Solubilization of Dodecane, Tetrachloroethylene, and 1,2

Environ. Sci. Technol. 1997, 31, 1382-1389

Solubilization of Dodecane, Tetrachloroethylene, and 1,2-Dichlorobenzene in Micellar Solutions of Ethoxylated Nonionic Surfactants KURT D. PENNELL* School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ANDREW M. ADINOLFI,† LINDA M. ABRIOLA, AND MAMADOU S. DIALLO‡ Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109

Although surfactants have received considerable attention as a potential means for enhancing the recovery of organic compounds from the subsurface, only limited information is available regarding the micellar solubilization of common groundwater contaminants by nonionic surfactants. The purpose of this study was to examine the influence of surfactant properties and environmental factors on the solubility of dodecane, tetrachloroethylene (PCE), and 1,2-dichlorobenzene (DCB) in micellar solutions of Witconol 2722, Tergitol NP-15, and Witconol SN-120. A matrix of batch experiments was performed at 10 and 25 °C and in the presence of CaCl2 for surfactant concentrations ranging from 0.5 to 15% by weight. Although the hydrophile-lipophile balance (HLB) values of the surfactants are similar, Witconol 2722 solubilized approximately three times more organic than the other surfactants, which was attributed to the greater alkyl chain length and ethoxylation of Witconol 2722. Results of HLB scans, conducted using Tergitol NP surfactants, showed that solubilization capacity was related to the micelle core volume and that cloud point effects can reduce the aqueous solubility of PCE. These findings demonstrate the importance of considering the specific surfactant-organic interactions, cloud point temperature, and macroemulsion formation when selecting nonionic surfactants for use in subsurface remediation applications.

Introduction The long-term persistence of organic contaminants in the subsurface is now recognized as a significant impediment to conventional remediation technologies. Recent evaluations of pump-and-treat systems, in which water is withdrawn from zones of contamination, have revealed that this approach is unlikely to be effective in the presence of strongly-sorbed and non-aqueous phase liquid (NAPL) contaminants (1, 2). * Corresponding author: telephone: (404) 894-9365; fax: (404) 894-8266; e-mail address: [email protected]. † Present address: GEI Consultants, 1021 Main St., Winchester, MA 01890. ‡ Present address: California Institute of Technology, Beckman Institute 139-74, Pasadena, CA 91225.

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These findings have prompted intensive study of innovative remediation strategies. One promising alternative involves the use of aqueous surfactant solutions to enhance the efficiency of conventional pump-and-treat systems. This approach is based on the capacity of surfactants, at concentrations above the critical micelle concentration (cmc), to increase the aqueous solubility of hydrophobic organic compounds via micellar solubilization and to mobilize entrapped NAPLs due to interfacial tension reductions. In laboratory studies, the introduction of aqueous surfactant solutions has been shown to significantly reduce the number of fluid pore volumes required to remove entrapped NAPLs and sorbed contaminants from soil columns (3, 4). Although micellar solubilization was the primary mode of recovery in these experiments, mobilization of entrapped NAPLs has also been observed (5, 6). In the latter experiments, the reduction in interfacial tension between the organic and the aqueous phase was sufficient to induce mobilization of the entrapped NAPL. Although both recovery processes could theoretically be utilized to remove residual NAPLs from porous media, mobilization of NAPLs may extend the zone of contamination within an aquifer formation. This is of particular concern in the case of dense NAPLs, such as chlorinated solvents, that could migrate downward through an aquifer formation upon mobilization (7). For this reason, it may be advantageous to develop and test surfactant formulations that act primarily to solubilize entrapped NAPLs. The efficiency of surfactant-enhanced solubilization will be governed, in large part, by the capacity of a particular surfactant to solubilize organic compounds under field conditions. The solubilization capacity of a surfactant can be represented as the ratio of the moles of organic solubilized to the moles of surfactant in micellar form. A molar solubilization ratio (MSR) may be defined as (8):

MSR )

Co - Co,cmc Csurf - Csurf,cmc

(1)

where Co is the molar concentration of the organic in solution, Co,cmc is the molar concentration of organic at the cmc, Csurf is the molar concentration of surfactant in solution, and Csurf,cmc is the molar concentration of surfactant at the cmc. Although the values of Co,cmc and Csurf,cmc may not be known, the MSR can be obtained from the slope of the solubility curve above the cmc. The MSR has been widely used to characterize the solubility of hydrocarbons in micellar solutions. Few studies, however, have addressed the effects of surfactant structure and organic properties on the solubilization of NAPLs by nonionic surfactants. Fountain et al. (5) investigated the influence of surfactant hydrophile-lipophile balance (HLB) on the solubilization of tetrachloroethylene (PCE) and trichloroethylene (TCE) using mixtures of two commercially-available nonylphenol ethoxylates with ethylene oxide (EO) numbers of 9.5 and 20. The maximum solubility of PCE was obtained at an HLB of 14, while TCE exhibited maximum solubility at an HLB of 15. However, no explanation was given for the observed differences in the HLB solubility curves, and it is unclear whether or not these data can be extended to other nonionic surfactant systems. Recently, Diallo et al. (9) reported MSR data for the solubilization of 11 NAPLs by a homologous series of dodecyl alcohol ethoxylates with EO numbers ranging from 6 to 31. By conducting detailed HLB scans, the authors were able to relate changes in the MSR to the volume of the micellar core and potential interactions between EO head groups and polarizable hydrocarbons. Further experimentation is required to

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confirm these findings for a wider range of nonionic surfactants and to explore the distribution of organic contaminants between the hydrophobic core and the EO mantle of nonionic surfactant micelles. A second approach used to quantify the solubilization capacity of a surfactant is based on the micelle-water partition coefficient (Kmw), which represents the distribution of organic between surfactant micelles and the aqueous phase:

Kmw ) Xm/Xa

(2)

Here Xm is the mole fraction of organic in the micellar phase and Xa is the mole fraction of organic in the micelle-free aqueous phase. In general, the value of Kmw has been found to be constant at surfactant concentrations above the cmc for a given surfactant-organic system (10). The micellar mole fraction of the organic (Xm) may be described as the number moles of organic solubilized per total moles of organic and surfactant in the micelle. The value of Xm can be calculated from the MSR (8):

Xm )

MSR 1 + MSR

(3)

For dilute solutions, the mole fraction of organic in the aqueous phase (Xa) may be estimated as

Xa ) Co,cmcVw

(4)

where Vw is the molar volume of water (0.01805 L/mol at 25 °C). Several investigators have reported a linear relationship between log Kmw and the log of the octanol-water partition coefficient (Kow), a measure of solute hydrophobicity (8, 1113). If generally applicable, such a relationship could provide a quantitative tool for predicting micellar solubilization of hydrocarbons in field applications of surfactant-based remediation technologies. An analogous approach has been widely used to estimate soil-water and sediment-water partition coefficients (Kp) based on the Kow of the solute (e.g., ref 14). The application of such an approach to micellar solubilization by ethoxylated nonionic surfactants, however, is complicated by the potential for specific interactions between polar or polarizable hydrocarbons and the EO mantle of nonionic surfactant micelles. In addition, the surfactant molecular structure is known to influence micellar properties, such as aggregation number and core volume, which could result in different solubilization capacities for each surfactant-organic system. Jafvert et al. (13) recognized the importance of these issues and limited their treatment to nonpolar hydrocarbons, which would be solubilized primarily within the hydrophobic micelle core. Their analysis resulted in the development of a semi-empirical correlation that related experimentally-determined values of Kmw to the Kow of the solute and to the number of alkane carbons and EO groups in the surfactant. The inclusion of surfactant properties in the correlation represents a significant improvement over previous correlations and suggests that it may be possible to account for specific interactions within the EO mantle by incorporating additional terms related to hydrocarbon polarity. Only limited experimental data are available, however, to evaluate correlations such as those developed by Jafvert et al. (13) and to assess the effects of hydrocarbon properties on micellar solubilization. In addition, much of the published solubilization data for nonionic surfactants has focused on alkylphenol ethoxylates, such as the Triton X, Tergitol NP, and Igepal CA series (8, 15-17). This family of nonionic surfactants is unlikely to be suitable for aquifer remediation due to its resistance to biodegradation (18) and the potential for transformation of these compounds to toxic alkylphenol monoethoxylates and diethoxylates under anaerobic condi-

tions (19). More recent studies have examined nonionic surfactants that have received U.S. Food and Drug Administration (FDA) approval for specific food and pharmaceutical applications (4, 6, 20, 21) and linear alcohol ethoxylates (3, 9), which are known to be readily degraded by soil microorganisms (22). Furthermore, only limited experimental data are available regarding the effect of surfactant properties and environmental factors, such as temperature and background electrolytes, on the solubilization of chlorinated solvents and normal alkanes by these surfactants. The purpose of this study was to investigate the micellar solubilization of dodecane, tetrachloroethylene (PCE), and 1,2-dichlorobenzene (DCB) by several classes of commercially-available nonionic surfactants. The surfactants selected for study included polyoxyethylene (20) sorbitan monooleate (Witconol 2722), a lauryl alcohol ethoxylate (Witconol SN-120), and a nonylphenol ethoxylate (Tergitol NP-15). These surfactants were chosen, in part, because they possess similar HLBs but contain different hydrophobic moieties. In addition, a homologous series of Tergitol NP surfactants was utilized to evaluate the effect of surfactant HLB on micellar solubilization capacity. The specific objectives of this research were to (i) investigate the effects of surfactant molecular structure and organic properties on micellar solubilization; (ii) quantify the influence of temperature and a background electrolyte (CaCl2) on solubilization capacity over a range of surfactant concentrations; and (iii) evaluate the utility of correlations developed for predicting the distribution of organic compounds between surfactant micelles and water.

Materials and Methods Materials. Micellar solubilization studies were conducted using three nonionic surfactants: Witconol 2722, Witconol SN-120, and Tergitol NP-15. A homologous series of Tergitol NP surfactants with EO numbers from 8 to 40 was employed for the HLB solubility scans. Witconol 2722 and Witconol SN-120 were provided by Witco Corp., while the Tergitol NP surfactants were obtained from the Union Carbide Corp. Relevant properties of these surfactants are given in Table 1. All of the surfactants were used as received from the supplier without further purification. Thus, the molecular formulas given in Table 1 represent average values that may vary with respect to the degree of ethoxylation and the nature of the hydrophobic group. Witconol 2722 is a food-grade (edible) surfactant used in the manufacture of shortenings, whipped toppings, and dietary supplements. Lauryl alcohol ethoxylates, such as Witconol SN-120, are commonly used as household detergents and have been tested for the remediation of soils contaminated by PCBs and transmission fluid (3). Due to their low toxicity and potential for biodegradation (22, 26), both Witconol 2722 and Witconol SN-120 are well-suited for aquifer remediation applications. As noted previously, the resistance of alkylphenol ethoxylates to biodegradation and toxicity issues could limit the use of this type of surfactant for remediation purposes. However, Tergitol NP surfactants were chosen for study to allow for comparisons with published data and because a homologous series of this surfactant is commercially available. Three organic compounds, dodecane, tetrachloroethylene (PCE), and 1,2-dichlorobenzene (DCB), were employed in the solubilization studies. Dodecane is representative of the saturated hydrocarbons present in gasoline and jet fuel, while PCE and DCB are chlorinated solvents frequently found at DNAPL-contaminated sites. Selected physical and chemical properties of the organic compounds are given in Table 2. Dodecane (HPLC-grade) was obtained from Aldrich Corp., while 14C-labeled dodecane was purchased from Sigma Chemical. Dichlorobenzene (HPLC-grade) and PCE (spectrophotometric-grade) were supplied by the Aldrich Corp. and Kodak, respectively. All aqueous solutions were prepared

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TABLE 1. Selected Properties of Nonionic Surfactants Used for Solubilization Experiments trade name

av molecular formula

MW (g/mol)

HLBa

cmcb (mg/L)

Nac

Witconol 2722 Witconol SN-120 Tergitol NP-8 Tergitol NP-9 Tergitol NP-10 Tergitol NP-13 Tergitol NP-15 Tergitol NP-40

C18H34O2C6H10O4(CH2CH2O)20 C10-12H21-25O(CH2CH2O)9H C9H19(C6H4)O(CH2CH2O)8H C9H19(C6H4)O(CH2CH2O)9H C9H19(C6H4)O(CH2CH2O)10.5H C9H19(C6H4)O(CH2CH2O)13H C9H19(C6H4)O(CH2CH2O)15H C9H19(C6H4)O(CH2CH2O)40H

1310 569 573 619 683 793 881 1983

15.0 14.0 12.3 12.9 13.6 14.4 15.0 17.8

13 54 na na 57 na 97 461

110 105 474 326 216 138 100 26

a Hydrophile-lipophile balance, calculated as HLB ) % wt EO/5 (10). b Critical micelle concentration (24), na, not available. c Aggregation number: Witconol 2722 estimated using correlation of Arnarson and Elworthy (23) for long chain ethoxylated nonionic surfactants, Witconol SN-120 estimated using correlation derived from measured data of Becher (24), Tergitol series estimated using correlation derived from measured data of Schick et al. (25).

TABLE 2. Selected Physical and Chemical Properties of Organic Compounds Used in Solubilization Experiments

a

NAPL

formula

MW (g/mol)

dodecane PCE 1,2-DCB

C12H26 C2Cl4 C6H4Cl2

170.34 165.85 147.01

Weast (27).

b

densitya (g/cm3)

aqueous solubility (mg/L)

log Kow

IFT (dyn/cm)

VPa (mmHg)

0.75 1.62 1.31

0.0037b 151c 144c

6.51d 2.88c 3.38c

52.8e 47.5e 23.2

0.1 14.0 1.0

Riddick et al. (28). c Schwarzenbach et al. (29).

d

Lyman et al. (30). e Demond and Linder (31).

with distilled water that was purified using a Barnstead Nanopure II treatment system. Experimental Methods. The equilibrium solubilities of dodecane, PCE, and DCB in aqueous solutions of Witconol 2722, Witconol SN-120, and Tergitol NP-15 were measured in batch reactors maintained at 10 and 25 °C. The effect of calcium chloride (500 mg/L) on the solubilization capacity of each surfactant was evaluated at 25 °C. Surfactant solutions ranged in concentration from 5 to 150 g/L or approximately 0.5-15% by weight. These concentrations are above the cmc of the surfactants studied and constitute a concentration range anticipated for use in laboratory column experiments and field applications of surfactant flushing. For the HLB scans, a constant surfactant concentration of 40 g/L was employed. Preliminary solubilization experiments, conducted over mixing periods ranging from 1 to 14 days, indicated that constant or “equilibrium” solubilities were achieved after 2 days of mixing. Therefore, all solubilization experiments were performed with a minimum mixing period of 48 h. For the dodecane solubilization studies, 1 mL of 14C-labeled dodecane was contacted with 20 mL of surfactant solution in 25-mL borosilicate glass vials equipped with open-top screw caps and Teflon-backed septa. The specific activity of the 14C-labeled dodecane was approximately 350 nCi/mL. Reactor vials were prepared in duplicate for each surfactant concentration considered. The contents of each vial was mixed for at least 48 h on an oscillating shaker (LabQuake) located in a temperature-controlled room. Sample vials were then inverted for 48 h to allow unsolubilized dodecane to separate from the aqueous phase. The inverted settling procedure provided for syringe access into the aqueous phase without disturbing excess dodecane liquid accumulated at the water surface. Duplicate 1-mL samples were taken from the aqueous phase of the inverted vials and transferred to scintillation vials containing 10 mL of Ecolume scintillation cocktail (ICN Biomedicals). The activity of dodecane in the aqueous phase was assayed by standard liquid scintillation counting (LSC) procedures using an LKB Wallac Rackbeta Model 1219 scintillation counter. All scintillation vials, including reference standards, samples, and blanks were counted twice for disintegrations per minute (dpm). Micellar solubilization of PCE and DCB was measured in 25-mL Corex high-speed centrifuge tubes equipped with open-top screw caps and Teflon-backed septa. For these

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systems, 2.0 mL of neat organic liquid was added to 20 mL of aqueous surfactant solution, prepared in triplicate for each surfactant concentration. The contents of each tube was mixed on an oscillating shaker as described for the dodecane experiments. In the PCE- and DCB-surfactant systems, however, macroemulsions formed upon mixing. The macroemulsions consisted of a milky-white phase that existed at the interface between the organic and aqueous phases. In order to break the macroemulsions, the organic and aqueous phases were separated by centrifugation at 7500 rpm for 45 min using a Sorvell RC-5B (Dupont Instruments) centrifuge. The aqueous-phase concentrations of PCE and DCB were determined by gas chromatography, using a Hewlett-Packard (HP) Model 5890 Series II GC equipped with an HP 7636 autosampler, 15-m megabore DB-5 column (J&W Scientific), a flame ionization detection (FID), and electron capture detector (ECD). A precolumn trap was employed to prevent surfactant fouling of the GC inlets, column, and detector. The surfactant trap consisted of a glass inlet liner packed with a 3-cm bed of 80-100 mesh Porpak Q (Alltech Assoc.), located within the heated zone of the split/spitless injection port. Analytical samples were prepared by adding 0.4 mL of the aqueous phase to 1.2 mL of 2-propanol in glass autosampler vials (∼1.7 mL capacity). The aqueous sample, consisting of water, surfactant, and PCE or DCB, completely dissolved in the 2-propanol to yield a single phase. The GC vials were immediately sealed with Teflon-lined aluminum caps to minimize volatile losses. Differences between the area response of duplicate samples obtained from the same centrifuge tube were less than 5%. A minimum of four PCE or DCB standards, initially prepared in 2-propanol and then mixed with water and surfactant to give the same background solution present in experimental samples, were analyzed in duplicate during each GC sample run. A linear, least squares regression procedure (SYSTAT) was employed to develop standard calibration curves for PCE and DCB. The PCE and DCB solubilization studies were initially performed using 14C-labeled organic compounds purchased from Sigma Chemical. However, the resulting PCE and DCB solubility curves intercepted the y-axis well above their reported solubilities in water. Subsequent experiments yielded aqueous solubilities of approximately 1500 and 2500 mg/L for DCB and PCE, respectively. These findings indicated that a 14C-labeled contaminant, more soluble in water than

TABLE 3. Solubilization Ratios and Micelle-Water Partition Coefficients for Organic-Surfactant Systems at 10 and 25 °Ca dodecane surfactant

1,2-DCB

WSR

MSR

log Kmw

WSR

MSR

log Kmw

WSR

MSR

log Kmw

25 10 25 10 25 10

0.041 0.034 0.048 0.033 0.090 0.064

0.21 0.18 0.16 0.11 0.69 0.49

8.65 8.58 8.55 8.40 9.02 8.93

0.44 0.32 0.61 0.40 0.91 0.61

2.34 1.72 2.14 1.41 7.18 4.81

4.63 4.58 4.62 4.55 4.73 4.70

* * * * 1.74 1.24

* * * * 15.51 11.32

* * * * 4.73 4.72

Tergitol NP-15 Witconol SN-120 Witconol 2722 a

PCE

T (°C)

An asterisk indicates that values were not reported due to nonlinear solubility curves.

FIGURE 1. Aqueous solubilities of (a) dodecane, (b) PCE, and (c) DCB in solutions of Witconol 2722 at 10 and 25 °C and in the presence of 500 mg/L CaCl2 at 25 °C. Dotted lines represent 95% confidence intervals for the linear regressions. PCE or DCB, was present in the working solutions. Such behavior was not observed for dodecane, and thus, LSC procedures were employed only for the dodecane solubilization experiments.

Results and Discussion Effect of Surfactant and Organic Properties on Micellar Solubilization. Equilibrium solubility curves for dodecane, PCE, and DCB in micellar solutions of Witconol 2722 at 25 °C are shown in Figure 1. The aqueous solubilities of these compounds increased linearly over the range of surfactant concentrations studied. Similar trends were observed for the solubilization of dodecane and PCE in solutions of Tergitol NP-15 and Witconol SN-120. The linear enhancements in solubility above the cmc are consistent with solubilization data reported for other hydrophobic organic compounds of environmental concern (8, 11, 12, 20). This behavior is generally attributed to the incorporation or partitioning of organic solutes within surfactant micelles (e.g., refs 11 and 32). At a surfactant concentration of 50 g/L, the solubility of

dodecane was approximately 6 orders of magnitude greater than the reported solubility of dodecane in water (3.7 µg/L) (Figure 1a). These data illustrate the potential capacity of nonionic surfactants to enhance the recovery of NAPLs in conventional pump-and-treat remediation systems. Laboratory studies have confirmed that flushing with a 4% aqueous solution of Witconol 2722 substantially reduces the number of pore volumes of aqueous solution required to remove residual dodecane and PCE from soil columns packed with Ottawa sand (4, 6). It should be noted, however, that micellar solubilization may be rate-limited under typical flow regimes. The potential effects of rate-limited solubilization on NAPL recovery and the design of surfactant flushing schemes have been explored by Abriola et al. (33). To further investigate the role of surfactant structure and organic properties in micellar solubilization, solubility data were expressed in terms of the molar solubilization ratio (MSR). Initially, weight solubilization ratios (WSRs) were computed from the solubility curves shown in Figure 1 using a least-squares, linear regression procedure (SYSTAT). In general, r 2 values of greater than 0.995 were obtained for all organic-surfactant systems studied, except for DCB in Witconol SN-120 and Tergitol NP-15. The WSR values were then converted to MSRs using the appropriate molecular weight of each surfactant and organic compound. The resulting MSR data (shown in Table 3) indicate that, although the HLB values of each surfactant are similar, Witconol 2722 solubilized 2-3 times more dodecane and PCE than Tergitol NP-15 and Witconol SN-120. The larger solubilization capacity of Witconol 2722 was attributed to the greater alkyl chain length (C18) and EO number (20) of this surfactant. Since the reported micelle aggregation numbers of the surfactants are similar (Table 1), the longer hydrophobic chain of Witconol 2722 results in a substantially larger micelle core volume. This explanation is consistent with micellar solubilization data presented by Ismail et al. (34), who found that the solubility of barbiturates increased with the hydrophobic chain length of polysorbate surfactants. These findings demonstrate that surfactant HLB should not be utilized as the sole selection criterion for nonionic surfactants and that surfactant structure must be considered when evaluating surfactants for micellar solubilization capacity. For all of the surfactants examined, significantly more PCE was solubilized than dodecane on both a weight and a molar basis (Table 3). It has been proposed that saturated aliphatic hydrocarbons are solubilized within the hydrophobic core of surfactant micelles, while polarizable hydrocarbons, such as PCE, may be distributed between the polyoxyethylene (POE) mantle and hydrophobic core (35-38). Dodecane is extremely hydrophobic (log Kow ) 6.51), and thus, solubilization would be expected to occur primarily within the hydrophobic core of surfactant micelles. In contrast, PCE (log Kow ) 2.88) is likely to be incorporated within both the POE mantle and the hydrophobic core of the micelle. These interactions may account for the greater overall solubilization capacity observed for PCE, even though PCE is less hydrophobic than dodecane. Although DCB (log Kow ) 3.38) is slightly more hydrophobic than PCE based on log Kow values, DCB was more soluble in


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FIGURE 2. Effect of surfactant HLB on the solubilities of (a) dodecane and (b) PCE in 40 g/L solutions of Tergitol NP surfactants at 25 °C. The relationship between surfactant HLB and micelle core volume is shown in panel. micellar solutions of Witconol 2722 than PCE. This behavior may have resulted from the greater potential for specific interactions between DCB and hydrophilic EO groups, as indicated by the greater dielectric constant and dipole moment of DCB relative to PCE (9.93 vs 2.28 and 2.5 vs 0.0 D). Thus, a larger amount of DCB may have been incorporated within the POE mantle. These results suggest that complex interactions may occur between ethoxylated nonionic surfactant micelles and polarizable hydrocarbons, such as DCB and PCE. Hydrophile-Lipophile Balance (HLB) Scans. The effects of surfactant HLB on the solubility of dodecane and PCE in 40 g/L solutions of nonylphenol ethoxylates are shown in Figure 2. Each HLB value corresponds to a distinct Tergitol NP series surfactant with an average EO number ranging from 8 to 40 (see Table 1). The capacity of the nonylphenol ethoxylates to solubilize dodecane was found to decrease as the HLB or EO number increased (Figure 2a). A similar trend has been observed for the solubilization of heptane by nonylphenol ethoxylates (15). To further investigate this relationship, the micelle core volume (Vc) of each nonylphenol ethoxylate was estimated using equations presented by Tanford (32):

Vc ) NaXVh ) Na[27.4 + 26.9 (Nc - 1)]

(5)

where Na is the aggregation number, Vh is the volume of the hydrophobic group, and Nc is the number of carbon atoms in the hydrophobic group. The values of Na given in Table 1 were interpolated from aggregation data reported by Schick et al. (25) for a series of nonylphenol ethoxylates at 25 °C. These calculations show that the volume of the hydrophobic core decreases as the HLB increases, due primarily to a reduction in the aggregation number. When compared to the solubility data shown in Figure 2a, the decrease in micellar

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core volume closely matches the observed reduction in solubilization capacity. A similar relationship was reported by Diallo et al. (9) for the solubilization of hexane, decane, and dodecane in 0.01 M solutions of dodecyl alcohol ethoxylates. These results indicate that the capacity of ethoxylated nonionic surfactants to solubilize alkanes is governed primarily by the volume of the micellar core. The HLB solubility scan for PCE in 40 g/L solutions of Tergitol NP surfactants is shown in Figure 2b. As the HLB increased from 12.3 to 17.8, the solubility of PCE rapidly attained a maximum value of ∼130 g/L and then declined gradually to a value ∼5 g/L. The observed maximum PCE solubility at HLB ) 12.9 is consistent with solubilization data reported by Saito and Shinoda (15) for a nonylphenol ethoxylate with an EO number of 9.2 (HLB ) 13.0) (Figure 2b). In their work, Saito and Shinoda (15) observed a reduction in the cloud point of the nonylphenol ethoxylate from 56 to 31 °C in the presence of PCE. The cloud point is defined as the temperature at which the surfactant solution separates into two distinct phases; a micelle-free phase containing surfactant monomers at a concentration equal to the cmc and a surfactant-rich micellar phase that may have a thick cloudy or white particulate appearance (10). In general, the cloud point temperature increases as the surfactant HLB becomes larger (i.e., as the surfactant becomes more soluble in water). At the lowest HLB examined (12.3), visual observations indicated that the cloud point was exceeded. Upon centrifugation, the surfactant-rich phase separated from the aqueous phase, resulting in the extremely low solubility of PCE (0.3 g/L) observed for Tergitol NP-8. At an HLB of 12.9 phase separation was not observed, indicating that the cloud point temperature was greater than 25 °C. The large amount of PCE solubilized at HLB ) 12.9 can be attributed to the increased aggregation number at low HLBs and the shift from spherical to asymmetric micelles commonly observed as the cloud point temperature is approached (10, 39). These two factors result in the formation of larger micelles that can accommodate a greater amount of PCE. As the HLB was further increased from 12.9 to 17.8, the surfactant aggregation number decreased (Table 1), resulting in a gradual decline in PCE solubility similar to that observed for dodecane. The results of a similar HLB scan, conducted by Fountain et al. (5) at 18 °C, are shown in Figure 2b. An additional data point, obtained for the solubility of PCE in Tergitol NP-10 at 17 °C (16), is also included. Although the general trends observed in the HLB solubility scans are similar, the maximum PCE solubility reported by Fountain et al. (5) was substantially less than the value obtained for the Tergitol NP surfactants at 25 °C. The observed differences in the magnitude and position of maximum PCE solubility are most likely due to the lower temperature at which the experiments of Fountain et al. (5) and West (16) were conducted. Below the cloud point, a temperature increase often results in a moderate increase in hydrocarbon solubilization. This trend has been attributed to increased thermal agitation of the surfactant molecules within micelles or a gradual increase in the aggregation number with temperature (10, 40). Macroemulsion Formation. Macroemulsions were formed when either PCE or DCB was equilibrated with the aqueous surfactant solutions using an oscillating shaker. The macroemulsions appeared as a milky-white phase that existed between the aqueous and organic phases and, in some cases, consumed the entire organic phase. In systems containing macroemulsions, the aqueous phase was cloudy or opaque in appearance. For all samples containing PCE and DCB, centrifugation was utilized to break the thermodynamically unstable macroemulsions and to allow for sampling of the aqueous phase (5, 41, 42). In systems containing PCE, macroemulsion formation was not as prevalent in systems containing DCB, and linear solubility curves were obtained for all of the surfactants evaluated. In addition, consistent

FIGURE 3. Aqueous solubility of DCB in solutions of Tergitol NP-15 and Witconol SN-120 at 25 °C. The effect of increasing the amount of DCB in each reactor from 2 to 4 mL on measured values of DCB solubility is shown. PCE solubility curves were obtained when either 0.5, 1.0, 1.5, or 2.0 mL of PCE was added to 20 mL of surfactant solution. The apparent solubility of DCB in aqueous solutions of Witconol SN-120 and Tergitol NP-15, however, approached zero at surfactant concentrations below 40 g/L and yielded nonlinear solubility curves (Figure 3). When the volume of DCB added to the surfactant solution was increased from 2 to 4 mL, further reductions in DCB solubility were observed. These results suggest that significant amounts of Witconol SN-120 and Tergitol NP-15 were incorporated within DCB macroemulsions or partitioned into the DCB liquid phase. The role of macroemulsions in surfactant remediation technologies has received limited attention in the environmental literature. Fountain et al. (5) and Pennell et al. (6) observed the formation of macroemulsions during surfactant flushing of soil columns containing PCE. However, micellar solubilization and NAPL mobilization were the dominant recovery processes in these experiments, and the presence of macroemulsions did not significantly alter flow and transport properties. More recently, Okuda et al. (17) reported that macroemulsions accounted for up to 30% of the PCE recovered during flushing with solutions of Triton X-100. Taken in concert with data presented herein, these observations indicate the need to carefully evaluate the influence of macroemulsion formation on NAPL recovery during surfactant flushing, particularly when dealing with chlorinated solvents and other polarizable hydrocarbons. Effect of Temperature and Calcium Chloride on Solubilization Capacity. The effect of temperature and calcium chloride on the solubilization capacity of Witconol 2722 was also investigated (Figure 1). An increase in temperature from 10 to 25 °C resulted in a 30% increase in the WSR values for all systems studied with the exception of dodecane in Tergitol NP-15, which increased on the order of 20%. This effect can be attributed to (a) changes in the aqueous solubility of the organic compound and (b) changes in the properties of surfactant micelles with temperature. For the systems examined here, it is likely that the latter mechanism is dominant due to the tendency for surfactant aggregation number to increase with temperature (40). Shinoda and Takeda (43) reported a 60% enhancement in the MSR of decane in methoxydodecaoxyethylene decyl ether when the temperature was increased from 10 to 30 °C. This effect corresponded to an increase in the aggregation number of the surfactant from 78 to 85. As noted in the previous section, increased temperature may also lead to enhanced solubilization due to thermal agitation of surfactant molecules (10). These results clearly demonstrate that the effect of groundwater temperature on solubilization capacity must be ac-

FIGURE 4. Relationship between (a) micelle-water (Kmw) and octanol-water (Kow) partition coefficients for this and several other studies and (b) measured and predicted values of log Kmw based on the correlation developed by Jafvert et al. (13). counted for in the design and evaluation of remediation strategies based on surfactant flushing. The addition of calcium chloride, at levels representative of natural groundwater concentrations (500 mg/L), had no significant effect on the micellar solubilization of dodecane, PCE, and DCB by Witconol 2722 (Figure 1, Table 3). At high concentrations (>60 g/L), CaCl2 has been shown to enhance the solubility of heptane in 1% solutions of a nonylphenol ethoxylate surfactant (43). This behavior is most commonly attributed to competition between the ether oxygen of the EO group and cations for water. The competitive effect results in “salting out” of the surfactant, which is often manifested as a lowering of the cloud point and an increase in the aggregation number (40). Data shown in Figure 1, however, demonstrate that for typical groundwater systems the amount of salt present will not be sufficient to influence the solubilization capacity of nonionic surfactants. This lack of sensitivity to background electrolyte concentration is one of the properties that makes nonionic surfactants attractive for subsurface remediation applications. In contrast, both the solubilization capacity and phase behavior of anionic surfactants are extremely sensitive to the presence of salts. Relationship between Octanol- and Micelle-Water Partition Coefficients. The capacity of each surfactant to solubilize dodecane, PCE, and DCB was expressed in terms of the micelle-water partition coefficient (Kmw) based on eq 2 (Table 3). The measured values of Kmw are plotted versus the octanol-water partition coefficient (Kow) of each organic compound in Figure 4a. Valsaraj and Thibodeaux (12) performed a similar analysis using an anionic surfactant, sodium dodecyl ulfate (SDS), and found that log Kmw was linearly related to log Kow for 11 hydrophobic organic compounds. A linear log-log relationship was also obtained by Edwards et al. (8) for the solubilization of several polycyclic


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aromatic hydrocarbons in aqueous solutions of Triton X-100 (Figure 4a). The intercept reported by these investigators was slightly larger than that derived by Valsaraj and Thibodeaux, indicating that Triton X-100 has a greater solubilization capacity than SDS. Although the log Kmw values obtained for PCE and DCB in the present study were consistent with the correlation of Edwards et al. (8), the log Kmw values for dodecane were considerably larger than would be anticipated based upon their correlation. Data reported by Diallo et al. (9) for the solubilization of several alkanes and aromatic hydrocarbons by a dodecyl alcohol ethoxylate (HLB ) 14.2) are also plotted in Figure 4a. The log Kmw-log Kow correlation obtained from those data is consistent with the experimental values of log Kmw obtained for dodecane in the present study. This appears to indicate that the surfactants evaluated herein have a greater capacity to solubilize extremely hydrophobic compounds, such as dodecane, than Triton X-100. The similarities in the molecular structure of Triton X-100 and Tergitol NP-15, however, lead to the conclusion that the properties of the organic compound may also influence micelle-water partitioning. To further investigate the role of surfactant and organic properties in the solubilization of hydrophobic organic compounds, an attempt was made to relate the micellewater partition coefficient and surfactant molecular structure to the octanol-water partition coefficient (Kow) of the organic compound using an empirical correlation developed by Jafvert et al. (13):

Kmw ) Kow[aNc - bNh]

(6)

Here a and b are fitted coefficients, Nc is the number of carbons in the hydrophobic group, and Nh is the number of hydrophilic groups (sorbitan carbons and ethoxy groups). Jafvert et al. (13) defined the micelle-water partition coefficient in units of liters per mole (i.e., Kmw ) MSR/Co,cmc), and thus, their Kmw values were divided by (1 + MSR)Vw to allow for equivalent comparisons with the data obtained in this study. The coefficients a and b were determined using a nonlinear regression procedure (SYSTAT) with a loss function of the form

[(Kmw - Kmw,estimated)/Kmw]2

(7)

The coefficients a and b derived from the hexachlorobenzene data reported by Jafvert et al. (13) were 1.65 and -0.30, respectively. A comparison of experimental and predicted Kmw values based on this correlation is shown in Figure 4b. Solubilization data reported by Edwards et al. (8) and Kile and Chiou (11) are also plotted on this graph. The solid symbols shown in Figure 4b correspond to the dodecane, PCE and DCB solubilization data obtained at 25 °C and the HLB scans for PCE and dodecane. The experimental values of log Kmw for dodecane and PCE measured in this study were greater than those predicted by the correlation of Jafvert et al. (13). Values of log Kmw obtained in this study were also larger than those reported by Edwards et al. (8) for a similar log Kow range (Figure 4b). The coefficients a and b obtained from a least-squares fit of eq 6 to the dodecane, PCE, and DCB solubilization data were 24.2 and -7.6, respectively. The discrepancy in the fitted coefficients suggests that the correlation employed by Jafvert et al. (13) is not suitable for the hydrophobic organic compounds considered in the present study. It is important to recognize that the data used to develop the correlation presented by Jafvert et al. (13) did not include any chlorinated solvents or normal alkanes. This analysis suggests that factors in addition to organic hydrophobicity (represented by the octanol-water partition coefficient) and the number of “hydrophobic” and “hydrophilic” carbons in a surfactant may need to be considered. For example, it may be necessary to directly

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account for specific interactions between the POE shell and polarizable organic compounds and the observed effects of molar volume on the solubilization of alkanes (9, 34, 44).

Acknowledgments The authors thank Mr. Tom Yavarski, Ms. Carol Steer, Ms. Paulina Belave, and Ms. Michele Vose for their contributions to this work and the Witco and Union Carbide Corporations for donating surfactants. Funding for this work was provided by the U.S. EPA Kerr Environmental Research Laboratory under Cooperative Agreement CA-818467 and by the Great Lakes and Mid-Atlantic Hazardous Substance Research Center (HSRC) under Grant R-815750. Partial funding for HSRC research activities was also provided by the Michigan Department of Natural Resources and the U.S. Department of Energy. This paper has not been subject to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.

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(31) Demond, A. H.; Linder, A. S. Environ. Sci. Technol. 1993, 27, 2318-2331. (32) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980; Chapter 6. (33) Abriola, L. M.; Dekker, T. J.; Pennell, K. D. Environ. Sci. Technol. 1993, 27, 2341-2351. (34) Ismail, A. A.; Gouda, M. W.; Motawi, M. M. J. Pharm. Sci. 1970, 59, 209-224. (35) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620-1627. (36) Mukerjee, P. In Solution Chemistry of Surfactants, Vol. 1; Mittal, K. L., Ed.; Plenum Press: New York, 1979; pp 153-174. (37) Nagarajan, R.; Ruckenstein, E. In Surfactants in Solution, Vol. 2; Mittal, K. L., Ed.; Plenum Press: New York, 1984; pp 923-947. (38) Xia, J.; Hu, Z. In Surfactants in Solution, Vol. 5; Mittal, K. L., Bothorol, P., Eds.; Plenum Press: New York, 1986; pp 10551065. (39) Clint, J. H. Surfactant Aggregation; Chapman and Hall: New York, 1992; pp 82-129.

(40) Mackay, R. A. In Non-ionic Surfactants: Physical Chemistry; Surfactant Science Series, Vol. 23; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; pp 297-367. (41) Chaiko, M. A.; Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1984, 99, 168-182. (42) Amos, D. A. Solubilization of Chlorinated Hydrocarbons in Aqueous Micellar Solution. M. S. Thesis, University of California at Berkeley, Berkeley, CA, 1989. (43) Shinoda, K.; Takeda, H. J. Colloid Interface Sci. 1970, 32, 642646. (44) Klevens, H. B. Chem. Rev. 1950, 47, 1-74.

Received for review July 10, 1996. Revised manuscript received December 30, 1996. Accepted January 14, 1997.X ES960604W X

Abstract published in Advance ACS Abstracts, March 15, 1997.


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Solubilization of Dodecane, Tetrachloroethylene, and 1,2

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