Sink or Surf: Atmospheric Implications for Succinic ... - ACS Publications

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Sink or Surf: Atmospheric Implications for Succinic Acid at Aqueous Surfaces Patrick G. Blower, Stephanie T. Ota, Nicholas A. Valley, Suzannah R. Wood, and Geraldine L. Richmond* Department of Chemistry, University of Oregon, Eugene, Oregon 97403, United States S Supporting Information *

ABSTRACT: Small organic compounds are increasingly being invoked as important players in atmospheric processes that occur on aerosol surfaces. The diacid succinic acid is one such constituent that is prevalent in the troposphere, surface active, and also water-soluble. This article presents a thorough examination of the surface characteristics of succinic acid at the vapor/water interface using a combination of theoretical simulation and experiments using vibrational sum frequency spectroscopy and surface tension. The adsorption and orientation of succinic acid at the water surface is characterized for a series of aqueous solution compositions relevant to atmospheric conditions. Fully protonated succinic acid is found to be particularly surface active. A new computational technique is introduced that provides a detailed picture of the different surface species that are contributing to the experimentally derived spectroscopic measurements. Additional results are presented for how SO2, a copollutant of succinic acid in the atmosphere, behaves at a water surface in the presence of surface adsorbed succinic acid.

1. INTRODUCTION Aqueous aerosols are prevalent in the troposphere and form a platform for many of the reactions that govern atmospheric chemistry. They can serve as cloud condensation nuclei (CCN),1,2 which grow into aerosols that contain both an aqueous bulk phase as well as a large surface area for heterogeneous reactions. The smallest aerosols detected (30− 50 nm) are often mostly surface with very little bulk.3 Recent experimental evidence3−7 has shown that the surface of aerosols can partake in unique chemical reactions, which may be important reaction pathways for aerosols in the atmosphere. The composition of these particles can be quite complex, including many different inorganic and organic species. Organic acids, and more generally organic material, are key players in aerosol chemistry and physics, and their effects on aerosol growth and chemistry are commonly studied.8−17 However, the nature of their influence on aerosol surface chemistry is still not well understood.18−22 In fact, a recent perspective studying the transport of persistent organic compounds in the atmosphere indicates that interfacial properties, such as adsorption probabilities and reaction potentials, are the least understood of the factors determining the fate of such species in the environment.23 The effects of organics on aqueous surface chemistry are difficult to study because of the vast diversity of atmospherically relevant species and the correspondingly high number of relevant pathways to explore. For example, long chain surfactants are expected to behave quite differently from small, semisoluble organic acids, and the level of surface © 2013 American Chemical Society

coverage is likely to be important in determining whether, and how effectively, molecules act as a barrier to the absorption of gases.24 Organic compounds with a relatively high oxygen/ carbon ratio, such as dicarboxylic acids and short-chain aldehydes, are the most likely to persist in the atmosphere,25 and researchers continue to work toward a better understanding of how these compounds, and their derivatives, impact the environment.26−34 Measurements of aerosol components have pointed to a prevalence of dicarboxylic acids, including succinic acid, in the troposphere.25,35−40 The complexity afforded by organic functionality is further complicated when surface chemistry differs from that in solution. The role of organics on surface behavior is often difficult to extrapolate, as most measurements are unable to distinguish surface contributions from those of the bulk. Thus, many of the gaps in our understanding of aerosol surface chemistry are related to the behavior and influence of organic constituents on interfacial behavior. Succinic acid is the second-most highly detected dicarboxylic acid, with only oxalic acid being more prevalent in the environment. Succinic acid is known to be surface active,41−43 but few studies have investigated its surface behavior at a molecular level. Understanding how succinic acid surface orientation and surface density changes as a function of concentration and pH may have important environmental Received: May 22, 2013 Revised: July 12, 2013 Published: July 22, 2013 7887

dx.doi.org/10.1021/jp405067y | J. Phys. Chem. A 2013, 117, 7887−7903

The Journal of Physical Chemistry A

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the sum frequency response, N, and the orientationally averaged molecular susceptibility, β (eq 2).

implications. In addition to being produced from the photodegradation of larger organics in the atmosphere, succinic acid is a primary pollutant with both natural and industrial sources. The main anthropogenic sources of succinic acid are the combustion of wood, coal, and gasoline for heat and automobile fuel. Thus, it is a copollutant of SO2, which is derived from many of the same sources.35−37,44 SO2, a natural and industrial pollutant, is known to form surface complexes with water at aqueous surfaces; an interaction that has been investigated previously using vibrational sum frequency spectroscopy.45−47 SO2(g) uptake by an aqueous system (e.g., aerosols) is a function of many parameters including gas-phase diffusion, bulk solubility, mass accommodation probability, and bulk reaction rates. In addition, these factors are in turn affected by the aerosol composition, temperature, and pH. Recent studies in this laboratory have shown that acidification of the aqueous surface does not hinder the SO2:H2O surface complex, which forms at the very top of the air/water interface.47 However, the effect of SO2(g) uptake on an aqueous succinic acid surface is unknown and will be investigated in this work. The results demonstrate the unique nature of the air/water interface and atmospheric implications of these results are discussed. This work seeks to characterize the adsorption of succinic acid, a prevalent dicarboxylic acid, to aqueous surfaces as a function of concentration and pH. Also investigated is the role of surface adsorbed succinic acid on the uptake of SO2(g). The results provide detailed insights into how succinic acid behaves as it “surfs” the surface, how it impacts the water surface, and the conditions that cause it to sink into the bulk solution and away from the surface. The studies are conducted using a combination of vibrational sum frequency spectroscopy (VSFS), computational modeling, and surface tension. In addition, an effective method for computing VSF spectra for succinic acid is detailed, and results are compared to experiments with good agreement. The powerful combination of experiments and computational methods allows a far deeper understanding of the surface orientation and solvation environment of succinic acid than can be obtained by VSFS alone.

χ (2) =

∑ χR,(2)i i

(2)

Because resonant modes can overlap with one another as well as the nonresonant background, it is necessary to employ spectral fitting to deconvolve individual peaks. Therefore, a fitting procedure51 is employed that accounts for homogeneous and inhomogeneous linewidths of vibrationally active modes (eq 3). χ

(2)

=

(2) χNR

e

iψ

+

∑∫ ν

∞

−∞

2

A ν eiϕν e−[ωL − ων / Γν] dω L ωL − ωIR + i ΓL

(3)

The first term is the nonresonant susceptibility (which contains an amplitude and phase). The second term, the resonant susceptibility, is the sum over all SF active vibrationally resonant modes. The resonant susceptibility is a convolution of the homogeneous (Lorentzian) line widths of the individual molecular transitions (ΓL) with inhomogeneous broadening (Γν). For a vibrational mode to be sum frequency active, both a change in hyperpolarizability (Raman) and a change in net dipole (IR) must occur. This is modeled as the transition strength Aν and is proportional to the orientationally averaged IR and Raman transition probabilities. The frequencies of the Lorentzian, resonant modes, and IR are ωL, ων, and ωIR, respectively. Each resonant mode also has a phase value, ϕν. By selecting specific incoming and outgoing polarizations of the beams, information about the net molecular orientation of the probed local modes can be ascertained. This is due to the fact that of the 27 elements of χ(2) only four are simultaneously nonzero and unique (χzzz, χxxz, χxzx, and χzxx). These unique elements can be probed using incoming polarized visible and IR light and outgoing polarized sum frequency light. The polarization schemes are denoted as S (perpendicular to the plane of incidence) or P (parallel to the plane of incidence) and are given in the order of sum frequency, visible, and IR. Selecting the incoming polarizations to be S polarized (visible) and P polarized (IR), while the output is S polarized (SSP), the VSF response allows one to probe dipole components that are perpendicular to the plane of the interface, whereas the SPS polarization combination will probe dipole components that are parallel to the plane of the interface. Both SSP and SPS configurations were used in this study. Two different laser systems were used in these VSFS studies. The first (BBSFG or Broad Band Sum Frequency Generation) is used to collect data in the region between 1600 and 1900 cm−1, probing the carboxylic acid CO vibrational stretching modes. This system is upgraded from a version that has been described elsewhere.52,53 Briefly, a CW Nd:YVO4 laser (Millennia 5sJ, Spectra Physics) is used to pump a Ti:sapphire oscillator (Tsunami, Spectra Physics), which is tuned to produce ∼100 fs pulses centered at 800 nm. These pulses are then amplified using a regenerative amplifier (Spitfire Pro XP, Spectra Physics) to produce nominally 3 W of 800 nm light with a bandwidth of ∼12 nm. The output beam is split with ∼1 W going to a home-built slicer that temporally broadens the pulse to ∼2 ps and ∼1 W going to an optical parametric amplifier (OPA-800C, Spectra Physics) for DFG mixing and subsequent IR generation. Both the IR and visible pulses are then propagated to the air/water interface in a copropagating geometry at 45° and 60°, respectively, from the surface normal,

2. EXPERIMENTAL SECTION 2.1. Surface Spectroscopy. The surface specificity of VSFS makes it an excellent tool for exploring aqueous interfaces since under the dipole approximation the nonlinear processes it probes are forbidden in centrosymmetric media such as bulk water. It has grown in the past decade to be a highly versatile method for studying a variety of processes at water surfaces.48−50 A brief description is given below as it pertains to the experiments conducted in this study. The VSFS experiments conducted in this report involve an 800 nm beam of light overlapped in time and space with a variable frequency IR beam at the surface of the aqueous solution. The intensity of the resulting sum frequency signal is proportional to the square of the second-order susceptibility, χ(2), which has both a resonant and nonresonant component (eq 1). (2) χ (2) = χNR +

N β ε0

(1)

The resonant susceptibility, χ(2) R , and nonresonant susceptibility are proportional to the number of molecules contributing to 7888



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over multiple days to ensure reproducibility and reduce the signal-to-noise ratio. The phase of the resonant component is fixed to be π out of phase with the nonresonant background. The Lorentzian widths are fixed at 2 cm−1. Global fitting is performed on spectra in similar chemical environments (i.e., concentration or pH dependence) for SSP and SPS, respectively, with the above constrained parameters to provide more confidence in the fit. 2.4.2. CH and OH. Sum frequency intensities were measured using a thermoelectrically cooled CCD camera with a 2 s exposure time. Intensities were recorded in 3 cm−1 steps over a range from 2800 to 3900 cm−1. Sum frequency data are normalized to account for spatial variation between the visible and IR while scanning the IR frequency, temporal lengthening of IR pulses by water vapor, absorption of IR energy by SO2 and/or water vapor, and the frequency dependence of the optics used for filtering the SF light. In these experiments, SF spectra were normalized by the nonresonant response from an uncoated gold surface. Spectra presented are averages of 3 to 12 spectra taken over multiple days to ensure reproducibility and to reduce the signal-to-noise ratio. The parameters used to fit the neat vapor/water interface in SSP polarization were determined in previous isotopic dilution studies.59−61 Each resonant peak contains five variables (eq 3); thus, there may be nonunique fitting solutions. To reduce the number of variables associated with the fits, the phases are fixed at either π (for peaks between 3200−3600 cm−1) or 0 (for peaks below 3200 or above 3600 cm−1). In addition, Lorentzian widths are fixed at either 12 cm−1 (for the free OH) or 5 cm−1 for the remaining OH stretches. Global fitting routines are employed to constrain parameters where possible. Additional resonant peaks are added when they are both phenomenologically logical and necessary to achieve agreement between the data and the fits. 2.4.3. Fitting Parameters for the Neat Vapor/Water Interface. There are many discussions about the sum frequency response of the vapor/water interface62−68 in the literature, but a brief overview is presented here to describe the basic parameters upon which the subsequent analysis will develop. Water in the interfacial region occupies a wide range of molecular environments depending on orientation, hydrogen bonding and coordination to other molecules, and solvation of species such as ions. The sum frequency response from the OH stretching region is correspondingly broad, resulting in some ambiguity regarding the interpretation of VSFS data. However, valuable information regarding interfacial behavior can be inferred from examining spectral changes that occur when the aqueous surface is perturbed. To this end, a number of parameters have been determined to describe the spectroscopic response from the vapor/water interface. Support for this description of the VSFS data comes from previous isotopic dilution studies, as well as from MD simulations. The fitting parameters that will be applied here are as follows: 1. The sharp peak at ∼3700 cm−1 (the free OH) is attributed to unbound OH oscillators with an average orientation away from the bulk. This mode is highly sensitive to weakly bound species at the surface. 2. The mode opposite the free OH mode (the companion OH) points into the bulk and gives rise to broad spectral intensity at ∼3460 cm−1, a frequency consistent with measurements of the OH of uncoupled HOD in liquid water.69 MD calculations support this conclusion and go further to indicate that such highly oriented water molecules interact weakly with neighboring

where they are overlapped in time and space to produce sum frequency pulses. The resulting sum frequency pulses are filtered by an edge filter, collected by a lens and focused into a spectrograph that disperses the signal onto a liquid nitrogen cooled charge-coupled device (CCD). Samples were placed in a specially designed cell constructed of KEL-f fitted with CaF2 (entrance port) and BK7 (exit port) windows. This cell minimizes contamination and evaporation during data acquisition and was thoroughly cleaned before use. The laser system used to probe the region between 2700 and 4000 cm−1 has been described extensively in previous publications.54−56 Sum frequency light is generated by overlapping ∼10−100 mJ of 800 nm (2.6 ps, 1 kHz repetition rate) and 3−20 mJ tunable (2700−4000 cm−1) infrared light in a copropagating geometry at 56° and 67° from the surface normal, respectively. After filtering any reflected 800 nm light, the resultant sum frequency light is collected with a thermoelectrically cooled CCD camera (Princeton Instruments). After filtering any reflected 800 nm light, the resultant sum frequency light is collected with a thermoelectrically cooled CCD camera (Princeton Instruments) in 3 cm−1 increments over a range from 2700 to 3900 cm−1. Gas flow experiments were conducted at atmospheric pressure with a constant SO2 flow rate of ∼10 standard cubic centimeters per minute (sccm). To minimize contamination, samples are poured into scrupulously clean glass dishes contained in a nitrogen purged Kel-f cell fitted with CaF2 windows. The Kel-f cell has three gas ports, two of which are used for gases, and the remaining port is vented via Teflon tubing to a fume hood. There is an additional port to accommodate a clean Teflon coated Type T thermocouple probe to monitor sample temperature. Data collection is facilitated using a Lab View program that records CCD intensity and monitors temperature for each data point. The data presented here were taken at 23 °C. 2.2. Surface Tension. Surface tension measurements were performed using the Wilhelmy plate method.57 A force balance (KSV Instruments) was used to measure the surface tension. The solutions were placed in a clean glass dish, and great care was taken to ensure that the plate was oriented correctly (such that the bottom perimeter of the plate is normal to the surface) to the interface. The samples were allowed to equilibrate before the measurement was taken. The Pt plate was cleaned by being flamed until glowing orange and rinsed repeatedly in >18 MΩ·cm water between measurements. 2.3. Samples. Succinic acid (SigmaUltra, ≥99%) was purchased from Sigma-Aldrich. Deuterated succinic acid ((COOH)2C2D4) was purchased from ISP. NaOH was purchased from Mallinckrodt Chemicals (AR). All solutions were prepared fresh with >18 MΩ·cm water and used within 72 h. All glassware was cleaned with concentrated H2SO4 and NOCHROMIX and was thoroughly rinsed with >18MΩ·cm water. pH adjusted solutions were calculated with a pH calculator58 and then checked with litmus paper. Gases were purchased from AirGas, nitrogen (cylinder); and Air Liquide, SO2 (lecture bottle, 99.99%). 2.4. Analysis. 2.4.1. Carbonyl (CO) Vibrational Stretch. Sum frequency intensities were measured using a liquid nitrogen cooled CCD camera with a 20 min exposure time. All spectra are normalized to the nonresonant response of an uncoated gold surface and are calibrated using a polystyrene standard and absorption lines from ambient water vapor. Spectra presented here are averages of 3 to 9 samples taken 7889



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program package.80 Full geometry optimization and frequency calculations for isolated succinic acid molecules were performed using the B3LYP exchange-correlation functional and a 6-311+ +G(2d,2p) basis set. Polarizabilities and dipole moments at displaced geometries are calculated using the same level of theory. Vibrational sum-frequency intensities are calculated by inspecting the second order linear susceptibility tensor. The tensor was constructed using polarizability and dipole moment derivatives with respect to the vibrational normal coordinates combined according to

molecules via hydrogen bonds through both the hydrogen and oxygen.70−72 Small contributions to this peak can also come from loosely coupled water molecules in the more coordinated region of the interface. 3. Loosely bound water molecules that are nearly parallel to the interface are observed at 3580 cm−1 and primarily contribute to data taken in the sps-polarization scheme.70,71 4. More coordinated water molecules, sometimes referred to as tetrahedrally bound water, reside deeper in the interfacial region and give rise to two modes at ∼3330 and ∼3200 cm−1. While the molecular origins of the intensity at 3200 cm−1 are not well understood, the consensus from isotopic dilution studies of the OD73,74 and OH59,61,75 stretching regions and recent MD simulations71 is that the intensity of this mode increases with stronger hydrogen bonding and increased intermolecular coupling. It should be noted that there is an apparent discrepancy between these phase assignments and the phase assignments determined by recent phase-sensitive experiments, which detect a phase shift below 3200 cm−1, which is not accounted for in this description. However, it should be noted that the signal amplitude in this frequency region is very low; thus, the use of an additional peak would be inconsequential for the overall interpretation. 2.5. Computational Methods. 2.5.1. Molecular Dynamics Methods. Classical molecular dynamics (MD) calculations were performed using the Amber 12 suite of programs.76 Starting configurations were created using the PACKMOL77 program with 900 water molecules and 2, 4, or 8 succinic acid molecules in a 30 Å cube. These configurations correspond to concentrations of approximately 0.12, 0.25, and 0.5 M, respectively. A water slab with two surfaces was created by expansion of one of the box dimensions to 120 Å and applying periodic boundary conditions. Energy minimization of the initial system was performed using a combination of steepest descent and conjugate gradient methods. Minimized structures were equilibrated by evolution through 2 ns of MD simulation. Each system was further evolved for 50 ns, with atomic coordinates recorded every 100 fs for a total of 500 000 data points. The simulations were performed using a time step of 1 fs. Fully polarizable models were used for both the water and succinic acid molecules. Water was simulated using the POL3 model,78 and the acid molecules were constructed using a fully atomistic model based on the Amber FF02EP force field.79 The parameters used in the force field are available in the Supporting Information. The system temperature was set at 298 K, and Langevin dynamics were used to propagate dynamics via a leapfrog integrator. The particle mesh Ewald technique was used for calculating long-range electrostatic interactions with a force cutoff set to 10 Å. Waters were held rigid by means of the SHAKE algorithm to increase computational throughput and speed of data collection. Distances from the water surface are calculated using the Gibbs dividing surface, which is determined from a hyperbolic tangent fit to the water density profile. To correct for possible drift of the water surface over the length of the simulation, the coordinates of each data point are shifted so that the center of mass of the water system remains constant. Data is collected using both water/vacuum interfaces, and angles relative to the surface are measured from the surface normal pointing into the vacuum phase. 2.5.2. Quantum Mechanical Methods. Calculations presented in this work have been performed using the NWChem

χijk(2) ∝

∑ Cabc a,b,c

∂αab ∂μc ∂Q q ∂Q q

(4)

where α is the molecular polarizability, μ is the dipole moment, ∂Qq is the displacement of normal mode q, and C is a geometrical factor relating the molecular and laboratory reference frames. Derivatives are calculated using three-point finite-differentiation. No attempt is made to incorporate phase in the calculation of VSF spectra. To avoid improper cancellation, only the squared real component is considered. Calculated vibrational frequencies have been shifted by a factor of 0.96.

3. RESULTS AND DISCUSSION The adsorption of succinic acid to the air/water interface is examined by probing two different spectroscopic regions. The first region probes the CO stretch mode of the carboxylic acid moieties on succinic acid in the range from 1600 to 1900 cm−1. The second region probes the CH2 and OH local modes of succinic acid as well as the OH modes of water in the range from 2700 to 3800 cm−1. Isotopic substitution experiments performed in this region allow for complete determination of the CH2 and OH modes on succinic acid. Molecular dynamics as well as DFT calculations are presented to strengthen and confirm spectral interpretations. 3.1. CO Region. VSFS (BBSFG) spectra of aqueous succinic acid were measured in the CO stretch region using the SSP polarization scheme and are shown in Figure 1a along with the fitted curves (solid lines). The intensity of the signal from the carbonyl (CO) modes rises monotonically as the concentration increases from 0.1 to 0.5 M. Because of the limited solubility of succinic acid in water (∼0.7 M at room temperature),81 the highest concentration studied here is 0.5 M. In VSFS experiments, intensity arises from the number density at the interface as well as a net orientation of molecules probed. The polarization combinations selectively probe the direction of the net orientation. On the basis of the presence of signal, it is clear that succinic acid is adsorbing to the interface. Furthermore, the presence of signal under the SSP polarization indicates that one or both of the CO carboxylic modes have a net orientation of its dipole normal to the interface. According to the global spectral fits, there is one peak centrally located at 1722 ± 2 cm−1 with a Gaussian width of 30 ± 2 cm−1. There is no observed shift in the frequency of the VSFS signal when bulk concentration is increased. The carboxylic acid CO mode was also probed in the SPS polarization scheme to measure changes in the molecular dipole moment with components parallel to the plane of the interface. Two different concentrations (0.5 and 0.25 M) with their corresponding fitted curves are shown in Figure 1b. The signal intensity for these modes was less than that probed in SSP. It is 7890



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of saturated succinic solutions show the CO mode is located at 1717 cm−1 as well.89 The VSFS results presented here yield slightly higher frequencies at 1722 cm−1. This is not surprising due to the weaker solvation that is typical for an air/water interface. However, the difference in frequencies is only ∼5 cm−1 indicating that the solvation of the CO carboxylic mode is only slightly less well solvated than that of a bulk solvated succinic acid molecule. Surface IR studies of long-chain fatty acid monolayers at an air/water interface90−92 have shown that specific ranges of C O frequencies correspond to different hydrogen bonding environments. The region from ∼1735−1739, ∼1715−1720, and 1700−1704 cm−1 are assigned as having non-H-bonded, singly H-bonded, and doubly H-bonded character, respectively. Although long-chain fatty acids and short-chain dicarboxylic acids have different solvation properties, the trend with respect to hydrogen bonding interactions will be similar for both. VSFS experiments probing other carboxylic acid containing molecules show a range of frequencies as well. The CO vibrational stretching mode for the monocarboxylic acids, acetic82 and hexanoic52 acid, has a central frequency of 1720 cm−1. The C O stretching modes for malonic acid, the aliphatic C3 dicarboxylic acid, have frequencies in the 1730−1740 cm−1 range, indicating that it is weakly solvated at the air/water interface.83 So while the CO stretch vibrations for succinic acid have frequencies at ∼1722 cm−1, indicating weak solvation at the air/water interface, the CO mode for succinic acid is more solvated than its slightly smaller counterpart, malonic acid. 3.2. CH/OH Region. Probing the spectral region between 2700 to 3900 cm−1 provides information about the water OH stretching modes, as well as the succinic acid CH and OH stretching modes. Figure 2 shows representative spectra of 0.25 M succinic acid solutions (green) compared to spectra of the neat vapor/water interface (black) in both the SSP (top) and SPS (bottom) polarization schemes. At the native pH, 2.4, succinic acid is expected to be fully protonated. Visual inspection of these spectra show that succinic acid does perturb the interfacial water environment, with the most obvious spectral changes being a decrease in the intensity of the more coordinated water OH stretching region and a sharp feature in the CH-stretching region, but that the water surface is not fully disrupted (as indicated by the strong free OH signal). While these results provide clear evidence that succinic acid is present in the interfacial region, fitting the spectra is nontrivial. The following discussion describes the use of isotopic substitution to further characterize the spectral response in this region. Deuterated analogues of succinic acid and water are chemically equivalent but vibrationally distinct from their fully hydrogenated counterparts. Substituting hydrogen atoms with deuterium results in lower energy vibrations and can be used to decrease interference between vibrational modes that are similar in energy. Spectra of 0.25 M C2H4(COOD)2 in D2O were used to isolate spectral contributions from the succinic acid CH2 groups by removing interference from the OH vibrational stretching region and is shown in Figure 3 (left). The intense peak at 2730 cm−1, which is present in spectra of D2O as well as that of succinic acid, is attributed to the free OD stretch and is analogous to the free OH stretch from H2O. The succinic acid spectrum also contains two other modes. A narrow peak at 2945 cm−1, which is assigned to the CH2 stretching mode, and a second, broader peak at ∼2695 cm−1,

Figure 1. VSFS Spectra of the CO region of aqueous succinic acid for (a) SSP and (b) SPS. Fits are shown in black.

important to note that for the VSFS experiments done in the CO stretch region, the angles used for the visible and infrared beams (and hence the corresponding Fresnel factors) are such that the intensities for the SSP and SPS polarization schemes are comparable. This may not always be the case for VSFS experiments. As stated earlier, the SPS scheme interrogates modes that are in the plane of the interface. The presence of signal under the SPS polarization scheme indicates that one or both of the CO carboxylic acid modes have a net orientation of its molecular dipole moment with components that are parallel to the plane of the surface. The spectra reveal a peak centrally located at 1722 ± 3 cm−1 with a Gaussian width of 30 ± 1 cm−1. The VSFS intensity increases as the bulk concentration of succinic acid increases, as was seen with the SSP signal. VSFS SPS spectra taken at a concentration of 0.1 M did not have strong enough intensity to permit analysis. Both the SSP and SPS polarization schemes show a peak located at 1722 cm−1 with comparable Gaussian widths. This indicates that the CO modes for the carboxylic group are experiencing similar solvation environments. It should be noted that the signals detected here are relatively weak so that the normally negligible nonresonant signal interferes with the resonant signal. This manifests itself spectrally by the signal not returning to zero on the blue side of the spectra. This has been seen before in previous VSFS experiments probing CO modes52,82,83 as well as NO253 modes at the air/water interface. It has been postulated84 that since the nonresonant signal is not spectrally flat over this spectral region, it may be due to the presence of libration overtones and the bending modes of water, similar to IR and Raman spectra of bulk water over the same region.85−87 Infrared studies of aqueous succinic acid have shown the C O vibrational stretch mode for the fully protonated form is located at 1717 cm−1 and the CO vibrational stretch mode for the singly protonated form at 1715 cm−1.88 Raman spectra 7891



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region does not change for the deuterated sample but is absent from spectra taken in D2O, indicating that it arises from vibrational stretching of the succinic acid OH groups. In the gas phase, carboxylic acid OH stretches are seen at 3500 cm−1,93 but hydrogen bonding interactions between the carboxyl OH and water or neighboring acid molecules are known to cause red-shifts in the OH-stretch frequencies observed for other aqueous dicarboxylic acids.94 Thus, the broad peak at 2940 cm−1 is assigned to the solvated carbonyl OH stretch. This assignment is consistent with measurements and calculations based on studies of crystalline succinic acid, where intermolecular hydrogen bonding is expected to decrease the energy of this vibration. The succinic acid spectral assignments were determined by fitting the spectra for the deuterated analogues and then applying the same fitting parameters to peaks for the fully hydrogenated compound. The resonant components used for fitting of the aqueous succinic acid SSP spectrum are shown in Figure 4. Succinic acid is clearly surface active and adopts a net orientation such that the CH2 groups give rise to a single broad peak, which is seen in both the SSP and SPS polarization schemes. The carboxylic acid OH stretch is assigned to a broad peak at 2940 cm−1. The peak at ∼3080 cm−1, which is assigned to highly coordinated and strongly hydrogen bound water, supports an interfacial picture where water forms hydrogen bonds to succinic acid, thereby increasing the level of water coordination deeper into the interface. Spectral contributions from the water OH stretching in the region below 3400 cm−1 are still relatively high for the succinic acid solutions; however, interference with the solvation peak at 3660 cm−1 results in an apparent decrease in the intensity of this region. This is supported by the SPS polarization spectra, which shows a large increase in the peak due to weakly coordinated water molecules lying nearly parallel to the surface when succinic acid is added to the water. These spectral results point to an interfacial picture where the topmost region is populated by weakly coordinated water molecules solvating hydronium ions, with larger succinic acid molecules hydrogen-bonding to water slightly deeper in the interfacial region. 3.3. Surface Tension of Succinic Acid and Comparison to VSFS Spectra. VSFS measurements reflect both the number density (or surface concentration) and the net

Figure 2. VSFS spectra of the CH/OH region for aqueous succinic acid for SSP (above) and SPS (below). The black trace is the pure water spectrum. Fits are shown as solid lines.

which is attributed to D2O solvating ions. This assignment is consistent with the analogous ion solvation peak at ∼3660 cm−1 identified in earlier studies.46,60,75 Similarly, the spectral response from the succinic acid OH groups was determined by using partially deuterated succinic acid of the form HOOC(CD2)2COOH in water to remove the CH stretch response from the spectra seen in Figure 3 (right). The spectra of the deuterated and protonated succinic acid are very similar; the main spectral change is the sharp feature at 2945 cm−1, which is attributed to CH vibrational stretching from the CH2 groups. The relatively broad peak in that same

Figure 3. VSFS spectra of succinic acid in D2O is shown in the left panel. VSFS spectra of aqueous succinic acid and aqueous partially deuterated succinic acid are shown in the right panel. The gray traces are of pure D2O (left) and pure H2O (right). 7892



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Figure 4. VSFS spectra of aqueous succinic acid (green) and water (black) are shown in the left panel. The resonant components used for fitting the spectra are shown in the right panel. Included as an inset are the resonant components for the water region for comparison.

in the surface pressure as well as both SSP and SPS CO VSFS signals. Because of the limited solubility of succinic acid in aqueous media (∼0.7 M),81 it is impossible to attain bulk concentrations in the molar range. In addition, solutions higher than 0.5 M were close enough to the solubility limit that issues with crystallization became apparent in samples prepared for both VSFS experiments as well as surface tension measurements. The plots show a similar trend with respect to increased bulk concentration, which indicates that the orientation of succinic acid does not undergo large changes to the orientation as a function of concentration. To confirm the invariance of orientation as a function of concentration, the Frumkin equation was used to determine the surface excess concentrations for each bulk concentration95

molecular orientation at a surface, while surface tension is a macroscopic measurement that reflects only the surface concentration of the adsorbed species. Combining these two techniques will allow for the decoupling of changes in SF intensity due to orientational changes from changes due to changes in surface concentration of succinic acid. Surface tension data was measured for concentrations of succinic acid at 0.1, 0.25, and 0.5 M and is in good agreement with previous experiments.41−43 The Gibbs adsorption equation is used to determine the surface area per molecule via the surface excess95 ⎛ 1 ⎞⎛ ∂π ⎞ ⎟⎜ Γi = −⎜ ⎟ ⎝ RT ⎠⎝ ∂ln ci ⎠

T

(5)

⎡ Γ⎤ π2 = −RT Γi ln⎢1 − 2 ⎥ Γi ⎦ ⎣

where Γi is the maximum (limiting) surface excess, π is the surface pressure in mN/m, and ci is the concentration. By plotting the surface pressure versus the natural log of the concentration, the maximum surface excess can be calculated. Using Γi, the area per molecule is calculated95 to be 191 Å2/ molecule. This comparatively large molecular area corroborates the VSFS data from the water region where the free OH signal did not disappear and confirms that succinic acid does not pack tightly at the air/water interface. In Figure 5 are the surface pressure values of succinic acid (left) plotted with the square root of VSFS fitted amplitudes (right) for SSP and SPS experiments in the CO region. The square root of the intensity is used since the number density is proportional to the sum frequency field.96 As the bulk concentration of succinic acid is increased, there is an increase

(6)

where π2 is the surface pressure in mN/m and Γ2 is the surface excess at that concentration. The fitted VSFS amplitudes of the CO stretch vibration are plotted against the surface excess of succinic acid for the SSP (red) and SPS (green) spectral data in Figure 6. The linear dependence of the data for both

Figure 6. Square root of VSFS fitted amplitude for SSP (red) and SPS (green) versus the surface coverage of aqueous succinic acid. Linear fits are shown for SSP (red) and SPS (green).

polarizations suggests that there are no large changes in orientation of succinic acid at the vapor/water interface as the bulk concentration is increased. Claims made about linear changes in orientation from 0.1 M (which there was insufficient signal to permit analysis for SPS) to 0.25 M are not possible

Figure 5. Surface pressure (blue) and VSFS amplitudes (red) are plotted as a function of concentration of aqueous succinic acid. 7893



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except through extrapolation. This will be examined in further detail in the computational section. The surface tension experiments performed as a function of concentration on succinic acid adsorption at the air/water interface confirm and enhance the results from VSFS experiments. Succinic acid is shown to be weakly solvated at the air/water interface with CO frequencies similar to monocarboxylic acids previously studied via VSFS. However, the area per molecule is quite large for such a small molecule indicating that it is isolated at the surface. The constant frequency and Gaussian width for the various concentrations support the assertion that the orientation at the interface does not change dramatically as a function of concentration for succinic acid. In addition, the presence of a clear free OH signal indicates that succinic acid does not pack tightly at the interface resulting in destruction of the free OH signal.97 3.4. Computational Results. 3.4.1. Orientational and Conformational Analysis. Confirmation of experimentally observed VSFS signal from succinic acid molecules through MD and DFT calculations requires an understanding of both molecular conformation and orientation relative to the water surface. A density profile was computed for the system (Figure S1 in Supporting Information). Analysis of the computed density profile for succinic acid in water shows an increase in concentration of acid molecules at the water/vacuum interface (located from ∼5 Å below the interface to directly at the interface), which after a small dip in concentration, levels off to a moderate concentration in the bulk. Only a small percentage of succinic acid molecules appear above the water surface. Stability of the calculations is confirmed by fitting the water density profile to a hyperbolic tangent function as can be seen in the Supporting Information (Figure S1). These results confirm that the simulations are behaving similarly to experimental observations. Molecular conformations were studied using both density functional theory and MD calculations. Geometries of stationary point structures from the DFT calculations were used as a set of structures to represent those found in the MD trajectory. This fitting was performed through examination of numerous torsions and dihedral angles in the molecules. A range was defined around the values of each angle obtained from the DFT calculations. Only conformations that substantially contribute (>2% of total) to the average state of the molecule are discussed here. Ignored structures account for approximately 3% of all structures in the molecular dynamics calculations. Figure 7 displays structures of six of the stationary points found for succinic acid in the DFT calculations. All geometries shown correspond to local minima on the gas-phase potential energy surface. The gas phase contributions (by percent) of each conformation are included, where a Boltzmann distribution is assumed, and degeneracy is considered. Structures A (denoting an anti conformation backbone) and G (denoting a gauche conformation backbone) are found to have the greatest contributions and account for nearly 75% of the total structures. Both geometries correspond to structures where the noncarbonyl C−O bonds in the carboxylic acid moiety are synperiplanar with the C2−C3 bond. The remaining structures all have at least one of these bonds eclipsed (denoted with an e) with an adjacent C−H bond. Structures A and Ae have the C−C bond in the carbon backbone in an anti conformation. The backbone is in a gauche conformation for the rest of the structures.

Figure 7. Conformations and populations of succinic acid (from stationary points of DFT calculations) based on MD of aqueous succinic acid.

After each conformation in the MD trajectory was matched with one of the DFT structures, the percent contribution at different depths relative to the surface in the MD calculations was analyzed. The results for each concentration are shown in Figure 8. In comparison with the gas phase quantum

Figure 8. Conformation and percent population of aqueous succinic acid as a function of depth (relative to the surface) for three different concentrations based on the center of mass from MD.

mechanics, structures with an anti carbon backbone have greater contributions at the lowest concentration especially for structure A. The higher concentrations have conformational distributions similar to what is seen in the gas phase DFT calculations. At 0.12 M, structure A is the most common conformation at all depths except in the topmost surface layer, followed by conformations G and Ae. At 2 Å below the water surface, structure A sees an increase to nearly 55% of all structures, up from 44% in the bulk. At this depth, when the molecule is in conformation A, both CO (and therefore the entire carbon backbone) tend to be in the plane of the surface. Conformation G is always the most common structure at the higher 7894



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Figure 9. Computed VSFS SSP of succinic acid for the CO region (left panel) and for the CH region (right panel) for three different concentrations. The composite spectrum is shown in black, with contributions from G (red), Ae (green), Ge1 (blue), and Ge2 (yellow).

interrogation of specific and individual contributions. On the basis of the population percentages plotted for the conformations (Figure 8), there are differences in the relative populations as a function of depth for a given concentration. However, these are small changes and are not indicative of large-scale reorientation. There is almost no difference from 0.25 to 0.5 M for a given conformation. While there is a change between the percentages of conformations in going from 0.1 to 0.25 M (or 0.5 M), the main difference is a prevalence of the A conformation over the G conformation. As addressed above, the A conformation should lead to small or negligible contributions for the SPS CO region due to oppositely oriented dipoles. The VSFS signal intensity for the SPS CO is at the detection limit, and the MD results helps to explain why: the orientation of the most prevalent conformation was unlikely to give strong VSFS intensity. These results confirm the experimental conclusions derived from VSFS spectra. The CO signal intensity for succinic acid was shown to be equivalent for SSP and SPS active modes. The peak frequency (∼1722 cm−1) showed a CO mode that was less solvated than bulk succinic acid (∼1717 cm−1). A weak but detectable CH2 signal was found in VSFS, and the structures that would contribute to this intensity were found to increase at the interface. Additionally, the depth specific percentages of conformations for the different concentrations confirm and elucidate the surface tension results. 3.4.2. Procedure for Calculating Spectra. Our current approach to computing VSFS spectra involves a direct weighted summation of the VSFS spectral intensities of static DFT structures. First, each classical MD structure is represented by one of the DFT optimized conformers, as in the previous analysis. The acid structures from the MD are then analyzed to determine which orientations, with respect to the water surface, are the most populous (approximately similar to the conformations in Figure 7). This single orientation is assumed

concentrations (0.25 and 0.5 M) generally representing nearly 45% of all structures. This increase occurs mainly at the expense of the A (27−32%) and Ae (∼12%) conformers. As the concentration of succinic acid is increased, the population percentage of the family of G structures increases, while the population percentage of the family of A structures decreases as these molecules approach the interface. The methylene backbone for the G family of structures is twisted such that there is a net dipole orientation with contributions to both SSP and SPS. This is unlike the A family, which would have a weak or negligible net dipole from the methylene backbone due to oppositely oriented dipoles. Similarly, Ae would contribute a net dipole for SSP and SPS for the CO, but A would again have the two CO dipoles oppositely oriented resulting in a weak or negligible net dipole. Meanwhile, the G family of structures contains CO orientations that would result in a net dipole oriented such that it would be active in SSP and SPS (with obvious dependence on the conformation). Looking at the orientation of each CO in the molecule relative to the surface, the family of G conformations generally have one CO that is in the plane of the surface and one that is pointing into the bulk (Figures S2 and S3, Supporting Information). These structures are shown to not only increase at the interface but also maintain an orientation that would give rise to VSFS signal for SSP and SPS in both the CO and CH vibrational stretch regions. In addition to comparing the MD results with VSFS experiments, further insight into surface tension measurements can also be gained. In the preceding section, evidence was given for a lack of orientational change at the interface as a function of concentration. This is based on linear changes from the VSFS experiments (number density) versus surface tension (number density). However, both of these techniques are measuring ensemble averages, while the MD allows for 7895



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Figure 10. Computed VSFS SPS of succinic acid for the CO region (left panel) and for the CH region (right panel) for three different concentrations. The composite spectrum is shown in black, with contributions from G (red), Ae (green), Ge1 (blue), and Ge2 (yellow).

SSP and SPS vibrational sum frequency spectra for the six main conformations presented here (A, G, Ae, Ge1, Ge2, and Gee) were calculated using density functional theory. For each conformation, the average percent contribution and orientation between 6 Å below and 1 Å above the water surface gleaned from the classical dynamics calculations is assumed. Composite spectra were made from the sum of the individual spectra at each concentration. SSP spectra are presented in Figure 9 and SPS spectra are presented in Figure 10. The composite spectra should be a reasonable representation of what one observes experimentally for a given polarization scheme. The structures with anti conformation backbones (A and Ae) both lie in the plane of the surface. Symmetry of the backbone leads to very weak signal in the CH region of the SSP spectrum. In contrast, the conformations with a gauche backbone have a strong peak corresponding to the symmetric CH2 stretching mode. The spectra of conformations A and G are weak in the carboxylic acid stretching region, as the CO in structures A and G point in opposite or nearly opposite directions. It can be seen, however, that the SSP contribution for G is not zero (Figure 9) because, although the two carbonyls point nearly opposite, there is still a net dipole normal to the interface. This is not the case for SPS (Figure 10) where the net dipoles that would be active in SPS are canceled due to symmetry. Nearly all the intensity in the CO region arises from structures with at least one CO bond eclipsed with a C−H bond (Ae, Ge1, Ge2, and Gee). With the exception of Ae, these structures become more popular at the interface (Figure 8). As noted above, since this treatment uses static structures with no solvating water molecules around them, there will be changes in the structure from solvation that can lower the high symmetry in the A and G structures and lead to stronger contributions from all conformers. This is currently being investigated in more depth (no pun intended).

for each DFT structure. Each DFT conformer is then rotated into the determined orientation. This sets up a geometry where the molecular and experimental frames coincide. A VSFS spectrum for each conformer is then calculated (as outlined in the computational details) and is weighted by its populations in the MD calculations. These spectra are summed to create the composite spectrum for a given polarization scheme. The procedure followed here is meant to provide a qualitative comparison with experiment and give insight to the vibrational modes and conformational species responsible for the VSFS spectrum. Further work on this method of computing VSFS spectra for succinic acid and other related acids will be addressed in a future publication. 3.4.3. Computed Spectra. The conformation and orientation analysis provided above confirm experimental spectral results; however, it is also possible to directly compute VSF spectra. While this procedure is difficult and computationally expensive due to the number of degrees of freedom in both the orientation and conformation of the molecule, it can provide further confirmation of experimental spectra. To compute these spectra, the conformation and orientation must be determined first. Once known, the static structures can then be interrogated for VSF spectra calculation. Additionally, these structures represent nonsolvated conformations and are therefore likely to adopt slightly different conformations as explicit solvation is introduced. These structures represent extremes wherein the true conformation is a mix or conglomeration of structures. For example, structures A and Ae represent an extreme of an eclipsed carboxylic and noneclipsed carboxylic. Solvation would weaken the intramolecular restrictions on conformation leading to an average solvated structure. The proceeding spectra represent a small portion of the available structures for calculation, and further work on computed VSFS spectra for this and other related acids will be addressed in a future publication. 7896



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3.5. Effects of pH on Succinic Acid Adsorption. 3.5.1. CO and CH VSFS. The previous sections detail the adsorption of succinic acid as a function of concentration. Since succinic acid is a diprotic acid, knowing the protonation state of the adsorbed species is necessary to fully characterize the adsorption characteristics. When the pH of these solutions is adjusted to higher pH values, the carboxylic acid will form a resonance stabilized carboxylate ion thus removing any response from VSFS experiments in the carbonyl region. There are two carboxylic acid moieties and therefore two different pKa values. The pKa values for succinic acid are 4.19 and 5.48.98 At a pH of 4, the fully protonated species represent about 60% with the remaining species being singly prononated. At a pH of 5, the fully protonated species only makes up about 12%, with the singly protonated species dominating the solution at around 72% and the fully dissociated species making up about 16%. At a pH of 6, there is virtually no doubly protonated species left (0.5%), with a third of the species being singly protonated and the remaining two-thirds completely dissociated. According to bulk IR studies,88 there are frequency differences between the fully and singly protonated species. By adjusting the bulk pH of the solutions and monitoring the adsorption of succinic acid through VSFS experiments and surface tension measurements, the adsorption characteristics can be fully understood. The bulk pH was adjusted using NaOH, while maintaining a constant succinic acid concentration (0.5 M), and VSFS data was taken in both SSP and SPS schemes. Figure 11a shows the spectral response of succinic acid in the CO region for SSP as a function of bulk adjusted pH as well as the respective fits. The spectral fits reveal a peak centrally located at 1724 ± 2 cm−1 with a Gaussian width of 35 ± 2 cm−1. There is little to

no change in intensity for pH of 3, and the intensity decreases monotonically as the pH increases. At a pH of 5.5, there is essentially no VSFS signal remaining in the CO region. Figure 11b shows the spectral response of succinic acid in the CO region for SPS as a function of bulk adjusted pH as well as the respective fits. The spectral fits reveal a peak centrally located at 1722 ± 2 cm−1 with a Gaussian width of 35 ± 2 cm−1. As was seen for SSP, the change in intensity for pH of 3 is negligible, and the intensity decreases monotonically as the pH increases. At a pH of 5.5, there is once again no VSFS signal remaining in the CO region. Attempts were made to probe the carboxylate region (∼1400 cm−1) for succinic acid solutions of a high pH (4 and above), but no VSFS signal was found. The resulting VSFS in the CO region therefore provides evidence that the surface-active species of succinic acid is the fully protonated moiety. This property, while surprising, stems from the higher energy required to solvate a charge at a surface versus the hydrophobic effect of the alkyl backbone. For typical surfactants, the hydrophobic alkyl chain can overcome the weak solvation and remain at the interface under charged conditions. Malonic acid83 displayed no discernible signal arising from CH stretch modes in the region between 2700 and 3000 cm−1. However, there is intensity from the CH stretching of methylenes in the alkane backbone of succinic acid. Therefore, it is possible to interrogate the pH dependence of succinic acid using the CH stretch region. The VSFS SSP response in D2O was measured between 2700 and 3200 cm−1 (CH region) and is shown in Figure 12. There are four traces overlaid with each

Figure 12. VSFS spectra of succinic acid (0.25 M) in D2O in the CH region. Pure D2O is shown in black.

other. The black trace shows the response to D2O only, and the red trace shows the response of succinic acid (0.25 M) in D2O. The pH was adjusted by NaOD to a pD of 4 and 6, with the green trace showing succinic acid adjusted to pD of 4 and the blue trace showing succinic acid adjusted to pD of 6. As the pD is adjusted from native to pD 4, there is a decrease in the CH signal as was seen with the CO experiments. When the pD was further adjusted to pD 6, the response from succinic acid disappears. These results provide further evidence that the surface-active species is the fully protonated succinic acid moiety. 3.5.2. Surface Tension of Bulk pH Adjusted Succinic Acid. The pH adjusted VSFS experiments provide convincing evidence that the surface-active species of succinic acid is the fully protonated moiety. This can be confirmed by employing surface tension measurements. The surface pressure as a function of pH is displayed as well as the percentage of protonated species in the bulk (based off pKa values) are shown

Figure 11. VSFS spectra of the CO region of aqueous succinic acid at different bulk adjusted pH values for (a) SSP and (b) SPS. Fits are shown in black. 7897



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pH dependence of succinic acid was explored, and we found that the surface-active species is the nondissociated species. 3.6. Aqueous Succinic Acid/Vapor Interface Exposure to SO2. With the surface adsorption of succinic acid fully characterized in the preceding sections, focus can now be moved to the effect that this organic acid has on the uptake of SO2(g). Shown in Figure 15 is aqueous succinic acid before, during, and after exposure to SO2(g). Two spectral changes occur when the aqueous succinic acid (0.25 M) surface is exposed to SO2(g): increased broadening and red-shifting of the free OH, and increased spectral intensity below 3400 cm−1. These changes are consistent with the changes observed upon adsorption of SO2 to the neat water surface, indicating that SO2 surface complexes are forming and that SO2 is being absorbed and reacting with the aqueous phase. As shown in Figure 16, the partially deuterated forms of succinic acid show a similar spectral response to exposure to SO2. Notably, the only spectral shift observed is in the free OH region; there is no change in the succinic acid peak parameters. Thus, it appears that SO2 interacts primarily with water at the surface. Fits to the data, shown in Figure 15 (right panels) show that the free OH peak is broadened and red-shifted from its initial position. In addition, there is an increase in the amplitude of the peaks at 3200 and 3080 cm−1, consistent with the production of HSO3− and H+ from the reaction of SO2 and water. Succinic acid appears to have very little effect on the adsorption to, and subsequent uptake of, SO2 by water. This observation is confirmed by spectra taken after the removal of SO2 from the system shown in Figure 15. The free OH region returns to its initial shape, but the intensity increases due to solvated SO2 species are maintained. This picture is confirmed by the reversible nature of the surface complex formation; the peak at 3650 cm−1 decreases in intensity upon removal of SO2 from the system. Moreover, the increased coordination induced by solvated SO2 species persists as the intensity for the lower energy modes remain higher than for the SO2-free succinic acid solutions.

in Figure 13. The decrease in surface pressure as the bulk pH is increased follows the percentage of fully protonated species

Figure 13. Surface pressure and percentage of fully protonated species plotted versus bulk pH values.

Figure 14. Surface pressure and square root of VSFS SSP and SPS fitted amplitudes versus bulk-adjusted pH of aqueous succinic acid.

closely. Shown in Figure 14 is the surface pressure of bulk pH adjusted succinic acid (left axis) as well as the square root of the fitted amplitudes for both SSP and SPS CO VSFS (right axis). The surface pressure decreases as the pH increases indicating that succinic acid is desorbing from the interface as the bulk pH of the solutions is adjusted. The square root of the fitted amplitudes from the VSFS CO experiments follow the decrease in surface pressure as the bulk pH is increased as well. By using VSFS in the CO and CH region as well as surface tension measurements, it is now clear that the surface-active species of succinic acid is the fully protonated species. By investigating the concentration and pH dependence of succinic acid through VSFS and surface pressure measurements, the adsorption of succinic acid is fully characterized. The VSFS concentration data reveals that both the SSP and SPS CO signals yield similar frequency responses indicating that the carboxylic CO experience the same solvation environment whether it is pointing normal to the interface or parallel to it. By coupling the surface tension data to the VSFS data, it has been shown that the orientation of succinic acid does not change greatly as a function of concentration. In addition, the adsorption of succinic acid disrupts the water network at the air/water interface and gives rise to a weak but detectable CH signal. The use of computational calculations confirm the experimental VSFS results and provide further illumination on molecular geometries responsible for VSFS signal. Finally, the

4. ATMOSPHERIC IMPLICATIONS Because of the very small volumes that aerosols can exist as in the atmosphere, the pH of aerosols is a difficult area of study.99 Despite these difficulties, measurements have been made on larger fog, cloud, and rain droplets.100 Depending upon the location, time of day, season, and other environmental factors, the pH can vary by multiple pH units.101,102 In general, the pH of large aerosols is in the range of 3−5,103,104 with smaller aerosols being more acidic. Since dicarboxylic acids such as succinic acid are preferentially separated in the aerosol phase due to their low vapor pressure105 and contain more than one acid group, the surface behavior as a function of pH is fundamentally important. The pH experiments in this work provide a complete description of the pH dependence of succinic acid adsorbed to an air/water interface and can be used as a model for aerosol surfaces. Most importantly, the surfaceactive form of succinic acid has been shown to be the fully protonated species. This means that surface reactions that may involve succinic acid will only take place under acidic conditions. SO2 uptake is known to result in acidification of aqueous aerosols. However, the uptake of SO2 remains difficult to determine due to the myriad of factors involved. On the basis of this study, succinic acid does accumulate at the aqueous surface, but it does little to change the adsorption of SO2 to this 7898



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Figure 15. VSFS spectra of the CH/OH region for aqueous succinic acid for SSP (top left), aqueous succinic acid with SO2 flowing (middle left), and aqueous succinic acid after SO2 exposure (bottom left). The resonant components for the fits are shown in the corresponding right panels.

water interface. The physical structure of aqueous succinic acid at the vapor boundary is that of an acid that orients with gauche conformations along the alkyl backbone as a result of the preferred solvation of both carboxylic acid head groups. This orientation is thus flat at the interface and stands in contrast to monocarboxylic acids (and other surfactants) where the alkyl chain will orient away from the bulk phase. This has obvious implications on surface reactions that may take place on an aerosol surface since the alkyl component is surfing along the surface. In addition, the surface activity of succinic acid is completely dependent upon the protonation state and will sink into the bulk solution if an acidic proton is removed. When considering traditional surfactants, pH dependence often concerns the packing between molecules but not an actual change in surface adsorption. As was seen with malonic acid,83 the complete loss of surface active succinic acid with changes to pH will play a dramatic role in aerosol dynamics and formation. In terms of Kohler theory, the decrease in the Kelvin effect may play a large role in the ability of these acids to become activated as cloud condensation nuclei. The uptake of gases on aerosol surfaces remains a fundamental question, which is crucial for understanding aerosol stability as well as chemical reactions that can take place at these surfaces. The results from this study indicate that the uptake of SO2 is not hindered by the presence of succinic acid at the water surface. Being isolated and weakly solvated at the surface, succinic acid does not act as a significant barrier to gas uptake and is unlikely to hinder water evaporation. The combined experimental and computational methods provide a

interface. Some questions that arise from this work are as follows: How do more reactive organic species impact surface adsorption of gaseous species, and how is surface adsorption tied to water surface coverage? As a sparingly soluble organic compound, succinic acid does adsorb to the water surface, but its surface coverage is low. It is possible that more surfactantlike species with tighter surface packing might have more of an effect on gas adsorption and uptake to aqueous surfaces. The results found for the aqueous succinic acid interface present two interesting consequences for atmospheric aerosols. First, the weak solvation of isolated succinic acid molecules result in only the fully protonated form being surface active. At low pH values for a given aerosol, succinic acid would be expected to be at the surface, while at neutral and high pH values succinic acid would not be expected to be surface active. Second, succinic acid alone does not disrupt the surface complexation that occurs for SO2(g) uptake under these conditions. The isolated, weakly solvated succinic acid molecules do not inhibit these reactions, and therefore, it would be expected that aerosols containing succinic acid could become acidified via SO2, leading to surface adsorption for more alkaline aerosols.

5. SUMMARY Succinic acid is widely found in various aqueous forms throughout our environment, and this study characterizes the surface adsorption via a combination of surface spectroscopy, thermodynamics, and computational modeling. The results from these experiments provide a complete picture of succinic acid showing that it is weakly solvated and isolated at the air/ 7899



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(3) Finlayson-Pitts, B. J. Reactions at Surfaces in the Atmosphere: Integration of Experiments and Theory as Necessary (But Not Necessarily Sufficient) for Predicting the Physical Chemistry of Aerosols. Phys. Chem. Chem. Phys. 2009, 11, 7760−7779. (4) Donaldson, D. J.; Valsaraj, K. T. Adsorption and Reaction of Trace Gas-Phase Organic Compounds on Atmoshperic Water Film Surfaces: A Critical Review. Environ. Sci. Technol. 2010, 44, 865−873. (5) Hayase, S.; Yabushita, A.; Kawasaki, M.; Enami, S.; Hoffman, M. R.; Colussi, A. J. Weak Acids Enhance Halogen Activation on Atmospheric Water’s Surfaces. J. Phys. Chem. A 2011, 115, 4935−4940. (6) Martins-Costa, M. T. C.; Anglada, J. M.; Francisco, J. S.; RuizLopez, M. F. Reactivity of Atmospherically Relevant Small Radicals at the Air−Water Interface. Angew. Chem., Int. Ed. 2012, 51, 5413−5417. (7) Monge, M. E.; Rosenorn, T.; Favez, O.; Muller, M.; Adler, G.; Riziq, A. A.; Rudich, Y.; Hermann, H.; George, C.; D’Anna, B. Alternative Pathway for Atmospheric Particles Growth. Proc. Natl. Acad. Sci. 2012, 109, 6840−6844. (8) Jacobson, M. C.; Hansson, H.-C.; Noone, K. J.; Charlson, R. J. Organic Atmospheric Aerosols: Review and State of the Science. Rev. Geophys. 2000, 38, 267−294. (9) Ming, Y.; Russell, L. M. Organic Aerosol Effects on Fog Droplet Spectra. J. Geophys. Res. 2004, 109, D10206/10201−D10206/10214. (10) Stephanou, E. G. The Decay of Organic Aerosols. Nature 2005, 434, 31. (11) Maria, S. F.; Russell, L. M.; Gilles, M. K.; Myneni, S. C. B. Organic Aerosol Growth Mechanisms and Their Climate-Forcing Implications. Science 2004, 306, 1921−1924. (12) Goss, K.-U. Predicting Adsorption of Organic Chemicals at the Air−Water Interface. J. Phys. Chem. A 2009, 113, 12256−12259. (13) Ellison, G. B.; Tuck, A. F.; Vaida, V. Atmospheric Processing of Organic Aerosols. J. Geophys. Res. 1999, 104, 11633−11641. (14) Hyvärinen, A.-P.; Raatikainen, T.; Laaksonen, A.; Viisanen, Y.; Lihavainen, H. Surface Tensions and Densities of H2SO4 + NH3 + Water Solutions. Geophys. Res. Lett. 2005, 32, L16806. (15) Eliason, T. L.; Gilman, J. B.; Vaida, V. Oxidation of Organic Films Relevant to Atmospheric Aerosols. Atmos. Environ. 2004, 38, 1367−1378. (16) Molina, M. J.; Ivanov, A. V.; Trakhtenberg, S.; Molina, L. T. Atmospheric Evolution of Organic Aerosol. Geophys. Res. Lett. 2004, 31, L22104/22101−L22104/22105. (17) O’Dowd, C. D.; Facchini, M. C.; Cavalli, F.; Ceburnis, D.; Decesari, S.; Fuzzi, S.; Yoon, J. J.; Putaud, J.-P. Biogenically Driven Organic Contribution to Marine Aerosol. Nature 2004, 431, 676−680. (18) Donaldson, D. J.; Vaida, V. The Influence of Organic Films at the Air−Aqueous Boundary on Atmospheric Processes. Chem. Rev. 2006, 106, 1445−1461. (19) Bluhm, H.; Siegmann, H. C. Surface Science with Aerosols. Surf. Sci. 2009, 603, 1969−1978. (20) Fuzzi, S.; Andreae, M. O.; Huebert, B. J.; Kulmala, M.; Bond, T. C.; Boy, M.; Doherty, S. J.; Guenther, A.; Kanakidou, M.; Kawamura, K.; Kerminen, V.-M.; Lohmann, U.; Russell, L. M.; Pöschl, U. Critical Assessment of the Current State of Scientific Knowledge, Terminology, and Research Needs Concerning the Role of Organic Aerosols in the Atmosphere, Climate, and Global Change. Atmos. Chem. Phys. Discuss. 2005, 5, 11729−11780. (21) Rudich, Y. Laboratory Perspectives on the Chemical Transformation of Organic Matter in Atmospheric Particles. Chem. Rev. 2003, 103, 5097−5124. (22) Rudich, Y.; Donahue, N. M.; Mentel, T. F. Aging of Organic Aerosol: Bridging the Gap Between Laboratory and Field Studies. Annu. Rev. Phys. Chem. 2007, 58, 321−352. (23) Valsaraj, K. T.; Thibodeaux, L. J. On the Physicochemical Aspects of the Global Fate and Long-Range Atmospheric Transport of Persistent Organic Pollutants. J. Phys. Chem. Lett. 2010, 1, 1694−1700. (24) Gilman, J. B.; Eliason, T. L.; Fast, A.; Vaida, V. Selectivity and Stability of Organic Films at the Air−Aqueous Interface. J. Colloid Interface Sci. 2004, 280, 234−243.

Figure 16. Partially deuterated succinic acid (CD4(COOH)2) before and during exposure (top) and before and after exposure to SO2(g) (bottom).

complete picture of the physical structure and adsorption of succinic acid and provide a framework with which to better understand atmospherically relevant, low molecular weight organic adsorption to the air/water interface.

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ASSOCIATED CONTENT

S Supporting Information *

Density profile and CO pointing angles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(G.L.R.) Tel: 541-346-4635. Fax: 541-346-5859. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support for this work from the National Science Foundation (CHE-1051215) and to Prof. Fred Moore of Whitman College for assistance with the manuscript.

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REFERENCES

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