Selective Solubilization of Polycyclic Aromatic Hydrocarbons from

Oct 5, 2004 - Montreal, Quebec, Canada H3A 2K6. This research investigates ... water equilibrium partition coefficient of the PAHs increased with the ...
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Environ. Sci. Technol. 2004, 38, 5878-5887

Selective Solubilization of Polycyclic Aromatic Hydrocarbons from Multicomponent Nonaqueous-Phase Liquids into Nonionic Surfactant Micelles† LETICIA A. BERNARDEZ AND SUBHASIS GHOSHAL* Department of Civil Engineering, McGill University, Macdonald Engineering Building, 817 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6

This research investigates the equilibrium solubilization behavior of naphthalene and phenanthrene from multicomponent nonaqueous-phase liquids (NAPLs) by five different polyoxyethylene nonionic surfactants. The overall goal of the study was to achieve an improved understanding of surfactant-aided dissolution of polycyclic aromatic hydrocarbons (PAHs) from multicomponent NAPLs in the context of surfactant-enhanced remediation of contaminated sites. The extent of solubilization of the PAHs in the surfactant micelles increased linearly with the PAH mole fraction in the NAPL. The solubilization extent and micellewater equilibrium partition coefficient of the PAHs increased with the size of the polar shell region of the micelles rather than the size of the hydrophobic core of the micelle. The presence of both PAHs in the shell region of the micelles was confirmed by 1H NMR analysis. This is an important observation because it is commonly assumed that in multi-solute systems the solutes with relatively greater hydrophobicity are solubilized only in the micellar core. A comparison of the 1H NMR spectra of pure surfactant solutions and solutions contacted with various NAPLs demonstrated that the distribution of PAHs between the shell and the core changed with the concentration of PAHs in the micelles and in the NAPL. Competitive solubilization of the PAHs was observed when both PAHs were present in the NAPL. For example, in surfactant solutions of Brij 35 and Tween 80, the solubilization of phenanthrene was decreased in the presence of naphthalene as compared to systems that contained phenanthrene as the only solute. In contrast, with micellar solutions of Tergitol NP-10 and Triton X-100, phenanthrene solubilization was enhanced in the presence of naphthalene. The activity coefficients of the PAHs in the micellar phase were generally found to increase with PAH concentrations in the micelle.

Introduction At many sites across North America chemically complex nonaqueous phase liquids (NAPLs) such as coal tar, creosote, * Corresponding author phone: (514)398-6867; fax: (514)398-7361; e-mail: [email protected]. † This paper is part of the Walter J. Weber Jr. tribute issue. 5878

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and petroleum liquids have been released in large volumes into the environment from accidental spills and leaks. These NAPLs are complex mixtures containing a number of polycyclic aromatic hydrocarbons (PAHs), several of which are known carcinogens. While the concentration of each PAH compound in NAPLs may be small, collectively they often comprise a significant fraction. For example, PAH compounds may constitute up to 75 wt % of coals tars (1). PAHs are generally slightly water-soluble and have low volatility and thus are difficult to extract or eliminate at NAPL-contaminated sites. Micellar surfactant solutions can solubilize PAHs several hundred times in excess of their true aqueous solubility, and thus surfactant flushing is a promising approach for remediation of NAPL-contaminated sites. Assessing the equilibrium partitioning of individual PAH solutes from multicomponent NAPLs into micellar solutions is necessary for quantifying equilibrium and nonequilibrium surfactant dissolution of target PAHs. Moreover, a characterization of how the micellar solubilization extent of each PAH solute changes with its relative abundance or mole fraction in the NAPL is important because the mole fraction of each solute in a multicomponent NAPL changes with continued dissolution. A large body of literature exists on the equilibrium partitioning of various environmentally significant solutes, including PAHs, in micellar surfactant solutions. The vast majority of these studies have employed solubilization in systems containing only one solute. Studies where co-solubilization of multiple solutes has been examined have reported that the extents of solubilization of the solutes differ from those in single-solute systems (2-7), as described in the following paragraphs. It has been suggested that the location of solutes within micelles is an important factor influencing the micellar partitioning of solutes in multi-solute systems (2, 5, 6). A two-state model described by Mukerjee (8) considers the hydrophobic core of a micelle and the hydrophilic outer shell as two distinct environments for solubilization, and solutes may be solubilized in either region depending on their molecular properties such as polarity (8, 9). Chaiko et al. (5) and Nagarajan et al. (6) reported that when NAPLs comprised of benzene, hexane, and cyclohexane were contacted with solutions of various cationic and anionic surfactants, it was observed that the extent of solubilization of benzene in the surfactants was not influenced by the presence of cyclohexane and hexane. However, those authors witnessed a synergistic increase in the solubilization of hexane from NAPLs containing small mole fractions of benzene. It was concluded that benzene solubilized in the outer micellar layers caused an increase in the micellar core volume, which in turn increased the solubilization of hexane. McCray et al. (7) investigated the solubilization of toluene, ethylbenzene, and buytlbenzene in solutions of a rhamnolipid biosurfactant and found that in the presence of multiple solutes the relatively hydrophobic compounds experienced solubility enhancements greater than those compared to single-solute systems. The authors attributed aqueous-phase interactions between the co-solutes and the biosurfactant micelles as being responsible for the competitive solubilization but concluded that partitioning of the solutes into the micellar shell did not play a role in the competitive solubilization. To date, only a few studies have systematically investigated the co-solubilization of multiple PAHs in surfactant micelles. Guha et al. (4) investigated the partitioning of aqueous-phase naphthalene, phenanthrene, and pyrene into micelles of the 10.1021/es0497429 CCC: $27.50

 2004 American Chemical Society Published on Web 10/05/2004

TABLE 1. Physicochemical Properties of NAPL Componentsa compounds

mol wtb (g/mol)

solid/liquid fugacity ratioc

log Kow

CAQ,sati e (µmol/L)

KNi e (µmol/L)

γNi e

hexadecane naphthalene

226.45 128.19

1 0.306

6.6d 3.36b

228.7 ( 9.7

phenanthrene

178.24

0.279

4.57b

1240 ( 43.4 (1172 ( 36.9)f 85.5 ( 2.9 (97.7 ( 5.4)f

1.65 ( 0.13 (1.64 ( 0.07)f 4.12 ( 0.24 (4.32 ( 0.22)f

a Error values indicate 95% confidence intervals. in a three-component NAPL.

b

Ref 11. c Ref 12.

6.32 ( 0.22

d

Ref 13. e Ref 2. f Values of KNi and γNi in the presence of the other PAH solute

nonionic surfactant Triton X-100. It was found that in some cases an increase in the solubilization capacity of Triton X-100 for phenanthrene occurred in the presence of naphthalene. Those authors concluded that naphthalene solubilized at the micelle-water interface or in the shell region of the micelles likely increased the solubilization of the other more hydrophobic PAHs. The inverse trend in competitive solubilization was observed by Hill and Ghoshal (2) in systems containing micellar solutions of Brij 35 contacted with NAPLs comprised of hexadecane, naphthalene, and/or phenanthrene. The micellar partitioning of phenanthrene was decreased in systems containing the three-component NAPL in comparison to systems containing a two-component NAPL comprised of hexadecane and phenanthrene. However, the micellar partitioning of naphthalene from the threecomponent NAPL was similar to the two-component NAPL. The striking differences in PAH partitioning patterns in the two studies (2, 7) suggests that selective solubilization of PAHs may be strongly influenced by the micellar characteristics of the nonionic surfactant employed. Results from a prior study (2) show that in NAPLs containing hexadecane and a single solute, such as naphthalene or phenanthrene, the extent of micellar partitioning of naphthalene and phenanthrene increased linearly with their NAPL mole fraction. This linear partitioning behavior was consistent for the five different nonionic surfactants employed, although the extent of partitioning differed from one surfactant to another. Furthermore, from a limited investigation of solubilization of naphthalene and phenanthrene from three-component NAPLs, selective solubilization of naphthalene was observed. This research attempts to improve the understanding of partitioning of PAHs in nonionic surfactant solutions in bisolute systems. The specific objectives were to (i) investigate how equilibrium partitioning of PAHs into micelles of different nonionic surfactants is influenced by changes in the PAH mole fractions in NAPLs containing multiple PAHs; (ii) explain how partitioning of PAHs are influenced by the micellar structure of the different nonionic surfactants; (iii) characterize the extent of selective solubilization of PAH solutes in the micelles of different nonionic surfactants; and, (iv) identify if the locus of solubilization of the PAH solutes within the micelles influences partitioning behavior. NAPLs were synthesized in the laboratory by dissolving naphthalene and phenanthrene in hexadecane at different mole fractions. The synthesized NAPLs were used as surrogate for complex multicomponent NAPLs. Five different nonionic surfactants (Brij 30, Brij 35, Tergitol NP-10, Triton X-100, and Tween 80) were used. These surfactants have been employed in many studies on surfactant solubilization (2-4, 10). The equilibrium partitioning of naphthalene and phenanthrene was characterized by the molar solubilization ratio (MSRi) defined as the ratio of the number of moles of solute (i) solubilized into micelles per mole of surfactant in the micelle form and a micelle-water partitioning coefficient (KMi) that describes the partitioning of a given solute between the micelle, aqueous pseudophase, and the NAPL (2). The extent

of selective solubilization of naphthalene and phenanthrene in different surfactant solutions was determined by comparing the KMi of each solute in the three-component NAPL with those obtained from single-solute, two-component NAPL systems. The locus of solubilization of naphthalene and phenanthrene in the micelles of surfactant solutions contacted with the different NAPLs were obtained from measurements of proton shifts in NMR analysis. Calculations of the activity coefficient of the PAHs in the micellar phase were made, and the changes in the magnitude of the activity coefficients with different PAH mole fractions in the NAPL were used to assess how the affinity of the PAHs to the micellar environment was altered under various conditions. The efficiency of surfactant-enhanced remediation of PAHs at NAPL-contaminated sites will depend to a large extent on the equilibrium solubilization capacity of surfactant micelles for different PAHs. Prior studies discussed above suggest that the competitive solubilization of multiple PAHs may affect the extent of solubilization. Thus an in-depth understanding of the factors that control the extent of partitioning of PAH solutes between the NAPL, aqueous, and micellar phases will provide a rational basis for selecting the optimum surfactant and its dose for enhanced solubilization of PAHs from NAPLs.

Materials and Methods Chemicals. Naphthalene (purity >99%), phenanthrene (purity >98%), n-hexadecane (purity 99.9%), and HPLC-grade methanol were purchased from Sigma Aldrich Chemical Company. Chemicals were used without further purification. The physicochemical properties of these compounds are presented in Table 1. Five nonionic surfactants were employed: Brij 30, Brij 35, Tween 80, Tergitol NP-10, and Triton X-100 (Sigma Aldrich Chemical Co.). Some relevant properties of these surfactants are listed in Table 2. NAPL Synthesis and Surfactant Solution Preparation. Three-component NAPLs comprised of varying amounts of naphthalene, phenanthrene, and hexadecane were prepared. Hexadecane is a liquid at room temperature and has negligible aqueous solubility (∼0.9 µg/L) as well as negligible solubility in the micelles of the nonionic surfactants employed. Thus the synthesized NAPL allowed investigation of solubilization of naphthalene and phenanthrene with little interference of any other solute. The NAPLs were synthesized by mixing various quantities of individual crystalline PAHs with hexadecane in sealed, 8-mL Pyrex vials. The mixture was then heated to 60 °C until all the PAH crystals were completely dissolved into hexadecane. The vials were then mixed in a rotary shaker at 200 rpm at 25 °C to obtain a uniform composition. The relative amounts of the PAHs were varied to obtain NAPLs of different composition. Thirty-five combinations of mole fractions were used with naphthalene varying from 0.01 to 0.17 and phenanthrene varying from 0.01 to 0.05. Experiments were carried out to determine the maximum amount of each PAH that could be dissolved into hexadecane in order to have a range of PAH mole fraction VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Characteristics and Properties of Surfactants Used surfactants

CMC (mol/L)

mol wt (g/mol)

Tween 80 Tergitol NP-10 Triton X-100 Brij 35 Brij 30

1.2 × 10-5 5.0 × 10-5 1.7 × 10-4 9.2 × 10-5 2.0 × 10-5

1310 683 625 1198 363

HLB

EO chain length

alkyl chain length

surfactant doses employed (CMC)

15.0 13.6 13.5 16.9 9.7

20 10.5 9.5 23 4.0

18 9 8 12 12

1, 5, 10, 20, 50, 100 1, 5, 10, 20, 50, 100 1, 10, 20, 50, 100, 200 1, 5, 10, 20, 50, 100 1, 10, 20, 50, 100, 200

a Calculated using methods described by Nagarajan and Ruckenstein (14). described by Tanford (16).

for the partitioning experiments. Naphthalene mole fractions up to 0.17 and phenanthrene mole fractions up to 0.05 were achieved in hexadecane without formation of any microcrystals of the PAHs. Surfactant stock solutions were prepared by mixing a known weight of surfactant into 1 L of deionized water for different surfactant concentrations ranging from 1 to 200 times the critical micelle concentration (CMC). Samples were stirred for 24 h until all of the surfactant dissolved. Equilibrium Partitioning Experiments. The experiments were conducted in 40-mL glass vials sealed with Teflon-lined caps to determine equilibrium partitioning of naphthalene and phenanthrene in NAPL-water-micelle systems. Two milliliters of NAPL was added to 25 mL of surfactant solution. For each surfactant, solutions prepared at specific surfactant doses were contacted with NAPLs of varying PAH content in a series of vials. For each of the five surfactants, equilibration experiments were conducted with 140 NAPLs with different PAH mole fraction combinations. Of these, 30 NAPLs were evaluated with duplicate systems, and 18 were evaluated with triplicates. The vials were mounted at on a rotary shaker set at 150 rpm for a predetermined equilibration time of 96 h. Segregated NAPL and micellar surfactant solution phases were maintained during mixing. The initial PAH mole fractions in the NAPL were reduced as a result of solubilization by an average of 2.5% and no more than 5% in any system. After equilibration, aliquots of 1 mL were taken from aqueous phase in each vial using a syringe, centrifuged, diluted with HPLC-grade methanol, and transferred to a 2-mL HPLC vial. Methanol dilution increased CMC of the surfactants to a point where micelles present in the samples disintegrated as a result of the dilution. All glassware was acid-washed and rinsed with water prior to use. All experiments were conducted in a temperature-controlled walk-in chamber at 25 ( 1 °C. Analytical Methods. The aqueous phase in surfactantfree systems, and the surfactant solution phase in other systems were analyzed for naphthalene and phenanthrene using Agilent 1100 series HPLC operated at 1 mL/min flow rate with a mobile phase gradient with 50% water and 50% of acetonitrile for 5 min, increasing to 60% acetonitrile with a linear gradient. A reverse-phase column with 5 µm packing diameter and dimensions of 250 mm × 2.1 mm (VYDAC 201 TP 52) maintained at 35 °C was used. Each run lasted 33 min to wash off the surfactants in the column. Diode array (DAD) and fluorescence (FLD) detectors were used for the quantification of the PAHs. The DAD was set at wavelength equal to 240 nm, and FLD was set at excitation 280 nm and emission 389 nm wavelength. Calibration plots bracketing the range of unknown concentrations were prepared with PAH/ methanol/water external standards. The amounts of hexadecane partitioned into the micellar phase of Tween 80, Tergitol NP-10, Triton X-100, Brij 30, and Brij 35 were determined using radiolabel trace techniques. Known amounts of 14C-labeled hexadecane were added to the three-component NAPLs, which was then used in equilibrium partitioning experiments as described above. 5880

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b

vol of 1 micelle (10-19 cm3) total core PEO 11.5a 3.06b 3.06b 2.61b 2.68a

0.28c 0.24c 0.51b 0.13b 1.79a

11.2 2.82 2.55b 2.48b 0.90

Values listed in Saitoh et al. (15). c Calculated using methods

The surfactant solutions were analyzed using a liquid scintillation counter (LS6500, Beckman-Coulter Inc.). The radioactivity of the surfactant solution (measured in disintegrations per minute) was then converted to the equivalent concentration of hexadecane in the micellar solutions. Variations in the aqueous concentrations measured by the HPLC and the liquid scintillation counter were less than 3% in duplicate measurements. 1H NMR Experiments. NMR analyses were performed on surfactant solutions contacted with two different threecomponent NAPLs, one with naphthalene and phenanthrene mole fraction each equal to 0.01 and the other with a naphthalene mole fraction of 0.17 and phenanthrene mole fractions of 0.05. NMR analyses were also performed on surfactant solutions contacted with two-component NAPLs. The two-component NAPLs had naphthalene at mole fractions equals to 0.01 or 0.17 and phenanthrene at mole fractions equal to 0.01 or 0.05. Surfactant solutions were prepared by mixing a known weight of surfactant into D2O (99.9% purity). The surfactant solutions were equilibrated with the various NAPLs as described above. Aliquots of 3 mL of the surfactant solution were then removed from the vials and centrifuged, and 600 µL of that solution was transferred to 5-mm NMR tubes. Proton NMR chemical shift measurements were carried out at 500 MHz on a Varian Spectrometer spectrum. In this method, the abscissa values of the spectra peaks corresponding to protons at certain locations on the surfactant molecules in a micelle were determined, and changes in the abscissa values due to the presence of different amounts of PAH compounds (naphthalene and phenanthrene) were measured as aromatic ring current-induced 1H chemical shifts along the surfactant chain (17, 18). This technique has been previously used to investigate the locus of solubilization of benzene in micelles of the cationic surfactant CTAB (19). The spectra were recorded for pure 9.2 mM Brij 35, 2 mM Brij 30, 17 mM Triton X-100, 6.46 mM Tergitol NP-10, and 1.2 mM Tween 80 solutions and also for each surfactant solution equilibrated with a NAPL. The D2O peak was assigned to 4.68 ppm in all spectra.

Results and Discussion Equilibrium Partitioning of PAHs in Micelle-NAPL-Water Systems. A linear relationship presented by Hill and Ghoshal (2) that describes the partitioning of PAHs between NAPLs and micellar surfactant solutions as a linear function of the PAH mole fraction in the NAPL, was employed. The micellewater equilibrium partition coefficient (KMi (L3/M)) for PAHs in the three-component NAPL and micellar surfactant solution systems was calculated as:

KMi ) MSRi/(KNiXNi) ) MSRi/CAQi

(1)

where KNi (M/L3) is the Raoult’s law based ratio of the equilibrium concentration of solute i in the true aqueous phase (CAQi (M/L3)) and the mole fraction of the solute in the NAPL (XNi) and MSRi is the ratio of number of moles of solute i per mole of surfactant in the micellar phase.

to methods proposed by Zimmerman et al. (20) (demonstrated in the Supporting Information). Error bars are presented for data points that correspond to systems that were employed in duplicate or triplicate. The KNi and KMi of each PAH are presented in Tables1 and 3. The naphthalene and phenanthrene MSRs and NAPL mole fractions fit the linear relationship in eq 1 closely, as shown in Figures 1 and 2. The MSR of each PAH in the presence of the companion was dependent on its relative abundance in the NAPL phase, but they were not influenced by the changes in the mole fraction of the other PAH (data not shown). The MSRs for both PAHs are greatest for Tween 80 and are followed by Tergitol NP-10, Triton X-100, Brij 35, and Brij 30. Brij 30 with the lowest HLB (hydrophile-lipohile balance) of 9.7 was the only surfactant that partitioned significantly in the NAPL. Surfactants with lower HLB are more hydrophobic. The apparent CMC was found to be 28 times greater than that reported in the literature. The concentration of surfactant and the PAH mole fraction in the NAPLs were thus corrected to take into account the partitioning of Brij 30.

FIGURE 1. Naphthalene MSR values in different surfactant solutions contacted with NAPLs with various naphthalene mole fraction and phenanthrene mole fraction equal to 0.01. Error bars represent one standard deviation.

FIGURE 2. Phenanthrene MSR values in different surfactant solutions contacted with NAPLs with various phenanthrene mole fraction and naphthalene mole fraction equal to 0.01. Error bars represent one standard deviation. Hill and Ghoshal (2) observed a good linear fit of the MSRi and XNi data pairs and eq 1 for systems containing micellar solutions of same surfactants used in this study and twocomponent NAPLs comprised of naphthalene or phenanthrene dissolved in hexadecane. In this study, eq 1 was fitted to a large MSRi and XNi data set obtained from systems containing three-component NAPLs comprised of naphthalene, phenanthrene, and hexadecane. The MSRi of naphthalene or phenanthrene at several mole fractions in the NAPL for each surfactant are shown in Figures1 and 2. Figures 1 and 2 show results for only the lowest solute mole fraction of the companion PAH, but the same trend was observed for all other intermediate PAH mole fractions (data presented in the Supporting Information). MSRi was determined as the slope of the linear plot of the total concentration of the PAH in the bulk aqueous phase versus the corresponding surfactant concentration above the CMC, according

Relationships between the surfactant structure and the solubilization extent have been reported for alkanes, monocyclic aromatic hydrocarbons, and substituted monocyclic aromatic hydrocarbons (3, 15) but not for PAHs. For nonpolar compounds such as alkanes where solubilization takes place only in the core of micelles, relationships between the extent of solubilization and compound properties or the surfactant structure have been reported (3). For monocyclic aromatic hydrocarbons and PAHs, solubilization in nonionic surfactant micelles may occur in the core as well as in the shell region of the micelles because these compounds are slightly polar due to the resonance of π-electrons in the aromatic ring(s) (21, 22). Table 2 lists the core, shell, and total micelle volume and the properties for the surfactants used in this study. The data on the total micelle volume and shell volume for Triton X-100 and Brij 35 and for total volume of Tergitol NP-10 were directly obtained from Saitoh et al. (15), and the remaining volumes were estimated from the data and methods described by various studies (14-16, 23). It is evident from Table 2 and from Figures 1 and 2 that the relative magnitude of the MSR for naphthalene and phenanthrene in various surfactant solutions is closely related to the volume of the shell of the surfactant micelles. For the polyoxyethylenes (POE), the largest portion of a micelle is formed by the solvated PEO part, and it is highly probable that the slightly polar PAH solutes will be located in the shell instead of the core (15). Tween 80, which exhibited the highest solubilization extent, is a much larger and complex molecule than the other surfactants used in this study. As shown in Table 4, a short hydrophobic chain consisting of 18 carbons comprises the core of Tween 80. In contrast, the hydrophilic head is much larger and is comprised of three free PEO chains with an average five EO groups per chain and an aromatic ring linking the head and tail. The free PEO chains when aggregated in micelles are oriented in parallel to the alkyl tail forming a very dense hydrophilic mantle (24). This structure allows Tween 80 micelles to solubilize large amounts of PAHs. Tergitol NP-10 had the second-highest solubilization extent. Tergitol NP-10 is an alkylphenylethoxylate ether possessing a hydrophilic moiety that consists of a POE chain with an average of 10.5 oxyethylene units per molecule, and its hydrophobic moiety consists of nine carbons and a shell volume forming more than 90% of the total volume of empty micelles. The next in terms of solubilization extent was the octylphenylethoxylate Triton X-100, which had a hydrophilic part consisting of a POE chain with an average of 9.5 oxyethylene units per molecules and a hydrophobic moiety consisting of an alkyl chain of eight carbons. Thus, the shell volume forms more than 80% of the total volume. Brij 35 had a significantly sized shell, comparable to that of Triton X-100. VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Micelle Partitioning Coefficients (KMi) for Naphthalene and Phenanthrene in Two- and Three-Component NAPLs XNnaph

XNphen

Brij 35

0.17 0.17 0.17 0.17 0.17 0.17

0.01 0.02 0.03 0.04 0.05

861 ( 40.4 874 ( 41.9 871( 80.5 865 ( 33.9 896 ( 40.7 879 ( 87.8

0.01 0.03 0.05 0.09 0.11 0.13 0.17

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

18300 ( 280 14500 ( 816 15800 ( 900 14400 ( 958 15500 ( 937 14300 ( 506 15800 ( 802 15200 ( 437

Brij 30

Tween 80

KMnaph (L/mol) 861 ( 48 1840 ( 174 750 ( 112.7 1790 ( 76.6 783 ( 288 1740 ( 113.7 817 ( 62.4 1740 ( 104.8 862 ( 363 1630 ( 118.4 824 ( 72.9 1760 ( 98.7 KMphen (L/mol) 6650 ( 487 27300 ( 426 8230 ( 1621 24800 ( 439 5720 ( 591 24200 ( 429 4400 ( 795 na 6040 ( 1156 24400 ( 811 5670 ( 1591 na 6320 ( 1751 24900 ( 405 6850 ( 217 23900 ( 863

Tergitol NP-10

Triton X-100

1080 ( 76.8 1080 ( 58.3 1100 ( 54.1 1060 ( 76.1 1120 ( 47.4 1160 ( 55.9

936 ( 56.8 917 ( 78.9 976 ( 47.4 944 ( 30.1 973 ( 37.7 991 ( 23.3

16000 ( 962 22000 ( 1080 18300 ( 598 19400 ( 928 20000 ( 954 19400 ( 631 19600 ( 919 19700 ( 863

14071 ( 965 16400 ( 976 15300 ( 1181 16700 ( 898 17400 ( 855 17500 ( 191 17600 ( 841 17800 ( 819

TABLE 4. Surfactant Molecule Structure and Segments at Which Aromatic Ring Current Shifts Were Observed in 1H NMR Analysis

Of the surfactants employed in this study, Brij 30 had the smallest shell and the smallest extent of solubilization. The selective solubilization effects are evident from a comparison of the KMi values obtained from fitting eq 1 to data such as those in Figures 1 and 2, for each PAH of the three-component NAPL containing naphthalene and phenanthrene, with those for either PAH in the two-component NAPLs. Table 3 lists the KMnaph values obtained for a series of NAPLs of XNnaph ) 0.17 and varying XNphen, and the KMphen values for a series of NAPLs with XNphen ) 0.05 and varying XNnaph. The KMi values for other NAPLs are not shown, but identical trends were noted. For Brij 35 and Tween 80 the KMnaph values at a fixed phenanthrene mole fraction in the two- and three-component NAPLs are very similar; however, the KMphen values are consistently lower. The KMnaph and KMphen for systems containing the surfactant Brij 30 were relatively constant for both the two-component and three-component NAPLs although there was significant variability in KMi and is attributable to the partitioning of the surfactant into the NAPLs. For Tergitol NP-10 and Triton X-100, the KMnaph values 5882

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obtained for the three-component NAPLs are very similar to those obtained for the two-component NAPLs. However, the KMi values for phenanthrene were greater with the threecomponent NAPL for all mole fractions of phenanthrene as compared to the two-component NAPL, for those surfactants. Reduced solubilization of phenanthrene in surfactant solutions of Brij 35 and Tween 80 and the enhanced solubilization of phenanthrene in surfactant solutions of Triton X-100 and Tergitol NP-10 was likely a result of competition between the solutes for space within micelles. Locus of Solubilization. The selective solubilization effects may be explained by the changes in locus of solubilization of PAHs in the micelles of the nonionic surfactants. The 1H NMR proton shift analyses demonstrate the locus of solubilization of PAH solutes in the micelle. The abscissa of the NMR spectra represents the chemical shift, which is a dimensionless number equal to the ratio of the difference in the frequencies of the signal and the reference to the operating frequency. The chemical shift is expressed in parts per million (ppm), and the frequency scale decreases from left (upfield)

TABLE 5. Changes in Proton Chemical Shifts in the Surfactants (ppm) Contacted with Different Three-component NAPLsa Brij 35

Brij 30

XNAPLPAHs XNAPLPAHs segment pure 0.01/0.01 0.05/0.17 H8 H7 H6 H5 H4 H1 H2 H3

0.75 1.15 1.40 3.30 3.44 3.52 3.70

0.75 1.15 1.40 3.28 3.40 3.52 3.68

0.70 1.05 1.32 3.20 3.36 3.52 3.70

pure 0.75 1.15 1.40 N/a 3.44 N/a N/a

Tween 80

Tergitol NP-10

Triton X-100

XNAPLPAHs XNAPLPAHs XNAPLPAHs XNAPLPAHs XNAPLPAHs XNAPLPAHs XNAPLPAHs XNAPLPAHs 0.01/0.01 0.05/0.17 pure 0.01/0.01 0.05/0.17 pure 0.01/0.01 0.05/0.17 pure 0.01/0.01 0.05/0.17 0.72 1.10 1.40 3.05 3.18 3.32 3.52

0.68 1.05 1.25 3.05 3.18 3.32 3.52

0.72 1.12 1.44 1.84 2.16 3.54 4.04 5.16

0.72 1.12 1.44 1.84 2.16 3.52 4.04 5.16

0.68 1.04 1.28 1.76 2.00 3.52 4.00 5.10

0.60 1.45 3.50 3.60 3.82 6.60 7.00

0.60 1.40 3.40 3.56 3.82 6.60 6.96

0.50 1.30 3.40 3.58 3.70 6.50 6.86

0.50 1.10 1.45 3.50 3.60 3.82 6.60 7.00

0.48 1.06 1.45 3.47 3.60 3.82 6.60 7.00

0.44 1.00 1.36 3.45 3.56 3.68 6.48 6.84

a Boldface type indicates shifts corresponding to the protons from the core. Italic type indicates shifts corresponding to the protons from the shell.

TABLE 6. Changes in Proton Chemical Shifts in the Surfactants (ppm) Contacted with Different Two-Component NAPLs Brij 35

XNnaph

Triton X-100

XNphen

XNnaph

XNphen

segment pure 0.01 0.17 0.01 0.05 pure 0.01 0.17 0.01 0.05 H8 H7 H6 H5 H4 H1 H2 H3

0.75 1.15 1.40 3.30 3.44 3.52 3.70

0.75 1.15 1.40 3.30 3.42 3.52 3.70

0.75 1.12 1.40 3.27 3.40 3.52 3.70

0.75 1.15 1.40 3.30 3.42 3.52 3.70

0.75 1.10 1.37 3.27 3.40 3.52 3.70

0.50 1.10 1.45 3.50 3.60 3.82 6.60 7.00

0.50 1.10 1.45 3.47 3.60 3.82 6.60 7.00

0.50 1.05 1.42 3.45 3.55 3.77 6.57 6.95

0.50 1.10 1.45 3.47 3.60 3.82 6.60 7.00

0.50 1.05 1.42 3.47 3.55 3.80 6.60 7.00

a Boldface type indicates shifts corresponding to the protons from the core. Italic type indicates shifts corresponding to the protons from the shell.

to right (downfield). The aromatic ring current shift on a segment of the surfactant molecule is evidence that on average the aromatic solute is located at or near that segment. The absence of shift on a surfactant segment indicates the absence of any significant amount of aromatic solutes at that segment (18). The signals corresponding to groups of protons on the NMR spectra for the different segments of a surfactant molecule are presented in Table 4. The changes in proton shifts in each of the signals for surfactants in solution containing various amounts of PAHs solubilized from the three-component NAPLs are presented in Table 5. The Tween 80 molecule is too complex for labeling segments not only because of its size, but also because a superposition of -CH2peaks of the free poly(ethylene oxide) (PEO) groups occurs at a peak corresponding to 3.54 ppm, of aliphatic CH3 peaks occurs near 0.72 ppm, and of aliphatic (non-EO) -CH2- peaks occurs near 1.12 ppm. The proton shift changes for surfactant solutions equilibrated with various two-component NAPLs are presented in Table 6. The proton shift changes were obtained by comparing the NMR spectra for the pure surfactant solutions with those for surfactant solution containing PAHs. The change in proton shifts in the NMR spectra indicates the presence of the PAH solutes in the micelle but does not distinguish between the presence of naphthalene and phenanthrene. As shown in Table 5, for micellar solutions of Brij 35 upon solubilization of naphthalene and phenanthrene from a NAPL containing the PAHs at lowest mole fraction (XNnaph ) 0.01, XNphen ) 0.01) changes in the proton shifts were observed on the H4, H1, and H3 segments of the surfactant that comprises the shell region of the micelle (Table 4). This suggests that the naphthalene and phenanthrene partitioned in the shell region of the Brij

35 micelles. For surfactant solutions equilibrated with a NAPL containing the highest possible mole fractions of naphthalene and phenanthrene (XNnaph ) 0.17 and XNphen ) 0.05) the chemical shifts were at the H7, H6 and H5 segments of the surfactant that comprise the core region in a micelle, suggesting that PAHs partitioned in the core region of the Brij 35 micelles. Additional shifts were observed on segments H4 and H1, suggesting that PAH solubilization also occurred in the shell region at elevated PAH concentrations. That phenanthrene was present in the shell region of the micelle at low mole fractions and both in the shell and core regions at high mole fractions, indicates a relationship between PAH concentration and the locus of solubilization with in the micelles. This is an important finding because other studies (4, 5) suggest that phenanthrene, the more hydrophobic solute, should be present exclusively in the core region. Aromatic hydrocarbons are polarizable because of the resonance effects in their structures, and the slight polarity allows these compounds to be located both in shell region of the micelle as well as the hydrophobic interior core (8, 14, 15, 25). Recent studies have shown that compounds such as benzene and naphthalene are slightly polar and participate in hydrogen bonding with water because of the presence of the π-electrons in the aromatic ring(s) (21, 22). Since the resonance energy of the π-electrons of naphthalene and phenanthrene are of similar magnitude to that of benzene, PAHs such as naphthalene and phenanthrene may partition in the relatively polar shell region of the micelle, as does benzene (26). The resonance of the π-electrons in the PAHs may facilitate weak bonding with the surfactant headgroups and thus encourage solubilization of the compounds in the shell of the micelle. The micellar shells of all of the nonionic surfactants employed in this study are large enough to accommodate molecules of the size of naphthalene and phenanthrene, which are estimated to have an approximate molecular volume of 2.25 × 10-22 cm3‚and 3.31 × 10-22 cm3, respectively. As discussed in the previous section, there is a strong co-relation between the extent of solubilization and the shell volumes of the various surfactants. For surfactant Brij 30 proton shift changes were distributed over the core and shell regions at various PAH concentrations in the micellar phase (Table 5). For Tween 80 the patterns of proton shift changes were similar to that of Brij 35. In surfactant solutions equilibrated with NAPLs containing naphthalene and phenanthrene at the lowest mole fractions (XNnaph ) 0.01, XNphen ) 0.01), the proton shift changes were observed in the surfactant segments that comprise the shell region of the Tween 80 micelle. However, for surfactant solutions contacted with NAPLs at the highest naphthalene and mole fractions, the proton shift changes were noted in all segments of the surfactants, suggesting partitioning of the PAHs in the core and the shell. VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Although the analysis of the NMR shift data demonstrated solute solubilization in the shell as well as the core, the analyses do not reveal the relative location of naphthalene and phenanthrene in the micelles. Thus the activity coefficients of the PAHs in the micellar phase were assessed to evaluate selective solubilization patterns of the PAHs. The changes in the magnitude of the activity coefficient of the PAH solutes (γMi) for a given mole fraction of the solute in the micelles (XMi) can indicate if with additional partitioning the individual solutes are being located in different environments within the micelles. An increase in the activity coefficients with increasing solute concentration in micelles suggests that a larger portion of the solute is incorporated into a less favorable environment and vice-versa. Dunaway et al. (27) concluded from the trends in the micellar phase activity coefficient of benzene with its increasing mole fractions in cationic micelles of cetylpyridinium chloride that benzene had a greater affinity to the shell regions of the micelles at low mole fractions, but at higher mole fractions it partitioned to the core. The activity coefficient of the PAH solute was calculated as:

γMi ) CAQi(fSi/fLi)/(XMiCAQ,sati)

(2)

where CAQi (M/L3) is computed using Raoult’s law as CAQi ) KNiXNi; CAQ,sati (M/L3) is the aqueous solubility of solute i, which was reported by Hill and Ghoshal (2) for the same two- and three-component NAPL systems employed in this study, and XMi is the molar fraction of the solute in the micelle and was calculated by:

XMi ) ni/(ni + nj + nk + nsurf) ) MSRi/(MSRi + MSRj + MSRk + 1) (3) In eq 3, ni, nj, nk, and nsurf are the number of moles of naphthalene, phenanthrene, hexadecane, and surfactant in the micelle. ni, nj, nk, and nsurf were determined from the plots of solute concentrations with surfactant dose, which was used for calculating the MSRs. The fugacity ratio (fSi/fLi) can be estimated based on thermodynamics properties and the ratios used for naphthalene and phenanthrene were equal to 0.306 and 0.279, respectively (12). Equation 3 assumes that the interior of the micelle behaves like an organic liquid phase, and thus liquid-liquid partitioning theory can be applied to a micelle-water system. 3-D plots relating the micellar activity coefficient of each PAH in the three-component NAPL, as a function of the micellar mole fraction of both PAHs are presented in Figures 3-6. For surfactants Brij 35 and Tween 80, an increasing trend in the naphthalene activity at the low micellar mole fraction range of both naphthalene and phenanthrene is evident from Figures 3a and 4a. The values of the activity coefficients of naphthalene at higher micellar mole fractions of naphthalene and phenanthrene are relatively constant. The trends in the activity coefficient of phenanthrene are similar to that of naphthalene in that an increasing trend with greater mole fractions of both naphthalene and phenanthrene was observed, as shown in Figures 3b and 4b. The lower activity of both PAHs at low mole fraction is attributable to the partitioning of the solutes in the outer layers of micelles, which has a larger volume than the core. The shift data for Brij 35 in Table 6 suggests that naphthalene successfully competes for space in the micelle shell region. Because naphthalene has a lower molecular volume than phenanthrene, it successfully competes for the intertial spaces in the shell region, over phenanthrene. Micellar solutions contacted with two-component (single-solute) NAPLs at a high mole fraction of naphthalene (XNnaph ) 0.17) resulted in a change in proton shift at segment H6, corresponding to 5884

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FIGURE 3. Micellar phase activity coefficient of (a) naphthalene and (b) phenanthrene in Brij 35 surfactant solutions contacted with three-component NAPLs of various PAH mole fractions. The surfaces have been fitted to the data points to guide the eye. the core. However for a high mole fraction of phenanthrene (XNphen ) 0.05), the changes in proton shift were observed over two segments H5 and H6. For the Brij 35 solutions contacted with the three-component NAPL, there is even more crowding in the micelle core at high PAH mole fractions as indicated by the shifts on all three segments associated with the core (Table 5). The higher activity coefficients for the solutes at higher micellar (or NAPL) mole fractions of naphthalene and phenanthrene is likely because the solutes experience space restrictions in the core which is relatively smaller than the shell. For systems containing Brij 30, the KMi values show no distinct trend with changing mole fractions, and the significant variability in those values make interpretation of the data difficult. The variability in the partition coefficient is likely due to the significant solubilization of the surfactant into the NAPL. The NMR proton shift changes for Triton X-100 and Tergitol NP-10 were similar to each other and indicate that the partitioning of solutes occurred in the core and shell regions of the micelles (Table 5). In contrast to Brij 35 and Tween 80, it was observed that, in micellar solutions of Triton X-100 and Tergitol NP-10 equilibrated with NAPLs containing low mole fractions of naphthalene and phenanthrene, some of the PAHs partitioned into the core. For example, for Triton X-100 prominent changes were observed for the proton shift of the H8 and H7 segments, indicating solute partitioning in the core in addition to partitioning in the shell (segment H5).

FIGURE 4. Micellar phase activity coefficient of (a) naphthalene and (b) phenanthrene in Tween 80 surfactant solutions contacted with three-component NAPLs of various PAH mole fractions. The surfaces have been fitted to the data points to guide the eye.

FIGURE 5. Micellar phase activity coefficient of (a) naphthalene and (b) phenanthrene in Triton X-100 surfactant solutions contacted with three-component NAPLs of various PAH mole fractions. The surfaces have been fitted to the data points to guide the eye.

At the highest mole fractions the changes in chemical shifts were observed over a larger region of the core (segments H8, H6, and H7) and all segments corresponding to the shell region. The NMR spectrum of pure solution of Triton X-100 was characterized by a poor resolution of peaks, or overlapping peaks, for segments H4 and H5. The overlapping peaks for pure solutions of Triton X-100 may be the result of its unique structure. It has been reported that self-aggregated Triton X-100 possess hydrophilic polyoxyethylene chains that are coiled and a dense shell surrounding the hydrophobic core is formed (28, 29). The distance between protons on the hydrophilic and hydrophobic chains in Triton X-100 micelles calculated from the 2D NOESY 1H NMR spectra (data not shown) are significantly shorter. The space restrictions in the dense shell of these surfactant micelles also explain why at low mole fractions of PAHs in the micelles the PAHs partitioned into the core. However, some solubilization does occur in the dense shell of Triton X-100 because shifts in the shell but not in the core were observed for the two-component NAPL at low mole fraction of both PAHs (Table 6). Furthermore, as in Brij 35, naphthalene appears to successfully compete with phenanthrene for space in the shell region. Table 6 shows that, at the highest mole fractions of naphthalene, there was a change in proton shift at segments H6 and H7 corresponding to the core. However for the highest mole fraction of phenanthrene (XNphen ) 0.05), the changes

in proton shift were observed over all three segments of the core (H8, H6, and H7). In micelles of surfactants Triton X-100, the activity coefficient of naphthalene and phenanthrene increased with their respective mole fraction in the micelles (Figure 5a,b). For Tergitol NP-10, the activity coefficients of both naphthalene and phenanthrene increased at the lower mole fractions but were fairly constant at higher mole fractions (Figure 6a,b). This trend is unlike that of Triton X-100 but similar to that of Brij 35 and Tween 80. This may be because, at the low mole fraction range, relative to Triton X-100 more PAHs partition in the shell of Tergitol NP-10. It has been suggested that micelles of Tergitol are flexible (30), and the micellar shape may change to allow accommodation of large amounts of phenanthrene in the micellar core. The enhanced solubilization of phenanthrene, as in Tergitol NP-10 and Triton X-100, seems to be associated with the presence of PAH solutes in the micelle core at low PAH mole fractions in the micelle. Reduced solubilization of phenanthrene, as in Brij 35 and Tween 80, is observed when there is an absence of PAH solutes in the micelle core at low PAH mole fractions. It may be argued that factors such as partitioning of hexadecane into the micelles and increases in the micellar volume with PAH partitioning could potentially contribute to enhanced solubilization of phenanthrene. Long chain molecules such as hexadecane if solubilized into micelles in VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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volume. As a result, the change in the phenanthrene KMi values cannot be attributed to the changes in micelle volume. An interesting observation is that, although the activity coefficient of PAHs in the micelle change with PAH concentrations in the micelle due to partitioning of PAHs in different regions of the micelle which have different polarities and space constraints for accommodating solutes, a linear partition coefficient (KMi) can be employed to describe partitioning between the NAPL and the micellar phase. The KMi did not change as significantly at different mole fractions, as the activity coefficients. As explained elsewhere (2), an increase in the activity coefficient of the solute in the micelle was accompanied by a decrease in the molar volume of the micelle (data not shown), and this resulted in a relatively constant ratio between the concentration of the solute in the micellar phase and in the aqueous phase. The presence of cosolutes resulted in changes to the magnitude of the partition coefficient of phenanthrene, but in the range of PAH concentrations studied, the partition coefficient still represents a linear relationship between the concentration of the solute in the micelle and in the aqueous phase. Thus although the partitioning behavior of mixtures of PAHs in nonionic surfactant micelles may be complex, a single partitioning coefficient can still be used for each PAH solute.

Acknowledgments Funding for this research was provided by National Science and Engineering Research Council, Canada. L.A.B. was financially supported by scholarships from CNPq, Brazil, and by FQRNT, Quebec. We thank Dr. Paul Xia (Department of Chemistry, McGill University), for his assistance with NMR analysis.

Supporting Information Available

FIGURE 6. Micellar phase activity coefficient of (a) naphthalene and (b) phenanthrene in Tergitol NP-10 surfactant solutions contacted with three-component NAPLs of various PAH mole fractions. The surfaces have been fitted to the data points to guide the eye. significant quantities could lead to reorganization of the micellar structure and an increase in the micellar volume. This could result in selective solubilization of one solute over another (31, 32) as observed for Tergitol NP-10 and Triton X-100. However, these phenomena did not occur to any significant extent in the systems studied. The MSR of hexadecane, in surfactants Brij 35, Brij30, Tween 80, Tergitol NP-10, and Triton X-100 were very low and equal to 0.005, 0.024, 0.024, 0.016, and 0.002, respectively, suggesting that the amounts of hexadecane solubilized were minimal. Further more, no changes in the CMC of Brij 35, Tween 80, Tergitol NP-10, and Triton X-100 occurred as a result of PAH partitioning, suggesting that changes to the micellar volumes did not occur. It has been proposed that solubilization of slightly polar compounds such as benzene in the shell region of micelles of ionic surfactants may reduce the core-water interfacial tension and increase the micellar cavity volume and thus increase the micellar partitioning of more hydrophobic compounds (5). However, this phenomenon is not applicable to the nonionic surfactant systems as explained elsewhere (2), and no systematic change in the CMC of the surfactants was observed in this study due to PAH solubilization (data presented in Supporting Information section), which indicates that there was no significant change to the micelle 5886

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All MSR data plots; an example plot of solute concentration in the surfactant solution vs surfactant dose used for computing MSR; and measured CMC of surfactants in NAPLsurfactant solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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(24) Prouzet, E.; Cot, F.; Nabias, G.; Larbot, A.; Kooyman; Pinnavaia, T. J. Assembly of mesoporous silica molecular sieves based on nonionic ethoxylated sorbitan esters as structure directors. Chem. Mater. 1999, 11, 1498-1503. (25) Cang, H.; Brace, D. D.; Fayer M. D. Dynamic partitioning of an aromatic probe between the headgroup and core regions of cationic micelles. J. Phys. Chem. B 2001, 105, 10007-10015. (26) Streitwiese, A., Jr.; Heathcock, C. H. Introduction to Organic Chemistry. 3rd edition, Macmillan Publishing Co.: New York, 1992. (27) Dunaway, C. S.; Christian, S. D.; Scamehorn, J. F. Overview and History of the Study of Solubilization. In Solubilization in Surfactant Aggregates, Christian, S. D., Scamehorn, J. F., Eds.; Surfactant Series 55; Marcel Dekker Inc.: New York, 1995; Chapter 1. (28) Yuan, H. Z.; Cheng, G. Z.; Zhao, S.; Miao, X. J.; Yu, J. Y.; Shen, L. F.; Du, Y. R. Micellization of sodium dodecyl sulfonate and Triton X-100 in polyacrylamide water solution studied by 1H NMR relaxation and two-dimensional nuclear overhauser enhancement spectroscopy. Colloid Polym. Sci. 1999, 277, 10261032. (29) Yuan, H. Z.; Cheng, G. Z.; Zhao, S.; Miao, X. J.; Yu, J. Y.; Shen, L. F.; Du, Y. R. Conformational dependence of Triton X-100 on environment studied by 2D NOESY and 1H NMR relaxation. Langmuir 2000, 16, 3030-3035. (30) Chen, B. H.; Miller, C. A.; Garrett, P. R. Rates of solubilization of triolein into nonionic surfactant solutions. Colloids Surf. A 1997, 128, 129-143. (31) Weiss, J.; Coupland, J. N.; Brathwaite, D.; McClements D. J. Influence of molecular structure of hydrocarbon emulsion droplets on their solubilization in nonionic surfactant micelles. Colloids Surf. A 1997, 121, 53-60. (32) Weiss, J.; McClements, D. J. Mass transport phenomena in oilin-water emulsions containing surfactant micelles: solubilization. Langmuir 2000, 16, 5879-5883.

Received for review February 18, 2004. Revised manuscript received August 10, 2004. Accepted August 17, 2004. ES0497429

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