Miscibility of Perfluorododecanoic Acid with Organic Acids at the Air

J. Phys. Chem. C 2007, 111, 9975-9980

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Miscibility of Perfluorododecanoic Acid with Organic Acids at the Air-Water Interface Nabilah Rontu and Veronica Vaida* Department of Chemistry and Biochemistry and CIRES, Campus Box 215, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: March 6, 2007; In Final Form: April 30, 2007

Field measurements have established that a significant percentage of atmospheric aerosol composition consists of organics. Perfluorinated compounds have been used in industry over the past 50 years, and recently, they have been detected in the atmosphere, environmental waters, the Arctic food chain, tissues of animals, and human blood. In general, fluorinated compounds are better surfactants than their hydrocarbon analogs. In this work, isotherms of perfluorododecanoic acid, stearic acid, and lauric acid were studied using a LangmuirBlodgett trough. Isotherms of the pure and mixed films at varying concentrations were collected and characterized to determine their stability and miscibility. Our results indicate that perfluorododecanoic acid forms miscible films in mixtures with stearic and lauric acid. The impetus for this research was to better understand the heterogeneous chemistry of mixed films containing perfluorinated compounds. Consequences of these results for the fate of perfluorinated acids in the environment are discussed.

Introduction Over the past 50 years, fluorinated alkyl substances have had numerous industrial applications.1 Fully fluorinated organic compounds exhibit unique physical and chemical properties, such as high stability due to their thermal and chemical inertness, low solubilities for long-chain perfluoroalkyl compounds in polar and nonpolar solvents, and high densities and compressibilities compared to their hydrocarbon counterparts.2 Their uses are many. For example, perfluorinated carboxylic acids or their salts are used in the synthesis of fluoropolymers;3 small-chain perfluoroalkyl sulfonates act as surface protectors in carpets, leather, paper, packaging, and upholstery;4 and fluorotelomer alcohols and olefins are intermediates in products such as surfactants and polymers.5 Recently, some perfluorinated acids have been detected in the tissues of animals, in environmental waters, and in precipitation.6 For example, trifluoroacetic acid was observed in the 45-433 ng/L concentration range in the rivers and lake systems of California and Nevada.7 Typical wastewater concentrations are in the 0.34-3.35 mM range for perfluorooctanoic acid8 and 400 ng/L for perfluorooctane sulfonate.9 Low levels (approximately 1-1000 ng/L) of shorterchain (2-7 carbons) perfluorocarboxylic acids are present in rainwater,10 and other perfluorinated compounds have even been observed in the Arctic food chain7,11 and detected in human blood12-14 and the tissues of animals.15,16 Generally, perfluorinated carboxylic acids are very efficient surfactants.17 Surfactants effectively lower the surface tension of a medium by selectively adsorbing on the interface.4 Perfluorinated carboxylic acids are amphiphiles, unique molecules that contain both a hydrophobic tail and a hydrophilic head group. In aqueous systems, the antipathy of the hydrophobic group for water combined with the affinity of the hydrophilic head group for water gives rise to surfactant properties. Amphiphiles have an intrinsic ability to form selfassembled films and partition to the interface. When adsorbed * To whom correspondence should be addressed. Tel.: (303) 492-8605. Fax: (303) 492-5894. E-mail: [email protected].

on substrates, fluorinated surfactants not only repel water but repel oils and fats as well. Fluorinated surfactants are much more surface-active than their hydrocarbon counterparts, efficiently lower the surface tension of aqueous systems at very low concentrations,4 and exhibit unique partition and adsorption behavior.18 Perfluorinated carboxylic acids are known to move to the air-water interface and thereby lower the surface tension of aqueous solutions dramatically.19 A significant fraction of atmospheric aerosol composition is known to consist of organics,20-34 and amphiphiles constitute a large portion of the organic content found on collected atmospheric aerosols.35-39 The organics can either partition to the interface27,40-42 or reside in the aqueous bulk.43,44 The airwater interface of atmospheric aerosols has the ability to concentrate organics at the surface. “Inverted micelle” structures consisting of an aqueous core and an organic surface film for aqueous atmospheric aerosols were previously proposed by Gill et al.45 and Ellison et al.46 In these representations, interactions between the polar head group and the aqueous ionic brine core hold the film of hydrocarbons that coats the aerosol particle to the surface, while the hydrophobic tails are exposed to the air. Exposure of the hydrocarbon chain to the highly oxidizing troposphere leads to atmospheric processing of the organic aerosol particle.46 The chemical species residing on the surface of the aerosol particle dictates many properties of the aerosol, such as its reactivity toward oxidative gases and transport of gases across the interface.47,48 When a substance is in the gas phase or sorbed on a particle or water in the atmosphere, longrange transport can occur via air. Because these compounds are well-mixed and long-lived in the troposphere, water solubility, adsorption, and vapor pressure are important parameters in considering the potential for long-range transport of perfluorinated compounds. Organic-rich atmospheric aerosol particles might increase the long-range transport and widespread global distribution of perfluorinated compounds. In this study, perfluorododecanoic acid [CF3(CF2)10COOH] was used as a model for perfluoro acids. The ability of the airwater interface to concentrate perfluorododecanoic acid in pure

10.1021/jp0718395 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007

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Figure 1. Molecular orientation of surfactants at the air-water interface as a function of barrier positions.

and binary mixed films with stearic and lauric acid was investigated with the use of a Langmuir-Blodgett trough. Differences in the shape of the isotherms, along with the molecular footprints and the surface pressures of the carboxylic acids studied, are discussed in detail. Perfluorododecanoic acid was used at varying concentration in the binary mixed films to determine its miscibility with the other two organics. The results obtained from this study are intended to serve as a representative model of aqueous atmospheric aerosol particles containing perfluorinated compounds. A significant outcome of the ability of the air-water interface to concentrate and stabilize perfluorododecanoic acid at the surfaces of atmospheric aerosol particles is the potential for their transport, distribution, and deposition. Experimental Methods All reagents and solvents were used as received without further purification. Stearic acid (octadecanoic acid, >98%), lauric acid (dodecanoic acid, 99.5%), and perfluorododecanoic acid (95%, denoted PFDDA) were purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI). ACS-grade hexanes were obtained from Fisher Scientific (Pittsburgh, PA), and ethanol was purchased from AAPER Alcohol and Chemical Co. (Shelbyville, KT). HPLC-grade water was acquired from Brudick and Jackson (Muskegon, MI). Two single-component surfactant-spreading solutions were made with stearic and lauric acid in hexanes. The perfluorododecanoic acid solution was made from a 9:1 (v:v) mixture of hexanes and ethanol. All solutions had a concentration of 10-3 M. Binary solutions of stearic and perfluorododecanoic acid and lauric and perfluorododecanoic acid were prepared from their respective 10-3 M solutions so that the ratio of perfluorinated carboxylic acid to either stearic or lauric acid composed 25%, 50%, and 75% of the mixture. The instrument used for forming the films consisted of a Langmuir trough (52 × 7 × 0.05 cm) with two computercontrolled mechanical barriers. The barrier positions ranged from 327.2 cm2, the maximum allowable, to 16.8 cm2 when closed. A Wilhelmy plate of chromatography paper (Whatman Chr 1) suspended from a balance measured the surface pressure as a function of surface area between the trough barriers. The computer interface and the software used were purchased from NIMA Technology Ltd., Coventry, U.K. Values of the molecular weight, concentration, and volume for the surfactant-spreading solutions were entered into the software, which calculated the molecular areas at the various barrier positions. See Figure 1 for details pertaining to the molecular orientation of the surfactant as a function of barrier position. An aspirator was used to remove surface contaminants on the subphase between the trough barriers. A known volume (50-200 µL) of each surfactant-spreading solution was added dropwise onto the HPLC-grade water subphase between the open barriers and allowed to spontaneously spread across the interface and self-assemble to form an organic film. The solvent was allowed to evaporate for 20 min prior to collection of the isotherms of the films. Surface pressure-area isotherms were collected at room temperature as the film was compressed at a constant speed of 100 cm2/min. The pH of the subphase was

Figure 2. Equimolar isotherms of stearic, lauric, and perfluorododecanoic acid (PFDDA).

monitored and remained constant (approximately 5) throughout the course of collecting the isotherms. For each film component, three reproducible isotherms were obtained. Results and Discussion The shape of a surface pressure vs molecular area isotherm at the air-water interface relates to the general molecular orientation of a surfactant due to the barrier positions and is shown in Figure 1. Isotherms of typical fatty acids usually exhibit three distinct regions. As the mechanical barriers are being closed, there is a gradual onset of surface pressure until an approximately horizontal region is reached. This is known as the disordered region where the hydrophobic chains are lifted away from the surface. Because very weak interactions exist between the water and the tail groups, this region is often referred to as a 2-D gas. As the barriers continue to compress the film, a second abrupt transition occurs, causing another rise of the surface pressure, forming a liquid-expanded state. In this region, the hydrophobic tails of the molecules experience more intermolecular attractions than in the 2-D gas state, but the polar head groups are not quite in contact with one another. Continued compression of the barriers leads to the last phase change, to the liquid-condensed region, signaled by a sharp increase of the slope. The molecules now form a solid-like arrangement and are in rigid contact with one another. If the film is compressed beyond the liquid-condensed region, the phenomenon of collapse occurs, causing the molecular layers to ride on top of each other and form disordered multilayers. A linear regression of the steep, linear part of the isotherm where the last phase transition arises yields the surface area per molecule, also known as the molecular footprint. Pure Acids. The surface pressure-area isotherms of pure equimolar (10-3 M solutions) samples of stearic, lauric, and perfluorododecanoic acid were collected and are shown in Figure 2. As can be seen in the figure, the isotherms of the three acids have very different shapes and molecular footprints. Perfluorododecanoic acid has the largest molecular footprint and collapse pressure, followed by stearic acid and then lauric acid. Of the carboxylic acids reported here, the isotherm of stearic acid is a textbook example of the three distinct regions, clearly showing the disordered (0-0.1 mN/m), liquid-expanded (0.127 mN/m), and liquid-condensed phases (27-63 mN/m). By linear regression of the steep, liquid-condensed part of the Langmuir isotherm, the molecular footprint for stearic acid was calculated as 22 Å2/molecule which is in excellent agreement with the literature value.49 The stearic acid film collapses at approximately 60 mN/m as expected.49 Even though stearic acid has the longest carbon chain of the carboxylic acids studied,

Miscibility of Perfluorododecanoic Acid with Organic Acids PFDDA, with six fewer carbons in its backbone, has the largest molecular footprint and collapse pressure for reasons explained later. The isotherm of lauric acid has a much different shape than that of stearic acid even though the molecules differ in size only by a six-carbon chain. The carbon chain length affects the molecular packing and the degree of molecular interactions between the individual components. Even though lauric acid does collapse at around 37 mN/m, it does not exhibit the three characteristic phase regions as shown in the stearic acid isotherm. It is difficult to determine accurately the molecular footprint when phase distinctions cannot be made from the isotherm. Fatty acids are known to have a molecular footprint of approximately 20-22 Å2/molecule, which is consistent with our data for stearic acid. However, in a study by Hutchinson,50 the area per molecule of lauric acid was determined as a function of mole fraction. The trend between the mole fraction and the molecular footprint showed an exponential relationship, with the footprint decreasing at increased mole fractions. By extrapolating the exponential fit to a mole fraction of 1, the calculated molecular footprint of lauric acid based on Hutchinson’s data was determined to be 16.8 Å2/molecule. A linear regression of the steep portion of the experimental isotherm yields a molecular footprint of 16.6 Å2/molecule. PFDDA not only has a much higher surface and collapse pressure but also has a more structured isotherm than its hydrocarbon analog, lauric acid. The efficiency of a fluorinated surfactant in lowering the surface tension is mainly determined by the structure of the hydrophobic group. The increase of the collapse pressure is due to the strong electronegative nature of the fluorine atoms.2,51 Similarly to stearic acid, the isotherm of PFDDA shows the three distinct phase regions even though that of lauric acid does not. As a result of the hydrophobic chain of PFDDA being more densely packed than that of lauric acid, fluorination forms more stable monolayers.2 The extrapolated surface area of PFDDA is 31.6 Å2/molecule, which is in good agreement with previous data.2,52-57 As shown in the isotherm of PFDDA, a slight bump is present in the liquid-expanded region. We are certain that the bump is not due to an impurity because it consistently appeared in the same region every time an isotherm of PFDDA was collected and also because it has been reported previously.52 The reason for this feature is unknown; however, it is a characteristic of the presence of fluorination, as will be seen in the section on mixed films. The molecular footprint of perfluorododecanoic acid is approximately twice as large as that of its hydrocarbon analog acid. Perfluorocarbon chains are apolar and interact only by dispersive forces. Perfluoroalkanes have a much smaller van der Waals interaction energy per molecular contact area as a result of the highly rigid perfluorocarbon chain. Therefore, fluorocarbons are known to form more expanded monolayers than their hydrocarbon analogs and to have a higher density than their corresponding alkanes.58-61 Compared to hydrocarbon surfactants of comparable chain length, fluorocarbons act as better surfactants. They are much more surface-active than their corresponding hydrocarbons analogs.62 The relative affinities with water for perfluorocarbons and hydrocarbons can be expressed in terms of the free energy required to separate a unit area of oil-water contact. The free energy is the work of adhesion, Wa, given by63

Wa ) γoa + γwa + γow

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Figure 3. Isotherms of stearic acid/perfluorododecanoic acid at different concentrations.

chain, its interaction with water is weaker than that of the hydrocarbon chain. The low cohesive energy per volume of fluorocarbons is also a result of their low polarizability and is reflected in the values of the surface tensions, as fluorocarbon values are approximately 6-8 mN/m lower than those of their corresponding alkanes,63 and consequently, fluorocarbons have higher surface pressures. Mixed Films. Three combinations of a binary mixture of perfluorododecanoic acid with stearic and lauric acid were studied to determine miscibility. Figure 3 shows the isotherms of the two-component mixture of perfluorododecanoic and stearic acid. As is apparent from the figure, the isotherms of the mixed film are not as structured as the pure isotherm of stearic acid and more closely resemble the shape of the PFDDA isotherm. There is also a slight bump in the isotherms, which indicates the presence of the perfluorinated carboxylic acid. Regardless of the percent of PFDDA in the mixture, the isotherms of the mixed films studied are all similar in their structure, shape, collapse pressure, and footprints. The average molecular footprint of the mixture increases as a function of the percent of stearic acid in the mixture. At 25%, 50%, and 75% stearic acid, the average molecular footprints of the mixtures are 30.6, 31.7, and 33.8 Å2/molecule, respectively. Two liquids that are ideally miscible are usually physically and chemically similar and exhibit identical intermolecular forces. In the case where a binary mixture shows complete immiscibility, there are strong attractive interactions among like molecules and almost no interaction between the unlike molecules.64 Because the isotherms of the mixed films shown in Figure 3 are more expanded than their respective individual isotherms, we ruled out the possibility that the different molecules do not interact. Furthermore, the collapse pressure is a very useful guide to determine miscibility. Resultant monolayers of two components that are immiscible have welldefined and different collapse pressures. The isotherm of such a film would show collapse at the lower value of one component followed by collapse at the higher value of the other component. If the materials are immiscible, the components will exist as domains. A true mixture yields only a single collapse as exhibited in Figure 3. Assuming ideal behavior, the average area per molecule of any mixture is the weighted sum of the areas occupied by the individual species at the surface, as given by

Aavg ) N1A1 + N2A2

(2)

(1)

where γoa, γwa, and γow are the surface tensions of the oilwater, water-air, and oil-water interfaces, respectively. As a consequence of the low polarizability of the perfluorocarbon

where Aavg is the calculated average area occupied per molecule of the mixed monolayer, N1 and N2 are the corresponding mole fractions of the single components, and A1 and A2 are the areas per molecule of the pure components. The two components in

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TABLE 1: Comparison of Experimental and Calculated Average Molecular Footprints of Stearic Acid/PFDDA Mixture stearic acid (%)

experimental footprint (Å2/molecule)

calculated footprint (Å2/molecule)

AEx

25 50 75

30.6 31.7 33.8

29.3 26.8 24.4

1.3 4.9 9.4

the mixture can be completely miscible or show complete phase separation and form an isolated monolayer of one compound suspended in a monolayer of the other component.65 The additivity rule gives the expected area per molecule for ideal mixing or complete phase separation. The excess area of mixing, AEx, provides a measure of nonideality and is defined as

AEx ) Aact - Aavg

(3)

where Aact is the actual area per molecule of the binary system. If a binary mixture of molecules exhibits either ideal mixing or complete demixing, then AEx ) 0.66 Whereas an immiscible film follows the additivity rule, a miscible film deviates from this behavior, indicating that there are molecular interactions between the film components. Table 1 summarizes the experimental and calculated molecular footprints of the stearic/perfluorododecanoic acid mixture. As shown in the table, the mixture of stearic acid with perfluorododecanoic acid shows a positive deviation from ideality. This occurs because of a repulsive interaction between the two components. Shibata et al.52 reported complete miscibility in a binary mixture between a shorter perfluorocarbon chain and a longer hydrocarbon chain, which is consistent with our results that show deviation from the additivity rule line. Similar to our results for this work, previous studies of the mixing of fatty acids with significantly different chain lengths showed a positive deviation.67 Literature studies have also shown that the introduction of perfluoroalkyl groups promotes the tendency of phase separation when mixed with hydrocarbon surfactants.68-70 In addition to arguments based on the additivity rule described above, several other factors were used to determine the miscibility of the films. For example, the breakpoint of the phase transition from the liquid-expanded to the condensed film state, K1, is a useful guide in determining miscibility.71 If K1 is a function of concentration, then the isotherms point to complete miscibility between the two components at equilibrium. For the binary mixture of stearic and perfluorododecanoic acid, K1 is a function of concentration, which is a sign of miscibility of the two compounds at the air-water interface. Last, the collapse pressure of the film can also be used as a criterion of miscibility. Similarly to the breakpoint of phase transition K1, a collapse pressure that is a function of concentration is a sign of miscibility.71 Although the collapse pressures of the three different stearic/perfluorododecanoic acid mixtures were too close in value to signify any quantitative comparison, the qualitative analysis of the presence of only one collapse pressure suggests a miscible film. Thus, the additivity rule, K1, and collapse pressure all point to miscibility for binary mixtures of stearic and perfluorododecanoic acid. Isotherms of a binary mixture of lauric acid and perfluorododecanoic acid were also collected at three different concentrations. In this case, the two surfactants have the same number of carbon atoms. Figure 4 shows the surface pressure-area isotherms of the binary mixture. As indicated in the figure, the shape of the isotherm of the mixture is much more structured than the isotherm of the pure lauric acid sample, and the collapse

Figure 4. Isotherms of lauric acid/perfluorododecanoic acid at different concentrations.

TABLE 2: Comparison of Experimental and Calculated Average Molecular Footprints of Lauric Acid/PFDDA Mixture lauric acid (%)

experimental footprint (Å2/molecule)

calculated footprint (Å2/molecule)

AEx

25 50 75

29.5 21.1 13.1

27.9 24.2 20.4

1.6 -3.1 -7.3

pressure of the mixture is significantly higher than that of lauric acid. The increase in the surface area per molecule as a function of percentage of PFDDA helps point to the fact that lauric acid is stabilized by the presence of the perfluorinated carboxylic acid. The isotherms of the mixture fall between the isotherms of the pure samples. Hussain et al.66 showed that the presence of a good surfactant helps stabilize films that are not very structured on their own. Gilman et al.72 also showed the same trend in a different system. In this case, PFDDA is the better surfactant of the two and stabilizes the film of lauric acid. This confirms that fluorinated compounds are much better surfactants than their hydrocarbon counterparts. The average molecular footprints of the lauric/perfluorododecanoic acid mixture at 25%, 50%, and 75% lauric acid were determined to be 29.5, 21.1, and 13.1 Å2/molecule, respectively. The work of Shibata et al., carried out in a different system than ours (pH 1, carbon chain length ) 14), indicated that, in the case where mixtures of perfluorinated and hydrocarbon carboxylic acids have the same carbon chain length, the film is immiscible and the two phases are in complete separation.52 Our experimental conditions are different, with the subphase pH at approximately 5 and the carbon chain length of the carboxylic acids at 12. Using the additivity rule, our results, on the other hand, reveal that the mixture is completely miscible. The collapse pressures of the individual components and the mixed films studied here does not point to immiscibility. Furthermore, K1 changes with concentration, thus indicating complete miscibility. The experimental and calculated average molecular footprints of the lauric/perfluorododecanoic acid mixture are summarized in Table 2. The average molecular footprint of the mixture decreases as the percentage of lauric acid in the mixture increases. At larger concentrations of lauric acid, there is a negative deviation from ideality that signifies not only an attractive interaction but also miscibility in both the liquid-expanded and liquid-condensed states. In the atmosphere, the surface activity of perfluorinated compounds residing at the air-water interface can have several important consequences. We realize that atmospherically relevant concentrations of perfluoro acids are significantly smaller than those reported here, but the results can be extrapolated to lower concentrations. More importantly, we feel that the

Miscibility of Perfluorododecanoic Acid with Organic Acids behavior of perfluoro acids in mixed films would not be drastically affected at smaller concentration and would still bear interesting atmospheric outcomes. First, atmospheric aerosol particles have the potential to transport perfluorinated compounds and contribute to their growing environmental distribution. In addition to aquatic transport, degradation of volatile precursors, and direct release into the atmosphere, aqueous aerosol particles are a plausible candidate for transporting and releasing these compounds in the atmosphere. The perfluorinated compounds might represent only a small percentage of the organic content found on atmospheric aerosols, but with regard to fluorinated compounds as a whole, they might constitute a large fraction. Because longer-chain perfluorocarboxylic acids are not very water soluble, atmospheric aerosol particles have the potential to concentrate, transport, and distribute these compounds. For example, Rontu et al.73 recently showed that fluorotelomer alcohols, which, although very volatile, are surface-active, partition to the air-water interface, and are stabilized in mixed films, have the potential to be transported in the troposphere via atmospheric aerosol particles. Next, because perfluorinated compounds act as better surfactants than their hydrocarbon analogs (as demonstrated in this study), they have the ability to stabilize organic compounds at the air-water interface. This means that molecules have the potential to undergo further chemistry at the air-water interface that would not have been possible without the presence of the fluorinated compound. Most importantly, little work has been done on understanding the heterogeneous chemistry of perfluorinated compounds although extensive studies of their gas-phase chemistry have been performed.74-79 In Earth’s atmosphere, rapid oxidization of biogenic and anthropogenic emissions by OH, O3, and NO3, produces oxidized organic and inorganic compounds. The processing of organic films with OH and O3,80-86 heterogeneous reaction kinetics,87-89 and uptake across the interface90-92 are all under study, but less is known about fluorinated compounds. Compared to the C-H bond, the C-F bond is much stronger and more anharmonic,93 and thus, in terms of oxidation reactions, one does not expect these compounds to react by abstraction by OH or O3 as do their hydrocarbon analogs. The heterogeneous chemistry of molecules is vastly different from their gas-phase chemistry, and therefore, it is crucial to investigate reactions of perfluorinated compounds at the airwater interface to fully understand their atmospheric fate and contribution to fluorinated compounds as a whole. Conclusion Few data are available concerning the atmospheric transport mechanisms of perfluorinated compounds. In this study, we used Langmuir films at the air-water interface as a representative model of organic films on environmental surfaces, models for films found on oceans and aqueous aerosols. Recent field studies have shown that atmospheric aerosol particles contain large mass fractions of organics.20-34 If the aerosol particles contain any perfluorinated compounds, they have the potential to transport and distribute them and account for their growing environmental distribution. We have shown in this present work that PFDDA is a very good surfactant and demonstrated that it is more surface-active than its hydrocarbon counterpart. PFDDA can act as a co-surfactant in mixtures with other organics. It was also shown to have a stabilizing effect in mixtures with other organics, which can have important atmospheric consequences. To fully understand the fate of perfluorinated compounds at the air-water interface, further research needs to be done on

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