Langmuir 1996,11, 2800-2805
2800
Electron Transfer between Amphiphilic Ferrocenes and Electrodes in a Bicontinuous Microemulsion Giv Gounili, James M. Bobbitt, and James F. Rusling" Department of Chemistry, University of Connecticut (U-60), Storrs, Connecticut 06269-3060 Received January 27, 1995. I n Final Form: April 21, 1 9 9 P Kinetics of electrochemical oxidations of ferrocene (Fc) and 2- and 5-(ferrocenylcarboxy)dodecyltrimethylammonium nitrates (2-Fc and 5-Fc, respectively) were studied to yield information about the electrode-fluid interface in a bicontinuous microemulsion. Apparent standard heterogeneous electrontransfer rate constants (lz" ') for Fc, 2-Fc, and 5-Fc at glassy carbon electrodes were similar in homogeneous acetonitrile, as found earlier in DMSO. In a bicontinuous microemulsion ofn-tetradecane, water, pentanol, and cetyltrimethylammoniumchloride (CTAC),electron-transfer rates for Fc, 2-Fc, and 5-Fcwere an order of magnitude smaller than in acetonitrile. Ferrocene had a K" 'twice as large as 2-Fc and 5-Fc in the CTAC microemulsion. Small differences in rates of 2-Fc and 5-Fc cannot be explained by a head down-tail up orientation at the time of electron transfer, as proposed to explain kinetics in micellar CTAEi solutions. Results suggest the possibility of increased disorder and mobility in the electrode-fluid interface in the CTAC microemulsion compared to micellar CTAB solutions.
Introduction Microemulsionsare thermodynamically stable, optically clear fluids made from water, oil, and surfactant. They sometimes require a cosurfactant. Although they appear homogeneous,they are microscopically heterogeneous and are useful for bringing together reactants of widely different p~larities.'-~ Bicontinuous microemulsions are continuous in both oil and water. They feature an intimately mixed, dynamic, microheterogeneous network of oil and water with surfactant a t the i n t e r f a ~ e . ~ . ~ Microemulsions are of considerable interest for secondary oil recovery,6 detergency, cosmetics, and drug formulations1Z2and as media for chemica17-10and electrochemical rea~ti0ns.ll-l~ The interfacial properties of these fluids, Abstract published inAdvance ACSAbstracts, J u n e 15,1995. (1)Hiemenz, P. C. Principles ofsurface and Colloid Chemistry; Marcel Dekker: New York, 1986. (2) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (3) Evans, D. F.; Mitchell, D. J.;Ninham, B. W. J.Phys. Chem. 1986, 90, 2817. (4) Shinoda, K.; Lindman, B. Langmuir 1987,3,135. ( 5 ) Rusling, J. F. Acc. Chem. Res. 1987,24,75. (6) Miller, C. A.; Qutubuddin, S. In Interfacial Phenomena in Apolar Media; Eicke, H. F., Parfitt, G. D., Eds.; Marcel Dekker: New York, 1987; pp 117-185. (7) Luisi, P. L.; Magid, L. J . CRC Crit. Rev. Biochem. 1987,20,409. (8) Friberg, S. E. Adu. Colloid Interface Sci. 1990,32,167. (9) Fendler, J. H.; Kurihara, K. In Metal Containing Polymeric Systems; Sheats, J. E., Carraher, C. E.; Pittman, C. U.,Eds., Plenum Press: New York, 1985; pp 341-353. (10) O'Connor, C. J.; Lomax, T. D.; Ramage. R. E. Adu. Colloid 21. Interface Sci. 1984., 20. (11)Mackay, R. A. Colloids Surf. 1994,82,1. (12) Rusling, J . F. In Electroanalytical Chemistry; Bard, A. J. Ed.; Marcel DekkG, New York, 1994; Vol. 18, pp 1-88: (13) Rusling, J . F. In Modern Aspects of Electrochemistry, No. 26; Conway, B. E., Bockris, J. O'M., Eds.; Plenum Press: New York, 1994; pp 49-104. (14) Iwunze, M. 0.;Sucheta,A,; Rusling, J. F. Anal. Chem. 1990,62, f-i_ A A* . (15) Kamau, G. N.; Hu, N.; Rusling, J. F. Langmuir 1992,8,1042. (16) Zhang, S.; Rusling, J . F. Enuiron. Sci. Technol. 1993,27, 1375. (17) Owlia, A.; Wang, 2.; Rusling, J. F. J.Am. Chem. Soc. 1989,111, 5091. (18)Zhou, D.-L.; Gao, J.;Rusling, J. F. J.Am. Chem. Soc. 1995,117, 1127-1134. (19) Garcia, E.; Oppenheimer, L. E.; Texter, J. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH Publishers: New York, 1992; pp 257-272. (20) Myers, S. A.; Mackay, R. A.; Brajter-Toth, A.Ana1. Chem. 1993, 65. ~. , 3447. - - ~ (21) Mackay,R.A.;Myers, S.A.; Bodalbhai, L.;Brajter-Toth,A.Anal. Chem. 1990,62,1084. @
> -
both a t oil-water and fluid-solid interfaces, are of central importance in many of their applications. Microemulsions are being investigated as less toxic and less costlp substitutes for organic solvents in electrochemical reactions. 13~15-18 Examples include conversion of chlorobiphenyls to biphenyl,16formation of olefins from alkyl vicinal dihalide~,'~J'J~ and the production of latex p01ymers.l~In these reactions, a reactant is activated by electron transfer a t an electrode, and this active species reduces or oxidizes another reactant. This latter reaction is generally rate determining. It can be kinetically enhanced when reactant preconcentration occurs a t the electrode surface.15 When rate-determining steps take place in the bulk microemulsion, they can be controlled by reactant phase distributi0n.l' However, recent work showed that in a bicontinuous microemulsion with a large interfacial area, rates of electrochemically initiated bimolecular reactions were controlled by the formal potential of the active species produced a t the electrode.18 Formal potentials of reactants can be manipulated by tailoring the composition of the microemulsion.20,21 Applications of microemulsions to electrochemical synthesis involve generation of active species or products a t electrodes. The rate of this generation should depend on the nature and dynamics of the electrode-microemulsion interface and the orientation of the reactant at the electrode at the time of electron transfer.12J3 Recent studies in water-in-oil microemulsions demonstrated that electrode reactions can be facilitated by increasing interfacial disorder and p e r m e a b i l i t ~ . ~ ~ - ~ ~ We previously used amphiphilic ferrocenes 2-Fc and 5-Fc to investigate interfacial aspects of electron transfer in micellar solutions of cationic surfactant and water.25 CH3 Y C H -3
I
+
CH3 OOCFC 1 (2-FC)
CH3 CH 1-3CH3
OOCFc (5-9
Heterogeneous electron- rans er rates of 2-Fc, 5-Fc, and ferrocene (Fc) were nearly the same in homogeneous (22) Garcia, E.; Song, S.; Oppenheimer, L. E.; Antalek, B.; Williams, A. J.; Texter, J. Langmuir 1993,9, 2782.
0743-746319512411-2800$09.00/0 0 1995 American Chemical Society
Electron Transfer in a Bicontinuous Microemulsion DMSO at Pt electrodes, suggesting that specific reactant orientation did not occur on the electrode. However, in micellar CTAB, electron-transfer rate constants were found in the order Fc > 2-Fc > 5-Fc,with 10-fold differences between each successive compound. Results suggested that differences in rates were caused by increases in the distance of electron transfer in the sequence Fc < 2-Fc < 5-Fc. A model featuring coadsorbed CTAB and 2-Fc and 5-Fc oriented with head groups down on the electrode explained the data.25 The present paper reports an attempt at elucidating the influence of interfacial molecular orientation on electron-transfer kinetics at an electrode in a microemulsion by using electroactive surfactants 2-Fc and 5-Fc. This was done by comparing kinetics of electron transfer in a bicontinuous microemulsion, in an organic solvent, and in the previous aqueous CTAB solutions.
Experimental Section Chemicals. Acetonitrile (Baker Analyzed), benzene (Baker Analyzed), carbon tetrachloride (EM Science), cetyltrimethylammonium chloride (CTAC, Kodak), diethyl ether anhydrous (Baker Analyzed), dimethylamine hydrochloride (Aldrich),(+Id-dodecalactone (Aldrich), l&epoxydodecane (Aldrich), ferrocenemonocarboxylic acid (Sigma),iodomethane (Janssen Chimica), lithium aluminum hydride (Aldrich), lithium perchlorate (Aldrich), 1-pentanol(Aldrich),silver nitrate (Fisher Scientific), tetrabutylammonium perchlorate (TBAP, Kodak), and n-tetradecane (Fisher Scientific) were used as received. All other chemicals were reagent grade. Thionyl chloride (Aldrich) was distilled before use. Water with a specific resistance =- 12 MQ cm prepared with a Sybron/Bamstead Nanopure water purification system was used for the preparation of all solutions. Synthesis of 2-Fc and 5-Fc were improved to provide higher yields than reported previou~ly.~~ Methods and analytical results are reported in the appendix. Apparatus and Procedures. A BAS-100 electrochemical analyzer with positive feedback compensation for 100%of the ohmic drop of the cell was used for cyclicvoltammetry (CV).The workingelectrodewas a highly polished glassy carbon disk (GCE, A = 0.072 cm2). The electrode surface was polished on a metallographic wheel, successively using Sic paper, diamond paste, and alumina as described previously.26 Polishing was repeated before each voltammetric scan. The area of the GCE was estimated electrochemicallyby using the Randles-Sevcik equation27with the CV peak current at 25 "C for the reversible oxidation of 1mM ferrocene in acetonitrile ( D = 2.4 x cm2 The counter electrode was a platinum wire. All potentials were measured with respect to a saturated calomelreference (SCE). The cell was thermostated to *0.1 "C. All solutions were purged with purified nitrogen for at least 15 min prior to electrochemical experiments. A YSI Model 3400 conductivity cell and a YSI Model 31 conductivity bridge were used for conductance measurements. The microemulsion used was previously characterized as bicontin~ous.~9It was prepared by initially mixing cetyltrimethylammonium chloride (CTAC),1-pentanol, and n-tetradecane in the weight ratios 1.412.811.0. This mixture was titrated with water until the final weightcompositionof CTAC11-pentanol/ (23) Garcia, E.; Texter, J. J . Colloid Interface Sci. 1994,162, 262. (24) Antalek, B.; Williams, A. J.; Garcia, E.; Texter, J. Langmuir 1994,IO, 4459. (25)(a) Abbott, A. P.; Gounili, G.; Bobbitt, J. M.; Rusling, J. F.; Kumosinski, T. F. J . Phys. Chem. 1992,96,11091.(b) Roughly similar kinetic differences to those found in ref 25a were indicated by CV in CTAC micelles, but a micelle-amuhiuhile dissociation reaction ureceding electron transfer for ferrocene amphiphiles, as well a i large background currents, precluded quantitative studies in this medium. (26) Kamau, G. N.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1985, 57, 545. (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (28) Kuwana, T.; Bublitz, D. E.; Hoh, G. J. J . Am. Chem. SOC.1960, 82,5811. (29) Ceglie, A.; Das, K. P.; Lindman, B. Colloids Surf. 1987,28, 29.
Langmuir, Vol. 11, No. 7, 1995 2801 25 1
-35 '
-
I
-
0.80
"
0.60 E, V
0.40
' "' '
0.20
""
'I
0.00
vs SCE
Figure 1. Cyclic voltammograms in CTAC microemulsion at 25 "C and 500 mV s-l at a glassy carbon electrode for (a) 1.06
mM ferrocene, arrows show directions of scans, (b) 2.08 mM 2-Fc, (c) 1.1mM 5-Fc, and (d) background. n-tetradecanelwater was 18.1/36.3/13.0/32.6. A clear fluid was Q-1 cm-l obtained which had a specificconductanceof 3.1 x at 25 "C. Analysis of Data. Methods for estimating formal potentials (E"'1, apparent standard heterogeneous rate constants (k" ') for electron transfer, and apparent diffusion coefficients (D') from cyclic voltammetric data were similar to those described previ0us1y.~~ Reversible CVs at scan rates of 0.1-0.6 V s-l where peak separations were 170 mV were used to estimate D' from the Randles-Sevcik equation. At least five data points were used for each estimate. CVs at scan rates ( v )that gave anodiccathodicpeak separations (AE,)> 70 mVwereused for estimating k" ' by comparison of experimental values to theoretical curves of AE, vs the kinetic parameter = k" ' l [ D g ~ v ( n F l R T ) ] ~These '~. theoretical curveswere constructed at each temperature by digital simulation of CVs, assuming an electrochemical transfer coefficient ( a )of 0.5 and equality of diffusion coefficients of oxidized and reducedforms. Digisim 1.0software(BioAnalyticalSystems) was used for simulations along with experimentally measured temperature, area of electrode, formal potential, D', and concentration.
Results Cyclic voltammograms of Fc, 2-Fc, and 5-Fc were characteristic of reversible electron transfer at scan rates of 0.5 V s-l (Figure 1)in the CTAC microemulsion, with equal anodic and cathodic peak heights. Peak currents were proportional to the square root of the scan rate. Formal potentials of 2-Fc and 5-Fc taken as midpoints between cathodic and anodic peaks are similar to one another but more positive than that of ferrocene. As the scan rate was increased, separations between anodic and cathodicpeaks (AE,) increased, characteristic of diffusionkinetic control of the electrode reaction^.^' A theoretical curve of AEp vs 1 ~ ,= Lo '/[D,,~v(~FIRT)I~/~ used to estimate rate constants for electron transfer is shown in Figure 2. The simulated curve shows excellent agreement with the published theoretical results of Nicholson,30showingthe reliability of the simulation. Data for all compounds gave good agreement with theory (Figure 2). Scatter in these data is most probably caused by the inherent difficulty of reproducing the electrede surface preparation.26 Individual values ofk" ' showed no dependence on either scan rate (Table 1) or concentration, indicating the applicability of the diffusion-kinetic model for data analysis and the absence of errors from ohmic drop. In addition, experimental and simulated currents showed good agreement throughout the full scan rate range. (30)Nicholson, R. Anal. Chem. 1965,37,1351.
Gounili et al.
2802 Langmuir, Vol. 11, No. 7, 1995 2oo
a
i 60
2-Fc
5-FC
0
FC
A
1
I
su
100
0
50 -1.00 -0.50 "
"
"
"
'
15
"""""" 0.00
0.50
log cy Figure 2. Plot of AE, vs VI = k" 'l[Donv(nFlRT)lu2 at 25 "C with a = 0.5. Solid line is theoretical curve from ref 30 with a = 0.5;dotted line computed from digital simulation (see text). Data are for ferrocene(O), 2-Fc (A), and 5-Fc (+)fork" 'values given in Table 2 in CTAC microemulsion. Table 1. Apparent Standard Heterogeneous Rate Constants Estimated at Different Scan Rates in the CTAC Microemulsion" at 25 "C k" ', cm s-1 scan rate, V ssl E" V vs SCE I,
2-FC 10.24 12.047 20.480 25.600 25.600
0.604 0.603 0.606 0.606 0.605
0.037 0.033 0.039 0.042 0.029
0.605 i 0.0013
0.036 & 0.005
34.133 34.133 40.960 40.960 40.960 51.200 51.200
0.600 0.602 0.603 0.603 0.602 0.603 0.601
0.037 0.031 0.025 0.035 0.034 0.023 0.025
av
0.602 i: 0.001
0.030 f 0.006
av 5-Fc
CTACA-pentanoVn-tetradecane/water (18.1/36.3/13.0/32.6). Table 2. Formal Potentials, Diffusion Coefficients, and Apparent Standard Heterogeneous Rate Constants at 25 "C
5-Fc
35
30
40
45
T, O C 6 6
E $ 4 0
c E
3
' I
0
10
20
30
T,
40
50
60
'C
Figure 3. Influence of temperature on (a)apparent diffusion
coefficientsof electroactivespeciesin the CTAC microemulsion and (b) specific conductance of the CTAC microemulsion.
0.80
0.00
L
290
300
310
320
T, O K
Figure 4. Influence of temperature on apparent standard heterogeneous rate constants for electron transfer at a glassy
0.692 f 0.002 0.677 i 0.003
carbon electrode in the CTAC microemulsion for amphiphilic ferrocenes. Points are experimental data and solid lines are the best fits to those points by nonlinear regression analysis onto eq 3.
1.1i 0.1 x 1.2 f 0.1 x
>0.45 0.45 f 0.11
CTAC Microemulsion" ferrocene 0.363 f 0.0025 3.1 & 0.2 x 2-FC
25
E" ', V vs SCE
D"', cm2 k" ', cm Acetonitrile ferrocene 0.327 f 0.002 2.5 x (ref.28) 20.45 2-FC 5-Fc
20
1.00
0.604 i 0.001 0.601 i 0.002
0.59 f 0.04 x 0.54 f 0.05 x
lou6
0.074 f 0.004 0.036 i 0.005 0.030 i:0.006
a CTAC/l-pentanoVn-tetradecandwater (18.1/36.3/13.0/32.6). Selfdiffusion coefficients from NMR were reported in ref 29 as 106D cm2 ssl for CTAC microemulsion: surfactant, 0.65 f 0.02; ntetradecane,2.51 f 0.06; 1-pentanol,2.16 i 0.03; DzO, 2.73 i0.04
at 25 "C.
Table 2 summarizes electrochemical parameters for Fc, 2-Fc, and 5-Fc at 25 "C. Rates of electron transfer were similar for all these compounds in acetonitrile and larger than in the microemulsion. The electron-transfer rate for ferrocene in the microemulsion was about twice those for 2-Fc and 5-Fc. Similarities of diffusion coefficients of 2-Fc and 5-Fc with reported self-diffusion coefficient^^^ of CTAC in the microemulsion (Table 2, footnote a) suggest that these amphiphiles travel with the surfactant. The D-value for ferrocene itself is larger than that of 2-Fc and 5-Fc and
similar to that of the oil, suggesting that ferrocene is dissolved in the oil phase of the microemulsion. Studies were done to confirm that the microemulsion remained bicontinuous at different temperatures. The apparent diffusion coefficients of Fc, 2-Fc, and 5-Fc increased linearly with temperature (Figure 3), as did the specific conductance of the microemulsion. These linear increases with temperature are expected for a n isotropic fluid.lJ The high conductivity shows that the fluid could not have changed to a water in oil system as T increased. The linear increase in D of Fc is inconsistent with conversion to a oil-in-water system, since in that case Fc would travel with the oil droplet and D should decrease to values similar to those of the amphiphilic probes. Similarly, if the system became homogeneous, D values for 2-Fc and 5-Fc would increase to about half of that of Fc (cf. Table 2). Thus, results suggest that the microemulsion remains bicontinuous between 15 and 50 "C. Heterogeneous electron-transfer rate constants between 19 and 40 "C were always larger for 2-Fc than for 5-Fc (Figure 4).
Electron Transfer in a Bicontinuous Microemulsion
Langmuir, Vol. 11, No. 7, 1995 2803
Discussion
In micellar CTAB solutions, the free energies of activation for electron transfer at Pt were 56.2 k J mol-l for 2-Fc As reported for kinetics in standard heteroand 58.6 k J mol-l for 5-Fc. This suggested that reorgageneous rate constants (k" '1 of Fc, 2-Fc, and 5-Fc were nization energies for the two compounds were not signearly indistinguishable in homogeneous acetonitrile nificantly different in this medium.25 In the CTAC (Table 2). In the microemulsion, k" '-values were about microemulsion, the 7.7 k J mol-' difference in AG* for the 10-fold smaller. At 25 "C, the k" 'for ferrocene was about two amphiphiles is significant and may reflect a difference twice that of 2-Fc. The k" ' of 2-Fc was consistently larger in 1. In any case, a distance dependence for the electronthan that of 5-Fc by small amounts at all temperatures transfer rate, as featured in the CTAB micelle system, (Figure 4) in the microemulsion. This behavior is very does not seem to explain the kinetic differences in the different from that in CTAB micelles, in which 10-fold CTAC microemulsion. Roughly 10-fold differences ink" ' differences in k" ' were found in the order Fc > 2-Fc > in the order Fc > 2-Fc > 5-Fc are predicted from Marcus ~-Fc.'~ theory if the amphiphiles are oriented head down-tail up The temperature dependence of the kinetic data was analyzed by using Marcus electron-transfer t h e ~ r y . ~ l , ~on ~ the electrode surface.25The differences ink" 'between Fc, 2-Fc, and 5-Fc in the CTAC microemulsion are too Accordingly, the rate constant for outer-sphere electron small to be explained in this way. transfer is related to the free energy of activation, AG*, Values ofk" 'for Fc, 2-Fc, and 5-Fc in the microemulsion by3' were smaller than in homogeneous acetonitrile. Similar k" ' = K~,K,,exp(-AG*/RT) (1) rate decreases have been noted previously in microemulwhere K p is the precursor equilibrium constant, un is the sions for a variety of molecules and This could nuclear collision frequency factor, K,I is the electronic result from partial inhibition by microemulsion compotransmission coefficient, R is the gas constant, and T i s nents adsorbed onto the e l e c t r ~ d e . ~ ~ J ~ temperature in Kelvins. The free energy of activation is It seems reasonably certain that the ferrocene amgiven by phiphiles in the CTAC microemulsion do not achieve AG* = (AGO A)2/4A specific orientations on glassy carbon with different (2) distances of electron transfer. The results could be where AGO is the standard free energy of electron transfer interpreted by a partly random distribution of orientations and A is the reorganization free energy. a t the time of electron transfer. We might expect a more A reaction is considered nonadiabatic when the donor disordered or dynamic fluid interface at the electrode in and acceptor are farther apart than the distance of closest the CTAC microemulsion compared to CTAB micelles. approach. This is a reasonable assumption for an electrode This could be induced a t the electrode-fluid interface by reaction in which the electrode is separated from the oil and/or cosurfactant, which is not present in a micellar reactant by a t least an adsorbed layer of ions or molecules. solution. Specific adsorption of the amphiphiles may also In such cases, un
