Environ. Sci. Technol. 1991, 25, 127-133
Solubilization of Polycyclic Aromatic Hydrocarbons in Micellar Nonionic Surfactant Solutions David. A. Edwards, Rlchard G. Luthy," and Zhongbao Liu
Department of Civil Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
w Experimental data are presented on the enhanced apparent solubilities of naphthalene, phenanthrene, and pyrene resulting from solubilization in aqueous solutions of four commercial, nonionic surfactants: an alkyl polyoxyethylene (POE) type, two octylphenol POE types, and a nonylphenol POE type. Apparent solubilities of the polycyclic aromatic hydrocarbon (PAH) compounds in surfactant solutions were determined by radiolabeled techniques. Solubilization of each PAH compound commenced at the surfactant critical micelle concentration and was proportional to the concentration of surfactant in micelle form. The partitioning of organic compounds between surfactant micelles and aqueous solution is characterized by a mole fraction micelle-phase/aqueousphase partition coefficient, K,. Values of log K , for PAH compounds in surfactant solutions of this study range from 4.57 to 6.53. log K , appears to be a linear function of log KO,for a given surfactant solution. A knowledge of partitioning in aqueous surfactant systems is a prerequisite to understanding mechanisms affecting the behavior of hydrophobic organic compounds in soil-water systems in which surfactants play a role in contaminant remediation or facilitated transport.
Introduction Contamination of soil by toxic and/or hazardous organic pollutants is an environmental concern. Hydrophobic organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), are of special interest because they are strongly sorbed to soil or sediments ( I , 2). As a consequence, remediation of hydrophobic organic contamination in soil-water systems is often dependent on desorption of the contaminant from the soil surface and subsequent incorporation of the pollutant into the bulk aqueous phase. Once in the bulk aqueous phase, engineered treatment systems may be used to effect remediation. Surfactants may be beneficial for use in soil-washing or soil-flushing pump-and-treat technologies by assisting solubilization of sorbed hydrophobic contaminants (3-5). Surfactants may also impact microbial remediation of hydrophobic organic contaminants in soils by affecting the accessibility of the organic compound to microorganisms. Implementation of surfactant treatment for soil remediation must account for a number of factors, including efficient surfactant recovery and reuse. Other factors that must be considered include the surfactant modification of hydrophobic organic compound soillwater partitioning, clay interactions with surfactant and injection water, and surfactant sorption, biodegradation, and effects on biota. These various issues are not addressed in this paper, the scope being limited to a single process among the many physical, chemical, and biological processes that may take place in the course of successful surfactant remediation of soil. In addition to the potential benefit that may result from deployment of surfactants in remediation of contaminated soil and sediments, there is also interest in understanding the role of surfactants in facilitating the transport of hydrophobic organic compounds. The immense quantity of surfactants used in industrial and household applications 0013-936X/91/0925-0127$02.50/0
gives rise to concern for the effects of these compounds in the environment (6). Aggregates of surfactant molecules, or micelles, may act as organic colloids and increase the mobility of contaminants in the subsurface (7). Presently, there is little understanding of the interaction of surfactants and hydrophobic organic compounds in soil or sediment systems. The purpose of this investigation is to explore the solubilization of PAH in nonionic surfactant solutions in order to determine mole fraction micelle-phase/aqueous-phase partition coefficients. These data can then be coupled with additional information from PAH solubilization experiments in soil-water systems in order to allow prediction of PAH compound partitioning between water, surfactant micelles, and soil.
Surfactant Monomers and Micelles A surfactant molecule is amphiphilic, having two distinct structural moieties, one polar and the other nonpolar. The polar moiety of the molecule has an affinity for water and other polar substances, while the nonpolar moiety is hydrophobic. As a result of its amphiphilic nature, a surfactant molecule may dissolve in water as a monomer, adsorb at an interface, or be incorporated with other surfactant molecules as part of a micelle. At surfactant concentrations less than a compound-specific threshold value, surfactant molecules exist predominantly in monomeric form, with some fraction being adsorbed at system interfaces. The surfactant concentration at which monomers begin to assemble in ordered, colloidal aggregates is termed the critical micelle concentration (CMC). The CMC represents a narrow concentration range over which the partial derivatives with respect to surfactant concentration of many solution properties, e.g., surface tension, display abrupt changes in value (8). In micelle-forming solutions, the CMC approximates monomeric solubility. At surfactant concentrations greater than the CMC, additional surfactant is incorporated into the bulk solution through micelle formation (9). The average number of surfactant molecules in a micelle is called the aggregation number. Nonpolar surfactant moieties spontaneously associate with each other in the process of micellization to form organized, dynamic chemical structures having such geometrical configurations as spheres, oblate spheroids, or prolate spheroids (10). The hydrophobic portion of each molecule in the micelle is directed inward, toward the center of the aggregate, forming with the other hydrophobic moieties a liquid core, which has a fairly smooth boundary with the outlying hydrophilic chains and polar solvent (11, 12). The central region of the micelle thus constitutes a hydrophobic pseudophase that is distinct in its properties from the polar solvent (13). The hydrophilic portion of a micellar molecule of a nonionic surfactant, the type used in this study, is a hydrated, oxygen-containing chain directed outward toward the solvent (8). The surfactant concentration at which micelle formation starts is a function of surfactant chemistry, temperature, ionic strength, and the presence and type of organic additives (8). CMC values for a number of surfactant solutions at different temperatures are tabulated in the compilation
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 25,
No. 1, 1991 127
of Mukerjee and Mysels (14).
Table I. Nonionic Surfactants Employed in This Study
Solubilization of Polycyclic Aromatic Hydrocarbons The apparent solubility of otherwise slightly soluble hydrophobic organic compounds may be dramatically enhanced in solutions of surfactants at concentrations greater than the CMC. The hydrophobic core of each micelle can accommodate a certain amount of lipophilic organic compound as a solubilizate (8). The amount of organic compound that is solubilized depends on surfactant structure, aggregation number, micelle geometry, ionic strength and chemistry, temperature, solubilizate chemistry, and solubilizate size (15). Solubilization of hydrophobic substances commences at the CMC and in general is a linear function of surfactant concentration over a wide range of surfactant concentrations greater than the CMC (8). With highly hydrophobic compounds, e.g., DDT, a lesser degree of apparent solubility enhancement may also occur in monomeric solution (6). Surfactant solubilization results in an isotropic colloidal solution, which is stable in the sense that it has the lowest possible sum of free energies of its components (15). Polycyclic aromatic hydrocarbons are examples of hydrophobic compounds that may be solubilized by surfactants. As early as 1934, researchers began experimenting with surfactants to enhance the apparent solubility of PAH compounds (16). An example of work regarding surfactant solubilization of PAH is that of Klevens ( I n , who reported solubilization of benzene and 12 different PAH compounds in potassium laurate solution.
PAH Partitioning between Micellar and Aqueous Pseudophases A measure of the effectiveness of a particular surfactant in solubilizing a given solubilizate is known as the molar solubilization ratio (MSR). The molar solubilization ratio is defined as the number of moles of organic compound solubilized per mole of surfactant added to solution (15). The increase in solubilizate concentration per unit increase in micellar surfactant concentration is equivalent to the MSR. In the presence of excess hydrophobic organic compound, the MSR may be obtained from the slope of the curve that results when the solubilizate concentration is plotted against surfactant concentration. The MSR for solubilization of PAH compounds may be calculated as follows: where is the apparent solubility of a PAH compound in moles per liter at the CMC; SpAH,,ic is the total apparent solubility of the PAH compound in moles per liter in micellar solution at a particular surfactant concentration greater than the CMC; and Csd is the surfactant concentration at which SpAH,mic is evaluated. An alternative approach in quantifying surfactant solubilization consists of characterizing the partitioning of the organic compound between micelles and monomeric solution with a mole fraction micelle-phase/aqueous-phase partition coefficient. The micelle-phase/aqueous-phase partition coefficient, Km,is the ratio of the mole fraction of the compound in the micellar pseudophase, Xm,to the mole fraction of the compound in the aqueous pseudophase, Xa (18). The value of Kmis dependent on surfactant chemistry, solubilizate chemistry, and temperature. Km may be calculated from experimental measurements by using the following formula: Km = Xm/Xa 128
Environ. Sci. Technol., Vol. 25, No. 1, 1991
(2)
surfactant
symbol
Brij 30 Igepal CA-720 Tergitol NP-10 Triton X-100
C12& C8PElz CSPEl0,, C8PE9.5
av mol formula
av MW
CizHzsO(CHzCHz0)4H C8H17C6H40(CHzCH20)12H C9H19C&0(CHzCHz0)1~.5H C,H,,C,H,O(CHzCHzO)ssH
363 735 683
625
The mole fraction of PAH compound in the micellar pseudophase, X,, may be calculated as xm = (SPAH,mic - SPAH,cmc) / (Csurf - CMC -k SPAH,mic SPAH,cmc) (3) or, in terms of the MSR, as X, = M S R / ( l + MSR) (4) The mole fraction of PAH in the aqueous pseudophase, Xa, is approximated for dilute solutions by xa = SPAH,cmcVw (5) where V, is the molar volume of water, e.g., 0.01805 L/mol at 25 "C. An expression for Km is thus Km = (SPAH,mic - SPAH,cmc) / [tCsurf - CMC + SPAH,mic SPAH,cmc)(SPAH,cmcVw)l (6) This paper presents experimental observations for batch-test surfactant solubilization of PAH using 1 2 surfactant-PAH combinations in aqueous solution. K, values are derived for each combination from the data. The PAH compounds of this study consist of a two-ring compound, naphthalene; a three-ring compound, phenanthrene; and a four-ring compound, pyrene. The surfactants of this study are all polyoxyethylene (POE) nonionic compounds. Nonionic surfactants chosen for batch tests include one alkyl POE type, two octylphenol POE types, and one nonylphenol POE type. Together, these three groups represent more than 70% of the 1.72 billion pounds of U S . nonionic surfactant production in 1986 (19). Nonionic surfactants may have specific advantages compared to anionic or cationic surfactants in regard to certain aspects of engineered remediation of contaminated soils because of differences in surfactant charge, CMC range, toxicity, and biodegradability. The selection of the surfactants employed in this investigation was guided by prior experimentation dealing with solubilization of PAH compounds in soil/water suspensions (20). Experimental Procedures The surfactants employed in batch tests of this study are described in Table I. The surfactants were used as received from supplier or distributor without further purification. It is assumed that the number distribution of oxyethylene groups per molecule of each type of surfactant was heterogeneous since this is the case with nearly all commercially available nonionic surfactants (14). The PAH compounds of this study had purities greater than 98% and were obtained from Aldrich Chemical Co. The formulas and selected properties of the PAH compounds are listed in Table 11. 14C-labeledPAH compounds were acquired from Amersham Corp. with specific activity values of 4.5 mCi/mmol for naphthalene, 11.3 mCi/mmol for phenanthrene, and 56 mCi/mmol for pyrene. Batch tests for solubilization of PAH in surfactant solution were performed at 25 "C for 12 distinct systems, with each system comprising one of the four surfactants of Table I and one of the three PAH compounds of Table 11. Each surfactant-PAH system involved 8-10 batch tests with surfactant solutions having a range of concentrations above and below the CMC. Replicate measurements were performed for each test. An individual batch test sample
Table 11. Formulas and Properties of Polycyclic Aromatic Hydrocarbons of This Study
compd naphthalene phenanthrene pyrene
mol formula CloHs C14H10 CI6HlO
MW
solubility," mol/L
log Kowb
128 178
2.5 X lod 7.2 X lo4 6.8 X lo-'
3.36 4.57 5.18
202
"Solubilities as reported by Mackay and Shiu (21). *log KO, values from Karickhoff et al. (22).
consisted of a 5-mL solution containing deionized water, PAH stock, and surfactant stock, all of which were in a glass vial having a capacity of 8 mL. The vial was sealed with an open-port screw cap, which was fitted with a Teflon-lined septum to prevent loss of PAH from solution. An individual PAH stock solution consisted of a mixture of a 14C-labeledPAH compound, nonlabeled PAH compound, and methanol. Each I4C-labeled PAH compound was supplied as a solid and was extricated from its packaging with either 10 or 20 mL of methanol to prepare a 14C-labeledsolution. The radiolabeled solution was kept refrigerated in a foil-wrapped glass vial equipped with a Teflon-lined septum to protect the PAH in the solution from loss. The activity of the solution was determined before the experiments were conducted by counting the decay rate of several solution samples in a Beckman LS 500 TD liquid scintillation counter (LSC). A sample was prepared by expressing a measured volume of the solution (e.g., 10 pL) into a scintillation vial containing 10 mL of Scintiverse I1 liquid scintillation cocktail obtained from Fisher Scientific. The LSC counted the decay events per minute (DPM) of the radiolabeled PAH by employing the H-number quench monitor and compensation technique. The average background decay rate of 40 DPM was subtracted from the measured DPM. The 14C-labeledsolution activity was obtained by dividing the corrected DPM by the solution volume and then converting this value to the equivalent number of moles of PAH per liter of solution by using the conversion factor of 2.22 X lo6 DPM/pCi and the specific activity of the radiolabeled PAH compound. With a knowledge of the radiolabeled solution activity, each PAH stock, consisting of a predetermined mass ratio of radiolabeled to nonlabeled PAH, was prepared in deionized water so as to ensure that the batch-test sample activity would be at least 1-2 orders of magnitude greater than the background decay rate, and that the PAH mass in each batch test would be 20-80 times the PAH mass required to attain aqueous solubility, in order that the progress of solubilization upon addition of surfactant could be observed over a surfactant concentration range several orders of magnitude in value. The mass ratio of total PAH to radiolabeled PAH in each PAH stock was on the order of 102-103,but varied for the different PAH compounds. Batch tests for a particular PAH compound and a given surfactant combination employed a duplicate series of 8-10 vials, each consisting of solutions of varying surfactant concentration. Two surfactant stocks, one with a dilution factor of 1000 and the other with a dilution factor of 100, were made up in deionized water for each surfactant. A measured volume of surfactant stock was expressed by syringe into each batch-test vial. The more dilute surfactant stock was used to make the four or five solutions in a given series that were to have the lower surfactant concentrations, and the less dilute stock was used to make the remaining solutions that were to have the higher surfactant concentrations. The volume of surfactant stock to be added was calculated beforehand such that the re-
sultant surfactant concentrations would span a range of several orders of magnitude: e.g., and lo-' M. The appropriate volume of water was added so that after addition of PAH stock, the total solution volume per vial would be 5 mL. A uniform volume of PAH stock prepared as previously described was then added to each vial in the two series. The prepared sample vials were placed in a water bath a t 25 "C and reciprocated at 80 cycles/min for approximately 24 h. Individual sample solutions were then processed in the following manner. Sample solution was withdrawn by syringe and expressed through a Teflon membrane filter of 0.22-pm pore diameter, and then the solution was discarded, this process being repeated three times consecutively in order to allow sorption saturation of all internal surfaces of the syringe, filter, and needle. This was found in control tests to be necessary so as to prevent experimental artifacts. Duplicate aliquots were then withdrawn from the sample with the same syringe and needle and expressed through the preconditioned filter in order to remove solid-phase PAH and to pass only dissolved PAH in the aqueous pseudophase, and solubilized PAH in the micellar pseudophase, into two prepared scintillation vials. The volume of sample in each aliquot was selected to be either 0.5,1, or 2 mL, depending on the series of experiments, but within a given series the aliquot volume was uniform. DPM values were then measured in the LSC to at least the 95% confidence level and recorded for subsequent background correction and conversion to PAH concentration units. Background DPM rates were periodically measured in scintillation vials containing scintillation cocktail but no sample. The influence of methanol, present in each vial at approximately 1'YO by volume, on CMC value and solubilization effectiveness was evaluated for pyrene, the least soluble of the PAH compounds used in these experiments, and for which the potential influence of methanol would be the most apparent. The tests were performed by adding pyrene stock solution to each vial and allowing the solvent to evaporate to dryness over a period of several hours. After this evaporation step, C8PEIzstock solution in various predetermined amounts was added to the vials along with sufficient deionized water to bring the total solution volume of each vial to 5 mL. The entire process was then repeated as before, with the exception that the methanol fraction of the added pyrene stock solution was not permitted to evaporate before being mixed with surfactant solution. Surface tension experiments to evaluate the CMC values of the four commercial surfactants of this study were conducted in a temperature-regulated laboratory at 24-25 " C with a Fisher Tensiomat Model 21 Du Nouy ring tensiometer. Surfactant solutions of varying concentration were made with surfactant stock and deionized water and were allowed to equilibrate for approximately 2 h before measurements were made. All glassware was cleaned with chromic acid solution, and the ring was cleaned with acetone and heated to redness in a gas flame. Multiple testing of each surfactant solution was performed to ensure that consistent readings were obtained, and corrections were made for the dial reading and the ring geometry in order to arrive at surface tension values (23).
Results and Discussion Approximately 1% methanol was used in each batch test. Methanol was employed as a means of extricating 14C-labeledPAH compound from its commercial packaging; methanol was also used in the preparation of highconcentration nonlabeled PAH solution. which was mixed Environ. Sci. Technol., Vol. 25,
No. 1 , 1991 129
Table 111. PAH Solubility and Mole Fraction Micelle-Phase/Aqueous-PhasePartition Coefficients surfactant
PAH compd
Brij 30
PAH solubility," mol/L no surfactant Csurf= CMC
naphthalene phenanthrene pyrene naphthalene phenanthrene pyrene naphthalene phenanthrene pyrene naphthalene phenanthrene pyrene
Igepal CA-720 Tergitol NP-10 Triton X-100
3x 9x 1x 3x 1x 8X 3x 1x 8X 3x Ix 1x
10-4 10-6 10-6 10-4
10-5 10-4 10-5 lo-' 10-4
10-5 10-6
3.4 x 2.0 x 1.1x 3.2 x 1.1x 2.1 x 4.0 x 1.5 x 1.2 x 3.2 x 1.3 x 1.9 x
10-4 10-5 10-6 10-4 10-5 10-6
10-4 10-5 106
10-4 10-5
10-6
MSR 3.17 X 1.52 X 7.15 X 3.23 X 1.04 X 4.25 X 3.68 X 1.60 X 5.76 X 3.38 X 1.11 x 3.52 X
lo-' lo-'
lo-' lo-' lo-'
lo-' lo-'
10-1 low2
log K , 4.59 5.57 6.53 4.63 5.68 6.01 4.57 5.72 6.41 4.64 5.70 6.03
With 1% by volume methanol. 0 Wio Methanol With Methanol
2e-5 lgepal CA-720
-2 E
v
a,
1
Tergitol
- 2e-4
NP-10
(C9PE10.5)
le-5 -
C
2
x
a
.
AqueousSolubility
Oe-3 Oe-5 Oe-4
2e-4
4e-4
6e-4 Be-4 Surfactant Concentration (moliL)
Flgure 1. Solubilization of pyrene by C8PE,, nonionic surfactant in the presence and absence of 1 % by volume methanol. (Axis notation denotes base 10 exponentiation.)
with the radiolabeled solution to create the PAH stock. This allowed for dosing sufficiently high PAH concentrations such that batch-test solutions would have apparent PAH solubilities 20-80 times in excess of aqueous solubility as needed to measure the progress of solubilization. Methanol was a preferred solvent, as tests discussed below showed no effect on surfactant solubilization; most higher alcohols could not be utilized since they affect CMC values significantly (8). Although for batch tests for low-volatility PAH compounds it would have been possible to selectively evaporate methanol from PAH stock and still retain much of the PAH compound prior to addition of surfactant and water, this process would not be not feasible using higher volatility PAH compounds such as naphthalene because of loss to the atmosphere while methanol was being evaporated. Table I11 provides measured values of apparent PAH solubility in solutions prepared with methanol but without surfactant. These values may be compared to reported aqueous solubilities given in Table I. Measured apparent PAH solubilities at zero surfactant concentration in the presence of 1% by volume methanol appear to be enhanced by -20-30% relative to reported solubilities in pure water. This result is consistent with other experimental data for naphthalene solubility in the presence of 1%methanol (24). The partitioning properties of PAH compounds are considered to be affected only slightly in the presence of a small amount of methanol (25). An assessment of ' methanol in a batch test would measurably whether 1% alter either the observed CMC or the slope of the solu130
Environ. Sci. Technol., Vol. 25, No. 1, 1991
1e-3
2e-3
Surfactant Concentration (moliL) Flgure 2. Solubilization of phenanthrene by C,PE,,5 nonionic surfactant. The slope of the solubilization curve is equal to the molar solubilization ratio. (Axis notation denotes base 10 exponentiation.) 7e-5
/
I
Oe-5 Oe-4
2e-4
4e-4
6e-4
8e-4
Surf act ant Concentration ( mo IiL) Figure 3. Solubilization of phenanthrene by COPE,, nonionic surfactant. The CMC is indicated by the intersection of the two linear regions of the solubilization relationship. (Axis notation denotes base 10 exponentiation.) bilization curve is displayed in Figure 1 for a solution of pyrene and octylphenyl POE surfactant. It is apparent from this figure that the presence of methanol did not shift the CMC, nor did it change the slope of the solubilization curve. The molar solubilization ratio (MSR) remained constant. Solubilization Relationships. PAH solubilization was plotted as a function of surfactant solution concentration for each data set. Plots of apparent phenanthrene solubility versus concentration of the surfactants CgPE10.5and C8PEI2are shown in Figures 2 and 3, respectively. In Figure 2, solubilization of phenanthrene by micellar C9PE10,5is characterized by the MSR for that portion of the apparent solubility curve that correlates with CgPE10,5
Table IV. Critical Micelle Concentration Measurements for Commercial Nonionic Surfactants of This Study As Determined by Solubilization and Surface Tension Methods CMC, mol/L surf tension tests
surfactant
solub tests
Brij 30 Igepal CA-720 Tergitol NP-IO Triton X-100
2 x 10-6 (2-3) x 10-4 5 x 10-6 2 x 10-6
2.3 X 2.3 x 10-4 5.4 x 10-5 1.7 x 10-4
concentrations in excess of the CMC. The value of the CMC for this surfactant is approximately 5 X 10" mol/L, denoted as in other tests by a sharp increase in the apparent solubility at this concentration and confirmed by independent surface tension measurements. It is evident that the relationship between the apparent phenanthrene solubility and the C9PE10,5concentration above the CMC is linear. Figure 3 shows for C8PE12that there is only a very small increase in phenanthrene apparent solubility at surfactant concentrations less than the CMC, compared to increases in solubility at surfactant concentrations in excess of the CMC. The slopes of all apparent PAH solubility curves of this study are linear at concentrations above the CMC, except for surfactant concentrations close to the CMC where there may be small curvature in the solubilization relationship due to surfactant inhomogeneities. The proportional dependence of PAH solubilization on micellar surfactant concentration results from the mass of surfactant that is added in excess of the mass of surfactant needed to attain the CMC, being manifest in bulk solution by the formation of micelles. This results in increased micelle volume, and the greater micelle volume present in bulk solution provides greater volume of hydrophobic micellar pseudophase available for PAH solubilization, with the extent of PAH partitioning per micelle being effectively constant. When there is a diffuse rather than sharp inflection of the apparent solubility curve in the vicinity of the CMC of a commercial surfactant, it is believed that this is the result of either organic impurities in the surfactant solution or a polydisperse oxyethylene number distribution for the hydrophilic moieties of the surfactant molecules of the product (6, 14). The presence of polyoxyethylene chains of various lengths in a surfactant solution depresses to some extent the CMC in comparison to the CMC of specially prepared and purified homogeneous surfactants denoted by the same chemical formula (414). An example of an indistinct curve inflection near the CMC is shown in Figure 3. In such a case, the CMC is obtained by taking the intersection of the projections from the linear portions of the apparent solubility curve above and below the CMC and confirming this estimate by surface tension measurements. Critical Micelle Concentrations. Table IV shows CMC values estimated from solubilization data. Shown also for comparison are CMC values for the identical commercial surfactant products as determined by surface tension experiments in deionized water. Figures 4 and 5 show examples of determination of CMC values from surface tension experiments for solutions of C,PE,,, and C12PE4,respectively, in deionized water. Data from these experiments allow determination of the CMC as the surfactant concentration denoted by the intersection of the two linear portions of a curve showing variation in surface tension as a function of the logarithm of C, If the effect of hydrocarbon solubilizates on CMC values is slight, as indicated in ref 8, then the CMC values in deionized water
20
-7
-6 -5 -4 -3 Log (Surfactant Concentration, moliL)
Flgure 4. Determination of CMC by surface tension measurements for C,PE,,, nonionic surfactant.
-
. 6
70
60 x
-0
v
c 50
.-0 ln
+ 40 er 2 30 c L
3 v)
20
I
-6
-5 -4 -3 -2 -1 Log (Surfactant Concentration, moliL)
Figure 5 . Determination of CMC by surface tension measurements for C12E4nonionic surfactant.
should approximate CMC values in the presence of PAH. If, on the other hand, the amount and type of hydrocarbon solubilizate affects the CMC value of a surfactant, then there may be differences between CMC values in the presence of PAH solubilizates and CMC values in water. There may also be minor differences in CMC values for solutions with different solubilizates, as can be seen in Figures 1 and 3. In general, however, the CMC values obtained from the surface tension data for solutions of deionized water and surfactant show fairly close agreement to the CMC values inferred from the PAH solubilization data. MSR and Micelle-Phase/Aqueous-Phase Partitioning. Determination of the slope of the linear portion of the apparent solubility curve at concentrations greater than the CMC for a given surfactant-PAH combination provides a direct numerical value for the MSR. The MSR reflects the capacity of 1 mol of a particular surfactant in micelle form to accommodate a given PAH solubilizate. Quantitatively, the MSR represents the average number of molecules of solubilizate per micelle divided by the aggregation number. The numerical value of the mole fraction micelle-phase/aqueous-phase partition coefficient, K,, may be calculated from the MSR by using the following formula derived from eqs 1, 2, 4, and 5: Krn = (55.4/S~~~,crnc)[MSR/(l + MSR)I
(7)
Table I11 shows values of measured aqueous PAH soluMSR, r n c ,and log K , for the 12 surfactantbility, S P ~ ~ , c PAH combinations used in this study. The parameter K , represents organic compound partitioning between nonpolar and polar pseudophases. Thus, for a given surfactant, K , values can be correlated with other hydrophobic-hydrophilic partitioning coefficients Environ. Sci. Technol., Vol. 25, No. 1, 1991
131
Triton X-100 (C8 P E 9,5) 6
Naphthalene
1p
4 ! 3
G D D S (Valsaraj an5 TI bodeaux, 1989)
l 4
5
6
compounds in C,PEg,, nonionic surfactant solution. The values of log K, for 1,2,3-trichlorobenzene and DDT are determined from solubility data of Kile and Chiou (6).
such as the octanol-water partition coefficient, KO,. Figure 6 displays log K, values plotted against log KO, values for solubilization of five hydrophobic organic compounds in Triton X-100 at 25 "C. Three of the data points in Figure 6 represent the PAH compounds of this study with log KO, values from Karickhoff et al. (22) and log K , values as given in Table 111. The other two data points are for solubilization of DDT and 1,2,3-trichlorobenzene. The log KO, values for DDT and 1,2,3-trichlorobenzene are 6.19 (26) and 4.14 (271, respectively. The log K , values for solubilization of these two compounds by Triton X-100 are computed from solubility data of Kile and Chiou (6). The solute solubility at the CMC is obtained by projecting the linear part of the pre-CMC portion of the apparent solubility curve to its intersection with the projection of the linear part of the post-CMC portion of the apparent solubility curve, a step that is necessary because of the heterogeneous OE number distribution of Triton X-100. The MSR value is obtained from the slope of the linear part of the post-CMC portion of the curve. The log K, value for DDT in Triton X-100 is calculated as 6.87 by using a SDDT,cmc value of 9.35 X mol/L and a MSR value of 1.28 X lo-*. A log K , value of 5.17 is obtained for 1,2,3trichlorobenzene. Figure 6 shows that the relationship between log K, and log KO, in Triton X-100 solution is highly linear, and the data for the other surfactant solutions suggest similar linear relationships between log K, and log Kow. log K,-log KO,Correlation. The findings shown in Figure 6 are in accord with the results of Valsaraj and Thibodeaux (et?),who demonstrated a linear relationship between log K , and log KO,for various hydrophobic organic compounds in micellar sodium dodecyl sulfate solution. This relationship is displayed in Figure 7 along with that for Triton X-100 solution. The slope obtained by plotting log K, against log KO, for the sodium dodecyl sulfate solution is nearly equivalent to the slope for the Triton X-100 solution, whereas the intercept is smaller. Such correlations may be dependent on the type of surfactant (34,as well as the units of expression of the partitioning relationship and whether there is an effect of solubility in the micelles due to high Laplace pressures in the micelles because of the curved interfaces (28). An additional observation from this study is that the mole fraction of PAH in the micelle pseudophase in these solutions is negatively correlated with log Kow. The mole fraction of PAH in the micelle pseudophase is given by xm = KmVwSPAH,cmc (8) A plot of the values of K,VwSpAH,cmcfor solubilization of 132
Environ. Sci. Technol., Vol. 25, No. 1, 1991
1
7
Log Kow Figure 8. Correlation of log K, and log KO, for five hydrophobic
"
I
' 2
l
.
8
3
,
4
I
5
,
I
6
,
7
Log Kow Comparison of log K,-log KO, correlations for hydrophobic
Figure 7. solubilizates in octylphenol polyoxyethylene surfactant (C,PEg,,) sodium dodecyl sulfate solutions.
.-C
I
a a S
0
2
0.2 -
a
and
improved mechanistic models for understanding these phenomona in environmental systems.
Conclusion The apparent solubilities of naphthalene, phenanthrene, and pyrene were measured in solutions of nonionic polyoxyethylene surfactants. PAH solubility increased linearly with surfactant dose at bulk aqueous concentrations of surfactant in excess of the critical micelle concentration. The slope of such a relationship for a each solution of surfactant and PAH compound was used to determine the molar solubilization ratio, MSR, and the PAH mole fraction micelle-phase/aqueous-phase partition coefficient,K,. The PAH compounds were solubilized in the range of -0.044.4 mol of PAH/mol of micellar surfactant with K, values in the range of 104.6-106.5.Values of log K , for a particular surfactant-PAH system appear to be correlated with PAH octanol-water partition coefficients. These data can be used with additional information on surfactant and PAH sorption on soil to estimate PAH solubilization in soil-water-surfactant systems. Acknowledgments We express our appreciation to Annette M. Jacobson and Shonali Laha who provided useful comments. Registry No. Brij 30, 9002-92-0; Igepal CA-720, 9036-19-5; Tergitol NP-10,9016-45-9; Triton X-100,9002-93-1;naphthalene, 91-20-3; phenanthrene, 85-01-8; pyrene, 129-00-0.
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