Langmuir 1995,11, 1791-1796
1791
Voltammetric Study of the Two-Dimensional Phase Formed by the Cation Radical of Methyl Viologen on Mercury in the Presence of Iodide Ions R. Salas, M. Sanchez-Maestre, R. Rodriguez-Amaro, E. Mufioz, J. J. Ruiz, and L. Camacho* Departamento de Quimica Fisica y Termodinamica Aplicada, Facultad de Ciencias, Universidad de Cbrdoba, Avda S u n Alberto Magno s In, E-14004 Cbrdoba, Spain Received October 13, 1994. I n Final Form: February 8, 1 9 9 P Methyl viologen (W+) gives a very narrow, sharp, tailless reversible peak by cyclic voltammetry at a mercury electrode. The peak arises from the one-electron reduction of the molecule to its cation radical (Mv"),which forms a two-dimensional (2D)phase over the electrode. In this work, we tested various analytical criteria in order to validate this assignation and fit the voltammetric peak numerically to the theoretical model developed for this purpose. The formation of the two-dimensional condensed phase of MV'+ over mercury was found to depend on temperature. Like three-dimensional phases, 2D phases are not formed above a given, critical temperature. Thus, the voltammetric peak disappeared abruptly above 67 "C, which can therefore be adopted as the critical temperature for the 2D phase of methyl viologen.
Introduction In a n aqueous medium, l,l-dimethyl-4,4'-bipyridine, also called methyl viologen (hIv2+),can be electrochemically reduced via two clearly resolved one-electron processes, in such a way that the species formed at the electrode over a wide potential range is the cation radical of the molecule (MV'+):1-5
MY+
+ e-===MV
(11)
In dc polarography4 and normal pulse polarography,6 the first electrode process for M V + (step I) is associated with a n adsorption prewave, which was ascribed by Heyrovsky et ale4to the formation of a two-phase (2D) crystalline layer of the MV'+-anion salt on account of its anomalous features; the assignment, however, was not experimentally supported. In cyclic voltammetry over a mercury electrode, methyl viologen gives a narrow, sharp peak a t a similar potential to that of the adsorption p r e w a ~ e . Such ~ . ~ a peak, however, has been assigned to the preferential adsorption of the cation This peak will henceforward be referred to as peak A (e.g. see Figure 1). No such peak, however, has been observed for methyl viologen over a carbon e l e ~ t r o d e . ~ All alkyl viologens,8as well a s 4,4'-bipyridine in an acid medium ( B P H Z ~ +give ) , ~ a peak similar to peak A for MV2+ by cyclic voltammetry over a mercury electrode. For heptyl and benzyl viologenlO and BPH22+,9such a peak has been found to arise from the formation of a 2D phase of the
* Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, April 15,1995. (1)Elofson, R. M.; Edsberg, R. L. Can. J. Chem. 1967,35,646. (2)Bird, C. L.; Kuhn, A. T. Chem. SOC.Rev. 1981,101,49. (3) Kaifer, A. E.; Bard, A. J.J. Phys. Chem. 1986,89,4876. ( 4 ) . Heyrovsky, M.; Novotny, L. Collect. Czech. Chem. Commun. 1987,52,1097. (5) Cotton, T. M.; Kim, J. H; Uphaus, R. A.Microchem. J . 1990,42, @
-
Ad-.
(6)Kobayashi, K. Chem. Lett. 1988,1243. (7)Kobayashi, K.; Fujisaki, F.; Yoshimine T.; Niki, K. Bull. Chem. SOC.Jpn. 1986,59,3715. (8) Kitamura, F.: Ohsaka, T.: Tokuda. K. J . Electroanal. Chem. 1993, 347,371. (9) Sbnchez-Maestre, M.; Rodriguez-Amaro, R.; Mufioz, E.; Ruiz J. J.; Camacho, L. (a) J. ElectroanaL Chem. 1993,359, 325 and (b) Langmuir 1994,10,723.
cation radical over the electrode. The primary aim of this work was to characterize peak A for Mv2+ and check whether it arises from the transition to a 2D phase as in the previous compounds. For this purpose, we used chronoamperometry and cyclic voltammetry, the latter of which was applied in conjunction with a theoretical 2D phase transition model previously developed by our group.11 Integration of peak A obtained in 0.1 M S042-allows the surface excess of cation radical molecules to be calculated (ea. 1.4x mo[cm2).8 Hence each molecule occupies an area of ca. 118 A. In theory, one molecule arranged with its two aromatic rings parallel or normal to the electrode must take ca. 84-88 or 34-38 A, re~pectively.~Therefore, based on literature values obtained by integration of the voltammetric peak, viz. 118 A, the area occupied by a single molecule is somewhat larger than that for a molecular monolayer lying parallel to the electrode surface.8 Kobayashi et al. used chronocoulometricmeasurements to estimate the surface excess o f W +and its cation radical Mv'+over Hg in a 0.1M Po43-medium. They obtained a surface excess of 4 x mol/cm2for the cation radical, which is equivalent to ca. 42 Nmolecule. For this purpose, they used a n applied potential of -800 mV, where MV'+ species accumulate a t the electrode, and then a potential jump to -300 mV, where the cation radical is oxidized to W + .The above molecular area is consistent with a roughly normal orientation of M V + species relative to the electrode, which contradicts the results obtained by integrating the voltammetric peak;8 however, the two experimental determinations involved different media. By reproducing some of the experiments used for this purpose, we determined the actual orientation ofthe cation radical relative to the electrode. The more or less sharp appearance of peak A and its distance to the diffusion peak for the first one-electrode process undergone by Mv2+ (peak B in Figure 1) are markedly dependent on the anion present in the medium as counterion. Thus, in a phosphate medium a t room temperature, peak A is merely 50 mV away from the (10) Scharifier, B.; Wehrmann, C. J. Electroanal. Chem. 1986,185, 93. (11)Sbnchez-Maestre, M.; Rodriguez-Amaro, R.; Mufioz, E.; Ruiz J. J.; Camacho, L. J. Electroanal. Chem. 1994,373,31.
0743-746319512411-1791$09.00/0 0 1995 American Chemical Society
Salas et al.
1792 Langmuir, Vol. 11, No. 5, 1995 4
IA
-675 mV
-2 -4
-6
n -600
-800
- 1000
-1200
E (mV v s SCE) Figure 1. Cyclic voltammogram obtained for 0.5 mM M V + in 0.1 M KI at T = 25 "C and u = 50 mV/s.
diffusion peak,'whereas in a sulfate medium, the distance is ca. 100 mV;8 in addition, the peak is not as narrow and sharp as those observed for other alkyl viologens (see Figure 2 in ref 8). In various experiments we found low temperatures and/or highly adsorptive anions such as iodide, bromide, or even chloride to result in increased resolution of peak A from the diffusion peak, a s well a s in a taller, sharper peak than in the presence of less adsorptive anions such a s sulfate or fluoride. Finally, in this work we studied the influence of temperature on the appearance of peak A in a n iodide medium and found it to disappear above 67 "C. Such a temperature must thus be close to the critical temperature ( T J where the 2D phase of MY'+ is formed. The peak corresponding to the 2D phase transition for B P H P in 0.1 M iodide splits into two above 15 oC.gb This phenomenon was not observed for W +a,t least not above 2 "C.
Experimental Section Practical-grade methyl viologen (purum grade '97%) was purchased from Fluka and used without further purification. Potassium iodide was Merck analytical reagent-gradeand also employed as supplied. All solutions were made in 0.1 M KI. Mercury was purified in dilute nitric acid and triply distilled in vacuo. Solutionswere all made in bidistilled water supplied by a Milli-Q system from Millipore and deaerated by bubbling gaseous nitrogen through them. The measuring cell (Amel 494 model) was thermostated to within *O.l "C. Voltammetric measurements were carried out by using an electronic system consisting of an HQ Instruments 305 potentiostat, an HQ Instruments 105 wave generator, a Norland Prowler digital oscilloscope, and a Houston Instruments 2000 recorder. A static mercury drop electrode (SMDE)with an area of 2.2 f 0.05 mm2 was used as the working electrode, and saturated calomel and platinum wire were employed as the reference and auxiliaryelectrode,respectively. A potential where no Faradaic reaction took place was applied for 2 s in order to allow the mercury drop to grow and the SMDE to stabilize (delay time). A potential E , was then applied over an interval t , = 2 s (equilibration time) and the cyclic voltammogram was recorded between E , and a potential Ef. I-t curves were recorded with the aid of a PAR M270 potentiostat, with automatic correction for the ZR drop. The curves were acquired by means of the above-described digital
0.00
0.01
0.02
t
0.03
0.04
(s)
Figure 2. I-t curves obtained at 25 "C. The potential applied before the potential jump was -600 mV and was held for 5 s. The final potential for each curve is shown in the figure. All other conditions are as in Figure 1.
B is typical of a diffusion-controlled one-electron process, whereas peak C is consistent with stripping of the insoluble form of M V . The voltammogram includes three further, smaller peaks. The presence of such peaks is related to the particular anion present in the medium; thus, none is observed in a chloride solution. None of these peaks is dealt with in this work. The narrow, sharp shape of peak A is typical of an electrode process involving immobilized molecules a t the electrode. Thus, the peak lacks a diffusion tail and the current drops to zero after it. This is the only peak analyzed in this work. Peak A is observed above a n MV2+ concentration of 3 x M. Also, under the experimental conditions of Figure 1, the peak potential for process A, E,, is independent of the iodide concentration over the range from 0.05 to 0.5 M. Figure 2 shows I-t potentiostatic curves recorded at T = 25 "C. The curves were experimentally obtained by applying a potential of -600 mV for 2 s, followed by a potential pulse up to a potential immediately following that of appearance of peak A. The final potential of the potentiostatic jump is shown in the figure. As can be seen, the current exhibited a charging component that dropped to zero within the first few milliseconds and was independent of the applied potential, as well as a Faradic component that was potentialdependent and exhibited a maximum typical of 2D phase transitions.12 Under constant-potential conditions, if the nucleation rate is very high (i.e. nucleation is instantaneous), the current is given by12
where I, and t, are the current and time, respectively, a t the maximum of the I-t curves of Figure 2. On the other hand, if the nucleation rate tends to zero (progressive nucleation), the current is given by12
oscilloscope.
Results and Discussion Figure 1shows a voltammogram obtained for 5 x MW+ in 0.1 M KI over a n Hg electrode a t u = 50 mV/s and I: = 25 "C. Peaks B and C in the voltammogram correspond to the two one-electron processes above (steps I and 11). Peak
Figure 3 shows the predictions of the above equations and the experimental results of Figure 2 after subtraction of the contribution from the charging current. (12)Harrison, J. A.; Thirsk, H.R. In Electroanalytical Chemistry; Bard, A. J., Ed.;Marcel Dekker: New York, 1971; Vol. 5 .
Voltammetric Study of Methyl Viologen
1 .oo
0
*
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A
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Langmuir, Vol. 11, No. 5, 1995 1793
mV mV mV mV mV
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v = 2 8 3 mV/s
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\
mV/s
/--v=500
W
-640
-680
-660
-
1 -700
0.00 0
1
2
3
E (mV vs SCE) Figure 3. Plot of IIIm us tlt, for progressive (curve a) and instantaneous (curve b) nucleation, as well as for the curves in Figure 2.
Figure 4. Cyclic voltammograms obtained for 0.5 m M M F in 0.1 M KI at T = 25 "C. The scan rate is shown for each voltammogram. I
As can be seen, the experimental data fit neither of the theoretical models described by eqs 1 and 2. This behavior was previously observed for heptyl viologenlO and B P H P at room t e m p e r a t ~ r e . ~ ~ Figure 4 shows three voltammograms obtained for peak A at different scan rates. The recordings in Figure 4 exhibit some typical features for 2D nucleation processes in cyclic voltammetry. Thus, the peak half-width (i.e. the width, in mV, a t half-height) and the voltammogram hysteresis (i.e. the distance between the oxidation and reduction peak potentials) increase with a n increase in the scan rate, u. In previous work,ll our group developed a theoretical model for 2D phase transitions taking place via nucleation mechanisms in cyclic voltammetry. The model is based on a straightforward nucleation kinetics and involves some approximations, the most significant of which is assuming the number of nuclei of critical size present a t any time to be that corresponding to a n equilibrium situation as regards the potential. This assumption precludes application of the model a t a high scan rate, where such a n equilibrium condition is probably not fulfilled. The fundamental equation for the model is1'
1.5 h
4
a
W
a 1 .o
H
w 0
4
0.5
*
n I
I
,
,
1.o
1.5
2.0
2.5
E
W
a
W
a M 0 3
n
>
E
W
1 .o
P M 0
4
0.5
3.0
3.5
log -v (mv/s) where f = RTInF, qmis the overall charge exchanged, Eo the standard reduction potential for the phase transition process, and b ( V I S 2 ) a constant dependent on the kinetics of charge transfer and the number of critically sized nuclei a t a zero overpotential.ll The plus sign in the exponential and pre-exponential term applies to the reduction process. In this case, eq 3 only possesses physical significance a t negative overpotentials &e. E < EO),where the reduction current is assumed to be positive. On the other hand, the minus sign in the above two terms applies to the oxidation process, so the equation only has physical significance at a positive overpotential (i.e. E > EO),where the oxidation current is taken to be negative. It should be noted that, as a rule, the kinetics of formation and destruction of 2D phases are different, so constant b must also be different for reduction and oxidation peaks,ll regardless of the apparent implications of eq 3. If the peak current is denoted by I,, the peak half-width by W, and the distance between the oxidation and
Figure 5. Plots of log I p ,log a,,and log W us log u for peak A. Conditions are as in Figure 4.
reduction peak potentials by AEp, then from eq 3 it follows that plots of log I,, log W, and log hE, against log u should be linear and of slope 0.6, 0.4, and 0.4, respectively.ll Figure 5 shows three such plots obtained under the same experimental conditions as Figure 4. As can be seen, all the plots were linear; also, the slopes were 0.59 (logI , us log u ) , 0.33 (log hE, us log u ) , and 0.40 (log W us log u ) and hence very close to the predictions of eq 3. One additional analytical criterion that can be applied to process A involves numerically fitting experimental peaks to eq 3. Figure 6 shows the results of one of the numerical fittings carried out at C m = low3M, u = 81 mVIs, and T = 25 "C. The figure compares experimental data obtained after subtraction of the charging current (circles) and the
Salas et al.
1794 Langmuir, Vol. 11, No. 5, 1995 I
I
I
I
t
II
1
A
I
2.5
I
I
i
lo 51 0
-7.5
1
1
-650
I
-660
I
-670
I
I
I
I
10
20
30
I
I
I
40
50
60
\
I 70
I
-680
E (mv> Figure 6. Cyclic voltammogramobtained for 0.5 mM Mv2+ at T = 25 "C and u = 81 mV/s after subtraction of the charging current (circles).The solid line corresponds to the predictions of eq 3 for b = 2.1 x lo6 mv2/s2and Eo= -663 mV (reduction peak) and b = 7.8 x lo6 mWs2 and Eo= -663 mV (oxidation peak).
predictions of eq 3 (solid line). The parameter values obtained in the fitting were b = (2.1 f 0.2) x lo6 mv2/s2 and Eo = -663 f 1mV, for the reduction peak, and b = (7.8 f 0.4) x lo6 mv2/s2and Eo = -663 f 1mV, for the oxidation peak. As can be seen, the theoretical predictions and experimental data were quite consistent. The above experimental results clearly confirm that the reduction peak for process A arises from the formation (condensation) of a two-dimensional phase of the cation radical a t the electrode, whereas the oxidation peak is due to the phase decomposition (fusion). Despite the fact that the I-t curves in Figure 3 conform to neither the instantaneous nor the progressive nucleation model,12the voltammetric peaks fit the predictions of eq 3 quite well. This is related to the above-mentioned fact that eq 3 is applicable under experimental conditions where the number of critically sized nuclei is always in equilibrium with the overpotential. Equation 3 is therefore independent of whether the nucleation kinetics is of the instantaneous or progressive type (or an even more complex type) under potentiostatic conditions.'l Therefore, the predictions of eq 3 are more universal than those of the above-described instantaneous and progressive nucleation models. Integration of the I - t curves of Figure 2 after removal of the charging current and extrapolation of the current to t = 0 allowed us to calculate the overall amount of charge exchanged in the potentiostatic transition, which turned out to be Q = 23.2 f0.2pC/cm2. This charge value was the same whether the potential jump was taken from more positive potentials than those of the appearance of peak A to more negative values than the latter or vice versa and was equivalent to a n area of ea. 69 &molecule. Integration of the voltammetric peak with respect to time also allowed us to calculate the amount of charge exchanged throughout peak A. Figure 7 shows a plot of Q @C/cm2),obtained by the above-described method, against T ( "C) a t u = 100 mV/s, all other conditions being identical with those of Figure 4. As can be seen, Q = 22 f 1pC/cm2 below 30 "C and decreases with an increase in T above such a temperature. This variation is discussed below. It should be noted that the Q value obtained by integrating the voltammetric peak
Figure 7. Variation of the charge with temperature for 0.5 mM M V + in 0.1 M KI at u = 50 mV/s.
at T < 30 "C is somewhat smaller than that resulting from integration of the I-t curves and is equivalent to a n area of ca. 73 &molecule. The Q values obtained above 30 "C by integrating the voltammetric peak were independent of the scan rate, u , and the Mv2+ concentration above 3 x M. Therefore, the calculated molecular areas (69-73 &molecule) lie midway between the expected values for a parallel (84-88 &molecule) and a perpendicular orientation (34-38 &molecule) relative to the e l e ~ t r o d e . ~ Accordingly, Mv'+adopts a tilted orientation with respect to the electrode surface, as previously observed in BPHz2+, which allows adjacent molecules to interact laterally to some e ~ t e n t . ~ Our molecular area values (69-73 &molecule) are much smaller than those obtained by Kitamura et aL8 using a sulfate medium (118 &molecule). For contrast, we also made Q measurements in 0.2 M S042-(the same medium used by these authors) by integrating the voltammetric peak; however, our measurements were made a t T = 2 "C, where peak A was clearly resolved from peak B. The charge for peak A obtained under suchsonditions was 17 f 1pC/cm2and thus equivalent to 94 Mmolecule, which is quite similar to the theoretical value for a molecular monolayer lying parallel to the electrode. The difference betw2en this value and that reported by Kitamura et a l a (118 Nmolecule) must be related to the low resolution of peak A from peak B a t room temperature used by these authors (see Figure 2 in ref 8). The Q value obtained in an Nos- medium was similar to that found in S042-, whereas that obtained in C1- was similar to that observed in I-. Kobayashi et al. hypothesized a normal orientation of Mv'+ molecules relative to the electrode on the basis of chronocoulometriccurves obtained in a phosphate medium at pH 7 using an applied potential following that of appearance of peak B (-800 mV) for 2 s and then a potential jump to -300 mV, where Q was measured as a function of time. This experiment was reproduced by us under exactly the same experimental conditions used in ref 7; weobtained a nonlinear plot of Q us tU2a t E = -300 mV. As a result, Q could hardly be extrapolated to t = 0 in order to estimate the contribution of adsorbed molecules a t the electrode. In this respect, we should note that keeping the potential a t -800 mV for 2 s did not result in a current variation conforming to the Cottrell equation. In fact, the current was higher than that predicted by such an equation, which suggests the occurrence of a catalytic effect. This phenomenon was not investigated
Langmuir, Vol. 11, No. 5, 1995 1795
Voltammetric Study of Methyl Viologen
Figure 10 shows the variation of AG with T obtained from the experimental data shown in Figure 9. As can be seen, AG scarcely changes with T a t low temperatures; hence, the 2D condensation of MY+ over
Hg is essentially an enthalpic phenomenon a t such temperatures. Above 30 "C, AG increases abruptly with increasing T , so A S is negative. This may be the result of the entropy consisting of a t least two major contributions, viz. that of condensation (AS < 0) and that of the displacement of adsorbed I- ions from the electrode surface (AS > 0). The two contributions virtually cancel out a t low temperatures. This reasoning entails neglecting the contribution of solvating water molecules. An increase in T should result in a sharp decrease in the number of adsorbed I- ions and hence in a marked decrease in the contribution of this term to the overall entropy change for the process. An increase in the temperature also decreases the number of MY+molecules that form a 2D phase, consistent with the decrease in Q with increase in T (see Figure 7). However, this effect is much less significant than the previous one since, as experimentally observed, A S is negative. Extrapolation of AG to 0 allowed us to calculate the temperature a t which AG = 0; such a value, which represents the critical temperature (TJfor the formation of a 2D phase of MY+ over Hg, turned out to be 67 "C. As can be inferred from the above reasoning, T,was a function of the particular anion (counterion) and its concentration present in the medium. The critical temperature obtained ( T , = 67 "C) is closer to the values found for 2D phases of planar molecules (33-55 "C)than for those of spherical molecules (86-125 "C).13 This is seemingly consistent with the tilted orientation of MY+ in the 2D phase. At this point, we should note that the critical temperature for the 2D phase formed by B P H Z ~ ca. + , ~73~ "C, is somewhat higher than that found in this work for the species MY+ as the likely result of a high surface density of the former, which facilitates lateral interactions between molecules. Unlike with peak A undergoes no splitting a t low temperatures, a t least not above 2 "C. Peak A for methyl viologen was not observed when a carbon electrode was used.3 This may have been the result of the nonuniform surface of the carbon electrode hindering the formation of a laterally arranged condensed phase. On the other hand, the occurrence of MY+-Hg specific interactions, which would favor the formation of such a n orderly structure a t the Hg electrode, cannot be ruled out. However, the fact that peak A disappears above 67 "C suggests that, if they do occur, such interactions must be very weak. The 2D phase formed after peak A appears consists of MV'+only, as shown by the fact that the E, value for such a peak is independent of the iodide concentration. Accordingly, the condensed phase is positively charged, in contrast with the behavior of other charged species, which form a 2D phase with the counterion in solution (e.g. guanidinium nitrate over Hg15 1. The 2D phases formed by neutral species,whether single molecules or salts, over Hg appear in potential regions near the electrocapillary maximum (ECM).15J6 On the other hand, the 2D phase of the species M V appears a t more negative potentials than the ECM (Figure 91, which suggests that the electrode negative charge offsets the excess positive charge of the condensed phase. As noted earlier, MV+ molecules in solution undergo two one-electron transfers that give rise to peaks B and C (see Figures 1 and 8). However, only the first such transfer (peak A) for the MV+ molecules forming the condensed phase could be observed over the potential
(13) Stenina, E. V.; Damaskin, B. B. J. Electroanal. Chem. 1993, -34.9. - - , 31. - -. (14) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods; Wiley: New York 1980.
(15) Wandlowski, T.; Jameson, G. B.; de Levie, R. J. Phys. Chem. 1993, 97, 10119. (16)Buess-Herman,C . InAdsorption of molecules ut metal electrodes; Lipkowski, J.; Ross, P. N., Ed.; VCH: New York, 1992.
I'
I
I
I
~n
-4L ' -600
t
I
-800
- 1000
I
I
- 1200
E (mV vs SCE) Figure 8. Cyclic voltammogram obtained for 0.5 mM W + in 0.1 M KI a t T = 67 "C and v = 50 mV/s.
in this work. In any case, the charge values obtained in the experiment' cannot be related to the interpretation given by the authors as regards the orientation of the species MY+. The temperature is highly influential on the phase formation processes. Thus, no 2D phase is formed above a given value known as the critical temperature (T,).l3As can be seen from the Q values shown in Figure 7 , Q decreased with increasing T above 30 "C; in fact, peak A disappeared above 65 "C. Figure 8 shows a voltammogram obtained for M V + a t T = 67 "C, all other conditions being identical with those of Figure 1. As can be seen, no nucleation peak (peak A) is observed, so the voltammogram is much simpler than that of Figure 1. In addition, the oxidation peak for process C exhibits a different shape that is typical of a diffusion-controlled process. The peak reduction potentials for processes A and B can be used to determine the formal reduction potential for both. Thus, for a diffusion-controlled process (peak B), on the assumption that the diffusion coefficients for species MV+ and MY+ are identical, EBO=E, 0.095T (mV).14 For a 2D phase transition process (peak A), E 2 D 0 =E,+ W/0.5263,where Wis the peak half-width.ll Figure 9 shows the variation of Eo and Epwith T for peaks A and B. As can be seen, the difference between E, and Eofor peaks A and B decreases with increasing T , the trend being more marked inEo. The function ~F(E~DO-EBO) must be proportional to the free energy change for the cation radical condensation. Such a change also includes a term accounting for the energy required to displace the iodide ions adsorbed a t the electrode and another reflecting the change in the number of molecules that solvate the different species involved:
+
MY+xH20
+
+ (ny - x)H20 m + 2 D
+ TZI-yHzO
bG = - F ( E 2 D 0 - E;)
Salas et al.
1796 Langmuir, Vol. 11, No. 5, 1995 -800
Peak B
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Figure 10. Plot of AG us T ( "C) run from the experimental data shown in Figure 9.
range studied (from -500 to -1600 mV), i.e. the first oneelectron transfer for such molecules (peak A) is advanced relative to the reduction of the molecules in the homogeneous phased (peak B), whereas the second one-electron transfer in the condensed phase is sufficiently delayed, relative to peak C, so a s not to be observed. This suggests
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that the species MV+ in the condensed phase is more stable against reduction and oxidation than in a homogeneous phase. The species MV.+ in solution is known to form a reversible dimer with a stacked s t r u ~ t u r e . l ' - ~However, ~ the W + concentration used in this work (5 x loT4M) was too low for the local concentrations of M Y + needed for appreciable dimerization to be electrochemically reached, a t least in regard to diffusion-controlled waves. This, however, does not apply to molecules deposited on the electrode, the local concentrations of which can be quite high. In fact, one may hypothesize that the condensed phase starts to be form around condensation
Acknowledgment. The authors wish to express their gratitude to the Spanish DGICyT for financial support of this research in the framework of Project PB91-0834. LA940798Y (17) Kosower, E. M.; Cotter, L. L. J.Am. Chem. SOC.1964,86,5524. (18) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem. SOC.Perkin 2 1977,1787.
(19)Wolszczak, M.; Stradowski, Cz. Radiat. Phys. Chem. 1989,33, 355.
