Effect of the Micellar Electric Field on Electron-TransferProcesses. A Study of the Metal-to-Metal Charge Transfer

within the Binuclear ComplexPentaammineruthenium(III)-(µ-Cyano)pentacyanoruthenium(II)in Micellar Solutions of Sodium Dodecylsulfate (SDS) and

Hexadecyltrimethylammonium Chloride (CTACl)

Francisco Sanchez, Manuel Lopez-Lopez, and Pilar Perez-Tejeda*

Department of Physical Chemistry, Faculty of Chemistry, University of Sevilla,C/ Profesor Garcia Gonzalez 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 containingsodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (CTACl). The redox potentialsof both ruthenium centers were also measured. From the optical and electrochemical (redox) data, boththe reorganization free energy and the reaction free energy corresponding to the electron transfer wereobtained. It was found that in solutions containing the anionic surfactant the relevant parameters forthe electron-transfer process, that is, the free energies of reorganization and reaction, are quite similarto those corresponding to aqueous electrolyte solutions. In the case of the solutions containing the cationicsurfactant, these parameters are quite different from those of the aqueous electrolyte solutions. Theresults are interpreted taking into account the effects of the micellar electric field on both the binuclearcomplex and the medium (solvent) surrounding it. These effects depend on the (average) position of thebinuclear complex in relation to the micelle: in the case of cationic micelles the binuclear complex penetratesthe 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 anarea of growing interest in the past few years.1 In relationto electron-transfer processes, the field can affect all therelevant parameters that modulate the rate of reaction.So, the solvent reorganization energy depends on thedielectric characteristics of the surrounding medium,2 andthese characteristics can be modified by the field throughsolvent saturation effects.3 On the other hand, the freeenergy of the reaction is dependent on the field, becausethe free energies of the reactant and product states alsodepend on the dielectric constant of the medium andbecause of the interaction of the dipole moment of theelectronic transition (optical or thermal) with the field.4Moreover, this field can change the adiabaticity of thereaction through the polarization of the orbitals of thereactants involved in the electron transfer:4 this polariza-tion can produce an increase (or decrease) of the overlap-ping between the intervening orbitals of the electron donorand acceptor, which would produce changes in adiaba-ticity.4 On the other hand, the dynamics of the solvent(and so the preexponential term in the rate constant5) canalso be changed by the field. Morever, the diffusion

coefficients of the intervening species corresponding tothe 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 presenceof a field can also be rather different from those in theabsence of a field.6 Finally, it has been suggested that thefluctuation dissipation theorem and other importanttheorems of statiscal mechanics may no longer be validin the presence of a strong field.6

From the above-mentioned considerations there is aclear interest in studying the effect of electric fields onelectron-transfer reactions, from a theoretical7 as well asan experimental1c point of view. On the other hand thistopic is of interest as well in relation to some importantquestions, such as molecular electronics4 and biological(electron transfer) processes. For example, long-rangeelectron transfer in photosynthetic reaction centers isinfluenced by the electric field arising from the permanentdipole of the R-helical section of the proteins sourrondingit. In this case, the influence of the field on the rate comesfrom the (favorable) modification of the driving force ofthe electron transfer.8

However, to produce the effects mentioned, in particularchanges 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 regionclose to an electrode surface9a and also in the vicinity ofthe surface of the (charged) micelles.9b

* 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. InElectron 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) Bottcher, 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.

(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.

3762 Langmuir 1998, 14, 3762-3766

S0743-7463(98)00018-3 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 06/10/1998

In this paper the effect of the latter fields on the opticalelectron-transfer process within the binuclear complexpentaamminerutenium(III)-(µ-cyano)pentacyanoruthe-nium(II) (RuII-CN-RuIII) is presented. This compoundwas chosen because it shows a clear metal-to-metal chargetransfer (MMCT) band in water10,11 and because it belongsto class II in the Robin-Day classification12 (that is, theinteraction between the two metal centers, measured bythe degree of ground-state delocalization R2, is lower than2 × 10-2), which is essential to use of eq 8 below. Theselected system also permits the experimental estimationof the free energy change ∆G°′ corresponding to thedifference between the free energies of the reactant and(equilibrated) product states of the intervalence transition

since both ruthenium centers exhibit a reversible behaviorfrom an electrochemical point of view. So, the free energyof reorganization for the metal-to-metal electron-transferprocess canbeobtained fromthereversibleelectrochemicaldata of the complex. In this way, the influence of the fieldon the relevant parameters contributing to the activationfree energy of the associated thermal electron-transferprocess can be obtained.

The results presented in this study correspond to twokinds of micellar solutions: those formed by anionic (SDS)and cationic (CTACl) surfactants. Given that the bi-nuclear complex under study bears a negative charge, itcan be supposed that the anionic micelles will interactwith it only via long-range (Coulombic) interactions. Onthe other hand, in the case of cationic micelles, bothCoulombic interactions and short-range effects are to beexpected. Therefore, a study of both long-range and short-range electric field effects seems feasible. The latter willcome through saturation effects produced by the electricfield on the solvent sourronding the binuclear complex.

Experimental SectionMaterials. The mixed valence compound Na[(NH3)5RuIII-

NC-RuII(CN)5]‚3H2O was prepared and purified according tothe method described in ref 10. The visible spectrum shows aMMCT band with an absorption maximum at 683 nm (εmax )2850 ( 50 mol-1 dm3 cm-1) in water. This corresponds to a valueof Eop ) hυmax of 175 kJ mol-1.

Hexadecyltrimethylammonium chloride (CTACl) was pur-chased from Fluka as a solution containing approximately 25%in the surfactant. This solution was titrated with AgNO3 andused without further purification. Sodium dodecyl sulfate wasfrom Merck and was recrystallized from ethanol, washed withdiethyl ether, and dried in vacuo over P2O5. Throughout thestudy, double-distilled and deionized water, obtained from aMillipore Mili-Q system, with a resistivity lower than 107 Ω cm,was used.

Spectra. The spectra of the binuclear complex in the differentsurfactant solutions were recorded in a Hitachi 150-20 UV-visspectrophotometer at 298.2 K. The concentration of this complexin the SDS solutions was always 8.7 × 10-4 mol dm-3, whereasin the CTACl solutions the concentration varied from 2.3 × 10-4

to 3.6× 10-4 mol dm-3. These concentrations were the maximumpossible ones compatible with the stability of the micellar (CTACl)solutions. The concentrations of the SDS and CTACl solutionsranged from 1 × 10-2 to 2 × 10-1 mol dm-3. Neither the molar

absorption coefficient at the maximum of the band εmax nor thefull-width at half-height ∆υ1/2 showed significant modificationsin SDS and CTACl micellar solutions, in relation to the valuesof these parameters in water (εmax ) 2850 ( 50 mol-1 dm3 cm-1

and ∆υ1/2 ) 4600 ( 100 cm-1). A representative spectrum isgiven in Figure 1.

Electrochemistry. The redox potentials of the two ruthe-nium centers in the binuclear complex were obtained bydifferential-pulse (DPV) and cyclic voltammetry techniques inSDS and CTACl solutions. The former technique was used forthe RuII cyanide center, and both DPV and cyclic voltammetrywere 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 thesame as those in the experiments in which the spectra wereobtained. A representative voltammogram is given in Figure 2.

The relationship between the peak and half-wave potentialsfor a reversible system is given by15

where ∆E is the voltage amplitude of the pulse (2 mV in ourexperiments), thus Epeak≈ E 1/2. The estimated uncertainty inredox potentials is about ( 10 mV.

ResultsSpectra. The binuclear complex studied in this work

shows a MMCT band corresponding to the process RuII

t2g6 f RuIII t2g

5. Table 1 gives the energies Eop ) hυmaxcorresponding to the absorption maximum. As can beseen in the table, Eop is a constant independent of thesurfactant concentration. However, the values of Eop aredifferent in the presence of the cationic and anionicsurfactants. The values ofEop corresponding to the anionicsurfactant are similar to those corresponding to aqueoussolutions of electrolytes. In this sense for comparativepurposes the values of Eop corresponding to aqueoussolutions (LiNO3) of the binuclear complex are also givenin the table.

(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.

(13) Roldan, E.; Dominguez, M.; Gonzalez-Arjona, D. Comput. Chem.1986, 10, 187.

(14) Sanchez-Burgos, F.; Galan, M.; Dominguez, M.; Perez-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.

[(NH3)5RuIII-NC-RuII(CN)5]- 98

hν

[(NH3)5RuII-NC-RuIII(CN)5]- (1)

Figure 1. Experimental MMCT band for the binuclear complex[(NH3)5RuIII-NC- RuII(CN)5]- in 0.1 mol dm-3 CTACl solutionat 298.2 K.

Epeak ) E1/2 + ∆E2

(2)

Metal-to-Metal Charge Transfer Langmuir, Vol. 14, No. 14, 1998 3763

Table 1 also gives Eopcorr calculated as

The δ parameter in this equation represents a correctionfor the spin-orbit coupling of the ruthenium(III) cyanidecenter in the excited state (see eq 1): in the octahedralsymmetry, the d5T state of the metal is split by spin-orbit coupling into a higher degenerate E state and a lowerA state, the energy separation of these states being 3/2 δ,where δ is the spin-orbit coupling parameter.16,17 Becauseof the existence of two excited states, the experimentalcharge-transfer band is the sum of two bands, corre-

sponding to transitions from the ground state to the Eand A states. These bands are at δ/2 (higher) and δ (lower)energies, respectively, than the maximum absorptionobserved 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 Eop

corr and ∆G°′ (see below) correspond to the sameprocess.

Electrochemistry. The half-wave potentials of bothruthenium centers are given in Table 2. From these data,the value of the free energy change corresponding to theelectron-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 Ruais the ruthenium ammine center and Rub is the ruthe-niumcyanide center. The values of ∆G°′ are collected inTable 2.

It is worth pointing out that the redox potentialsobtained for the ruthenium ammine center correspond tothe process

[(NH3)5RuIII-NC-RuII(CN)5]- \

e-

-e-

[(NH3)5RuII-NC-RuII(CN)5]2- (5)

and that those for the ruthenium cyanide center cor-respond to

Therefore, the determination of ∆G°′ by the precedingmethod involves implicitly the following assumption: theredox potential of each ruthenium is not affected by theoxidation state of the other. Strictly speaking, the valuesof ∆G°′ obtained from eq 4 correspond rather to thecomproportionation reaction

than to the redox process

(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.

Figure 2. Differential pulse voltammogram for the binuclearcomplex [(NH3)5RuIII-NC-RuII(CN)5]- in 0.1 mol dm-3 CTAClsolution at 298.2 K: (a) peak potential corresponding to theruthenium ammine center; (b) peak potential corresponding tothe ruthenium cyanide center.

Table 1. Experimental and Corrected EnergiesCorresponding to the Maximum of the MMCT Band, Eop

and Eopcorr, for the Electron Transfer within the

[(NH3)5RuIII-NC- RuII(CN)5]- Binuclear Complex at298.2 K

conc/mol dm-3 Eop/kJ mol-1 Eopcorr/kJ mol-1

SDS0.010 175.4 161.10.025 175.9 161.60.030 175.4 161.10.050 175.6 161.30.10 175.6 161.30.20 176.1 161.8

CTACl0.010 170.6 156.30.025 170.6 156.30.030 170.6 156.30.054 170.6 156.30.103 170.6 156.30.205 170.4 156.1

LiNO30.2 176.4 162.10.6 176.7 162.41.0 177.5 163.22.0 178.8 164.53.0 179.3 165.04.0 180.1 165.85.0 180.7 166.4

Eopcorr ) Eop - δ (3)

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

conc/mol dm-3 E1/2(Rua)/V E1/2(Rub)/V ∆G°′/kJ mol-1

SDS0.010 -0.027 1.107 109.50.025 -0.033 1.098 109.20.030 -0.034 1.083 107.80.050 -0.044 1.083 108.80.10 -0.045 1.084 108.90.20 -0.062 1.067 108.9

CTACl0.010 -0.069 0.890 92.50.025 -0.014 0.875 85.80.030 -0.002 0.877 84.80.054 0.011 0.870 82.90.103 0.005 0.859 82.40.205 -0.001 0.850 82.1

[(NH3)5RuIII-NC-RuII(CN)5]- \

-e-

e-

[(NH3)5RuIII-NC-RuIII(CN)5]0 (6)

[(NH3)5RuII-NC-RuII(CN)5]2- +

[(NH3)5RuIII-NC-RuIII(CN)5]0 f

[(NH3)5RuII-NC-RuIII(CN)5]- (7a)

[(NH3)5RuIII-NC-RuII(CN)5]- f

[(NH3)5RuII-NC-RuIII(CN)5]- (7b)

3764 Langmuir, Vol. 14, No. 14, 1998 Sanchez et al.

However, it has been stated that the differences in thefree energies corresponding to both processes depend onthe donor number of the solvent.18 Such a difference isminimum for solvents whose donor numbers are about15.19 In view of the donor number of water, which is 14,19

the differences in free energies of processes 7a and 7b, inthe present case, should be small. On the other hand, thedifference between the free energies of both processes ifany should be a constant, independent of the medium aslong as the coupling between the ruthenium centers isconstant. So, as we are interested in changes in the freeenergies of reaction and reorganization, our approachshould be valid, if the coupling is a constant. Thisconstancy of the coupling between the centers follows fromthe fact that the intensity of the band, as measured by themolar extinction coefficient, does not change in the seriesof experiments presented here, as indicated in theExperimental Section.

Reorganization Free Energies. The reorganizationfree energies corresponding to the electron-transfer pro-cess 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 interactionwith the solvent is strong enough to ensure a structurelessband.21 In fact this equation has been used in previousstudies of this complex in water.11 However, there hasbeen some controversy over this equation, because it iswritten in terms of energy (Eop

corr) and free energy (∆G°′and λ). For this reason it was pointed out that on theright side of eq 8, instead of λ and ∆G°′, the correspondingenergetic magnitudes should appear. Nevertheless, Mar-cus and Sutin have convincingly argued that the param-eters λ and ∆G°′, appearing on the right-hand side of eq8, are better viewed as free energies.22 It can also beargued23 that λ depends on the optical and static dielectricconstants of the medium and ∆G°′ mainly on the staticdielectric constant. As the temperature coefficients ofthese dielectric parameters are small, the entropic termsin λ and ∆G°′ must also be small, and they, indeed,compensate to some extent. Finally, for an optical electrontransfer, the nucleus is frozen so the corresponding(electronic) entropy change would be

where Ω is the spin multiplicity of the correspondingexcited or ground state. Thus, the corresponding freeenergy term should be ∼RT, which is small in comparisonto the Eop and λ values. Consequently, λ and ∆G°′ will betaken, in agreement with Marcus and Sutin22 and oth-ers,11,23 as free energies.

Discussion

First, the results corresponding to anionic micelles willbe considered. As can be seen in Tables 2 and 3, the valuesof ∆G°′ and λ are practically constants in the range ofconcentrations of SDS studied here. The values of bothparameters are, indeed, close to the values observed inelectrolyte solutions that are given in Table 4.14 Thesedata seem to support the point of view of a similar behaviorof electrolyte and micellar solutions, the latter beingconsidered as highly asymmetric electrolytes.24

The behavior of E1/2 for both ruthenium centers inmicellar (SDS) solutions is the expected one, consideringthe charges at each center: the ruthenium ammine(cationic) center becomes a less potent oxidant when theSDS concentration increases. This is in agreement withexpectations taking into account that the half- wavepotentials in the tables are given by

In the presence of a negatively charged micelle both γoxand γred must decrease (γred corresponds to the reducedform of the ruthenium(III) ammine center, that is, to aruthenium(II) ammine species) but the decrese in γox willbe greater than the one corresponding to γred. So a decreaseof E1/2 results. As to the ruthenium cyanide center of thebinuclear complex, which has an anionic character, thereduced form will have a higher negative charge. So, thisform will be more destabilized by the interaction with thefield 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 half-wave potentials in both ruthenium centers point out thatalthough the binuclear complex must be outside the Sternlayer of the micelles,25 it is under the influence of themicellar electric field.

On the other hand, from the values of λ and ∆G°′ inTables 2 and 3 it is possible to calculate the free energyof activation corresponding to the thermal electron-

(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.

(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 thesize of the surfactant head group, contains the ionic head groups of theamphiphile, 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.

Table 3. Reorganization Free Energy λ for the ElectronTransfer within the [(NH3)5RuIII-NC-RuII(CN)5]-

Complex in Micellar Solutions of SDS and CTACl at298.2 K

[SDS]/moldm-3

λSDS/kJmol-1

[CTACl]/moldm-3

λCTACl/kJmol-1

0.010 51.6 0.010 63.80.025 52.4 0.025 70.50.030 53.3 0.030 71.50.050 52.5 0.054 73.40.10 52.4 0.103 73.90.20 52.9 0.205 73.9

∆S ) R ln Ωexc

Ωg(9)

Table 4. Free Energy Change ∆G°′ and ReorganizationFree Energy λ for the Electron Transfer within the

[(NH3)5RuIII-NC-RuII(CN)5]- Complex in LiNO3Solutions at 298.2 K

[LiNO3]/mol dm-3 ∆G°′/kJ mol-1 λ/kJ mol-1

0.2 111.5 50.60.6 111.7 50.71.0 112.5 50.72.0 112.8 51.73.0 113.3 51.74.0 114.1 51.75.0 114.4 52.0

E1/2 ) Eo + RTF

lnγox

γred(10)

Metal-to-Metal Charge Transfer Langmuir, Vol. 14, No. 14, 1998 3765

transfer process using26

The values of ∆Gq thus obtained appear in Table 5. Thesevalues first decrease, reaching a minimum value at thesurfactant concentration 3 × 10-2 mol dm-3, and thenrise. So, a maximum in the rate of the thermal electron-transfer rate should be expected. However, the variationsof ∆Gq are small, and it is not particularly safe to speculateabout this point, since their variations are close to thelimits of the error in this parameter (∼2 kJ mol-1), arisingfrom the error in E1/2 (∼10 mV). In any case the influenceof the anionic micelles on the rate of the thermal electrontransfer is small, as expected, given the negative chargeof the binuclear complex.

As to the results corresponding to solutions of thecationic surfactant, Tables 2 and 3 show that variationsin both λ and ∆G°′ are much greater than those observedin the case of SDS, as expected. According to these valuesand eq 11, the thermal reaction (eq 7b) becomes morefavorable from a thermodynamic point of view as well asfrom a kinetic point of view (see Tables 2 and 5). Themore favorable thermodynamic character of the reactionarises from the fact that the ruthenium ammine centerbecomes a more potent oxidant and the ruthenium cyanidecenter becomes a more potent reductant (see Table 2).The variations in the redox potential of the oxidant arethose to be expected on the basis of the electrostaticinteraction of this center with the positively chargedmicelle field. However, the ruthenium cyanide center atthe binuclear complex bears a negative charge, which isgreater in the reduced form than that in the oxidized formof this couple. So, γred should be expected to decreasemore than γox for this center, as a consequence of theinteraction with the electric field arising from a positivelycharged micelle. This would produce, according to eq 10,an increase in E1/2 which is opposite to the observeddecrease. However, the behavior of E1/2 for the rutheniumcyanide center observed in this work has been observedpreviously: the redox potentials of the couples of oppositesign charge to the charge in the micelles vary in the oppositesense expected under electrostatic grounds only.27,28 In-deed, we have measured the redox potential of theFe(CN)6

3-/4- couple in CTACl solutions, and in agreementwith data in Table 2, a decrease in the redox potentialsof this couple was found (Ef° ) 0.406 V for [CTACl] ) 0.01mol dm-3 and Ef° ) 0.393 V for [CTACl] ) 0.2 mol dm-3).These results are explained by considering that the centersof opposite charge sign to the micellar charge are placedat the Stern layer of the micelles.28 In this layer, the

strong micellar field causes a strong decrease of the localdielectric constant, as a consequence of the dielectricsaturation effects. This decrease in the local dielectricconstant will produce an increase of both γox and γred ineq 10, but a more marked one in γred because thiscorresponds to a species with a higher charge (in absolutevalue). So the decrease in the redox potential of thereductant is due not so much to the direct effect of the fieldon this center as to the effect of the field on the solvent,which produces a dielectrically more saturated state of themedium (solvent).

As to the λ values observed in the presence of CTACl,they show a marked increase of about 10 kJ mol-1. Thisimplies that the reorganization energy, when the reactantsof an electron transfer are close to a micelle, increases.This effect is similar to the ionic cloud effect on electron-transfer reactions (that is, the observed effect on λ whenelectrolytes are present in the medium: an additionalcontribution of the reorganization energy appears, owingto the reorganization of the ion positions29) (see Table 4),but much bigger as a consequence of the strongerinteraction with the micelle, and to the lower mobility ofthe latter.

Finally, a word in relation to the data in Table 5. Thevalues in ∆Gq in the presence of CTACl decrease about 10kJ mol-1. This would imply an increase in the rate of the(thermal) electron-transfer reaction by a factor of 50; thatis, a catalytic micellar effect appears. This effect appearsto be even greater when the values of ∆Gq in CTAClsolutions are compared with those corresponding toaqueous solutions: in this case, an increase in rate by afactor of ∼108 (!) is calculated from data in Table 5.

Summarizing: the effects of both cationic and anionicmicelles have been studied on optical electron transfer inthe binuclear complex RuII-CN-RuIII. Results obtainedin the presence of the anionic surfactant are similar to theones corresponding to electrolyte solutions. In the pres-ence of a cationic surfactant the relevant parameters inrelation to electron-transfer reactions, λ and ∆G°′, changemarkedly, much more than in the presence of salts. Thesechanges produce a big decrease in the free energy ofactivation for the thermal process in the presence of thecationic surfactant; that is, a catalytic micellar effectoperates. In fact, the catalytic micellar effect resemblesthe phenomena of two-phase catalysis30 and molecularrecognition.31 Like them, it can have important practicalapplications.

Acknowledgment. This work was financed by theD.G.I.C.Y.T. (Grant PB-95-0535) and the Consejerna deEducacion y Ciencia de la Junta de Andalucia.LA980018B

(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.

(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.

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

0.010 125.7 0.010 95.7 0.2 129.80.025 124.6 0.025 86.6 0.6 130.00.030 121.7 0.030 85.4 1.0 131.30.050 123.9 0.054 83.2 2.0 130.80.10 124.1 0.103 82.6 3.0 131.60.20 123.7 0.205 82.5 4.0 132.9

5.0 133.1

∆Gq )(λ + ∆G°′)2

4λ)

(Eopcorr)2

4λ(11)

3766 Langmuir, Vol. 14, No. 14, 1998 Sanchez et al.