Amphiphilic Ferrocene Alcohols as Electroactive Probes in

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Langmuir 1997, 13, 3693-3699

3693

Amphiphilic Ferrocene Alcohols as Electroactive Probes in Micellar Solutions and Microemulsions Xiaolin Zu and James F. Rusling* Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-4060 Received November 22, 1996. In Final Form: April 29, 1997X 2-Ferrocenyl-2-decanol (FcOHC10), 2-ferrocenyl-2-tetradecanol (FcOHC14), and 2-ferrocenyl-2-octadecanol (FcOHC18) were synthesized and evaluated as electrochemical probes for the behavior of alcohols in micellar solutions and microemulsions. One-electron oxidations of the ferrocene moieties were nearly reversible and controlled by diffusion in microemulsions. The electrode reactions were more complex in micellar solutions and were influenced by diffusion, adsorption onto the electrode, and possibly chemical reactions coupled to electron transfer. Amphiphilic ferrocene alcohols were reasonably informative electrochemical probes in bicontinuous microemulsions made with cationic and anionic surfactants. Apparent diffusion coefficients suggested that the alcohols are distributed between the oil-water interfaces and the oil phase of the microemulsions, with increases in chain length favoring binding at this interface. Little evidence was found for adsorption of any ferrocene alcohols at any electrode-microemulsion interface. In comparison to microemulsions of cationic surfactants, the ferrocene alcohols were more easily and rapidly reduced in an SDS microemulsion. This may be attributed to Coulombic interactions between the ferricinium oxidation product and dodecyl sulfate.

Introduction Micellar solutions and microemulsions are surfactantorganized fluids with possible applications as less toxic, less costly substitutes for organic solvents in electrochemical synthesis.1-7 These fluids can have a profound influence on the electrochemistry of solutes. For example, electroactive solutes bound to micelles have much smaller effective diffusion coefficients, yielding smaller currents for diffusion-controlled reactions than in homogeneous solvents. Furthermore, electron transfer rates between electrodes and reactants can be mediated by surfactant structures.2-4 In some cases, the kinetics2,3,5,6 and stereochemistry7 of products of electrochemical reactions can be controlled by the composition of the microemulsion. Amphiphiles containing an electroactive ferrocene moiety are useful electrochemical probes for investigating surfactant-based fluids because they can undergo nearly reversible oxidations. This allows facile estimation of micellar and droplet diffusion coefficients.3-5 Information about surface dynamics and structure can also be inferred from kinetic studies. Recent experiments employing ferrocene amphiphiles suggest that micellar solutions of alkylammonium surfactants form ordered aggregates on platinum and carbon electrodes which can help to orient electroactive amphiphiles on electrodes.8-10 Studies of electrochemistry in water-in-oil microemulsions suggested that the presence of sufficient cosurfactant can improve interfacial fluidity and facilitate electron transfer at X

Abstract published in Advance ACS Abstracts, June 15, 1997.

(1) Mackay, R. A., Texter, J., Eds. Electrochemistry in Colloids and Dispersions; VCH Publishers: New York, 1992. (2) Rusling, J. F. Acc. Chem. Res. 1991, 24, 75-81. (3) Rusling, J. F. in Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 19, pp 1-88. (4) Mackay, R. A. Colloids Surf. 1994, 82, 1-23. (5) Rusling, J. F. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum Press: New York, 1994; No. 26, pp 49-104. (6) Zhou, D.-L.; Gao, J.; Rusling, J. F. J. Am. Chem. Soc. 1995, 117, 1127-1134. (7) Gao, J.; Rusling, J. F.; Zhou, D.-L. J. Org. Chem. 1996, 61, 59725977. (8) Abbott, A. P.; Guonili, G.; Bobbitt, J. M.; Rusling, J. F.; Kumosinski, T. F. J. Phys. Chem. 1992, 96, 11091-11095. (9) Peng, W.; Zhou, D.-L.; Rusling, J. F. J. Phys. Chem. 1995, 99, 6986-6993. (10) Peng, W.; Rusling, J. F. J. Phys. Chem. 1995, 99, 16436-16441.

S0743-7463(96)02037-9 CCC: $14.00

electrodes.11-13 Comparative electron transfer kinetics of cationic surfactants with ferrocene moieties at different distances from the head group suggested increased fluidity of the electrode-fluid interface in bicontinuous microemulsions compared to micellar solutions of cationic surfactants.14 Aliphatic alcohols often serve as cosurfactants in microemulsions.15 Questions then arise as to their influence on electrode reactions. For example, alcohols added to micellar solutions induced adsorption of hydrophobic electroactive solutes onto mercury and silver electrodes.16a Most of the ferrocene amphiphiles used as electrochemical probes in micellar solutions and microemulsions have had alkylammonium head groups.8-10,14 While electrochemistry of micelle-forming ferrocene alkylpolyoxyethylenes with OH groups at the opposite end of the alkylpolyoxyethylene chains has been reported,16b-c we are not aware of their use as probes in microheterogeneous fluids made with other surfactants. In the present work, we prepared long chain secondary aliphatic alcohols containing the ferrocene moiety (Chart 1) attached to the same carbon as the OH group. These molecules were investigated as probes of the behavior of alcohols in microheterogeneous fluids. Herein we report their electrochemistry in micellar solutions and microemulsions. Experimental Section Chemicals. Materials and their sources were acetylferrocene (Fluka), n-octyl bromide (Fluka), n-dodecyl bromide (Aldrich), n-hexadecyl bromide (Aldrich), magnesium (Fluka), petroleum ether (Baker), ethyl acetate (Baker), diethyl ether (anhydrous, (11) Garcia, E.; Song, S.; Oppenheimer, L. E.; Antalek, B.; Williams, A. J.; Texter, J. Langmuir 1993, 9, 2782-2785. (12) Garcia, E.; Texter, J. J. Colloid Interface Sci. 1994, 162, 262264. (13) Antalek, B.; Williams, A. J.; Garcia, E.; Texter, J. Langmuir 1994, 10, 4459-4467. (14) Guonili, G.; Bobbitt, J. M.; Rusling, J. F. Langmuir 1995, 11, 2800-2805. (15) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Dekker: New York, 1988. (16) (a) Rusling, J. F.; Couture, E. C. Langmuir 1990, 6, 425-432. (b) Saji, T. Chem. Lett. 1988, 693-696. (c) Yokoyama, S.; Kurata, H.; Harima, Y.; Yamashita, K.; Hoshino, K.; Kokado, H. Chem. Lett. 1990, 343-346. (d) Takeoka, Y.; Aoki, T.; Sanui, K.; Ogata, N.; Watanabe, M. Langmuir 1996, 12, 487-493.

© 1997 American Chemical Society

3694 Langmuir, Vol. 13, No. 14, 1997 Chart 1

Zu and Rusling Table 1. Oil-Water Distribution Coefficients of Ferrocene Alcohols at 25 °C Ra oil

FcOHC10

FcOHC14

FcOHC18

dodecane tetradecane

180 150

270 240

1250 1010

a R ) [FcOHC ] /[FcOHC ] x oil x water obtained by UV-vis spectroscopy of each separated phase after equilibration of 1 mM FcOH overnight in an oil-water system.

ACS reagent, Aldrich), ammonium chloride (Baker), sodium bicarbonate (Baker), silica gel 60 (EM), didodecyldimethylammonium bromide (DDAB, Acros), dodecane (Kodak), cetyltrimethylammonium chloride (CTAC, Acros), 1-pentanol (Aldrich), tetradecane (Fisher Scientific), sodium dodecyl sulfate (SDS, Kodak), sodium chloride (Baker), acetonitrile (Baker), and cetyltrimethylammonium bromide (CTAB, Acros). Water with a specific resistance > 12 MΩ‚cm prepared with a Sybron/ Barnstead nanopure water purification system was used for all solutions. Microemulsions used were previously characterized as bicontinuous.17-19 They were prepared by mixing surfactants (DDAB, CTAC, or SDS), oils (dodecane or tetradecane), and cosurfactant (1-pentanol) in appropriate weight ratios and titrating these mixtures with water or 0.1 M NaCl. Compositions and specific conductances (κ) of the microemulsions were DDAB/ dodecane/water (21/40/39), κ ) 1.0 × 10-3 Ω-1 cm-1; CTAC/1pentanol/tetradecane/water (17.5/35/12.5/35), κ ) 3.1 × 10-3 Ω-1 cm-1; and SDS/1-pentanol/dodecane/0.1 M NaCl (10/20/6/64), κ ) 7 × 10-3 Ω-1 cm-1. Preparation of Ferrocene Alcohols. Syntheses of FcOHC10, FcOHC14, and FcOHC18 (see Chart 1) were based on similar procedures reported for an amphiphilic ferrocene alcohol olefin.20 Methods were optimized for higher yields. To 8 mmol of magnesium filings were added 3 mL of dry ether and 1/20 of the total amount of 8 mmol of alkyl bromide. The mixture was gently heated until it became slightly turbid. The rest of the alkyl bromide was then added. The mixture was heated for 1 h more after this addition. Then, 4 mmol of acetylferrocene in 120 mL of ether was added over 2 h. The solution was refluxed overnight. After hydrolysis, washing, separation, and removal of the solvent, the residue of the ether phase was purified by flash chromatography on a silica gel column. The yields of the dark red viscous oils were as follows: 2-ferrocenyl-2-decanol, 66%; 2-ferrocenyl-2-tetradecanol, 46%; 2-ferrocenyl-2-octadecanol, 44%. 1H NMR (CDCl3, 270 MHz): δ/ppm ) 4.22 (s, 5 H, C5H5), 4.16, 4.10 (2s, 4 H, C5H4), 2.11 (s, 1 H, OH), 1.60 (m, 2 H, Fc-C-CH2), 1.48 (s, 3 H, Fc-C-CH3), 1.20 (s, 2n H, (CH2)n), 0.86 (t, 3 H, CH3). Apparatus and Procedures. A Bioanalytical Systems BAS100B/W electrochemical analyzer was used for cyclic voltammetry (CV) and chronocoulometry. In cyclic voltammetric measurements, electrochemical cell resistances were compensated by the BAS-100B system to 70 mV were used to estimate24 each k°′, assuming an electrochemical transfer coefficient R of 0.5. D′ was also estimated from chronocoulometric data after background subtraction by using nonlinear regression analysis, as described previously,25,26 by using a general program employing the Marquardt-Levenberg algorithm. Computations assumed absolute errors in measured charges and no errors in time.

Results Distribution Coefficients. The three ferrocene alcohols were not very soluble in water but could be dissolved in organic solvents. In a two-phase oil-water system, the distributions of each compound strongly favored the oil (Table 1). Oil-water distribution coefficients increased as the chain length of the alcohol increased. Electrochemistry in Micellar Solutions. Cyclic voltammetry was done for FcOHC10 and FcOHC14 dissolved in micellar CTAB and CTAC solutions. FcOHC18 was insufficiently insoluble in these media. All ferrocene alcohols were insoluble in micellar SDS solutions. Reversible, diffusion-controlled electrochemical behavior in cyclic voltammetry is characterized22 by peaks for oxidation and reduction having equal heights separated on the potential scale by 59/n mV at 25 °C, where n is the number of electrons transferred per reactant molecule. The curves have an unsymmetric shape, with a characteristic “tail” after each peak. Cyclic voltammograms of FcOHC10 (Figure 1) and FcOHC14 in micellar solutions of CTAB and CTAC were not characteristic of reversible electron transfer. At low scan rates, oxidation peaks had nearly sigmoid shapes at scan rates near 50 mV s-1 (Figure 1a), with a reduction peak of about the same height, but with peak separations (∆Ep) greater than 59 mV. As scan rate was increased (Figure 1b), the relative size of the cathodic peak decreased. Peak currents were not proportional to the scan rate, as (23) Kuwana, T.; Bublitz, D. E.; Hoh, G. J. J. Am. Chem. Soc. 1960, 82, 5811-5817. (24) Nicholson, R. Anal. Chem. 1965, 37, 1351-1355. (25) Sucheta, A.; Rusling, J. F. Electroanalysis 1991, 3, 735-739. (26) Rusling, J. F.; Kumosinski, T. F. Nonlinear Computer Modeling of Chemical and Biochemical Data; Academic Press: San Diego, CA, 1996. (27) Iwunze, M. O.; Sucheta, A.; Rusling, J. F. Anal. Chem. 1990, 62, 644-649.

Ferrocene Alcohols as Electroactive Probes

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Figure 2. Results of regression analysis on backgroundsubtracted chronocoulometric data for 1 mM FcOHC14 in aqueous 0.05 M CTAB at 25 °C. Points are experimental data; line is best fit onto eq 1. Table 3. Midpoint Potentials and Peak Separations in Micellar Solutions at 25 °C solution

probe

C, mM

Em, mV vs SCE

∆Ep, mV

CTAC (0.05 M)

FcOHC10 FcOHC14 FcOHC10 FcOHC14

1.0 1.0 1.0 1.0

332 430 331 411

90a 22a 95a 18b

CTAB (0.05 M) a

Figure 1. Cyclic voltammograms at glassy carbon electrodes of 1 mM FcOHC10 in 0.05 M CTAC at 25 °C and (a) 50 mV s-1, (b) 600 mV s-1. (- - -) Background in 0.05 M CTAC. Table 2. Slopes of log ip vs log ν Plots from CV Data at 0.01-0.5 V s-1 solventa

FcOHC10

FcOHC14

FcOHC18

0.05 M CTAC 0.05 M CTAB DDAB µE CTAC µE SDS µE

0.704 0.716 0.499 0.498 0.506

0.862 0.645 0.511 0.512 0.512

0.500 0.504 0.524

a See Experimental Section for compositions of microemulsions (µE).

expected for adsorption control, or to its square root, characteristic of diffusion control.22 Plots of log peak current (ip) vs log scan rate (ν) have theoretical slopes of 0.5 for diffusion-controlled voltammograms and 1 for ideal adsorption control.22 Table 2 shows that the slopes of log ip vs log ν plots in the two micellar solutions were between 0.5 and 1. Combined with the observations discussed above, this suggests that the electrode reactions of the ferrocene alcohols in micellar solutions are quite complex and involve combined diffusion, adsorption, and possibly participation of a chemical reaction. The method of single-potential step chronocoulometry measures charge (Q) vs time after the application of a large-amplitude potential step.22 It can be employed to obtain diffusion coefficients when diffusion and adsorption simultaneously control an electrode reaction,25,26 by fitting the model in eq 1 to the Q vs t data:

Q(t) ) b0 + b1t1/2 + b2t

(1)

where b0 ) Qdl + nFAΓR, b1 ) 2FAD1/2Cπ-1/2, b2 ) aFADC(πr)-1/2, F is Faraday’s constant, Qdl is the doublelayer charge on the electrode, A is the electrode area, r is

Scan rate, 50 mV s-1. b Scan rate, 200 mV s-1.

the electrode radius, C is the reactant concentration, D is the reactant’s diffusion coefficient, and ΓR is its surface concentration. The parameters b0, b1, and b2 can be obtained by nonlinear regression analysis.26 The complexity of the electro-oxidation process of the ferrocene alcohols in micellar solutions is further illustrated by poor fits of the adsorption-diffusion model in eq 1 to chronocoulometry data (Figure 2). It is clear that factors in addition to diffusion and ideal adsorption control the shape of the charge vs time curve. We shall see that this model gave excellent fits for the same compounds in microemulsions. Midpoint potentials between oxidation and reduction peaks of FcOHC10 were 80-100 mV more positive than those for FcOHC14 in the two micellar solutions (Table 3). Peak separations were much larger for FcOHC10 than for FcOHC14. This suggests stronger adsorption of the C14 alcohol, in accord with Traube’s rule of the increase of surface activity with increasing chain length.3 Electrochemistry in Microemulsions. CV scans of FcOHC10, FcOHC14, and FcOHC18 at scan rates < 500 mV s-1 were characteristic of reversible, diffusion-controlled electron transfer in all the microemulsions examined. Oxidation and reduction peaks were separated by 60-72 mV at low scan rates (Figure 3), and their ratio of heights was roughly 1. Slopes of log ip vs log ν plots were all close to 0.5 (Table 2). The formal potentials of FcOHC10, FcOHC14, and FcOHC18, obtained as midpoints between oxidation and reduction peaks, were similar to one another in a given microemulsion (Tables 4 and 5). Peak separations increased significantly with the increase of scan rate above 1 V s-1 in DDAB and CTAC microemulsions and >30 V s-1 in SDS microemulsions. These results suggest that the electrode reactions in microemulsions are controlled only by diffusion and/or electron transfer kinetics between 0.01 and 50 V s-1. Single-potential step chronocoulometry of ferrocene alcohols in all microemulsions (Figure 4) gave excellent fits to the model in eq 1. Apparent diffusion coefficients

3696 Langmuir, Vol. 13, No. 14, 1997

Zu and Rusling Table 5. Formal Potentials, Peak Separations, Diffusion Coefficients, and Electron Transfer Rate Constants in CTAC and SDS Microemulsiona at 25 °C ∆E,b mV

106D′, cm2 s-1

102k°′, cm s-1

C, mM

E°′, mV vs SCE

FcOHC10 FcOHC14 FcOHC18

4.19 3.78 3.99

CTAC microemulsion 399 ( 1 116 0.92 1.26 ( 0.01 408 ( 2 108 1.00 1.20 ( 0.01 402 ( 3 110 0.88 1.17 ( 0.01

0.74 ( 0.03 0.77 ( 0.02 0.67 ( 0.04

FcOHC10 FcOHC14 FcOHC18

3.95 1.76 3.02

SDS microemulsion 320 ( 1 70 1.69 1.68 ( 0.03 321 ( 1 73 1.64 1.79 ( 0.02 320 ( 1 70 1.19 1.25 ( 0.01

6.21 ( 0.34 6.62 ( 0.98 6.08 ( 0.84

CVc

CCd

a CTAC/1-pentanol/tetradecane/water (17.5/35/12.5/35), SDS/1pentanol/dodecane/0.1 M NaCl (10/20/6/64). b Scan rate, 200 mV s-1. c Cyclic voltammetry. d Chronocoulometry.

Figure 3. Cyclic voltammograms at glassy carbon electrodes in microemulsions at 25 °C: (a) 1 mM FcOHC10 in the DDAB microemulsion at 200 mV s-1; (b) 3.95 mM FcOHC10 in SDS microemulsion at 600 mV s-1. (- - -) Background in microemulsions alone.

Figure 4. Results of regression analysis on backgroundsubtracted chronocoulometric data in the DDAB microemulsion. Points are experimental data: (top curve) 1 mM FcOHC10; (bottom curve) 1 mM FcOHC14. Lines are best fits onto eq 1.

Table 4. Formal Potentials, Peak Separations, Diffusion Coefficients, and Electron Transfer Rate Constants in the DDAB Microemulsiona at 25 °C C, mM FcOHC10 0.25 0.5 1.0 2.0 avg (sd FcOHC14 0.5 1.0 2.0 avg (sd FcOHC18 0.25 0.5 1.0 2.0 4.0 avg (sd c

106D′, cm2 s-1

E°′, mV vs SCE

∆Ep,b mV

CVc

421 ( 2 416 ( 1 418 ( 1 418 ( 2 426 ( 2 421 ( 1 421 ( 2 423 ( 3 440 ( 3 432 ( 3 432 ( 1 433 ( 3 438 ( 4 435 ( 3

69 69 69 69 73 71 71 71 ( 1 74 74 71 71 71 72 ( 2

8.86 5.14 4.42 ( 0.13 2.98 ( 0.17 3.17 3.68 ( 0.10 2.23 ( 0.17 2.83 2.74 ( 0.10 2.39 ( 0.43 2.53 ( 0.26 3.30 3.37 ( 0.08 1.64 ( 0.07 2.35 2.74 ( 0.02 1.64 ( 0.10 2.32 2.66 ( 0.04 1.59 ( 0.18 1.62 ( 0.12 1.59 1.35 0.82 ( 0.11 1.38 1.46 ( 0.16 1.34 1.29 ( 0.20 1.28 0.85 ( 0.10 1.11 ( 0.14

CCd

102k°′, cm s-1

a DDAB/dodecane/water (21/40/39). b Scan rate, 200 mV s-1. Cyclic voltammetry. d Chronocoulometry.

(D′) were estimated from the parameter b1, obtained by nonlinear regression analysis of Q vs t data onto eq 1. Table 4 summarizes electrochemical parameters for different concentrations of ferrocene alcohols in the DDAB microemulsion. Formal potentials and peak separations increased very slightly with increasing chain length. D′ and k°′ were in the order FcOHC10 > FcOHC14 > FcOHC18 with very small differences between each successive compound. D′ values from cyclic voltammetry and chronocoulometry showed acceptable agreement. As the concentration increased, D′ deceased slightly for FcOHC10 and FcOHC14 but remained nearly constant for FcOHC18 (Figure 5).

Figure 5. Influence of concentration on apparent diffusion coefficients in the DDAB microemulsion: (b) FcOHC10; (+) FcOHC14; (O) FcOHC18.

Electrochemical parameters of the ferrocene alcohols in the CTAC and SDS microemulsions are summarized in Table 5. The SDS microemulsion gave faster electron transfer than the CTAC system. Electron transfer rate constants were in the order k°′SDS > k°′DDAB > k°′CTAC. D′ in the CTAC microemulsion behaved qualitatively similarly to the DDAB microemulsion with increasing probe concentration (Figure 6a). That is, D′ for FcOHC10 decreased, while D′ for FcOHC18 did not vary significantly, with increasing probe concentration. In the SDS microemulsion, D′ for both of these probes decreased with probe concentration, with D′FcOHC10 > D′FcOHC18 in the higher concentration range (Figure 6b).

Ferrocene Alcohols as Electroactive Probes

Langmuir, Vol. 13, No. 14, 1997 3697

surfactant is found in the D′ values (Tables 4 and 5). We first discuss the DDAB microemulsion. It is useful to compare the ferrocene alcohol D′ values in the microemulsion with their D values in dodecane. While the latter are not directly available, we have D values for a series of ferrocene alkylammonium surfactants of comparable size (Table 6). Walden’s rule was used with these data to estimate values in dodecane. Apparent D values of each ferrocene alcohol at the lowest concentration (Table 4, 0.25 mM) are comparable to the estimated values in dodecane (Table 6). In contrast, 1-hexadecyl-4-carbomethoxypyridinium ion is strongly bound to oil-water interfaces in this DDAB microemulsion and has a D′ of 0.14 × 10-6 cm2 s-1, compared to a value of 8 × 10-6 cm2 s-1 in homogeneous acetonitrile.28 DDAB residing in oilwater interfacial regions had self-diffusion coefficients of (0.3-0.4) × 10-6 cm2 s-1 in similar microemulsions.17b As the concentration of FcOHC10 or FcOHC14 increased in the microemulsion, apparent D values decreased (Figure 5). This suggests binding of the ferrocene alcohols to larger aggregates. Since the DDAB system is bicontinuous, we can envision binding of the ferrocene alcohols along with DDAB to interfacial subunits containing a series of unspecified interfacial binding units (IS) according to the following equation:

(nFcOHCx)oil + IS h (FcOHCx)nIS

(2)

This process can be described by the overall equilibrium constant26 Figure 6. Influence of concentration on apparent diffusion coefficients of (b) FcOHC10 and (O) FcOHC18 in (a) the CTAC microemulsion and (b) the SDS microemulsion.

Discussion Electrochemistry in Micellar Solutions. Ferrocene alcohols solubilized by aqueous micelles of cationic surfactants gave electrode reactions complicated by adsorption and possibly by chemical reactions coupled to the electron transfer. Thus, while the micelles impart solubilization, adsorption at the electrode-solution interface limits the usefulness of the ferrocene alcohols as probes in micellar solutions. Similarly, simple long chain aliphatic alcohols adsorbed on mercury and silver electrodes form cationic micellar solutions.16a An interesting contrast can be made with the electrochemistry of a C14 ferrocene alcohol with a terminal double bond [FcOHC11CdC]. This compound gave quasireversible electron transfer at a Pt electrode in methylene chloride, and no chemical complications or adsorption were observed.20 Electrochemistry in Microemulsions. In contrast to the case in micellar solutions, electrode reactions of ferrocene alcohols in microemulsions were nearly reversible and diffusion controlled at low scan rates. Oil-water partition coefficients (Table 1) suggest a strong preference for the oil phase and increased with chain length. The microemulsions appear to be good solvents for these compounds, which do not seem to adsorb to the electrodemicroemulsion interface. Diffusion coefficients can be used to help locate the solubilization sites of solutes in microemulsions.3-5 From the partition data (Table 1), it is unlikely that significant amounts of any of the ferrocene alcohols reside in the water phase. They must reside in the oil phase and/or at the oil-water interface. An indication of the binding of ferrocene alcohols at the oil-water interface of the microemulsions with the

K)

[(FcOHCx)nIS] [FcOHCx]n[IS]

(3)

An analogous overall equilibrium approach has been used to describe electrochemically measured diffusion coefficients of probe solutes bound to micelles or microemulsion droplets.3,5 In the present case, D′ can be represented by

D′ ) foDo + fbDI

(4)

where fo is the fraction of the probe in the oil phase, Do is the diffusion coefficient of the probe in the oil phase, fb is the fraction of the probe bound to the interface, and DI is the probe diffusion coefficient in the interface. The fractions fo and fb can be expressed in terms of K, leading to the expression3,26

D′ )

Do n

1 + [IS]K [FcOHCx]n-1

+

DI[IS]Kn[FcOHCx]n-1 1 + [IS]Kn[FcOHCx]n-1

(5)

Equation 5 is difficult to implement quantitatively in the present situation because the capacity of the binding sites IS is undefined. Nevertheless, this expression correctly predicts3,26 the observed dependence of D′ on probe concentration (Figures 5 and 6). Also, as the binding strength increases, i.e. as K becomes larger, eq 5 predicts that the dependence of D′ on concentration becomes less strong, and the D′ vs Cprobe plot flattens out.29 This is observed in the DDAB microemulsion (Figure 5) as chain length increased from (28) Gounili, G.; Miaw, C.-L.; Bobbitt, J. M.; Rusling, J. F. J. Colloid Interface Sci. 1992, 153, 446-456. (29) Rusling, J. F.; Wang, Z.; Owlia, A. Colloids Surfaces 1990, 48, 173-184.

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Zu and Rusling

Table 6. Diffusion Coefficients at 25 °C in Microemulsions and Homogeneous Solvents fluid 0.2 M NaCl(aq) dodecane acetonitrile DDAB µEc

CTAC µEc SDS µEc

probe

residence site

106D, cm2 s-1

oil phase oil phase oil phase oil phase oil phase interface interface water phase water phase water phase oil-water interface oil-water interface oil phase

5.5 4.5 3.1 4.1 3.4 2.3 8 6 5 6 5.5 8 0.3-0.4 0.14 0.3 0.55 0.7 0.59 0.54 3.3

+NFcC a 8 +NFcC a 12 +NFcC a 16 +NFcC a,b 8 +NFcC a,b 12 +NFcC a,b 16

1-hexadecyl-4-carbomethoxypyridinium ferrocene perylene pyrene dodecane 9-phenylanthracene DDAB 1-hexadecyl-4-carbomethoxypyridinium vitamin B12 Co(salen) Ru(NH3)63+ +N(2-Fc)C a,d 12 +N(5-Fc)C a,d 12 ferrocene

ref 9 9 9

28 27 27 27 17b 27 17b 28 6 6 27 14 14 19

a Cationic dimethyl ammonium surfactant ferrocenes of similar chain lengths as the ferrocene alcohols. b Estimated from Walden’s rule and D in water. c Compositions as in Experimental Section. d These derivatives have ferrocene moieties on the alkyl chain at the indicated positions.14

FcOHC10 to FcOHC18, in accord with Traube’s rule. That is, stronger interfacial binding occurs as chain length increases. Finally, as probe concentration is increased, D′ approaches a limiting value close to DI. D′ at the largest concentration of FcOHC18 is 1.3 × 10-6 cm2 s-1 (Table 4). This is still significantly larger than the (0.16-0.4) × 10-6 cm2 s-1 for interfacial surfactant in this system (Table 6). These results suggest that although FcOHC18 has the strongest interfacial binding, there still must be considerable FcOHC18 in the oil phase. Similar conclusions can be drawn from diffusion data obtained in the CTAC microemulsion (Table 5 and Figure 6a). The dependences on probe concentrations are qualitatively described by eq 5. As in the DDAB microemulsion, FcOHC10 shows a concentration dependence of D′ while FcOHC18 does not. Values of D′ at higher concentrations of the alcohols in the CTAC microemulsions are roughly 1 × 10-6 cm2 s-1 (Table 5), about twice the value of C12ammonium ferrocenes that reside mainly in the interface (Table 6). Results are consistent with distribution of all the ferrocene alcohols between the interface and the oil phase of the CTAC microemulsion. The SDS microemulsion showed a D′ dependence on concentration for all the ferrocene alcohols (Figure 6b). Limiting D′ values at high concentration are significantly smaller than that of ferrocene in the oil phase (Table 6), although we have no value for comparison with surfactant in the interfacial region. Again, results are generally consistent with the distribution of ferrocene alcohols between the oil-water interface and the oil phase. The above analysis suggests that the ferrocene alcohols are distributed between the oil phase and the oil-water interfacial region of all the microemulsions studied. As chain length increases, the distribution tends to favor the interfacial region. There is little evidence for adsorption of the ferrocene alcohols at the electrode-fluid interface. Electrochemical Kinetics and Formal Potentials in the Microemulsions. The most striking observation about the dependence of electrochemical kinetics on the nature of the microemulsion is that electron transfer rate constants were 3- to 9-fold larger in the SDS microemulsion than in the microemulsions made with cationic surfactants. Also, the formal potentials of the ferrocene alcohols were 80-100 mV more negative in the SDS microemulsion compared to values in microemulsions made with cationic

Scheme 1 (Fc+OHCx)IS + e– (KO)

(Fc+OHCx)o + IS + e–

(FcOHCx)IS (KR)

(FcOHCx)o + IS + e–

surfactants. Thus, electrochemical oxidation of the ferrocene alcohols is easier and faster in the SDS microemulsion than in the other fluids. Voltammetry of inorganic30 and organic31 ions suggested enhanced electron transfer rates for adsorbed surfactants with head groups of the opposite sign from that of the electroactive ions. Inhibition of electron transfer occurred if the charge signs on the reactant and surfactant were the same, suggesting Coulombic repulsion. The ferrocene alcohols are neutral, however, and adsorbed surfactant ions cannot exert specific Coulombic effects on them. Specific interactions between surfactant and the ferricinium alcohol oxidation products may be important in governing the reduction potential in the SDS microemulsion. A model for interactions of electroactive reactants with surfactant micelles,3 recently extended to microemulsions,19 may provide insight. Scheme 1 presents this model for our probes in bicontinuous microemulsions. (FcOHCx)IS and (Fc+OHCx)IS represent ferrocene derivatives bound to the interfacial surfactant monolayer (IS) separating the oil and water phases. KO and KR are partition coefficients between the probe that is free in either the oil or water phases (subscript “o”) or bound to interfacial surfactant layers (subscript “IS”):

KO ) [(Fc+OHCx)o]/ [(Fc+OHCx)IS] KR ) [(FcOHCx)o]/[(FcOHCx)IS] The reversible formal potential in the microemulsion (E°′) is given by3,19 (30) Rusling, J. F.; Zhang, H.; Willis, W. S. Anal. Chim. Acta 1990, 235, 307-315. (31) Marino, A.; Brajter-Toth, A. Anal. Chem. 1993, 65, 370-374.

Ferrocene Alcohols as Electroactive Probes

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E°′ ) E°′oil + (RT/F) ln{KO(1 + KR)/KR(1 + KO)} + (RT/2F) ln[DO/DR] (6) where is E°′oil is the formal potential in oil, and DO and DR are measured diffusion coefficients for oxidized and reduced forms of the ferrocene alcohol in the microemulsion. FcOHCx has zero charge and resides predominantly in the oil phase. Its oxidation product Fc+OHCx is positively charged, is not as soluble in oil as FcOHCx, and can interact with the negatively charged head groups of SDS. If we assume KR . 1 . KO, eq 6 becomes, to a first approximation,

E°′ ) E°′oil + (RT/F) ln KO - (RT/2F) ln[DO/DR]

(7)

We expect that KO , 1 in the SDS microemulsions. That is, there is more Fc+OHCx interfacially bound than free. Thus, E°′ should be more negative than E°′oil, consistent with the more negative experimental values of E°′ in the SDS microemulsion (Table 5). Approximating DO by the D′ of interfacial ferrocene surfactants of about 0.6 × 10-6 cm2 s-1 (Table 6) and taking DR as an average value of about 2 × 10-6 cm2 s-1 (Figure 6b), we estimate that the last term in eq 7 contributes about +20 mV. Weaker interactions between Fc+OHCx and cationic surfactants are expected. Thus, KO should be larger in microemulsions of cationic surfactants than in SDS. According to eq 6 or 7, this suggests that E°′ in microemulsions of cationic surfactants would be more positive than E°′ in SDS microemulsion, as observed.

This simplified analysis suggests that Coulombic interactions of the ferricinium alcohol with dodecyl sulfate make the formal potentials of the ferrocene alcohols in the SDS microemulsion more negative than those in microemulsions of the cationic surfactants. Similar arguments were proposed to explain the control of solute formal potentials, including that of ferrocene, in SDS microemulsions.19 Also, interactions between Co(salen)and DDA+ ions were proposed to explain positive shifts of the Co(salen) redox potential in DDAB microemulsions.6 Conclusions Amphiphilic ferrocene alcohols are reasonably informative electrochemical probes in bicontinuous microemulsions made with cationic and anionic surfactants. They mimic long chain aliphatic alcohols often used as cosurfactants in microemulsions. Electrochemical results suggested that they are distributed between the oil-water interface and the oil phase of the microemulsions, with increases in chain length favoring stronger binding at the interface. Little evidence was found for adsorption of ferrocene alcohols at any electrode-microemulsion interface, in contrast to observations in cationic micellar solutions. The ferrocene alcohols were more easily and rapidly reduced in the SDS microemulsion, partly because of Coulombic interactions between the ferricinium oxidation products and dodecyl sulfate. Acknowledgment. This work was supported by Grant Nos. CTS-9306961 and CTS-9632391 from the NSF. The authors thank James M. Bobbitt for helpful discussions on the synthetic aspects of this work. LA962037W