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J. Phys. Chem. B 1999, 103, 304-308
Aqueous Electrochemistry of a C60-Bearing Artificial Lipid Bilayer Membrane Film Immobilized on an Electrode Surface: Thermodynamics for the Binding of Tetraalkylammonium Ion to the Fullerene Anion Takashi Nakanishi, Hiroto Murakami, Takamasa Sagara,* and Naotoshi Nakashima*,† Department of Applied Chemistry, Faculty of Engineering, Nagasaki UniVersity, Bunkyo-cho, Nagasaki 852-8521, Japan ReceiVed: August 6, 1998; In Final Form: NoVember 9, 1998
Thermodynamics for the binding of electrolyte cations, in particular, tetraalkylammonium ions, to the electrogenerated radical anion and the dianion of the fullerene moieties in a molecular-bilayer membrane film of C60-bearing artificial lipid, 1, cast on an electrode surface was described. The theoretical treatment predicts a linear relationship between the half-wave potential (E1/2) for the electrode reaction and electrolyte concentration in the film (c) at higher concentrations and enables us to obtain the number of bound cation and binding constant, respectively, from the slope and intercept of the E1/2-ln c plot. We measured differential pulse voltammograms for a cast film of 1 on a basal plane graphite electrode in aqueous solution containing tetraethylammonium chloride (or tetra-n-butylammonium chloride) + KCl ) 0. 5 M and found that E1/2-ln cs plots for the modified electrodes are linear in a concentration range of ca. 0.002-0.14 M, where cs is the concentration of ammonium electrolytes in the solution phase. The analysis of the plot reveals that the fullerene radical anion forms a 1:1 complex with both tetraethylammonium and tetra-n-butylammonium cations and that the fullerene dianion forms a 1:2 complex with tetra-n-butylammonium cations. The binding constants K1 for the binding of tetraethylammonium and tetra-n-butylammonium cations to radical anions of the fullerene moieties was determined to be 7.93 × 103 and 1.71 × 108 M-1, respectively, provided that the partition coefficients (c/cs) of the tetraalkylammonium ions are unity.
Introduction
CHART 1
The chemistry and physics of fullerenes and fullerene films have been given increasing attention by many research groups.1 Our interest is focused on the combination of fullerene chemistry and the chemistry of lipid bilayer membranes.2 We have recently reported that the fullerene lipid 1 (see Chart 1) forms an organized mulibilayer membrane film which exhibits a main phase transition as well as a subphase transition that regulates the spectral properties of the fullerene moieties.3 The electrochemistry of fullerenes in solution phase and their films on electrode surfaces has been extensively studied and is currently the subject of intense research focus.4 Recently, Bard and coworkers4 summarized half-wave potentials (E1/2) of fullerenes in various media. Supporting electrolyte cations, solvents, and temperature affect the reduction potentials of fullerenes. Kadish and co-workers5 investigated the correlation between E1/2 values of the first three consecutive one-electron-transfer processes of C60 and the carbon chain length of tetraalkylammonium perchlorate supporting electrolytes in four different organic solvents. They found a monotonic positive shift of E1/2 for the first and second reductions with increasing the chain length. Fawcett and co-workers6 described thermodynamics of the reduction of C60 in benzonitrile containing tetraalkylammonium salts. They reported that C60•- and C602- are associated with two or more tetraalkylammonium cations. Kadish et al.7 investigated quantitatively the ion pairing of tetraalkylammonium cations with C60n- (n ) 1-4) in four different aprotic organic solvents by * To whom correspondence should be addressed. † E-mail:
[email protected].
cyclic voltammetry at a 10 mm diameter Pt disk microelectrode. The effect of cations, including Li+, Na+, K+, Rb+, Cs+ , Ca+, and Ba2+, on the reduction potential for a cast film of C60 on an electrode surface in an aqueous solution was examined by Lamberts and co-workers.8 They reported that the reduction peaks shifted to less-negative potentials with the decrease of the hydration energy of these cations. We have recently found that the formal potential of C60 embedded in cast films of ammonium lipids exhibits strong lipid chemical structure dependence.9 There is a tendency for the formal potential to be shifted to less negative in the presence of ammonium cation sites in the film. This may be due to strong Coulomb-coupled binding of the reduction product anions of C60 with ammonium cations. Besides the fullerene species, Coulomb interactions between radical anions and cations have been reported.10 In this paper, we describe the result of quantitative thermodynamic approach to the binding of electrolyte cations, in particular, tetraalkylammonium ions, to the electrogenerated radical anion and the dianion of the C60 moieties in a 1 film cast on an electrode surface. This is the first attempt to measure the binding of C60 anions with cations in aqueous media. The
10.1021/jp9833101 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998
C60-Bearing Artificial Lipid
J. Phys. Chem. B, Vol. 103, No. 2, 1999 305
cast film of 1 may provide an excellent system to the quantitative study of the binding, since (i) the electrochemical activity of C60 moiety in the film is quite stable in aqueous solutions, (ii) electrolytes would be permeable into the film in nature, and (iii) the total amount of the C60 moiety can be kept much less than the amount of cation of interest in the electrochemical cell throughout a wide concentration range of cation. Additional advantages of the present approach over the measurements in homogeneous systems6,7 may be that we can avoid the difficulty to dissolve fullerenes in water and thus a wide range of fullerene/ anion concentration ratio can be readily achieved.
where c is the concentration of the tetraalkylammonium ion in the film. Therefore, eq 5 is rewritten as
Eeq ) E10′ +
When Eeq ) E1/2,1, [C60] ) [C60•-] + [C60•- ... pX+], where E1/2,1 is the half-wave potential of the redox reaction 1 in the presence of X+ under an electrochemically reversible condition. Using this relationship, one can rewrite eq 7 as
Model and Theory In the absence of cation binding to the reduced forms of C60, the two consecutive one-electron-transfer reactions of C60 can be written as
C60 + e- a C60•-
E10′
C60•-
-
+e a
E20′
C602-
(2)
where E10′ and E20′ are the standard redox potentials for the two elemental redox processes. We are concerned with the binding of the tetraalkylammonium ion to the reduced forms of C60. The binding equilibrium can be expressed as
C60•- + pX+ a C60•- ... pX+
K1
K2
[C60] RT ln F [C •-]
(8)
When the concentration of X+ is so high that K1cp . 1, eq 8 can be approximated as
E1/2,1 ) E10′ +
RT pRT ln K1 + ln c F F
(9)
For the C60•-/C602- couple, the Nernst equation corresponding to eq 2 is given as
Eeq ) E20′ +
•RT [C60 ] ln F [C 2-]
(10)
60
The binding equilibrium for C602- given by eq 4 is expressed as
[C602- ... qX+] ) K2[C602-]cq
(4)
When Eeq ) E1/2,2, [C60•-] + [C60•- ... pX+] ) [C602-] + [C602- ... qX+], where E1/2,2 is the half-wave potential of redox reaction 2 in the presence of X+ under an electrochemically reversible condition. Therefore, using eqs 6, 10, and 11, one obtains
where X+ stands for a tetraalkylammonium ion, C60•- ... pX+ is the complex (ion pair) of C60•- and X+ with a number of bound X+ being p and the binding equilibrium constant K1, and C602- ... qX+ is the complex (ion pair) of C602- and X+ with a number of bound X+ being q and the binding equilibrium constant K2. The Nernst equation for C60/C60•- redox couple is written as
Eeq ) E10′ +
RT ln(1 + K1cp) F
(3)
and
C602- + qX+ a C602- ... qX+
E1/2,1 ) E10′ +
(1)
and
[C60] RT RT pRT ln K1 + ln c + ln •F F F [C60 ... pX+] (7)
(5)
60
E1/2,2 )
E20′
q RT 1 + K2c + ln F 1 + K cp
(11)
(12)
1
When the concentration of X+ is so high that K1cp . 1 and K2cq . 1, eq 12 can be approximated as
E1/2,2 ) E20′ +
RT K2 (q - p)RT + ln ln c F K1 F
(13)
where Eeq is the equilibrium potential, R is the gas constant, T is the temperature, and F is the Faraday constant. [C60] and [C60•-] represent the concentrations of the species in the brackets. Note that these concentrations are of the quantity expressing the concentration in the film of compound 1 in the present paper. Strictly speaking, if the film exhibits a reversible thin-layer electrochemistry, the concentration term should be substituted by surface excess. However, in the expressions of the equilibrium and half-wave potentials, these terms always appear in the equations in the form of ratio. Therefore, it should never affect the final form describing the half-wave potentials and the binding constants whether one use concentration or surface excess. The binding equilibrium for C60•- given by eq 3 is expressed as
The relationship between the half-wave potentials and c expressed by eqs 8 and 12 is schematically depicted in Figure 1. The number of bound cation can be obtained from the slope of the plot of E1/2 versus ln c. Binding constant can be calculated from the difference between E1/2 at ln c ) 0 and E°′. Experimentally, the variables necessary to have the E1/2-ln c plots can be obtained as follows. The half-wave potentials can be obtained by voltammetric measurements under electrochemically reversible conditions. They can also be obtained under electrochemically quasi-reversible conditions provided that the transfer-coefficient of the charge-transfer process is nearly 0.5. The way of the estimation of E°′ will be described in the section of results. The value of c is given as
[C60•- ... pX+] ) K1[C60•-]cp
where Kpar is the partition coefficient and cs is the concentration of the tetraalkylammonium ion in solution phase. Rigorously
(6)
c ) Kparcs
(14)
306 J. Phys. Chem. B, Vol. 103, No. 2, 1999
Nakanishi et al.
Figure 2. DPVs (25 mV/s scan rate, 50 mV pulse amplitude, 50 ms pulse width, 200 ms pulse interval) for 1-modified BPG electrodes in aqueous solution containing (a) 0.5 M n-Bu4N+Cl- or (b) 0.5 M Et4N+Cl- at 55 °C.
Figure 1. Theoretical prediction of the plots of the half-wave potentials versus ln c on a basis of eqs 8 and 12.
speaking, the value of Kpar may be a function of cs as well as the electrode potential, since the bound species is charged. Nevertheless, we assume that Kpar is independent of electrode potential and cs in the present paper as an approximation. The experimental examination of the rationale behind this assumption will be also described in the section of results. Experimental Section Compound 1 was available from our previous study.3 Tetran-butylammonium chloride (n-Bu4N+Cl-) and tetraethylammonium chloride (Et4N+Cl-) from Tokyo Kasei Co. were used as received. Modified electrodes were prepared as follows. Twenty microliters of 1.0 mM 1 in benzene were placed on a basal plane graphite (BPG) disk electrode9,11 (geometric area, 0.25 cm2) and then allowed to air dry. The modified electrode was then annealed in water at 50 °C for 30 min. The electrochemistry for the modified electrodes was examined using differential pulse voltammetry in the presence of given concentration of electrolytes by employing BAS-100BW electrochemical analyzer (Bioanalytical Systems). For the electrolyte solutions, various concentrations of KCl aqueous solutions as well as mixed aqueous solutions of n-Bu4N+Cl- + KCl and Et4N+Cl+ KCl were used. For the mixed solutions, the total concentration of the two 1:1-electrolytes was adjusted to be 0.5 M for the sake of achieving approximately a constant ionic strength. This rather high level of constant salt concentration may also be important to minimize the dependence of Kpar on cs by compressing the Gouy layer. A saturated calomel electrode (SCE) and a Pt plate electrode serve as the reference and the counter electrode, respectively. Temperature was maintained at 55 ( 0.1 °C. Results and Discussion Figure 2 shows differential pulse voltammograms (DPVs) for cast films of 1 on BPG electrodes in aqueous solution containing 0.5 M n-Bu4N+Cl- or Et4N+Cl-. The potential sweep range was limited to -1.0 V, since residual current due to the hydrogen evolution was obvious at more negative potentials. In the n-Bu4N+Cl- solution, the two reduction waves leading to the radical monoanion of the fullerene moiety in 1 and subsequently to the dianion are evident. In the Et4N+Clsolution, only the wave due to C60/C60•- couple is observed. The peak position in the n-Bu4N+Cl- solution is remarkably less negative than that in the Et4N+Cl- solution.
Figure 3. DPVs for a 1-modified electrode in aqueous solution containing (a) 500 mM n-Bu4N+Cl-, (b) 300 mM n-Bu4N+Cl- + 200 mM KCl, (c) 100 mM n-Bu4N+Cl- + 400 mM KCl, (d) 50 mM n-Bu4N+Cl- + 450 mM KCl, (e) 10 mM n-Bu4N+Cl- + 490 mM KCl, or (f) 2 mM n-Bu4N+Cl- + 498 mM KCl. Other experimental conditions are the same as in Figure 2.
A series of DPVs of the modified electrodes at various concentrations of n-Bu4N+Cl- in the aqueous solution are shown in Figure 3. With the increase of n-Bu4N+Cl- concentration, the peak potentials monotonically shifted to less-negative potentials. The total widths of the DPV peaks at the half-height (∆W1/2) are greater than the value expected for the reversible redox reaction, indicating that the DPV response is quasireversible. However, ∆W1/2 is independent of the concentration of X+. Therefore, the concentration dependent influence of the kinetic factors on the DPV curves, if any, is unnecessary to be taken into account. The DPV peak shape is symmetrical with respect to the peak potential. This indicates that the transfer coefficient is not largely different from 0.5 as well as that Kpar does not sharply depend on the electrode potential, since otherwise the DPV curve should have been distorted. The difference between the peak potentials of forward and reversal potential scan in DPV is nearly equal to the pulse amplitude of the DPV measurement (∆E). This fact confirms that E1/2 can be equated at a good approximation to Epeak + 0.5∆E for cathodic scan. DPV measurements were also conducted in Et4N+Cl- + KCl solutions and KCl solutions, and the values of E1/2 obtained from DPVs were plotted versus logarithm of supporting electrolyte concentration in Figure 4. The results presented in Figure 4 were analyzed and interpreted in the light of the model described in the previous section. It is evident that the E1/2-ln cs plots for the modified electrodes give linear relation at concentrations lower than ca. ln cs ) -2. This fact indicates the validity of our thermodynamic treatment. Such a linear relationship is consistent with our prediction represented
C60-Bearing Artificial Lipid
J. Phys. Chem. B, Vol. 103, No. 2, 1999 307 may include an attractive electrostatic interaction or a hydrophobic interaction between the fullerene anion moieties and the alkyl chains of the cation, i.e., rigorously speaking, a gain of hydration energy. The contribution from the π-σ interaction between the fullerene anion moieties and the alkyl chains might be possible. Although it is unwarranted to discuss the quantitative extent of the contribution of each of these interactions from the present results, we can at least conclude that the binding due to the electrostatic interaction is enhanced by increasing the alkyl chain length, since the value of ∆G1 increases significantly with increasing the chain length. Concluding Remarks
Figure 4. Plots of E1/2 of DPVs for 1-modified electrodes in aqueous solution as a function of ln cs. Open circles and squares denote the data for the first reduction waves in the solutions containing n-Bu4N+Cl- + KCl ) 0.5 M and Et4N+Cl- + KCl ) 0.5 M, respectively, with cs being the concentration of tetraalkylammonium cations. Lozenges denote the data for the first reduction waves in KCl with cs ) [K+]. Closed circles are the data for the second reduction waves in solution containing n-Bu4N+Cl- + KCl ) 0.5 M with cs ) [n-Bu4N+Cl-].
TABLE 1: Parameters for the Binding of the Reduced Forms of 1 and Tetraalkylammonium Ions at the Reduction Processes of 1 Films on BPG at 328 K electrolyte cation N+
Et4 n-Bu4N+
p
q
K1/M-1
∆G1,328K/ kJ mol-1
1.05 0.97
2.11
7.93 × 103 1.71 × 108
-25.7 -50.6
by eqs 9 and 13. The p and q values evaluated from the slopes (Table 1) are near to integers. This fact together with the linearity of the plot connotes that the dependence of Kpar on cs is actually insignificant. The result reveals that the fullerene radical anion forms 1:1 complex with both Et4N+ and n-Bu4N+ and the fullerene dianion forms 1:2 complex with n-Bu4N+. We need to know E10′ and E20′ values to determine K1 and K2 values, respectively. In this study, the pseudo E10′ value, -904.4 mV, which is obtained by the extrapolation of E1/2 to ln cs ) 0 for the modified electrode in aqueous solution containing solely KCl, was used for the determination of K1.12 Unfortunately, the value of K2 was impossible to obtain in the present work, since the second reduction peak was beyond the electrochemical window of the BPG electrode in the aqueous solution containing solely KCl. The binding constant K1 for the binding of radical anion of the fullerene moiety to Et4N+ or n-Bu4N+ was determined to be 7.93 × 103 and 1.71 × 108 M-1, respectively, proVided that Kpar is unity. This is the first example for the estimation of binding constants between fullerene radical anions and supporting electrolyte cations using the electrontransfer reaction of the fullerene at electrode systems. A great difference in K1 values between the two tetraalkylammonium electrolyte cations with the different carbon chain length is to be emphasized. The Gibbs free energy change of the binding reactions 3 and 4 are, respectively, ∆G1 ) -RT ln K1 and ∆G2 ) -RT ln K2. In this study, ∆G1 values for Et4N+ or n-Bu4N+ were calculated to be -25.7 and -50.6 kJ/mol, respectively. The Gibbs free energy change may involve the contributions from (i) the displacement of water molecules from the fullerene surface, (ii) the interaction between fullerene and tetraalkylammonium ion, and (iii) the relaxation of the polarization of the medium by the formation of ion pair. The interaction leading to the binding
We have described the theoretical treatment (thermodynamics) for the binding of electrolyte cations (tetraalkylammonium) to the radical anion and the dianion of the fullerene moieties in a 1 cast film on a BPG electrode in aqueous solution. We have determined the number of cations being involved in the reduction processes and have estimated the binding constants. Finally, we would like to emphasize that the thermodynamic treatment enables us to conduct quantitative analysis for the interaction between (multi)anions of fullerenes and related compounds in film states and cations in the electroreduction processes. Acknowledgment. This work was supported, in part, by the Grant-in-Aids from the Ministry of Education, Science, Sports, and Culture, Japan (for N.N.). References and Notes (1) (a) Hammond, G. S., Kuck, V. J., Eds. Fullerenes; ACS Symposium Series 481; American Chemical Society: Washington, DC, 1992. (b) Billups, W. E, Ciufolini, M. A., Eds. Buckminsterfullerenes; VCH Publishers: New York, 1993. (c) Prassides, K., Ed. Physics and Chemistry of the Fullerenes; Kluwer Academic Publishers: Boston, 1994. (d) Kadish, K. M.; Ruoff, R. S. Eds. Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials: The Electrochemical Society: Pennington, 1997. (e) Kroto, H. W. The Fullerenes; New Horizons for the Chemistry, Physics and Astrophysics of Carbon; Cambridge University Press: Cambridge, 1997. (2) (a) Hungerbu¨hler, H.; Guldi, D. M.; Asmus, K.-D. J. Am. Chem. Soc. 1993, 115, 3386-3387. (b) Bensasson, R. V.; Garaud, J.-L.; Leach, S.; Miquel, G.; Seta, P. Chem. Phys. Lett. 1993, 210, 141-148. (c) Garaud, J. L.; Janot, J. M.; Miquel, G.; Seta, P. J. Membr. Sci. 1994, 91, 259-264. (d) Niu, S.; Mauzerall, D. J. Am. Chem. Soc. 1996, 118, 5791-5795. (e) Janot, J. M.; Seta, P.; Bensasson, R. V.; Leach, S. Synthetic Met. 1996, 77, 103-106. (f)Tien, H. T.; Wang, L.-G.; Wang, X.; Ottova, A. L. Bioelectochem. Bioenerg. 1997, 42, 161-167. (g) Hetzer, M.; Bayerl, S.; Camps, X.; Vostrowsky, O.; Hirch, A.; Bayerl, T. M. AdV. Mater. 1997, 9, 913917. (3) Murakami, H.; Watanabe, Y.; Nakashima, N. J. Am. Chem. Soc. 1996, 118, 4484-4485. (4) Chlistunoff, J.; Cliffel, D.; Bard, A. J. In Handbook of Organic ConductiVe Molecules and Polymers; Nalwa, H. S. Ed.; John Wiley & Sons: Chichester, 1997; Vol. 1; pp 333-412. (5) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1992, 96, 7137-7145. (6) Fawcett, W. R.; Opallo, M.; Fedurco, M.; Lee, J. W. J. Am. Chem. Soc. 1993, 115, 196-200. (7) S.-Guillous, B.; Kutner, W.; Jones, M. T.; Kadish, K. M. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M.; Ruoff, R. S. Eds.; The Electrochemical Society: Pennington, 1994; pp 1020-1029. (8) Szucs, A.; Loix, A.; Nagy, J. B.; Lamberts, L. J. Electroanal. Chem. 1996, 402, 137-148. (9) (a) Nakashima, N.; Kuriyama,T.; Tokunaga, T.; Murakami, H.; Sagara, T. Chem. Lett. 1998, 663-664. (b) Nakashima, N.; Nonaka, Y.; Nakanishi, T.; Sagara, T.; Murakami, H. J. Phys. Chem. B 1998, 102, 73287330. (c) Nakashima, N.; Tokunaga, T.; Nakanishi, T.; Murakami, H.; Sagara, T. Angew. Chem., Int. Ed. Engl. 1998, 37, 2671-2673.
308 J. Phys. Chem. B, Vol. 103, No. 2, 1999 (10) (a) Bock, H.; Ha¨nel, P. Z. Naturforsch B. 1992 47, 288-300. (b) Sa¨uberlich, J.; Brede, O.; Beckert, D. Acta Chem. Scand. 1997, 51, 602609. (11) Scotch tape was used to expose a fresh basal plane. (12) The E1/2 values in KCl solution also depended on the KCl concentration. This fact may indicate that K+ also binds to C60•-, i.e., K+ is also the potential-determining ion. Therefore, we use the pseudo value. Note that the experimental results in the mixed solutions tell us that the binding of K+ is negligibly weak in comparison to those of tetraalkylammonium ions. Assuming that K+ and a tetraalkylammonium ion compete to form a 1:1 complex with C60•-, the half-wave potential should be written as
Nakanishi et al.
E1/2,1 ) E10′ +
RT ln{1 + (K1 - Kr)c + Kr(c + [K+])} F
where Kr is the binding constant between C60•- and K+. In the present experiment, only the second term in the logarithm is the function of c. If the binding constant for K+ was comparable to that for the tetraalkylammonium ion, we should have observed much smaller difference of E1/2 values between KCl solution and tetraalkylammonium salt solution. If the binding of K+ was stronger than tetraalkylammonium ion, the E1/2 value should have shifted to more negative with the increase in c. The results in Figure 4, therefore, indicate that Kr is in fact much smaller than K1.
