Reorientation of the Cation Radical of Heptyl Viologen on Mercury in

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Langmuir 1999, 15, 618-623

Reorientation of the Cation Radical of Heptyl Viologen on Mercury in Water/DMSO Mixed Media Juan Ignacio Milla´n, Rafael Rodrı´guez-Amaro, Juan Jose´ Ruiz, and Luis Camacho* Departamento de Quı´mica Fı´sica y Termodina´ mica Aplicada, Facultad de Ciencias, Universidad de Co´ rdoba, Avda San Alberto Magno s/n, E-14004 Co´ rdoba, Spain Received July 14, 1998. In Final Form: October 26, 1998 In aqueous mixed media containing DMSO in fractions by volume above 0.50 the cation radical of heptyl viologen (HV•+) can be absorbed in two different orientations on a mercury electrode. Thus, the cation radical can take a planar conformation relative to the surface; however, depending on the applied potential, it can reorient itself to adopt a different conformation where the bipyridine group lies normal to or at an angle with the electrode. In this work, the reorientation was found to take place via a two-dimensional phase transition mechanism. The experimental conditions under which the phenomenon occurs are analyzed.

Introduction Viologen compounds, which are 1,1-disubstituted 4,4bipyridinium ions (V2+), have aroused much interest as mediators on account of their negative redox potentials.1,2 Recently, the cation radicals of these compounds were shown to form two-dimensional (2D) phases on mercury in aqueous media.3-8 The 2D phase of heptyl viologen (HV2+) has been detected by using various electrochemical approaches including I-t curves,3 cyclic voltammetry,3,7 and capacitance-potential curves.7 Because the amount of charge exchanged during the formation of this 2D phase in aqueous solutions of bromide ion is 23 µC/cm2, the area occupied by a molecule of HV•+ must be approximately 0.7 nm;2,7 consequently, its bipyridine group must be at an angle with or normal to the electrode. The surface behavior of the cation radicals for various viologens adsorbed on a polished Ag electrode from aqueous solutions was analyzed elsewhere by using different vibrational surface spectroscopies.9-13 However, the orientation of the cation radical in the 2D phase was not studied, since no such phase was detected on Ags this, however, is seemingly dependent on the particular treatment to which the electrode surface is subjected.3 * To whom correspondence should be addressed. E-mail: [email protected]. (1) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (2) Andrieux, C. P.; Gallardo, I.; Save´ant, J. M.; Su, K. B. J. Am. Chem. Soc. 1986, 108, 638. (3) Scharifker, B.; Wehrmann, C. J. Electroanal. Chem. 1985, 185, 93. (4) Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1993, 359, 325. (5) Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. Langmuir 1994, 10, 723. (6) Salas, R.; Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. Langmuir 1995, 11, 1791. (7) Milla´n, J. I.; Sa´nchez-Maestre, M.; Camacho, L.; Ruiz, J. J.; Rodrı´guez-Amaro, R. Langmuir 1997, 13, 3860. (8) Kelaidopoulou, A.; Kokkinidis, G.; Coutouli-Argyropoulou, E. Electrochim. Acta 1998, 43, 987. (9) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. P. J. Phys. Chem. 1988, 92, 6978. (10) Lu, T.; Cotton, T. M.; Birke, R. L.; Lombardi, J. R. Langmuir 1989, 5, 406. (11) Cotton, T. M.; Lu, T.; Uphaus, R. A. Microchem. J. 1990, 42, 44 and references therein. (12) Misono, Y.; Shibasaki, K.; Yamasawa, N.; Mineo, Y.; Itoh, K. J. Phys. Chem. 1993, 97, 6054. (13) Osawa, M.; Yoshii, K.; Ataka, K.; Yotsuyanagi, T. Langmuir 1994, 10, 640.

By using STM and AFM, Tao et al.14 found the unreduced form of 4,4′-bipyridine to form a 2D condensed phase on Au(111). In this phase, the molecule lies normal to the substrate, with one of its nitrogen atoms facing the surface. A similar phase was also detected to occur on Hg.15 In this work, we studied the formation of the 2D phase of the heptyl viologen cation radical (HV•+) on mercury in water/DMSO mixtures of variable composition containing Br- ion with a view to determining whether the process is governed by the presence of water in the medium. In addition to a 2D phase, HV•+ forms insoluble 3D salts on an electrode immersed in an aqueous medium.16-19 In this work, we found the salts to be dissolved in media containing high proportions of DMSO. On the other hand, the 2D phase of HV•+ continues to be formed even in pure DMSO, yet its interfacial behavior in this medium is more complex than that in pure water and involves a reorientation in the condensed phase at intermediate DMSO proportions. In this work, the surface behavior of heptyl viologen cation radical in water/DMSO mixtures was analyzed and so were the structures of the phases involved. Experimental Section Practical-grade 1,1′-diheptyl-4,4′-bipyridinium dibromide (purum grade, 97%) was purchased from Aldrich, and dimethyl sulfoxide (puriss p.a.) was obtained from Fluka. Both were used as received. All other chemicals were Merck analytical reagentgrade and also used without further purification. Mercury was purified in dilute nitric acid and triply distilled in vacuo. Solutions were all made in double-distilled water supplied by a Milli-Q System from Millipore and deaerated by bubbling gaseous nitrogen through them. The measuring cell was thermostated to within (0.1 °C. Unless otherwise noted, the working temperature was 25 °C. Voltammetric measurements were made on a Quiceltro´n electronic system. A static mercury drop electrode (SMDE) with an area of 1.86 ( 0.05 mm2 was used as working electrode. A (14) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; D’Agnese, J. Langmuir 1996, 12, 6418. (15) Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1993, 359, 325. (16) Jasinski, R. J. Electrochem. Soc. 1977, 124, 637; 1979, 126, 167. (17) Bruinik, J.; Kregting, C. G. A. J. Electrochem. Soc. 1978, 125, 1397. (18) Barna, G. G. J. Electrochem. Soc. 1980, 127, 1317. (19) Bewick, A.; Lowe, A. C.; Wederell, C. W. Electrochim. Acta 1983, 28, 1899.

10.1021/la980884o CCC: $18.00 © 1999 American Chemical Society Published on Web 12/19/1998

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Figure 2. Plots of Ep versus XDMSO for peaks A1 (O), C1 (4), D1 (]), B (0), and F (b). All other conditions as in Figure 1.

Figure 1. Cyclic voltammograms for 1 mM HV2+ in 0.2 M KBr at T ) 16 °C and v ) 100 mV/s in (a) XDMSO ) 0.40, (b) XDMSO ) 0.55, and (c) XDMSO ) 0.60. potential Ee was applied over an interval te = 2 s (equilibration time), and a cyclic voltammogram was then recorded between Ee and the final potential (Ef) in each run. Ag/AgCl and platinum wire were used as reference and auxiliary electrode, respectively. All measurements were made in a nitrogen atmosphere. Capacitance-potential curves were recorded by using a PAR M273 potentiostat with automatic correction for the iR drop, equipped with a PAR 5210 lock-in amplifier. The frequency and amplitude of ac modulation were fixed at 277 Hz and 10 mV, respectively. This instrumental setup was governed by M270 Research Electrochemistry Software.

Results and Discussion Figure 1 shows selected cyclic voltammograms obtained at T ) 16 °C and a scan rate v ) 100 mV/s for 10-3 M HV in water/DMSO mixtures of variable composition containing 0.2 M Br-. In a medium containing a DMSO fraction by volume XDMSO ) 0.4 (curve a), HV gave two reduction peaks (A1 and B) and two oxidation peaks (A2 and K). The behavior of HV in this medium was identical with that in pure water described elsewhere.7 Peaks A1 and A2 are sharp and narrow and have been shown to arise from the reduction of heptyl viologen and the formation of a 2D condensed phase of its cation radical (peak A1) and from the oxidation and subsequent destruction of the condensed phase (peak A2).7 Integration of peak A1 after eliminating the background current provides a charge of 20 µC/cm2, which is very close to the 23 µC/cm2 found in pure water.7 Peak B was previously assigned to the one-electron reduction of molecules reaching the electrode by diffusion

and to the precipitation of the cation radical formed, and peak K was attributed to the oxidation of the radical and the subsequent stripping of the precipitate formed.7,16-19 At XDMSO ) 0.55 (Figure 1b), peak A1 in the reduction scan is small relative to the size in the previous medium; also, two new peaks appear at a more positive (peak C1) and a more negative potential (peak D1) than that of A1. On the other hand, peak A2 disappears from the oxidation scan, seemingly by splitting into two at a more positive (peak C2) and a more negative potential (peak F) than the parent one (see Figure 1b). The areas of peaks C1 and C2 are similar under all experimental conditions, whereas that of peak F is roughly equal to the combined areas of peaks A1 and D1. In media of XDMSO above 0.60, peak A1 disappears and the reduction scan exhibits peaks C1 and D1 (Figure 1c). The charges obtained by integrating C1 and D1 are roughly the same and close to 10 µC/cm2. The oxidation scan continues to exhibit peaks C2 and F, the areas of which are now roughly identical with those of the previous peaks. Also, it should be noted that a new, small, inverted peak is observed at potentials between those for peaks C2 and F. In addition, the shapes of peaks B and K differ from those in the previous media (compare curves a and c in Figure 1). In fact, at XDMSO g 0.60, these peaks exhibit the typical shapes for a diffusion-controlled redox process. This shape change must be related to the differential solubility of the cation radical in these media. Figure 2 shows the variation of the peak potential Ep for peaks A1, C1, D1, B, and F as a function of XDMSO. As can be seen, Ep for peak C1 shifts to more positive potentials with an increase in XDMSO. Under the working conditions used, this peak is not observed at XDMSO > 80 because it is masked by the discharge of Br- ion. At XDMSO values below 0.70 and above 0.85, Ep(D1) shifts to more negative potentials as the DMSO content is raised. Over the XDMSO range from 0.70 to 0.85, however, Ep(D1) is roughly constant. At DMSO fractions close to 0.50, the peak potential for F, Ep(F), is about 20 mV more positive than that for A1, Ep(A1). At XDMSO > 0.65, Ep(F) shifts abruptly to more negative potentials, so much so that, at XDMSO > 0.90, this peak potential is about 30 mV more positive than Ep(D1). Ep(A1) shifts to slightly more negative potentials with an increase in XDMSO and eventually disappears at XDMSO > 0.60. Also, the inverted peak is not observed at XDMSO > 0.70. Finally, Ep(B) shifts to negative potentials at XDMSO < 0.50 and to positive potentials at XDMSO > 0.50. It is also worth noting that Ep(B) is more positive that Ep(D1) at XDMSO > 0.75.

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Figure 3. Capacitance (C) versus potential (E) plot obtained in media of XDMSO ) 0.40 (solid line), XDMSO ) 0.55 (dashed line), and XDMSO ) 0.60 (dotted line). C was determined at a frequency of 277 Hz and a scan rate of 2 mV/s, using a 10 mV pulse. All other conditions as in Figure 1.

Figure 3 shows selected capacitance-potential (C-E) curves obtained under the same conditions as those for the previous figures, using different water/DMSO mixtures. As can be seen, the capacitance is roughly constant and dependent on the DMSO content in the medium at positive potentials. At potentials close to -365 mV, the capacitance drops abruptly. This drop is concomitant with the appearance of peak A1 at XDMSO ) 0.40 and that of C1 at XDMSO g 0.55. At more negative potentials, the scan includes a region where the capacitance again remains roughly constant; however, C continues to decrease at potentials above -400 mV in media with XDMSO g 0.55. The potential at which the new capacitance drop occurs coincides with that at which peak D1 appears if the voltammogram is obtained at the same scan rate (v ) 2 mV/s). Figure 4 shows three voltammetric recordings obtained at a variable scan rate and a fixed DMSO proportion (XDMSO ) 0.55) on constancy of all other conditions. As can be seen, only peaks C1 and D1 are observed in the reduction scan performed at v ) 25 mV/s (curve a). When v is raised, peak A1 appears at potentials between the previous ones (v ) 75 mV/s, Figure 4b), in such a way that, at v ) 175 mV/s (Figure 4c), this peak is the dominant in the scan. The oxidation scan exhibits peaks C2 and F (as well as the inverted peak) throughout. Raising the temperature or decreasing the HV or Brconcentration has the same effect as using a higher scan rate. Thus, peak A1 is not observed at XDMSO ) 0.55, v ) 100 mV/s, and T ) 9 °C (all other conditions being equal to those of Figure 1); at T ) 25 °C, however, only peak A1 is observed in the reduction scansboth C1 and D1 disappeared on increasing the temperature. In previous work,20 different criteria for characterizing 2D phase transitions on electrodes by cyclic voltammetry were derived. Thus, if Ip is used to denote the peak current, W is used to denote the width at half-height of the peak, and the hysteresis of the voltammogram is defined as ∆Ep ) Epa - Epc (Epa and Epc being the oxidation and reduction peak potential, respectively), then the log-log plots of Ip, W, and ∆Ep versus v must all be linear and have slopes of x, 1 - x, and 1 - x, respectively, where x g 0.6. In the ideal situation (a high nucleation rate and a low v), x ) (20) Sa´nchez-Maestre, M.; Rodrı´guez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1994, 373, 31.

Figure 4. Cyclic voltammograms for 1 mM HV2+ in a medium of XDMSO ) 0.55, obtained at a scan rate of (a) 25, (b) 75, and (c) 175 mV/s. All other conditions as in Figure 1.

0.6.20 In any case, x must fit experimental parameters equal to or slightly greater than 0.6.20-22 The criteria based on Ip and W cannot be applied to the data of Figure 4, since these parameter values depend on the total amount of charge exchanged20 and, in our case, such charge values varied with v at XDMSO ) 0.55. On the other hand, Ep is independent of the amount of charge exchanged, so the criteria based on it can indeed be used. Thus, for a 2D phase transition,

Epc ) E° - pvr

Epa ) E° + pvr

∆Ep ) Epa - Epc ) 2pvr

(1a) (1b)

where E° is the standard phase transition potential, r ) 1 - x e 0.4 (a parameter to be determined by numerical fitting) and p is a previously defined constant,20 which, for the ideal case (r ) 0.4), is given by

p)

( )( RT F

3/5

6 πk0 kn(nc + β) 2

)

1/5

(2)

where β is a transfer coefficient, nc is the number of molecules that form a critically sized nucleus, kn is the number of nuclei formed at very low overpotentials, and (21) Demir, U.; Shannon, C. Langmuir 1996, 12, 6091. (22) Hatchett, D. W.; Uibel, R. H.; Stevenson, K. J.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062.

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Figure 5. Plots of Ep versus vr for 1 mM HV2+ in a medium of XDMSO ) 0.55 corresponding to peaks C1 (r ) 0.31), C2 (r ) 0.31), A1 (r ) 0.32), F (r ) 0.32), and D1 (r ) 0.36). All other conditions as in Figure 4.

k0 is the heterogeneous rate constant for the charge transfer. Therefore, p is a constant related to the nucleation kinetics. As a rule, the greater is p, the slower will be the kinetics of the 2D nucleation process. Equation 1b (∆Ep ) 2pvr) is correct provided the reduction and oxidation peaks arise from the same electrode process and E° vanishes in subtracting the peak potentials. This relation is obvious for a simple 2D phase transition and can be of assistance in assigning peaks obtained under more complex conditions involving phase transitions (e.g. ours). A plot of log[Ep(C2) - Ep(C1)] versus log v at XDMSO ) 0.55 was found to be linear with the slope 0.31. A plot of log[Ep(F) - Ep(A1)] versus log v was also linear, with the slope 0.32. These relations suggest that peaks C1 and C2 are mutually related and that both have the same E°(C) value. This is also the case with peaks A1 and F, which possess the same E°(A) value. On the other hand, peak D1 cannot be directly related to any of the oxidation peaks obtained in this medium; however, as noted earlier, the area under peak F is the sum of those under peaks A1 and D 1. To ascertain that peak D1 also corresponded to a 2D phase transition, Ep(D1) values were fitted to vr using eq 1a and r turned out to be 0.36. Figure 5 shows the plots of Ep versus vr for the five peaks; r was 0.31 for C1 and C2, 0.32 for A1 and F, and 0.36 for D1. As can be seen, the linearity was excellent in all cases, which suggests that all the peaks considered are due to 2D phase transitions. The plots of Figure 5 can be used as an analytical criterion for 2D phase transitions. In addition, it allows one to assign reduction and oxidation peaks and to determine E° from the intercept. Figure 6 shows three voltammetric recordings obtained at variable scan rates in a medium of XDMSO ) 0.70, all other conditions being identical with those of the previous figures. As can be seen, peak A1 is not observed in this medium. Also, the width of peaks C1 and D1 in the reduction scan increases with increasing scan rate. This is also the case with peak C2 in the oxidation scan. This phenomenon is typical of 2D phase transitions.20 However, peak F exhibits the opposite trend; that is, its width decreases with an increase in v. It is also worth noting that the inverted peak is not observed at low scan rates but only at high ones. The log-log plots of Ip and W against v for peak C1 in this medium were linear and of slope 0.63 and 0.39,

Figure 6. Cyclic voltammograms for 1 mM HV2+ in a medium of XDMSO ) 0.70, obtained at a scan rate of (a) 25, (b) 75, and (c) 175 mV/s. All other conditions as in Figure 1.

respectively. So were the plot of log[Ep(C2) - Ep(C1)] versus log v, with a slope of 0.32, and the log-log plots of Ip and W against v for peak D1, with slopes of 0.7 and 0.3, respectively. On the other hand, the plot of log[Ep(F) Ep(D1)] versus log v was not linear. For peak D1 Ep(D1) values were numerically fitted to vr using eq 1a, and an r value of 0.34 was obtained. However, this type of fitting gave unacceptable results for peak F. Figure 7 shows the plot of Ep versus vr for peaks C1, C2, D1 and F in a medium of XDMSO ) 0.70, r being 0.32 for peaks C1 and C2 and 0.34 for D1 and F. As can be seen, the linearity of the plot is excellent for peaks C1, C2, and D1 but not for peak F. The plot for this last, however, is indeed linear at high scan rates. In this region of v values, peak F is narrow and the voltammograms exhibit the inverted peak (see Figure 6c). At low scan rates, Ep(F) shifts abruptly to negative values and the extrapolation of Ep(F) at v ) 0 appears to converge with that of Ep(D1) at a zero scan rate. In pure DMSO, the log-log plot of Ep(F) - Ep(D1) versus v has been experimentally found to be linear and of slope 0.4, which confirms that peaks F and D1 are mutually related in this medium and that both have the same E°(D) value. Assignment of Peaks And Mechanisms. Peaks A1 and A2. As noted earlier, peak A1 is due to the singleelectron reduction of HV2+ and the simultaneous formation of a 2D condensed phase (HV•+-Br-) on the electrode.7 The amount of charge exchanged in this process, close to 20 µC/cm2, results in a surface concentration Γ ) 2.07 ×

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Scheme 1. Proposed Structures for Phases r and β, and Assignation of the Different Peaks Observed

Figure 7. Plots of Ep versus vr for 1 mM HV2+ in a medium of XDMSO ) 0.70 corresponding to peaks C1 (4, r ) 0.32), C2 (2, r ) 0.32), D1 (], r ) 0.34), and F (b, r ) 0.34). All other conditions as in Figure 4. The dashed line is the linear extrapolation of Ep(F) at high scan rates.

10-10 mol/cm2 and a surface area of approximately 0.8 nm2 per molecule. Such an area is consistent with a configuration where the plane containing the bipyridine group is edge-on or tilted relative to the electrode surface. Unfortunately, no specific model for the orientation of HV•+ could be established, owing to the variety of conformations the alkyl chains in the molecule can take.7,11 We shall designate the condensed phase formed at potentials more negative than those of the appearance of peak A1 as “phase R”. Scheme 1 depicts such a phase in an edge-on configuration, which is consistent with a surface area per molecule of 0.8 nm2. An edge-on orientation was previously assumed for the adsorption of 4,4′bipyridine cation radical on Ag in the presence of I- ions.10 In any case, both the edge-on and the tilted configuration enable stacking interactions between the planar structures of bipyridine groups in neighboring radicals. Peak A2 must be related to the oxidation of HV•+ and the resulting destruction of phase R, which involves the desorption of the HV2+ formed. Peaks C1 and C2. The above-described properties of peak C1 allow us to assign it to the single-electron reduction of

HV2+ and the subsequent formation of a new condensed phase on the electrode. At potentials more positive than those of the appearance of peak C1, the capacitance is similar to that of the supporting electrolyte in the absence of HV (data not shown), which suggests weak or no adsorption of HV2+ on the electrode. At potentials more negative than those of the appearance of peak C1, the capacitance drops abruptly (see Figure 3), which can be ascribed to the formation of the condensed phase. The amount of charge exchanged in this process at XDMSO > 0.70 is approximately 10 µC/cm2, so the surface concentration will be Γ ) 1.04 × 10-10 mol/cm2 and the surface area 1.6 nm2 per molecule. This area is consistent with an entirely planar configuration of the molecule on the electrode. The theoretical area for such a configuration is about 1.7 nm2. The configuration is depicted in Scheme 1, where the phase formed between the neutral salt HV•+Br- in this configuration is named “phase β”. The fact that Ep(C1) shifts to more positive values with an increase in XDMSO (see Figure 2) suggests that phase β is especially stable in this medium. Whether phase R or phase β is initially formed depends on the DMSO content in the medium. Peak C2 must be assigned to the reverse process, that is the oxidation of HV•+ and the resulting destruction of phase β. Peak D1. Like the previous ones, this peak meets the criteria for 2D phase transitions. In fact, the amount of charge obtained by integrating it at XDMSO > 0.70 is about 10 µC/cm2, so the amount of HV•+ that is incorporated onto the electrode is equivalent to Γ ) 1.04 × 10-10 mol/ cm2. The overall concentration on the electrode after this peak will be the sum of this value and the mass of the molecules incorporated along peak C1, that is Γtotal ) 2.08 × 10-10 mol/cm2, which is the concentration exactly needed for phase R to be formed. We believe peak D1 should be assigned to the transition from phase β to phase R of HV•+ (see Scheme 1). This transition involves the reorientation of the cation radical to a more compact structure, with the consequent incorporation of new HV2+ molecules, which must be reduced. This hypothesis is supported by the fact that the capacitances obtained at potentials more negative than those of the appearance of peak D1 are roughly coincident with those obtained after peak A1 (see Figure 3).

Cation Radical of Heptyl Viologen on Mercury

The properties of peak D1 are altered above an XDMSO value of 0.70. Thus, Ep(D1) is roughly constant over the range 0.70 < XDMSO < 0.85 (see Figure 2). In addition, the charge obtained upon integration changes with respect to the above-mentioned value (10 µC/cm2).23 These changes are a result of this peak and diffusion peak B intersecting at XDMSO ) 0.75 (See Figure 2). Thus, when peak B appears at potentials more positive than does peak D1, the HV•+ concentration around the electrode rises abruptly and formation of the more compact R phase is favored. The properties and structures of phases R and β in pure DMSO will be studied in future work.23 Peak F. As before, this peak exhibits characteristics typical of 2D phase transitions. Thus, at XDMSO ) 0.55, where phases R and β can coexist, the area under peak F is approximately the sum of those under peaks A1 and D1, which suggests a complex underlying mechanism. At XDMSO < 0.70, the normal potential for peak F coincides with those for peaks A1 and A2 (see Figure 5); consequently, the overall process associated with peak F under these conditions must be controlled by a mechanism similar to that for peak A2, viz. the oxidation of (HV•+)R to nonadsorbed HV2+sthe overall process is more complex than this, however. In fact, as the oxidation develops, the surface concentration of (HV•+)R on the electrode decreases, which must allow the radical to reorient itself to (HV•+)β; under these conditions, the reorientation of the cation radical will therefore be kinetically controlled by the oxidation of phase R. This allows the oxidation of (HV•+)R to develop to a greater extent than strictly needed for phase β to be formed. Thus, if the surface concentration of phase R is ΓR ) 2.07 × 10-10 mol/cm2 and that of phase β is Γβ ) 1.04 × 10-10 mol/cm2, then an amount of molecules slightly greater than the difference between these two concentrations will be oxidized. When this happens, an additional amount of HV2+ must be subsequently reduced for the formation of phase β to complete. This phenomenon produces an inverted peak that is observed provided peak F is controlled by the oxidation of phase R. At XDMSO > 0.70, the normal potential of peak F is the same as that for peak D1 (see Scheme 1), which suggests a change in the mechanism associated with this peak in (23) Millan, J. I.; Rodrı´guez-Amaro, R.; Ruiz, J. J.; Camacho, L. Manuscript in preparation.

Langmuir, Vol. 15, No. 2, 1999 623

relation to that described above. Under these conditions, it is the reorientation of HV•+ molecules from phase R to phase β that controls the processsthe reverse of the one that takes place along peak D1. This involves the release of some molecules from the monolayer owing to the lower surface density of phase β. Such molecules are immediately oxidized and give peak F. In this case, the amount of molecules that are oxidized coincides exactly with the difference between the surface concentrations of both phases, so the inverted peak does not appear. In fact, the inverted peak was experimentally found to disappear when the normal potential of peak F coincided with that of D1 (see Figures 6 and 7). The broadening of peak F at low scan rates in XDMSO ) 0.70 (Figure 6) is also a consequence of the mechanistic change in this peak, so it must be related to the differential nucleation kinetics20 of the mechanisms controlling the reorientation of the cation radical. Conclusions HV cation radical exhibits two different types of 2D condensed phases on Hg in the presence of water/DMSO mixtures and Br-. The surface area of the molecule in phase R is approximately 0.8 nm2 (see Scheme 1). Such an area is consistent with a configuration where the plane containing the bipyridine group is edge-on or tilted relative to the electrode surface. Unfortunately, no specific model for the orientation of HV•+ in phase R could be established, owing to the variety of conformations the alkyl chains in the molecule can adopt. In any case, the edge-on or tilted configuration allows for stacking interactions between the planar structures of bipyridine groups in neighboring radicals. Only phase R is observed in an aqueous medium. Phase β occurs at XDMSO > 0.50. The surface area of the molecule in this phase is approximately 1.6 nm2, consistent with a planar configuration on the electrode that hinders stacking interactions between bipyridine groups in the cation radical. Accordingly, an increased content in DMSO must somehow favor the occurrence of this condensed phase structure. Acknowledgment. The authors wish to express their gratitude to Spain’s DGICyT for funding this research in the framework of Projects PB94-0448 and PB97-0453. LA980884O