Langmuir 1992,8, 1633-1636
1633
Dechlorination of 9-Chloroanthracene in an Adsorbed Film of Cationic Surfactant on an Electrode Artur Sucheta,?Inam U1 Haque,*and James F. Rusling' Department of Chemistry (U-60),University of Connecticut, Storrs, Connecticut 06269-3060 Received February 3,1992.I n Final Form: March 27, 1992 Direct electrolytic dechlorination of 9-chloroanthracene at a mercury electrode was investigated by cyclic voltammetry in a solution containing a single distribution of large cationic micelles. The dechlorination reaction occurred at about -1.65 V vs SCE in a layer of adsorbed cetyltrimethylammoniumbromide on the electrode surface, with only a few percent fractional coverage of the 9-chloroanthracene. Dechlorination of the 9-chloroanthracene anion radical was too fast to observe directly by cyclic voltammetry with scan rates up to 51 V s-l. However, the lower limit for the rate constant was estimated at 200 5-1. A rate constant in this range suggests that anion radical dechlorination occurs in a relatively polar microenvironment, not unlike that found for hydrophobic solutes in ionic micelles. Introduction The urgent need to remove toxic organic chemicals from the natural environment has caused renewed interest in dechlorination reactions.'-' Such reactions can cleanly produce hydrocarbons and chloride ions by reduction of toxic organohalides. Alternatively, incineration of aryl halides can produce small amounts of highly toxic chlorodioxins in effluentsY2and prior dechlorination of aryl halides could decrease the impact on air quality. The present work is part of a program designed to develop surfactant solutions as media for dechlorinations of environmental substances contaminated with chlorinated biphenyls (PCBs) and other aryl halide pollutants. Surfactant solutions can solubilize nonpolar organohalides in water and facilitate their conversion to hydrocarbon^.^ This paper addresses the influence of a cetyltrimethylammonium bromide solution containing large micelles on the kinetics of electrochemical dechlorination of the model aryl halide 9-chloroanthracene. Electrochemical reductive dehalogenation of aryl halides (ArX) in organic solvents occurs by the general pathway in Scheme An electron is first transferred to ArX (eq 11, followed by cleavage of the anion radical in eq 2 to yield radical Ar' and halide ion. Equation 3 represents the several possible paths for addition of an electron and a proton to the radical, as documented for aryl halides.5a Rate constants for cleavage of PCB anion radicals have not been measured, partly because their values are likely to be very large. However, electrochemical reduction of 9-chloroanthracene occurs by a similar pathway to PCBs.5-6 Its k-value has been shown to be rather susceptible to solvent effects.' This is readily seen when In k is plotted vs Taft's solvent dipolaritylpolarizability8parameter T* (Figure 1). Using the T* value of 1.09for water, this roughly linear relation predicts a k value in water about 10-fold + Present address, Department of Chemistry, University of California at Imine. f On leave from University of Engineering and Technology, Lahore, Pakistan. (1) Daley, P. S. Enuiron. Sci. Technol. 1989, 23, 912.
(2) Hites, R. A. Ace. Chem. Res. 1990, 23, 194. (3) Ruling, J. F. Acc. Chem. Res. 1991, 24, 75. (4) Ruling, J. F.; Arena, J. V. J.Electroanal. Chem. 1985, 186, 225. (5) (a) Andrieux, C. P.; Saveant,J. M.; Zann, D. Nouu. J. Chem. 1984, 8,107. (b)Andrieux, C. P.; Saveant, J. M.; Su, K. B. J.Phys. Chem. 1986, 90,3815, and references therein. (6) Sucheta, A.; Ruling, J. F. J. Phys. Chem. 1989,93, 5796. (7) Wipf, D. 0.; Wightman, R. M. Anal. Chem. 1990,62,98.
(8)Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983,48,2877.
0743-7463/9212408-1633$03.00/0
1
3 ' 0.40
1.20
0.80 7T*
Figure 1. Influence of solvent polarity on rate constant (k)for decomposition (eq 2) of 9-chloroanthraceneanion radical. Data from refs 5-7. Solvents with Taft ?r* values:8 DME, 0.53;THF, 0.58; chlorobenzene, 0.71; MeCN, 0.75;CHzC12,0.82;DMF, 0.88; DMSO, 1.00.
Scheme I ArX + e e ArX'-
k
ArX'-
Ar'
Ar' (+HS or e/H+)
(1)
+ X-
-
ArH (fast)
(3)
larger than in weakly polar solvents such as tetrahydrofuran (THF) or dimethoxyethane (DME). This suggests that a polar reaction environment might be used to practical advantage to increase rates of dechlorination reactions. With the above concepts in mind, we set out to investigate the electroreduction of 9-chloroanthracene in an aqueous medium containing cationic micelles to solubilize the reactant. Our goal was to compare the kinetics of dehalogenation of the 9-chloroanthracene anion radical in micellar media to those in organic solvents. We previously showed that rates of bimolecular catalytic dechlorination of PCBs are enhanced in cationic surfactants, but not in anionic or neutral surfactants. Rate enhancement was caused mainly by preconcentration of the two reactants in the second-order rate-determining steps a t high concentrations in a surfactant coating on the e l e ~ t r o d e With . ~ 9-chloroanthracene, we hoped to be able to observe directly the influence of the micellar medium on the first-order dehalogenation step in eq 2. For this, 0 1992 American Chemical Society
Sucheta et al.
1634 Langmuir, Vol. 8, No. 6,1992 we chose a monodisperse cationic micellar system, 0.15 M cetyltrimethylammonium bromide/O.l M tetraethylammonium bromide, shown previously to contain a single distribution of rather large micelles. Micelles in this system had a diffusion coefficient of 0.33 X lo* cm2 s-l and are presumably rod shaped. Experimental Section Chemicals and Solutions. Anthracene (An) was from Aldrich and used as received. 9-Chloroanthracene (9-CLAn) was Aldrich technical grade and was recrystallized 4 times from ethanol before use. The purified material showed a single spot by thin-layer chromatography and had a melting point of 104.5105.5"C (lit. 106OC).l0. Tetraethylammonium bromide (TEAB) was from Eastman Kodak and used as received. Hexadecyltrimethylammoniumbromide (cetyltrimethylammonium bromide, CTAB) was from Eastman Kodak (99+%). Distilled water purified with a Sybron/Barnstead Nanopure system to a specific resistance >12 MQ cm was used. CTAB solutions were prepared by dissolving appropriate amounts of CTABand TEAB in water at 50-60 "C in volumetric flasks.The solutionwasslowlycooledto 30OC andwater was added to volume. Micellarsolutions of electroactive species (9-ClAnand An) were prepared with the aid of sonication and equilibrated in a water bath at 30 OC one or more days prior to electrochemical experiments. Equilibration was considered complete when voltammetric peaks at 30-50 V s-l became reproducible with time. Apparatus and Procedures. ABioanalyticalSystems BAS100 electrochemical analyzer was used for cyclic voltammetry. The water-jacketed electrochemical cell was kept at 30.0 0.2 "C. Athree-electrode cell was used with a hanging drop mercury (HDME) working electrode (A = 0.0182 cm*),a Pt wire counter electrode, and a lab-made miniature aqueous saturated calomel electrode (SCE) as the reference. The SCE terminated in a tube containing an internal agar plug saturated with KC1, ending in a glass frit. This SCE contacted the solution in the cell by a salt bridgef i e d with the electrolyteused. All potentials are reported against SCE. Resistance of cells containing the 0.15 M CTAB/ 0.1 M TEAB electrolyte was about 400 Q and was compensated by the BAS-100. Care was needed to maintain the HDME in functional conditionin the micellar solution. Contamination of the capillary caused surfactant solution to creep up the mercury column, requiring extensive cleaning of the HDME." Results Cyclic voltammograms (CVs) of 9-chloroanthracene (9ClAn) in 0.15 M CTAB/O.l M TEAB a t scan rates >10 V s-l showed two symmetric reduction peaks at about -1.7 and -1.9 V (all vs SCE), and a third peak at about -2.2 V with a characteristic asymmetric diffusion controlled shape (Figure 2a,b). A reverse anodic peak at about -2.1 V accompanied the third peak.12 The potentials for the first two peaks are similar to those for 9-ClAn in DMF.5a CVs of anthracene at high scan rates in 0.15 M CTAB/O.l M TEAB show a symmetric peak at -1.9 V and the reversible couple at -2.2 V (Figure 2c). Their symmetric shapes are characteristic of reductions of species adsorbed or confined to a thin layer at the electrode,13 and this is borne out by additional data presented below. By analogy with the CVs in DMF,5the first peak is attributed to reduction of 9-ClAn and the second to reduction of anthracene, the product of 9-ClAn reduction.
*
(9) Ruling, J. F.; Wang, Z.;Owlia, A. Colloids Surf.1990,48, 173. (10) (a) Handbook of Chemistry and Physics, 53rd ed.; CRC Press: Cleveland, OH, 1972. (b) Connors, T. F. M.S.Thesis, University of Connecticut, 1983. (11)Ruling, J. F.; Shi, C.-N.; Gosser, D. K.; Shulda, S. S. J. Electroanol. Chem. 1988,240, 201. (12) A fully reversible third peak for 9-ClAn is found only when the surfactant solution is equilibrated for several days at the temperature of the experiment. This has ala0 been found for 9-phenylanthracene in 0.1 M CTAB." However, adsorption peaks were reproducible after overnight equilibration. (13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.
1.00
UE
i
-
-
I
0.50
'
-0.50 1.50
'
I
'
1.70
1.90
-E, V
VS.
2.10 SCE
0.50 I
-0.50 I 1.50
2.30
I
'
'
1-70
'
'
1.90
-E, V
vS.
2.10
2-30
SCE
Figure 2. Cyclic voltammograms on HDME in 0.15 M CTAB/ 0.1 M TEAB: (a) background charging current at 51 V s-l; (b) 0.5 mM 9-chloroanthracene at 51 V s-l; (c) 0.5 mM anthracene at 41 V s-l. An apparent diffusion coefficient of 7 X cm2s-l for 9-ClAn was estimated from the Randles-Sevcik equation13 and the heights of the third peak. This is much larger cm2s-l for 9-ClAn in DMF.lob than the value of 1.3 X This suggests reduction of the reactant in a thick film of CTAB which forms on the electrode at potentials negative of -2 V.3 Similar behavior has been found" for 9-phenylanthracene in 0.1 M CTAB/O.l M TEAB. The diffusion controlled third peak for 9-ClAn has a ratio to the second symmetric peak that is nearly twice that found for An. This may be because 9-chloroanthracene attains a higher concentration in the surfactant film than anthracene or because diffusion-controlled reduction of both 9-ClAn and An occurs in the third peak in the 9-ClAn solutions. However, our main interest is in the dechlorination at the electrode surface, and the rest of this paper concerns the surface electrochemistry represented by the first two peaks. By scanning between -0.5 and -2 V, reasonably well defiied symmetric adsorption peaks were obtained a t scan rates above about 5 V s-l, although charging current was relatively large (Figure 3). No reoxidation peak was observed in this potential range throughout the available scan range (0.001 I v I 51.2 V/s) of our instrumentation. An experiment was done in which CVs of a micellar solution containing 0.228 mM 9-ClAn and 0.305 mM An were compared to those of solutions containing only 9ClAn. In the 9-ClAn solution, the ratio of the height of the first peak to the second is about 2, while in the mixed solution the second peak increased in height and was larger than the first because of the presence of anthracene. This confirms the peak assignments made previously. The first peak involves reduction of 9-ClAn, while the second peak is for anthracene formed by reduction of 9-ClAn. Plots of the current of the current peak i,l vs v1I2 were nonlinear. Plots of i,l vs v showed gradual transition from parabolic to linear behavior as Y increased above 8 V s-l (Figure 4). The peak current for a diffusion limited process should be proportional to the square root of scan rate (v), but for thin-layer systems the peak current is proportional to the first power of v.l3 The observed linearity of peak
Dechlorination of 9-Chloroanthracene
'
15 0.50
Langmuir, Vol. 8, No. 6, 1992 1635
I 1.00
1.50
2.00 Y
-E, V VS. SCE
a,
Q
-1.50
' -5
I
1
-3
-1
1
3
In ( l / v ) / W V ) 15 r
I
Figure 6. Dependence of dechlorination peak potential on In (l/v) in 0.15 M CTAB/O.l M TEAB for the three concentrations of 9-chloroanthracene in Figure 4. Regression line equation for h e a r portion: Ep = -1.6047 + 0.02729 In (l/v); r = 0.999.
scan rate is increased, the chemical reaction has less time
'
-15 0.50
to occur and consequently has a smaller influence on peak
I 1.00
-E, V
1.50 VS.
potential. Thus, the peak shifts in a negative direction. For areversible electron transfer the EC mechanism leads to the e x p r e ~ s i o n ~ ~ J ~
2.00
SCE
Figure 3. Cyclic voltammograms on HDME in 0.15 M CTAB/ 0.1 M TEAB at 17 V s-l: (a) background charging current; (b) 0.48mM 9-chloroanthracene, fiist peak -1.659 V, second peak -1.875 V. 10
Ep= Eo' - 0.780(RT/nF) + (RTl2nF) In k (RTl2nF)In ( l l v ) (4) where R is the gas constant, T is temperature in kelvin, n is the number of electrons transferred per molecule, and
F is Faraday's constant. Equation 4 predicts that a plot of E, vs In ( l l v ) for reversible electron transfer in an EC 5
r a,
Q
0 0
10
20
30
40
50
60
scan rate, V/s Figure 4. Dependence of peak current i, on scan rate for 0.23 mM (0),0.39 mM (01,and 0.48 mM ( 0 )9-chloroanthracene in 0.15 M CTABlO.1 M TElB.
current vs v a t high scan rates suggests that the electrochemical reaction occurs with 9-ClAn confined to a thin layer on the electrode surface. Since the initial steps in dechlorination of 9-ClAn (SchemeI) are reduction at the electrode and decay of the resultant anion radical, the simplest pathway is the socalled EC (electrochemicalstep, chemical reaction) mechanism.13 One way to get rate constant k (eq 2) would be from ratios of the reverse anodic peak of the anion radical to that of the f i t cathodic peak.14 However, no anodic peak was ever observed for 9-ClAn anion radical in micellar solutions, even at the the highest scan rates. This is presumably because of the high rate of the chemical reaction. In cases where no reverse peak is found, an estimate of the rate constant of the chemical step in an EC process can be obtained from the shift of cathodic peak potential (E,) with scan rate (v). The chemical reaction tends to shift the potential positive of Eo' (ArX/ArX'-) (eq 1). As (14) Chronocoulometry, used previously to measure dechlorination kinetics for aryl halides in isotropic DMF,B did not give usable results in the surfactant medium, presumably because of the very large charging currents.
process should have a slope of 12.6 mV/ln (llv) at 30 OC. An alternative is the ECE mechanism,for which the same slope is predicted.le However, when the electron transfer rate at the electrode is not fast enough for the reaction to be fully reversible,17the slope of E, vs. In ( l l v ) becomes larger. Digital simulations1s for an EC mechanism (Scheme I) showed that k of 200 s-l was the lowest value at which a reverse anodic peak would be barely discernible at our highest experimental scan rate of 51 V s-l. Since we never observe an anodic peak for the anion radical, 200 s-l represents a lower limit of k for decomposition of 9-ClAn anion radical in the CTAB medium. Using this k with a standard heterogeneous rate constant of 0.01 cm s-l and an electrochemical transfer coefficient of 0.5, digital simulations predicted a slope of 25.6 mV/ln ( l l v ) between 5 and 52 V s-l for the Eplvs In ( l l v ) plot. Analysis of the voltammetric peak potential data for the first peak provides a picture consistent with EC (Scheme I) or ECE mechanisms (Scheme I followed by a second electron transfer). In accord with the theoretical prediction that Eplshould have a range where it depends linearly on ln W v ) , the three concentrations of 9-ClAn used show such a dependence (Figure 5 ) . No reversible half wave potential Elp (=Eo')can be estimated since reversibility is not achieved a t high scan rates. However, a linear dependence of E,1 on In ( l l v ) was found at higher scan rates. The slope of the linear portion is 27.3 mV/ln ( l l v ) , in good agreement with the EC prediction of 25.6 mV/ln ( l / v ) for ko = 0.01 cm s-l and k = 200 s-l. (15) Nicholson, R.S.;Shain, I. Anal. Chem. 1964, 36, 706. (16)Amatore, C.; Gareil, M.; Saveant, J. M. J . Electroanol. Chem. 1983, 147, 1. (17) Values of ko > 0.5 cm 8-l are usually considered to reflect full reversibility in cyclic voltammetry. (18)Gosser, D. K.; Zhang, F. Talanta 1991,38, 715.
Sucheta et al.
1636 Langmuir, Vol. 8, No. 6, 1992 Table I. Apparent Surface Concentrations and Coverages for 9-Chloroanthraceneon H g in CTAB Solution I9-CLAnl. m M ~~
0.23 0.39 0.48
@ / A , UC cm-2 10131'~,bmol cm-2 0.010 i 0.001 1.0 0.020 0.002 0.017 i 0.005
a By integration of CVs a t Y I 5 V 8-l. charge density 0.739 pC cm-2 (see text).
% coverage*
2.1 1.8
1.4 2.7 2.3
* For n = 1, full coverage
Simulations using these parameters also predicted the observed curvature a t low v. Again, the observed voltammetric behavior is consistent with an EC or ECE reaction in which the first electron transfer is not fully reversible. The two symmetric peaks were integrated to obtain the total charge Q under each peak. Because of the large charging current and resulting poor signal to noise ratio in the data (cf. Figure 31, the error in these integrations is estimated at about f30%. However, charges for each peak were reasonably constant at scan rates above 5 V s-l. The charge under the first peak was an average of 2.5 times greater than that of the second. Because of the poor signal to noise, definitive conclusions based on this result are risky. The Q values were also used to obtain the amount of coverage of the electrode with 9-ClAn. The relevant expression is13 Q = nFAr0 (5) where ro is the surface concentration of 9-ClAn on an electrode of area A. Thus, the measured charge Q is directly proportional to ro. The theoretical value of ro for a monolayer of 9-ClAn molecules lying flat on the Hg electrode surface and separated by twice the van der Waals radii of the outer atoms is 0.739 pC for n = 1. Integrals obtained at all three concentrations of 9-ClAn are only a small fraction of this value (Table I).
Discussion The shape and scan rate dependence of the first voltammetric peak in the micellar solution above 8 V s-l show that 9-ClAn is reduced in a thin layer on the electrode surface. However, in a 9-ClAn concentration range where nearly full surface coverage would be expected for similar sized adsorbate molecules in homogeneous solutions, the maximum coverage of 9-ClAn is only a few percent of its theoretical full coveragefor a flat orientation on the surface. Strong adsorption of CTAB on negatively charged metal electrodes has been demonstrated clearly by electrochemical and spectroscopic experiments.lSz2 At potentials where 9-ClAn is reduced, the first layer of surfactant is likely to be adsorbed with head groups facing down on the electrode.21 A monolayer that presents extended hydrocarbon chains to the aqueous phase would be thermodynamically unfavorable, so it is likely that at least a bilayer of CTAB is present on Hg at these potentials and surfactant concentration^.^ The most probable explanation of the voltammetric results is that 9-ClAn is coadsorbed with CTAB on the Hg surface in the potential range where the symmetric reduction peaks are observed. Similar coadsorption with low fractional coverage on metal electrodes was found for 1,2-dicyanobenzene and perylene in 0.1 M CTAB solutions containing alcohols.20 We now make a rough comparison of the concentration of 9-ClAn in micelles with its concentration in the surface (19)Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J. Electroanal. Chem. 1981, 125, 73. (20) Rusling, J. F.; Couture, E. C. Langmuir 1990, 6, 425. (21) Sun, S.; Birke, R. L.; Lombardi, J. R. J . Phys. Chem. 1990, 94, 2005. (22) Rusling, J. F.; Ahmadi, M. F. Langmuir 1991, 7, 1529.
film. Using an aggregation number of 150estimated from the diffusion coefficient of the micelle^,^ Tanford's method23agives a hydrophobic volume fraction of 0.039 for the CTAB micelles in our system. Assuming that all 9-ClAn in the 0.48 mM solution is dissolved in micelles, its estimated concentration in the micelles is 0.012 M. A bilayer of CTAB on the electrode would have a thickness of about 44 A, the length of two Crs chains. Using this thickness and the 2 X mol cm-29-ClAn measured on the electrode (Table I) gives a concentration of 0.045 M 9-ClAn in a hypothetical surfactant bilayer on the electrode surface. Given the roughness of these estimates, we conclude that the concentration of 9-ClAn in the surfactant film on the electrode is about the same as in micelles in solution. The amount of preconcentration of 9-ClAn in the film is small. Voltammetric data on the first symmetric reduction peak allowed a semiquantitative analysis of the kinetics of dechlorination (eq 2) of the anion radical of 9-ClAn in a CTAB film on the electrode. The reduction occurs by an EC or ECE mechanism, but distinguishing between the two is difficult because of the inherently high charging currents encountered. The lower limit of the k value was 200 s-l, estimated by comparison of CV data with digital simulations for a reasonably fast but not fully reversible heterogeneous charge transfer (eq 1). Equation 4 could not be used to estimate k because E O ' was not determinable in the surfactant medium. However, the above analysis suggests a rate of reaction roughly similar to that in N,N-dimethylformamide (DMF). A value of -1.76 V has been found for the Eo' of 9-ClAn in DMF.5 By use of this value and the experimental slope from Figure 5 in eq 4, a k value on the order of lo3 s-l is estimated. Although this must be considered the roughest of estimates, it is consistent with a lower limit on k of 200 S-1.
The estimated range of k in the CTAB film is similar to the k values found in isotropic acetonitrile and DMF (cf. Figure l), suggesting a relatively polar microenvironment for the reaction on the electrode surface. Such a range for k suggests that anion radical dechlorination (eq 2) occurs in the vicinity of the head groups of CTAB, which also must be associated on the electrode surface with water molecules and bromide ions. Such a microenvironment is in agreement with proposed solubilization sites of other hydrophobic ions and molecules in micelle^.^^--"^ A similar averge microenvironment for ferrocene during electron transfer to a platinum electrode in micellar CTAB solutions was inferred from kinetic data.26 In summary, 0.15 M CTAB/O.l M TEAB provides an adsorbed surfactant layer on a Hg electrode surface at -1.7 V vs SCE that allows only a small fractional coverage of 9-ClAn to occur. The rate of dechlorination of the anion radical in this film is consistent with a relatively polar environment for the reaction. Acknowledgment. This work was supported by U S . PHS Grant ES03154 awarded by the National Institute of Environmental Heath Sciences and by award of a Fulbright Research Fellowship (1991-1992) to I.U.H. from CIES, Washington, DC. Registry No. 9-ClAn, 716-53-0; anthracene anion radical, 34509-92-7. (23) (a) Tanford, C. The Hydrophobic Effect, 2nd ed.; Wiley: New York, 1980. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (24) Fendler, J. H. J . Phys. Chem. 1985, 89, 2730. (25) Kalyanasundarurn, K. Photochemistry in Microheterogeneous Systems; Academic: New York, 1987. (26) Abbott, A. P.; Miaw, C.-L.; Rusling, J. F. J.Electroanal. Chem., in press.
