Selectivity Across the Interface: A Test of Surface Activity in the

Apr 19, 2016 - Selectivity Across the Interface: A Test of Surface Activity in the Composition of Organic-Enriched Aerosols from Bubble Bursting .... ...
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Selectivity Across the Interface: A Test of Surface Activity in the Composition of Organic-Enriched Aerosols from Bubble Bursting Richard E. Cochran,† Thilina Jayarathne,‡ Elizabeth A. Stone,*,‡ and Vicki H. Grassian*,† †

Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, United States Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States

‡

S Supporting Information *

ABSTRACT: Although theories have been developed that describe surface activity of organic molecules at the air−water interface, few studies have tested how surface activity impacts the selective transfer of molecules from solution phase into the aerosol phase during bubble bursting. The selective transfer of a series of organic compounds that differ in their solubility and surface activity from solution into the aerosol phase is quantified experimentally for the first time. Aerosol was produced from solutions containing salts and a series of linear carboxlyates (LCs) and dicarboxylates (LDCs) using a bubble bursting process. Surface activity of these molecules dominated the transport across the interface, with enrichment factors of the more surface-active C4− C8 LCs (55 ± 8) being greater than those of C4−C8 LDCs (5 ± 1). Trends in the estimated surface concentrations of LCs at the liquid−air interface agreed well with their relative concentrations in the aerosol phase. In addition, enrichment of LCs was followed by enrichment of calcium with respect to other inorganic cations and depletion of chloride and sulfate.

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function of size. Second, we test model frameworks of the air− water interface that have been proposed to explain the transfer of organic compounds from the liquid−air interface to the aerosol phase. Third, we determine factors that control selectivity of inorganic ions present in the solution. During the bubble bursting process, two mechanisms occur that produce aerosols of different size.6 First, the bubble film breaks, releasing film drops. Then the bubble cavity is immediately filled, producing a jet of ocean water from which droplets are released. While consistent trends between bubble diameter and drop size have been observed for jet drops, film drops have been suggested to form through a more complex process.6 Many of the variations observed in the film drop diameters and composition suggest that the bubble film composition as well as the bubble production method plays a large role in determining the formation of film drops. In comparing the type of organics transferred into the aerosol phase, film drops have been shown to contain more aliphaticrich organic molecules, while those in jet drops are more oxygen-rich.10,11 Recent efforts have identified saturated and unsaturated linear carboxlyates (LCs) and various derivatives as well as linear dicarboxylates (LDCs) as components of this aliphatic-rich fraction of nascent SSA.12 In an effort to determine the extent by which the surface behavior of a given molecule influences its concentration in SSA, we show here the selective enrichment of a series of LCs

ea spray aerosol (SSA), one of the most abundant naturally produced aerosols in the atmosphere, is mainly formed through the bubble bursting process in the open ocean.1 SSA chemically consists of organic matter and salts, whose relative ratios vary as a function of size.1,2 Currently, the chemical properties that drive the transfer of specific organic molecules at the liquid−air interface into the aerosol phase were still largely unknown. Theories to describe the transfer of molecules into the gas phase across the liquid−air interface have been described by Raoult’s law for ideal solutions and are proportional to the vapor pressure and concentration in solution. However, nonideal behavior due to intermolecular interactions within the solution phase can lead to large deviations from theoretical models.3,4 The transfer of molecules at the liquid−air interface into an aerosol depends on the details of the aerosol production mechanism. 5,6 It is hypothesized that the surface activity of a given molecule plays a major role in the efficiency of its transfer to aerosol during the bubble bursting process.7,8 Therefore, surfactants present at the air−sea interface would be more efficiently transferred into the aerosol phase relative to other less surfaceactive molecules. While recent models based on simple competitive Langmuir adsorption have been developed that estimate the transfer of surface-active organic molecules into aerosol,5,7,9 few experiments if any have tested these models by comparing their concentrations in the aerosol and solution phases to their behavior at the liquid−air interface. The focus of this Letter is several-fold. First, we quantitatively measure the chemical composition of aerosol formed from the liquid−air interface and how it varies as a © XXXX American Chemical Society

Received: March 1, 2016 Accepted: April 19, 2016

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DOI: 10.1021/acs.jpclett.6b00489 J. Phys. Chem. Lett. 2016, 7, 1692−1696

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The Journal of Physical Chemistry Letters and LDCs in aerosol particles produced under a controlled laboratory environment. Aerosol was generated from solutions mimicking the ocean in both its salt content as well as acidity. Species within these two classes of organic acids were chosen to represent a wide range of surface activities as well as water solubilities. In comparing LC and LDC species with the same number of carbon atoms, the LDCs exhibit greater water solubility while the LCs are more surface-active, leading to higher LC surface excess concentrations (Γ) at the liquid−air interface. The two-dimensional van der Waals model, together with the Gibbs adsorption equation, relates the surfactant concentration (c) to its adsorption Kc =

⎛ αΓ 2β Γ ⎞ αΓ ⎟ − exp⎜ ⎝ 1 − αΓ 1 − αΓ kT ⎠

(1) Figure 1. (a) Mole fractions of the total organic constituents (summed linear carboxylates and linear dicarboxylates) and inorganic salts in the bulk seawater solution (left) and SSA generated with the sintered glass bubbler (right). Concentrations of organic species in size-resolved sea aerosol are shown for individual LCs (b) and LDCs (c). The total concentrations of the LCs and LDCs in aerosol are shown in (d). The data points in (b−d) are shown at the lower cutoff size of the MOUDI impactor stage. Error bars represent the 95% confidence intervals calculated using the standard deviation from triplicate experiments.

where K is the surface adsorption constant, α is the excluded area per molecule, β is a parameter accounting for the interaction between adsorbed molecules, Γ is the surface concentration, k is the Boltzmann constant, and T is the solution temperature. While the values of Γ for alkanoic acids and LCs are not expected to be the same, the trend whereby an increase in the number of methylene units results in greater surface concentration is assumed to be similar. Therefore, values of Γ reported for linear alkanoic acids by Danov et al.13 (using α = 22.61 Å2 and 2β/αkT = 0.4751) were compared to the measured concentrations of the carboxylates in the aerosol generated within the bubbler system ([LC]aerosol). To test the relationship between the surface activity of a surfactant and its concentration in the aerosol phase, a method that promotes the formation of film over jet drops is ideal. Therefore, in this work, aerosol was generated using a sintered glass bubbler that selectively generates film drops through the consistent formation of a bubble foam.14,15 Aerosol generated in the bubbler system was collected using a multiple-stage deposition impactor where the bulk aerosol was fractioned by size into 11 separate size bins. Then the organic carboxylates and major inorganic ions in the aerosol phase were quantified using gas chromatography mass spectrometry (GC-MS) and ion exchange chromatography (IEC) with conductivity detection, respectively. Compared to their relative levels in the aqueous solution, the total organic fraction was enriched in the aerosol phase (Figure 1a; a speciated distribution is shown Figure S1a). The distributions of the inorganic salts and organic fraction differed among the collected size ranges (Figure S1b). Unique distributions were observed for the LC and LDC fractions (Figure 1d; values are averages over triplicate experiments with error bars representing the 95% confidence interval). The LC fraction had a more defined peak in its concentration in the 0.56−1 μm size range (Figure 1d), with both octanoate and hexanoate showing the same trend. Butanoate was observed at much lower concentrations and did not exhibit a well-defined trend as observed for the other LCs (Figure 1b). LDCs were observed at much lower concentrations in the aerosol phase and did not exhibit a clearly defined maximum as did the LCs. However, hexanedioate and octanedioate were observed at slightly higher concentration in the lower SSA size bins, 0.1− 01.8 μm (Figure 1c), relative to larger SSA (1.8−18 μm). Given the higher concentrations of the more surface-active LCs in the aerosol phase, we turn to a theoretical model that estimates the surface concentration of surfactants at the liquid− air interface in an effort to explore the relationship between

surface activity of a given compound and its concentration in the aerosol phase. Figure 2a shows the modeled Γ values for alkanoic acids as a function of their concentration in bulk solution. The Γ values for the three LC species used in this study (for c = 1 mM) are then compared to the measured LC concentration in the aerosol (shown in Figure 2b). This relationship provides a visual representation of how the surface activity of a given molecule can translate into its concentration in the aerosol generated from bubble films at the liquid−air interface. As the carbon number of the LC increases, the surface activity increases as well, yielding a higher Γ value. Assuming that the mechanism of transferring organics into aerosol produced from a bubble film is mainly a function of surface concentration, then the relative concentrations of the LCs measured in the aerosol phase are expected be similar to their relative Γ values. Figure 2c shows that the trend in the values of Γ is comparable to that for [LC]aerosol (background corrected; see the SI for method details), with the most surface-active LC (octanoate) being present at higher concentrations than the other less surface-active species. According to the van der Waals model and experimental observations, butanoic acid, being considered infinitely soluble in pure water, requires large concentrations (≫1 mM) before appreciable surface adsorption is observed.16 In this respect, for butanoate, Γ ≈ 0. However, recent work has shown that even formic acid, this simplest and most water-soluble carboxylic acid, has some affinity for the air−water interface,17 explaining the observation of butanoate in the aerosol phase in this work. Alternatively, nonequilibrium mixing of bulk solution within the bubble film may also contribute to the transfer of more soluble constituents, such as butanoate, into film drops. The trend observed for the selective transfer of LCs from the air−water interface to the aerosol phase was not similarly observed for LDCs. This suggests that for LDCs, the lower surface activity results in a decrease in their transfer across the air−water interface. While a similar trend is observed between the estimated Γ and [LDC]aerosol, slight deviations between the relative 1693

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Figure 3. Enrichment factors (calculated by eq 2) for (a) the total linear LCs, (b) chloride (Cl−) and sulfate (SO42−), and (c) individual inorganic cations relative in size-resolved aerosol generated by a sintered glass bubbler system. EF values for the upper three stages (aerosol diameter ≥ 5.6 μm) are not shown as the measured concentration of either the species of interest or Na+ was below the method detection limit. Error bars represent the 95% confidence intervals for triplicate experiments.

Figure 2. (a) Estimated surface excess concentrations (Γ; moles of acid per surface area of solution) for linear alkanoic acids (n = 5−12) as a function of solution concentration as described by van der Waals adsorption isotherms. Estimated surface concentrations for linear alkanoic acids with carbon numbers 4−9 at the solution concentrations used in this study ([LC]i,solution = 1 mM) are shown on the left in (b) and compared to the measured concentrations of the LCs in the aerosol phase ([LC]i,aerosol; moles of LC per volume of air sampled from the sintered glass bubbler system). The plot in (a) was recreated with the permission from ref 12, with the curve for butanoic acid estimated in this work based on the models described for carbon numbers 5−12.

+

potassium (K+) (EFKNa+ = 0.72 ± 0.05) and magnesium (Mg2+) 2+ (EFMg Na+ = 0.83 ± 0.05) and follows a similar trend as the enrichment of the total LC fraction across the size-resolved aerosol (Figure 3c). This agrees well with previous evidence showing stronger interactions between carboxylate moieties (COO−) and Ca2+ compared to Mg2+.20 Furthermore, Shaloski et al. have recently shown that the surface activity of anionic surfactants is enhanced in the presence of Ca2+ compared to that for Na+.21 For monovalent cations, K+ was depleted in the + aerosol phase relative to Na+ (EFKNa+ = 0.72 ± 0.05) across the full aerosol size range, which agrees well with previous work reporting stronger binding affinities of Na+ with COO− compared to those of K+.22 The trend in the calculated values in size-resolved aerosol shows depletion of chloride (Cl−) relative to Na+ in the aerosol size ranges where LC enrichment is greatest (Figure 3b). Sulfate (SO42−) was also depleted relative to Na+ in the same aerosol size ranges as Cl−. Here, we hypothesize that the enrichment of the negatively charged COO− (LCs) at the liquid−air interface causes inorganic anions to be depleted. Taking these data all together, the enrichment of LCs and inorganic cations as well as depletion of inorganic anions in the aerosol phase, a representation of the liquid−air interface of a bubble showing the relative distribution of LCs, LDCs, and inorganic salts is shown in Figure 4. Here, LCs at the surface are coordinated with mono- and divalent inorganic cations. LDCs reside more in the bubble film than at the surface and may also complex with inorganic cations. Free inorganic anions are then distributed away from the surface to avoid excess negative charge. In summary, estimated surface adsorption is compared to measured concentrations in aerosol to better understand the selectivity by which a molecule at the liquid−air interface is

[LC]aerosol and Γ values are apparent. This is likely due to the presence of a more complicated and synergistic mechanism involving other interactions including those with cations present in the seawater solution and other species in solution for even more complex solutions. This added complexity would further alter expected concentrations from an ideal solution assumed in estimating Γ. With the pH of the seawater environment at 8.1, nearly all (>99.9%) of the LCs and LDCs are in their carboxylate form. Therefore, both mono- and divalent cations present in the seawater solution may favorably complex with the negatively charged carboxylate groups of the LCs and LDCs.18,19 These counterions will then accompany the surface-active LCs as they are transferred into the aerosol phase. To compare the selectivity in the transfer of species (x) across the air−sea interface, enrichment factors (EFs) normalized to sodium (Na+) were calculated for the aerosol relative to the bulk solution (sol) with eq 2 x + EFNa

=

[x]i ,aerosol /[Na +]aerosol [x]i ,sol /[Na +]sol LC EFTotal Na+

(2)

EFion Na+

Calculated values of and for size-resolved aerosol are shown in Figure 3. The enrichment of calcium 2+ (Ca2+) (EFCa Na+ = 1.14 ± 0.02) is greater than that of both 1694

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The Journal of Physical Chemistry Letters Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation through the Centers of Chemical Innovation Program under Grant CHE1305427.

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(1) Quinn, P. K.; Collins, D. B.; Grassian, V. H.; Prather, K. A.; Bates, T. S. Chemistry and Related Properties of Freshly Emitted Sea Spray Aerosol. Chem. Rev. 2015, 115, 4383−4399. (2) Prather, K. A.; Bertram, T. H.; Grassian, V. H.; Deane, G. B.; Stokes, M. D.; Demott, P. J.; Aluwihare, L. I.; Palenik, B. P.; Azam, F.; Seinfeld, J. H.; et al. Bringing the Ocean Into the Laboratory to Probe the Chemical Complexity of Sea Spray Aerosol. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 7550−7555. (3) Jungwirth, P.; Tobias, D. J. Specific Ion Effects at the Air/Water Interface. Chem. Rev. 2006, 106, 1259−1281. (4) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Design and Characterization of Crystalline Thin Film Architectures at the Air-Liquid Interface: Simplicity to Complexity. Chem. Rev. 2001, 101, 1659−1696. (5) Donaldson, D. J.; Vaida, V. The Influence of Organic Films at the Air-Aqueous Boundary on Atmospheric Processes. Chem. Rev. 2006, 106, 1445−1461. (6) Lewis, E. R.; Schwartz, S. E. Sea Salt Aerosol Production: Mechanisms, Methods, Measurements and Models, 1st ed.; American Geophysical Union: Washington, DC, 2004. (7) Burrows, S. M.; Ogunro, O.; Frossard, A. A.; Russell, L. M.; Rasch, P. J.; Elliott, S. M. A Physically Based Framework For Modeling the Organic Fractionation of Sea Spray Aerosol From Bubble Film Langmuir Equilibria. Atmos. Chem. Phys. 2014, 14, 13601−13629. (8) Elliott, S.; Burrows, S. M.; Deal, C.; Liu, X.; Long, M.; Ogunro, O.; Russell, L. M.; Wingenter, O. Prospects for Simulating Macromolecular Surfactant Chemistry at the Ocean−Atmosphere Boundary. Environ. Res. Lett. 2014, 9, 064012/1−064012/9. (9) Gilman, J. B.; Tervahattu, H.; Vaida, V. Interfacial Properties of Mixed Films of Long-Chain Organics at the Air-Water Interface. Atmos. Environ. 2006, 40, 6606−6614. (10) Wang, X.; Sultana, C. M.; Trueblood, J.; Hill, T. C. J.; Malfatti, F.; Lee, C.; Laskina, O.; Moore, K. A.; Beall, C. M.; McCluskey, C. S.; et al. Microbial Control of Sea Spray Aerosol Composition: A Tale of Two Blooms. ACS Cent. Sci. 2015, 1, 124−131. (11) Russell, L. M.; Hawkins, L. N.; Frossard, A. A.; Quinn, P. K.; Bates, T. S. Carbohydrate-like Composition of Submicron Atmospheric Particles and Their Production From Ocean Bubble Bursting. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6652−6657. (12) Cochran, R. E.; Laskina, O.; Jayarathne, T.; Laskin, A.; Laskin, J.; Lin, P.; Sultana, C. M.; Lee, C.; Moore, K. A.; Cappa, C. D.; et al. Analysis of Organic Anionic Surfactants in Fine (PM2.5) and Coarse (PM10) Fractions of Freshly Emitted Sea Spray Aerosol. Environ. Sci. Technol. 2016, 50, 2477−2486. (13) Danov, K. D.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Interpretation of Surface-Tension Isotherms of N-Alkanoic (Fatty) Acids by Means of the van Der Waals Model. J. Colloid Interface Sci. 2006, 300, 809−813. (14) Collins, D. B.; Zhao, D. F.; Ruppel, M. J.; Laskina, O.; Grandquist, J. R.; Modini, R. L.; Stokes, M. D.; Russell, L. M.; Bertram, T. H.; Grassian, V. H.; et al. Direct Aerosol Chemical Composition Measurements to Evaluate the Physicochemical Differences Between Controlled Sea Spray Aerosol Generation Schemes. Atmos. Meas. Tech. 2014, 7, 3667−3683. (15) Keene, W. C.; Maring, H.; Maben, J. R.; Kieber, D. J.; Pszenny, A. A. P.; Dahl, E. E.; Izaguirre, M. A.; Davis, A. J.; Long, M. S.; Zhou, X.; et al. Chemical and Physical Characteristics of Nascent Aerosols Produced by Bursting Bubbles at a Model Air-Sea Interface. J. Geophys. Res. 2007, 112, 1−16.

Figure 4. Cartoon representation of LC and DLC molecules along with the inorganic salts within the bubble film. LCs at the air−water interface are more efficiently transferred into the aerosol phase relative to the DLC molecules residing in the bulk solution away from the interface. Strong interactions between LCs and calcium at the air− water interface lead to the selective enrichment of calcium in the resulting aerosol.

transferred to the aerosol phase during the bubble bursting process. For the most part, theory explains well the trends observed in the aerosol composition. This suggests that surface activity plays a key role in the transfer of molecules from the air−water interface to film drops produced during the bubble bursting process. However, nonequilibrium mixing of bulk solution with molecules at the air−liquid interface during film drop formation results in the transfer of a small amount of the more water-soluble components, such as butanoate. Furthermore, interactions between cations and the LCs and DLCs are not accounted for in these thermodynamic calculations, and only through more molecular level probes of the interface can these interactions be better understood. The measurements shown here support the need for further development of interfacial probes and a more detailed quantum chemical approach in understanding the detailed interaction between the organic surfactant and inorganic ions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00489. Descriptions of the aerosol generation methods, online aerosol measurement systems, and the methods for quantifying the organic and inorganic constituents in the aerosol and solution (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel. 858-534-2499 (V.H.G.). *E-mail: [email protected]. Tel. 319-384-1863 (E.A.S.). 1695

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