1896
Langmuir 2006, 22, 1896-1902
Double-Layer Effects and Distance Dependence of Electron Transfer in Reduction of Nitro Aromatic Radical Anions Petra Morˇkovska´,† Magdale´na Hromadova´,† Lubomı´r Pospı´sˇil,*,† and Stefania Giannarelli‡ J. HeyroVsky´ Institute of Physical Chemistry, DolejsˇkoVa 3, Prague, Czech Republic, and Department of Chemistry and Industrial Chemistry, UniVersity of Pisa, Via Risorgimento 35, Pisa, Italy ReceiVed August 11, 2005. In Final Form: NoVember 24, 2005 The first example of the effect of an electric double layer on the reduction of electrochemically generated radical species is reported. The anion radical of methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (pesticide bifenox) is electrochemically reduced in acetonitrile to a phenylhydroxylamine derivative in a process involving three electrons. This heterogeneous reaction is strongly influenced by the concentration and nature of the cation of the indifferent electrolyte. Depending on the type of tetraalkylammonium cation, the redox potential changes by 0.45 V. The kinetic parameters were obtained for five tetraalkylammonium hexafluorophosphate salts. The Frumkin correction, which assumes that the outer Helmholtz plane coincides with the reaction site, was applied to kinetic data of the radical anion reduction. The correction of the apparent rate accounted for the observed effect only in the case of tetramethylammonium salt. The presence of higher tetraalkylammonium homologues causes deviations from the predicted dependence of the electron-transfer rate on the φ2 potential of the outer Helmholtz plane. Hence, the nature of the cation of the electrolyte exerts a further effect extending beyond the electrostatic repulsion only. The corrected rate of electron transfer decreases exponentially with increasing size of the alkyl chain of the indifferent electrolyte cation in the order methyl > ethyl > propyl > butyl > hexyl. The rate decay is characterized by an exponent β ) 0.83. This confirms that the reaction plane for the reduction of the bifenox radical anion is different for each electrolyte. Due to this fact the Frumkin correction cannot fully account for the observed dependence of the heterogeneous rate on the solution composition. The observed effect is not specific to the bifenox radical. A similar influence of the concentration and nature of the cation of the indifferent electrolyte was observed for other nitro compounds, namely, nitrobenzene, nitrobenzoate, and nitrofen.
Introduction The reduction of aromatic nitro compounds was investigated for decades, and reduction products in different media are wellknown.1,2 Effects of the double-layer structure on the electrontransfer reactions of neutral aromatic nitro compounds were discovered 30 years ago by Kojima and Bard.3 They clearly demonstrated the importance of the Frumkin correction4 for the correct evaluation of electrochemical kinetic parameters of the primary reduction step:
Ar-NO2 + e- S Ar-NO2•-
(1)
The second reduction step leading to a dianion of nitro compounds is not observed at ordinary experimental conditions, and it was detected only in liquid ammonia:5
Ar-NO2•- + e- S Ar-NO22-
(2)
In nonaqueous aprotic solvents, such as acetonitrile used in this study, the anion radical is further reduced by an overall addition of three electrons and four protons, yielding phenylhydroxylamine:5-7
Ar-NO2•- + 3e- + 4H+ f Ar-NHOH + H2O † ‡
(3)
J. Heyrovsky´ Institute of Physical Chemistry. University of Pisa.
(1) Kemula, W.; Krygowski, T. M. Encyclopedia of Electrochemistry of Elements; Marcel Dekker: New York, 1979; pp 77-130. (2) Lund, H. In Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.; Marcel Dekker: New York, 1991; Chapter 9. (3) Kojima, H.; Bard, A. J. J. Am. Chem. Soc. 1975, 97, 6317. (4) Frumkin, A. N. Discuss. Faraday Soc. 1947, 1, 57.
Many researchers following the pioneering work of Kojima and Bard3 studied different aspects of the formation of aromatic nitro anion radicals8-15 (reaction 1). This included the consideration of the adsorption influence,9,11,16 the effect of the size of the reactant,13 the solvent effect,14 the double-layer effect resulting from different planes of the closest approach for different electrolyte cations,17 and the effect of the ion pair formation between the anion radical species and the cation of the supporting electrolyte8,10 on the electron-transfer rate. The transfer of the second electron to nitrobenzene as the rate-determining step influenced by the double-layer structure was assumed15 in aqueous solutions at high pH. Continuous studies of the reduction of nitro aromatics are motivated by the industrial importance of many organic nitro compounds. Full understanding of the reactivity of the nitro aromatic radical anion is a prerequisite for the application of radical reactions in various fields including pharmacology, food production, and/or military industries. Bifenox, methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (Scheme 1), is a pesticide18 that works on a principle of the interruption of the photosynthetic chain in weed plants through the reactivity of its photogenerated nitro radical. However, bifenox also inhibits heme (5) Smith, W. H.; Bard, A. J. J. Am. Chem. Soc. 1975, 97, 5203. (6) Holleck, L.; Becher, D. J. Electroanal. Chem. 1962, 4, 321. (7) Geske, D. H.; Maki, A. H. J. Am. Chem. Soc. 1960, 82, 2671. (8) Fawcett, W. R.; Lasia, A. J. Phys. Chem. 1978, 82, 1114. (9) Petersen, R. A.; Evans, D. H. J. Electroanal. Chem. 1987, 222, 129. (10) Chauhan, B. G.; Fawcett, W. R.; Lasia, A. J. Phys. Chem. 1977, 81, 1476. (11) Fawcett, W. R.; Fedurco, M.; Opallo, M. J. Phys. Chem. 1992, 96, 9959. (12) Kwiatek, B.; Kalinowski, M. K. J. Electroanal. Chem. 1987, 226, 61. (13) Kraiya, C.; Singh, P.; Evans, D. H. J. Electroanal. Chem. 2004, 563, 203. (14) Kapturkiewicz, A.; Opallo, M. J. Electroanal. Chem. 1985, 185, 15. (15) Pezzatini, G.; Guidelli, R. J. Electroanal. Chem. 1979, 102, 205. (16) Evans, D. H.; Gilicinski, A. G. J. Phys. Chem. 1992, 96, 2528. (17) Corrigan, D. A.; Evans, D. H. J. Electroanal. Chem. 1980, 106, 287. (18) Worthing, C. R.; Phil, D. The Pesticide Manual; British Crop Protection Council: Hampshire, U.K., 1987.
10.1021/la0521894 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006
Reduction of Nitro Aromatic Radical Anions Scheme 1
synthesis, leading to the accumulation of porphyrin,19 which causes the disease porphyria. Bifenox contains nitro and carboxyl groups in ortho proximity on the aromatic ring and chlorine substituents on the other aromatic ring. Our previous paper20 described in detail the mechanism of bifenox reduction. The nitro group of bifenox is reversibly reduced in acetonitrile to the anion radical at -1.45 V against Fc/Fc+ followed by an irreversible three-electron reduction between -1.9 and -2.3 V. The formation of an amine derivative, the ultimate product of the uptake of six electrons,2,21,22 was not observed in our study. In general, the electrochemical processes require the presence of a redox-inactive electrolyte, which ensures the proper solution conductivity. The concentration of a strong electrolyte determines the structure of the electrode double layer, which in most cases has a negligible influence on the rate of electron transfer. However, in certain circumstances the influence of the double-layer structure on the redox reactions4,23,24 and also on the coupled chemical reactions25,26 of charged reactants has to be taken into account. The theoretical aspects of the double-layer effects were thoroughly worked out, and hence, we refrain from reviewing them here. The models assume that the reaction site of a heterogeneous electron-transfer reaction is located at the outer Helmholtz plane (OHP) of the electrode double layer. A local potential at the OHP depends on the electrolyte concentration and hence influences the concentrations of charged reactants at the reaction site. A verification of the double-layer effects was intensively investigated in the 1960s on systems including mainly inorganic polyelectrolytes, such as Zn2+, Eu3+, In3+, and S2O82-. This paper is aimed at elucidation of the effects of the solution composition, namely, the concentration and type of the indifferent electrolyte (the effects of the ionic strength), on the second threeelectron reduction of the anion radical of bifenox. It is to the best of our knowledge the first paper on the double-layer influence on the further electron-transfer reaction (reaction 3) to the electrochemically generated organic nitro radical anion. The phenomenon is more general, and we were able to observe it also for simpler structures such as nitrobenzene, nitrobenzoate, and the pesticide nitrofen. Experimental Section Materials. Bifenox was purchased as a pesticide reference material (with the 99.0% purity certificate) from Dr. Ehrenstorfer, GmbH (Augsburg, Germany). Tetraalkylammonium hexafluorophosphates were obtained from Sigma and dried before use. Acetonitrile (Sigma) as received contained 900 ppm water, whereas the treatment with (19) Sˇ ivikova´, K.; Dianovsky´, J. Mutat, Res. 1999, 439, 129-135. (20) Morˇkovska´, P.; Hromadova´, M.; Pospı´sˇil, L. J. Electroanal. Chem. 2005, 582, 156. (21) Kastening, B. Electrochim. Acta 1964, 9, 24. (22) Geske, D. H.; Maki, A. H. J. Am. Chem. Soc. 1960, 82, 2671-2676. (23) Delahay, P. Double Layer and Electrode Kinetics; J. Wiley: New York, 1965; Chapter 9. (24) Gierst, L. In Transactions of the Symposium on Electrode Processes; Yeager, E., Ed.; J. Wiley: New York, 1961; p 109. (25) Nu¨rnberg, H. W. Discuss. Faraday Soc. 1965, 39, 136. (26) Pospı´sˇil, L.; Ku˚ta, J. Collect. Czech. Chem. Commun. 1969, 34, 742.
Langmuir, Vol. 22, No. 4, 2006 1897 3 Å activated molecular sieves (Lachema, Brno) decreased the water content below 20 ppm. Techniques. Electrochemical measurements were done using an electrochemical system for cyclic voltammetry, phase-sensitive ac polarography, and dc polarography. It consisted of a fast rise-time potentiostat and a lock-in amplifier (Stanford Research, model SR830). The instruments were interfaced to a personal computer via an IEEE interface card (PC-Lab, AdvanTech model PCL-848) and a data acquisition card (PCL-818) using 12-bit precision. A threeelectrode electrochemical cell was thermostated using Ministat CC (Huber, Germany). The reference electrode, Ag|AgCl|1 M LiCl, was separated from the test solution by a salt bridge, and the halfwave potential of ferrocene against it was +0.56 V (checked daily). The working electrode was a valve-operated static mercury electrode (SMDE2, Laboratornı´ Prˇ´ıstroje, Prague) with an area of 1.13 × 10-2 cm2 and a mechanically controlled drop time of 1 s. The auxiliary electrode was a cylindrical platinum net. Oxygen was removed from the solution by passing a stream of argon. The electrode charge q and the potential of the outer Helmholtz plane φ2 as a function of the applied potential E were obtained by integration of the capacitance against potential curves. The sinewave frequency and amplitude for the capacitance measurements were 320 Hz and 5 mV, respectively. The potential of zero charge Epzc (where q ) 0), needed for the integration of C-E data, was identified as a potential of a minimum on the C-E curves or from the potential of the electrocapillary maximum. Determination of Epzc was done by computer-controlled drop-time measurements of a part of the electrocapillary curve. The drop-time measurement was based on a sharp change of the cell admittance Y. The program loop evaluated repeatedly two subsequent readings of Y against a threshold difference. The number of readings of Y between two subsequent drop falls was proportional to the drop time and to the surface tension. The estimated error of drop-time measurements by this method was 0.2%. Integration of capacitance-potential curves was performed using the program Origin (MicroCalc).
Results and Discussion The reduction of bifenox, methyl 5-(2,4-dichlorophenoxy)2-nitrobenzoate, proceeds in a sequence of redox steps common to nitroaromatic compounds. The reduction is characterized by a reversible monoelectronic formation of a reactive anion radical at -1.45 V against the Fc/Fc+ couple according to eq 1. The anion radical is characterized by an EPR spectrum with the hyperfine coupling constants aN ) 9.358 G, aH ) 3.31 G, and 2aH ) 1.07 G. The EPR spectrum is consistent with the spectra of radicals of aromatic nitro compounds. The anion radical is further reduced at more negative potentials (-1.9 V or less against Fc/Fc+) in an irreversible three-electron process described by eq 3. Details concerning the reaction mechanism, EPR spectra, and product identification were thoroughly described in our previous paper.20 In this paper we report the analysis of the influence of electrolytes on the reduction of the anion radical (reaction 3) with the aim to distinguish the effects of the nature of the electrolyte from the Coulombic double-layer effects. Figure 1 shows dc polarograms measured at the constant concentration of 0.02 M for five different tetraalkylammonium (TAA+) electrolytes.27 The second reduction step shows a strong dependence of the half-wave potential on the type of alkyl chain of the TAA+ cation of the indifferent electrolyte. The smallest tetramethylammonium cation (TMA+) yields the second reduction step at the least negative reduction potential. Only in the presence of TMA+ a complete reduction by six electrons can be observed (see a third two-electron wave at -2.6 V in Figure 1). All other homologues of TAA+ shift the reduction to more negative (27) The concentration of bifenox was chosen to be correspondingly low (0.1 mM). This concentration ratio of electroactive and indifferent compounds is needed for avoiding migration effects that influence the faradic currents.
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Figure 1. dc polarograms of 0.1 mM bifenox and 0.02 M tetraalkylammonium hexafluorophosphate in dry acetonitrile. Curves 1-5 correspond to tetramethyl-, tetraethyl-, tetrapropyl-, tetrabutyl-, and tetrahexylammonium salts, respectively.
potentials. The influence of the nature of TAA+ illustrates that the cation may not be as “indifferent” as supposed. Many possible reasons could be invoked as a starting hypothesis to explain such a behavior. For example, the cation of TAA+ could participate in the ion-pairing equilibrium with the radical anion
Ar-NO2•- + TAA+ T [Ar-NO2•-, TAA+]
(4)
which would lead to a formally uncharged aggregate. It is wellknown that the ion-pairing interactions are strongest among ions of the opposite charge, which have the most similar solvation properties. The solvation depends on the ion surface charge density and hence on the ionic size. An estimated molecular dimension of the bifenox radical is 13 Å, which implies that the preferred interacting cation would be THA+. Hence, the electrostatic repulsion of the electrogenerated anion radical should be the least pronounced in solutions of THA+. Experimental data clearly indicate the opposite effect: the strongest repulsion and the most negative reduction potentials are observed in the presence of THA+. Bieman and Fawcett confirmed ion-pairing effects in reduction of polyvalent anions present in the bulk of the solutions.28 The present case differs from conditions where the ion pairing was proven because the anion radical of bifenox is generated at the electrode interface under the influence of a strong intensity of the electric field. In this preelectrode space the equilibrium constant for the ion pair formation is strongly influenced by the Wien effect,29 and as a result the association is disfavored. The observed dependence of the second reduction step on the electrolyte concentration is independent of the bulk concentration of bifenox. If ion pairing is significant, an increase of the electrogenerated surface concentration (with the increase of the bulk concentration) should shift the equilibrium of reaction 4 in favor of neutral associates and the electrostatic repulsion should be diminished. Ion pair formation is sensitive to the dielectric permittivity of the solvent. Further argument for the absence of the ion-pairing effects stems from insensitivity of the repulsion to the solvent type. Effects described above in acetonitrile were observed also in DMSO, which suggests that ion pair equilibrium is not the dominant factor responsible for such a large change of reduction potentials shown in Figure 1. Another hypothesis is that a different rate of protonation participating in reaction 3 is responsible for the observed effect. (28) Bieman, D. J.; Fawcett, W. R. J. Electroanal. Chem. 1972, 34, 27. (29) Harned, H. S.; Owen, B. B. The Physical Chemistry of Electrolytic Solutions, 2nd ed.; Reinhold Publishing Corp.: New York, 1960; p 214.
Figure 2. dc polarograms of 0.1 mM bifenox in acetonitrile at different concentrations of electrolytes: (A) 1, 3, 10, 20, 50, and 100 mM tetramethylammonium hexafluorophosphate; (B) 1, 3, 10, 30, 100, and 300 mM tetrabutylammonium hexafluorophosphate. Curves 1-6 correspond to increasing concentrations.
This is rather unlikely because the negative electrode charge should contribute to an acceleration of the overall process, not to its retardation. Most likely, it is the electrostatic repulsion of the anion radical at the negatively charged electrode interface which plays the decisive role. Hence, our further investigation was directed along this line. The influence of the double-layer structure can be disclosed as a change of the electron-transfer rate with the ionic strength of the indifferent electrolyte. Also the rate change in electrolytes of different natures suggests that the double-layer structure plays an important role in the overall mechanism. The present case of the bifenox reduction is influenced both by the ionic strength and by the electrolyte type. The dependence of the polarograms on the ionic strength was investigated for all five electrolytes. We used the concentration range from 0.5 mM to 0.3 M or to the solubility limits of a given salt. The maximum difference of the redox potential of the reduction of the bifenox anion radical to phenylhydroxylamine amounts to 0.45 V. We stress that all curves were recorded with an automatic compensation of the ohmic potential drop due to the solution resistance. A low bifenox concentration and a correspondingly low faradic current allowed an efficient compensation. Two typical results for TMA+ and tetrabutylammonium (TBA+) hexafluorophosphate are given in Figure 2. Tetraethyl- and tetrapropylammonium hexafluorophosphates behaved similarly to the TBA+ cation. Differences were only in the amount of the potential shift of the second reduction step, and they will be illustrated in figures given below. A different picture was observed only at the lowest concentrations of tetrahexylammonium cation (THA+) (Figure 3). There the second reduction is displaced to the most negative potentials where the electrostatic repulsion is expected to be quite pronounced. However, at the lowest concentrations of THA+ a single three-electron wave develops a prewave showing a marked
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Langmuir, Vol. 22, No. 4, 2006 1899
Figure 3. dc polarograms of 0.1 mM bifenox in acetonitrile at different concentrations of tetrahexylammonium hexafluorophosphate: 0.5 mM (curve 1), 1 mM (curve 2), 3 mM (curve 3), 10 mM (curve 4), and 20 mM (curve 5).
Figure 5. Differential capacitance curves (A) and corresponding values of the φ2 potential at the outer Helmholtz plane (B) as a function of the applied potential for different concentrations of tetraethylammonium hexafluorophosphate: 0.5, 1, 2, 3, 5, 10, 20, 30, 50, and 100 mM. Curves 1-10 correspond to increasing electrolyte concentrations.
Figure 4. Differential capacitance curves (A) and corresponding values of the φ2 potential at the outer Helmholtz plane (B) as a function of the applied potential for different concentrations of tetramethylammonium hexafluorophosphate: 1, 3, 5, 10, 20, 30, 50, 70, and 100 mM. Curves 1-9 correspond to increasing electrolyte concentrations.
minimum.30 The absence of a limiting mass-transfer-controlled current (a potential-independent current plateau) of this prewave is a typical indication of a strong electrostatic repulsion. Similar minima on limiting currents were observed for double-layer repulsion of several negatively charged polyanions (S2O82-, S4O62-, Fe(CN)62-, PtCl42-, and others).31 In those cases the decisive role of the double layer was proven. The current minimum observed in the present system further supports a hypothesis that the repulsion due to the double-layer structure is a dominant factor responsible for salt effects. (30) Low concentrations of THA+ allow design of a system which yields oscillatory behavior of bifenox radical reduction. The complexity of it requires further work and will be reported in the future. (31) Heyrovsky´, J.; Ku˚ta, J. Principles of Polarography; Publishing House of the Czechoslovak Academy of Sciences: Prague, 1965; pp 244-253.
The reduction potential of the anion radical of bifenox is located at the negatively charged electrode interface. The negative potential of the outer Helmholtz plane φ2 causes the electrostatic repulsion of the anion radical from the reaction plane. This is the most likely reason the observed current-voltage curves depend so strongly on the ionic strength and a minimum appears at the lowest THA+ concentrations. The quantitative verification of this hypothesis is not a simple task since it requires detailed data on the double-layer structure at all concentrations and in all electrolytes used in this study. The effects of the φ2 potential on the electrode kinetics were thoroughly treated and verified several decades ago. However, those studies were confined mainly to inorganic compounds and to aqueous solutions, where the φ2 potentials were known. Similar double-layer data for nonaqueous solvents are sparse32,33 and mainly limited to TBA+ salts, which are the most widely used indifferent electrolytes in aprotic solvents. Verification of double-layer effects in our set of data required rather laborious evaluation of φ2 potentials in acetonitrile for all salts and all their concentrations. This was accomplished by measuring the double-layer capacity C as a function of the electrode potential E and concentration c of the tetraalkylammonium hexafluorophosphates in acetonitrile. The resulting C-E curves were integrated to yield the surface charge q from which the dependence of the φ2 potential on E and c was obtained. Double-layer data (C-E and φ2-E curves) for all five indifferent electrolytes in acetonitrile are summarized in Figures 4-8. The current-voltage dependence of a slow electron transfer, such as that in the present case, under the influence of the φ2 potential was treated first by Frumkin.4,23,24 The Frumkin (32) Fawcett, W. R.; Loufty, R. O. Can. J. Chem. 1973, 51, 230. (33) Gambert, R.; Baumgartel, H. J. Electroanal. Chem. 1985, 183, 315.
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Figure 6. Differential capacitance curves (A) and corresponding values of the φ2 potential at the outer Helmholtz plane (B) as a function of the applied potential for different concentrations of tetrapropylammonium hexafluorophosphate: 1, 3, 5, 10, 20, 30, 50, 70, and 100 mM. Curves 1-9 correspond to increasing electrolyte concentrations.
correction assumes that (i) the electron-transfer reaction is driven by the effective potential (E - φ2) instead of the externally applied potential E, (ii) the redox reaction takes place at the distance of the closest approach of the reactant to the electrode, and (iii) the concentration of electroactive species at the reaction site is given by the Boltzmann distribution, which includes the φ2 potential. The relation of the observed apparent rate constants k′(E) to a true electron-transfer rate at a given applied potential k(E) was derived as
[
]
(Rn - z)F φ2 k′(E) ) k(E) exp RT
(5)
where R is the transfer coefficient, n is the number of electrons involved in the electrode reaction, and z is the charge of the electroactive species. The theory predicts that the experimentally observed shift of the polarographic half-wave potential ∆E1/2, or more generally the shift of the potentials at which the slow electron-transfer rate is constant, changes with the φ2 potentials according to the relationship
(
∆E1/2 ) 1 -
z ∆φ2 Rn
)
(6)
where ∆φ2 is the difference of φ2 in the corresponding solutions. Figure 9 shows plots of E1/2 against φ2 according to eq 6 for all five electrolytes and concentrations used here. Correlations are indeed observed; however, the resulting linear plots are different for each electrolyte. Also the extrapolation of E1/2 to φ2 ) 0 does not yield a single value of a corrected half-wave potential. This may suggest that the Frumkin correction is not fully obeyed and other factors should be considered.
Figure 7. Differential capacitance curves (A) and corresponding values of the φ2 potential at the outer Helmholtz plane (B) as a function of the applied potential for different concentrations of tetrabutylammonium hexafluorophosphate: 1, 3, 5, 10, 20, 30, 50, 100, 200, and 300 mM. Curves 1-10 correspond to increasing electrolyte concentrations.
We performed the analysis of the φ2 effects and estimation of the charge of species involved in the rate-determining step by evaluation of the dependence of the apparent electron-transfer rate k′(E) on the φ2 potential at a constant applied potential E, which should obey eq 5. For the present case of an irreversible reduction of the anion radical, the apparent rate constants are rather low and the dc polarography is an adequate measuring technique. The apparent rate constant of the electron transfer at a given potential is calculated from polarograms using the relation
i t 1/2 ) 0.886k′(E) id - i D
()
(7)
where i is the current at a given potential, id is the diffusionlimited current, t is the drop time, and D is the diffusion coefficient. The plot of log k′ vs φ2 at a constant potential E is indeed linear; however, it is different for each electrolyte (Figure 10). The apparent electron-transfer rate at a constant φ2 decreases in the order methyl > ethyl > propyl > butyl > hexyl. Hence, there are still other size-specific factors which influence the electrontransfer kinetics of the nitro anion radical. Lower values of the φ2-corrected rate in the presence of higher homologues of tetraalkylammonium cations could be attributed to enhanced adsorption and inhibition. This argument evidently is not applicable to the present case. The surface coverage and therefore also the inhibition effects should increase with the increasing bulk concentration of tetraalkylammonium cations. However, an opposite trend is observed for all salts we used. This excludes the interpretation in terms of the inhibition caused by the adsorption of the supporting electrolyte as was reported in the case of nitromesitylene reduction11 in propylene carbonate, a more polar solvent. Also the specific adsorption of TAA+ cations is known to be negligible in acetonitrile.32 The absence
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Figure 10. Dependence of the apparent heterogeneous rate of the bifenox anion radical reduction at an applied potential of -2.0 V on the φ2 potential of the outer Helmholtz plane in the presence of (b) tetramethylammonium, (O) tetraethylammonium, (2) tetrapropylammonium, (0) tetrabutylammonium, and (9) tetrahexylammonium hexafluorophosphate. The solid line has a slope of 39 mV/decade, which corresponds to z ) -1, n ) 1, and R ) 0.51.
Figure 8. Differential capacitance curves (A) and corresponding values of the φ2 potential at the outer Helmholtz plane (B) as a function of the applied potential for different concentrations of tetrahexylammonium hexafluorophosphate: 0.5, 1, 2, 3, 5, 10, 20, 50, 100, and 200 mM. Curves 1-10 correspond to increasing electrolyte concentrations.
Figure 11. Dependence of the apparent heterogeneous rate k′ of the nitro anion radical of bifenox reduction at E ) -2.0 V and φ2 ) -0.150 V on the number of carbon bonds N of the tetraalkylammonium cation. The data were taken from Figure 10.
Figure 9. Dependence of the half-wave potential of the bifenox anion radical reduction on the φ2 potential of the outer Helmholtz plane in the presence of (b) tetramethylammonium, (O) tetraethylammonium, (2) tetrapropylammonium, (0) tetrabutylammonium, and (9) tetrahexylammonium hexafluorophosphate.
of ion-pairing effects was already addressed in the discussion of the data in Figure 1. The theory of the electrode kinetics influenced by the φ2 potential of the electrode double layer was developed under the assumption that the reaction plane for an electron-transfer reaction coincides with the outer Helmholtz plane. Data presented here indicate that ionic size effects have to be taken into account. Larger bulky cations, such as tetrahexylammonium, cause the reaction plane to be located considerably further away from the electrode surface than in the case of cations with a smaller ionic radius. This hypothesis is in line with previous observations when the increased size of the nitro derivatives caused a decrease of the electron-transfer rate constant of the first reduction step (generating the radical anion) due to a change of the distance of
the reaction plane from the electrode surface.13 Figure 11 shows the dependence of k′(E) (at a constant E and a constant φ2 potential) on the number N of carbon bonds of the tetraalkylammonium cation. The estimated value of the exponent of an exponential decay is β ) 0.83. A similar linear dependence holds also for a plot against the ionic diameter of TAA+ cations. A good linear dependence is in agreement with recent observations of the dependence of the electron-transfer rate on the distance from the source of electrons. Our data show that similar principles derived from the behavior of redox-active monolayers of different thicknesses apply also to cases where the reaction plane distance is determined by an “ordinary” component of the double layer, the cations of an indifferent electrolyte at the negatively charged surface. The kinetics of a redox reaction indeed probes the doublelayer structure in a sensitive manner. The unresolved problem of the present study remains the mechanistic origin of the minima on i-E curves at the lowest concentration of THA+ cations, which under certain conditions yield even oscillatory behavior. We plan to address this issue in the future.
Conclusions Anion radicals of bifenox, methyl 5-(2,4-dichlorophenoxy)2-nitrobenzoate, and also of nitrobenzene are irreversibly reduced in acetonitrile in a three-electron process which is strongly dependent on the concentration and cation size of the indifferent
1902 Langmuir, Vol. 22, No. 4, 2006
electrolyte. The lower the concentration of the indifferent electrolyte, the slower the electron-transfer rate. The correction of the apparent rate constants for the influence of the φ2 potential at the outer Helmholtz plane (the Frumkin correction) holds for the case of tetramethylammonium hexafluorophosphate and is consistent with reaction 2 being the rate-determining step. The electron-transfer rates corrected for the value of the φ2 potential are still different for larger cations and decrease in the series tetramethyl- > tetraethyl- > tetrapropyl- > tetrabutyl- > tetrahexylammonuim. Corrected rate constants decay exponentially with the number of carbon bonds of the indifferent cation with an exponent β ) 0.83. The results show that a seemingly
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established reaction kinetics of aromatic nitro compounds is quite complex in many aspects. Acknowledgment. This work was supported by the Grant Agency of the Czech Republic (Grant 203/03/0821), Grant Agency of the Academy of Sciences of the Czech Republic (Grant A400400505), and Ministry of Education of the Czech Republic (Grant LC510). Joint cooperation was made possible on the basis of the cooperation agreement between CNR Rome and the Academy of Sciences of the Czech Republic. LA0521894
