Effects of Sodium Dodecyl Sulfate, SDS, Micelles on the

The effects of sodium dodecyl sulfate, SDS, on the reductive electrochemistry of p-nitrobenzenediazonium tetrafluoroborate, PNBD, were investigated at...
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Langmuir 2002, 18, 10311-10317

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Effects of Sodium Dodecyl Sulfate, SDS, Micelles on the Electrochemical Behavior of a Model Arenediazonium Ion. 2. Estimation of the Association Constant of the Electrochemically Generated Aryl Radicals with SDS Micelles Elisa Gonza´lez-Romero,† Ma Begon˜a Ferna´ndez-Calvar,† and Carlos Bravo-Dı´az*,‡ Universidad de Vigo, Facultad de Ciencias, Departamento de Quı´mica Analı´tica y Alimentaria, and Departamento de Quı´mica Fı´sica, 36200 Vigo, Spain Received July 29, 2002. In Final Form: October 7, 2002 The effects of sodium dodecyl sulfate, SDS, on the reductive electrochemistry of p-nitrobenzenediazonium tetrafluoroborate, PNBD, were investigated at two different pHs by employing differential pulse polarography (DPP). At pH ) 2, the reduction peaks of the -N2+ and -NO2 groups are completely overlapped. Upon addition of SDS ([SDS] < critical micelle concentration, cmc), their peak potentials, Ep, are shifted in opposite directions, Ep(-N2+) toward more anodic values and Ep(-NO2) toward more cathodic values, and two polarographic peaks are clearly observed, suggesting that PNBD interacts with the sulfate group of SDS leading to the formation of an ion-pair. At the same pH but at [SDS] > cmc, the peak potentials shift in the reverse directions as those below the cmc and the polarographic peaks overlap again, an effect that is interpreted in terms of the incorporation of PNBD to the SDS micellar aggregates. At pH ) 5, the reduction peaks of the -N2+ and -NO2 are clearly separated. Addition of SDS shifts their peak potentials in opposite directions, the same as those at pH ) 2, up to a maximum (-N2+) or minimum (-NO2) at [SDS] ) cmc, after which the directions are reversed getting a constant value. Peak currents, ip, for either -N2+ and -NO2 decrease smoothly reaching a plateau region. Quantitative analyses of the effects of SDS on Ep and on ip at [SDS] > cmc allowed estimations of the association constants of the parent PNBD and of the electrochemically generated aryl radical, Ar•, with SDS micelles. The association constant of the aryl radical is higher than that of PNBD by a factor of ∼2, indicating that the nitrobenzene radicals are preferentially stabilized in SDS micelles. The results obtained, together with those in previous work, suggest that the combination of DPP with arenediazonium ions as probe molecules provides a rapid, easy, and low-cost method to estimate the stability constants of a large number of aryl radicals to biomimetic systems, information that may be valuable for understanding relevant redox reactions in the more complex biological systems.

Introduction Micelles and other colloidal and macromolecular systems have been extensively exploited as models to investigate the effects of heterogeneous environments and microenvironments on a large variety of reactions, providing relatively simple standard systems for understanding electron transfer and other processes that are important for grasping the complex behavior encountered in biological assemblies.1-5 Redox reactions of micelle-solubilized organic compounds have been an object of study for more than three decades by employing electrochemical techniques, and a number of reviews on the topic are available.6-10 Two * Corresponding author. Phone: +34+986+812303. Fax: 34+986+812 382. E-mail: [email protected]. † Departamento de Quı´mica Analı´tica y Alimentaria. ‡ Departamento de Quı´mica Fı´sica. (1) Fendler, J. Membrane Mimetic Chemistry; Wiley: New York, 1982. (2) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (3) Tascioglu, S. Tetrahedron 1996, 52, 11113. (4) Zana, R. Surfactant Solutions: New Methods for Investigation; Marcel Dekker: New York, 1985. (5) Savelli, G.; Germani, R.; Brinchi, L. Reactivity Control by Aqueous Self-Assembling Systems. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel-Dekker: New York, 2001. (6) Mackay, R.; Texter, J. Electrochemistry in Colloids and Dispersions; VCH: New York, 1992. (7) Mackay, R. A. Colloids Surf., A 1994, 82, 1.

major lines of research appear evident: first, examination of micellar effects on half-wave potentials,11-14 rate constants,15-18 diffusion coefficients,19-21 and so forth of well-known redox couples, and second, studies concerned with the stability of the electrogenerated anion or cation radicals.8,22-24 Meyer et al.22 have reported the remark(8) Russling, J. F. Electrochemistry in Micelles, Microemulsions and Related Microheterogeneous Fluids. In Electroanalytical Chemistry, Vol. 18; Marcel Dekker: New York, 1994. (9) McIntire, G. L. Crit. Rev. Anal. Chem. 1990, 21, 257. (10) Kaifer, A. E.; Go´mez-Kaifer, M. Supramolecular Electrochemistry; Wiley-VCH: New York, 1999. (11) Pouillen, P.; Martre, A. M.; Martinet, P. Electrochim. Acta 1982, 27, 853. (12) Bencheikh-Sayarh, S.; Pouillen, P.; Martre, A.; Martinet, P. Electrochim. Acta 1983, 28, 627. (13) Georges, J.; Desmetre, S. Electrochim. Acta 1984, 29, 521. (14) Mousty, C.; Devaux, B.; Mousset, G.; Pouillen, P.; Martinet, P. Electrochim. Acta 1985, 30, 1733. (15) Bravo-Diaz, C.; Gonzalez-Romero, E. Anal. Chim. Acta 1999, 385, 373. (16) Pazo-Llorente, R.; Bravo-Dı´az, C.; Gonza´lez-Romero, E. Fresenius’ J. Anal. Chem. 2001, 369, 582. (17) Romero-Nieto, M. E.; Bravo-Diaz, C.; Gonzalez-Romero, E. Int. J. Chem. Kinet. 2000, 32, 419. (18) Bravo-Dı´az, C.; Gonza´lez-Romero, E. Electroanalysis, in press. (19) Zana, R.; Mackay, R. A. Langmuir 1986, 2, 109. (20) Verral, R. E.; Milioto, S.; Giradeau, A.; Zana, R. Langmuir 1989, 5, 1242. (21) Charlton, I. D.; Doherty, A. P. Langmuir 1999, 15, 5251. (22) Meyer, G.; Nadjo, L.; Saveant, J. M. J. Electroanal. Chem. 1981, 119, 417.

10.1021/la026312s CCC: $22.00 © 2002 American Chemical Society Published on Web 11/21/2002

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able stabilization of the electrogenerated anion radical of phthalonitrile in the presence of cationic micelles.22 McIntire et al.23 examined the role of anionic, cationic, and nonionic micellar systems on the heterogeneous oneelectron-transfer process of nitrobenzene, NB, to yield the anion radical NB•-, and on the stability of NB•-.23 They showed, in contrast to Meyer’s report, that NB•- is stabilized in anionic sodium dodecyl sulfate (SDS) micelles with respect to consumptive homogeneous chemical reactions; meanwhile cationic CTAC micelles concentrate NB•- in the Stern layer causing its second-order decay process to proceed more rapidly. Kaifer et al., who studied the micellar effects on the reductive electrochemistry of methyl viologen, reported similar conclusions.24 Certainly, the possibility of stabilizing radical intermediates at the interfaces of micelles may have important consequences in many vital areas where electron-transfer reactions are relevant such as the biological, food, and energy-transfer areas. Such a possibility prompted Burkey et al. to propose micellar systems as devices for enhancing the lifetimes and concentrations of free radicals.25 The combined results suggest that the influence of micellar systems on the electrochemistry of a particular substrate cannot be predicted on the basis of simple electrostatic considerations and that micellar effects are strongly dependent upon the substrate and surfactant structures.24 The limited number of studies involving the stabilization of aryl radicals in colloidal systems, together with the well-known ability of arenediazonium ions to generate radicals that may be responsible, to some extent, for the mutagenic and carcinogenic properties of aromatic diazonium compounds,26-31 prompted us to study the effects of SDS micelles on the electrochemical behavior of a model arenediazonium ion. For this purpose, we have chosen the 4-nitrobenzenediazonium ion, PNBD, as a model substrate because the half-life for its spontaneous decomposition in aqueous systems, in the absence of catalysts, is extremely slow on the time scale of typical electrochemical experiments, t1/2 . 215 h at T ) 25 °C, and because it bears two electroactive groups, showing a number of polarographic peaks that can be employed to investigate the micellar effects.17,32 Further details on the reasons why PNBD can be considered as a molecule of choice can be found elsewhere.42 In addition, it has been reported that SDS micelles promote homolytic dediazoniation of PNBD, in contrast with the observed behavior for other arenediazonium ions with substituents in the same and in other positions.34-36 The reasons of this (23) McIntire, G. L.; Chiappardi, D. M.; Casselberry, R. L.; Blount, H. N. J. Phys. Chem. 1982, 86, 2632. (24) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985, 89, 4876. (25) Burkey, T. J.; Griller, D. J. Am. Chem. Soc. 1985, 107, 246. (26) Riordan, J.; Vallee, B. L. Methods Enzymol. 1972, 25, 521. (27) Augusto, O. Free Radical Biol. Med. 1993, 15, 329. (28) Kato, T.; Kojima, K.; Hiramoto, K.; Kigugawa, K. Mutat. Res. 1992, 268, 105. (29) Ohshima, H.; Friesen, M.; Malaveille, C.; Brouet, I.; Hautefeulli, A.; Bartsch, H. Food Chem. Toxicol. 1989, 27, 193. (30) Reszka, K. J.; Chignell, C. F. J. Am. Chem. Soc. 1993, 115, 7752. (31) Reszka, K. J.; Chignell, C. F. Chem.-Biol. Interact. 1995, 96, 223. (32) Bravo-Diaz, C.; Romsted, L. S.; Harbowy, M.; Romero-Nieto, M. E.; Gonzalez-Romero, E. J. Phys. Org. Chem. 1999, 12, 130. (33) Gonza´lez-Romero, E.; Ferna´ndez-Calvar, B.; Bravo-Dı´az, C. Prog. Colloid Polym. Sci., in press. (34) Bravo-Dı´az, C.; Sarabia-Rodriguez, M. J.; Barreiro-Sio, P.; Gonzalez-Romero, E. Langmuir 1999, 15, 2823. (35) Bravo-Dı´az, C.; Romero-Nieto, M. E.; Gonzalez-Romero, E. Langmuir 2000, 16, 42. (36) Romsted, L. S. Interfacial Compositions of Surfactant Assemblies by Chemical Trapping with Arenediazonium Ions: Method and Applications. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001.

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unexpected conduct are still unclear,35 and electrochemical studies on this system may provide new data to rationalize this anomalous behavior. Among the different electrochemical techniques available, differential pulse polarography, DPP, has been proved to be a convenient technique to study dediazoniation mechanisms and to obtain valuable information not only on the parent electroactive substrate but also on the electrochemically generated species concurrently. Previous work on arenediazonium, ArN2+, electrochemistry indicates that ArN2+ ions are easily reducible compounds, the first reduction process involving the transfer of one electron, yielding the arenediazenyl radical ArN2•, which further decomposes yielding N2 and aryl radicals, Ar•.33,37,38 Experimental Section Instrumentation. DPP measurements were obtained with a Metrohm E506 Polarecord, in conjunction with the cell systems described below. For DPP experiments, a 663 VA-Stand (Metrohm) equipped with a water-jacketed voltammetric cell was used. The multimode working electrode was used in the DME mode. The three-electrode system was completed by means of a glassy carbon rod (2 × 65 mm) auxiliary electrode and a Ag/AgCl (3 M KCl) reference electrode. Solutions used in polarographic measurements were bubbled with N2 gas (99.999%) for at least 10 min and kept under a nitrogen atmosphere during the electrochemical runs. pH was measured using a previously calibrated Metrohm model 744 pH-meter. All potentials given hereafter will be relative to the above-mentioned Ag/AgCl electrodes. Materials. Reagents were of maximum purity available and were used without further purification. The chemicals used in the preparation of the universal Britton-Robinson buffer, BRB, p-nitrophenol, NP, and nitrobenzene, NB, were purchased from Aldrich or Fluka. Other materials employed were from Riedel de Ha¨en. All solutions were prepared by using Milli-Q grade water (κ ) 1.2 µS cm-1). p-Nitrobenzenediazonium tetrafluoroborate, PNBD, was purchased from Aldrich (97%) and recrystallized twice with acetonitrile/cold ether as described elsewhere.39 PNBD was stored in the dark in a desiccator at low temperature and recrystallized periodically to minimize possible decomposition via the Schieman40 reaction and by reactions with water vapor. The purity of ArN2+ was checked periodically by employing DPP and vis-UV spectroscopy.

Results The basic electrochemical behavior of a number of ArN2+ ions in aqueous acid solution and in alcohol/water mixtures has been previously investigated.15,17,18,38,41,42 Two polarographic peaks are observed for the reduction of the -N2+ group; the first one, in the +0.05 to -0.02 V (vs Ag/AgCl) potential range, is associated with the one-electrontransfer process to yield the arenediazenyl radical, ArN2•. Following this reduction process, a second reduction peak (-0.4 to -0.9 V), involving three electrons and three protons, is observed, to give the corresponding phenylhydrazine.38,41 When the aromatic ring bears more than one electroactive center, a number of extra peaks can be (37) Galli, C. Chem. Rev. 1988, 88, 765. (38) Viertler, H.; Pardini, V. L.; Vargas, R. R. The Electrochemistry of Triple Bond in The Chemistry of Triple-Bonded Functional Groups, Supplement C.; Patai, S., Ed.; J. Wiley & Sons: New York, 1994. (39) Garcia-Meijide, M. C.; Bravo-Diaz, C.; Romsted, L. S. Int. J. Chem. Kinet. 1998, 30, 31. (40) Hegarty, A. F. Kinetics and Mechanisms of Reactions Involving Diazonium and Diazo Groups. In The Chemistry of Diazonium and Diazo Compounds; Patai, S., Ed.; J. Wiley & Sons: New York, 1978. (41) Fry, A. J. Electrochemistry of the Diazo and Diazonium Groups. In The Chemistry of Diazo and Diazonium Groups; Patai, S., Ed.; J. Wiley & Sons: New York, 1978. (42) Gonzalez-Romero, E.; Malvido-Hermelo, B.; Bravo-Dı´az, C. Langmuir 2002, 18, 46.

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Figure 2. Effects of SDS on the polarographic behavior of PNBD at pH ) 5 (BR buffer). The highest peaks are those in the absence of SDS. T ) 20 °C. Other conditions are as in Figure 1.

Figure 1. (A) Effect of addition of SDS on the polarograms of PNBD. Peak I is that in the absence of SDS. (B) Variation of the peak current at Ep ) -130 mV (peak I) for the reduction process of the -N2+ group (see text for peak identification). [PNBD] ) 1 × 10-4 M, [HCl] ) 0.01 M, T ) 20 °C, Ei ) +0.2 V, ∆U ) -1.5 V, td ) 0.4 s, ∆E ) -80 mV, v ) 7.5 mV s-1.

detected depending on the experimental conditions.42 Such is the case for PNBD: at pH < 3, the first polarographic peak of the -N2+ group is completely overlapped with that for the first reduction process of the -NO2 group; meanwhile at pH > 3 both peaks are perfectly separated (vide infra). Because of this, we have studied the effects of SDS on the polarographic behavior of PNBD at pH ) 2 and pH ) 5. Figure 1A shows the effect of SDS on the polarograms of PNBD at pH ) 2. The chemical processes associated with each polarographic are also indicated. In the absence of SDS and at pH ) 2, the observed polarographic peak has been denoted as peak I and, as mentioned before, contains the polarographic signals for the reduction of the -N2+ and -NO2 groups. Addition of SDS leads, however, to a symmetric decomposition of peak I in the two clearly observed polarographic peaks, peak IA, associated with the one-electron reduction of the -N2+ group, (43) Pazo-Llorente, R.; Gonza´lez-Romero, E.; Bravo-Dı´az, C. Int. J. Chem. Kinet. 2000, 32, 210.

and peak IB, associated with the four-electron reduction process of the -NO2 group, suggesting that at [SDS] < critical micelle concentration (cmc), PNBD interacts with the sulfate group of SDS, probably in the form of an ionpair. At [SDS] > cmc, the peak potential shifts in the reverse direction and peaks IA and IB overlap again until only one reduction peak is observed. Addition of SDS shifts the peak potential value of peak IA toward more positive potential values, in contrast with that for peak IB, which is shifted in the opposite direction. In addition, peak currents, ip, decrease, reaching a plateau region, Figure 1B. The decrease in ip can be a consequence of the combination of two effects, the adsorption of the surfactant monomers to the surface of the electrode and the formation of the ion-pair, whose diffusion coefficient should be much lower than that of the free ion. Formation of SDS/PNBD ion-pairs was not observed in previous conductometric studies on this system,35 but it has been proposed in a number of dediazoniation studies to explain the unexpected low yields observed in the course of some dediazoniations.35,44,45 The fact that when [SDS] > cmc the polarographic peaks are shifted in the opposite directions than those when [SDS] < cmc, overlapping again, strongly suggests that the PNBD ions are incorporated into the SDS micelles and that such an aggregation process is more favorable than the formation of ion-pairs, consistent with reported results obtained at [SDS] > cmc by employing 1 H NMR, vis-UV, and high-performance liquid chromatography.35,36 The observed shifts in Ep and the decrease in ip were confirmed when doing the experiments at pH ) 5; see Figure 2, which shows the polarograms for PNBD at different amounts of SDS. As mentioned before, at pH ) 5 and in the absence of SDS, the two polarographic peaks for the reduction of the -N2+ and -NO2 groups can be (44) Cuccovia, I. M.; Agostinho-Neto, A.; Wendel, C. M. A.; Chaimovich, H.; Romsted, L. S. Langmuir 1997, 13, 5032. (45) Cuccovia, I. M.; da Silva, M. A.; Ferraz, H. M. C.; Pliego, J. R.; Riveros, J. M.; Chaimovich, H. J. Chem. Soc., Perkin Trans. 2 2000, 1896.

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Figure 4. Effect of SDS on the peak currents for the reduction process of the -NO2 group of NB (b) and NP (O). [NP] ) [NB] ) 5 × 10-5 M, [HCl] ) 0.01 M, T ) 20 °C. Other conditions are as indicated in Figure 1. Scheme 1. Square Mechanism for the Electrochemical Reduction of ArN2+ Ions in the Presence of SDS Micelles Involving n Electrons and Reflecting the Possibility of Association of the Electrochemically Generated Aryl Radical to the Micellar Aggregatea

a E and E represent the formal potentials of the “free” and F C “associated” ArN2+.

Figure 3. Variation in the peak potentials (A) and peak currents (B) for the reduction processes of the -N2+ (O) and -NO2 (b) groups. Data are from Figure 2.

perfectly discriminated.18,42 Figure 3A shows that addition of SDS shifts peak IB toward more negative values up to a minimum after which the peak potential shifts slightly in the opposite direction reaching a plateau region. A similar behavior, but in the reverse direction, is observed for the reduction of -N2+, Figure 3A. Peak current values for peaks IA and IB decrease upon increasing [SDS], Figure 3B, reaching a plateau region after which peak currents are independent of [SDS]. Notice that in the presence of SDS, the morphology of the peak IA is essentially the same as that in its absence, indicating that SDS micelles do not modify the characteristics of the electrochemical processes. Peak current decrease is consistent with the incorporation of PNBD to the SDS micellar aggregates. Diffusion coefficients, which are related with peak currents through the Ilkovic46 equation, of the bounded substrates, DM, are usually much smaller than those of the “free” substrates, DF, since molecules of guest compounds are usually much smaller than those of the substrate/micelle assembly and thus diffuse more slowly toward the Hg electrode. Alternatively, the observed Ep shift is also consistent with the association of PNBD to the micellar aggregates showing that the -NO2 group is inserted into the SDS micelle and hence more difficult electron-transfer associated with the reduction of the nitro group would be expected. The results are consistent with those reported in previous studies on the location of PNBD in micellar aggregates.34,35 The observed minimum in the apparent peak potentials, Ep, versus [SDS] takes place at [SDS] ∼ 5 × 10-3 M, Figure 3A, and suggests the formation of micelles, that is, the cmc. This point was confirmed by analyzing the effects of (46) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L. Electrochemistry for Chemists, 2nd ed.; J. Wiley & Sons: New York, 1995.

SDS micelles on the electrochemical behavior of nitrobenzene and p-nitrophenol, Figure 4. Both NB and NP are known to associate to SDS micelles, and since both molecules bear the electroactive -NO2 group, they can be considered as model molecules to investigate the micellar effects on the electrochemical behavior of PNBD, allowing one to study independently the SDS effects on the -NO2 group. Cyclic voltammograms of NB in the presence of anionic, cationic, and nonionic micelles23 and the polarograms of NP and NB in aqueous acid solution have been already reported.17,33 Addition of SDS up to [SDS] ) 0.027 M leads to a shift in peak potentials toward more negative values of 120 mV (NB) and 102 mV (NP), reaching a plateau region; meanwhile peak currents decrease up to a minimum at [SDS] ∼ 2.5 × 10-3 M after which a slight increase in ip is observed reaching a plateau region, Figure 4. The results are in complete agreement with previous electrochemical studies of NB in micellar systems23 and with those obtained in this work for the -NO2 group of PNBD. Note that the minimum in the ip versus [SDS] plots takes place at a very similar SDS concentration than that for PNBD. Discussion When studying relevant electron-transfer reactions in biomimetic systems, one always has to bear in mind that the reduced, R, and oxidized, O, species may have different preferential solubilization sites and that their relative stability in that particular region may not be the same. A commonly proposed mechanism for the oxidationreduction process of a substrate in the presence of particular biomimetic system such as micelles, where the probe can be partitioned in two different regions, is shown in Scheme 1. The mechanism involves the electron transfer to/from the electrode from/to the probe in the region

Effects of SDS Micelles on Electrochemistry

Langmuir, Vol. 18, No. 26, 2002 10315 Table 1. Values for the Association Constants of PNBD and of the Electrochemically Generated Aryl Radical with SDS Micelles and for the Diffusion Coefficient Ratioa -N2+ -NO2 -N2+

KO/M-1

KR/M-1

DM/DF

180 ( 33 222 ( 38

314 ( 55 667 ( 92 760b

∼0 0.06 ( 0.02 0.03b

a Values were determined at pH ) 5 (Figure 2). b In the presence of β-cyclodextrin. See ref 42.

Figure 5. Calibration plot for peak I (Figure 1) in the presence of [SDS] ) 0.033 M. [HCl] ) 0.01 M, T ) 20 °C. Other conditions as indicated in Figure 1.

forming a reactive radical intermediate (e.g., a radical or radical anion/cation) that may be held within that particular region, Scheme 1.8,42,47 Certainly, the reactivity of this intermediate will differ from that of the uncomplexed intermediate and from that of the parent substrate, leading to substantial changes in the selectivity of the reaction, as shown for the β-cyclodextrin-mediated dediazoniation of p-nitrobenzenediazonium ions.42 Several approaches need to be taken into consideration to develop equations for Scheme 1 when micellar systems are employed. The first one is that micelles do not change the electrochemical reduction processes of the electroactive probes employed. The assumption has been proven to be likely because the morphology of the PNBD polarograms does not change when SDS micelles are present in the medium. The assumption is also consistent with results of other electroactive molecules in surfactant systems.11,13,24,48 A second supposition involved is that the PNBD probe does not change the micelle size and shape. This approach seems valid provided that the probe concentration is kept much smaller that the surfactant concentration and the surfactant concentrations employed are low enough so that micellar growth to rods can be neglected.19 The assumption was, however, checked experimentally by preparing Ilkovic plots, that is, by plotting ip values versus [PNBD] in the presence of surfactant; indeed, if the probe had an effect on the measured ip value, one would expect nonlinear plots even at low surfactant concentrations. Figure 5, chosen as representative, shows that this is not the case. The slight deviation from linearity observed at high [PNBD] is due to adsorption phenomena.17,18 The third point to be discussed is the distribution of the probe among micelles, which is not uniform but is of the Poisson type.2 Thus, for any probe concentration, there will be some micelles with no probe, some micelles with only one probe molecule, some others with two probe molecules, and so forth. If the time required for the probe reduction, tpr, is smaller than that for the diffusion of micelles away from the electrode, tdm, then all probes in a particular micelle will have time to be reduced. If the time scales are reversed, that is, if tpr > tdm, then at most only one probe will have time to be reduced. Certainly, the linear plot observed in Figure 5 strongly suggests that tpr < tdm and hence it is safe to use the stoichiometric (47) Matsue, T.; Osa, T.; Evans, D. J. Inclusion Phenom. 1984, 2, 547. (48) Mousty, C.; Pouillen, P.; Martre, A.; Mousset, G. J. Colloid Interface Sci. 1986, 113, 521.

probe concentration and not the effective probe concentration, [PNBD]/[Dn], where Dn is the micellized surfactant ([Dn] ) [SDS]T - cmc). Finally, we have taken into consideration the dynamic aspects of micellar systems. Micelles are dynamic systems whose lifetimes range between 10-3 and 1 s, depending on experimental conditions. Rates for entrance of substrates into the micelles are typically in the limit of diffusion control, that is, ∼109 L mol-1 s-1, but rates for the exit of substrates depend largely on their hydrophobicity.2,49 Residence times of anionic surfactants in micelles are in the 10-5-10-3 s range, increasing with chain length. Hence, it appears that our PNBD probe exits and enters the micelle several times during an average micellar lifetime, and hence the measured ip values, which are related to the diffusion coefficients through the Ilkovic equation, are average values taken over the whole distribution of micelles. The hypothesis is consistent with results obtained when employing a number of ArN2+ ions of different hydrophobicities.36 Bearing in mind the above assumptions and that PNBD partitions between the bulk aqueous and the micellar psudophases, Scheme 1, the apparent diffusion coefficient, Dapp, for any of the polarographic peaks of PNBD is given by eq 1.

Dapp [ST] ) DM [SM] + DF [SF]

(1)

where DM and DF represent the diffusion coefficients of the bounded (SM) and free (SF) substrate ([ST] ) [SC] + [SF]). To determine the association constant, eq 2 can be derived:42,47

ipc2

) 2

ip0

( )

ipc2 DM 1 1- 2 + DF KO[SDS] ip0

(2)

where ipc is the peak current when all the substrate is incorporated into the micelle, that is, when [SDS] . [ST], ip0 is the peak current in the absence of SDS, KO is the association constant, and [SDS] is the total surfactant concentration. Calculated KO and DM/DF values are given in Table 1. Analysis of ip and Eapp shifts, Figure 3, provides valuable information on the complexation of the electrogenerated species given that both the oxidized and reduced species can be incorporated into the SDS micelles, Scheme 1. Assuming that the diffusion coefficients of the parent substrate and the electrogenerated radical are equal to each other and that [SDS] . [PNBD], the apparent peak potential, Eapp, when all the substrate is incorporated into the micelle is given by eq 3:42 (49) Anniansson, E. A. G.; Wall, S. N.; Almgrem, M.; Hoffman, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905.

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Eapp ) EF +

RT KR ln nF KO

Gonza´ lez-Romero et al.

(3)

where EF is the peak potential in the absence of SDS and KO and KR are the binding constants for the oxidized and reduced forms of the substrate. From the data in Figure 3 and by use of eq 2, the KO value was obtained, and by use of eq 3, the KR value for the one-electron-reduced form of PNBD, Ar•, was estimated, Table 1. The ratio DM/DF is expected to be very low because the molecular size of the micelles/probe assembly is much larger than that of the free probe, and hence its diffusion coefficient should be much lower. This expectation is fulfilled on view of the data in Table 1. On the other hand, results in Table 1 indicate that very similar association constant values for PNBD with SDS micelles were obtained independently of the chosen polarographic peak but they are somewhat lower than that obtained by employing other techniques.42 The association constants for the electrochemically generated radicals cannot be directly compared to each other because they correspond to different specimens, but their values reflect that the radical intermediates are stabilized by a factor of ∼2-3 with respect to the parent molecules, consistent with previous results for other electroactive compounds in SDS micelles.23,24,50 Note that the radical intermediate derived from -N2+ is the aryl radical Ar•, and hence the calculated KR value represents the association constant of the aryl radical to the SDS micelles. To our knowledge, no association constants of aryl radicals with colloidal or macromolecular systems have been published to date except for the inclusion of Ar• with β-cyclodextrin, for which a KR ) 760 M-1 value was reported.42 Association constants of some radical anions and radical cations with micelles are known, but the experimental conditions, the parent substrates, and the radicals studied are completely different from those employed here; thus comparison of KR values is not reliable.23,24,50 The high KR value obtained for the ArN2• radical is, however, in keeping with the general conclusion pointed out in those studies: radicals and radical anions are included more tightly than the parent substrates. The fact that KR/KO ∼ 2 for PNBD provides a rationale for the tendency of SDS micelles to promote PNBD homolytic dediazoniations, an effect which has already been described.35 The ratio KR/KO for PNBD/SDS is about 20 times lower than that for PNBD/β-CD, reflecting the fact that in SDS micelles the heterolytic dediazoniation pathway is predominant and only a small percentage of the reaction proceeds through the homolytic pathway; meanwhile in the presence of β-CD the major dediazoniation pathway is the homolytic one.33,35,42 The results obtained together with those in previous work33,42 suggest that the combination of the polarographic technique with ArN2+ ions as probe molecules provides a general, rapid, relatively easy and low-cost method to determine the stability constants of a large variety of aryl radicals in biomimetic systems, data that may not be readily obtained when employing other techniques such as electron spin resonance (ESR), vis-UV spectroscopy, fluorescence, and so forth, and those are certainly needed to get a complete scope of the behavior of aryl radicals in multiphase systems. For one side, a large variety of arenediazonium compounds can be easily prepared from their precursors by employing a number of reactions and they can be safely (50) McIntire, G. L. Anal. Chem. 1990, 21, 257-277.

stored in the solid state under appropriate conditions for relatively long times.39,51-53 Previous electrochemical studies show that when an arenediazonium ion acquires an electron, the corresponding arenediazenyl, ArN2•, radical is formed, which further reacts to yield the aryl radical, Ar•, and molecular nitrogen.37,54 The rate of decay of the arenediazenyl radical into Ar• and N2 is not known exactly, but values in the 106-108 s-1 order of magnitude have been reported.37 Certainly, the value must be higher than 106 s-1 because of the irreversibility of the first reduction process of the -N2+ group, a process which is mainly diffusion-controlled.15-18,42 Consequently, a large variety of aryl radicals can be electrochemically generated by employing ArN2+ ions as parent substrates. The electrochemical generation of aryl radicals from arenediazonium ions has an additional advantage in view of the fact that very few organic compounds are electroactive in the same potential region where the one-electron reduction of ArN2+ takes place, typically +0.05 to -0.02 V versus Ag/AgCl,16,38,41,46 and hence hardly any interference from other analytes in the medium is expected.17,18 In addition, the effects of a number of experimental and instrumental parameters on the polarographic peaks of model arenediazonium ions were also investigated and knowledge on the nature of the polarographic peaks, that is, the reversibility/irreversibility degree, if they are diffusion-, adsorption-, or kinetically controlled, and so forth, is available.18 Alternatively, a number of reasons suggest DPP as the method of choice, including the low-demanding experimental setup, the speed of the analysis of a reaction mixture, the possibility to carry out real-time analysis directly in the reaction mixture, and principally the periodical renewal of the DME surface, minimizing problems with passivation and electrode fouling, and the low limits of detection that can be achieved.18,55-57 Moreover, half-lives for the spontaneous decomposition of arenediazonium ions in aqueous acid solution, in the absence of catalysts, are usually much higher than the time required for a typical DPP experiment,54 and they are even lower in the presence of SDS micelles,35,36,44 so electrochemical experiments can be safely carried out without significant decomposition of the substrates under a variety of experimental conditions. Conclusions In conclusion, we have shown that at [SDS] < cmc, PNBD forms an ion-pair with the sulfate headgroup of the surfactant monomer, while at [SDS] > cmc, PNBD is incorporated into the micellar aggregate with the -NO2 group pointing toward the interior of the micelle. The estimated binding constant for the p-nitrobenezene radical is higher by a factor of 2 with respect to that of the parent substrate, indicating that SDS micelles stabilize the electrochemically generated aryl radicals. For the mentioned reasons, it appears that the use of the polarographic technique to study the electrochemical behavior of the -N2+ group provides a very convenient (51) Schank, K. Preparation of Diazonium Groups. In The Chemistry of Diazonium and Diazo Compounds; Patai, S., Ed.; J. Wiley & Sons: New York, 1978. (52) Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Compounds, 3rd ed.; Edward Arnold: Baltimore, MD, 1985. (53) Colas, C.; Goeldner, M. Eur. J. Org. Chem. 1999, 1357, 7. (54) Zollinger, H. Diazo Chemistry I, Aromatic and Heteroaromatic Compounds; VCH: New York, 1994. (55) Zuman, P.; Ludvik, J. Electroanalysis 2000, 12, 879. (56) Barek, J.; Cvacka, J.; Muck, A.; Quasirova´, V.; Zima, J. Fresenius’ J. Anal. Chem. 2001, 369, 556. (57) Zuman, P. Electroanalysis 2000, 12, 1187.

Effects of SDS Micelles on Electrochemistry

technique to obtain the stabilization constants of a large variety of aryl radicals in a number of biomimetic systems, which otherwise are very difficult to obtain by employing other techniques such as ESR, pulse radiolysis, photolysis, and so forth, mainly due to the high instability of the aryl radicals. Further investigations in this direction by employing DPP combined with a variety of arenediazonium ions and biomimetic systems are in progress.

Langmuir, Vol. 18, No. 26, 2002 10317

Acknowledgment. Financial support from the following institutions is acknowledged: MCYT of Spain (BQU2000-0239-C02), Xunta de Galicia (XUGA 38301A92 and XUGA 38305A94), and Universidad de Vigo. One of us, M.B. F-C., thanks Xunta de Galicia for a graduate research training grant. LA026312S