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Electrochemical behavior of nitroxy radical monolayers prepared on tin oxide electrodes by the Langmuir—Blodgett method
J. Electroanal. Chem., 195 (1985) 157-163 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
157
Preliminars note ELECTROCHEMICAL BEHAVIOR OF NITROXY RADICAL MONOLAYERS PREPARED ON TIN OXIDE ELECTRODES BY THE LANGMUIRBLODGETI’ METHOD AN UNUSUAL EFFECT OF CONCENTRATION OF SUPPORTING ELECTROLYTES ON CYCLIC VOLTAMMOGRAMS
SO0 GIL PARK, KOICHI AOKI, KOICHI TOKUDA* and HIROAKI MATSUDA* Department of Electronic Chemistry, Graduate School at Nagatsuta, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227 (Japan) (Received 23rd July 1985)
INTRODUCTION
A number of procedures have been devised for immobilizing various species onto electrode surfaces [ 11. In our laboratory the Langmuir-Blodgett method [2,3] has been used to confine polypyridine ruthenium and osmium complexes to tin oxide electrodes. The electrode kinetics of these complexes [4] as well as electron mediated reactions of substrates in solution by the immobilized complexes [ 51 have been studied. This method has some advantages over other techniques because it enables us to confine species to electrodes in a monomolecular layer and in high densities as well as with high reproducibility. Recently Semmelhack and co-workers [6,7] reported that oxoimmonium cation formed by electrooxidation of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical, which is well known as an electron-spin probe, is an active oxidizing agent for many alcohols and amines and TEMPO is suitable as an electron mediator for electrooxidation of organic compounds. It is of great interest to provide a modified electrode with immobilized TEMPO moieties. In the present study we have synthesized a surface active nitroxy radical, 2,2,6,6-tetramethyl-4-octadecyloxy-l-piperidinyloxy (TEMOPO), and confined TEMOPO by itself or mixtures of TEMOPO and arachidic acid (AA) to tin oxide electrodes as a monolayer by the Langmuir-Blodgett method. Electrachemical study of such electrodes in various supporting electrolytes is under way. This communication describes our findings on the behavior of the electrodes in aqueous acidic media; an unusual effect of concentration of the supporting electrolytes on the progress of oxidation of TEMOPO has been found.
*To whom correspondence should be addressed. 0022-0728/85/$03.30
0 1985 Elsevier Sequoia S.A.
158 EXPERIMENTAL
TEMOPO was prepared from 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL), which was synthesized according to the procedure described in the literature [8], as follows: Sodium hydride (0.72 g) was washed with dry n-hexane and was immediately transferred into a four-necked flask under an atmosphere of nitrogen. A solution of 3.05 g TEMPOL in 30 cm3 anhydrous dimethyl formamide (DMF) was added dropwise (ca. 10 drops/min) under nitrogen atmosphere while stirring, and the reaction was allowed to continue for 6 h at a room temperature. A solution of 8.08 g octadecyl iodide in 30 cm3 DMF wad then added to the reaction mixture, which was stirred for 18 h at 50-6O”C under nitrogen atmosphere. The reaction mixture was filtered to remove precipitates of sodium iodide formed and the filtrate was mixed with 150 cm3 diethyl ether. This extraction procedure was repeated several times. After the ether was evaporated, the residue was dissolved into a 4:l mixture of n-hexane and diethyl ether and TEMOPO was isolated by column chromatography on silica gel (Merck, particle size 0.063-0.200 mm) and was eluted with dimethyl ether. This procedure was repeated twice. The elute was dried for 12 h in vacua. When cooled in an ice bath, crystals were formed, m.p. 40-42°C. Elemental analysis: Calculated for C27H54N02: C, 76.33; H, 12.82; N, 3.29. Found: C, 76.08; H, 13.52; N, 3.27. Other chemicals used are of reagent grade. Optically transparent tin oxide electrodes were purchased from the Matsuzaki Shinku Himaku Co., Ltd. A monolayer of TEMOPO or TEMOPO + AA mixtures spread on the surface of distilled water in a teflon coated trough was transferred to the tin oxide electrode under a constant surface pressure of 1.5 X lo-’ N/m using a Kyowa Kaimenkagaku surface pressure controller. Electrochemical cells and apparatus have been described elsewhere [ 41. Electrochemical measurements were performed at a room temperature of 25 f 0.5”C. RESULTS
AND DISCUSSION
Typical cyclic voltammograms for the oxidation and re-reduction of TEMOPO monolayer confined to a tin oxide electrode in 0.18 mol/dm3 HzS04 aqueous solution are shown in Fig. 1. This figure indicates that the current gradually increases on repeating the cyclic potential sweep between 0.3 and 1.0 V vs. SCE and eventually reaches the maximum wave. The anodic waves are not always welldefined and sometimes they exhibit two peaks, while the cathodic waves are rather symmetric in shape. The fact that these voltammograms do not show any diffusion tails, that the stirring of the solution does not affect the wave forms and that the peak current values for the maximum waves are almost proportional to potential sweep rate indicate that only the species confined to the electrodes take part in the electrode process. The anodic and cathodic peak potentials are about 0.82 V and 0.72 V, respectively. A cyclic voltammogram for oxidation and re-reduction of water soluble TEMPOL in 0.18 mol/dm3 H2S04 at a glassy cabon electrode with a sweep rate of 10 mV/s is almost reversible and has the anodic and cathodic peaks at 0.60 and 0.54 V, respectively. Although there are about 0.2 V differences in the peak potentials for TEMOPO and TEMPOL, it is reasonable to consider that they
159
Fig. 1. Cyclic voltammograms for oxidation and re-reduction of TEMOPO confined to a SnO, electrode in a monolayer. Supporting electrolyte: 0.18 mol/dm3 H,SO, aqueous solution. Potential sweep rate: 0.1 V/s. Electrode surface area: 0.363 cmz.
undergo the same one-electron oxidation and thus the electrode process for Fig. 1 may be written as N-O’
RO -c
;
ILO
RO
+e-
is:
The superficial concentration of TEMOPO confined to the electrode surface was calculated to be (1.55 &0.05) X lo-” mol/cm’ from graphical integration of the maximum cathodic waves with background current correction. Thus, the area occupied by one molecule of TEMOPO is calculated to be 1.07 + 0.03 nm*. Figure 2 shows voltammograms for the monolayer of aTEMOP + AA mixture (1:2 in mole ratio) adsorbed on the tin oxide electrode under the same conditions as Fig. 1. It is expected that dilution with AA will reduce interactions among TEMOPO molecules. The gradual increase in current with the repetition of potential sweep is again observed, the behavior being similar to that in Fig. 1. In this case, however, both the anodic and the cathodic waves are well shaped, being symmetric about the potential axis. There occurred a peak potential shift of about 0.1 V due to the dilution of TEMOPO with AA. The surface concentration of TEMOPO was evaluated as (1.24 f 0.05) X lo-” mol/cm* as described above. The maximum waves similar to those in Figs. 1 and 2,can be recorded if we first set the electrode potential at 1.0 V and keep the electrode at this potential for more than 5 min, which is considered to be long enough to oxidize all the nitroxy radicals, and then apply the cyclic potential sweeps between 1.0 V and 0.3 V, instead of the potential sweep cycling from the start. Once the maximum waves are reached, it is found that the repeated potential sweeps trace the same tracks for hours. After the maximum wave is reached, if we keep the potential at 0.1 or 0.0 V for 5 or 10 min in order to reduce all the
160 r
I
0.4
I
0.6
I
I I
l.0
E / V vs. S:i8
Fig. 2. Cyclic voltammograms for oxidation and re-reduction of a mixture of TEMOPO and AA confined to a SnO, electrode in a monolayer. [TEMOPO]/[AA] = 1:2. Supporting electrolyte: 0.18 mol/dm3 H,SO, aqueous solution. Potential sweep rate: 0.1 V/s. Electrode surface area: 0.363 cm*.
oxoimmonium cations back to nitroxy radicals again, and perform cyclic voltammetry, then the current which flows during the first cycle becomes very low and the resulting voltammograms are very similar to those shown in Figs. 1 and 2. The effect of the concentration of HzS04 as a supporting electrolyte on the voltammograms was studied next. Tin oxide electrodes coated with a monolayer of 1:2 TEMOPO + AA mixture and cyclic voltammetry with the potential sweep between 0.3 and 1.0 V at a sweep rate of 0.1 V/s were used in the experiments. The amount of charge passed, Q, was evaluated from graphical integration of the cathodic and anodic waves for each cycle with background correction. The values of Q obtained from the cathodic and anodic waves are almost equal to each other. The values of Q thus calculated are plotted in Fig. 3 against the number of potential sweep cycles for various concentrations of Hz SO,. Interestingly, it was found that the voltammograms obtained in dilute solutions, e.g., 1.0 and 2.0 mmol/dm3 HzS04, showed the greatest waves from the first sweep cycle. In 0.5 mol/dm3 HzS04 solution, however, it took at least 110 cycles to reach the maximum wave, the amount of charge for the maximum wave being smaller than those for the maximum waves observed in the dilute solutions. In 2 .O mol/dm3 H, SO,, the current passed was negligibly small and no peak was observed. One can see from Fig. 3 that the higher the HzS04 concentration, the more cycles are required to reach the maximum wave and that not all nitroxy radicals can react when the HzS04 concentration is higher than 0.5 mol/dm3. Concentration effects of supporting electrolytes on the voltammograms were also observed in hydrochloric acid and perchloric acid solutions. The behavior in these electrolytes was quite similar to that in sulfuric acid except for the fact that the peak potentials observed in perchloric acid solutions are at somewhat less positive potentials than in the other two acids. The pH range we investigated here is limited between about 0 and 3. In order to examine whether the effect is due to the change in pH or not, we added
161
4
c_l a
3
& j 1 i
Oli
2I
/
1111111 5
. .
l
50ttllll
10
20 I
I
Number
of
cycles
I
100
200 I
Fig. 3. The effect of sulfuric acid concentration on the progress of TEMOPO oxidation confined in a monolayer to a SnO, electrode. Amount of the electric charge, Q, passed during each potential sweep cycle is plotted as a function of number of potential sweep cycles between 0.3 V and 1 .O V vs. SCE at a sweep rate of 0.1 V/s. [TEMOPO] /[AA] = l/2. H, SO, concentration: (u) 0.005, (A) 0.01, (0)0.02, (010.05, (m) 0.1, (A) 0.2, (0) 0.5 and (*) 1.0 mol/dm’ .
crystals of potassium chloride progressively to a 0.005 mol/dm3 HCl solution and recorded voltammograms to find that retardation of the TEMOPO oxidation rate occurs similarly with the increase in the KC1 concentration. This finding suggests that the effect results from the change in the concentration of the supporting electrolytes or ionic strength. We used trifluoroacetic acid as a supporting electrolyte expecting a different result because the trifluoroacetate anion is an organic acid and is more hydrophobic than the inorganic anions employed in this study. However, the voltammetric behavior was similar to that in the inorganic acids. The plots of Q against the number of potential sweep cycles obtained for several concentrations of trifluoroacetic acid are shown in Fig. 4. Such behavior for the redox process of the species adsorbed in a monolayer to the electrode surface as found in Figs. 1 and 2, i.e., the gradual increase in current with repetition of potential sweep cycles, has not been reported. In the case of the adsorbed monolayer of surfactant polypyridine osmium and ruthenium complexes, the maximum surface waves were observed during the first or the second potential sweep [ 41. It may be reasonable to consider that the TEMOPO radicals and AA molecules in the monolayer are oriented at the tin oxide electrode surface with their less hydrophobic ends, N--O * and carboxyl groups, respectively, directed toward the electrode ahd their alkyl chains outward. This is because the surface of tin oxide is considered to be rather hydrophilic and because it was confirmed from the observation of the decrease in the area of the monolayer spread on the water surface under a controlled surface pressure that a portion of the monolayer was not transferred onto the tin oxide electrode when it was dipped into the water subphase but was transferred only when it was withdrawn upward from the subphase. Contrary to the polypyridine complexes we used in previous studies [4,5], the TEMOPO radical is neutral and the monolayer transferred onto the electrode
162
Number
of
cycles
Fig. 4. Effect of trifluoroacetic acid concentration on the progress of oxidation of TEMOPO confined in a monolayer to a SnO, electrode. Other conditions are the same as in Fig. 3. [TEMOPO]/[AA] = l/2. CF,COOH concentration: (0) 0.05, (A) 0.1, (0) 0.2, (0) 0.5 and (=) 1 .O mol/dm' .
surface may be neutral. Therefore, the electrode surface may not be covered by many solvent (water) molecules or ions of supporting electrolyte, when the electrode is mounted to the cell and comes into contact with the solution. In order for the oxidation of TEMOPO to its oxoimmonium ion to proceed, penetration of anions through the monolayer is necessary to neutralize the electric charge of the cations formed. This may be a possible explanation for the slow progress of the TEMOPO oxidation as visualized in Figs. 1 and 2 by the gradual increase in the current with the repeated cyclic potential sweep. Once the electrooxidation of TEMOPO commences, the solvent molecules and ions are able to enter into the monolayer gradually and the maximum wave is reached eventually. As stated above, it was found that the maximum wave form is repeatedly traced as long as the potential sweep is continued. It can be presumed from this finding that the monolayer of TEMOPO once oxidized can contain sufficient solvent molecules and ions of the supporting electrolyte which still remain in the monolayer even if the monolayer is almost in its neutral state when the potential being swept comes close to 0.3 V. However, if ample time is allowed for the complete reduction of any oxoimmonium ions formed to the neutral radicals while holding the electrode potential at e.g. 0.0 V, then not only anions but also cations and solvents seem to be expelled from the monolayer so that the original state of the electrode is recovered. The reason why the higher the concentration of acid solutions, the more retarded is the progress of TEMOPO oxidation, has not been clarified yet. A possible explanation for this behavior is an effect of concentration of electrolytes similar to the salting-out effect; in a two-component mixed solvent with different dielectric constants, if the solubility of one solvent (non-electrolyte) to the other (water) is limited then the solubility is very sensitive to the electrolyte added to the system and is, in general, decreased with the increase in the concentration of the added electrolyte.
163
At the monolayer/solution interface region, the alkyl chains in the monolayer having very low solubility to water can be regarded as a non-polar solvent. As the concentrations of the supporting electrolytes increase, the solubility of the alkyl chains becomes extremely 1o.w; in other words, the higher the concentration of the supporting electrolytes, the more the activity of water around the alkyl chain may be lowered and thus the penetration of ions through the monolayer may be hindered. If this is the case, such a drastic change in the activity of water, not in the bulk phase but around the alkyl chains, may be possible in the concentration range of electrolytes between 0.1 and 6.5 mol/dm3. It is worth mentioning that this behavior may be closely related to the buildup of the electric double layer at such electrodes coated with densely packed monolayers of neutral molecules. Further study along this line is needed. ACKNOWLEDGEMENTS
The authors are deeply grateful to Drs. Takeshi Endo and Tsutomu Nonaka of Tokyo Institute of Technology for their fruitful instructions in the synthesis of TEMOPO. This work was partly supported by Grants-in-Aid for Scientific Research No. 59470067 and No. 59390012 from the Ministry of Education, Science, and Culture, Japan.
REFERENCES 1 R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1983, p. 191. 2 G.L. Gaines, Jr., Insoluble Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. 3 H. Kuhn, D. Mobius and H. Buchner in A. Weissburger and B. Rossiter (Eds.), Physical Methods of Chemistry, Vol. 1, Part 3B, Wiley, New York, 1972, p. 688. 4 H. Daifuku, K. Aoki, K. Tokuda and H. Matsuda, J. Electroanal. Chem., 183 (1985) 1. 5. H. Daifuku, I. Yoshimura, I. Hirata, K. Aoki, K. Tokuda and H. Matsuda, J. Electroanal. Chem., in press. 6 M.F. Semmelhack, C.S. Chou and D.A. Cortes, J. Am. Chem. Sot., 105 (1983) 4492. 7 M.F. Semmelhack and C.R. Schmid, J. Am. Chem. Sot., 105 (1983) 6732. 8 E.G. Rozantsev, Free Nitroxyl Radicals, Plenum Press, New York, 1970.
157
Preliminars note ELECTROCHEMICAL BEHAVIOR OF NITROXY RADICAL MONOLAYERS PREPARED ON TIN OXIDE ELECTRODES BY THE LANGMUIRBLODGETI’ METHOD AN UNUSUAL EFFECT OF CONCENTRATION OF SUPPORTING ELECTROLYTES ON CYCLIC VOLTAMMOGRAMS
SO0 GIL PARK, KOICHI AOKI, KOICHI TOKUDA* and HIROAKI MATSUDA* Department of Electronic Chemistry, Graduate School at Nagatsuta, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227 (Japan) (Received 23rd July 1985)
INTRODUCTION
A number of procedures have been devised for immobilizing various species onto electrode surfaces [ 11. In our laboratory the Langmuir-Blodgett method [2,3] has been used to confine polypyridine ruthenium and osmium complexes to tin oxide electrodes. The electrode kinetics of these complexes [4] as well as electron mediated reactions of substrates in solution by the immobilized complexes [ 51 have been studied. This method has some advantages over other techniques because it enables us to confine species to electrodes in a monomolecular layer and in high densities as well as with high reproducibility. Recently Semmelhack and co-workers [6,7] reported that oxoimmonium cation formed by electrooxidation of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical, which is well known as an electron-spin probe, is an active oxidizing agent for many alcohols and amines and TEMPO is suitable as an electron mediator for electrooxidation of organic compounds. It is of great interest to provide a modified electrode with immobilized TEMPO moieties. In the present study we have synthesized a surface active nitroxy radical, 2,2,6,6-tetramethyl-4-octadecyloxy-l-piperidinyloxy (TEMOPO), and confined TEMOPO by itself or mixtures of TEMOPO and arachidic acid (AA) to tin oxide electrodes as a monolayer by the Langmuir-Blodgett method. Electrachemical study of such electrodes in various supporting electrolytes is under way. This communication describes our findings on the behavior of the electrodes in aqueous acidic media; an unusual effect of concentration of the supporting electrolytes on the progress of oxidation of TEMOPO has been found.
*To whom correspondence should be addressed. 0022-0728/85/$03.30
0 1985 Elsevier Sequoia S.A.
158 EXPERIMENTAL
TEMOPO was prepared from 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL), which was synthesized according to the procedure described in the literature [8], as follows: Sodium hydride (0.72 g) was washed with dry n-hexane and was immediately transferred into a four-necked flask under an atmosphere of nitrogen. A solution of 3.05 g TEMPOL in 30 cm3 anhydrous dimethyl formamide (DMF) was added dropwise (ca. 10 drops/min) under nitrogen atmosphere while stirring, and the reaction was allowed to continue for 6 h at a room temperature. A solution of 8.08 g octadecyl iodide in 30 cm3 DMF wad then added to the reaction mixture, which was stirred for 18 h at 50-6O”C under nitrogen atmosphere. The reaction mixture was filtered to remove precipitates of sodium iodide formed and the filtrate was mixed with 150 cm3 diethyl ether. This extraction procedure was repeated several times. After the ether was evaporated, the residue was dissolved into a 4:l mixture of n-hexane and diethyl ether and TEMOPO was isolated by column chromatography on silica gel (Merck, particle size 0.063-0.200 mm) and was eluted with dimethyl ether. This procedure was repeated twice. The elute was dried for 12 h in vacua. When cooled in an ice bath, crystals were formed, m.p. 40-42°C. Elemental analysis: Calculated for C27H54N02: C, 76.33; H, 12.82; N, 3.29. Found: C, 76.08; H, 13.52; N, 3.27. Other chemicals used are of reagent grade. Optically transparent tin oxide electrodes were purchased from the Matsuzaki Shinku Himaku Co., Ltd. A monolayer of TEMOPO or TEMOPO + AA mixtures spread on the surface of distilled water in a teflon coated trough was transferred to the tin oxide electrode under a constant surface pressure of 1.5 X lo-’ N/m using a Kyowa Kaimenkagaku surface pressure controller. Electrochemical cells and apparatus have been described elsewhere [ 41. Electrochemical measurements were performed at a room temperature of 25 f 0.5”C. RESULTS
AND DISCUSSION
Typical cyclic voltammograms for the oxidation and re-reduction of TEMOPO monolayer confined to a tin oxide electrode in 0.18 mol/dm3 HzS04 aqueous solution are shown in Fig. 1. This figure indicates that the current gradually increases on repeating the cyclic potential sweep between 0.3 and 1.0 V vs. SCE and eventually reaches the maximum wave. The anodic waves are not always welldefined and sometimes they exhibit two peaks, while the cathodic waves are rather symmetric in shape. The fact that these voltammograms do not show any diffusion tails, that the stirring of the solution does not affect the wave forms and that the peak current values for the maximum waves are almost proportional to potential sweep rate indicate that only the species confined to the electrodes take part in the electrode process. The anodic and cathodic peak potentials are about 0.82 V and 0.72 V, respectively. A cyclic voltammogram for oxidation and re-reduction of water soluble TEMPOL in 0.18 mol/dm3 H2S04 at a glassy cabon electrode with a sweep rate of 10 mV/s is almost reversible and has the anodic and cathodic peaks at 0.60 and 0.54 V, respectively. Although there are about 0.2 V differences in the peak potentials for TEMOPO and TEMPOL, it is reasonable to consider that they
159
Fig. 1. Cyclic voltammograms for oxidation and re-reduction of TEMOPO confined to a SnO, electrode in a monolayer. Supporting electrolyte: 0.18 mol/dm3 H,SO, aqueous solution. Potential sweep rate: 0.1 V/s. Electrode surface area: 0.363 cmz.
undergo the same one-electron oxidation and thus the electrode process for Fig. 1 may be written as N-O’
RO -c
;
ILO
RO
+e-
is:
The superficial concentration of TEMOPO confined to the electrode surface was calculated to be (1.55 &0.05) X lo-” mol/cm’ from graphical integration of the maximum cathodic waves with background current correction. Thus, the area occupied by one molecule of TEMOPO is calculated to be 1.07 + 0.03 nm*. Figure 2 shows voltammograms for the monolayer of aTEMOP + AA mixture (1:2 in mole ratio) adsorbed on the tin oxide electrode under the same conditions as Fig. 1. It is expected that dilution with AA will reduce interactions among TEMOPO molecules. The gradual increase in current with the repetition of potential sweep is again observed, the behavior being similar to that in Fig. 1. In this case, however, both the anodic and the cathodic waves are well shaped, being symmetric about the potential axis. There occurred a peak potential shift of about 0.1 V due to the dilution of TEMOPO with AA. The surface concentration of TEMOPO was evaluated as (1.24 f 0.05) X lo-” mol/cm* as described above. The maximum waves similar to those in Figs. 1 and 2,can be recorded if we first set the electrode potential at 1.0 V and keep the electrode at this potential for more than 5 min, which is considered to be long enough to oxidize all the nitroxy radicals, and then apply the cyclic potential sweeps between 1.0 V and 0.3 V, instead of the potential sweep cycling from the start. Once the maximum waves are reached, it is found that the repeated potential sweeps trace the same tracks for hours. After the maximum wave is reached, if we keep the potential at 0.1 or 0.0 V for 5 or 10 min in order to reduce all the
160 r
I
0.4
I
0.6
I
I I
l.0
E / V vs. S:i8
Fig. 2. Cyclic voltammograms for oxidation and re-reduction of a mixture of TEMOPO and AA confined to a SnO, electrode in a monolayer. [TEMOPO]/[AA] = 1:2. Supporting electrolyte: 0.18 mol/dm3 H,SO, aqueous solution. Potential sweep rate: 0.1 V/s. Electrode surface area: 0.363 cm*.
oxoimmonium cations back to nitroxy radicals again, and perform cyclic voltammetry, then the current which flows during the first cycle becomes very low and the resulting voltammograms are very similar to those shown in Figs. 1 and 2. The effect of the concentration of HzS04 as a supporting electrolyte on the voltammograms was studied next. Tin oxide electrodes coated with a monolayer of 1:2 TEMOPO + AA mixture and cyclic voltammetry with the potential sweep between 0.3 and 1.0 V at a sweep rate of 0.1 V/s were used in the experiments. The amount of charge passed, Q, was evaluated from graphical integration of the cathodic and anodic waves for each cycle with background correction. The values of Q obtained from the cathodic and anodic waves are almost equal to each other. The values of Q thus calculated are plotted in Fig. 3 against the number of potential sweep cycles for various concentrations of Hz SO,. Interestingly, it was found that the voltammograms obtained in dilute solutions, e.g., 1.0 and 2.0 mmol/dm3 HzS04, showed the greatest waves from the first sweep cycle. In 0.5 mol/dm3 HzS04 solution, however, it took at least 110 cycles to reach the maximum wave, the amount of charge for the maximum wave being smaller than those for the maximum waves observed in the dilute solutions. In 2 .O mol/dm3 H, SO,, the current passed was negligibly small and no peak was observed. One can see from Fig. 3 that the higher the HzS04 concentration, the more cycles are required to reach the maximum wave and that not all nitroxy radicals can react when the HzS04 concentration is higher than 0.5 mol/dm3. Concentration effects of supporting electrolytes on the voltammograms were also observed in hydrochloric acid and perchloric acid solutions. The behavior in these electrolytes was quite similar to that in sulfuric acid except for the fact that the peak potentials observed in perchloric acid solutions are at somewhat less positive potentials than in the other two acids. The pH range we investigated here is limited between about 0 and 3. In order to examine whether the effect is due to the change in pH or not, we added
161
4
c_l a
3
& j 1 i
Oli
2I
/
1111111 5
. .
l
50ttllll
10
20 I
I
Number
of
cycles
I
100
200 I
Fig. 3. The effect of sulfuric acid concentration on the progress of TEMOPO oxidation confined in a monolayer to a SnO, electrode. Amount of the electric charge, Q, passed during each potential sweep cycle is plotted as a function of number of potential sweep cycles between 0.3 V and 1 .O V vs. SCE at a sweep rate of 0.1 V/s. [TEMOPO] /[AA] = l/2. H, SO, concentration: (u) 0.005, (A) 0.01, (0)0.02, (010.05, (m) 0.1, (A) 0.2, (0) 0.5 and (*) 1.0 mol/dm’ .
crystals of potassium chloride progressively to a 0.005 mol/dm3 HCl solution and recorded voltammograms to find that retardation of the TEMOPO oxidation rate occurs similarly with the increase in the KC1 concentration. This finding suggests that the effect results from the change in the concentration of the supporting electrolytes or ionic strength. We used trifluoroacetic acid as a supporting electrolyte expecting a different result because the trifluoroacetate anion is an organic acid and is more hydrophobic than the inorganic anions employed in this study. However, the voltammetric behavior was similar to that in the inorganic acids. The plots of Q against the number of potential sweep cycles obtained for several concentrations of trifluoroacetic acid are shown in Fig. 4. Such behavior for the redox process of the species adsorbed in a monolayer to the electrode surface as found in Figs. 1 and 2, i.e., the gradual increase in current with repetition of potential sweep cycles, has not been reported. In the case of the adsorbed monolayer of surfactant polypyridine osmium and ruthenium complexes, the maximum surface waves were observed during the first or the second potential sweep [ 41. It may be reasonable to consider that the TEMOPO radicals and AA molecules in the monolayer are oriented at the tin oxide electrode surface with their less hydrophobic ends, N--O * and carboxyl groups, respectively, directed toward the electrode ahd their alkyl chains outward. This is because the surface of tin oxide is considered to be rather hydrophilic and because it was confirmed from the observation of the decrease in the area of the monolayer spread on the water surface under a controlled surface pressure that a portion of the monolayer was not transferred onto the tin oxide electrode when it was dipped into the water subphase but was transferred only when it was withdrawn upward from the subphase. Contrary to the polypyridine complexes we used in previous studies [4,5], the TEMOPO radical is neutral and the monolayer transferred onto the electrode
162
Number
of
cycles
Fig. 4. Effect of trifluoroacetic acid concentration on the progress of oxidation of TEMOPO confined in a monolayer to a SnO, electrode. Other conditions are the same as in Fig. 3. [TEMOPO]/[AA] = l/2. CF,COOH concentration: (0) 0.05, (A) 0.1, (0) 0.2, (0) 0.5 and (=) 1 .O mol/dm' .
surface may be neutral. Therefore, the electrode surface may not be covered by many solvent (water) molecules or ions of supporting electrolyte, when the electrode is mounted to the cell and comes into contact with the solution. In order for the oxidation of TEMOPO to its oxoimmonium ion to proceed, penetration of anions through the monolayer is necessary to neutralize the electric charge of the cations formed. This may be a possible explanation for the slow progress of the TEMOPO oxidation as visualized in Figs. 1 and 2 by the gradual increase in the current with the repeated cyclic potential sweep. Once the electrooxidation of TEMOPO commences, the solvent molecules and ions are able to enter into the monolayer gradually and the maximum wave is reached eventually. As stated above, it was found that the maximum wave form is repeatedly traced as long as the potential sweep is continued. It can be presumed from this finding that the monolayer of TEMOPO once oxidized can contain sufficient solvent molecules and ions of the supporting electrolyte which still remain in the monolayer even if the monolayer is almost in its neutral state when the potential being swept comes close to 0.3 V. However, if ample time is allowed for the complete reduction of any oxoimmonium ions formed to the neutral radicals while holding the electrode potential at e.g. 0.0 V, then not only anions but also cations and solvents seem to be expelled from the monolayer so that the original state of the electrode is recovered. The reason why the higher the concentration of acid solutions, the more retarded is the progress of TEMOPO oxidation, has not been clarified yet. A possible explanation for this behavior is an effect of concentration of electrolytes similar to the salting-out effect; in a two-component mixed solvent with different dielectric constants, if the solubility of one solvent (non-electrolyte) to the other (water) is limited then the solubility is very sensitive to the electrolyte added to the system and is, in general, decreased with the increase in the concentration of the added electrolyte.
163
At the monolayer/solution interface region, the alkyl chains in the monolayer having very low solubility to water can be regarded as a non-polar solvent. As the concentrations of the supporting electrolytes increase, the solubility of the alkyl chains becomes extremely 1o.w; in other words, the higher the concentration of the supporting electrolytes, the more the activity of water around the alkyl chain may be lowered and thus the penetration of ions through the monolayer may be hindered. If this is the case, such a drastic change in the activity of water, not in the bulk phase but around the alkyl chains, may be possible in the concentration range of electrolytes between 0.1 and 6.5 mol/dm3. It is worth mentioning that this behavior may be closely related to the buildup of the electric double layer at such electrodes coated with densely packed monolayers of neutral molecules. Further study along this line is needed. ACKNOWLEDGEMENTS
The authors are deeply grateful to Drs. Takeshi Endo and Tsutomu Nonaka of Tokyo Institute of Technology for their fruitful instructions in the synthesis of TEMOPO. This work was partly supported by Grants-in-Aid for Scientific Research No. 59470067 and No. 59390012 from the Ministry of Education, Science, and Culture, Japan.
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