Langmuir 1992,8, 2577-2581
2577
Properties of Reduced Viologens in Nafion Films Deposited on SnO2 Electrodes Oddvar Johansen, John W. Loder, Albert W.-H. Mau, Joseph Rabani,’ and Wolfgang H.F. Sasse Division of Chemicals and Polymers, CSIRO, Private Bag 10, Clayton, Victoria 3168, Australia Received September 14, 1990 Optical spectroscopy and cyclic voltammetry were used to study the monomer-dimer equilibrium of the one-electron reduction products of methyl viologen (MV2+), l-methyl-1’-(3-sulfonatopropyl)-4,4’bipyridinium (MPVS+),and l,l’-bis(sulfonatopropy1)-4,4’-bipyridinium(SPV) in Nafion films deposited on SnO2 electrodes. In Nafion the equilibrium constants are considerably smaller than in water and they are affected by the viologen loading.
Introduction Viologenscontinue to play an important role as electron relays in systems in which electron transfer is initiated by photochemical or electrochemical Of special significance are the reversibility of their redox processes and the chemicalpropertiesof their one-electron-reduction products. These include relative stability in the absence
of oxygen, characteristicoptical properties, and the ability to undergo catalyzed reactions with protons to give hydr~gen.l~-~~ Another characteristic feature is their tendency to form physical dimers:9J2s2*34 Vgn+ + e
-
vg(n-l)*+
(1)
K
* Author to whom correspondence should be sent at the Depart-
ment of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. (1)Summers, L. A. The Bipyridinium Herbicides; Academic Press: London, 1980. (2)Wayne, R. P. J. Photochem. 1985,29,1-265. (3)Michaelis, L.; Hill, E. S. J. Gen. Physiol. 1933,16,859. (4)Bird, C.L.; Kuhn, A. T. Chem. SOC.Reu. Org. 1976, 13,211. (5)Hillman, A. R. Electrochem. Sci. Technol. Polym. 1987,1, 103239.
2vg(n-1)*+e (vg(n-l)*+
12
K=
(2)
[(vg(”-”’+),]
1
[vg(n-l)*+ 2
The transformation of dimer from MV’+ in Ndion has been reported earlier by Gaudiello et ale9These workers assume the equilibrium constant (K)to be similar to that in aqueous solution. However, in view of the strong (6)Launikonis,A.;Loder,J.W.;Mau,A.W.-H.;Sasse,W.H.F.;Wella, D. Isr. J. Chem. 1982,22,158-162. electrostatic and hydrophobic interactions within Ndion, (7)Izelt, G.; Day, R. W.; Kinstle, J. F.; Chambers,J. Q.J . Phys. Chem. this assumption warranta closer experimentalexamination. 1983,87,4592-4598. We now report such a study, in which the formation of (8)Albery, W. J.; Compton, R. G.; Jones,C. C. J. Am. Chem. SOC.1984, 106,469-473. physical dimers in Ndion was examined with three (9)Gaudiello, J. G.; Ghosh, P. K.; Bard, A. J. J . Am. Chem. SOC. 1985, viologens. In this work Ndion membranes were deposited 107,3027-3032. on optically transparent electrodes. This approach has (10)White, J. R.;Bard, A. J. J. ElectroanaL Chem. Interfacial Electrochem. 1985,197,233-244. allowed us to simultaneously use electrochemical and (11)Tsou,Y. M.; Liu,H. Y.;Bard,A.J. J.Electrochem. SOC.1988,135, spectroscopic techniques. The viologens used in the 1669-1675. ~ . ~. . . present work were chosen so as to differ in their overall (12)Furlong, D. N.; Johansen, 0.; Launikonis, A.; Loder, J. W.; Mau, charges as follows: methyl viologen, MV2+, 1-methyl-1’A. W.-H.; Sasee, W. H. F. Aust. J. Chem. 1985,38,363-367. (13)Mau, A. W.-H.; Overbeek, J. M.; Loder, J. W.; Sasse, W. H. F. J . (sulfonatopropyl)-4,4’-bippidinium, MPVS+,and 1,l’-bisChem. SOC.,Faraday Trans. 2 1986,82,869-876. (sulfonatopropyl)-4,4’-bipyridinium,SPV. (14)Hoffman, M. Z.J. Phys. Chem. 1988,92,3458-3464. (15)Neshvad, G.; Hoffman, M. Z. J.Phys. Chem. 1989,93,2445-2452. (16)Matheson, M. S.;Lee, I. C.; Meisel, D.; Pelizzetti, E. J . Phys. Chem. 1983.87,394. (17)Brandeis, M.; Nahor, G. S.; Rabani, J. J . Phys. Chem. 1984,88, 1615. (18)Sassoon, R.E.;Gershuni, S.; Rabani, J. J. Phys. Chem. 1985,89, 1937. (19)Miller, D. S.;Bard, A. J.; McLendon, G.; Fergusson, J. J . Am. Chem. SOC.1981,103,5336. (20)Stramel, R.D.;Nguyen, C.; Webber, S. E.; Rodgers, M. A. J. J. Phys. Chem. 1988,92,2934. (21)Slama-Schwok,A,; Rabani, J. J. Phys. Chem. 1987,91, 43944398. (22)Johansen, 0.; Launikonis, A.; Loder, J. W.; Mau, A. W.-H.; Sasse, W. H. F.; Swift, J. D.; Wells, D. A u t . J. Chem. 1981,34,981. (23)Miller, D. S.;McLendon, G. J . Am. Chem. SOC.1981,103,6791. (24)Sassoon, R. E.;Lenoir, P. M.; Kozak, J. J. J. Phys. Chem. 1986, 90,4654. (25)Young, R.C.; Meger, J. J.;Whitten, D. G. J . Am. Chem. SOC.1975, 97,4781-4782. (26)Moradpour, A,; Amouyal, E.; Keller, P.; Kagan, H. N o w . Chim. 1978,2,547-549. (27)Kiwi, J.; Gratzel, M. J. Am. Chem. SOC.1979,101,7214-7217. (28)Nosaka, Y.; Fox, M. A. J. Phys. Chem. 1988,92,1893-1897. (29)See, e.g., (a) Heyrovsky, M. J. Chem. SOC.,Chem. Commun. 1987, 1856. (b) Mollin, J.; Pavelek, Z.; Kasparek, F. Collect. Czech. Chem. Commun. 1987,52,1097-1115.
Experimental Section Materials. A solution of Nafion was obtained from Solution Technology, Delaware, U.S.A.; NaC1, analytical grade, was obtained from Ajax Chemicals. Methyl viologen dichloride (MV2+)was a product of Fluka. MPVS+and SPV were prepared according to the literature procedure.36Water was triply distilled. All solutions contained 0.1 M NaCl and were deaerated by bubbling vigorously with NOfor at least 10 min prior to use. All experiments used 1.3 cm3 of solution and were carried out at 22 f 1 O C . To remove oxygen, all solutions were treated with Nz (99.998%)before being used. For the measurement of isosbestic points, residual oxygen was reduced electrolytically. (30)Watanabe, T.; Honda, K. J . Phys. Chem. 1982,86,2617-2619. (31)Kosower, E.M.; Cotter, J. L. J. Am. Chem. SOC.1964,86,5524. (32)Neta, P.; Richoux, M. C.; Harriman, A. J. Chem. SOC.,Faraday Trans. 2 1985,81,1427. (33)Bookbinder, D. C.;Wrighton, M. 5.J . Electrochem. SOC.1983, 130,1080. (34)Heyrovsky, M.; Novotny, L. ColZect. Czech. Chem. Commun. 1987, 52, 1096. (35)Zhuyin, L.; Wang, C. M.; Persaud, L.; Mallouk, T. E.; Thomas, E. J. J. Phys. Chem. 1988,92,2592-2597.
0743-7463/92/2408-2577$03.00/0Q 1992 American Chemical Society
Johansen et al.
2578 Langmuir, Vol. 8, No. 10, 1992 Apparatus. Working electrode potentials were controlled by a potentiostat (Mod 0152) and a sweep generator (Mod 0151) manufactured by Utah Electronics (Australia). Cyclic voltammogramswererecordedwithaRikadenkiX-Y plotter. AHewlettr Packard 8451A diode array spectrophotometer was used for absorbance measurements. Electrodes and Their Preparation. The working electrodes were prepared from plastic sheets, coated with SnOz (InzO3doped) (gift from Courtaulds Performance Film, Inc., California). In most experiments K-M high conductivity plastic based multilayer material was employed. Identical results were obtained from K-60and K-200sheets. The conducting SnO2 coated plastic sheets were cut to pieces 2.3 cm long and 0.87cm wide and copper wires were attached with conducting silver araldite. The SnOz surfaces were coated with Ndion solution and were left to dry at room temperature for 2 days. The Nafion thickness was calculated from the volume applied, using a specific gravity 2 for the Nafion. This calculation ignored swelling of Nafion in water. The Nafion-coated electrodes were normally stable in water and in aqueous electrolyte (0.1 M NaCl) solutions for at least two days; in the NaCl electrolyte Nafion is in its sodium form. All electrodes were examined microscopically for defects in the SnOz material and the Nafion layers before and after use. Results obtained with electrodes that were subsequently found to have defects were disregarded, but this was only usually observed after prolonged applications of negative voltages 2-0.9 V with respect to the Ag/AgCl reference electrode. The counter electrode was made from Pt. All solutions contained 0.1M NaCl. All potentials are quoted vs Ag/AgCl (0.1 M C1-) [redox potential in 0.1 M NaCl (+0.257 V) vs NHE]. Distribution of the Viologens between Nafion and the Aqueous Solution. The distribution of the viologens between the Nafion membrane and the bulk of the solution was estimated from absorption measurements. Electrodes coated with Nafion (1.3cm2 area and 2 X 10"' cm thickness) were immersed in the appropriate viologen solution (1.3cm3)for 20min withcontinuous mixing and then transferred into 1.3 cm3 deaerated 0.1 M NaC1. After removal of a Ndion-coated electrode that had been equilibrated with a viologen solution, e.g., MPVS+,the maximum at 260 nm (e = 20 800 M-1 cm-9 in the UV absorption spectrum of the viologen was used to determine the viologen concentration in the bulk solution. The concentration of the viologens in Nafion was measured after their total reduction at -0.9 to -1.0 V. The distribution of MV2+and MPVS+ between the bulk of a solution and a Ndion layer fits the Langmuir adsorption equilibrium
where [Vgn+],, is the concentration of the viologen in the bulk, [Vg"+]a is the equilibrium concentration of the viologen in Nafion, [Vgn+]d,ais the limiting concentration of the viologen in the Ndion membrane, reached at an infinite Wp+Ias. Kapp is an apparent association constant. The loading of MV2+follows eq 3 with [MV2+Ind,hf= (0.8 f 0.1)M and Kapp= (3.3f 0.7)X 103 M-1. The respective values for MPVS+ are 0.65 f 0.1 M and (7.7 & 1.5) X lo3 M-I. These values represent a remarkable enrichment by the two charged viologens. The comparable values of [Vgn+]a;inf,despite the 2-fold difference in the ionic charges of these viologens, implies that the capacity of the Nafion for a given viologen is not determined by the electrostatic attraction alone. The remarkable enrichment of the singly charged viologen ions is also reflected by a high specificity constant, [MPVS+lnc [Na+],/[MPVS+]aq[Na+]nd > lo4 in dilute aqueous solutions. High concentrations of the formally neutral SPV in Nafion are also observed. For example, with 1 X 10-3 M SPV remaining in the initial solution,the concentration of thisviologen in the Nafion is 0.3 M. However, unlike the charged viologens, most of the SPV is quickly removed from the Ndion when immersed in 0.1 M NaCl and nitrogen bubbled (e.g. 5 min of bubbling reduces [SPVInd from 0.3 to 0.1 M). The high concentrations of the viologens in the Ndion layers, and the fact that even the formally neutral SPV is highly concentrated, differ from observations made in zeolite^.^^^^ In (36)Gemborys, H.A.; Shaw, B.R.J . Electroanal. Chem. Interfacial Electrochem. 1986, 208, 95.
65
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Figure 1. Spectra taken after a pulse of negative voltage. Spectra were recorded at constant time intervals after electrolysis was discontinued. K(naf) = 5.4 M-l; K-60electrode; 0.6 M MPVS+ loaded into 2.07 X cm Nafion (concentration in the bulk, 2 x 10-3M). Curve 0 obtained after 3.8 s of electrolysis at -0.8 V. Subsequent spectra were recorded every 10 a. The numbers at the curves show the sequence of spectra in the order with which they were taken. (Not all spectra are shown.) the latter case,MV2+ions have been reported to exchangequickly with Na+ ions, even at relatively low Na+ concentrations. This difference is attributed to the combined effect of the electrostatic field of the Nafion and hydrophobic interactions. The latter are not important in zeolites. The preferred packing of the viologens in Nafion will depend on both electrostatic and hydrophobic interactions between the polymer and viologen and with each viologen the resultant effect will be different. With MV2+/MV+ the overall electrostatic interactions are attractive but with the zwitterionic viologens MPVS+and SPV the overall electrostatic attractive interactions become smaller; with SPV- they are repulsive. The hydrophobic character increases in the order MV2+, MPVS+, SPV, and reduction makes the first two viologens more hydrophobic; the in the opposite applies to SPV/SPV-. The high value of case of MPVS+,observed despite its only single net ionic charge, and the relative high enrichment of SPV demonstrate the importance of the hydrophobic interactions in the systems studied here. Reduction of Viologens within Nafion. When a NaFioncoated SnOz electrode is immersed in an aqueous solution of a viologen, the viologen accumulates in the membrane so that its concentration near the electrode surface is considerably higher than that in the bulk solution. If this enrichment were related only to the electrostatic interactions in the membrane, the maximum capacity of the polymer should be equivalent to the ionic charge, which is about 1 M for Ndion. In the present work this limit has in fact been reached with the viologens used but the fact that the formally uncharged SPV is also strongly concentrated in the membrane indicates that other effects are also important. Application of a sufficiently negative potential to the viologen in the membrane causes reduction and initially the products are found in a narrow zone along the oxide surface. With the circuit open the extent of reduction can be controlled and this allows the study of the equilibrium (2)within the Nafion membrane. Aspects of the diffusion of the reduction products in the Nafion will be dealt with separately. Results of a typical experiment are demonstrated in curve 0 of Figure 1 which shows the absorption spectrum observed after application of a short pulse of negative potential to a Nafon membrane containing MPVS+;similar curves were obtained with MV2+. These curves, recorded at different times, are all composite spectra of MPVS' and (MPVS*)2;the spectra of the singlespecies are derived below (cf. Figure 3). Under the conditions described in Figure 1only a fraction of the viologen was reduced, and the relative amounts of monomeric MPVS' (Ame 395 and 600nm) and dimeric (MPVS*)2(Amm 370 and 520 nm) are determined by the volume occupied by the reduced species at the moment when the electrolysis is interrupted (curve 1, Figure 1). Once the circuit is opened, this volume
Langmuir, Vol. 8,No.10, 1992 2679
Reduced Viologens on SnOz Electrodes 50000
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Figure 3. Absorption spectra of the monomer and dimer viologen species in Nafion. Spectra were obtained by successive
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linear combinations of monomer-dimer equilibria spectra. reduced, provided the duration of the initial electrolysiswaa kept short (
