3762
Langmuir 1998, 14, 3762-3766
Effect of the Micellar Electric Field on Electron-Transfer Processes. A Study of the Metal-to-Metal Charge Transfer within the Binuclear Complex Pentaammineruthenium(III)-(µ-Cyano)pentacyanoruthenium(II) in Micellar Solutions of Sodium Dodecylsulfate (SDS) and Hexadecyltrimethylammonium Chloride (CTACl) Francisco Sa´nchez, Manuel Lo´pez-Lo´pez, and Pilar Pe´rez-Tejeda* Department of Physical Chemistry, Faculty of Chemistry, University of Sevilla, C/ Profesor Garcia Gonza´ lez s/n, 41012 Sevilla, Spain Received January 5, 1998 The metal-to-metal charge-transfer band within the binuclear complex pentaammineruthenium(III)(µ-cyano)pentacyanoruthenium(II), a Robin-Day class II complex, was studied in solutions containing sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (CTACl). The redox potentials of both ruthenium centers were also measured. From the optical and electrochemical (redox) data, both the reorganization free energy and the reaction free energy corresponding to the electron transfer were obtained. It was found that in solutions containing the anionic surfactant the relevant parameters for the electron-transfer process, that is, the free energies of reorganization and reaction, are quite similar to those corresponding to aqueous electrolyte solutions. In the case of the solutions containing the cationic surfactant, these parameters are quite different from those of the aqueous electrolyte solutions. The results are interpreted taking into account the effects of the micellar electric field on both the binuclear complex and the medium (solvent) surrounding it. These effects depend on the (average) position of the binuclear complex in relation to the micelle: in the case of cationic micelles the binuclear complex penetrates the Stern layer. When the micelles are anionic, the binuclear complex is, on the average, outside this layer, but a residual influence of the micellar field seems to be operative.
Introduction The effects of electric fields on reactivity have been an area of growing interest in the past few years.1 In relation to electron-transfer processes, the field can affect all the relevant parameters that modulate the rate of reaction. So, the solvent reorganization energy depends on the dielectric characteristics of the surrounding medium,2 and these characteristics can be modified by the field through solvent saturation effects.3 On the other hand, the free energy of the reaction is dependent on the field, because the free energies of the reactant and product states also depend on the dielectric constant of the medium and because of the interaction of the dipole moment of the electronic transition (optical or thermal) with the field.4 Moreover, this field can change the adiabaticity of the reaction through the polarization of the orbitals of the reactants involved in the electron transfer:4 this polarization can produce an increase (or decrease) of the overlapping between the intervening orbitals of the electron donor and acceptor, which would produce changes in adiabaticity.4 On the other hand, the dynamics of the solvent (and so the preexponential term in the rate constant5) can also be changed by the field. Morever, the diffusion * To whom all correspondence should be directed. (1) See for example: (a) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 2, 1111. (b) Molecular Electronics in Science and Technology; Aviram, A., Ed.; A.I.P. Press: 1992. (c) Franzen, S.; Boxer, S. G. In Electron Transfer in Inorganic, Organic, and Biological Systems; Bolton, J. R., Mataga, N., McLendon, G., Eds.; American Chemical Society: Washington, DC, 1991; Chapter 9. (2) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155. (3) Bo¨ttcher, C. J. F. Theory of Electric Polarization, 2nd ed.; Elsevier Scientific Publishing Company: Amsterdam, 1973; Vol. 1, Chapter 7. (4) Lao, K.; Franzen, S.; Stanley, R. J.; Lambright, D. G.; Boxer, S. G. J. Phys. Chem. 1993, 97, 13165.
coefficients of the intervening species corresponding to the nonhomogeneous state (in the presence of the field) can be quite different from those of the homogeneous state (without the field).6 Indeed, the equilibrium correlations, such as the direct correlation functions, in the presence of a field can also be rather different from those in the absence of a field.6 Finally, it has been suggested that the fluctuation dissipation theorem and other important theorems of statiscal mechanics may no longer be valid in the presence of a strong field.6 From the above-mentioned considerations there is a clear interest in studying the effect of electric fields on electron-transfer reactions, from a theoretical7 as well as an experimental1c point of view. On the other hand this topic is of interest as well in relation to some important questions, such as molecular electronics4 and biological (electron transfer) processes. For example, long-range electron transfer in photosynthetic reaction centers is influenced by the electric field arising from the permanent dipole of the R-helical section of the proteins sourronding it. In this case, the influence of the field on the rate comes from the (favorable) modification of the driving force of the electron transfer.8 However, to produce the effects mentioned, in particular changes in the free energies of reaction and reorganization, the applied electric field must be quite strong, about 107 V/m.7 Such intense fields exist, for example, in the region close to an electrode surface9a and also in the vicinity of the surface of the (charged) micelles.9b (5) Weaver, M. J. Chem. Rev. 1992, 92, 463. (6) Bagchi, B.; Chandra, A. Adv. Chem. Phys. 1991, 80, 1. (7) Farazdel, A.; Dupuis, M.; Clementi, E.; Aviram, A. J. Am. Chem. Soc. 1990, 112, 4206. (8) Galoppini, E.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 2299.
S0743-7463(98)00018-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/10/1998
Metal-to-Metal Charge Transfer
Langmuir, Vol. 14, No. 14, 1998 3763
In this paper the effect of the latter fields on the optical electron-transfer process within the binuclear complex pentaamminerutenium(III)-(µ-cyano)pentacyanoruthenium(II) (RuII-CN-RuIII) is presented. This compound was chosen because it shows a clear metal-to-metal charge transfer (MMCT) band in water10,11 and because it belongs to class II in the Robin-Day classification12 (that is, the interaction between the two metal centers, measured by the degree of ground-state delocalization R2, is lower than 2 × 10-2), which is essential to use of eq 8 below. The selected system also permits the experimental estimation of the free energy change ∆G°′ corresponding to the difference between the free energies of the reactant and (equilibrated) product states of the intervalence transition hν
[(NH3)5RuIII-NC-RuII(CN)5]- 98 [(NH3)5RuII-NC-RuIII(CN)5]- (1) since both ruthenium centers exhibit a reversible behavior from an electrochemical point of view. So, the free energy of reorganization for the metal-to-metal electron-transfer process can be obtained from the reversible electrochemical data of the complex. In this way, the influence of the field on the relevant parameters contributing to the activation free energy of the associated thermal electron-transfer process can be obtained. The results presented in this study correspond to two kinds of micellar solutions: those formed by anionic (SDS) and cationic (CTACl) surfactants. Given that the binuclear complex under study bears a negative charge, it can be supposed that the anionic micelles will interact with it only via long-range (Coulombic) interactions. On the other hand, in the case of cationic micelles, both Coulombic interactions and short-range effects are to be expected. Therefore, a study of both long-range and shortrange electric field effects seems feasible. The latter will come through saturation effects produced by the electric field on the solvent sourronding the binuclear complex. Experimental Section Materials. The mixed valence compound Na[(NH3)5RuIIINC-RuII(CN)5]‚3H2O was prepared and purified according to the method described in ref 10. The visible spectrum shows a MMCT band with an absorption maximum at 683 nm (max ) 2850 ( 50 mol-1 dm3 cm-1) in water. This corresponds to a value of Eop ) hυmax of 175 kJ mol-1. Hexadecyltrimethylammonium chloride (CTACl) was purchased from Fluka as a solution containing approximately 25% in the surfactant. This solution was titrated with AgNO3 and used without further purification. Sodium dodecyl sulfate was from Merck and was recrystallized from ethanol, washed with diethyl ether, and dried in vacuo over P2O5. Throughout the study, double-distilled and deionized water, obtained from a Millipore Mili-Q system, with a resistivity lower than 107 Ω cm, was used. Spectra. The spectra of the binuclear complex in the different surfactant solutions were recorded in a Hitachi 150-20 UV-vis spectrophotometer at 298.2 K. The concentration of this complex in the SDS solutions was always 8.7 × 10-4 mol dm-3, whereas in the CTACl solutions the concentration varied from 2.3 × 10-4 to 3.6 × 10-4 mol dm-3. These concentrations were the maximum possible ones compatible with the stability of the micellar (CTACl) solutions. The concentrations of the SDS and CTACl solutions ranged from 1 × 10-2 to 2 × 10-1 mol dm-3. Neither the molar (9) (a) Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970; Vol. 2. (b) See for example: Grand, D.; Hautecloque, S. J. Phys. Chem. 1990, 94, 837 and references therein. (10) Vogler, A.; Kisslinger, J. J. Am. Chem. Soc. 1982, 104, 2311. (11) Tominaga, K.; Kliner, D. A. V.; Jhonson, A. E.; Levinger, W. E.; Barbara, P. F. J. Chem. Phys. 1993, 98, 1228. (12) Crutchley, R. J. Adv. Inorg. Chem. 1994, 41, 273.
Figure 1. Experimental MMCT band for the binuclear complex [(NH3)5RuIII-NC- RuII(CN)5]- in 0.1 mol dm-3 CTACl solution at 298.2 K. absorption coefficient at the maximum of the band max nor the full-width at half-height ∆υ1/2 showed significant modifications in SDS and CTACl micellar solutions, in relation to the values of these parameters in water (max ) 2850 ( 50 mol-1 dm3 cm-1 and ∆υ1/2 ) 4600 ( 100 cm-1). A representative spectrum is given in Figure 1. Electrochemistry. The redox potentials of the two ruthenium centers in the binuclear complex were obtained by differential-pulse (DPV) and cyclic voltammetry techniques in SDS and CTACl solutions. The former technique was used for the RuII cyanide center, and both DPV and cyclic voltammetry were used for the cationic RuIII ammine center. The equipment, procedure, and electrodes have been previously described.13,14 The concentrations of the mixed valence compound were the same as those in the experiments in which the spectra were obtained. A representative voltammogram is given in Figure 2. The relationship between the peak and half-wave potentials for a reversible system is given by15
Epeak ) E1/2 +
∆E 2
(2)
where ∆E is the voltage amplitude of the pulse (2 mV in our experiments), thus Epeak≈ E 1/2. The estimated uncertainty in redox potentials is about ( 10 mV.
Results Spectra. The binuclear complex studied in this work shows a MMCT band corresponding to the process RuII t2g6 f RuIII t2g5. Table 1 gives the energies Eop ) hυmax corresponding to the absorption maximum. As can be seen in the table, Eop is a constant independent of the surfactant concentration. However, the values of Eop are different in the presence of the cationic and anionic surfactants. The values of Eop corresponding to the anionic surfactant are similar to those corresponding to aqueous solutions of electrolytes. In this sense for comparative purposes the values of Eop corresponding to aqueous solutions (LiNO3) of the binuclear complex are also given in the table. (13) Rolda`n, E.; Dominguez, M.; Gonza´lez-Arjona, D. Comput. Chem. 1986, 10, 187. (14) Sa´nchez-Burgos, F.; Gala`n, M.; Dominguez, M.; Pe´rez-Tejeda, P. New. J. Chem., in press. (15) Geiger, W. E. Inorganic Reactions and Methods; Zuckerman, J. J., Ed.; VCH Publishers: Deerfield Beach, FL, 1986; Vol. 15, p 104.
3764 Langmuir, Vol. 14, No. 14, 1998
Sa´ nchez et al. Table 2. Redox Potentials versus NHE, E1/2 (Rua ) Ruthenium Ammine Center and Rub ) Ruthenium Cyanide Center), and Free Energy Change, ∆G°′, for the [(NH3)5RuIII-NC-RuII(CN)5]- Complex in Micellar Solutions of SDS and CTACl at 298.2 K
Figure 2. Differential pulse voltammogram for the binuclear complex [(NH3)5RuIII-NC-RuII(CN)5]- in 0.1 mol dm-3 CTACl solution at 298.2 K: (a) peak potential corresponding to the ruthenium ammine center; (b) peak potential corresponding to the ruthenium cyanide center. Table 1. Experimental and Corrected Energies Corresponding to the Maximum of the MMCT Band, Eop and Eopcorr, for the Electron Transfer within the [(NH3)5RuIII-NC- RuII(CN)5]- Binuclear Complex at 298.2 K conc/mol dm-3
Eop/kJ mol-1
Eopcorr/kJ mol-1
0.010 0.025 0.030 0.050 0.10 0.20
SDS 175.4 175.9 175.4 175.6 175.6 176.1
161.1 161.6 161.1 161.3 161.3 161.8
0.010 0.025 0.030 0.054 0.103 0.205
CTACl 170.6 170.6 170.6 170.6 170.6 170.4
156.3 156.3 156.3 156.3 156.3 156.1
0.2 0.6 1.0 2.0 3.0 4.0 5.0
LiNO3 176.4 176.7 177.5 178.8 179.3 180.1 180.7
162.1 162.4 163.2 164.5 165.0 165.8 166.4
E1/2(Rua)/V
0.010 0.025 0.030 0.050 0.10 0.20
-0.027 -0.033 -0.034 -0.044 -0.045 -0.062
0.010 0.025 0.030 0.054 0.103 0.205
-0.069 -0.014 -0.002 0.011 0.005 -0.001
E1/2(Rub)/V
∆G°′/kJ mol-1
1.107 1.098 1.083 1.083 1.084 1.067
109.5 109.2 107.8 108.8 108.9 108.9
0.890 0.875 0.877 0.870 0.859 0.850
92.5 85.8 84.8 82.9 82.4 82.1
SDS
CTACl
sponding to transitions from the ground state to the E and A states. These bands are at δ/2 (higher) and δ (lower) energies, respectively, than the maximum absorption observed in the composite band. A value of 1200 cm-1 ) 14.3 kJ mol-1 for δ was used.17 In this way, after correction, both Eopcorr and ∆G°′ (see below) correspond to the same process. Electrochemistry. The half-wave potentials of both ruthenium centers are given in Table 2. From these data, the value of the free energy change corresponding to the electron-transfer process can be calculated as
∆G°′ ) -nF∆E°′ = -nF∆E1/2
(4)
with n ) 1 and ∆E1/2 ) E1/2(Rua) - E1/2(Rub), where Rua is the ruthenium ammine center and Rub is the rutheniumcyanide center. The values of ∆G°′ are collected in Table 2. It is worth pointing out that the redox potentials obtained for the ruthenium ammine center correspond to the process e-
[(NH3)5RuIII-NC-RuII(CN)5]- {\ } -e
[(NH3)5RuII-NC-RuII(CN)5]2- (5) and that those for the ruthenium cyanide center correspond to -e-
[(NH3)5RuIII-NC-RuII(CN)5]- {\ } e
[(NH3)5RuIII-NC-RuIII(CN)5]0 (6)
Table 1 also gives Eopcorr calculated as
Eopcorr ) Eop - δ
conc/mol dm-3
(3)
The δ parameter in this equation represents a correction for the spin-orbit coupling of the ruthenium(III) cyanide center in the excited state (see eq 1): in the octahedral symmetry, the d5T state of the metal is split by spinorbit coupling into a higher degenerate E state and a lower A state, the energy separation of these states being 3/2 δ, where δ is the spin-orbit coupling parameter.16,17 Because of the existence of two excited states, the experimental charge-transfer band is the sum of two bands, corre(16) Curtis, J. C.; Meyer, T. J. Inorg. Chem. 1982, 21, 1562. (17) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J. Phys. Chem. 1986, 90, 3657.
Therefore, the determination of ∆G°′ by the preceding method involves implicitly the following assumption: the redox potential of each ruthenium is not affected by the oxidation state of the other. Strictly speaking, the values of ∆G°′ obtained from eq 4 correspond rather to the comproportionation reaction
[(NH3)5RuII-NC-RuII(CN)5]2- + [(NH3)5RuIII-NC-RuIII(CN)5]0 f [(NH3)5RuII-NC-RuIII(CN)5]- (7a) than to the redox process
[(NH3)5RuIII-NC-RuII(CN)5]- f [(NH3)5RuII-NC-RuIII(CN)5]- (7b)
Metal-to-Metal Charge Transfer
Langmuir, Vol. 14, No. 14, 1998 3765
Table 3. Reorganization Free Energy λ for the Electron Transfer within the [(NH3)5RuIII-NC-RuII(CN)5]Complex in Micellar Solutions of SDS and CTACl at 298.2 K [SDS]/mol dm-3
λSDS/kJ mol-1
[CTACl]/mol dm-3
λCTACl/kJ mol-1
0.010 0.025 0.030 0.050 0.10 0.20
51.6 52.4 53.3 52.5 52.4 52.9
0.010 0.025 0.030 0.054 0.103 0.205
63.8 70.5 71.5 73.4 73.9 73.9
However, it has been stated that the differences in the free energies corresponding to both processes depend on the donor number of the solvent.18 Such a difference is minimum for solvents whose donor numbers are about 15.19 In view of the donor number of water, which is 14,19 the differences in free energies of processes 7a and 7b, in the present case, should be small. On the other hand, the difference between the free energies of both processes if any should be a constant, independent of the medium as long as the coupling between the ruthenium centers is constant. So, as we are interested in changes in the free energies of reaction and reorganization, our approach should be valid, if the coupling is a constant. This constancy of the coupling between the centers follows from the fact that the intensity of the band, as measured by the molar extinction coefficient, does not change in the series of experiments presented here, as indicated in the Experimental Section. Reorganization Free Energies. The reorganization free energies corresponding to the electron-transfer process have been calculated through20
Eopcorr ) λ + ∆G°′
(8)
The values of this parameter are given in Table 3. Equation 8 can be applied, provided that the interaction with the solvent is strong enough to ensure a structureless band.21 In fact this equation has been used in previous studies of this complex in water.11 However, there has been some controversy over this equation, because it is written in terms of energy (Eopcorr) and free energy (∆G°′ and λ). For this reason it was pointed out that on the right side of eq 8, instead of λ and ∆G°′, the corresponding energetic magnitudes should appear. Nevertheless, Marcus and Sutin have convincingly argued that the parameters λ and ∆G°′, appearing on the right-hand side of eq 8, are better viewed as free energies.22 It can also be argued23 that λ depends on the optical and static dielectric constants of the medium and ∆G°′ mainly on the static dielectric constant. As the temperature coefficients of these dielectric parameters are small, the entropic terms in λ and ∆G°′ must also be small, and they, indeed, compensate to some extent. Finally, for an optical electron transfer, the nucleus is frozen so the corresponding (electronic) entropy change would be
∆S ) R ln
Ωexc Ωg
(9)
(18) Blackbourn, R. L.; Hupp, J. T. Chem. Phys. Lett. 1988, 150, 399. (19) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: 1978; p 20. (20) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (21) Walker, G. C.; Barbara, P. F.; Doom, S. K.; Hupp, J. T. J. Phys. Chem., 1991, 95, 5712. (22) (a) Marcus, R. A.; Sutin, N. Commun. Inorg. Chem. 1986, 5, 119. (b) Marcus, R. A., Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (23) Doong, Y.; Hupp, J. T. Inorg. Chem. 1992, 31, 332.
Table 4. Free Energy Change ∆G°′ and Reorganization Free Energy λ for the Electron Transfer within the [(NH3)5RuIII-NC-RuII(CN)5]- Complex in LiNO3 Solutions at 298.2 K [LiNO3]/mol dm-3
∆G°′/kJ mol-1
λ/kJ mol-1
0.2 0.6 1.0 2.0 3.0 4.0 5.0
111.5 111.7 112.5 112.8 113.3 114.1 114.4
50.6 50.7 50.7 51.7 51.7 51.7 52.0
where Ω is the spin multiplicity of the corresponding excited or ground state. Thus, the corresponding free energy term should be ∼RT, which is small in comparison to the Eop and λ values. Consequently, λ and ∆G°′ will be taken, in agreement with Marcus and Sutin22 and others,11,23 as free energies. Discussion First, the results corresponding to anionic micelles will be considered. As can be seen in Tables 2 and 3, the values of ∆G°′ and λ are practically constants in the range of concentrations of SDS studied here. The values of both parameters are, indeed, close to the values observed in electrolyte solutions that are given in Table 4.14 These data seem to support the point of view of a similar behavior of electrolyte and micellar solutions, the latter being considered as highly asymmetric electrolytes.24 The behavior of E1/2 for both ruthenium centers in micellar (SDS) solutions is the expected one, considering the charges at each center: the ruthenium ammine (cationic) center becomes a less potent oxidant when the SDS concentration increases. This is in agreement with expectations taking into account that the half- wave potentials in the tables are given by
E1/2 ) Eo +
γox RT ln F γred
(10)
In the presence of a negatively charged micelle both γox and γred must decrease (γred corresponds to the reduced form of the ruthenium(III) ammine center, that is, to a ruthenium(II) ammine species) but the decrese in γox will be greater than the one corresponding to γred. So a decrease of E1/2 results. As to the ruthenium cyanide center of the binuclear complex, which has an anionic character, the reduced form will have a higher negative charge. So, this form will be more destabilized by the interaction with the field of the negatively charged micelles, and consequently, a decrease of E1/2 is also found for this center. It is interesting to note that these variations of the halfwave potentials in both ruthenium centers point out that although the binuclear complex must be outside the Stern layer of the micelles,25 it is under the influence of the micellar electric field. On the other hand, from the values of λ and ∆G°′ in Tables 2 and 3 it is possible to calculate the free energy of activation corresponding to the thermal electron(24) See for example: Lin, Y. C.; Chen, S. H. Physica A 1996, 231, 277. (25) The interfacial region or Stern layer, having a width about the size of the surfactant head group, contains the ionic head groups of the amphiphile, a fraction of the couterions, and water. See for example: Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357.
3766 Langmuir, Vol. 14, No. 14, 1998
Sa´ nchez et al.
Table 5. Free Energy of Activation ∆Gq for the Thermal Electron Transfer within the [(NH3)5RuIII-NC-RuII(CN)5]Complex at 298.2 K SDS
CTACl
LiNO3
[SDS]/mol dm-3
∆Gq/kJ mol-1
[CTACl]/mol dm-3
∆Gq/kJ mol-1
[LiNO3]/mol dm-3
∆Gq/kJ mol-1
0.010 0.025 0.030 0.050 0.10 0.20
125.7 124.6 121.7 123.9 124.1 123.7
0.010 0.025 0.030 0.054 0.103 0.205
95.7 86.6 85.4 83.2 82.6 82.5
0.2 0.6 1.0 2.0 3.0 4.0 5.0
129.8 130.0 131.3 130.8 131.6 132.9 133.1
transfer process using26
∆Gq )
corr 2 (λ + ∆G°′)2 (Eop ) ) 4λ 4λ
(11)
The values of ∆Gq thus obtained appear in Table 5. These values first decrease, reaching a minimum value at the surfactant concentration 3 × 10-2 mol dm-3, and then rise. So, a maximum in the rate of the thermal electrontransfer rate should be expected. However, the variations of ∆Gq are small, and it is not particularly safe to speculate about this point, since their variations are close to the limits of the error in this parameter (∼2 kJ mol-1), arising from the error in E1/2 (∼10 mV). In any case the influence of the anionic micelles on the rate of the thermal electron transfer is small, as expected, given the negative charge of the binuclear complex. As to the results corresponding to solutions of the cationic surfactant, Tables 2 and 3 show that variations in both λ and ∆G°′ are much greater than those observed in the case of SDS, as expected. According to these values and eq 11, the thermal reaction (eq 7b) becomes more favorable from a thermodynamic point of view as well as from a kinetic point of view (see Tables 2 and 5). The more favorable thermodynamic character of the reaction arises from the fact that the ruthenium ammine center becomes a more potent oxidant and the ruthenium cyanide center becomes a more potent reductant (see Table 2). The variations in the redox potential of the oxidant are those to be expected on the basis of the electrostatic interaction of this center with the positively charged micelle field. However, the ruthenium cyanide center at the binuclear complex bears a negative charge, which is greater in the reduced form than that in the oxidized form of this couple. So, γred should be expected to decrease more than γox for this center, as a consequence of the interaction with the electric field arising from a positively charged micelle. This would produce, according to eq 10, an increase in E1/2 which is opposite to the observed decrease. However, the behavior of E1/2 for the ruthenium cyanide center observed in this work has been observed previously: the redox potentials of the couples of opposite sign charge to the charge in the micelles vary in the opposite sense expected under electrostatic grounds only.27,28 Indeed, we have measured the redox potential of the Fe(CN)63-/4- couple in CTACl solutions, and in agreement with data in Table 2, a decrease in the redox potentials of this couple was found (Ef° ) 0.406 V for [CTACl] ) 0.01 mol dm-3 and Ef° ) 0.393 V for [CTACl] ) 0.2 mol dm-3). These results are explained by considering that the centers of opposite charge sign to the micellar charge are placed at the Stern layer of the micelles.28 In this layer, the (26) Goldsby, K. A.; Meyer, T. J. Inorg. Chem. 1984, 23, 3002. (27) Ohsawa, Y.; Shimazaki, Y.; Aoyagui, S. J. Electroanal. Chem. 1980, 114, 235. (28) Davies, K. M.; Hussam, A.; Rector, B. R., Jr.; Owen, I. M.; King, P. Inorg. Chem. 1994, 33, 1741.
strong micellar field causes a strong decrease of the local dielectric constant, as a consequence of the dielectric saturation effects. This decrease in the local dielectric constant will produce an increase of both γox and γred in eq 10, but a more marked one in γred because this corresponds to a species with a higher charge (in absolute value). So the decrease in the redox potential of the reductant is due not so much to the direct effect of the field on this center as to the effect of the field on the solvent, which produces a dielectrically more saturated state of the medium (solvent). As to the λ values observed in the presence of CTACl, they show a marked increase of about 10 kJ mol-1. This implies that the reorganization energy, when the reactants of an electron transfer are close to a micelle, increases. This effect is similar to the ionic cloud effect on electrontransfer reactions (that is, the observed effect on λ when electrolytes are present in the medium: an additional contribution of the reorganization energy appears, owing to the reorganization of the ion positions29) (see Table 4), but much bigger as a consequence of the stronger interaction with the micelle, and to the lower mobility of the latter. Finally, a word in relation to the data in Table 5. The values in ∆Gq in the presence of CTACl decrease about 10 kJ mol-1. This would imply an increase in the rate of the (thermal) electron-transfer reaction by a factor of 50; that is, a catalytic micellar effect appears. This effect appears to be even greater when the values of ∆Gq in CTACl solutions are compared with those corresponding to aqueous solutions: in this case, an increase in rate by a factor of ∼108 (!) is calculated from data in Table 5. Summarizing: the effects of both cationic and anionic micelles have been studied on optical electron transfer in the binuclear complex RuII-CN-RuIII. Results obtained in the presence of the anionic surfactant are similar to the ones corresponding to electrolyte solutions. In the presence of a cationic surfactant the relevant parameters in relation to electron-transfer reactions, λ and ∆G°′, change markedly, much more than in the presence of salts. These changes produce a big decrease in the free energy of activation for the thermal process in the presence of the cationic surfactant; that is, a catalytic micellar effect operates. In fact, the catalytic micellar effect resembles the phenomena of two-phase catalysis30 and molecular recognition.31 Like them, it can have important practical applications. Acknowledgment. This work was financed by the D.G.I.C.Y.T. (Grant PB-95-0535) and the Consejern˜a de Educacio´n y Ciencia de la Junta de Andalucia. LA980018B (29) Marcus, R. A. J. Chem. Phys. 1965, 43, 679. (30) Herrmann, W. A.; Kohlpaintner, C. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1524. (31) (a) Nielson, R. M.; Hupp, J. T.; Yoon, D. I. J. Am. Chem. Soc. 1995, 117, 9085. (b) Nielson, R. M.; Lyon, L. A.; Hupp, J. T. Inorg. Chem. 1996, 35, 970.
