecular rate constant of (1.65 ± 0.64) × 10−16 cm3 molecules−1 s−1 is obtained at 2σ uncertainty level. This number can be converted to the commonly used reactive uptake coefficient γ, which is defined as γ=

k 2·[OA]·[O3]

number of O3 molecules reacting number of O3 molecules colliding with the surface

Table 2. Literature Data on OA Monolayer Ozonolysisa reference

sample

Wadia et al. González-Labrada et al.46 Voss et al.35 King et al.48 unpublished work, cited in refs 22 and 49 this work

OPPC OA OA d-OA d-MeOA OA

10

γ/10−6

method and measurement Langmuir trough, API-MS of nonanal pendant drop, surface pressure Langmuir trough, surface pressure, VSFG spectroscopy Langmuir trough, surface pressure, neutron reflection Langmuir trough, surface pressure, neutron reflection flow cell, quantitative VSFG spect., Langmuir trough, surface pressure

∼13b ∼0.27c ∼3d ∼4 ∼5 (4.7 ± 1.8)

surface products no minor, transient dense NA monolayer minor

a

Abbreviations: OPPC (1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine), OA (oleic acid), d-OA (deuterated OA), d-MeOA (deuterated methyl oleate), NA (nonanoic acid), API-MS (atmospheric pressure ionzation mass spectrometry). bγ is estimated from Figure 5 and Table 1 in ref 10; see the text. cγ is recalculated from the k2 constant in ref 46; see the text. dγ is estimated from the surface pressure curve displayed in Figure 5 of ref 35; see the text. 7870



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concentrations of about 10−5 mol L−1. For comparison, assuming a 100% yield of NA, the complete dissolution of NA in our experiments would have resulted in a roughly 10 times lower bulk concentration of 10−6 mol L−1. Finally, in order to exclude a concerted effect of NA and OA, we spread the same amounts of OA and NA simultaneously. Following an evaporation period of 5 min, the pressure area isotherm was found to be identical to the one observed for pure OA monolayers. Summing up, in agreement with most studies, we have found no indications that LMW products may reside at the air−water interface in significant amounts. Even if there were some remaining oxidation products at the interface, these products seemingly did not critically alter the SFG response of the OA film. The good agreement of the calibration curve data in Figure 3 with the oxidation experiment data shown in Figure 4 can be taken as a confirmation that potential products have not interfered with the time-resolved VSFG measurements. Most strikingly, the step-like intensity loss in Figure 3 (corresponding to the 2D phase transition) and the disappearance of surface pressure (corresponding to the lift-off point) coincide; exactly the same behavior is seen in Figure 4. Nevertheless, it should be kept in mind that the dynamic range of the measurements covers only the upper half of the possible OA surface concentrations and that some ambiguities remain. Compared to Figure 3, the crossing point of the CH3 and CH2 SFG intensity curves was found at lower surface pressure, the phase transition is somewhat blurred, and the CH2 signal level persists at long reaction times. These differences may well indicate a limited time response of the 2D phase state of the OA monolayer or the influence of reaction products. Especially, the persisting CH2 signal may arise from residual organics at the interface. Note, however, that in the time-resolved experiments performed in the closed-shell reactor, this CH2 signal level was always much lower than that in the less well defined test experiment shown in Figure 4. In a recent paper, the amount of residual organics is discussed by Dennis-Smither et al.55 For the ozone oxidation of mixed OA/sodium chlorid aerosol particles, they estimated that 50−85% of the initial organic volume remains involatile, depending on the ozone concentration and humidity. HWM products can be formed through polymerization processes initiated by the highly reactive Criegee intermediate, which can be described as a zwitterion as well as a biradical.7 In fact, HWM products have been identified in several heterogeneous OA−ozone oxidation experiments.13,14 They can be assumed to exhibit a higher surface preference than the LMW products, and their dissolution into the bulk liquid may be hampered by low diffusion coefficients. If present, the generated polymer or oligomer structures are very likely to be highly disordered and hence would result in weak SFG signals owing to the intrinsic sensitivity of VSFG spectroscopy to wellordered structures. We conclude that the role of HWM products in monolayer oxidation remains ambiguous, but obviously, they did not significantly affect the analysis of the time-resolved SFG signals. VSFG Spectroscopy as a Quantitative Tool for Interface Kinetics. The applicability of VSFG spectroscopy as a quantitative tool for interface kinetic studies had not been fully exploited as of yet. Whereas the intrinsic surface specificity of the SFG process response is ideal for monolayer reactivity studies, other features of VSFG spectroscopy are less advantageous. Next to interfering surface oxidation products,

products reside at the air−water interface, it has been claimed in a paper by King et al.48 that NA forms a dense product film. Oxidation Products. A basic assumption of our kinetic analysis of the time-resolved data is that the SFG signal intensity only stems from OA molecules and that the intensity calibration curve in Figure 3 is not altered by contributions from oxidation products. For the reaction of OA with ozone in a protic surrounding, the four low molecular weight (LMW) products16,17,20,25,50−52 azelaic acid, 9-oxononanoic acid, nonanal, and NA and high molecular weight (HMW) products13,14 such as oligomers or polymers are expected. The yields of these products depend on the exact reaction conditions,7 and their presence at the surface highly depends on molecular properties such as volatility and solubility. In contrast to the nonsoluble OA (5.6 mg/L, solubilities are for unbuffered solution at T = 25 °C as reported in the Scifinder database53), the partly difunctional and shorter-chain reaction products azelaic acid (8.7 g L−1), 9-oxononanoic acid (3.1 g L−1), and NA (1.0 g L−1) are comparatively soluble in water. Nonanal with a solubility of merely 0.33 g L−1 is a highly volatile compound (vapor pressure ≈ 0.35 mbar at 25 °C) and has been detected as a gas-phase product.10 Reported product yields are 1−6% for azelaic acid,12,13,50 14 and 42% for 9oxononanoic acid,12,13 2 and 9% for NA,12,50 and 25 to 42% for nonanal.12,16,51 Due to their high hydrophilicity and volatility, the LMW products should dissappear from the surface. This was confirmed by VSFG measurements of Voss et al.35 They found no CH stretching signals when spreading 5 monolayers (ML) equiv of NA or azelaic acid and 2 ML equival of nonanal at the air−water interface. Only at 3 ML equiv of nonanal was a distinct CH signature observed, which however was not consistent with their VSFG spectra measured during OA− ozone oxidation. Other experiments of González-Labrada et al.,46 who did not succeed in measuring the pressure−area isotherm of nonanal, corroborate the quick dissapearance of nonanal from the surface.46 NA as the nonvolatile LMW product with the lowest solubility deserves a closer look because King et al.48 suggested a monolayer of NA remains at the aqueous interface after the oxidation of the OA monolayer. They followed the ozone oxidation of a deuterated OA monolayer in a Langmuir trough by neutron reflection. Both their reflectivity data as well as the measured surface pressure curve revealed that some deuterated species with approximately half of the number of deuterium atoms present in deuterated OA remain at the surface as a final product. However, the presented surface pressure curve (Figure 2 in ref 48) with a plateau value of 14 mN m−1 at long reaction times clearly contradicts our own (see Figure 4) as well as the results of Voss et al.35 and González-Labrada et al.46 In all of these studies, the surface pressure dropped to zero at later stages of the reaction, indicating the absence of significant amounts of organic material. Intriguingly, in another neutron reflection study of Pfrang et al., which has been cited as unpublished work in refs 22 and 49, no surface-active products have been detected for the ozonolysis of a monolayer of the deuterated methyl ester of OA. In order to resolve this issue, we performed some additional test experiments by spreading several monolayer equivalents of NA without being able to measure any SFG signal or appreciable surface pressure rise during compression on the Langmuir through. This is also consistent with the published NA adsorption isotherm of Lunkenheimer et al.54 showing that the surface tension of an aqueous solution of NA does not change up to bulk 7871



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that were independent of the investigated solution subphases including pure water, artificial seawater, and salt solutions at diverse pH values but showed significant scatter. A direct comparison with our data reveals the good quality of the VSFG measurements. Nonetheless, the question arises if the remaining scatter and the differences of the obtained bimolecular rate constants for the three separate measurement series shown in Figure 6 may be ascribed to shortcomings of the calibration and data evaluation procedure or to other experimental artifacts. In order to assess the robustness of the data handling procedure, an alternative and simple two-point analysis of the time-resolved data can be performed. For this purpose, the start concentration at tstart = 0 (point 1) is set equal to the experimentally intended spreading OA concentration. The stop concentration was assumed to be 1.75 × 1014 molecules cm−2 (from 2D phase transition of the calibration curve), with the stop time tstop taken independently from the step-like features in the time-resolved SFG signals (point 2). Assuming a monoexponential decay of OA, the pseudo-firstorder rate constant can be directly calculated using k1 = ln([OA]spread/1.75 × 1014 molecules cm−2)/tstop. Example data are shown for series 2 as open circles in Figure 7. Good agreement between this simplified two-point analysis and the analysis based on the full calibration and normalization procedure has been obtained for all three measurement series. Hence, we conclude that the data handling yields reliable rate constant results and that the remaining uncertainties must have another reason. We tentatively ascribe the scatter to systematic uncertainties of the applied low ozone mole fractions or surface-active minor impurities at the air−water interface that are known to critically limit the attainable precision of monolayer experiments. Uptake Coefficient. Our final result for the reactive uptake coefficient, γ = (4.7 ± 1.8) × 10−6, is compared with literature data on OA monolayer oxidation and the oxidation of similar systems in Table 2. Uptake coefficients in the range of 2.7 × 10−7 ≤ γ ≤ 1.3 × 10−5 have been reported or have been deduced from these studies as outlined below. In 2000, Wadia et al.10 reported the first time-resolved monolayer study on the gas-phase products of the reaction of ozone with the unsaturated phospholipid 1-oleoyl-2-palmitoylsn-glycero-3-phosphocholine (OPPC) at the air−water interface. Nonanal was detected as the main gas-phase product by atmospheric pressure ionization mass spectrometry. Nonanal could be detected for approximately 5 min with x(O3) = 1 ppm and for >11 min with x(O3) = 0.25 ppm. No discernible trend of the reaction time with respect to the initial surface concentration of OPPC over the range of 40−158 Å 2 molecule−1 has been found. Assuming pseudo-first-order reaction conditions and 95% OPPC conversion after 4.6 min at 1 ppm O3 level (values deduced from Figure 5 and Table 1 in ref 10), a bimolecular rate constant of k2 ≈ 4.5 × 10−16 cm3 molecules−1 s−1 and with it an uptake coefficient of γ ≈ 1.3 × 10−5 can be estimated from this work. In another study, Thompson et al.58 investigated the ozone oxidation of the structural isomer 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) by means of neutron reflection and surface pressure measurements. Although no rate coefficients have been reported, an interesting result was that the unsaturated palmitoyl strands were lost from the air−water interface during the reaction. The authors explain this unexpected result from radical-initiated degradation reactions that may have been initiated by OH radicals formed from the ozone oxidation of

issues may arise from a too slow 2D surface dynamics of the monolayer and the well-known difficulty of nonlinear spectroscopic methods to obtain reproducible absolute signal intensity levels on a day-to-day basis. Again, we conclude from the ozone oxidation experiment shown in Figure 4 that the time scale of our experiments was long enough to ensure a steady reequilibration of the 2D structure of the OA monolayer during the reaction. Deviations of the overall signal trend would have been expected for a situation with a surface structure significantly different from the equilibrium structure probed by the calibration experiments. Molecular dynamics simulations of phospholipid monolayer oxidation performed by Cwiklik and Jungwirth56 and Khabiri et al.57 revealed that unoxidized chains preserved their ordered structure up to the point where half of the molecules are oxidized. In contrast, a fast reorientation of the oxidation products such as nonanal took place. Both aspects are consistent with our view of negligible signal contributions from the oxidation products. On the one hand, the dynamic range of our OA oxidation measurements was limited to about half of the initial OA surface concentrations anyway, and on the other hand, disordered reaction products are expected to yield low or no SFG intensity. Day-to-day intensity alterations of SFG signal intensity makes it difficult to extract accurate absolute OA surface concentrations from the SFG signal intensity alone. This problem was efficiently handled by using the ratio of I(CH3)/ I(CH2) as an internal standard. For the kinetics experiments, the initial OA surface concentration was accurately determined from the ratio measured during a prerun period first, and then, the total signal intensity was rescaled to be consistent with the normalized calibration curve. In this way, absolute concentration measurements were possible without the need for repeated laborious signal intensity calibrations. Overall, the linearity of the resulting pseudo-first-order plots of the three measurement series illustrated in Figure 6 and redrawn in Figure 7 for series 2 is good. In light of the typical difficulties in measuring surface reaction rates of organic monolayers, the obtained scatter of the data is comparatively small. For example, Figure 7 compares our series 2 data (solid circles) with a comprehensive data set taken from the most recent paper of King et al.48 They found overall rate constants (star symbols)

Figure 7. Comparison of pseudo-first-order rate constants k1 (series 2) with k1 values obtained by a simplified two-point analysis (open circles; see the text). Open stars refer to literature data taken from the neutron reflection study of King et al.48 7872



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the oleoyl strand. Initial excited-state Criegee intermediates may not be stabilized in the POPC monolayer and instead may break down to give highly reactive OH radicals. If this is true, a similar behavior should be expected for the OPPC monolayer as well. Therefore, as OH radicals can be expected to nonselectively react with both the saturated and unsaturated strands of OPPC, the observed nonanal detection time during OPPC oxidation in ref 10 may be somewhat biased by such a reaction pathway. In this case, the observed nonanal lifetime would not directly reflect the reactive ozone uptake coefficient of the oleyl strand, and the estimated uptake coefficient would be systematically too high. González-Labrada et al.46 followed OA monolayer oxidation by measuring the surface pressure decrease of a pendent drop and gave a value of γ = (2.6 ± 0.1) × 10−6. However, this value is not consistent with their reported bimolecular surface rate constant, k2 = (9.4 ± 0.5) × 10−18 cm3 molecules−1 s−1, and the observed time scale of their experiments. For example, even at an O3 mole fraction of 8 ppm, which is about 1 order of magnitude higher than the highest O3 mol fraction used in this work, an OA lifetime of 12 min was observed (Figure 5 and Table 1 in ref 46). Relying on their k2 value, we therefore recalculated their uptake coefficient to γ = (2.6 ± 0.1) × 10−7, which is 17 times lower than our value. The reason for this large discrepancy remains unclear. Voss et al.35 performed VSFG and surface pressure measurements similar to the measurements reported in this work. In Figure 5 of their paper, time-resolved surface pressure data are presented for an experiment with x(O3) ≈ 190 ppm. Assuming pseudo-first-order reaction conditions and applying our surface pressure calibration curve, a bimolecular rate constant of k2 ≈ 1.0 × 10−16 cm3 molecules−1 s−1 and with it an uptake coefficient of γ ≈ 3 × 10−6 can be estimated from their data. This value is in very good agreement with our measurements as well as the data from two neutron reflection studies. These time-resolved neutron reflection oxidation experiments on monolayers of deuterated OA and deuterated OA methyl ester performed by King et al.48 and Pfrang et al. (cited in refs 22 and 49) yielded γ ≈ 4 × 10−6 and 5 × 10−6, respectively. Overall, except for the data of González-Labrada et al.,46 our direct measurement of the surface rate constant and the reactive uptake coefficient agree very well with most recently published data. On the one hand, this good agreement can be taken as a confirmation of the feasibility of the new quantitative timeresolved VSFG approach presented in this paper. On the other hand, the data clearly corroborate the much lower reactive uptake coefficients of OA monolayers compared to those of pure OA film and aerosol samples.

may arise from surface-active reaction products and arising mixed monolayers. The OA−ozone oxidation reaction, O3(g) + OA(monolayer) → products, turned out to be an advantageous test system for the new data analysis method. The distinct 2D phase behavior accompanied by structural changes within the monolayer gives rise to a characteristic surface concentration dependence of different vibrational SFG modes such that measuring peak ratios instead of absolute intensities can be used as an internal standard for surface concentration determinations. In this way, the typical day-to-day intensity alterations of the nonlinear laser spectroscopic VSFG method could be taken into account and did not compromise the precision of the kinetic experiments. As no surface-active products have been found to interfere with the OA calibration curve, a straightforward and direct pseudofirst-order measurement of the surface rate constant, k2 = (1.65 ± 0.64) × 10−16 cm3 molecules−1 s−1, and of the reactive uptake coefficient, γ = (4.7 ± 1.8) × 10−6, was possible. Actually, the uncertainty and limited reproducibility of the absolute ozone concentration seemed to limit the precision of our experiments rather than the sophisticated VSFG detection scheme. Low CH2 signal levels observed at long reaction times in some experiments probably arise from minor polymeric products. The kinetic results agree very well with recently published data and thus can be taken as a confirmation of the accuracy of the quantitative time-resolved VSFG approach. Moreover, the outcome of this study corroborates the order of magnitude lower reactive uptake coefficient of OA monolayers compared to those of pure OA films and OA aerosol samples. Obviously, although difficult to perform, monolayer oxidation experiments are crucial to separate interface chemistry from bulk secondary chemistry and contribute to a better understanding of the roles of heterogeneous reactions in the environment. To further enhance the method, next to the installation of a revised ozone generation and metering system, the simple reactor setup used in this study should be enhanced by a Langmuir trough equipped with a purged hood to enable future experiments with well-define oxidation conditions and the opportunity to simultaneously measure time-resolved surface pressure and VSFG intensity curves. An improved precision of the extracted rate constants is essential to deduce structure− reactivity relationships by investigating and comparing the oxidation kinetics of different unsaturated lipids at the air− water interface. Moreover, quantitative data on reactivity trends of mixed monolayers are of high interest to provide reliable and realistic input parameters for atmospheric models.

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AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS It has been successfully demonstrated that VSFG spectroscopy as an intrinsically surface-sensitive method can serve as a quantitative and time-resolved detection tool to directly measure the rate constants of surface reactions at air−liquid interfaces. Due to the structure dependence of the SFG intensity, the conversion of signal intensity to surface concentrations has to rely on calibration curves. In simple cases, such calibration curves can be easily obtained by Langmuir trough compression experiments. Limitations of the method arise from the limited dynamic range of detectable surface concentrations and the distinct cross-sensitivity that

*E-mail: [email protected]. Phone: +49 431 880 7742. Fax: +49 431 880 7743. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Matthias Haffke for setting up the UV-LEDbased ozone absorption setup. Financial support by the German Science Foundation (DFG-EXC 80) in the framework of the cluster of excellence “The Future Ocean” is gratefully acknowledged. 7873



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Quantitative Time-Resolved Vibrational Sum Frequency Generation

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