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Carboxylate Ion Availability at the Air-Water Interface Shinichi Enami, Tomihide Fujii, Yosuke Sakamoto, Tetsuya Hama, and Yoshizumi Kajii J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08868 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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The Journal of Physical Chemistry

Carboxylate Ion Availability at the Air-Water Interface Shinichi Enami*a, Tomihide Fujiib, Yosuke Sakamotob,c, Tetsuya Hamad, and Yoshizumi Kajiia,b,c a

National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan

b

Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501, Japan

c

Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8316, Japan

d

Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan

*Author to whom correspondence should be addressed: S.E. [email protected], phone: +81-29-850-2770

ABSTRACT − Amphiphilic organic compounds at the air-water interface play key roles in the nucleation, growth and aging process of atmospheric aerosol. Surface-active species are expected to react preferentially with atmospheric oxidants, such as the OH-radical, at the air-water interface via specific mechanisms. Establishing the relative availability of the different amphiphilic species to gas-phase oxidants at the air-water interface under atmospherically relevant conditions is, however, challenging. Here we report the interfacial availability of atmospherically relevant carboxylate ions, Rn-COO(n = 1-7) and n-, cyclo-, aromatic-R6-COO- at the air-water interface via a novel application of mass spectrometry of aqueous microjets. The breakup mechanism of microjets lets us determine the relative interfacial affinities of carboxylate ions in equimolar solutions of the corresponding carboxylic acids in the 1 µM to 1 mM range under ambient conditions. We find that the interfacial affinity of Rn-COO- increases exponentially with both chain-length and solvent-accessible surface area (SASA), except in the case of R1-COO-. The relative interfacial affinities for n-heptanoate (n-R6-COO-) > cyclohexanecarboxylate (c-R6-COO-) > benzoate (Ar-R6-COO-) are also

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determined. We attribute the smallest availability of Ar-R6-COO- at the air-water interface among the three carboxylate ions to a strong π-H bonding between the aromatic ring and water molecule. Molecular mechanisms on the availability of carboxylate ions at the air-water interface and the atmospheric implications are discussed.

INTRODUCTION Some ions accumulate at a specific depth layer of water-hydrophobes (e.g., air, oil and biomembranes) interfaces, while other ions prefer to remain in bulk water and react homogeneously.1-11 Long-chain carboxylate ions, due to the amphiphilic property,12-14 play key roles in the nucleation, growth and photochemical aging process of atmospheric aerosol.15-20 For example, bubble-bursting from surface of the ocean is responsible for enriching marine aerosols with long-chain carboxylate ions,16-17, 21-22 thereby influencing the chemistry of marine boundary layers.17,

23-33

Long-chain

carboxylate ions act as a film covering atmospheric particles, directly confirmed by field measurement studies.34-38 Recent studies have suggested long-chain carboxylate ions could induce cloud droplet formation, thereby affecting global climate change.15, 39-41 Benzoate ion, C6H5-COO-, has been recently found to be one of the most abundant organic species observed in PM2.5 of polluted areas, where the concentration on PM2.5 exceeds 1 µg/m3.42 Previous surface tension measurements43-44

and core-level photoelectron

spectroscopy (PES)45 revealed that longer-chain alkylcarboxylate ions Rn-COO- (n ≥ 2) have increasingly larger propensities for the air-water interface, relative to shorter-chain formate (n = 0) and acetate (n = 1). A recent molecular dynamics 2 ACS Paragon Plus Environment

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simulation including polarizable models suggested that the anionic headgroups COO- of alkylcarboxylate ions are strongly oriented toward the aqueous core, while the hydrophobic alkyl chains are repelled into air.46 Furthermore, recent experiments on the heterogeneous OH-oxidation of aqueous mono-, di-carboxylate ions revealed that the extent of the oxidation for smaller acids (e.g., acetic and oxalic acids) are much smaller than longer-chain ones.18, 47 These results indicate surface-active species would be more reactive towards atmospheric oxidants, such as OH-radicals, at the air-water interface. Thus, a measure of carboxylate ions “availability” at the air-water interface seems critically important. Here we report the interfacial availability of atmospherically relevant carboxylate ions at the air-water interface by using a novel application of mass spectrometry of liquid water microjets.48-49 We base our analysis on the bag-breakup mechanism of primary droplets and determine relative interfacial affinities of carboxylate ions, Rn-COO- (n = 1 - 7) or n-, cyclo-, aromatic-R6-COO- from equimolar concentration mixtures of the corresponding carboxylic acids at sufficiently low concentrations, i.e., 1 µM to 1000 µM.

EXPERIMENTAL In our experiments we measure (in situ, via online electrospray mass spectrometry, ES-MS, at National Institute for Environmental Studies, NIES) relative anion populations in the water ‘films’ produced upon blowing up drops of Rn-COOH solutions by a high-speed nitrogen gas as a function of the concentration of equimolar mixture in the 1 – 1000 µM range.8 The experimental setup has been described in previous publications.6-7 Here we summarize the key mechanisms of droplet breakup that give 3 ACS Paragon Plus Environment


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mass spectral signals. Liquid solutions injected as microjets into the spraying chamber of the mass spectrometer are sheared into primary droplets by means of a co-directional high-speed nebulizer gas (N2) under ambient temperature and pressure. Fig. 1 shows a schematic mechanism of the initial droplet breakup.

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Figure 1 – Schematic illustration of a droplet breakup mechanism. In our experiments we sample the ions contained in the secondary droplets (d ≤ 1 µm) generated from the thin-film, which are detected in situ by online electrospray mass spectrometry. Coarser droplets (d > 1 µm) arising from the rims go down to sink and are not sampled. As the result, ions having the larger propensities for the air-water interface, such as larger n Rn-COO-, produce the more intense mass signals, i.e., [A]/[C] >> 1. See text for details.

These initial droplets are flattened by the moving N2 gas, and then stretched windward into rimmed thin-film bags (2 in Fig. 1).50 In this process, ions having smaller propensities (ion C in Fig. 1) accumulate in the rims whereas ions having larger


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propensities (ion A in Fig.1) preferentially distribute along the films. The rimmed bags are

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sub-micrometer-sized secondary droplets. It has been experimentally confirmed that in the case of neat water droplets the finer droplets originating from the film are negatively charged because they contain excess hydroxide anion (OH-), whereas the coarser ones arising from the rims carry net positive charge due to hydronium cations (H3O+).51-52 Since the kinetic energy of the gas can deform only initial drops of d ∼ 1 mm diameter,50 the breakup of the bag into charged secondary droplets is the one-time event in which net charges (those detected by mass spectrometry) are created from the inflowing solutions. Small (d ≤ 1 µm) secondary droplets rapidly shrink via solvent evaporation by drying gas, enhanced by their large surface/volume ratios, thereby crowding their excess charges. These secondary droplets eventually become Rayleigh-unstable and undergo a cascade of Coulomb explosions, ultimately resulting in the ejection of bare single ions to the gas-phase.53-54 By electrically biasing the inlet to the detection chamber of the mass spectrometer we can monitor the gas-phase ions. An important feature of our instrument is that the microjet issuing from the nozzle source is orthogonal to the polarized inlet to the mass spectrometer (see Figure S1). In the previous papers, we have confirmed that the modest polarizations of the initial droplets do not affect the observed phenomena.48, 55-57 For example, we confirmed that the kinetics of the reaction of dissolved α-tocopherol with gaseous ozone determined on the basis of negative and positive ion detection were identical.58 The fact that the titration curves of carboxylic acids and trimethylammonium obtained in the similar setup are identical with the ionization constants reported in the literature (i.e., pKa ∼ 4.8 and ∼ 9.8, respectively)48 indicates that solvent evaporation is negligible 5 ACS Paragon Plus Environment


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prior to droplet breakup. It has been demonstrated that the depth (or thickness) of the sampled interfacial layers can be controlled by varying the nebulizer gas velocity vg, as evidenced by the fact that both ion signal intensities and relative anion surface affinities increase with higher gas velocities vg and extrapolate to zero as vg → 0.6 In this context, our method is similar to sonic spray ionization mass spectrometry where mass signal intensity increases as a function of nebulizer gas flow rate.59-60 Online mass-based sampling from the surface of continually refreshed water droplets under ambient temperature and pressure makes our instrument a unique surface-sensitive technique. Further experimental details and validation tests could be found in elsewhere.6, 8, 48, 56, 61 Conditions in the present experiments were: drying N2 gas flow rate: 12 L min-1; drying N2 gas temperature: 340 oC; inlet voltage: + 3.5 kV relative to ground; fragmentor voltage value: 80 V. Mass spectra were recorded by selective ion mode (SIM) for accurate measurements. Acetic acid (> 99.5 %), propionic acid (> 98 %), butyric acid (> 98 %), pentanoic acid (> 95 %), hexanoic acid (> 99 %), heptanoic acid (> 98 %) and octanoic acid (> 97 %) were purchased from Wako Pure Chemical Industries, Ltd. and benzoic acid (≥ 99.5 %) and cyclohexanecarboxylic acid (> 98 %) were purchased from Sigma-Aldrich. All chemicals were used as received. All solutions were prepared in purified water (Resistivity ≥ 18.2 MΩ cm at 298 K) from a Millipore Milli-Q water purification system. For some experiments, samples were pH-adjusted by concentrated NaOH solution (1N, Wako Pure Chemical Industries, Ltd.) with a calibrated pH meter (Horiba, LAQUA F-74). Reported uncertainties were derived from 3 independent measurements. All experiments were performed at 298 + 3 K.


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Results and discussion

Fig. 2 shows negative ion mass spectra from aqueous A) 1 µM, B) 10 µM, and C) 100 µM equimolar Rn-COOH (n = 1-7) mixture solutions. 0.12

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The mass signals at m/z = 59, 73, 87, 101, 115, 129 and 143 are attributed to acetate (n = 1), propanoate (n = 2), butanoate (n = 3), pentanoate (n = 4), hexanoate (n = 5), heptanoate (n = 6) and octanoate (n = 7) ions, respectively. Clearly, the relative signal intensities of an equimolar mixture are not identical but increase as a function of n in these concentration ranges. Since all carboxylic acids have the very similar pKa ≈ 4.8 + 0.1,62 we ascribe the trend of relative signal intensities to the corresponding ion populations at the air-water interface of liquid films probed by the present method (see above).6-7 The observation that the interfacial affinity of larger Rn-COO- is larger than that of smaller one is consistent with the proposed droplet breakup mechanism (Fig. 1). We found that the trend of the observed interfacial affinity is unchanged by pH between 3.5 and 6.6 (Fig. S2). Figures 3A and 3B show the signal intensity of Rn-COO- as a function of the concentration of equimolar mixtures.


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Since R7-COO- signal intensity is ~80 and ~9 x 103 times larger than those of R1-COO- at 1 and 1000 µM equimolar concentration, respectively, the former is much more enriched at the surface, the divergence increasing with concentration.63 Thus, the larger carboxylate anions displace the smaller ones by outcompeting them from the interfacial layers.7 Figure 4 shows semi-log plots of χ(n), the ratios of mass spectral signal intensities I of individual Rn-COO- divided by the sum of Rn-COO- (n = 1-7) signal intensities, as a



function of the concentration of equimolar Rn-COOH solutions.

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Figure 4 – (A) Semi-log plots of χ(n), the ratios of mass spectral signal intensities of individual Rn-COO- divided by the sum of Rn-COO- (n = 1-7) signal intensities, as a function of concentration of equimolar Rn-COOH solutions. (B) Zooming-in plots at 0 < χ(n) < 0.3. (C) Zooming-in plots at 0 < χ(n) < 0.006.


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Rn-COO- (n ≤ 3) signals begin to fall off at 1 µM, while Rn-COO- (4 ≤ n ≤ 6) do so at larger concentration (Fig. 4B, C). R6-COO- signals shows a rise-and-fall (Fig. 4B) and R7-COOkeeps increasing as a function of concentration (Fig. 4A). Note that in none of the experiments the surface becomes saturated with carboxylates (e.g., saturation reached at ∼ 200 mM, ∼ 50 mM and ∼ 5 mM for acetic (n = 1), hexanoic (n = 5) and octanoic acids (n = 7), respectively).24, 62 Figure 5 shows the plots of ln χ(n) as a function of the distance between C-atom of COO- and the ω-C-atom of the terminal–CH3 group in different equimolar concentration mixture.

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Apparently, the interfacial affinity of Rn-COO- increases exponentially with the size (except for n = 1), consistent with previous measurements for those of inorganic electrolytes by Cheng et al. 3, 50 According to Cheng et al., the normalized anion fractionation factor would depend on the free energy associated with the segregation of anions from the bulk solution to the interface, ∆GB→IF.3, 63 The observed increase of χ(n) as a function of n is clearly controlled by increasing hydrophobicity of alkyl chain. Ottosson et al. found, by using PES, that R0-COO-/R1-COO- are not surface-enriched while R3-COO- coexists with the molecular form R3-COOH at the surface (within a few nm) of liquid microjet.45 Our results are consistent with Ottosson et al.45 If we assume that the droplet size is large enough and hence we could approximate the bulk concentration does not change by segregation to the interface, the slope in figure 5 could represent the dependence of free energies on the number of carbon, d∆GB→IF/dn.

d ln χ (n) d ln K B→IF (n) 1 d ∆GB→IF (n) ≈ =− dn dn RT dn

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Where R is molar gas constant, T is temperature and KB→IF(n) is equilibrium constant between the bulk and interface of Rn-COO-. The d∆GB→IF(n)/dn values may represent two factors; one is a stabilizing effect by CH2 group at the interface, and the other is a competitive displacing effect as shown in Figs. 3 and 4. Since 1 µM is low enough to assume the displacing effect is negligible, then the stabilization energy per CH2 group at the interface could be estimated to be ∼0.2 kcal mol-1 by the slope of Fig. 5A. This is consistent with the MD simulation by Houriez et al. who reported that the potentials 12 ACS Paragon Plus Environment

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of mean force, PMF, was reduced by about 0.3 kcal mol-1 at the minimum close to the air/droplet interface (with 1000 water molecules) and by about 0.5 kcal mol-1 on the air/liquid water interface per CH2 group, respectively.46 This trend would result from both enthalpic and entropic effects. Although enthalpic stabilization by ion/water dispersion is reduced at the air-water interface by 3 kcal mol-1 per CH2 group as a function of chain length (n from 0 to 5), the penalty is compensated by positive entropy effects over 3.3 cal mol-1 K-1 per CH2 group.46 They concluded that both the enthalpy and entropy effects are consistent with the loss of the structure-making effects of the alkyl chain by removal from solution.46 They suggested that the surface active trend of carboxylate is mainly dominated by ion carbon chain/water hydrophobic effects and thus the trend is in line with the solvation trends of neutral hydrocarbons, i.e. the ionic head, -COO-, and the alkyl substituent could be considered as solvated independently.46 Next, we analyze the data by considering a solvent accessible surface area (SASA) of the alkyl substituent of carboxylate ion. SASA is a commonly used scale factor for the bulk hydration properties of non-polar organic compounds.64-66 Here we adopt the SASA values of linear alkanes (from methane (n = 1) to heptane (n = 7)) for those of Rn-COO- (n = 1-7).67-68 Fig. 6 shows the ln χ(n) plots as a function of SASA (Å2) in different equimolar concentration mixture.



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Clearly, the plots for n ≥ 2 follow the linear regression line, supporting the suggestion by Houriez et al.46 It is notable, however, R1-COO- does not follow the linear regression line neither as functions of rn nor SASA (Figs. 5 and 6). The similar exceptional behavior was reported in the case of CF3-COO- among F(CF2)n-COO- (n = 1-8).69 From the slopes in Fig. 5, the difference in ln χ(1) between the measured and expected value derived from regression line is about 2.0 ~ 2.5, indicating that there may exist an additional factor that stabilizes R1-COO- more in the bulk. The difference may correspond to ∆GB→IF ~ -1 kcal mol-1. The observed negligibly small χ(1) < 0.005 (Fig. 4C) and the exceptional behaviors of R1-COO- (Figs. 5 and 6) among Rn-COO- (n = 1-7) is in 14 ACS Paragon Plus Environment

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agreement with surface tension experiments43-44 and the result of core-level photoelectron spectroscopy (PES) that showed both R0-COO- (formate ion) and R1-COO- are not surface-enriched at all.45 The MD simulation also predicted that the positive PMF on the air/droplet and air/liquid water interface for both R0-COO- and R1-COO-.46 It is noted that R1-COO-, acetate ion, is known as a Hofmeister ion where the interfacial affinity follows the order: R1-COO- < Br- < NO3- < I- < ClO4-.70 Previously, we confirmed R1-COO- has a much smaller interfacial propensity than Br- and I-.7 It is conceivable that the size of R1-COO- is too small to disrupt hydrogen-bonding network of water. In fact, the MD simulation by Chandler revealed a small solute such as methane molecule in bulk water, that only excludes the center of water molecules from a spherical volume less than 0.5 nm across, cannot disorder water’s hydrogen-bonding-network where each water molecule participates in four hydrogen bonds.71 Thus, fully miscible CH3-COO- is not surface-active and may not be preferentially oxidized by OH-radical coming from gas-phase at the air-water interface.18 Indeed, previous experiments on the OH-radical + carboxylate ions reaction at the air-water interface showed the extent of the oxidation, [Rn-COO-]/[Rn-COO-]0, for n = 1 is ~0.93 meaning only ~7 % of R1-COO- is oxidized, while for n = 7 is ~ 0.83, i.e., ~17 % of n = 7 is consumed under the identical condition, e.g., OH dose.18 We note, however, that neutral small organic species such as methanol and acetone may behave differently.72 Fig. 7 shows plots of the slope for n = 2-7 from Fig. 5 as a function of equimolar mixture concentration.



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Figure 7 – Plots of dlnχ(n)/dn as a function of concentration of equimolar mixture. The solid line is an exponential fitting curve.

The dlnχ(n)/dn value increases as a function of equimolar concentration and goes into a plateau at ~ 500 µM. This result implies that d∆GB→IF(n)/dn, that may represent the competitive displacing effect, reaches to 0.4 k cal mol-1 over this concentration. Finally, we investigate how structural difference influences on the interfacial availability. Figures 8A and 8B show the signal intensity of n-heptanoate (n-R6-COO-), cyclohexanecarboxylate (C6H11-COO-, c-R6-COO-) and benzoate (C6H5-COO-, Ar-R6-COO-) as a function of concentration of equimolar mixture of these three R6-COOH acids: n-heptanoic (pKa = 4.9), cyclohexane carboxylic (pKa = 4.9) and benzoic (pka = 4.2) acids solutions at pH 6.3 + 0.2 adjusted by NaOH.


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1.0

A

Benzoate c-Hexane carboxylate Heptanoate

0.8

0.6

0.4

0.2

5


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0.0

1

1

10

100

1000

1

10

100

1000

B

0.1

0.01

[mixture] / µM Figure 8 – (A) Semi-log and (B) log plots of mass spectral signal intensities of benzoate at m/z = 121, cyclo-hexane carboxylate at m/z = 127 and heptanoate at m/z = 129 as a function of concentration of equimolar benzoic, cyclohexane carboxylic and heptanoic acids solutions at pH 6.3 + 0.2. Error bars contained within symbols size.

Since the benzoic acid’s pKa 4.2 is considerably smaller than others, pH was adjusted to 6.3 + 0.2 by NaOH to see the effect of R-COO- alone. We obtained the clear tendency of the interfacial affinities: n-R6-COO- (m/z = 129) > c-R6-COO- (m/z = 127) > Ar-R6-COO(m/z = 121) at [mixture] ≥ 2 µM. Figure 9 shows the plots of χ(n) as a function of the distance between C-atom of COO17 ACS Paragon Plus Environment


and the farthest C-atom (r) and SASA in different equimolar concentration mixture. We adopt the SASA values of n-hexane, cyclohexane, and benzene for n-R6-COO-, c-R6-COO-, and Ar-R6-COO-, respectively.67-68 We found that the ln χ(n) data can be more smoothly proportional to the SASA, than the rn distance (Fig. 9).

-1.0

-1.0

A 10 µM

D 10 µM

-1.2

-1.2

Heptanoate

-1.4

Heptanoate

-1.6

-1.6

c-Hexane carboxylate

c-Hexane carboxylate -1.8

-1.8

-2.0

-2.0

-2.2

-2.2

Benzoate

Benzoate

-2.4 300

400

500

600

700

B 100 µM

0.0

lnχ(n)

-1.4

800 220

240

260

-2.4 280

300

320

E 100 µM

0.0

-0.5

-0.5

-1.0

-1.0

-1.5

-1.5

-2.0

-2.0

-2.5 300 0.0

-2.5 400

500

600

700

800 220

240

260

280

300

320

F 1000 µM

C 1000 µM

0.0

-0.5

-0.5

-1.0

-1.0

-1.5

-1.5

-2.0 300

lnχ(n)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

-2.0 400

500

600

700

800 220

240

260

280

300

320

2

r / pm

SASA / Å

Figure 9 – Plots of ln χ(n) as a function of the distance r (picometer) between C-atom of -COO- and the farthest C-atom (A, B, C) and as a function of SASA (Å2) (D, E, F) in different equimolar concentration mixture, A, D) 10 µM, B, E) 100 µM, and C, F) 1000 µM. The linear regressions are fitted for the three carboxylate ions.


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In association with the relative interfacial affinity n-R6-COO- > c-R6-COO-, it is known that cyclohexane is less hydrophobic than n-hexane,73 despite being that the interaction of linear and cyclic hydrocarbon with water is both dominated by dispersion force. Gallicchio et al. suggested that the larger solubility of the cyclic alkanes with respect to the linear alkanes is due to their more favorable solute-water interaction energies per unit surface area.65 For the relative interfacial affinity c-R6-COO- > Ar-R6-COO-, the ln χ(n) value of Ar-R6-COO- at a low concentration (i.e., 10 µM) is smaller than the expected linear regression line fitted by the plots of n-R6-COO- and c-R6-COO-. Clearly, in the comparison of cyclic vs. aromatic under the lower concentration, considering the size is not enough and we need additional factor. This exceptionally low interfacial affinity of Ar-R6-COO- can be explained by its aromaticity. π-electron density on aromatic ring creates a quadrupole moment with partial negative charge above both aromatic faces and a partial positive charge around the periphery.74-75 For the benzene-water system, the electrostatic interactions (i.e., the quadrupole–dipole interaction) predominantly contribute to the interaction energy,76 and the faces of benzene act as a hydrogen bond acceptor for a water molecule with the energy of ‒0.96 ~ ‒1.04 kcal mol‒1 (while the C-H periphery weakly interacts with water at -0.26 to -0.30 kcal mol‒1).77 For the cyclohexane-water system, on the contrary, the dispersion interactions are more relevant (‒0.34 ~ ‒0.49 kcal mol‒1).77 As a result of the π-H bonding, benzoate ion is not considered as a simple amphiphile, that has a separated hydrophobic and a hydrophilic part, like other acids tested here. In fact, Minofar et al. observed a relatively small decrease of the surface tension by adding Ar-R6-COO-, which implies that this anion has a considerably weaker surface-affinity than typical surfactants.43 19 ACS Paragon Plus Environment


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They inferred Ar-R6-COO- acts as a “hydrotrope”,43 where an aromatic ring has an affinity with water molecule via the strong π-H bonding.78 MD simulations demonstrated a preferential orientation of Ar-R6-COO- at the air-water interface, with the charged group pointing into the aqueous bulk and the hydrophobic aromatic ring being exposed toward the vapor phase.43 Fedotova and Kruchinin calculated the hydration of benzoic acid, and reported that water molecules preferably locate above and below the plane of the phenyl ring at a distance of ~0.3 nm from the ring center.79 In a larger concentration (i.e., 1000 µM), on the other hand, the relative interfacial affinity n-R6-COO- > c-R6-COO- > Ar-R6-COO- appears to be well proportional to their SASA of the alkyl substituents (Fig. 9). This may imply that the π-H bonding of Ar-R6-COO- may become less important. At a larger concentration, the surface starts to be covered by hydrophobic solutes. As a result, the number of Ar-R6-COO- ions on the surface that can create the π-H bonding with water would decrease compared with those at a lower concentration. The atmospheric implications of present findings are briefly discussed below. Our experiments are related to bubble-bursting mechanism that produces finer droplets containing surface-active organic species from ocean.20-21 Our results reveal that the larger carboxylate ions are increasingly enriched as a function of concentration at the air-water interface and hence would accumulate in finer droplets under ambient conditions. The enrichment factor will also be as a function of size (chain-length or SASA) of corresponding carboxylate ions. These results are consistent with recent experimental and field observational results on the alkylcarboxylate enrichment by bubble-bursting process.20-21 Cochran et al. reported that a considerable amount of R3-COO- (butanoate ion) is found in the aerosol phase in the process of bubble-bursting, 20 ACS Paragon Plus Environment

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implying R3-COO- evidently accumulate at the air-water interface of seawater.21 Our experiments imply the oxidation of “surface-available” carboxylate ions by ⋅OH(g) at the air-water interface may dominate its fate. Note that recent studies have revealed that the heterogeneous reaction of ⋅OH(g) with aqueous species proceeds via a Langmuir-Hinshelwood mechanism, where ⋅OH itself has a sufficiently long lifetime to react with a species at the air-water interface.18,

47, 80-84

The ⋅OH-oxidation of

surface-available carboxylate ions at the air-water interface would produce highly-oxygenated products, functionalized products, smaller-chain products and volatile organic compounds (VOCs) immediately emitted into gas phase.18, 47, 81 Thus, present results underscore the importance of interfacial ·OH-oxidation of surface-available carboxylate ions, such as octanoate ion. Acetate ion, on the other hand, will accumulate in atmospheric aqueous aerosol during photochemical aging process since it is protected from the attack of gas-phase OH-radical like oxalate ion, as already demonstrated in our laboratory.18, 47 Our results are fully consistent with field observations that showed the highest concentration of acetate ion among all the carboxylate ions in aqueous aerosol.85-86 We showed interfacial availabilities for alkylcarboxylate ions can be well predicted by chain-length and SASA, except for acetate. Benzoate ion, the most abundant organic species observed in PM2.5 of Beijing,42 is found to have a moderate surface-propensity in aqueous media. In the co-existence with more surface-available long-chain alkylcarboxylate ions, this ion would stay at relatively deeper layers of aqueous aerosol. We showed the interfacial availability for benzoate ion could not be predicted by the SASA in lower concentrations, e.g., ambient conditions.



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Conclusion

By utilizing the bag-breakup mechanism of droplets, we determined relative interfacial affinities of carboxylate ions, Rn-COO- (n = 1 - 7) and n-, cyclo-, aromatic-R6-COO-, from equimolar concentration mixtures of the corresponding carboxylic acids for the first time. We found the signal intensity of Rn-COO- increases as a function of the size (i.e., chain-length or solvent-accessible surface area) and the divergence becomes more intense as the equimolar concentration increases. R7-COOsignal intensity is ~80 and ~9 x 103 times larger than those of R1-COO- at 1 and 1000 µM equimolar concentration, respectively. The stabilization energy per CH2 group at the interface was estimated to be ∼0.2 kcal mol-1, which is consistent with the reported theoretical value.46 This stabilization at the interface is expected to be driven by positive entropic effect overcoming enthalpic destabilization.46 Interfacial availability of R1-COO- is found to be exceptionally small possibly due to the small size that prevents from disordering water’s hydrogen-bonding-network. The relative interfacial affinities for linear vs. cyclo vs. aromatic in R6-COO- is found to be: n-heptanoate

(n-R6-COO-)

>

cyclohexanecarboxylate

(c-R6-COO-)

>

benzoate

(Ar-R6-COO-). We attribute the smallest availability of Ar-R6-COO- among the three carboxylate ions to a strong π-H bonding between the aromatic ring and water molecule. Our results suggest that amphiphilic alkylcarboxylate ions increasingly locate at the air-water interface of atmospheric particles as functions of the concentration and size, consistent with field measurement studies. It is implied that the more available carboxylate ions to the surface, the more it enhances the heterogeneous OH-radical oxidations, resulting in the change of nucleation, growth and aging rates of 22 ACS Paragon Plus Environment

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atmospheric particles. On the other hand, small-size acetate ion would play very limited role in the heterogeneous aging process due to the negligibly small interfacial availability. As the result, acetate ion will accumulate in aqueous media, consistent with field observations.

SUPPORTING INFORMATION

The Supporting Information is available free of charge on the http://pubs.acs.org. Experimental setup and additional data.

AUTHOR INFORMATION Corresponding author

S.E. [email protected], phone: +81-29-850-2770

ACKNOWLEDGEMENT: S. E. is grateful to Dr. A. J. Colussi of Caltech and Drs. Satoshi

Inomata and Akihiro Fushimi of NIES for stimulating discussion. This work is partly supported by JSPS KAKENHI grant numbers 15H05328 and 15K12188.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 30 of 30

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Carboxylate Ion Availability at the AirâWater Interface - The Journal of

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