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The π-interaction of bipyridinium ions with the electrode surface and its effect on the electrode admittance
291
J. Electroanal. Chem., 255 (1988) 291-296 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Short communication
The T-interaction of bipyridinium ions with the electrode surface and its effect on the electrode admittance M. Heyrovskjr and L. Pospigil TheJ. Heyrovsk$ Institute of Physical Chemistry and Electrochemistry Dolejzkova 3, 182 23 Prague 8 (Czechoslovakia) (Received
Czechoslovak
Academy
of Sciences,
17 June 1988; in revised form 14 July 1988)
In our studies on the electroreduction of bipyridine derivatives we found that adsorption of these substances as well as of their reduced forms plays an important role in the electrode processes. While gathering results of various electrochemical methods we realized that depending solely on the measurements of electrode admittance can become misleading if the substance under study, when adsorbed, enters into a-interaction with the electrode surface. By electrocapillary measurements it had been demonstrated [l] that 4,4’-bipyridine is adsorbed on mercury both in neutral and in acid solutions. Figures 1 and 2 show the low frequency phase-sensitive ac polarograms of 4,4’-bipyridine in 0.1 M sodium sulphate and in 0.1 M sulphuric acid, i.e., in solutions in which the electrocapillarity had been measured. The curves were recorded with the frequency of the sine wave at 64 Hz and using automatic iR drop compensation via the positive feed-back loop of the potentiostat. At a 100 times higher frequency the qualitative picture of the electrode admittance in the two solutions remains unchanged. In order to make sure that adsorption equilibrium had been reached we used both the DME (with drop time 3.1 s) and a stationary mercury drop. Also, the capacity vs. time curves were recorded over a period of 1000 s after the electrode formation. At the DME in the neutral solution, equilibrium was attained within the drop time only at bipyridine concentrations of 0.16 mM or higher. In the acid solution the results with the dropping and the stationary electrodes coincided from the lowest concentrations of 4,4’-bipyridine. The electroreduction of 4,4’-bipyridine takes place at potentials more negative than about - 0.6 V in the acid and about - 0.9 V in the neutral solutions. With the neutral solution (Fig. 1) we observe the typical case of adsorption lowering the double layer capacitance in the region positive of the potential of electroreduction. In the acid medium (Fig. 2), on the other hand, no depression of capacitance occurs with respect to the pure supporting electrolyte in the non-faradaic region; instead, 0022-0728/88/$03.50
0 1988 Elsevier Sequoia
S.A.
-430 a’ dyn.cm-l
-
0
- 0.2
I
-0.4
I
- 0.6
I
I
- 0.6
- 1.0
400
I
-1.2
E/V
Fig. 1. Comparison of capacitance () and electrocapillary (- - -) curves of 4,4’-bipyridine in aqueous 0.1 M sodium sulphate. Concentrations of 4,4’-bipyridine were: (1) 0, (2) 5, (3) 10, (4) 20, (5) 40, (6) 80, (7) 160 PM; (a) 0, (b) 5, (c) 20 pM. The ac frequency was 64 Hz. Electrocapillarity was measured with drop times of 56-60 s [l]. Potentials vs. saturated AgCl electrode. 0.5Y” pFcm-
5
420
0 .4-
01.1L
I
- 0.2
I
-0.4
I
-0.6
-0.6
E/V
Fig. 2. Comparison of capacitance ( -) and electrocapillary (- - -) curves of 4,4’-bipyridine in aqueous 0.1 M sulphuric acid. Concentrations of 4,4’-bipyridine were: (1) 0, (2) 5, (3) 10, (4) 20, (5) 40, (6) 80, (7) 160 pM, (a) 0, (b) 20, (c) 100 pM. The ac frequency was 64 Hr. Electrocapillarity was measured with drop times of 56-60 s [l]. Potentials vs. saturated AgCl electrode.
293
Y yFcm0.:
0.
Fig. 3. Potential dependence of the out-of-phase (a) and in-phase (b) admittance components of aqueous 0.1 M sulphuric acid+ 2,2’-bipyridine at concentrations: (1) 0, (2) 5, (3) 10, (4) 20 PM. HMDE, scan rate 10 mV/s; ac frequency 1606 Hz; amplitude 5 mV. Potentials vs. saturated AgCl electrode.
the double layer capacitance increases with increasing additions of 4,4’-bipyridine. This result is similar to what can be seen [l] on dc polarograms or on equilibrium charge-potential curves. In acid solutions of pH < 2.7 the 4,4’-bipyridine molecules become doubly protonated [2] and are adsorbed reversibly in a position parallel to the electrode surface with the aromatic rings in r-interaction with the metal [l]. It has been known long since [3] that the adsorbed pyridinium cation in m-interaction with the electrode surface does not change the capacity of the electrode; the same could be expected to apply to the 4,4’-bipyridinium dication. However, an increase of the capacitance is observed with this species, which is presumably due to a surface electron transfer process, analogous to the case of methyl viologen mentioned below. The 2,2’-bipyridine behaves differently from its 4,4’-isomer: it displays adsorption on the admittance curves in neutral [4] as well as in acid solutions (Fig. 3). Its molecule has the tendency to form chelates with various cations in the solution, including protons; as an unsymmetrically charged entity the chelate cannot stretch out flat on the electrode surface and hence cannot share its rr-electrons with the surface. The cation of methylviologen, or l,l’-dimethyl-4,4’-bipyridinium, adsorbed flat
294
Y pF.cm-*
400
2oc
0 - 0.3
-0.4
- 0.5
-0.6
E/V
Fig. 4. The effect of frequency on the phase-sensitive ac polarograms of 0.5 mM methylviologen + aqueous 1 M potassium fluoride. (1,2) Out-of-phase, (1’,2’) in-phase admittance components. Frequency: (1) 6.4, (2) 6407 Hz. Potentials vs. saturated AgCl electrode.
on mercury, as found by the occupied surface area calculated from the electrocapillary measurements [5], does not give the usual adsorption picture on admittance curves either. Instead of lowering the capacitance in the potential region of adsorption it causes it to increase in the shape of a broad maximum at a potential more positive by almost 0.2 V than the redox potential [6-91. A similar maximum appears on the in-phase admittance component. These maxima become relatively more prominent than others on the curves when the frequency of the ac sine wave signal is increased by one or two orders of magnitude (Fig. 4). They were interpreted [5,9] as due to surface electron transfer, i.e. electron transfer to the methylviologen dication engaged in n-interaction with the electrode. A similar maximum appears on the capacitance curves of dibenzylviologen [6], but only at low concentrations of the substance, when the whole dication can adsorb flat on the electrode surface and its reducible dipyridinium part undergoes m-interaction with the metal. As shown by analysis of the charge transfer rate of reduction of benzylviologen [6,7], at higher concentrations of the cation in the adsorbed state
295
Fig. 5. The effect of frequency on the phase-sensitive ac polarograms of 0.5 mM l,l’-ethylene-2,2’-bipyridinium bromide+ aqueous 1 M potassium fluoride. (1,2) out-of-phase, (1’,2’) in-phase admittance components. Frequency; (1) 6.4, (2) 642 Hz. Potentials vs. saturated AgCl electrode.
the a-interaction with the electrode weakens on account of the interaction of the two terminal phenyl groups; the distance between the two pyridinium rings and the electrode plane increases so that the m-interaction and the surface electron transfer with the electroactive center of the cation cannot take place any longer. The occurrence of an analogous characteristic broad maximum on the admittance curves of the l,l’-ethylene-2,2’-bipyridinium dication indicates that this planar species in its adsorbed state can also enter into close contact with the electrode surface and share with it its a-electrons (Fig. 5). When an adsorbed species is in r-interaction with the electrode, admittance measurements are not sufficient by themselves: the absence of capacitance change would indicate no adsorption and the appearence of a new peak could lead to erroneous conclusions; in such a case information from other electrochemical methods must be used. On the basis of the quoted and the present experimental evidence we can suggest that the 4,4’-bipyridine dications adsorb flat with the aromatic rings sharing their n-electrons with the surface in a close contact. The 2,2’-bipyridine dication behaves so only when an ethylene bridge between the nitrogen atoms makes the whole structure planar. Neutral molecules of the bipyridines or their radical cations, formed in the faradaic process, presumably undergo a change of solvation and polarization at the interface and acquire a non-planar orientation in the adsorbed state.
296 REFERENCES 1 2 3 4 5 6 7 8 9
M. Heyrovskp and L. Novotn$, Collect. Czech. Chem. Commun., 52 (1987) 54. L.A. Ashton, J.I. Bullock and P.W.G. Simpson, J. Chem. Sot. Faraday Trans. 1,78 (1982) 1961. B.B. Damaskin, A.A. SurviIa, S. Ya. Vasina and A.I. Fedorova, Elektrokhimiya, 3 (1967) 825. E.A. Mambetkaziev, A.M. Shaldybaeva, V.N. Statsiuk and S.I. Zhdanov, Elektrokhimiya, 11 (1975) 1750. M. Heyrovsky and L. Novotnjr, Collect. Czech. Chem. Commun., 52 (1987) 1097. L. PospiXil, J. Ktita and J. VoIke, J. Electroanal. Chem., 58 (1975) 217. L. PospBil and J. K&a, J. Electroanal. Chem., 90 (1978) 231. V.N. Grachev, S.I. Zhdanov and G.S. Supin, Elektrokhimiya, 14 (1978) 1353. K. Kobayashi, F. Fuji&i, T. Yoshimine and K. Niki, Bull. Chem. Sot. Jpn., 59 (1986) 3715.
J. Electroanal. Chem., 255 (1988) 291-296 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Short communication
The T-interaction of bipyridinium ions with the electrode surface and its effect on the electrode admittance M. Heyrovskjr and L. Pospigil TheJ. Heyrovsk$ Institute of Physical Chemistry and Electrochemistry Dolejzkova 3, 182 23 Prague 8 (Czechoslovakia) (Received
Czechoslovak
Academy
of Sciences,
17 June 1988; in revised form 14 July 1988)
In our studies on the electroreduction of bipyridine derivatives we found that adsorption of these substances as well as of their reduced forms plays an important role in the electrode processes. While gathering results of various electrochemical methods we realized that depending solely on the measurements of electrode admittance can become misleading if the substance under study, when adsorbed, enters into a-interaction with the electrode surface. By electrocapillary measurements it had been demonstrated [l] that 4,4’-bipyridine is adsorbed on mercury both in neutral and in acid solutions. Figures 1 and 2 show the low frequency phase-sensitive ac polarograms of 4,4’-bipyridine in 0.1 M sodium sulphate and in 0.1 M sulphuric acid, i.e., in solutions in which the electrocapillarity had been measured. The curves were recorded with the frequency of the sine wave at 64 Hz and using automatic iR drop compensation via the positive feed-back loop of the potentiostat. At a 100 times higher frequency the qualitative picture of the electrode admittance in the two solutions remains unchanged. In order to make sure that adsorption equilibrium had been reached we used both the DME (with drop time 3.1 s) and a stationary mercury drop. Also, the capacity vs. time curves were recorded over a period of 1000 s after the electrode formation. At the DME in the neutral solution, equilibrium was attained within the drop time only at bipyridine concentrations of 0.16 mM or higher. In the acid solution the results with the dropping and the stationary electrodes coincided from the lowest concentrations of 4,4’-bipyridine. The electroreduction of 4,4’-bipyridine takes place at potentials more negative than about - 0.6 V in the acid and about - 0.9 V in the neutral solutions. With the neutral solution (Fig. 1) we observe the typical case of adsorption lowering the double layer capacitance in the region positive of the potential of electroreduction. In the acid medium (Fig. 2), on the other hand, no depression of capacitance occurs with respect to the pure supporting electrolyte in the non-faradaic region; instead, 0022-0728/88/$03.50
0 1988 Elsevier Sequoia
S.A.
-430 a’ dyn.cm-l
-
0
- 0.2
I
-0.4
I
- 0.6
I
I
- 0.6
- 1.0
400
I
-1.2
E/V
Fig. 1. Comparison of capacitance () and electrocapillary (- - -) curves of 4,4’-bipyridine in aqueous 0.1 M sodium sulphate. Concentrations of 4,4’-bipyridine were: (1) 0, (2) 5, (3) 10, (4) 20, (5) 40, (6) 80, (7) 160 PM; (a) 0, (b) 5, (c) 20 pM. The ac frequency was 64 Hz. Electrocapillarity was measured with drop times of 56-60 s [l]. Potentials vs. saturated AgCl electrode. 0.5Y” pFcm-
5
420
0 .4-
01.1L
I
- 0.2
I
-0.4
I
-0.6
-0.6
E/V
Fig. 2. Comparison of capacitance ( -) and electrocapillary (- - -) curves of 4,4’-bipyridine in aqueous 0.1 M sulphuric acid. Concentrations of 4,4’-bipyridine were: (1) 0, (2) 5, (3) 10, (4) 20, (5) 40, (6) 80, (7) 160 pM, (a) 0, (b) 20, (c) 100 pM. The ac frequency was 64 Hr. Electrocapillarity was measured with drop times of 56-60 s [l]. Potentials vs. saturated AgCl electrode.
293
Y yFcm0.:
0.
Fig. 3. Potential dependence of the out-of-phase (a) and in-phase (b) admittance components of aqueous 0.1 M sulphuric acid+ 2,2’-bipyridine at concentrations: (1) 0, (2) 5, (3) 10, (4) 20 PM. HMDE, scan rate 10 mV/s; ac frequency 1606 Hz; amplitude 5 mV. Potentials vs. saturated AgCl electrode.
the double layer capacitance increases with increasing additions of 4,4’-bipyridine. This result is similar to what can be seen [l] on dc polarograms or on equilibrium charge-potential curves. In acid solutions of pH < 2.7 the 4,4’-bipyridine molecules become doubly protonated [2] and are adsorbed reversibly in a position parallel to the electrode surface with the aromatic rings in r-interaction with the metal [l]. It has been known long since [3] that the adsorbed pyridinium cation in m-interaction with the electrode surface does not change the capacity of the electrode; the same could be expected to apply to the 4,4’-bipyridinium dication. However, an increase of the capacitance is observed with this species, which is presumably due to a surface electron transfer process, analogous to the case of methyl viologen mentioned below. The 2,2’-bipyridine behaves differently from its 4,4’-isomer: it displays adsorption on the admittance curves in neutral [4] as well as in acid solutions (Fig. 3). Its molecule has the tendency to form chelates with various cations in the solution, including protons; as an unsymmetrically charged entity the chelate cannot stretch out flat on the electrode surface and hence cannot share its rr-electrons with the surface. The cation of methylviologen, or l,l’-dimethyl-4,4’-bipyridinium, adsorbed flat
294
Y pF.cm-*
400
2oc
0 - 0.3
-0.4
- 0.5
-0.6
E/V
Fig. 4. The effect of frequency on the phase-sensitive ac polarograms of 0.5 mM methylviologen + aqueous 1 M potassium fluoride. (1,2) Out-of-phase, (1’,2’) in-phase admittance components. Frequency: (1) 6.4, (2) 6407 Hz. Potentials vs. saturated AgCl electrode.
on mercury, as found by the occupied surface area calculated from the electrocapillary measurements [5], does not give the usual adsorption picture on admittance curves either. Instead of lowering the capacitance in the potential region of adsorption it causes it to increase in the shape of a broad maximum at a potential more positive by almost 0.2 V than the redox potential [6-91. A similar maximum appears on the in-phase admittance component. These maxima become relatively more prominent than others on the curves when the frequency of the ac sine wave signal is increased by one or two orders of magnitude (Fig. 4). They were interpreted [5,9] as due to surface electron transfer, i.e. electron transfer to the methylviologen dication engaged in n-interaction with the electrode. A similar maximum appears on the capacitance curves of dibenzylviologen [6], but only at low concentrations of the substance, when the whole dication can adsorb flat on the electrode surface and its reducible dipyridinium part undergoes m-interaction with the metal. As shown by analysis of the charge transfer rate of reduction of benzylviologen [6,7], at higher concentrations of the cation in the adsorbed state
295
Fig. 5. The effect of frequency on the phase-sensitive ac polarograms of 0.5 mM l,l’-ethylene-2,2’-bipyridinium bromide+ aqueous 1 M potassium fluoride. (1,2) out-of-phase, (1’,2’) in-phase admittance components. Frequency; (1) 6.4, (2) 642 Hz. Potentials vs. saturated AgCl electrode.
the a-interaction with the electrode weakens on account of the interaction of the two terminal phenyl groups; the distance between the two pyridinium rings and the electrode plane increases so that the m-interaction and the surface electron transfer with the electroactive center of the cation cannot take place any longer. The occurrence of an analogous characteristic broad maximum on the admittance curves of the l,l’-ethylene-2,2’-bipyridinium dication indicates that this planar species in its adsorbed state can also enter into close contact with the electrode surface and share with it its a-electrons (Fig. 5). When an adsorbed species is in r-interaction with the electrode, admittance measurements are not sufficient by themselves: the absence of capacitance change would indicate no adsorption and the appearence of a new peak could lead to erroneous conclusions; in such a case information from other electrochemical methods must be used. On the basis of the quoted and the present experimental evidence we can suggest that the 4,4’-bipyridine dications adsorb flat with the aromatic rings sharing their n-electrons with the surface in a close contact. The 2,2’-bipyridine dication behaves so only when an ethylene bridge between the nitrogen atoms makes the whole structure planar. Neutral molecules of the bipyridines or their radical cations, formed in the faradaic process, presumably undergo a change of solvation and polarization at the interface and acquire a non-planar orientation in the adsorbed state.
296 REFERENCES 1 2 3 4 5 6 7 8 9
M. Heyrovskp and L. Novotn$, Collect. Czech. Chem. Commun., 52 (1987) 54. L.A. Ashton, J.I. Bullock and P.W.G. Simpson, J. Chem. Sot. Faraday Trans. 1,78 (1982) 1961. B.B. Damaskin, A.A. SurviIa, S. Ya. Vasina and A.I. Fedorova, Elektrokhimiya, 3 (1967) 825. E.A. Mambetkaziev, A.M. Shaldybaeva, V.N. Statsiuk and S.I. Zhdanov, Elektrokhimiya, 11 (1975) 1750. M. Heyrovsky and L. Novotnjr, Collect. Czech. Chem. Commun., 52 (1987) 1097. L. PospiXil, J. Ktita and J. VoIke, J. Electroanal. Chem., 58 (1975) 217. L. PospBil and J. K&a, J. Electroanal. Chem., 90 (1978) 231. V.N. Grachev, S.I. Zhdanov and G.S. Supin, Elektrokhimiya, 14 (1978) 1353. K. Kobayashi, F. Fuji&i, T. Yoshimine and K. Niki, Bull. Chem. Sot. Jpn., 59 (1986) 3715.