Langmuir
1992,8,
1291-1299
1291
Solubilization of Polycyclic Aromatic Hydrocarbons by Poly(ethylene oxide-propylene oxide) Block Copolymer Micelles: Effects of Polymer Structure Patricia N. Hurter and T. Alan Hatton' Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received December 19, 1991. I n Final Form: March 2, 1992 The partitioning behavior of three polycyclicaromatic hydrocarbons between water and micelles formed by poly(ethy1eneoxide-propylene oxide)block copolymersof varying structure and composition is reported. The micelle-water partition coefficient increased with increasing polypropylene oxide content of the polymer and with molecular weight. The structure of the polymer was important, with linear copolymers solubilizing naphthalene more effectively than branched copolymers. For the three polycyclic aromatic hydrocarbons studied (naphthalene, phenanthrene, and pyrene) there was a strong correlation between the micelle-water partition coefficient and the octanol-water partition coefficient. The strong solubilizing powers of block copolymer micellar solutions can be exploited to remove and/or recover halogenated and unhalogenated, aromatic, and polyaromatic hydrocarbons from contaminated coastal, surface, and groundwater sources, and from industrial and domestic effluents.
Introduction Block copolymers are high molecular weight polymers having two or more distinct regions of differing properties. For instance, an A-B diblock copolymer can be synthesized such that one region of the polymer, the A block, say, is lyophilic while the other, the B block, is lyophobic. When the B block is strongly incompatible with the solvent (water, in the discussion that follows),the molecules begin to aggregate to form micelles in which the lyophobic blocks cluster together within a nonpolar, hydrocarbon-like core, protected from the more polar solvent. The hydrophilic blocks extend into the surrounding aqueous solvent, forming a "corona" around the micelle interior (Figure 1). The core of the micelle constitutes a hydrophobic environment, which is extremely attractive to nonpolar solutes. The unusually high capacity of block Copolymer micelles for hydrophobic solutes can be exploited in the development of novel, effective ways to remove trace contaminants such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) from aqueous waste streams and contaminated sources. Pollution of the nation's harbors, waterways, and groundwater resources with these contaminants, owing tourban runoffs, waste disposal policies, and industrial and domestic effluents, has reached critical prop~rtionsl-~ and poses many serious environmental, health, and economic problems. The increasing awareness of these problems has focused attention on the need to identify effective and energy-efficient means for the removal of toxic substances from water resources and industrial effluents, and this is where block copolymer micellar solutions may play an important role. While many methods have been developed for the destruction of organic pollutants such as PCBs and PAHs, including supercritical water oxidation: ozonation?I6
* To whom all correspondence should be addressed.
(1) Hoffman, E. J.; Mills, G. L.; Latimer, J. S.; Quinn, J. G. Enuiron. Sci. Technol. 1984, 18, 580-687. (2) Boehm, P. D.; Steinhauer, W.; Brown, J. Organic Pollutant Biogeochemistry Studies Northeast US.Marine Environment Final Report Contract No. NA-83-FA-C-00022;N O M Washington, DC, 1984. (3) Shiaria,M. P.;Jambard-Sweet, D. Mar. Pollut. Bull. 1986,17,469472.
Hydrophilic blocks
& Hydrophobic blocks
Figure 1. Schematic diagram of a block copolymer micelle, with the hydrophobic core being shielded from the polar solvent by a hydrophilic corona.
irradiation with UV light, and incineration:^^ these methods are only feasible economically if applied to relatively concentrated solutions. PAHs, however, have water solubilities in the ppm range and are toxic a t even these low levels. It is thus apparent that a water-treatment method which concentrates trace quantities of organic solutes from aqueous solution could find widespread application. Two large-scale processes that can be used for treatment of contaminated aqueous sources are solvent extraction and activated carbon a d s ~ r p t i o n .Solvent ~~ extraction relies on the strong partitioning of solutes toward a waterimmiscible solvent phase for removal and concentration of the contaminant. It does not appear to be an attractive route for the removal of these toxic substances on a large scale, however, because of the problems associated with solvent losses caused by the finite, albeit very small, solubility of traditional solvents in water and because of (4) Johnston, J. B.;Hannah, R. E.; Cunningham, V. L.; Daggy, B.P.;
Sturm, F. J.; Kelly, R. M. BiolTechnology 1988,6, 1423-1427. (5) Rice, R. G. Ozone Treatment of Industrial Wastewater; Noyes
Data Corp.: Park Ridge, NJ, 1981. (6) Ackerman, D. G.; Scinto, L. L.; Bakshi, P. S.; Delumyea, R. G.; Johnson, R. J.; Richard, G.; Takata, A. M.; Swonyn, E. M. Destruction and Disposal of PCBs by Thermal and Non-Thermal Methods; Noyes Data Corp.: Park Ridge, NJ, 1983. (7) Piver, W. T.; Lindstrom, F. T. Enuiron. HealthPerspect. 1986,SS; 163-177. (8)Smith, E. H.; Weber, W. J. Enuiron. Sci. Technol. 1988,22, 313321. (9) Ruthven, D. M.; Ching, C. B.Chem. Eng.Sei. 1989,44,1011-1038.
0743-7463/92/2408-1291$03.00/00 1992 American Chemical Society
1292 Langmuir,
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VoZ.8, No. 5,1992
the invariable entrainment of finely dispersed solvent droplets by the water leaving the extractor.lO Indeed, this countercontamination of the water feed by the solvent may pose as severe a problem as that being treated. Adsorption using activated carbon and other adsorbents could be considered. However, the materials handling characteristics are such that fixed bed processes may not be favorable because of the sheer bulk of materials to be treated, the high costs associated with pumping the water through the bed, only part of which is “active”at any given time, and the cost of energy-intensive adsorbent regeneration.6911 In a recently developed process, we have used aqueous solutions of block copolymer micelles of the type discussed above, which act as selective solvents for organic polluta n t ~ . Since ~ ~ *the ~ ~micelles are, in essence, supramolecular aggregates dissolved in water, they are completely miscible with the water to be treated. This solution can be prevented from mixing with the bulk water feed, however, by retention within the lumens of hollow-fiber membranesof sufficientlylow molecular weight cutoff that even the individual polymer molecules themselves do not pass through the membrane pores (conventional surfactant micelles are not considered suitable for this application because the monomers with which they are in equilibrium would be lost by simple diffusion through the membranes to the feed stream). The membrane is permeable to the solutes, which are able to transfer from the solution external to the hollow fibers to the micellar solution, where they are solubilized by, and concentrated within, the micelles. Thus the aqueous micellar solution can be considered as a separate solvent phase in this application and can be treated as though it were immiscible with the contaminated feed phase. The high capacity of the micelles for organic solutes ensures that the driving force for mass transfer is high and that the water phase concentrations can be reduced significantly with only a small volume of the “solvent phase” relative to the bulk volume of the water being treated. The large interfacial areas required for the efficient contacting of the micellar solution with the water feed can be achieved readily using hollow fiber m0du1es.l~ The concept is illustrated more fully in Figure 2. Once the micelles are laden with solute, they can be processed to regenerate the “solvent phase” (i.e., the block copolymer micellar solution) by removing the toxic compounds. A number of regeneration methods could be feasible. It is possible to induce a phase change in the micellar system resulting in the expulsion of the solubilized contaminant species either as a separate phase, or as a solid or waxy precipitate. Temperature or salinity swings may be used in this step. Another possibility is to ultrafilter the micellar solution to remove most of the suspending water and then contact it with a low molecular weight solvent having a high capacity for the solutes. For instance, highly volatile solvents such as dichloromethane, or even supercritical fluids,15 could be used for micellar solution regeneration, with subsequent recovery of these solvents by flashing; a concentrated, ~
~~~~~~
~
~~
~
(10) Mackay, D.;Medir, M. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; John Wiley and Sons: New York, 1983; pp 619-625. (11) Schechter, R. S.,Center for Petroleum and Geosystems Engineering, University of Texas at Austin, personal communication, 1991. (12) Hatton, T.A.; Hurter, P. N.; Prioleau, G. The 1990lnternutional Congress on Membranes and Membrane Processes; Proceedings; North American Membrane Society: Chicago, IL, 1990. (13) Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. AIChE National Meeting; Los Angeles, paper no. 47e, 1991. (14) Dahuron, L.;Cussler, E. L. AIChE J. 1988,34,130-136. (15) McFann, G. J.; Johnston, K. P.; Hurter, P. N.; Hatton, T. A. Manuscript in preparation.
U
Figure 2. Contaminated water contacts the micellar solution in a hollow fiber membrane extractor. The micelles are retained within the hollow fibers. Solutes in the contaminatedfeed water (such as naphthalene) are free to pass through the membrane pores and are concentratedwithin the hydrophobic cores of the micelles.
nonvolatile contaminant residue would be obtained in th& way. Following this solvent extraction step, the residud solvent in the block copolymer phase can be flashed off to ensure no carryover to the treated water stream on recycling of the micellar solutions. Another potential method is to rely on the biodegradation of the pollutants, provided appropriate microorganisms can be identified for this purpose.16 In all cases, the high concentration of the recovered toxic organics in the micelles will ensure low processing volumes in this regeneration step. Other processes which rely on the high solubilization capacity of micelles include micellar-enhanced ultrafiltration, and soil washing using aqueous surfactant solutions. Scamehorn and co-workers have investigated the use of micellar-enhanced ultrafiltration to treat water contaminated with pollutants such as organicsl7 and multivalent metal ions.18J9 In this scheme the surfactants are added directly to the feed stream in sufficient quantity to ensure micelle formation, and the feed stream is ultrafiltered with retention of the solute-laden micelles by a membrane. McDermott et al.20 and Abdul and Gibson21 have investigated using aqueous surfactant solutions for extracting PCBs from contaminated soil. The soil is washed in a countercurrent extraction process, then the loaded surfactant solution is treated with calcium chloride to precipitate the PCBs and surfactant. In both of these examples, high contaminant removal efficiencies could probably be obtained using polymeric surfactants, or block (16) Vogel, T. M.; Criddle, C. S.;McCarty, P. L. Enuiron. Sci. Technol. 1987,21, 722. (17) Christian,S.D.;Scamehom,J. F. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989; Vol. 33, pp 3-28. (18) Scamehorn,J. F.;Christian, S. D.; Ellington,R. T. In SurfactantBased Separation Processes; Scamehorn,J. F., Harwell,J. H., Eds.; Marcel Dekker: New York, 1989; Vol. 33, pp 29-56. (19) Sasaki,K.J.; Burnett, S. L.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1989,5,363-369. (20) McDermott, J. B.;Unterman, R.; Brennan, M. J.; Brooks, R. E.; Mobley, D. P.; Schwartz, C. C.; Dietrich, D. K. Environ. Prog. 1989,8, 4651. (21) Abdul, A. S.;Gibson, T. L. Environ. Sci. Technol. 1991,25,665671.
Solubilization of Polycyclic Aromatic Hydrocarbons
copolymers, rather than conventional detergents, although this has not yet been addressed. For these processes to be evaluated as viable alternatives to conventionaltreatment methods,a sound understanding of the block copolymer micellization and solubilization properties must be developed. While some work has been reported on these topics, as noted below, there is still much to be done. In particular, the effect of polymer architecture on solubilization has not been investigated to any significant extent. In this paper we report results on a class of poly(ethy1ene oxide-propylene oxide) block copolymers. A number of researchers have investigated the formation of micelles by poly(ethy1ene oxide-propylene oxide) triblock copolymers. Some earlier studies which attempted to measure the critical micelle concentrations (cmc’s) of these polymers, using surface tension measurements and dye solubilizationtechniques, reported results varying over 2 orders of m a g n i t ~ d e . ~There ~ - ~ ~are a number of possible explanations for these discrepancies, one being that micelles are formed by single molecules at low concentrations and that multimolecular aggregates form at higher concentrations. Zhou and Chu26performed a light-scattering study on the temperature-induced micellization behavior of a triblock copolymer of ethylene oxide and propylene oxide and found that three temperature regions existed: a unimer region, a transition region, and a micellar region. They postulated that since different research groups concentrated their studies in only one of the regions, different results were obtained. Polydispersity in the samples could also lead to anomalous results being obtained by different techniques, while there is the possibility that the cmc is so low that values being reported are actually the limits of detection of the methods being used, rather than the true cmc. A recent study by Wanka et al.,27using surface and interfacial tension measurements, as well as static and dynamic light scattering and small angle neutron scattering techniques, found that micelles did form above a critical micelle concentration of around w t % ,with aggregation numbers ranging from 30 to 100. The polymers used in this study had molecular weights from 5000 to 13000 and ethylene oxide compositions ranging from 30 to 70 w t %. The nonpolar cores of these micelles can take up significant quantities of hydrophobic solutes, the solubilization efficiency being dependent on the relative sizes of the lyophilic and lyophobic blocks and on the overall block copolymer molecular weight. Nagarajan et al.28 report that aromatic solutes are solubilized selectively over aliphatic solutes,and they obtained solubilization of simple aromatics such as benzene and toluene of up to 1-2 g of solute/g of polymer in aqueouscopolymer solutions in equilibrium with bulk aromatic phases. The high capacities of block copolymer micelles for solutes of low polarity have also been noted by Collett and TobinB for para-substituted acetanilides and Lin and Kawashima30for indomethacin. Tontisakis et al.31found that o-xylene, which is a selective (22) Schmolka, I. R.; Raymond, A. J. J.Am. Oil Chem. SOC.1965,42, 1088-1091. (23) Schmolka, I. R. J. Am. Oil Chem. SOC.1977,54, 110-116. (24) Prasad, K. N.; Luong, T. T.; Florence, A. T.; Paris, J.; Vaution, C.; Seiller, M.; Puisieux, F. J. Colloid Interface Sei. 1979, 69, 225-232. (25) Anderson, R. A. Pharm. Acta Helv. 1972,47,304-308. (26) Zhou, Z.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171-180. (27) Wanka, G.; Hoffman, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101-117. (28) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986,2,210215. (29) Collett, J. H.; Tobin, E. A. J. Pharm. Pharmacol. 1979,31,174177. (30) Lin, S.; Kawashima, Y. Pharm. Acta Helv. 1985,60, 339-344. (31) Tontisakis, A.; Hilfiker, R.; Chu, B. J. Colloidlnterface Sci. 1990, 135,427-434.
Langmuir, Vol. 8, No. 5, 1992 1293
4iD
PEO
Pluronlc
I PPO
Tetronic
Figure 3. Structure of the Pluronic and Tetronic block copolymers. PEO refers to poly(ethy1eneoxide) and PPO refers to poly(propy1ene oxide).
solvent for poly(propy1eneoxide), is solubilized within the core of poly(ethy1ene oxide-propylene oxideethylene oxide) block copolymer micelles. There have also been studies of solubilization of organic molecules in micelles of traditional surfactants. Kile and C h o found ~ ~ ~significant solubility enhancement of DDT and trichlorobenzene above the critical micelle concentration, in a variety of commerical surfactants. The solubilization of phenol in polyethoxylated nonionic micelles was studied by Kandori et al.33 A linear correlation between the micelle/water partition coefficient and the octanol/water partition coefficient of the solute, for each surfactant, was found by Edwards et al.34for the solubilization of various PAHs in four low molecular weight (360-735) commercial surfactants. These studies confirm the ability of micelleforming amphiphilic molecules to enhance significantly the solubility of hydrophobic solutes in aqueous solution. In this paper the effects of polymer structure, composition, and molecular weight on the partitioning behavior of three PAHs in micellar solutions of poly(ethy1ene oxide propylene oxide) block copolymers are explored. The variation in the micelle-water partition coefficient of naphthalene, under saturation conditions, is reported as a function of the hydrophobic content of the polymer and as a function of polymer molecular weight. The solubilization efficiency of linear, triblock copolymers is compared with that of starlike branched copolymers. The effect of the solute structure is demonstrated by comparing the partition coefficients of naphthalene, phenanthrene, and pyrene, which have two, three and four fused benzene rings, respectively.
Experimental Section Materials. Copolymers of poly(ethy1ene oxide) (PEO)and poly(propy1ene oxide) (PPO) were used in this study. Pluronic and Tetronic block copolymerswere obtained as a gift from the BASF Wyandotte ChemicalCorp. These polymers are available in a range of molecular weights and poly(ethy1ene oxide)-poly(propyleneoxide) ratios; the general structure of these polymers is illustrated in Figure 3. PEO-PPO copolymers were also obtained from PolySciences, Inc. We were unable to ascertain from the suppliers whether these polymers were in fact block or random copolymers. Table I lists the physical properties of the polymers studied. To facilitate analysis of the results, a brief description of the system of nomenclature for the BASF Pluronic and Tetronic polymers follows. The Pluronic polymers, which are the triblock copolymers, start with the letters P (for paste), L (for liquid), or F (for flakes). The first two numbers are indicative of the molecular weight of the PPO block, and the last letter signifies the weight fraction of the PEO block. For example, P103 and P104 both have the same molecular weight of PPO (on the order of 3000) but P103 has 30 w t %I and P104 (32) Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1989,23,832-838. (33) Kandori, K.; McGreevy, R. I.; Schechter, R. S.J. ColloidInterface Sci. 1989, 132, 395. (34) Edwards, D. A.; Luthy,R. G.; Liu, Z. Environ. Sci. Technol. 1991, 25,127-133.
Hurter and Hatton
1294 Langmuir, Vol. 8, No. 5, 1992
Table I. Physical ProDerties of the Polsmers mol wt PPO content (wt % PPO mol wt P103 P104 P105 P123 F108 T904 T1304 T1504 T1107 T1307 PS16273 PS16276 PS16277 PS06522
4950 5900 6500 5750 14600 6700 10500 12000 15000 18000 1100 13300 8750 12500
70.0 60.0 50.0 70.0 20.0 60.0 60.0 60.0 30.0 30.0 89.8 30.5 . 20.9 36.1
3110 3072 2730 3612 3091 3488 5467 6248 3553 4263 951 3207 1405 3629
40 wt % PEO. The Tetronic copolymers, which all have four branches, all start with the letter T; the first two numbers represent the molecular weight of the PPO (X103)and the last two indicate the weight fraction of PEO, so that for example T15M has 40 wt % PEO. The PolySciences polymersare denoted by PS followed by their PolySciencescatalog number. The polycyclic aromatic hydrocarbons, naphthalene (scintillation grade, 99+%), phenanthrene (98+% pure), and pyrene (99+% pure) were obtained from Aldrich Chemical Co. and used without further purification. Partitioning Experiments. PAHs are only sparinglysoluble in water and have a tendency to adsorb to solid surfaces, which makes it difficult to prepare accurately solutions of these compounds with known concentration. The method of May et al.35936 for the preparation of saturated aqueoussolutionsof PAHs and other sparingly-soluble compounds appears well-suited for our purposes. In this approach, water is pumped through a generatorcolumn packed withglass beads coated with the desired organic compoundand leavesthe column as a saturated solution. Continualrecirculationof this solution ensures that the solution itself is saturated throughout and that the amounts adsorbed to the walls of the tubing and other solid surfacesare in equilibrium with the solution. To prepare the generator column, the organic solute was first dissolved in dichloromethane,3-mm glass beads were added, and finally the solvent was stripped in a rotary evaporator. The beads, which were now coated with the organic solute, were packed into a 50 cm long, 25 mm i.d. glass column, fitted with a glass frit at the downstreamend. In the experiments in which naphthalene was the solute, the column was simply packed with crystals of naphthalene, which were first sieved on a 12-mesh screen to remove small particles. A schematic of the apparatus used for measuring the partition coefficients of PAHs between water and polymer solutions of various strengths is shown in Figure 4. Polymer solution was circulated continuously through the generator column using a high flow piston-diaphragm pump with all Teflon wetted parts (Cole-Parmer,Model NY7149-20). A small stream was diverted to a continuous UV flow cell where total solute concentrations could be determined. Since the uptake of organic solute by the polymer solution was normally very high, flow cells with path lengths as low as 0.01 mm were used to maintain the absorbance reading below unity. The absorbancewas measuredon a PerkinElmer Lambda 3B UV/VIS spectrophotometer, using quartz quarasil flow cells with path lengths ranging from 0.01 mm to 1 cm, obtained from Hellma Cells, Inc. The measurements were made at the absorbance maximum for each compound: 276 nm for naphthalene; 274 nm for phenanthrene; 272 nm for pyrene. Once equilibrium had been reached, a small sample of the polymer solution was extracted and analyzed on a Waters 410 differential refractometer. Water was pumped continuously through the refractometer at 5 mL/min using an Eldex metering pump. The sample was then injected into the flow stream using a Rheodyne sampling valve with a 10-pL sample loop (obtained from Varian Instruments). .The refractometer was connected to a Shimadzu CR3A integrator, and both height and area of the (35) May, W. E.; Wasik, S. P.; Freeman, D. H. Anal. Chem. 1978,50, 175-179. (36) May, W. E.; Wasik, S. P.; Freeman, D. H. Anal. Chem. 1978,50, 997-1000.
Generating Column
Water Bath Micellar Solution
Figure 4. Apparatus for partitioning experiments. The polymer solution is saturated with naphthalene by circulating it through the generating column. A small stream is diverted to a flow cell in the spectrophotometer (UV), where the solute concentration is monitored continuously. Once equilibrium has been reached, a sample is withdrawn and analyzed on a differential refractometer (DR), to determine the polymer concentration. The system is maintained at 25 "C by immersing the polymer solution reservoir in a water bath. peak were recorded. With this apparatus, polymer concentrations could be measured to within *0.05 wt %. Measurements were made over a range of polymer concentrations by diluting the polymer solution and allowing the system to re-equilibrate. The temperature was maintained at 25 "C by immersing the polymer solution reservoir in a water bath.
Results and Discussion The partitioning experiments were conducted to determine the effect of both polymer and solute structure on the partitioning behavior. Triblock and branched copolymers of poly(ethy1ene oxide) (PEO) and poly(propylene oxide) (PPO) were used, with varying proportions of the hydrophobic and hydrophilic parts and with a range of overall molecular weights. In addition, copolymers of PEO and PPO were obtained from PolySciences, Inc., which are of unknown structure. The solutes chosen were three polycyclic aromatic hydrocarbons, naphthalene, phenanthrene, and pyrene, which consist of two, three, and four fused benzene rings, respectively. The aqueous solubility of these three PAHs decreases by approximately an order of magnitude per benzene ring addition. The enhanced solubility of naphthalene in Pluronics polymers is illustrated in Figure 5a. The ratio of the naphthalene concentration in the micellar solution,relative to the saturated concentration of naphthalene in water, is plotted as a function of polymer concentration. Four of these polymers, P103, P104, P105, and F108, have the same molecular weight of PPO, the hydrophobic part, but varying PE0:PPO ratios. The fifth polymer, P123, has the same proportion of PPO as the P103, but a slightly higher molecular weight. Figure 5b shows the relationship between the micelle/water partition coefficient of naphthalene, Kmw,and the PPO content of the polymer where
Kmw= c;/c; and ck and c; are the concentrations of naphthalene in the micelle (based on total polymer mass) and in the water, respectively. Since the polymer solution is in equilibrium with the bulk solute phase, the concentration of the solute in water is known and is the aqueous solubility of the PAH in water. The total concentration of the solute in the micellar solution is measured during the experiment and is equal to the water concentration plus the.micellar concentration, weighted by their respective mass fractions in solution. The concentration of naphthalene in the micelles can thus be found by difference. In the case where there is a linear relationship between the naphthalene solubility and the polymer concentration, the partition coefficient is constant and is derived from the slope of the graph. A nonlinear relationship indicates that the par-
Langmuir, Vol. 8, No. 5, 1992 1295
Solubilization of Polycyclic Aromatic Hydrocarbons
(KPo) and PEO (KEo)phases, respectively where 4po is the mass fraction of PPO in the polymer. Equation 2 can be rearranged to yield an expression for the normalized partition coefficient, KfmW
0
1
2
3
4
5
6
7
8
Polymer Concentration (wt%)
E
=‘$
3500
b
’
I
I
I
I
I
3000. 2500.
r
g
2000.
10
h 20
I
I
I
I
I
30
40
50
60
70
80
Polymer PPO Content (wt %)
Solubility of naphthalene in Pluronic polymers: 0, P123; w, P103; e, P104; A, P105; 0 , F108. Part a shows the enhanced solubility of naphthalene solutions as a function of polymer concentration, and part b shows the variation in the micelle/watsr partition coefficient, K,,, and the partition coefficientnormalized with the polymer PPO content,K‘,,, with PPO content of the polymer. For F108, K,, is plotted at 1%, 5 % , and 8% polymer concentration. F i g u r e 5.
tition coefficient is not constant, and in this case Km, is shown at three different polymer concentrations for illustrative purposes. Figure 5 shows that we can achieve significant concentration ratios between water and the micellar solution; for example, for a 6% solution of P103 the concentration in the polymer solution is about 140times that in pure water. As the proportion of PPO in the polymer increases, at a given total polymer concentration, the solubility of naphthalene is enhanced. This is not surprising; since naphthalene is hydrophobic, it is expected to associate with the PPO,so that increasing the hydrophobicity of the polymer by increasing the proportion of PPO should lead to an increase in the uptake of naphthalene. If Kmw is normalized with PPO content (denoted by K’,,), we still see a small increase in the partition coefficient with increased PPO content of the polymer, indicating probable changes in the micelle structure. For P103,P104,and P105,the molecular weight of the hydrophobic block remains constant, and the length of the ethylene oxide block changes. Since naphthalene is expected to reside mainly in the core of the micelle, and the chains making up the core are the same for each of theae polymers, one would not expect a large change in the normalized partition coefficient. If the solubilization behavior were governed by partitioning of naphthalene between the PPO core, the PEO in the corona, and water, then a mass balance would lead to an equation relating the partition coefficient based on the total polymer mass, K,,, and the partition coefficients of naphthalene between water and the PPO
According to this result, if the solubilization behavior were related simply to the composition of the solution, then the normalized partition coefficient, K’,,, should decrease as the mass fraction of PPO in the polymer increases. Figure 5b shows that there is actually a small increase in the normalized partition coefficient with increasing PPO content of the polymer, which suggests that structural changes within the micelle, as aresult of changes in polymer composition,must be influencing the partitioning behavior. For the majority of these polymers, the partition coefficient is constant over the entire polymer concentration range studied. This would indicate that the micellar structure does not change significantly in this concentration range and that increasing the polymer concentration leads to an increase in the number of micelles rather than micellar growth. This would give rise to a linear relationship between the polymer concentration and the amount of naphthalene solubilized. Leibler et al.37showed that at concentrations far from the critical micelle concentration, micellar properties such as the aggregation number are indeed only weakly dependent on the copolymer concentration. It could be expected that the structure of the micelles would be affected by the presence of solute. Al-Saden et al.38used light scattering to measure the variation in the hydrodynamic radius of Pluronic micelles with hexane uptake, and found that the micelle radius increases with an increase in solubilizate uptake (the hydrodynamic radius of a micelle formed from L64, a pluronic polymer with 40% PEO and a molecular weight of 2900, increased from approximately 6.7 nm to 8 nm, as the hexane concentration was increased from zero to the saturation limit). This size increase was postulated to result not only from the incorporation of the solutes but also from a simultaneous increase in the aggregation number of the micelle. The naphthalene-laden micelles are thus probably larger than solute-free micelles, though the size increase would be smaller for naphthalene than for hexane, which is solubilized in much larger amounts; the saturation concentration of hexane in L64 is approximately 25 % hexane, based on the micelle mass, whereas the naphthalene concentration in the micelle varies from 2 to 8% for pluronic polymers. The fact that the micelle-water partition Coefficients for P103,P104,and P105 are constant with an increase in polymer concentration indicates that the properties of the solute-laden micelles do not change with polymer concentration, but rather the number of micelles increases. The partition coefficient of naphthalene in F108 is not constant, and this can also be explained in terms of the theory of Leibler et al.3’ Their analysis showed that the degree of incompatibility of the copolymer with solvent can be characterized by the product xN,where x is the Flory-Huggins polymer-solvent interaction parameter, which is a measure of the heat of interaction between polymer and solvent, and N is the degree of polymeriza(37) Leibler, L.;Orland, H.; Wheeler, J. C. J. Chem. Phys. 1983,79, 3550-3557. (38) Al-Seden, A. A.; Whateley, T. L.; Florence, A. T. J. Colloid Interface Sci. 1982,90,303-309.
1296 Langmuir, Vol. 8, No. 5, 1992 tion. The cmc increases with a decrease in xN, and at the cmc, the micellar properties exhibit rapid continuous variations. In addition, for polymers with small incompatibility degree x N , the concentration of free copolymer molecules in equilibrium with the micelles increases with overall polymer concentration, whereas for polymers with large xN, the free copolymer concentration remains virtually constant. Thus for block copolymers with small xN, a larger variation in the micellar properties would be expected with a change in the polymer concentration, and one would not expect a constant partition coefficient. F108, which contains only 20% PPO, is not as solventincompatible as P103, P104, and P105, so it is likely that the micellar structure changes with polymer concentration, leading to a change in the solubilization behavior, and a dependence of K,, on polymer concentration. Cowie and Siriar1ni3~measured the weight average molecular weight of F108 in aqueous solution at 25 OC using light scattering and found no evidenceof micelle formation, but it is unclear what polymer concentrations were used. It is possible that raising the temperature of the solution could induce micelle formation and lead to enhanced naphthalene solubilization. McDonald and Wong4O and Al-Saden et al.38 found that L64 did not form micelles at 25 OC but formed stable aggregates with an aggregation number of 30 at 35 "C. The phase diagrams of both ethylene oxide and propylene oxide in water exhibit a lower critical solution temperature$l indicating that raising the temperature causes water to become a poorer solvent for these polymers. The decreased solubility of ethylene oxide could be due to the breaking of hydrogen bonds between the ether oxygen and water, or to conformational changes in the ethylene oxide chain structure as temperature is i n c r e a ~ e d .Polymers ~ ~ ~ ~ ~that are relatively hydrophilic, either due to a high PE0:PPO ratio or to a low overall molecular weight, might not aggregate at room temperatures, but raising the temperature could induce aggregation as water becomes a poorer solvent. Temperature effects on the partitioning of PAHs in block copolymer micelles will be investigated in a later paper. The final point of note in Figure 5 is the increased solubility of naphthalene in P123 as compared to P103, two polymers with the same composition ratios, but differing molecular weights. The molecular weight effect is investigated more fully with the Tetronic polymers and discussed below. Figure 6 shows the solubility of naphthalene in some Tetronic polymers. T1504, T1304, and T904 all have 40 wt 3'% PEO, but different molecular weights. T1307 and T1107 have 70 wt % PEO, but are of higher molecular weight than the 40 5% PEO series. There is clearly a strong molecular weight effect for Tetronic polymers; the partitioning of naphthalene into the micelles increases with an increase in the molecular weight of the polymer. The observed dependence of the partition coefficient on polymer molecular weight could be due either to a more favorable interaction between the solute and longer PPO chains or to changes in the micelle structure. The chemical potential of a solute in a polymer solution, according to the liquid lattice theory, is given by Fl0ry44 ~
(39) Cowie, J . M. G.; Sirianni, A. F. J. Am. Oil Chem. SOC.1966, 43, 572-575. (40) McDonald, C.; Wong, C. K. J.Pharm. Pharmacol. 1974,26,556557. 141) Malcolm, G. N.; Rowlinson, J. S. Trans. Faraday SOC.1957,53, 921-931. (42) Karlstrom, G. J . Phys. Chem. 1985,89, 4962-4964. (43) Samii,A. A,; Karlstrom,G.; Lindman, D. Langmuir 1991,7,10651071. (44) Flory,P. J . Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.
Hurter and Hatton
--
a
Q
-
140
2.
120
b
*
I
I
-
2 4 6 8 Polymer Concentration (wt%)
0
Y
I
I
m
I
e
t
10
I
60% PPO
Tetronics
c-
E
Tetronlcs
4
a/
I
30% PPO) L
h
I *
0 1 5
I
RI
10 15 Molecular Weight (kDa)
I
20
Figure 6. Solubilityof naphthalenein Tetronics polymers. The closed symbols represent polymers with 60% PPO and the open symbols represent polymers with 30% PPO: 0 T1504; T1304; T904; o T1307; T1107. In part b, the partition coefficient was calculated at 1% , 5 % , and 8% polymer concentration for the cases where the partition function is not constant. The partition coefficient of P104, denoted by A, is also compared with that of the Tetronic surfactants.
+
(4) where p1 is the chemical potential of the solute, and pl* is the pure component standard state chemical potential, uz is the volume fraction of polymer, x is the degree of polymerization, and x is the Flory-Huggins interaction parameter. If it is assumed that naphthalene is solubilized within a melt of hydrophobic chains in the cores of the micelles, then eq 4 shows that if two polymer core solutions are compared, where the volume fraction of polymer in each is the same, but the polymers differ in their molecular weight, the solute chemical potential will be lower in the solution with shorter chains. Solutes would thus partition favorably into a solution where the polymer was of lower molecular weight. The observed increase in the partition coefficient with polymer molecular weight thus cannot be explained by simply considering the interaction between the polymer segments forming the core and the solutes. Clearly the molecular weight of the polymer affects the micelle structure, which has an effect on the partitioning behavior of the solute. Theoretical studies of block copolymer micelle formation have suggested that the micelle size increases with molecular weight.37.4514'3 Since the incompatibility of a polymer with a solvent is characterized by the product x N , the hydrophobicity of a polymer must increase with molecular weight, making micelle formation more favorable owing to the lower free energy required to form mi(45) Noolandi, J.; Hong, K. M. Macromolecules 1983,16,1443-1448. (46) Nagarajan, R.; Canesh, K. J. Chem. Phys. 1989,90, 5843-5856.
Solubilization of Polycyclic Aromatic Hydrocarbons
celles, compared to the polymers being uniformly distributed in the solution. Micellization results in a lower heat of mixing due to fewer PPO-water contacts, which is favorable, but this is offset by the entropic penalty of confining the polymers to the micelle. Since in equilibrium the free energy of formation of a micelle is balanced by the translational entropy of the micelles and the free copolymer molecules in solution,47 a smaller free energy of micelle formation leads to the creation of fewer, larger micelles. In fact, the analysis of Noolandi and Hong46 predicts that the size of a micelle scales as Wl3,where N is the total number of monomer units in the polymer. There are a number of possible reasons why larger micelles would solubilize more naphthalene. The excess pressure inside a spherical droplet with radius r is given by the Laplace equation4
AP = 2y/r (5) where y is the interfacial tension. It has been assumed here that the interfacial tension is curvature independent, which is reasonable for nonionic surfactants where there is no electric double layer at the interfa~e.'~At equilibrium the chemical potential of the solute in the core of the micelle (phase 1)is equal to that in bulk solution (phase 2) plo
+ Vi(P,-Po)+ kT In u1= p p o + V@p -Po)+
kTlna, (6) where plo and ppo are the standard state chemical potentials and a1 and up the activities of the solute in each phase, where the standard state is defined as an ideal solution of unit molality at pressure POand temperature T. The partial molar volume of the solute is denoted by Vi. In dilute solution, the activity coefficients are unity, so that the natural logarithm of the partition coefficient is
-I [Ap" + ViAP] (7) kT The variation in partition coefficient with radius of the micelle is thus given by taking into account eq 5
The first term in this equation accounts for changes in the composition of the core of the micelles with a change in micelle radius: this term will be discussed later. The second term is positive, and inversely proportional to the micellar radius, showing that a smaller droplet leads to a higher droplet pressure and consequently favors partitioning into the bulk solution rather than the droplet (Figure 7a). If the radius of the core does scale as WI3, the change in core radius between the T1504 and the T904 is probably of the order of 30 A. Some simple calculations show that for this effect to account for the observed change in partition coefficient for naphthalene between these two polymers, the interfacial tension would need to be of the order of 40 dyn/cm. The interfacial tension between PPO and PEO is around 2 dyn/cm,M so one would expect the interfacialtension between the core, which consists chiefly (47) Hall,D. G.; Pethica, B.A. In Nonionic Surfactants; Schick, M. J., Ed.;Marcel Dekker: New York, 1967; pp 233-296. (48)Hiemenz,P.C.A.inciplesofColloidandSurface Chemhtry;Marcel Dekker: New York, 1988. (49) Overbeek,J.T.C.;Verhoeckx,G. J.;DeB~yn,P.L.;Lekkerkerker, H. N. W. J . Colloid Interface Sci. 1987, 119, 422-441. (50)Bailey, A. I.; Saleme,B.K.; Walsh, D. J.; Zeytountaian,A. Colloid Polym. Scr. 1979, 257,948-952.
Langmuir, Vol. 8, No. 5, 1992 1297
unavailable
-@
"wwted" P W in interfacial @on
Figure 7. Illustration of the effect of micelle size on the solubilization of naphthalene: (a) the increase in droplet pressure with decrease in droplet radius leads to a lower partition coefficient; (b) exclusion of the naphthalene from the corona leads to an effective "dead volume" in the core, which becomes a larger fraction of core volume with decreasing radius; (c) the fraction of PPO used to form the core/corona interface, which is less readily available to naphthalene, becomes larger with a smaller droplet size.
of PPO, and the corona, which consists of a PEO/water mixture, to be slightly higher than the PEO/PPO value. It is unlikely, however, that the interfacial tension would be as high as 40 dynlcm. The increased droplet pressure due to a smaller droplet radius thus probably contributes to the decreased partition coefficient,but not significantly, and other effects must also come into play. Since the solute is apolar, it will probably avoid the polar corona. Small amounts of naphthalene could be solubilizedin the corona of the micelle, however the strong correlation between K,, and the PPO content of the polymer (Figure 5b) tends to indicate very little interaction between naphthalene and the PEO segments. If there is a significant energetic disadvantage in penetrating the corona, then there will be an effective dead volume in the core (a shell of thickness equal to the radius of the solute molecule), which will be less attractive to the center of mass of the solute molecule (Figure 7b). Since this dead volume is a shell of constant thickness, the fraction of the core volume that it occupies will increase with a decrease in core radius. One would therefore predict a decrease in the solubilization of hydrophobic solutes with a decrease in micelle radius. The molecular length of naphthalene so that the shell thickness would be 4 A in this is 8 case. Once again assuming a core radius change from 80 to 50 A, the decrease in core volume available to the solute is of the order of lo%, which is fairly small compared to the observed decrease in the micelle loading. If one assumes a fairly constant interfacial thickness, then the ratio of the interfacial volume to core volume will increase with a decrease in core radius (Figure 712). Since some PPO segments will be involved in forming the interface, this means less PPO available to form the core. This too will lead to a decrease in the solubilizationcapacity of the micelle. Nagarajan and Ganeshsl have proposed a simple theoretical model of solubilization in diblock copolymer mi(51) Nagarajan,R.; Ganesh, K. Macromolecules 1989,22,4312-4325.
1298 Langmuir, Vol. 8, No. 5, 1992
celles, where the micelle is modeled as comprising two regions, each of constant polymer density. The micelle core contains solvent-incompatible A blocks and solute, and the corona contains solvent and solvent-compatible B blocks. The free energy of solubilization is calculated as the sum of the contributions from the changes in state of dilution and deformation of the A blocks in the core and the B blocks in the corona, the entropic reduction due to localization of the joints linking A and B blocks in the core-corona interface, and the free energy of formation of the core-solvent interface. By correlating numerical results for a model system of PEO-PPO diblock copolymers in water with benzene as the solubilizate, scaling relations between the polymer molecular weight and the amount of benzene solubilized were obtained. These results predict that for polymers with equal block sizes, the amount solubilized should scale as 2v0.l5, where N is the total polymer molecular weight. This very slight effect of polymer molecular weight on solubilization is not borne out by our results, for which an exponent of 1.6 is obtained. It should be emphasized, however, that Nagarajan’s model is for diblock copolymers. Clearly, a more detailed theoretical analysis is required to capture the behavior of the branched tetronic polymers. There is thus a combination of effects which could explain the decrease in the micellelwater partition coefficient with a decrease in the molecular weight of the block copolymer, although the simple calculations we have performed seem to indicate that none of these explanations can account completely for the observed dependence of solubilization on the molecular weight of the Tetronic polymers. An explanation could be that since the Tetronic molecules have a branched structure, the molecular weight effect is more pronounced than it would be for linear molecules. One would expect these multibranched polymers to become less flexible as the molecular weight decreases, which would lead to a decrease in the number of possible conformations in the core of the micelle, and hence a greater increase in the free energy of micelle formation than would be the case for linear polymers. The branched molecules might also be expected to form micelles with a “looser” structure, due to steric hindrance of the branches, which would lead to a higher water concentration in the core of the micelle. A less hydrophobic core environment would have a lower affinity for hydrophobic solutes such as naphthalene. These composition effects would be reflected in the standard state chemical potential of the solute in the micellar phase, defined in eq 8. At a smaller micelle radius, with higher water core concentrations, plo would increase due to more unfavorable naphthalene-water interactions, which would lead to a decrease in the partition coefficient. Further evidence of the importance of molecular structure has been provided by ten Brinke and Hadziioannou,52 who showed that for triblock copolymers where the lyophilic block is in the center of the molecule, the reduction in entropy due to loop formation of the middle block causes a significant increase in the cmc and, in some cases, will prevent micelle formation altogether. We are currently investigating theoretically the structural effects of polymers on solubilization in using the selfconsistent mean field theory of Scheutjens and Fleer.54 It is anticipated that these detailed calculations will shed some light on the solubilization behavior observed here. In addition to illustrating the molecular weight effect, (52) ten Brinke, G.; Hadziiannou, G. Macromolecules 1987,20, 486489.
(53)Hurter,P. N.; Scheutjens,J. M. H. M.; Hatton,T. A. In preparation. (54) Scheutjens, J. M. H. M.; Fleer, G . J. J. Phys. Chem. 1979, 83, 1614-1635.
Hurter and Hatton
Figure 6 shows again the effect of PPO content on solubilization; the polymers with 30 % PPO have significantly lower partition coefficients than those with 60% PPO, for the same molecular weight. For the polymers with a relatively low PPO content (the T1307 and T1107) and the polymer with relatively low molecular weight (the T904), the partition coefficient is not constant, and not very high. This could indicate micellar growth with an increase in the polymer concentration. In Figure 6b, the partition coefficient of naphthalene in P104 is plotted for comparison purposes. This polymer also has 60% PPO, but it is a linear triblock copolymer, as opposed to the branched structure of the Tetronicseries. For the same PPO content and molecular weight, the Pluronic polymers have a significantly higher capacity for naphthalene than the Tetronic polymers. This indicates that all facets of the polymer architecture play a role in determining the affinity of the solute for the block copolymer; not only are the composition and molecular weight important, but also the microstructure of the polymer. As mentioned above, the Tetronic polymers probably do not micellize as readily as the Pluronics, due to their branched structure, which could lead to steric hindrance between the molecules and a decrease in the number of possible conformations. The third series of polymers investigated was a range of PEO-PPO copolymers of unknown structure obtained from PolySciences, Inc. Turro and ChungS5 studied the behavior of one of these PolySciences poly(ethy1eneoxide propylene oxide) copolymers in water, using photoluminescent probes. The polymer used in this study had a total molecular weight of 3000, with a molar ratio of PEO to PPO of 0.8:1.0, and was purported to be a triblock copolymer,PEO-PPO-PEO. However it has been claimed elsewhere28 that these polymers are diblock, and the suppliers (PolySciences)were unable to confirm that these polymers are indeed block copolymers rather than random copolymers. Using a variety of fluorescence probes which were sensitive to the environmental micropolarity, Turro and Chung identified three regions: Below a polymer concentration of 1 wt % , the probes resided in a highly polar region, suggesting that no aggregation occurs. In the region between 1% and 10% polymer, there is a blue shift in the fluorescence maximum, indicating that the probe is being drawn into a hydrophobic environment. It is suggested that monomolecular micelles are formed in this region. Above a polymer concentration of l o % , a further, sharper blue shift is observed, which is believed to indicate the formation of polymolecular micelles. From the time decay of pyrene monomer emission, the aggregation number of the micelles formed at high polymer concentration was determined to be 52. Nagarajan et al.28 also found enhanced solubility of some simple aromatics in a PolySciences ethylene oxidepropylene oxide copolymer. Thus it seems that these polymers either form micelles or a t least structured solutions with hydrophobic regions which are attractive to hydrophobic solutes. The enhanced solubility of naphthalene in these polymers is illustrated in Figure 8. The results confirm previous trends, as the PS16273 which has a low molecular weight (1100) has a low affinity for naphthalene despite its relatively high PPO content (89.8 wt 9%). The R16277, which contains only 21 wt % PPO also does not solubilize naphthalene effectively. However the PS16276 and PS06522, both of which have reasonable molecularweights and 30.5 and 36.1 wt % PPO, respectively, appear to solubilize naphthalene fairly well. Figure 9 compares the solubilities of naphthalene, (55) Turro, N. J.; Chung, C. Macromolecules 1984, 17, 2123-2126.
Solubilization of Polycyclic Aromatic Hydrocarbons
3
Langmuir, Vol. 8, No.5, 1992 1299 Table 11. Properties of the PAHE. log log micelle load, g of aqueous K,, KO, solute/g of polymer solubility,g/L PFene 4.94 5.18 0.0155 3.2 x 10-2 phenanthrene 4.60 4.57 0.0399 1.0 x 10-3 naphthalene 3.31 3.35 0.0703 1.3 X lo4 K,, and micelle load are for solutions of P103,KO,is reported by Kamlet et al." and aqueous solubilities by May et al.35
70
s 60
-a -a
I
50 40
0
30
v,
al c a, 2 0 m
of the same order of magnitude (in fact decreasing slightly with an increase in hydrophobicity) and the dramatic 0 increase in the partition coefficient is chiefly due to the 2 0 0 2 4 6 8 10 decrease in water solubility (Table 11). The slight decrease Polymer Concentratlon (wt%) in the micellar concentration is easily understood by comparingthe enthalpic and entropic interactions between Figure 8. Variationin the solubility of naphthalenein the PolySciences polymers, as a function of polymer concentration: . , the solute and the micellar core. For any molecule, the PS16276;A, PS06522;0 , PS16273;0 , PS16277. heat of interaction between each benzene ring and the PPO segmentswill be about equal. However as thenumber of benzene rings per molecule is increased, there is an entropic penalty to being placed in the micelle owing to the larger molecular volume of the molecule. It is thus 3 energetically more advantageousto solubilizeslightly more 6000 of the smaller molecules.
g
10
P
s
Conclusion
0
6
b
2
4 6 8 10 P i 0 3 Concentration (wtY0) I
I
1
3
4
5
12
5 1 yE 4
-m 0
3
2 2
6
log Kow
Figure 9. Solubility of some polycyclic aromatic hydrocarbons
+,
naphthalene. Part b in P103: 0, pyrene;W! phenanthrene; shows the relationship between the octanol/water partition coefficient and the micelle/water partition coefficient.
phenanthrene, and pyrene in P103, the pluronic polymer with 70 wt % PPO. This figure shows a significant increase in the solubility enhancement for the more hydrophobic PAHs. Figure 9b illustrates a strong correlation between of the solute the octanol-water partition coefficient,Kow,sG (a standard measure of hydrophobicity) and the micellewater partition coefficient. Similar relationships have been found for micelle/water partition coefficients of various hydrophobic solutes in commercial surfactants such as Triton X-100(an octylphenol poly(oxyethy1ene) surfactantu and sodium dodecyl sulfate.s7 Note that while the partition coefficient increases with the hydrophobicity of the solute, the micellar concentration of solute c: remains (56)Kamlet,M. J.;Doherty,R.M.;Carr,P. W.;Mackay,D.;Abraham,
M. H.; Taft, R. Enuiron. Sci. Technol. 1988,22, 503-509. (67) Valearaj,X. T.; Thibodeaux, L. J. Water Res. 1989, 23, 183.
The solubilization of polycyclic aromatic hydrocarbons in block copolymer micelles of poly(ethy1ene oxide-propylene oxide) depends strongly on the structure of the micelle-forming polymers, whether they are linear or branched, and on their relative compositionratios. Linear triblock copolymers are more accommodating than the branched tetronic polymers, which is attributed to their ability to form tighter micelles. It is believed that the configurational constraints on branched polymers in micellar solutions will lead to the formation of looser aggregates which will not provide as apolar an environment in the core as found in the case of the linear copolymers. As anticipated, the partitioning of PAHs to the micelles is enhanced aa the PPO content of the block copolymers increases, and this partitioning behavior correlates well with the octanol-water partition coefficient, Kow. A detailed theoretical analysis will shed more light on the reasons for the observed trends in partitioning behavior with changes in polymer composition, molecular weight, and structure; such a study is currently underway.I3 The large micelle-water partition Coefficientsmeasured in this work indicate that the extraction of hydrophobic pollutants from contaminated water and soils using block copolymer micellar solutions as novel solvents could be effective for the treatment of industrial effluents, and surface and groundwaters in environmental remediation and control strategies. One such application, the use of block copolymer micellar solutions in a hollow fiber membrane extractor (see Figure 2), will be discussed in a later paper. Acknowledgment. This work was funded (in part) by the M.I.T. Sea Grant College Program, under Federal Grant Number NA90-AA-SG-D424from the National Sea Grant College Program, from the National Oceanic and Atmospheric Administration, U.S.Department of Commerce. Additional funding was provided by Union Carbide as matching funds for an NSF Presidential Young Investigator Award to T.A.H. Rsgistry No. PEO/PPO (block copolymer), 106392-12-5; naphthalene, 91-20-3;phenanthrene,85-01-8;pyrene, 129-00-0.
