Preconcentration of Organic Compounds from Water across Dia~sisMembranes into TERESA M. PEKOL AND JAMES A. COX* Department of Chemistry, Miami University, Oxford, Ohio 45056
Interaction with micellar Brij 35 (0.20 M) enhances the transport rate of naphthalene across a cellulose ester dialysis membrane (500 Da cutoff) by a factor of 29 over that into water. This transport proceeds even when the naphthalene concentration in the receiver solution exceeds that in the sample; hence, naphthalene is preconcentrated from aqueous samples into micellar receivers held in dialysis bags. Overthe range6.5 x 10-6to 1.3 x 10-4Mnaphthalene, an enrichment factor of 3.4 f 0.3 (pooled standard deviation of six sets of dialyses over the stated concentration range, 4 replicates per set) is obtained for a 60-min dialysis of a 200-mL sample into a 2-mL receiver, micellar Brij 58 (0.050 M). Comparable results are obtained with p-dichlorobenzene as the test compound. With hexane as the receiver, the analogous enrichment factor is 5.3 f 0.3 ( n = 4). Data are presented which demonstrate that nonspecific interactions between naphthalene and the cellulose ester membrane significantly perturb the flux into micellar and hexane receivers. Solid-phase extractions of 1.3 x to 3.2 x M naphthalene in 2% ethanol solutions with 0.50 g of cellulose acetate powder were characterized by a distribution constant of 1.8.
Introduction Membrane-based preconcentrators for ionic species are well known (I). These devices are based on using chemical or electrical methods to drive test species from aqueous samples across membranes into receiver solutions. To achieve preconcentration, “uphill transport” is required that is, the flux across the membrane continues even when the analytical concentration in the receiver solution exceeds that in the sample. In addition, the volume of the receiver must be less than the volume of the sample. Most studies of uphill transport have been performed with ion-exchange or with supported-liquidmembranes. With ion-exchange membranes, electrodialysis and Donnan dialysis are the most common driving forces, whereas transport of metal ions across supported-liquid membranes generally is facilitated by complexationreactions. Donnan dialysis has been used for speciation studies as well as for analytical preconcentration (1). A common objective of liquid membrane studies is the recovery of metals from waste streams (21, but analytical preconcentrations also are being investigated ( I ) . Only in recent years have analogous methods for sampling neutral organic compounds in water been reported. Perhaps the first report, which appeared in 1987 (3), was a test of whether a hexane-filled dialysis bag simulated the bioaccumulation of selected hydrophobic compounds. In experiments where a continuously-flowed sample was allowed to equilibrate with the hexane-filled dialysis bag, preconcentration of chlorinated organic compounds occurred. The stability of the membrane-based system made it a promising alternative to biological indicators. Subsequently, Johnson (4) presented a fugacity-based theory of the preconcentration into these samplers. In that study he also reported a 25-fold preconcentration of Arochlor 1248 over a 5-day exposure of a hexane-filled, 1000 Da cutoff dialysis bag to a spring that contained a constant polychlorinated biphenyl level (4). Huckins et al. have studied the transport of nonpolar chlorinated organic compounds in mixtures with fish lipids across polyethylene membranes into cyclopentane (5). Transport rates were dependent on molecular size. For example, recoveries of dibenz[a,hlanthracene (molecular mass, 278 Da) were comparable to those of naphthalene (molecular mass, 128 Da) only when the dialysis time was extended from 4 to 24 h. Over 95% of the lipid was retained in the sample chamber after a 24-h dialysis. A variation of the experiment was the use of model lipids as receiver solutions in the transport of test compounds such as chlorinated hydrocarbons across polyethylenemembranes (6, 7). This approach was used to better mimic biopreconcentration. It was found that lipid-filled receiver chambers have an advantage over those filled with organic solvent in that less of the receiver material was transported * Corresponding author; e-mail address: ACS.MUOHIO.EDU.
0013-936W95/0929-0001$09.00/0
0 1994 American
Chemical Society
JCOX@MIAMIU.
VOL. 29, NO. 1, 1995 ENVIRONMENTAL SCIENCE & TECHNOLOGYm i
into the sample solution. As was the case with samplers filled with organic solvents, preconcentration of test compounds was observed, but as an analytical technique, the lipid-filled samplers were less convenient because of the need to recover the test compound from the receiver prior to its determination. The above approaches to preconcentration of hydrophobic compounds are not compatible with analytical methods that generally are performed in electrolytic solution orwith some approaches to wastewater treatment. For example, a goal of our program is to use membranebased preconcentration followed either by electrochemical detection of targeted compounds after liquid chromatographic separation or by electrochemical conversion of these compounds into environmentally benign substances. Neither lipids nor organic solvents are satisfactoryreceivers for these applications. The present study is the first step in this program. Micellar solutions are attractive alternatives as receivers for fugacity-based membrane preconcentrators. It is wellknown that surfactants at levels above their critical micelle concentration (cmc) mimic organic solvents in terms of solubilization of hydrophobic compounds. This property has recently been discussed in terms of the advantages of micellar media over the use of organic solvents for soil and water remediation (81. Specifically, the enhancement of the solubility of hydrophobic compounds such as toluene, naphthalene, and phenanthrene by the addition of Nvinylpyrrolidonelstyrene to water was demonstrated (8). Micellar solutions also are compatible with electrochemical methodology. For example, the electrochemical dehalogenation of organic compounds in aqueous solutions is assisted by the presence of micelles (9, 10). Further, we have recently found an electrocatalytic method for the oxidation of N-nitrosamines that permits their determination in micellar solutions (11). There is a precedent for fugacity-driven transport into micellar receivers. In a previous study, we characterized the dialysis of an amino acid (phenylalanine] across a cation-exchange membrane into micellar sodium dodecyl sulfate (12). Phenylalanine was selected as the test analyte because its aromatic ring provided a hydrophobic site that could interact with micelles. When the sample and receiver solutions were at a pH where phenylalanine was protonated, interaction with micelles was demonstrated to promote transport against the concentration gradient of the analyte (uphill transport). This result allowed the prediction that uphill transport of a neutral, hydrophobic compound across a neutral membrane into a micellar medium will occur. Presented herein is evidence that micellar media compare well with organic solvents as receivers for fugacitybased membrane preconcentrators of hydrophobic compounds. In addition, the present study evaluates the utility of transport of neutral compounds across dialysis membranes into micellar media as an analytical preconcentration method. This evaluation required investigations of membrane memory effects and of the solid-phase extraction of targeted analytes onto cellulose acetate, a major component of the dialysis membranes that were used.
Experimental Section The cellulose ester dialysis membranes (Spectra/Por)were from Spectrum Medical Industries, Inc. In all cases the molecular mass cutoff was 500 Da. Prior to use, the membranes were soaked in water to remove the sodium 2 . ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 1, 1995
azide preservative. Generally, 10-cm strips of the tubes with a flat-width of 1.6 cm and thickness of 20-65 pm (as per Spectrum Medical Industries, Inc.) comprised the cell. The strips were clamped at the bottom, 2 mL of the receiver was introduced, and then the tops were clamped. The surface area of the membrane in contact with the receiver was about 26 cm2. The dialyseswith this membrane system were performed by placing the clamped tubes into a 200mL sample. Convection was provided by magnetic stirring; the lower clamp, which was magnetic, served as the stirring bar. Some experiments were performed with a Spectrum Medical Industries MicroDialyzer. Here, sheet-type membranes were used. The material was that described above, but because it was mounted tightly in a rigid spacer by the manufacturer, the actual membrane thickness was probably less than when used as a dialysis bag. The microdialyzer had cylindricalwells with areas of 0.65 cm2that were closed with the membrane. The receiver volumes were 250 pL. The 30-mL samples were magnetically stirred. Solid-phase extraction was performed by mixing various quantities of either cellulose acetate (CAI powder or cellulose triacetate (CT) pellets with 200-mL samples of naphthalene in (a) 2% ethanol, (b) 0.25 M sodium dodecyl sulfate in 2% ethanol, or (c) hexane. Because the sorption after 1h was the same as that after 2 h, the former time was used for allreported experiments. After the extraction step, the phases were separated with a nylon syringe filter. Control samples, which were identical to those on which extraction was performed except the solid phase was not added, were run in parallel to the extraction experiments to provide ameans of correction forvolatilityandlorfiltering losses of naphthalene. After the extraction, sample absorbances were measured, and the millimoles of naphthalene remaining in solution were determined. The millimoles on the solid phase were calculated by the difference. The distribution constant, K, was determined; K is the ratio of the millimoles on the solid phase to the millimoles remaining in solution. The surfactants were generally either Brij 58 (IC1 Americas, Inc.) or Brij 35 (Aldrich Chemical Co.). The former, polyoxyethylene(20)cetyl ether, has a molecular mass of 1123.52 Da, and the latter, polyoxyethylene(23)lauryl ether, has a molecular mass of 1199.57 Da. In certain experiments, the surfactant was sodium dodecyl sulfate, SDS (98%,AldrichChemicalCo.1, forwhich the molecular weight is 288.38. Bovine albumin (98-99%) was from Sigma Chemical Co. The other chemicals, naphthalene (scintillation grade, 99+%), ethanol (spectrophotometric grade),hexane (95+%, spectrophotometric grade), cellulose acetate powder (acetyl content, 39.8%; molecular mass, 30 m a ) , and cellulose triacetate pellets, were from Aldrich Chemical Co. Unless otherwise noted, all samples and receivers were prepared in 2% (volume) ethanol with in-house distilled water that was purified with a Barnstead NANOpure I1 system. Dialyses were initiated by contacting the receiver cell to the sample. Unless otherwise noted, the dialysis times were 60 min. When dialyses into the 2-mL receivers were performed, the determinations of naphthalene were made by Wvisible spectrophotometry at 279 nm with either a Milton Roy Spectronic 3000 or a Hewlett Packard 845212 diode array spectrophotometer using 10-mm cuvets. Microdialysis experiments were quantified by flow injection analysis using
TABLE 1
Dialysis of Naphtbalene into Various Single Component and Mixed Micellar Receiver9
5 t
4
I
0 0
receiver
I
A
0
*t
O A
0.00001
0.0001
0.01
0.001
Brij Conc.,
I
II
0.1
M
FIGURE 1. Effect of the surfactant concentration in the receiver on the transport of naphthalene across a dialysis membrane. Sample, 200 mL of 1.2 x lo-' M naphthalene; receiver, 2 mL of (0)Brij 58 (cmc = 4 p M ) or (A) Brij 35 (cmc = 0.1 mM) at the plotted concentration; membrane, 500 Da cutoff cellulose ester; dialysis time, 60 min.
a Tecator FIAstar 5010 analyzer with a Shimadzu SPD-6AV spectrophotometric detector set at 279 nm. The data on preconcentrations are reported as enrichment factors (EFs). EF is the ratio of the concentration of the analyte in the receiver after dialysis to the initial concentration of the analyte in the sample. These concentrations were determined using calibration curves from standards that were prepared in matched matrices. As demonstrated herein, the combinations of sample volumes, receiver volumes, and dialysis times used in the present study permit quantlfylng the uphill transport on the basis of a fured-timekinetic model; that is, the quantity of material transported into the receiver is directly proportional to dialysis time and to the initial concentration of the analyte in the sample ( 1 ) . When these conditions hold, the EF is independent of the initial concentration of the analyte in the sample over a wide range. This model requires that transport is by a chemical driving force rather than just passive diffusion, that the system is not near equilibrium, and that the average concentration of the analyte in the sample during the dialysis is not significantly different from its initialvalue. The alternative to the furedtime kinetic method is to allow the system to reach equilibrium. Except where the receiver volume is extremely small (on the order of microliters), the equilibrium method requires a several-hour dialysis. In addition, the position of equilibrium is more dependent upon the overall composition of the sample than is the transport rate at times well short of equilibrium ( 1 ) . Hence, the fured-timekinetic method is preferable for analytical studies.
Results and Discussion The first goal of the study was to test the hypothesis that uphill transport of a neutral organic compound across a dialysis membrane into a micellar solution will occur. Naphthalene was the primary test analyte. This test was performed by varying the concentration of a surfactant in the receiver cell in a manner that bracketed its cmc. Brij 58 was selected as a surfactant because its molecularweight is well-above the cutoff of the dialysis membrane that was employed. The results are shown in Figure 1. At concentrations above about 1 mM, uphill transport (EF =. 1)was achieved. However, in contrast to the study on the transport
0.05 M Brij 58 0.04 M Brij 58 0.25 M SDS
+ 0.02 M SDS
EF
SD
trials
4.5
0.3 0.3 0.3
4 4 4
4.5 4.3
a Sample, 200 mL of 1.3 x lo-' M naphthalene; receiver volume, 2 mL; dialysis time, 60 min.
of protonated phenylalanine across a cation-exchange membrane into micellar sodium dodecyl sulfate (SDS) receivers, where the threshold concentration of SDS to promote uphill transport was its cmc (121, an EF above unity was observed only when the Brij 58 concentration exceeded its cmc, 4 pM (131,by more than a factor of 100. A possible cause for this difference in behavior was that the onset of promotion of uphill transport by Brij 58 was at a concentration where this surfactant is a dispersion rather than a true solution. This presents the possibility that the uphill transport of naphthalene was promoted by interaction between naphthalene and the dispersed solid rather than with micellar Brij 58. To differentiate between these proposed driving forces, the experiments were repeated using Brij 35 as the surfactant. Even at concentrations as high as 0.2 M, this surfactant is soluble in water. As shown in Figure 1, the onset of uphill transport of naphthalene into Brij 35 solutions is at about 0.01 M, which iswellaboveitscmc ofO.1 mM (14). Apparentlywitheither Brij 35 or Brij 58, the equilibrium constant for the naphthalene-micelle interaction is not large enough to make the rate-limiting step of the transport process independent of the micelle concentration until the number of micelles in the receiver greatly exceeds that present at concentrations just above the cmcs of these surfactants. Brij 58 generally was used in subsequent work. The small amount of dispersed solid did not interfere with the analytical methods. The above interpretation of Figure 1 suggests that at a sufficiently high micelle concentration a constant EF will be reached. That is, the equilibrium for the interaction between naphthalene and the micelles will be shifted to a position where the free naphthalene concentration approaches zero. That the highest two Brij 58 concentrations in Figure 1 yield the same EF supports this hypothesis; however,the presence of the dispersed solid phase at these concentrations makes it uncertain whether the micelle concentration was, in fact, increasing. To verify our interpretation of Figure 1, it was necessary to find another means of varying the concentration of Brij 58 micelles. The use of mixtures of surfactants, where the concentration of one component is above its cmc, provides a route to increasing the number of micelles over the value for a single surfactant at a given concentration. For example, the addition of a poly(oxyethy1ene)-type surfactant to micellar SDS decreases the aggregation number of SDS, thereby increasing the number of SDS micelles (15, 16). Table 1summarizesthe results of dialyses into various single component and mixed surfactant receivers. In all cases, the EFs for naphthalene were statistically identical, which supports the hypothesis that the equilibrium constant for the naphthalene-micelle interaction governs the shape of the plot shown in Figure 1. VOL. 29, NO. 1, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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When the dialyses are performed at surfactant concentrations well-above their cmcs and for a time much less than that for equilibrium between the sample and receiver solutions,the role of the equilibrium between naphthalene and the micelles is limited to the promotion of uphill transport across the CE membrane. The actual values of the EFs obtained depend upon the rate of the transport. That is, these experiments were performed in the fixedtime kinetic mode. A basic assumption when experiments are quantified by the fixed-time kinetic method is that the dialysis time must be in a region where the EF is proportional to time, which signifies that the flux of the analyte across the membrane is independent of dialysis time. This condition was tested by measuring the moles of naphthalene transported into the receiver during dialyses that ranged from 0.5 to 25 h. AU samples were 1.4 x M naphthalene in 2%ethanol. Experiments performed at six dialysis times over the range 1.0-5.0 h yielded a constant flux, 3.9 f 0.4pmol.cm-l.h-’, which demonstrates that the fixed-time kinetic model is followed for preconcentrations at any prescribed dialysis time in that range. The flux over a 25-h dialysis was significantly lower, 1.0 pmol*cm-l.h-l, which is a result of the approach to equilibration of naphthalene between the sample and receiver solutions slowing the transport after some time greater than 5 h. The flux measured after 0.5 h was 1.4 pmol-cm-lah-l. An initial value lower than that which is characteristic of the flux over times where the fixed-time kinetic model is valid suggests the presence of a step in the process that does not yield the measured product (naphthalene in the receiver solution in this case). A possible cause of this low initial flux is that some naphthalene is sorbed to the membrane by nonspecific interaction with CE rather than crossing into the receiver. Such “loading” of the CE by naphthalene will produce a memory effect upon re-use of a membrane; hence, the experiments reported above were performed using a new membrane for each dialysis. This interpretation of the low flux observed with a 0.5-h dialysis was verified by the following experiment. A 1.6 x 10-4 M naphthalene sample was dialyzed for 1.0 h into 0.010 M Brij 58; the quantity of naphthalene transported into the 2-mL receiver was 0.58 f 0.02 pmol ( n= 3). After replacing the sample with a blank solution (2% ethanol), a second dialysis was performed using a new aliquot of the receiver with each of the three membranes; the average quantity of naphthalene that was leached into the 2-mL receivers was 0.45 f 0.03 pmol. Blank dialyses on new membranes under the same conditions did not yield an absorbance above that of the 0.010 M Brij 58 reference. Clearly, retention of naphthalene on CE is significant, so experiments with the objective of obtaining analytically useful preconcentrations must be designed to avoid memory effects on the observed EF values. Although using eachmembrane only once eliminates concern over memory effects, we investigated other routes to alleviating this problem. Apossible approach to minimizing memory effects is to pretreat the solid phase with a substance that occupies the active sites. Hayashita and Takagi (17) found that (NH4)zZII(SCN)~has a particular affinity for CE; however, pretreating a new membrane by soaking it overnight in a 0.6 mM solution of this complex and rinsing it with water did not influence the EFs obtained in subsequent dialyses of naphthalene. The CE membranes also were treated with a general surfactant, albumin, prior to transport experi4
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1. 1995
TABLE 2
Solid-Phase Extraction of Naphthalene on Cellulose Estersa solid phase 0.1 g of CA 0.5 g of CA 0.5 g of CA 0.5 g of CA 5.0 g of CT 0.5g of CA 0.5 g of CA
solute (initial) naphthalene concn, M
1.3 1.3 6.3 x 3.2 1.3 1.6 1.6 x
10-4 10-4 10-5 10-5 10-4 10-4 10-4
solvent
water water water water water 0.25 M SDS
hexane
K 0.46 1.8 1.6 1.9 0.14 0.00 0.00
The water and the 0.25 M SDS solutions contained 2% ethanol; K, mmols of naphthalene on solid phasehnmols in solution;CA, cellulose acetate powder; CT, cellulose triacetate pellets; equilibrium time, 60 min; data correctedfor 3.3% loss of naphthalene from a control sample. a
ments with 1.4 x loT4M naphthalene (in 2% ethanol) samples. A control experiment yielded an EF of 2.9 f 0.2 (n= 4). When the membranes were soaked in 1%albumin ovemight prior to the dialyses, the EFs were 3.3 k 0.4 ( n = 8). The increase in the mean EF is statistically significant at the 95%confidence level, which is evidence of nonspecific interaction between naphthalene and the membrane. However, the increase in EF by pretreatment is too small to merit the extra experimental time. Using 0.050 M Brij 58 rather than a ( N H ~ ) z Z ~ ( S Cor N )albumin ~ solution to condition the membrane also failed to change the dialysis results. By conditioning the membrane with the analyte, the EF values can be increased over those obtained with a new membrane. This can be done by performing a series of dialyses using fresh portions of the sample and receiver but the with the same membrane. For example, with a 1.4 x M naphthalene sample and a 0.010 M Brij 58 receiver, consecutive dialyses yielded EFs of 2.0, 3.9, and 4.2. Although a single trial is sufficient to load the membrane, the time involved to obtain a reliable preconcentration in this manner is excessive, given that each dialysis is for 60 min. Hence, in experimentswith the objectiveof evaluating this membrane transport system as a means of performing analytical preconcentrations (rather than of characterizing the system),the reported data are for the first trial on a new membrane. The data obtained with albumin as a conditioning agent and with pretreatment of the membrane with analyte suggest that the memory effect involves solid-phase extraction of the naphthalene from the sample onto the membrane rather than entrainment of the naphthalene in the pores of the membrane. To test the proposed solidphase extraction mechanism, a series of experiments were performed with granular cellulose acetate, which is less porous than the membrane material. A distribution constant, K, was determined for the partitioning of naphthalene from a sample solution onto the powders. By definition, Kfor a solid-phase extraction is the ratio of the millimoles of the solute transferred to the solid phase at equilibrium to the millimoles that remain in solution. The results are summarized in Table 2. Two factors are important. First, the results follow the typical pattern for solid-phase extraction. A higher surface area increases the amount of sorption, and the sorption is characterized by a distribution constant, K, that is independent of the naphthalene concentration. Therefore, the memory effect observed with the membranes is not simplydue to a physical
. .
0 ’ 3.00-06
I 1.00-01
1.00-04
Naphthalene Conc., M FIGURE 2. Influence of the initial concentration of naphthalene in the sample on its transport into a micellar receiver. Receiver, 2 mL of 0.050 M Brij 58; other conditions are the same as those in Figure 1. Error bars represent the 95% confidence limits.
factor such as entrainment in the pores. Nonspecific interaction between neutral solutes and the membrane probably is a general factor in fugacity-driven dialysis of these compounds. Second, as expected, partitioning of naphthalene onto CA does not occur from hexane or micellar solutions, which supports the use of such solvents as receivers in fugacity-driventransport of neutral molecules across membranes. That extensive solid-phase extraction of naphthalene onto CE occurs does not preclude application of micellar receivers to sampler design or to analytical preconcentration devices. To evaluate the potential applicability of micellarbased systems as sampling devices for organic compounds in water and wastewater, a direct comparison was made to hexane-filled CE bags. As previously discussed, organic solvents as receivers in polyethylenebags have been shown to be useful for this purpose. Again, naphthalene was used as a representative polyaromatic hydrocarbon. With 1.4 x M naphthalene as the sample, a 0.050 M Brij 58 receiver yielded an EF of 3.5 0.3 ( n = 4). With the same experimental design except that 2 mL of hexane served as the receiver, an EF of 5.3 f 0.3 was obtained. The use of hexane as the receiver component did not eliminate the memory effect from nonspecific interaction between naphthalene and the CE. A second set of trials with the same membrane yielded an EF of 8.8 f 0.2. Hence, the greater EFs with the hexane receiver than with micellar solutions is a result of the entropy effect on solubility of hydrophobic compounds being greater with a hydrocarbon solvent than with micelles. Nevertheless, the similarity of the EF values that were observed suggests that the micellar medium may be practical for applications such as samplers and analytical preconcentrators. The second objective of the present study is to evaluate micellar receivers for application in analytical preconcentrators. This requires determining whether the quantity of analyte transported into the receiver in a fixed time is directly proportional to the initial concentration of the analyte in the sample over a wide range. Under this condition, the EFs are independent of the initial concentration of the analyte in the sample. With 6.5 x to 1.3 x M naphthalene as the analyte, EF values were determined after 60-min dialyses into 0.050 M Brij 58 (Figure 2). The pooled mean and standard deviation of these data
are 3.4 & 0.3. The deviations are comparable to those we observed with similar experimental designs for Donnan dialysis and for facilitated transport across supported liquid membranes when the experimentswere performed at room temperature andwithmagnetic stirring (1,18,19). The EF values do vary from lot to lot of nominally identical membranes. A repeat of the experiment in Figure 2 with CE from a different lot yielded an EF for naphthalene of 4.5 M f 0.4 (n = 4). The same experiments with 2.1 x p-dichlorobenzene gave an EF = 5.0 f 0.6, which suggests that similar enrichments of awide variety of neutral organic compounds in water may be achieved, which if verified can simplify screening experiments on environmental samplesby obviating the need for several calibration curves. An evaluation of the potential of a membrane transport method as an analytical preconcentration technique cannot be made solely on the basis of the EF value. Factors such as the thickness of the membrane and the ratio of the receiver volume to the area of the membrane influence the EF. For example, a 1.0-hmicrodialysisexperiment with 30 M naphthalene as the sample and 250 pL mL of 1.3 x of 0.2 M Brij 35 as the receiver yielded an EF of 4.1, a value comparable to that shown in the Figure 1 experiment. If the membrane areas and thicknesses were the same, the EF would have been expected to be eight times higher (the volume ratio) in microdialysis. An alternative way to evaluate the present method is to compare the flux into a micellar receiver to the flux into a water receiver. In the latter case, passive diffusion is the driving force. A repeat of the microdialysis experiment with water as the receiver yielded a concentration of 1.8 x M naphthalene in the 250-pL receiver after 1.0 h (an EF of 0.14). The micellar medium therefore accelerated the flux by a factor of 29 relative to the transport into water. This significant enhancement of the rate of transport of naphthalene across the CE membrane by the micellar solution suggests that these preconcentration experiments can be designed to significantlyimprove upon the determination of trace levels of polyaromatic hydrocarbons and other hydrophobic compounds in water. In conclusion, micellar receivers provide a driving force which is similar to that of an organic solvent for transport of hydrophobic compounds across dialysis membranes. The surfactants that were employed are more environmentally benign than organic solvents. Moreover, the leakage of such high molecular weight surfactants as Brij 35 and Brij 58 across a 500 Da cutoff membrane is very limited (it was not observed during a 1-h dialysis in the present study). Hence, the described fugacity-driven process has promisefor application to continuous sampling. This membrane-based system also possesses the basic characteristics necessary for analytical preconcentrators. However, further study is needed to determine whether the influence of nonspecific interaction between hydrophobic compounds and the membrane will preclude use of the fixed-time kinetic method for quantifymg the preconcentrations of trace levels of these analytes.
Acknowledgments The work was supported by Grant R816507 from the U.S. Environmental Protection Agency, Office of Exploratory Research. The study has not been reviewed by the Agency, so an endorsement should not be inferred. Certain experiments were performed by K. E. Shanks, a participant in the National Science Foundation’s Research Experience VOL. 29, NO. 1. 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
I
for Undergraduates program at Miami University (Grant CHE9000865).
literature Cited Cox, J. A. In Preconcentration Techniques for Trace Elements; Alfassi, Z. B., Wai, C. M., Eds.; CRC Press: Boca Raton, FL, 1992; pp 301-331. Babcock, W. C.; Friesen, D. T.; LaChapelle, E. D. J. Membr, Sci. 1986,26, 303-312. Sodergren, A. Environ. Sci. Technol. 1987, 21, 855-859. Johnson, G. D. Environ. Sci. Technol. 1991, 25, 1897-1902. Huckins, J. N.; Tubergen, M. W.; Lebo, J.A.; Gale, R. W.; Schwartz, T. R. 1.Assoc. Off: Anal. Chem. 1990, 73, 290-293. Huckins, J. N.; Tubergen, M. W.; Manuweera, G. K. Chemosphere 1990, 20, 533-552. Huckins, J. N.; Manuweera, G. K.; Petty, J. D.; Mackay, D.; Lebo, I. A. Environ. Sci. Technol. 1993, 27, 2489-2496. Haulbrook W. R.; Feerer, J. L.; Hatton, T. A,; Tester, J. W. Environ. Sci. Technol. 1993, 27, 2783-2788. Sucheta, A.; Haque, I. U.; Rusling, J. F. Lungmuir 1992,8, 16331636. Rusling, J. F. Acc. Chem. Res. 1991, 24, 75-81. Cox, J. A.; Lewinski, K. Electroanalysis, in press.
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Pekol, T.; Poopisut, N.; Cox, J. A. Tulantu 1994, 41, 663-668. Egan, R. W.; Jones, M. A.; Lehninger, A. L. J. €301. Chem. 1976, 251,4442-4447. Hinze, W. L. In Ordered Media in Chemical Separations; Hinze, W. L., Armstrong, D. W., Eds.; American Chemical Society SymposiumSeries 342;American ChemicalSociety: Washington, DC, 1987; p 4. Xia, J.; Dubin, P. L.; Kim,Y.1.Phys. Chem. 1992,96,6805-6811. Holland, P. M.; Rubingh, D. N. In Mixed Suflactunt Systems; Holland, P. M., Rubingh, D. N., Eds.; American ChemicalSociety Symposium Series 501;American Chemical Society: Washington, DC, 1992: p 3. Hayashita, T.; Takagi, M. Tuluntu 1985, 32, 399-405. Cox, J. A.; DiNunzio, J. E.Ana1. Chem. 1977, 49, 1272-1275. Cox, J. A.; Bhatnagar, A.; Francis, R. W. Tulanta 1986,33, 713716.
Received for review January 21, 1994. Revised manuscript received August 15, 1994. Accepted September 15, 1994.@
ES940045Y Abstract published in AdvanceACSAbstructs, November 1,1994.
