Langmuir 1988,4,663-667
663
Dimerization of the Methylviologen Cation Radical in Anionic Micellar and Polyelectrolyte Solutions Pablo A. Quintela, Abigail Diaz, and Angel E. Kaifer* Department of Chemistry, University of Miami, Coral Gables, Florida 33124 Received September 25, 1987. I n Final Form: December 21, 1987 The dimerization behavior of the methylviologen cation radical ( M V )in the presence of sodium dodecyl sulfate (SDS),sodium decyl sulfate (SdecS), and sodium poly(styrenesu1fonate) (PSS) was examined by using the visible spectral characteristics of solutions containing electrogenerated M V . The results indicate that the concentration of either SDS or SdecS micellar aggregates exerts a strong effect on the extent of dimerization. Thus, negligible dimerization was found at high levels of surfactant while extensive dimerization was observed at surfactant concentrations slightly above the corresponding critical micelle concentrations. This behavior was reasonably well simulated by a simple computational model based on the Poisson distribution. The dimerization pattern in the presence of PSS was quite different. The polyelectrolyte was found to increase the dimerization level, throughout the entire concentration range surveyed (5-100 mM), over that found in isotropic aqueous solutions. This was interpreted as the result of MV+clustering triggered by the polyelectrolyte chains.
Introduction The N,N’-dialkyl-4,4’-bipyridiniumions (viologens) constitute a well-known class of electron acceptors.’ Several viologens have been used frequently as electrontransfer mediators (relays) in experimental schemes aimed at the photochemical splitting of water.2 Viologens are very attractive materials for this purpose because they can be reduced easily to yield cation radicals which in turn are thermodynamically capable of reducing hydrogen ions (at neutral of acidic pH) to generate molecular hydrogen. Extensive research efforta have been devoted to the search for effective ways to catalyze this last r e a ~ t i o n . ~ Viologen cation radicals are also known to dimerize in aqueous media.4 This is probably the result of the increased hydrophobic nature of the cation radicals as compared to the parent dications. The reactivity and properties of the resulting dimers are mostly unknown. Several authors have reported that the dimers are ESR ~ i l e n t . ~ The optical absorption differences between dimers and monomers are also well established? It must be recognized that the formation of dimers can strongly affect the course of any further reactions in which the viologen cation radicals are to partake. As part of our studiesG8 on the electrochemistry of viologens in organized media we have reported previously that anionic sodium dodecyl sulfate (SDS) micelles prevent the dimerization of the cation radical of N,”-dimethyl4,4‘-bipyridinium ion (methylviologen, Mv2+).6 Quite recently, we found that sodium decyl sulfate (SdecS) micelles have a similar effect.8 However, in this last paper, we noted that the extent of dimerization is highly dependent on the concentration of micellar aggregates; that is, a high concentration of micelles essentially suppresses dimerization whereas a low concentration of micelles results in a dimerization level much higher than that observed in micelle-free, aqueous solution. All of these findings stimBird, C. L.; Kuhn, A. T. Chem. SOC.Reo. 1981,10,49. (a) Gratzel, M. Acc. Chem. Res. 1981,14,376. (b)Matauo, T.Pure Chem. 1982,54,1693. (c) Calvin,M. Photochem. Photobiol. 1983, 37,-349. (3) Photogenerution of Hydrogen; Harriman, A,, West, M. Q.,Eds.; Academic: London, 1982. (4) Kosower, E. M.; Cotter, J. L. J. Am. Chem. SOC.1964, 86, 5524. (5) Rieger, A. L.; Rieger, P. H. J. Phys. Chem. 1984, 88, 5845 and
references therein. (6) Kaifer, A. E.; Bard, A; J. J. Phys. Chem. 1985, 89, 4876. (7) Kaifer, A. E. J. Am. Chem. SOC.1986,108, 6837. (8)Quintela, P. A.; Kaifer, A. E. Lungmuir 1987,3, 769.
dated us to start a more detailed study on the dimerization of the cation radical of methylviologen in the presence of variable concentrations of micellar and other colloidal aggregates. We report here the results of this investigation in media containing SDS, SdecS, and sodium poly(styrenesulfonate) (PSS). The latter was selected because it constitutes a system with similar electrostatic but different aggregation properties from those of the two anionic surfactants.
Experimental Section Materials. Methylviologen was purchased from Aldrich and
recrystallized from methanol. SDS (Baker) and SdecS (Kodak) were recrystallized from absolute ethanol and dried at 80 “C in a vacuum oven. Polystyrene sulfonate (sodium salt, nominal MW 70000) was purchased from Scientific Polymer Products (Ontario, NY) and used without further purification. Distilled water was further purified by passage through a pressurized,four-cartridge Barnstead Nanopure system. All other chemicals were of analytical grade. Equipment. Optical spectra were recorded with a HewlettPackard 8452A spectrophotometer interfaced to a Leading Edge personal computer. A Princeton Applied Research Model 173 potentiostat was used to drive the electrochemical generation of the methylviologen cation radical. The instrumentation for cyclic voltammetry has been previously described.* Surface tension measurements were done with a Fisher Model 20 tensiometer, using platinum-iridium rings. Procedures. The methylviologen cation radical (MV‘+)was electrochemically generated from deoxygenated solutions of the parent dication. In order to take advantage of the rapid spectral acquisition capabilities of the HP 8452A spectrophotometer system, a conventional 1.0-cm cuvette was used as the electrolysis cell (see Figure 1). Two class slides covered with indium oxide (transmittance 90% , Delta Technologies) were affixed to the optical windows of the cuvette in such a way that the conductive surfaces faced each other. These two slides were connected as the working and the auxiliary electrodes of the cell. The tip of the reference electrode (a sodium chloride saturated calomel electrode, SSCE) was then immersed in the solution filling the cell. At this point, under open circuit conditions, a blank spectrum (in the range 300-800 nm) was obtained and stored in the computer memory. The potential of the working electrode was then set at -0.80 V vs SSCE to initiate the one-electron reduction of MV2+. After 30-60 s of electrolysis, a visible spectrum was recorded. The optical spectra recorded in this way were found to be of similar shape (dimer/monomer ratio) to those recorded, in separate experiments, in a vacuum electrolysis cell, after exhaustive reduction of the MV2+solutions. In 1964, Kosowar and Cotter described the spectral changes associated with the dimerization of the methylviologen cation
0743-7463I88 l2404-0663%01.60 - . ~ ~, ~IO . 0 1988 American Chemical Society , - - I - - - -
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664 Langmuir, Vol. 4, No. 3, 1988
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Quintela et al.
1'
i
1.24
o
0.6
0.2
DIM
Figure 1. Scheme of the electrochemical cell employed for the . working optically transparent electrode; generation of W +W, C, a u x d h y optically transparent electrode; R, reference electrode; B, spectrophotometer beam; S, glass spacer. See text for additional details. radical? The monomer shows absorption maxima at 394 and 602 nm, while the dimer exhibits absorption maxima at 364 and 550 nm. Thus, either the 350-400-nm or the 540-610-nm spectral regions are suitable to estimate the relative amounts of dimer and monomer present in a reduced methylviologen solution. Our experimental design is not adequate for conventional two-component analyeis using spectral data from two wavelengths because of the variable effective optical pathlength in our cell (only the diffusion layer contains the absorbing species). However, we devised a simple procedure to approximate the dimerlmonomer concentration ratio. Let us assume that Am and A394are the absorbances at these two wavelengths for a sample having equilibrium dimer and monomer concentrations equal to D and M, respectively. Thus
1.0 r
0.8
Ratio
Figure 2. Am/Aw VB DIM concentration ratio. The points were obtained with solutions of methylviologen whose concentration was adjusted to produce varying dimerlmonomer ratios. The medium was aqueous 50 mM NaCl.
Table I. Data for the Dimerization of the Methylviologen Cation Radical in the Presence of Variable Concentrations of SDS [SDSI, [SDSI, mM AmlA394 D I M mM A364IA394 DIM 0.1 2.0 4.0 6.0 8.0 10.0 12.0
0.57 1.70 1.57 1.33 0.84 0.69 0.62
0.21 a a a
0.64 0.40 0.28
14.0 16.0 18.0 20.0 35.0 70.0
0.54 0.51 0.50 0.48 0.42 0.42
0.16 0.12 0.11 0.066
b b
"Undefined. The dimer concentration is too large to be measured by our method. *Denotes an undetectably small concentration of dimer. the errors involved in their determination.
where the E values represent the corresponding molar absorptivity coefficients. If the second term in this equation is negligibly small compared to the first one, it can be easily shown that AW = -€384m +-A394
4394m
D 6 3 ~ 4 mM
(2)
Thus, a plot of Am/ASQ4vs DIM should be linear. In order to verify the validity of eq 2, solutions of varying MV2+concentrations in 50 mh3 NaCl were prepared, electrochemically reduced, and analyzed for their spectral characteristics in the 350-400-nm region. The ratio Am/A394 was then plotted against the concentration ratio DIM, calculated from the analytical MV2+concentration of each solution (thisis a crude approximation because complete conversion of MV+ to MV" is only accomplished at the electrode surface, and the concentration of MV+ drops off exponentiallyas the distance from the surface increases) and the reported aqueous equilibrium constant for the dimerization reaction (Koeower and Cotter quoted a value of 380 M-' based on unpublished studies by W. M. Schwartz: and we used it as the only available valueg). The resulting graph is shown in Figure 2 and is linear for DIM values in the range 0 . 8 . For larger dimer concentrations the plot is no longer linear as expected because of the increasing magnitude of the second term in eq 1. Therefore, this linear graph was employed as a calibration curve to evaluate the DIM ratios from the experimentally obtained Am,IAm ratios. The resulting ratios are excellent qualitative indicators of the extent of dimerization in the surveyed media. However, their use for quantitative purposes is probably not advisable because of (9) One of the reviewers pointed out that Schwartz's value waa determined for ethylviologenin a medium containing l M KCI and 0.1 M boric acid/sodium borate buffer (pH8.56). Indeed, the ionic strength in our solutions waa much lower, and this can change the dimerization
constant value.
Results The results obtained for the dimerization of methylviologen cation radical in the presence of variable concentrations of SDS are shown in Table I. All of these experiments were done with solutions containing MV2+at a concentration of 1.0 mM. Therefore, the D I M ratio in each solution is determined by using the calibration curve of Figure 2 and the experimentally obtained absorbance ratio. The results in Table I are in excellent agreement with our previous qualitative findingsa6T8At 70 mM SDS, the ratio of absorbances reveals an undetectable concentration of dimer. A similar situation is observed at 35 mM SDS. At about 20 mM SDS the dimer becomes detectable, and any further decrease in the concentration of SDS, with the associated reduction in the concentration of micellar aggregates, substantially increases the concentration of dimer. At concentrations of SDS below 8.0 mM the ratio D I M is too large t o be measured by our method. The critical micelle concentration (cmc) of the SDS/50 mM NaCLl1.0 mM MV+ system was determined through the concentration dependence of the solution's surface tension and found to be 1.0 mM SDS, not far from the reported value in the absence of MV2+(2.25 mM SDS).'O The high extent of dimerization in the proximity of the cmc, although previously unreported for SDS, is not surprising because of our earlier findings with the SdecS system? It must be pointed out that in the concentration range 0.5-2.0 mM SDS the electrogenerated methylviolo(IO) Britz, D.; Mortenaen, J.
J. Electroanal. Chem. 1981, 127, 231.
Dimerization of Methyluiologen Cation Radical
Langmuir, Vol. 4, No. 3, 1988 665
Table 11. Data for the Dimerization of the Methylviologen Cation Radical in the Presence of Variable Concentrations of SdecS [SdecS], [SdecS], mM AwIA9w DIM mM A w / A s w DIM 1 0.54 0.16 26 0.67 0.37 28 0.62 0.28 20 1.01 a 0.90 a 30 22 0.60 0.25 24 0.76 0.51 70 0.45 0.017
I
c c L Y
m
"Undefined. The dimer concentration is too large to be measured by our method.
PL 0
Table 111. Data for the Dimerization of the Methylviologen Cation Radical in the Presence of Variable Concentrations of PSS
5 10 20 35
1.70 1.39 1.13 0.91
a
a a
50 75 100
0.80 0.76 0.66
0.57 0.51 0.39
/I:*
I
-0.9
-015
-0:7
a
"Undefined. The dimer concentration is too large to be measured by our method.
gen cation radical is weakly adsorbed on the electrode surface? Under these conditions, the adsorbed cation radicals are expected to dimerize extensively because of their agglomeration in a small volume. Therefore, it is clear that misleadingly large DIM ratios, i.e., values much larger than the actual D I M values in solution, can result in cases where adsorption of the cation radicals takes place. Hence, values obtained in those regions should be taken with due care. Note, however, that a t sufficiently low concentrations of SDS (0.1 m M entry in Table I) the extent of the dimerization reaction returns to the level found in micelle-free aqueous solution. The data for the dimerization equilibrium in the presence of variable amounts of SdecS are given in Table 11. Again, a very low concentration of dimer is present a t 70 mM SdecS, but a decrease in the surfactant concentration below this level causes a concurrent increase in the dimer concentration. The concentration of dimer becomes too large to be measured below 24 mM SdecS. Note, that, for SdecS, the maximum D I M values occur a t larger surfactant concentrations than for SDS. This is a reflection of the lower surfactant activity (and higher cmc) of the former. In fact, the cmc of the SdecS/5O mM NaCl/1.0 mM MV2+system has been previously reported by our group to be 11mM SdecS." As with the longer chain surfactant, a t sufficiently low concentrations of SdecS (1.0 mM entry in Table 11),the extent of dimerization recedes to the level of isotropic aqueous solution. The data for the dimerization equilibrium in the presence of the anionic polyelectrolyte PSS are given in Table III. The level of dimer in all the PSS-containing solutions was found to be higher than the level in isotropic aqueous solution. This is not unexpected since the methylviologen cation radical should exhibit higher local concentrations in the vicinity of the negatively charged PSS chains, and thus the extent of dimerization must be enhanced. However, the magnitude of the PSS effect on the dimerization level is much larger than can be accounted for on the basis of the increased local concentration of cation radicals in the proximity of the PSS chains. Since our experimental procedure would produce anomalously high D I M ratios in the case of adsorption or deposition of the cation radicals a t the electrode surface, we decided to check for this possibility by using cyclic voltammetry. The MV2+/Mv'+ redox couple showed diffusion-controlled, reversible be-
E,
V
VI
SSCE
Figure 3. Cyclic voltammogram on glassy carbon of 1.0 mM MV2+/2.0mM PSS/50 mM NaC1. Scan rate, 50 mV/s. Notice the parabolic shape of the anodic peak revealing the weak adsorption of the oxidized M V + species.
"-
1
0.s
'
g
I
I U
[Pss]
,
mM
Figure 4. Normalized peak current for the first reduction of MV2+in the presence of variable PSS concentrations. All data points were recorded at 50 mV/s with a glassy carbon electrode (projected area, 0.08 cm2) in 50 mM NaC1.
havior, with no evidence of either MV2+or M V + adsorption, in the concentration range 5-100 mM PSS. Interestingly, we found weak adsorption of the cation radical for PSS concentrations below 2 mM (see voltammogram in Figure 3), a behavior reminiscent of that observed with SDS6 (in a similar concentration range) and with SdecS8 (at higher surfactant concentrations). However, the adsorption of the M V species takes place a t PSS concentrations below those surveyed in Table I11 and does not provide an explanation for the high levels of dimer detected experimentally. In order to assess the degree of methylviologen association to the PSS chains, we recorded the voltammetric peak currents for the first reduction peak of methylviologen (at the 1.0 mM level) in the presence of PSS a t variable concentrations. The results are plotted in Figure 4. Note that the decrease in the peak current is fast so that, at 5.0 mM PSS, the peak current is about half of that recorded in the absence of polyelectrolyte. Indeed, these
666 Langmuir, Vol. 4, No. 3, 1988
data clearly indicate that the methylviologen dication associates strongly to the negatively charged polyelectrolyte chains. MV2+appears to interact preferentially with the sulfonic sites if the presence of hydrophilic ions (Na+, present at the 50 mM or higher level) is taken into account. The interaction of M V with PSS can be quite different from that of the parent dication and the polyelectrolyte; however, the extensive dimerization observed points also to the strong association of the MV'+ species with the polyelectrolyte.
Quintela et al.
0.8 l '
O
I
-
-
-
l
Discussion The dimerization pattern observed for the methylviologen cation radical in the presence of either SDS of SdecS micelles can be easily rationalized. The cation radical has been shown to associate strongly with the negatively charged micellar aggregates.6i8 Thus, if the concentration of micelles is comparable to the concentration of cation radicals, the Poisson distribution predicts that most micelles will contain none, one, two, or three cation radicals;" that is, the micelles act mostly as spacers, keeping the reactive species apart from each other and preventing dimerization. On the other hand, if the concentration of micelles is much lower than the concentration of cation radicals, the micellar aggregates will concentrate the M V + radicals, increasing substantially the probability of dimerization. The steep increase in the concentration of dimer is expected to peak very near the cmc. In our experiments, we could not determine the exact position of the peak because the D I M ratios in this concentration region were too large to be measured with our method. However, the region of high dimer concentrations extends below the cmc in our experiments, probably as a result of the already reported adsorption of MV'+ in this concentration range. Nonetheless, if the concentration of anionic surfactant is made low enough in order to avoid MV'+ adsorption, the dimerization equilibrium returns to levels similar to that of micelle-free aqueous solution. Other interesting conclusions can be drawn from the data in Tables I and 11. For instance, a t 70 mM SDS essentially no dimer was detected in our experiments. However, according to the Poisson distribution, 15.8% of the micelles will contain two MV'+ions, 4.6% will contain three, and minor percentages will contain more than three cation radicals. This indicates that two, and perhaps three, cation radicals can be solubilized in the same micellar assembly without formation of dimers. Thus, we tried to estimate how many cation radicals per micelle are needed to trigger dimerization. One can assume that dimerization only takes place in micelles containing more than a threshold number of cation radicals. The probability associated with this crowding level can be simply calculated from the Poisson distribution and compared to the experimental probability of a M V ion being found in the dimeric state, PD,which in turn can be obtained from the D I M ratios as 2D/M PD = (3) 2D/M + 1 In the case of SDS, the plot of experimental PD values as a function of surfactant concentration is shown in Figure 5. The solid lines were calculated from the Poisson distribution assuming a cmc of 1 mM, a mean aggregation number of 6012for the SDS micelles throughout the whole (11) For a micelle/MV'+ concentration ratio of 1, the Poisson distribution predicts that 36.8% of the micelles will contain no cation radical; 36.8%w ill contain one cation radical; 18.4%, two; 6.170,three; and 1.5%, four.
Lo
0 0
20
40
60
I S D S I , mM
Figure 5. Probability of M V +dimerization as a function of the SDS concentration: ( 0 )experimental values; (-) calculated with the Poisson model described in the text, using 7,8, and 9 as the threshold occupancies for dimerization.
"1 0.4
0.2
4
6
i \!I ISdecSI, mM
Figure 6. Probability of MV" dimerization as a function of the SdecS concentration: ( 0 )experimental values; (-) calculated with the Poisson model described in the text, using 4, 5, and 6 as the threshold occupancies for dimerization. concentration range surveyed, and the threshold occupancies for dimerization that are given in the figure. The experimental points fit reasonably well the lines calculated for threshold occupancies of eight and seven cation radicals per micelle. The model used to simulate the experimental data is very simple, but it reproduces well the concentration dependence of the dimerization for relatively large threshold occupancies (7-8). Perhaps the crudest approximation of the model is that the micellar aggregation number is assumed constant over the entire concentration range. Indeed, the presence of methylviologen cation radicals can alter the aggregation number of SDS micelles, particularly at concentrations slightly above the cmc where the micelle/MV'+ concentration ratio is very low. A similar treatment was applied to the dimerization data for SdecS solutions. The experimental PD values are plotted in Figure 6. The solid lines were calculated assuming a cmc of 11 mM, a mean aggregation number of 5013 for the SdecS micelles, and the threshold occupancies given in the figure. In this case a good fit is obtained for (12)Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1979,100,5951. (13) Tartar, H.V.; Lelong, L. M. J. Phys. Chem. 1955,59, 1185.
Dimerization of Methyluiologen Cation Radical a threshold occupancy of five. Note that although the model is very simple it predicts correctly the expected effects of the micellar aggregation number on the extent of dimerization. Thus, the smaller SdecS micelles show a lower threshold occupancy than the bigger SDS micelles. Indeed, the absolute threshold occupancies estimated here are only approximate values because of the errors involved in the determination of the D/M ratios. Nonetheless, it should be quite clear that SDS and SdecS micellar assemblies appear to be capable of solubilizing more than one cation radical (maybe as many as three or four) and still prevent their dimerization. The results obtained with PSS solutions reveal a completely different dimerization behavior. The dimerization level in all PSS solutions surveyed was relatively high. Even a t the 100 mM PSS concentration level, the extent of dimerization is still higher than that found in isotropic aqueous solution. This behavior clearly differs from that observed with both anionic surfactants for which the dimerization can be essentially suppressed a t high concentrations of surfactant. The voltammetric data in Figure 4 indicate the strong association of the methylviologen dication with the negatively charged PSS chains. The high degree of dimerization suggests also the strong association of the cation radical with the PSS chains. Aggregates formed by the interaction of amphiphilic ions and polyelectrolytes have been the subject of limited study. Photochemical t e c h n i q ~ e s ' and ~ surfactant-selective electrodes16 have been used to obtain information on the structure and properties of these aggregates. In a recent report Abuin and Scaianol6have examined the aggregates of PSS and dodecyltrimethylammonium bromide (DTA+Br-). They demonstrated that the association of the DTA+ ions with the anionic polyelectrolyte chains is strong and that the aggregates formed are constituted by (14) (a) Okubo, T.; Turro, N. J. J. Phys. Chem. 1981, 85, 4034. (b) Turro, N. J.; Okubo,T.;Chung, C.-J.;Emert, J.; Catena, R. J. Am. Chem. SOC.1982,104,4799. (c) Turro, N. J.; Okubo, T. J. Phys. Chem. 1982, 86, 1535. (d) Turro, N. J.; Pierola, I. F. J. Phys. Chem. 1983,87, 2420. (15) (a) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982,86,3866. (b) Shirahama, K.; Yuasa, H.; Sugimoto, S. Bull. Chem. SOC.Jpn. 1981, 54, 375. (16) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. SOC. 1984,106, 6274.
Langmuir, Vol. 4, No. 3, 1988 667 a large number of small (7-10 units) surfactant clusters. These clusters behave like "minimicelles" and form well below the surfactant's cmc due to the cooperative effect of the PSS chains. The dimerization pattern of M V in the presence of PSS can be explained by the formation of similar small M V + clusters containing several cation radical units. The aggregation of the cation radicals into these clusters (which in turn are associated to the polyelectrolyte chains) would then explain the extensive dimerization level observed in these solutions. The increase in the concentration of PSS is expected to decrease the probability of cluster formation, thus decreasing the probability of dimerization, as was observed in our experiments.
Summary This work has shown that the extent of MV'+ dimerization in anionic micellar media is highly dependent on the surfactant concentration. SDS and SdecS micelles, when present a t concentration levels similar to that of M V , substantially depress its dimerization. Conversely, a t concentrations slightly above their respective cmc's, these two surfactant systems enhance substantially the dimerization level of the methylviologen cation radical. This behavior was interpreted as the result of a strong interaction between M V + and either SDS or SdecS micelles. In fact, the concentration dependence of dimerization is reproduced satisfactorily by a simple computational model, which assumes that the solubilization of the cation radicals by the micellar aggregates proceeds according to the Poisson distribution and that dimerization takes place only in those micelles containing more than a certain number of M V ions. The extensive dimerization in PSS solutions was interpreted as a result of the formation of small M V clusters in the vicinity of the PSS chains. The association of methylviologen with PSS was independently verified through voltammetric measurements. Acknowledgment. This research was supported by a Bristol-Myers Company Grant of Research Corporation. Registry No. MV'+, 25239-55-8; SDS, 151-21-3; SdecS, 14281-0; PSS, 9080-79-9.
