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Chemical Composition and Properties of the Liquid – Vapor Interface of Aqueous C1 to C4 Monofunctional Acid and Alcohol Solutions Ming-Tao Lee, Fabrizio Orlando, Luca Artiglia, Shuzhen Chen, and Markus Ammann J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b09261 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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The Journal of Physical Chemistry

Chemical Composition and Properties of the Liquid – Vapor Interface of Aqueous C1 to C4 Monofunctional Acid and Alcohol Solutions Ming-Tao Lee§,#,†, Fabrizio Orlando§, Luca Artiglia§,£, Shuzhen Chen§,&, Markus Ammann§,* §

Laboratory of Environmental Chemistry, Paul Scherrer Institute, Villigen, Switzerland

#

Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland

£

Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, Villigen,

Switzerland. &

Institute of Atmospheric and Climate Sciences, ETH Zürich, Zürich, Switzerland

KEYWORDS. Alcohols, carboxylic acids, atmospheric chemistry, surface tension, aqueous solution, X-ray photoelectron spectroscopy, liquid jet

1 ACS Paragon Plus Environment


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ABSTRACT. The liquid – vapor interface is playing an important role in aerosol and cloud chemistry, in cloud droplet activation by aerosol particles and potentially also in ice nucleation. We have employed the surface sensitive and chemically selective X-ray photoelectron spectroscopy (XPS) technique to examine the liquid-vapor interface for mixtures of water and small alcohols or small carboxylic acids (C1-C4), abundant chemicals in the atmosphere, in concentration ranges relevant for cloud chemistry or aerosol particles at the point of activation into a cloud droplet. A linear correlation was found between the headgroup carbon 1s core-level signal intensity and the surface excess derived from literature surface tension data, with the offset being explained by the bulk contribution to the photoemission signal. The relative interfacial enhancement of the carboxylic acids over the carboxylates at the same bulk concentration was found to be highest (nearly 20) for propionic acid/propionate and still about five for formic acid/formate, also in fair agreement with surface tension measurements. This provides direct spectroscopic evidence for high carboxylic acid concentrations at aqueous solution – air interfaces that may be responsible for acid catalyzed chemistry under moderately acidic conditions with respect to their bulk aqueous phase acidity constant. By assessing the ratio of aliphatic to headgroup C 1s signal intensities XPS also provides information about the orientation of the molecules. The results indicate an increasing orientation of alcohols and neutral acids towards the surface normal as a function of chain length, along with increasing importance of lateral hydrophobic interactions at higher surface coverage. In turn, the carboxylate ions exhibit stronger orientation towards the surface normal than the corresponding neutral acids, likely caused by the stronger hydration of the charged headgroup.


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Introduction A better understanding of the molecular properties of organic compounds at the liquid-vapor interface and of the impact of their presence on the properties of this interface is important in atmospheric chemistry and physics. Small oxygenated volatile organic compounds (OVOC), such as alcohols and acids, are important products of atmospheric oxidation cycles. They typically occur in the low ppbv partial pressure range in the gas phase and are thus linked via Henry’s law to aqueous phase concentrations in the µM range.1 As a compound class, soluble oxygenated organic compounds often comprise a major fraction of solutes in aerosol or cloud water.2 Their amphiphilic nature due to the presence of a hydrophilic head-group and hydrophobic aliphatic carbon tail is controlling their propensity for the aqueous solution – vapor interface. Organic surfactants may affect cloud condensation nucleus activation3-4 and the phase transfer of major and trace species,5-6 or facilitate non-conventional reaction pathways at the aqueous solution – vapor interface of aerosol particles.7-13 The role the organic surfactants play in detail have not been sufficiently elucidated so far, mostly related to the lack of experimental methods and approaches to selectively probe the interfacial region and processes thereon. Traditionally, the propensity of solutes for the liquid – vapor interface can be assessed through measurement of the surface tension, σ, for the solutions of interest. For aspects of aerosol microphysics or cloud condensation nucleation activation, the surface tension is also of direct interest.3-4 Surface tension measurements allow deriving the surface excess, Γi of solutes i, which expresses the increased or decreased number of solute molecules within the volume above the Gibbs dividing surface relative to their activity, ai, in the bulk of the solution, via the Gibbs equation:

 a  ∂σ   Γ i = − i   RT  ∂ai 

(1).



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While comparatively easy to measure, surface tension falls short of selectivity for the chemical properties, for the detailed depth profile of components at the interface contributing to it, or for further details, such as molecular orientation. Surface tension measurements become difficult to interpret for more complex solutions with multiple solutes. Estimating surface composition and surface tension from thermodynamic bulk solution and solute properties is inherently difficult and require experimental verification.14-16 X-ray photoelectron spectroscopy (XPS) likely provides the most chemically selective information from the interfacial region17. XPS is surface-specific due to the short inelastic mean free path (IMFP, λ) of photoelectrons in condensed matter, typically very few nm, which is still a significant depth on molecular length scales, though. XPS quantitatively measures the amount of molecules within the probe depth (defined as 3 × λ), since the intensity of core-level photoelectron (PE) signals is proportional to the atom density. Note since for liquids λ is not well known the probe depth may be better referred to as information depth. Apart from the selectivity for the chemical element, XPS is sensitive to the chemical environment and oxidation state; for instance, for soluble organic compounds, it distinguishes between carbons in hydroxyl (-C-OH), carboxyl (-C-OOH), carboxylate (-C-OO-), and aliphatic (-CH2)n functional groups. The development of liquid jets provides the opportunity of probing a continuously renewed surface free of contamination or beam damage that would significantly affect XPS experiments with static liquids18. XPS and surface tension thus provide complementary information about the liquid - vapor interface especially for organic compounds. XPS provides additional information about electronic structure, orientation, co-solutes and other interface properties, and has thus been applied to aqueous solutions of butanol,19 formic acid,20 acetic and butyric acids,21 decanoic acid,3 various alcohols22-23 or citric acid.24 Thus, XPS will lead to further elucidation of the


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surface specific processes mentioned above that are playing an important role in atmospheric chemistry. As pointed out in these previous studies, the measured photoemission intensity is sensitive to both the number density of molecules at the surface itself, and bulk molecules residing within the probe depth with a concentration nb (in molecule cm-3). In the simplest picture of the interface, the number density of molecules at the surface is equal to the surface excess, Γi, constituting the surface tension as given by equation (1). If attenuation of surface molecules is neglected, and if the attenuation of the bulk molecules leads to an exponentially decreasing contribution under the assumption of constant density with depth, z, the photoemission signal, Ii, related to a core level excitation of solute i, becomes:20 ஶ

‫ܫ‬௜ = ‫ܣ‬൫߁௜ + ‫׬‬଴ ݊௕ ݁ ି௭/ఒ ݀‫ݖ‬൯ = ‫ܣ‬ሺ߁௜ + ߣ݊௕ ሻ

(2)

where A denotes a proportionality constant related to the overall measurement efficiency, including excitation cross section, transmission and detection efficiency of the electron analyzer. Since λ is not well established for liquids,25-27 absolute quantification of the surface composition is difficult, but eq. (2) predicts a linear relationship between measured photoemission intensity and the surface excess. In this study, we report C 1s core-level photoemission spectra of C1 to C4 monoacids (formic, acetic, propionic, and butyric acid), their respective (sodium) carboxylate conjugate base forms (formate, acetate, propionate, and butyrate), and C1 to C4 alcohols (methanol, ethanol, 1propanol, 2-propanol, and 1-butanol) in aqueous solution, all at a bulk concentration of 0.5 M. Atmospheric concentrations of these OVOC species vary widely among aerosol particles, droplets at the point of activation or cloud droplets.2-3, 28 Organic compounds constituting aerosol and cloud water are typically more complex species2 than the simple monofunctional compounds



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studied here, which serve rather to benchmark the technique and investigate the effect of individual functional groups. Also, from surface tension measurements, it is clear that at the selected concentration, the smaller species are within the linear part of the surface excess versus concentration isotherms, while especially the C4 species already exhibit saturating surface excess.29 The focus here is on the comparison among the different species, and concentration effects will be addressed where appropriate. We have a detailed look on how the XPS technique may help to explain the origin of the largely different surface tensions of neutral carboxyls versus the deprotonated carboxylate ions and clearly differentiate the organic anions from the inorganic cations which are convoluted in surface tension measurements. Furthermore, XPS provides information about the orientation of surface molecules for different functional groups and chain lengths, which is driven by lateral hydrophobic interactions. This study relies on liquid jet XPS experiments performed in vacuum, which means that the aqueous solution is injected into a vacuum chamber and travels as an about 20 µm diameter laminar jet during a few hundred microseconds before being probed by XPS, which is considered an equilibrated liquid-vapor interface in the following sense.25,

30

First, we note that the time

scale of evaporation is slow enough that it does not significantly change the composition nor the temperature of the solution during the residence time in the chamber before analysis. Second, the rate of evaporation of H2O molecules from the surface is orders of magnitude slower than that of the hydrogen bond network dynamics in the bulk liquid (picosecond time scale) and comparable to the rate of bulk diffusion of solutes and H2O over tens of nanometer, so that the near-surface bulk liquid can easily cope with the evaporative loss of solvent. Third, the residence time of the liquid jet in the chamber is long enough to allow building up the surface excess of the solute by diffusion from the bulk without significantly depleting its concentration there.24 Therefore,


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experiments performed in vacuum and under equilibrated conditions with respect to water vapor return consistent results.31 However, experiments in vacuum are spectroscopically much more efficient in terms of time, signal to noise ratio and level of detail in comparison to experiments under equilibrated conditions, where signal to noise ratios are lower due to scattering of photoelectrons in the gas phase. The conditions mentioned above are easily met for the species considered in this study at the concentrations used, but need to be assessed separately for other solutions of different viscosity, solvent vapor pressure and surface active solute concentration.



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Experimental section X-ray photoelectron spectroscopy (XPS). XPS experiments using a liquid micro-jet18 were performed at the Surfaces/Interfaces: Microscopy (SIM) beamline32 of the Swiss Light Source (SLS) using the near ambient pressure photoemission (NAPP) endstation.33 A 19 µm liquid jet operating at 279 K (measured immediately before entry into the ionization chamber) with a flow rate of 0.30 ml/min and in a chamber vacuum of < 1.0 × 10−4 mbar was used. The entrance orifice diameter of the hemispherical electron energy analyzer and the working distance to the liquid jet were both 500 µm. The Scienta HiPP-2 analyzer was operated at 100 eV pass energy in 0.1 eV step size. All aqueous solutions (methanol and ethanol, ACS reagent, ≥99.8%, SigmaAldrich; 1 and 2-propanol, ACS reagent, ≥99.5%, Sigma-Aldrich; 1-bunanol, 99.9%, SigmaAldrich; formic acid, ACS reagent, ≥98%, Sigma-Aldrich; acetic acid, ACS reagent, ≥99.7%, Sigma-Aldrich; propionic acid, ACS reagent, ≥99.5%, Alfa Aesar; butyric acid, ≥99+%, Alfa Aesar) were prepared using Milli-Q water (Millipore, 18.2 MΩ cm at 25 °C) at 0.5 M in 0.05 M NaCl (Sigma-Aldrich, ACS reagent, ≥99%). Sodium chloride at 0.05 M was used to ensure adequate conductivity of the solutions that prevents charging of the liquid micro-jet in the X-ray beam in absence of other ionic solutes.34 Bulk pH values were measured using a Mettler Toledo Expert Pro electrode that was calibrated using a four point curve at room temperature. We used a combination of first- and second-order synchrotron light.20, 24, 34-35 The primary photon beam was set to 450 eV to ionize the C 1s orbital of aqueous organic solutions with a kinetic energy of ca. 155 eV20. Second order light, 900 eV, that passes through the beamline optics with ca. 10 % intensity of the primary energy was used to ionize the O 1s orbital with kinetic energy of ca. 362 eV. The kinetic energy of the C 1s spectra were energy calibrated against the binding energy of the O 1s in liquid water at 538.1 eV.18 The C 1s and O 1s regions were collected in parallel by


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setting the hemispherical energy analyzer to take a single sweep of the C 1s region followed by a single sweep of the O 1s; spectra were obtained from averaging at least 15 such sequences. Because the C 1s and O 1s orbitals were collected from different electron kinetic energies (KE’s) and therefore different depths into solution, we report only a relative surface concentration of the organic. That is, a C 1s / O 1s ratio of the integrated peak areas that assumes the O 1s signal is dominated by H2O independent of organic content. This serves to normalize to the overlap between the X-ray beam and the liquid and thus takes into account the small spatial fluctuations of the liquid jet or the variations of this overlap from experiment to experiment. These C 1s / O 1s ratios will be referred to as the measured (functional group) C 1s intensity in the following sections. The C 1s / O 1s intensity ratios are not interpreted quantitatively to obtain atomic ratios and thus absolute surface coverages. Derivation of surface excess. The surface excesses were derived from measurements of surface tension reported in the literature for the alcohols,29,

36-40

carboxylic acids29,

41-42

and

carboxylates.43-44 The uncertainty of the surface excess was estimated from the standard deviation of values derived from different studies. More details about the derivation of the surface excesses are given in the supporting material.



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Results and discussion

Figure 1. Carbon 1s photoemission spectra (dots) of 0.5 M aqueous solutions of organics taken at a photon energy of 450 eV for (a) alcohols, (b) carboxylic acids, and (c) ionic (sodium) carboxylates. All the spectra are normalized to the peak area of the functional group carbon (C4) within the same functional group family. Black lines present the fits, red and blue lines their methyl carbon and functional group carbon contributions, respectively. Dashed lines represent their gas phase counterparts’ contribution to the alcohol series in (a). O 1s and C 1s photoelectron spectra. An exemplary O 1s spectrum that is representative of all investigated solutions is shown in the supporting material Fig. S1. There are two peaks in the O 1s region, assigned to gas phase water at lower KE and condensed liquid water at higher KE.45 The noticeable contribution from gas phase water in the O 1s spectrum of Fig. S1 results from the fact that the X-ray beam (FWHM = 100 µm)46 not only hits the around 20 µm liquid jet but also partially ionizes the gas phase water (and organic) envelope that surrounds the liquid jet as it


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propagates through the measurement chamber. Within the O 1s liquid peak two components should be present: oxygen of solvent water at ~55 M and oxygen of carboxyl and alcohol groups of the organic solute at 0.5 M. We are, however, unable to resolve the individual contributions (water dominates) and fit the liquid region with a single component (see Supporting Information). The C 1s photoelectron spectra from (a) the alcohols, (b) the carboxylic acids and (c) the carboxylates are presented in Figure 1 as a function of kinetic energy. Up to four peaks are present, two from the condensed phase (functional group carbon and the aliphatic chain carbon) and two from the gas phase for the alcohol series (Figure 1a). The pH of the measured solutions (SI Table S1) ensured that the acid-base equilibrium is shifted predominantly in favor of the carboxylic acids47 in Figure 1b (97% for formic acid and >99% for the others) and entirely to that of the carboxylates (100%) in Figure 1c. The carbon atom of the head group (fit shown in blue) is well resolved from the carbon backbone (red), where applicable, with the former appearing at lower kinetic energy (KE) due to its higher binding energy (BE) reflecting its more oxidized state. Further confirmation of this assignment comes by noting the peak position for the solutes that contain only the head group carbon, methanol (lower panel of Fig. 1a), formic acid (lower panel of Fig. 1b) and formate (lower panel of Fig. 1c). Similar to the case of the O1s spectrum mentioned above, also for the C1s spectra, the presence of evaporating organics in the gas phase surrounding the liquid jet leads to additional components obvious for the alcohol series. Qualitatively, the ratios of the gas phase contributions to that of H2O(g) can be compared. The gas-phase contribution for the acids is much smaller than for the alcohols in general and negligible. This is consistent with the equilibrium vapor pressure derived from the Henry’s law constants for the 0.5 M acids (0.11-8.4×10-2 mbar) being smaller



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than for 0.5 M alcohols (2.3-4.6 mbar)1. Within the series of alcohols themselves, the equilibrium vapor pressure increases with increasing chain length (due to decreasing Henry’s law constants 1) leading to a slight increase of the relative contribution of the gas phase peaks in each spectrum from methanol to butanol. The C 1s kinetic (binding) energy splitting between the aliphatic and the functional group carbons for neutral acids, deprotonated acids and alcohols, about 3.9, 3.5 and 1.4 eV respectively, are in line with previously reported values for similar systems.21-22,

48

A more

detailed investigation of the small changes in kinetic (binding) energy of headgroup and aliphatic carbon as affected by solvation and concentration22 goes beyond the scope of this work, which is focused on the relative changes in C1s peak areas for the series of species and functional group families and their relation to surface tension. We measured a marked difference in normalized C1s signal intensity from the head group carbon atom between organics at the aqueous solution – vapor interface (SI Table S1). These measurements were done at a fixed photoelectron kinetic energy (inelastic mean free path of around 1 nm, see also below) and thus fixed information depth into the bulk. Furthermore, the bulk concentration for all solutions was kept constant. Thus, these changes in intensity between the different organic solutes are easiest to interpret as arising from substantially different propensities for the interface. Based on equation (2), we expect a linear correlation between photoemission signal intensity and the surface excess derived from surface tension measurements (equation (1)). The results are shown in Figure 2, where the intensity of the functional group C1s signal (values given in SI Table S1) is plotted against Γi. The uncertainty of Γi was obtained from the standard deviation of the values returned from the different surface tension measurements available. The uncertainty of


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the photoemission intensity was estimated from the variation of the peak areas obtained under slightly varying fitting constraints for peak widths, peak shapes and background subtraction method to be about 8% of the full scale value of figure 2a, thus being relatively larger for the less surface active species with smaller photoemission intensity. This conservative error was larger than the standard deviation of replicate measurements in most cases.

Figure 2. (a) Measured functional group C 1s intensity of organics versus the surface excess as a function of organic species (red: alcohols, black: carboxylic acid, blue: sodium carboxylates; squares: C1, circles: C2, upward triangles: C3, downward triangles: C4). The dashed line represents a linear fit through the data, with the red area representing the 95% confidence intervals. The horizontal dotted line indicates the intersect of the linear fit line with the C1s intensity scale at zero surface excess, interpreted as bulk contribution to the C1s intensity. (b)



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conceptual model of how bulk and surface molecules contribute to the signal intensity according to equation (2). A.W.I. indicates the air (vapor) – water – interface.

At 0.5 M, the number of solute molecules contained in a 1 nm thick and 1 cm2 wide slice is about 3×1013 molecules, which is of the same order of magnitude as the surface excesses encountered for the present range of solutions. Thus, the signal measured for species with low surface excess is dominated by the contribution of the bulk molecules at 0.5 M, taking into account that based on the integration of the signal contributions with depth (see conceptual model in Fig. 2), 95% of the signal comes from a depth down to 3λ. Within this simple concept, λ is assumed to remain constant with depth, and other processes, such as elastic scattering, are not considered.27 Due to the uncertainty around the inelastic mean free path and the impact of the more detailed molecular level density profile,27 we refrain from quantitatively interpreting the offset of the linear fit as in equation (2) and thus left it as a free parameter in the linear fit shown in Fig. 2. For the same reason, we also refrain from quantitatively interpreting the photoemission intensity to obtain absolute surface coverage to compare with the surface excess data. Fig. 2 demonstrates the nice correlation of the C 1s signal with surface excess, including its extension into negative surface excess values, as expected from equation (2). Even if we do not quantitatively interpret it, the horizontal dashed line, at the intersect of the linear fit line with the C1s intensity scale at zero surface excess, would correspond to the intensity of a species with a perfectly uniform distribution across the entire solution at 0.5 M concentration. This thus represents the bulk contribution to the signals measured for species with Γi > 0. Negative surface excess values need to be understood in the context of the more general definition of surface excess, which is the relative departure of the concentration of a solute at and near the surface


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(averaged over a finite depth into the bulk) from that deep in the bulk.49-50 Negative values thus correspond to a net reduction of the bulk solute molecules close to the surface for instance due to repulsion from the interface. Within the simple model of the photoemission signal expressed in equation (2), the surface contribution would be zero in this case, while the bulk contribution is reduced due to solute molecules not being present in a thin bulk layer close to the surface, thus effectively shifting the lower integration bound in equation (2) to the depth of such a depleted layer. Since for the data presented here only one solute shows significant negative surface excess, formate, a quantitative elaboration is not within the scope of this study. Overall, the XPS data presented here nicely confirm the concept of surface excess as embedded in the Gibbs equation by a direct and chemically selective surface composition measurement. It is well established that the presence of inorganic salts have salting out effects on other solutes. The solutions of the present study contained 0.05 M NaCl to assure electrical conductivity of the liquid as mentioned in the experimental part, while the literature surface tension data used to derive surface excesses refer to solutions in absence of NaCl. On one hand, in the concentration range where the surface excess increases linearly with concentration of the organic, this increase is steeper as compared to the slope in absence of salt. On the other hand, in presence of salt, saturation sets in at already lower organic concentration, but the saturating surface excess is not much affected (see, e.g., the effect of KI on butanol reported by Krisch et al.19). Therefore, the largest effect is expected for formic acid, where the surface excess at 0.5 M is increased by 12% in presence of 0.05 M NaCl,20 but gradually less for the other more surface active species. Since we do not have salting data for all organics of this study for the low 0.05 M salt content, and in view of the substantial uncertainties in both photemission signal and surface tension measurements, the surface excess data were not corrected for this.



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As apparent from Fig. 2, the surface propensity increases with increasing chain length within the same functional group family. While alcohols and carboxylic acids span about the same range of Γi and C1s signal intensity, the sodium carboxylate solutions remain below those. We start with a more detailed discussion of the data for the carboxylic acids and their conjugate bases. Photoelectron spectroscopy has been used to study the liquid-vapor interface of aqueous carboxylic acid solutions previously.20-21,

51

All the carboxylic acids were found with higher

affinity for the surface relative to their conjugate base carboxylates related to the larger hydration free energies of the charged carboxylate group compared to the neutral acids52-54 and in line with the simple electrostatic picture that ions are repelled from the interface. In the present investigation, the substantially larger measured functional group C 1s intensity for the acids than for their conjugate carboxylate bases is qualitatively in line with the previous studies mentioned above. Fig. 3a shows the same data as in Fig. 2 but now plotted as the measured C 1s photoemission signals of the 0.5 M formic, acetic, propionic, and butyric acids at low pH and the signals of the corresponding conjugate carboxylate solutions at high pH, respectively, using the carbon chain length to categorize the data. The chain length is also a measure of the ratio of hydrophobic to hydrophilic interaction options. The measured functional group C 1s intensity increases with increasing aliphatic chain length for both neutral carboxylic acid and charged carboxylates. Formate ion exhibits positive surface tension difference to pure water at 0.5 M (Fig. 2), indicative of the extent of electrostatic repulsion of the negatively charged formate ion away from the interface and of the strong free energy gain from complete hydration.55 For propionate and butyrate, the increasing hydrophobic carbon chain leads to a net positive attraction towards the interface, counteracting charge repulsion and demonstrating the balance between structure


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breaking hygrophilic solvation of the anionic headgroup and the independent structure making hydrophobic solvation of the alkyl chain.54

Measured functional group C 1s intensity

(a) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 C1

C2

C4

C3

(b) 22

C1s [CCOOH ] / C1s [C COO-]

20 18

0.05 M

16 14 12 10 0.2 M

8 6

0.5 M

4 C1

C2

C4

C3

(c) 90 Ratio of surface contributions to C1s of carboxylic acid / carboxylate

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80 70 60 50 40 30 20 10 0 0.0

0.2

0.4

0.6

butyric acid concentration [M]

Figure 3. (a) Measured functional group C 1s intensity of solutions containing the carboxylic acids at pH 2 (black) or their conjugate carboxylate ions at pH 12 (blue) against chain length. (b) Ratio of functional group C 1s signal intensity of the acids at pH 2 to that of their corresponding conjugated base at pH 12 against chain length (green symbols). Open symbols refer to measurements with 0.2 M (bold edge) and 0.05 M (thin edge) measurements. (c) Ratio of the 17 ACS Paragon Plus Environment


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surface contributions to the C1s signals for butyric acid and butyrate similar to that in (b), but after subtracting the bulk contribution, as a function of the bulk solute concentration, based on functional group C1s intensity (filled symbols) and aliphatic chain C1s intensity (open symbols). The solid line depicts the ratio of the corresponding surface excesses with the shaded area giving the 95% confidence interval. Fig. 3b represents the ratios of the C 1s signals of each pair of carboxylic acid and its conjugate base, measured separately at around pH 2 and 12, respectively (see SI Table S1). The relatively large uncertainties of the ratios result from propagating the errors shown in Fig. 2 and 3a, especially for formate and acetate. Based on the 0.5 M data, this ratio tends to first increase with chain length from formic/formate to propionic/propionate and then drops again towards butyric/butyrate. A constant ratio would indicate that the electrostatic and hydration effects have always the same effect on neutral acid and the carboxylate ion, independent of the chain length, while an increase/decrease could indicate that a longer hydrophobic aliphatic carbon chain has a relatively stronger/weaker effect on the neutral acids than on the carboxylates. The butyric acid to sodium butyrate ratio measured at 0.5 M may be lower because the surface excess of butyric acid exhibits saturation at 0.5 M (see SI Fig. S4 and S4). This saturation of the surface excess is likely not due to micelle formation, since the critical micelle concentrations for butyric acid and sodium butyrate are well above 1 M.56-57 As mentioned upfront, we do not quantitatively assess surface coverages from the photoemission signals so that we are unable to judge whether a full monolayer is established or not at saturation as done by Walz et al..22 Given the surface excesses in the range of a few 1014 molecules cm-2 it is likely that saturation is due to lateral interactions among the alkyl chains. Thus, the surface propensity is not solely governed by the interplay between hydrophilic and hydrophobic solvation of the carboxylate head group and the alkyl


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chain, respectively. We therefore additionally measured solutions with lower concentrations for both butyric/butyrate and propionic/propionate, which led to a strong increase of the ratio, as indicated by the open symbols in Fig. 3b. Even though the low concentration ratios show a smaller relative decrease than those at higher concentration, the saturation and lateral interactions among the alkyl chains is not the only reason for the decrease from C3 to C4. We note that also the ratio of the surface excesses strongly decreases from C3 to C4 at all concentrations (see SI Figures S4 and S5 for reference). Therefore, the relative enhancement of the neutral carboxylic acid over the carboxylate ions is going through a maximum between C1 and C4 species. The ratio for formic acid to formate and acetic acid to acetate reported here is consistent with data from both Ottosson et al.21 and Brown et al.51 (both measured at pH ≈ pKa). Ottosson et al. measured the carboxylic acid to carboxylate PE signal ratio for a total bulk concentration of 1 M at pH ≈ pKa. Their assumption was that the neutral carboxylic and charged carboxylate are coexisting in the bulk at equal concentrations of ca. 0.5 M in the same solution. Therefore, their ratio has been affected by saturation on the surface and by competition between carboxylate and carboxylic acid at the interface. The ratios reported here are based on measurement at low and high pH for the same bulk concentration for both individually, so that lateral interactions were not playing a role at low enough concentration. Since the surface propensity also of the neutral formic acid is not very high (so that formate does not feel the presence of formic acid at the interface), the data for formic acid / formate and acetic acid / acetate are consistent among the studies. We also note that Ottosson et al. used a different photoelectron kinetic energy of 70 eV for C 1s in their study, compared to KE = 155 eV in this study. Their enhanced surface sensitivity leads to a different proportion of the bulk contribution to their C 1s signal, and explains why their ratios at 0.5 M tend to be higher than ours for C2 – C4. Therefore, to become



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independent of the bulk contributions, the true ratio of the surface concentrations of butyric acid at low pH and of butyrate at high pH was estimated in Fig. 3c as a function of bulk concentration by subtracting the bulk contribution to the C1s signal. Based on Fig. 2 for the 0.5 M solution, this is straightforward, and the constant value represented by the horizontal dashed line was subtracted, and a 10% error was allowed on this bulk contribution to propagate the error. For the 0.2 and 0.05 M solutions, this value was scaled down linearly before subtraction. The justification for subtracting the bulk contribution in this way is given by the fact that our photoemission signals for both acids and carboxylate ions lie on the same correlation line of the plot in Fig. 2. This method has not been used to correct the data for the C2 and C3 species, since the surface excesses of the carboxylate solutions are negative or too low, respectively, and applying a 10% error to the bulk contribution would lead to insignificant numbers for the corresponding ratios. As Fig. 3c indicates, the neutral acid to carboxylate ratio decreases strongly from low to high concentration, even though these ratios estimated from the C1s signals remain significantly below the ratios calculated from the surface excesses (based on the data shown in SI Figures S4 and S5). The carboxylic acids may act as Brønsted acids (proton donors) in aqueous solution, and their conjugate bases may act as Brønsted bases (proton acceptors), whereas the sodium ions have practically no acid/base properties in this respect. The acid-base equilibrium and dissociation and protonation kinetics in the bulk aqueous phase are well known in the aqueous phase. However, whether the asymmetric hydrogen bonding environment at the aqueous solution – air interface changes these is not well established. Whether the availability of protons at the interface is higher remains also open. The debate is ongoing whether protons themselves prefer the interface or not,58-61 or whether very fine structure exists in the interfacial region of the proton density


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profile with depth.62 Mass spectrometry results from nanodrops emerging from microdroplets of hexanoic acid solutions in the ionization environment of an electro-spray led Mishra et al.63 to suggest that the aqueous solution – vapor interface (on the vapor side) is Brønsted neutral for bulk pH values around 3, qualitatively consistent with previous21,

51, 64

and the present study

based on photoemission. As a direct consequence, enhanced reaction rates for acid catalyzed reactions may become important for solutions containing organic acids.65-66 Thus, this work confirms the enhanced presence of neutral organic acids at the interface and elucidates the strong increase of this enhancement with chain length. It also emphasizes the role XPS plays to directly probe the protonation state at the aqueous solution – vapor interface. In principle, the C 1s core-level excitation cross section is independent of the chemical environment carbon is engaged in, even though oscillations of the cross section due to scattering of the outgoing photoelectron wave at nearby atoms have been reported based on gas phase spectra.67 This is likely relevant only for heavy substituents, such as halogen atoms attached to the carbon chain. Thus, we could safely expect that the C 1s photoemission intensity is reflecting the molecular structure, i.e., the ratio of the C 1s peak areas assigned to aliphatic carbon to that assigned to the functional group carbon should increase from one to three for the C2 to C4 species, respectively. In Fig. 4a-c, the aliphatic to functional group C 1s signal intensity ratios normalized to their stoichiometric value are plotted against the number of total carbon atoms. Uncertainties were estimated as described above; they are smaller than above, because ratios are obtained from the two peaks from the same individual C1s spectra. The ratios exceed stoichiometry for all species. This is in line with the intuitive expectation that the preferred orientation of the amphiphilic molecules is with the functional group carbons pointing towards the bulk side of the interfacial region68 and the aliphatic chain pointing rather outward away from



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it into vacuum. Thus, aliphatic C 1s photoelectrons are less attenuated than functional group carbon C 1s photoelectrons. Since bulk molecules are assumed to be oriented randomly, i.e., should exhibit a normalized ratio of one, a linear mixing rule can be applied as also recently done so by Walz et al.23 to calculate the ratio for the surface molecules only by taking the bulk and surface contributions to the C1s intensity as described above (open symbols in Fig. 4a-c). This analysis could not be applied to acetate and propanoate due to the absent or small surface contributions, as mentioned above. For butyric acid and sodium butyrate, these ratios are shown also separately as a function of bulk solute concentration in Fig. 4d. The net upward orientation of surface molecules increases with increasing chain length for each species family, in line with theoretical expectations,69-72 surface tension measurements38 and SFG experiments.73 While previous studies mostly addressed alcohols and to lesser extent carboxylic acids, we also extend observations here to carboxylate solutions. For them, the upward orientation may be the consequence of the negatively charged headgroup being drawn more deeply into solution than for its neutral carboxyl conjugate and at the same time minimizing the volume of hydrophobic water displacement. This is especially apparent for the bulk-corrected ratio displayed in Figure 4d for butyric acid and butyrate. At the lowest bulk concentration, butyrate shows significantly higher upward orientation than butyric acid, while at higher concentration, where the surface excess of butyric acid is in saturation, the lateral interactions lead to similarly strong upward orientation for butyric acid. The fact that the preferential orientation of the aliphatic backbone of the molecules leads to less attenuation of photoelectrons from aliphatic carbon than of those from the functional group carbon does not necessarily mean that we may have even underestimated the relatively higher preference of the neutral acids versus their conjugate bases based on Fig. 3; as indicated in Fig. 3c, the analysis of


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the butyric acid to butyrate ratio for the surface contributions only based on the C1s data for aliphatic carbon does not deviate from that based on the functional group carbon for this pair of species. Turning the discussion towards the alcohol series, again, for those with the alcohol in the headgroup, the ratio increases as for the acids, albeit less strongly. Walz et al.23 have very recently measured this ratio for the series of butanol to hexanol as a function of concentration, indicating an increase up to the point where a monolayer is formed, similar to the discussion above for the acid series. Similar to the acids, the increasing orientation along the surface normal with increasing concentration is due to lateral interactions, as documented by the two maxima in surface entropy of butanol and propanol solutions derived from surface tension measurements. 38, 74

For propanol, we also report a measurement for 2-propanol, where the OH group is on the

center carbon of this C3 alcohol. As expected, for 2-propanol, the aliphatic carbons being bound on both sides of the functional group, the ratio is lower than for 1-propanol, which offers more flexibility to orient the two aliphatic carbons away from the interface.



(b)

(a)

1.8 carboxylic acids I (C1s, -CH2) / I (C1s, -COOH)

1.6 1.4 1.2 1.0

C2

C3

/ number of aliphatic carbons

-

I(C1s, -CH2) / I(C1s, -COO )

/ number of aliphatic carbons

1.8 carboxylates

1.6 1.4 1.2 1.0

C2

C4

(c) I(C1s, -CH2)/I(C1s, head group carbon): surface contributions only

I (C1s, -CH2) / I (C1s, -C-OH)

1.6 1.4 2-PrOH

1.2 1.0

C2

C4

C3

(d)

1.8 alcohols / number of aliphatic carbons

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Page 24 of 39

C3

C4

1.8 1.6 1.4 1.2 1.0 0.0

0.2

0.4

0.6

Concentration (M)

Figure 4. Relative enhancement of integrated -C-CH2 C1s PE intensity to -C-functional-group C1s PE intensity ratio as function of functional group and number of aliphatic carbon atoms (solid symbols; blue, black, red for carboxylates (a), carboxylic acids (b) and alcohols (c), respectively). The open symbols show the ratios after subtraction of the bulk contribution from the C1s signal intensities. Some of the downward error bars were omitted for clarity. (d) Aliphatic to headgroup carbon C1s intensity ratio evaluated for the surface contribution only (see text) for butyric acid (black triangles) and sodium butyrate (blue triangles).


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Conclusion and atmospheric implications XPS on aqueous solutions of a range of short-chained C1 to C4 alcohols and carboxylic acids confirm the surface composition given by the surface excess derived from surface tension measurements. The XPS measurements reassure the manifold enhanced surface concentrations of carboxylic acids compared to their conjugate bases, leading to very high interfacial densities of protonated carboxylic acid head groups at atmospherically relevant acid concentrations and pH values. The XPS measurements further provide us information about the orientation of the molecules consistent with the picture that the aliphatic backbone of the organics point away from the interface. For the neutral acids and alcohols, this is mainly driven by the increasing lateral interactions at higher surface coverage. For the carboxylate solutions, the upward orientation is already strong at low surface excess, possibly related to the charged headgroup being more deeply drawn into the bulk liquid. The surface density of neutral carboxylic acids largely exceeds that of the corresponding conjugate carboxylate salt solutions, in line with other results using XPS21,

51, 64, 75

and other

techniques63 and of course all surface tension measurements discussed here. It indicates that acid catalyzed chemistry may occur at the aqueous solution – air interface under conditions where the bulk phase acidity would not be strong enough10,

76-77

and influence radical chain reactions

propagated in the particular environment at organic acid rich aqueous solution – air interfaces.8-9 While the OVOC species family investigated here are still the most simple possible representation of atmospheric aqueous organic solutions, this study provides the foundation for using this technique also for more complex and thus more realistic mixtures, the modelling of which provide substantial theoretical obstacles.14 In terms of concentration space, the technique



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used here is directly applicable to those relevant for cloud water and for the point of activation of aqueous aerosol particles into cloud droplets. For the higher solute strength solutions of aerosol particles at lower relative humidity, XPS experiments are also feasible on a droplet train, where droplets may become equilibrated with a given water vapor pressure before measurement.78 In such a configuration, XPS provides unique information from supersaturated solutions, their surface structure and composition. ASSOCIATED CONTENT Supporting Information with O1s photoemission spectra and details of the derivation of surface excess from surface tension measurements is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Markus Ammann, [email protected], Laboratory of Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland. Present Addresses †Ming-Tao Lee, [email protected], Chemical Physics Division, Department of Physics, Stockholm University, Stockholm, Sweden. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources 26 ACS Paragon Plus Environment

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This project was supported by the Swiss National Science Foundation (grant no 149492). Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was performed at the SIM beamline of the Swiss Light Source, Paul Scherrer Institute, Villigen PSI, Switzerland. Matthew Brown initiated and continuously supported this work. The authors are grateful to Andreas Türler, Shunsuke Kato, Armin Kleibert, Inga Jordan, Amaia Beloqui Redondo. This project was supported by the Swiss National Science Foundation (grant no 149492). REFERENCES 1.

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Synopsis: Liquid jet X-ray photoelectron spectroscopy experiments on aqueous solutions of C1 to C4 carboxylic acid, sodium carboxylate and alcohol solution demonstrate correlation of carbon 1s core level photoemission intensity with surface excess as derived from surface tension measurements, the strongly enhanced preference for the aqueous solution – vapor interface of the carboxylic acids as well as upward orientation of these surface active molecules with increasing chain length.


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Chemical Composition and Properties of the Liquid ... - ACS Publications

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