Electrochemical Investigation of Amphiphilic Cobalt (III) Complexes

The dependence of measured D0 and E1/2 values on microemulsion composition has ... In contrast to the cobalt(III) complexes, higher D0 values are obse...
0 downloads 0 Views 211KB Size
Download PDF
Langmuir 1997, 13, 4729-4736

4729

Electrochemical Investigation of Amphiphilic Cobalt(III) Complexes and Ferrocene Derivatives in Sodium Dodecyl Sulfate Microemulsions Dilip M. Shah, Keith M. Davies,* and Abul Hussam Department of Chemistry, George Mason University, Fairfax, Virginia 22030 Received February 20, 1997. In Final Form: May 23, 1997X Cyclic voltammetry and chronocoulometry have been used to measure diffusion coefficients (D0) and half-wave potentials (E1/2) of a series of amphiphilic cobalt complexes and ferrocene derivatives in sodium dodecyl sulfate (SDS)/dodecane/water/1-pentanol/NaBr microemulsions. The electrochemical behavior of the cobalt(III) complexes, [Co(phen)3](ClO4)3, [Co(4,7-Me2-phen)3]Cl3, and [Co(4,7-phen2-phen)3]Cl3, has been compared with that of bis(pentamethylcyclopentadienyl)iron, Fe[C5(CH3)5]2 (PMFc), and (ferrocenylmethyl)trimethylammonium bromide, [C5H5FeC5H4CH2N(CH3)3]Br ([TMAFc]Br), in similar microemulsion environments. The dependence of measured D0 and E1/2 values on microemulsion composition has been used to provide information on the dynamic structure of the microemulsions and to examine the details of the solubilization processes involved for the different redox probes. D0 values obtained for [Co(phen)3]3+ (4.7 ((1.6) × 10-7 cm2/s), [Co(4,7-Me2-phen)3]3+ (1.1 ((0.2) × 10-7 cm2/s), and [Co(4,7phen2-phen)3]3+ (1.4 ((0.3) × 10-7 cm2/s) are largely independent of microemulsion composition varying in water content from 87 to 35%. Such behavior is consistent with association of the complexes with the charged surfactant phase of the microemulsion, though the smaller D0 values obtained with the hydrophobic methyl- and phenyl-substituted complexes suggest some partitioning into the oil phase. For PMFc and [TMAFc]Br, the dependence of D0 values on the oil volume fractions has been related to changes in the hydrodynamic radii of the O/W droplets. Estimates of the interfacial area per surfactant head group at the droplet surface have also been made. In contrast to the cobalt(III) complexes, higher D0 values are observed for PMFc (1.8 × 10-6 cm2/s) and [TMAFc]Br (9.1 × 10-7 cm2/s) in low water content microemulsions. Differences are discussed in terms of possible microstructures existing at these compositions. Binding constants, K1, and partition coefficients, Kd, have also been determined for the association of TMAFc2+ and the ferrocenium cation, Fc+, with SDS micelles from the dependence of measured D0 values on the SDS concentration. K1 values obtained for Fc+ (120 ((5) M-1) and TMAFc2+ (1180 ((240) M-1) are compared with values previously determined for ferrocene itself and for amphiphilic cobalt complexes.

Introduction The use of redox-active probes to investigate microemulsion microstructure through electrochemical measurement of diffusion coefficients and half-wave potentials has been well demonstrated.1-6 Electrochemical methods offer fast and simple means of microemulsion characterization, as the diffusion coefficient of an electroactive probe will reflect the self-diffusion of the phase in which the probe resides. Since diffusion behavior depends directly on the organized character of microemulsions, changes in measured D0 values with microemulsion composition can afford insight into transitions from oil-in-water (O/W) to water-in-oil (W/O) droplet microemulsions, through bicontinuous microemulsion structures. Electrochemical measurements can also help to elucidate the details of the solubilization process itself through information they provide on probe partitioning between aqueous and surfactant pseudophases and on the nature of the interactions between redox probes and micellar and microemulsion aggregates. In this study, different composition sodium dodecyl sulfate (SDS) 1-pentanol/dodecane/water/NaBr microemulsion systems have been investigated through cyclic X

Abstract published in Advance ACS Abstracts, August 1, 1997.

(1) Mackay, R. A., Texter, J., Eds. Electrochemistry in Colloids and Dispersions; VCH Publishers, Inc: New York, 1992. (2) Mackay, R. A.; Myers, S. A.; Bodalbhai, L.; Brajter-Toth, A. Anal. Chem. 1990, 62, 1058. (3) Dayalan, E.; Qutubuddin, A.; Hussam, A. Langmuir 1990, 6, 715. (4) Chokshi, K.; Qutubuddin, S.; Hussam, A. J. Colloid Interface Sci. 1989, 129, 315. (5) Rusling, J. F.; Kamau, G. N. J. Electroanal. Chem. 1985, 187, 355. (6) Gounili, G.; Bobbitt, J. M.; Rusling, J. F. Langmuir 1995, 11, 2800.

S0743-7463(97)00181-9 CCC: $14.00

voltammetric and chronocoulometric measurement of diffusion coefficients and half-wave potentials of a series of cobalt(III) complexes and ferrocene derivatives. The cobalt complexes studied, [Co(phen)3](ClO4)3, [Co(4,7-Me2phen)3]Cl3, and [Co(4,7-phen2-phen)3]Cl3, exhibit common formal ionic charges and 3+/2+ redox states but display a considerable variation in size and hydrophobic nature through their ligand structure. The electrochemical behavior of the ferrocenium tetrafluoroborate and hexafluorophosphate salts, [Fc]BF4 and [Fc]PF6, and the ferrocene derivatives, PMFc and [TMAFc]Br, varying in both their ligand structures and ionic charges, has also been determined in SDS micellar and microemulsion media. In a previous electrochemical investigation of cobalt complexes in SDS surfactant solutions,7,8 the balance between electrostatic and specific hydrophobic interactions in the micellar binding of the cobalt probes [Co(phen)3](ClO4)3 and [Co(terpy)2](ClO4)2 was examined. The present study extends our earlier investigation to a wider range of amphiphilic behavior and broadens its focus from SDS micellar solutions to SDS microemulsions. Experimental Section Materials and Sample Preparation. Ultrapure-grade SDS was obtained from Boehringer Mannheim Biochemical. NaBr crystal, reagent-grade dodecane (anhydrous, 99%), and 1-pentanol (99%) were obtained from J. T. Baker Chemical, Aldrich Chemical Co., and Fisher Scientific, respectively. All were used as received. The Co(III) complexes, [Co(phen)3(ClO4)3]‚2H2O, [Co(4,7-Me2phen)3]Cl3, and [Co(4,7-phen2-phen)3]Cl3, were pre(7) Davies, K. M.; Hussam, A. Langmuir 1993, 9, 3270-3276. (8) Davies, K. M.; Hussam, A.; Rector, B. R.; Owen, I. M.; King, P. Inorg. Chem. 1994, 33, 1741-1747.

© 1997 American Chemical Society

4730 Langmuir, Vol. 13, No. 17, 1997

Shah et al.

pared by literature procedures9,10 and analyzed spectrophotometrically. The ligands 1,10-phenanthroline (phen), 4,7-dimethyl1,10-phenanthroline (4,7-Me2-phen), and 4,7-diphenyl-1,10phenanthroline (4,7-phen2-phen) were obtained from GFS Chemicals and recrystallized from ethanol. Ferrocene (Aldrich) was purified by sublimation. [Fc]BF4 and [Fc]PF6 (from Aldrich) and PMFc and [TMAFc]Br (from Strem Chemical Co.) were used as received. Microemulsion Solution Preparation. All microemulsions studied contained the surfactant sodium dodecyl sulfate (SDS), 1-pentanol (n-amyl alcohol) as cosurfactant, NaBr as electrolyte, and n-dodecane as the oil. The composition is given as a weight percent basis unless specified otherwise. The composition of the microemulsion was varied by changing the ratio of dodecane to water from 1:20 to 1:7, at constant SDS/1-pentanol. This range of composition corresponds to a relatively low dodecane content (e10% oil) and a range of water content from 87% to 35%. In the text, microemulsions are referred to by their ratio of water to oil. For example, a microemulsion with 79.2% water and 4.0% dodecane oil is a 79.2/4.0 microemulsion. Microemulsion solutions were prepared by combining required weights of the surfactant, water, NaBr, and cosurfactant, with constant stirring. To the above aqueous solution, the required weights of the oil, dodecane, were added dropwise until clear and homogeneous solutions were obtained. Microemulsions, stored in tightly stoppered glass bottles, appear to be stable indefinitely. No evidence of phase separation (turbidity), change in UV-visible absorption, or lack of reproducibility of electrochemical data was found using microemulsions which had been stored at room temperature for several months. Ferrocene derivatives and cobalt complexes, used as the electroactive probes, were added directly to a measured volume of microemulsion in an electrochemical cell and stirred. Ultrasonication was used, in some instances, to aid the dissolution. Microemulsion solutions containing a dissolved probe were used immediately after preparation. Solutions were not routinely deaerated, due to the complication of sudsing. In trial experiments with each probe, when deaeration was carried out, the presence of dissolved oxygen did not change the electrochemical response in the potential range studied. Micellar solutions at different SDS concentrations were generated in the electrochemical cell by adding appropriate volumes of ca. 0.5 M SDS to the cell containing the probe in 0.10 M NaBr using a microsyringe. Electrode Preparation. The glassy-carbon working electrode was polished prior to each use with 1 and 0.3 µm alumina paste. It was then washed with doubly distilled water, ultrasonicated for at least 3-5 min, rinsed with distilled water and acetone, and allowed to air dry. Equipment and Instrumentation. Electrochemical investigation of the cobalt complexes and ferrocene derivatives in microemulsion solutions was carried out by cyclic voltammetry and chronocoulometry. Cyclic voltammetry was performed using a BAS (Bioanalytical systems) Model 100B electrochemical analyzer and BAS Model CV1B voltammograph connected to a Houston Instrument Model 2000 omnigraphic recorder. Measurements were made in a jacketed electrochemical cell, thermostated at 25.0 ( 0.1 °C. A glassy-carbon working electrode, a Pt wire auxiliary electrode, and a Ag/AgCl (saturated sodium chloride) reference electrode (all BAS) were used. Scan rates between 10 and 250 mV/s were employed. Measurement of Diffusion Coefficients. Cyclic Voltammetry (CV). Diffusion coefficients of the cobalt(III) complexes and ferrocene derivatives in microemulsions were obtained from the Randles-Sevcik equation.11

ip ) 2.69 × 105n3/2AC0D1/2 V1/2

(1)

diffusion coefficient in cm2/s, and V ) scan rate in V/s. Diffusion coefficient values were obtained from a linear regression of the slope of ip versus V1/2 plots using the known surface area of the electrode. Half-wave potentials of the redox probes were obtained from the mean of the cathodic and anodic peak potentials, E1/2 ) (Epa + Epc)/2. Chronocoulometry. Diffusion coefficients in microemulsion solutions were also measured by chronocoulometry,12,13 with a large-amplitude double potential step. Measurements were made using a custom-built computerized electrochemical system14 and a BAS electrochemical analyzer, Model 100B. In a typical experiment with ferrocene, the potential is stepped from an initial value, 0 mV, at which no redox process occurs at the electrode (i ) 0) to a second final value 600 mV, where diffusion-controlled oxidation occurs and is enforced for a fixed period (the pulse width). The potential is then stepped back to the initial potential 0 mV where reduction occurs, again at limiting rate. For ferrocene, the potential steps were 0 to 600 to 0 mV, with 200 charge data points collected on each step at 1 ms intervals. A total of 200 charge data points were similarly collected during both oxidation and reduction steps, for all other probes over the particular potential range of interest: PMFc (-300 to +300 mV), TMAFc (0 to 700 mV), [Co(phen)3](ClO4)3 (400 to 0 mV), [Co(4,7-phen-phen)3]Cl3 (350 to 0 mV), and [Co(4,7-Me2-phen)3]Cl3 (+300 to -100 mV). For a large potential step at a planar electrode, the measured charge removed with time, for a reduction step, is given by

Q ) -2nFAC0(Dt/π)1/2 + Qdl + Qads

(2)

where A, C0, D, and n are as in eq 1. -2nFAC0(Dt/π)1/2 is the cumulative charge, from t ) 0, passed in reducing the diffusing electroactive probe, Qdl is the charge flowing to the interfacial capacitance (the electrode double layer), and Qads is the charge resulting from reduction of any adsorbed electroactive probe, )nFAΓ, for Γ as mol/cm2 of adsorbed oxidant. In the reverse step, the measured charge removed is given by the equation

Qr(t>τ) ) Qτ - Q(t>τ) Qr(t>τ) ) Qdl + (2nFAD01/2C0/π1/2) [τ1/2 + (t - τ)1/2 - t1/2] + nFAΓ (3) where Qdl is the charge due to the double layer, D0 is the diffusion coefficient of the oxidant, C0 is its bulk concentration, and τ is pulse reversal time. Equation 3 shows that the plot of residual charge (Q) versus the term τ1/2 + (t - τ)1/2 + t1/2 should be a straight line, from the slope and intercept of which one can obtain D0 and Qdl, respectively. In the absence of product adsorption, the difference of intercepts of the Q versus t1/2 plot for eq 2 and the Qr versus τ1/2 + (t - τ)1/2 + t1/2 plot for eq 3 gives a direct measurement of adsorbed species.15 The average double-layer capacity (Qdl) of the glassy-carbon electrode, obtained with a typical probe (PMFc) in microemulsions containing various concentrations of dodecane (0 -10 w/w %), is about 60 ( 10 µF/ cm2 and shows no trend in Qdl as a function of dodecane concentration. This indicates no significant change in the nature of the glassy carbon surface during electron transfer. This is consistent with our previous study using both glassy-carbon and Pt as the working electrode,7 where insignificant adsorption of oil or probe was found. Nonlinear parameter estimation was performed with MINSQ program (Micromath Scientific Software, Utah) as described previously.7

Results and Discussion where ip) peak current in amps, n ) number of electrons transferred, A ) area of the working electrode in cm2, C0 ) initial bulk concentration of the electroactive species in mol/cm3, D ) (9) Baker, B. R.; Basolo, F.; Neumann, H. M. J. Phys. Chem. 1959, 63, 371. (10) Warren, R. M. L.; Lappin, A. G.; Dev Mehta, B.; Neumann, H. M. Inorg. Chem. 1990, 29, 4185. (11) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980.

Diffusion Coefficient Measurements in SDS Microemulsions. Diffusion coefficients measured for cobalt phenanthroline complexes and ferrocene derivatives in (12) Anson, F. C. Anal. Chem. 1966, 38, 54. (13) Turner, J. A.; Turner, B. A.; Darkinson, B. A. J. Electroanal. Chem. 1983, 150, 611. (14) Hussam, A. Anal. Chem. 1988, 60, 2776. (15) Anson, F. C.; Osteryoung, R. A. J. Chem. Educ. 1983, 60 (4), 293.

Cobalt(III) Complexes and Ferrocene Derivatives

Langmuir, Vol. 13, No. 17, 1997 4731

Table 1. Measured Diffusion Coefficients and Half-Wave Potentials of [Co(phen)3](ClO4)3a in SDS/1-Pentanol/ Dodecane/NaBr Microemulsions diffusion coefficient (10-7 cm2 s-1) b

water/dodecane D(R) 87.0/2.0f 79.2/4.0g 77.2/6.0g 75.7/7.5g 74.4/8.8g 73.2/10.g 39.0/6.0h 35.0/10.0h

3.6 5.6 6.2 6.0 5.8 5.6 4.8 4.5

D(O)b 3.0 5.4 7.1 7.7 7.6 5.4 5.2 4.9

c

half-wave potential (mV vs Ag/AgCl)

D(R)

D(O)c

E1/2d

∆Ep

2.8 4.3

3.0 4.5

4.6 2.8 2.7

4.5 3.8 3.5

227 226 219 215 215 219 239 262

64 70 78 70 70 67 73 75

Table 4. Measured Diffusion Coefficients and Half-Wave Potentials of (Ferrocenylmethyl)trimethylammonium Bromidea in SDS/1-Pentanol/Dodecane/NaBr Microemulsions diffusion coefficient (10-7 cm2 s-1)

e

a Probe concentration ) 1.0 mM. b Chronocoulometry. c Cyclic voltammetry. d E1/2 ) (Epa + Epc)/2. e ∆Ep ) Epa - Epc. f 3.32% SDS; 6.68% 1-pentanol; 1% NaBr. g 5.50% SDS; 10.3% 1-pentanol; 1% NaBr. h 18.0% SDS; 36.0% 1-pentanol; 1% NaBr.

half-wave potential (mV vs Ag/AgCl)

water/dodecane

D(R)b

D(O)b

D(R)c

E1/2d

∆Epe

87.0/2.0f 79.2/4.0g 77.2/6.0g 75.7/7.5g 74.4/8.8g 73.2/10.0g 39.0/6.0h

3.5 7.5 7.0 6.1 5.2 4.8 9.1

4.9 6.9 6.7 5.9 5.3 4.9 9.4

2.8 5.5 4.6 3.7 3.5 3.4 5.6

440 450 448 453 448 459 473

60 60 65 65 65 60 65

a Probe concentration ) 0.78 mM. b Chronocoulometry. c Cyclic voltammetry. d E1/2 ) (Epa + Epc)/2. e ∆Ep ) Epa - Epc. f 3.32% SDS; 6.68% 1-pentanol; 1% NaBr. g 5.50% SDS; 10.3% 1-pentanol; 1% NaBr. h 18.0% SDS; 36.0% 1-pentanol; 1% NaBr.

Table 2. Measured Diffusion Coefficients and Half-Wave Potentials of [Co(4,7-Me2-phen3)]Cl3a and [Co(4,7-phen2-phen)]Cl3a in SDS/1-Pentanol/Dodecane/ NaBr Microemulsions diffusion coefficient (10-7 cm2 s-1)

half-wave potential (mV vs Ag/AgCl)

water/dodecane D(R)a D(O)a D(R)b D(O)b

E1/2c

∆Epd

87.0/2.0f 79.2/4.0g 73.2/10.0g 39.0/6.0h

[Co(4,7-Me2-phen)3]Cl3 0.9 1.1 1.0 1.2 1.4 1.5 1.2 1.7 1.2 1.3 1.1 1.5 1.4 1.5 0.8 1.2

106 107 101 134

84 102 95 148

87.0/2.0f 79.2/4.0g 73.2/10.0g 39.0/6.0g

[Co(4,7-phen2-phen)3]Cl3 0.9 1.1 1.2 1.0 1.4 1.9 1.6 1.3 1.2 1.6 1.4 1.1 2.0 3.1 1.8 1.7

200 206 205 241

54 61 61 66

a Probe concentration ) 1.0 mM. b Chronocoulometry. c Cyclic voltammetry. d E1/2 ) (Epa + Epc)/2. e ∆Ep ) Epa - Epc. f 3.32% SDS; 6.68% 1-pentanol; 1% NaBr. g 5.50% SDS; 10.3% 1-pentanol; 1% NaBr. h 18.0% SDS; 36.0% 1-pentanol; 1% NaBr.

Table 3. Measured Diffusion Coefficients and Half-Wave Potentials of Bis(pentamethyl)ferrocenea in SDS/ 1-Pentanol/Dodecane/NaBr Microemulsions diffusion coefficient (10-7 cm2 s-1)

half-wave potential (mV vs Ag/AgCl)

water/dodecane

D(R)b

D(O)b

D(R)c

E1/2d

∆Epe

87.0/2.0f 79.2/4.0g 77.2/6.0g 75.7/7.5g 74.4/8.8g 73.2/10.0g 39.0/6.0h

2.7 7.0 6.1 5.1 5.0 4.1 18

2.0 7.7 7.1 6.4 5.6 4.4 19

2.2 6.3 5.6 5.0 4.8 4.4 12

-60 -49 -44 -39 -35 -35 -28

60 65 68 68 70 70 65

a Probe concentration ) 1.0 mM. b Chronocoulometry. c Cyclic voltammetry. d E1/2 ) (Epa + Epc)/2. e ∆Ep ) Epa - Epc. f 3.32% SDS; 6.68% 1-pentanol; 1% NaBr. g 5.50% SDS; 10.3% 1-pentanol; 1% NaBr. h 18.0% SDS; 36.0% 1-pentanol; 1% NaBr.

SDS/dodecane/1-pentanol/NaBr microemulsions are summarized in Tables 1-4. Three types of microemulsion solutions were examined; high water content (87% H2O), lower water content (79.2-73.2%) with variable dodecane (2-10%), and low water content (35-39% H2O). All solutions had the same 1:2 ratio of surfactant/cosurfactant, though the total emulsifier content varied in the three, with 3.3%, 5.5%, and 18.0% SDS, respectively. Diffusion coefficients were measured by both cyclic voltammetry and chronocoulometry. Good general agreement was found between the two techniques, with values typically within ca. 10% of each other, although those obtained by chronocoulometry were generally higher.

Figure 1. Scan rate dependence of cathodic peak current in SDS microemulsions (H2O/dodecane/SDS/1-pentanol) with 1% NaBr: (a) PMFc (39/6/18/36); (b) TMAFc (87/2/3.3/6.7); (c) [Co(4,7-phen2-phen)3]Cl3 (73.2/10/5.5/10.3).

Considering the relatively low probe concentrations employed (1 mM) and the background corrections that were made, differences are not considered significant. Cyclic voltammograms showed close to reversible behavior in all cases with constant ip/V1/2 ratios at all scan rates between 10 and 250 mV/s and anodic and cathodic peak separations, ∆Ep ) Epa - Epc, between 60 and 70 mV. Typical data are illustrated in Figure 1. Electron transfer and probe partitioning between phases are assumed to be rapid. Double potential step chronocoulometry indicated the charge resulting from probes adsorbed on the electrode surface to be on the order of 4 µC/cm2, which for the probe concentrations employed are considered negligible. Electrochemical measurements carried out on PMFc as a function of probe concentration (0.5, 1.0, 2.0, and 5.0 mM) showed no significant differences in the D0 values ((4.80 ( 0.29) × 10-7 cm2/s) obtained. This suggests that the microemulsion shape and size were not perturbed by the probe in this concentration range. Diffusion Data for [Co(phen)3](ClO4)3, [Co(4,7-Me2phen)3]Cl3, and [Co(4,7-phen2-phen)3]Cl3 in High Water Content Microemulsions. Diffusion coefficients measured for the three cobalt(III) complexes, in microemulsion solutions ranging in water content from 87% to 73.2% H2O, did not vary with changes in the microemulsion composition. There was no dependence on oil/water ratios at constant SDS, as found for the ferrocene derivatives (see below), and the location of the solubilized probe does

4732 Langmuir, Vol. 13, No. 17, 1997

Shah et al.

not appear to be affected by the size of the oil core. With [Co(phen)3]3+ as the probe, the diffusion coefficient (D0 ) 2.8 × 10-7 cm2/s) obtained in the highest water content microemulsion (87% H2O /2.0% dodecane /3.3% SDS) resembled the limiting value (D0 ) 2.2 × 10-7 cm2/s) previously determined in a 0.0204 M SDS/0.20 M LiCF3SO3 micellar solution.7 An O/W droplet microstructure is assumed at all the high water content compositions. The observed overall insensitivity of D0 values to the microemulsion composition resembles the behavior of the methylviologen cation MV2+ (1,1′-dimethyl-4,4′-bipyridinium ion) in anionic SDS microemulsions (D0 ) 2.0 ((0.5) × 10-6 cm2/s) and Fe(CN)63- in cationic CTAB microemulsions (D0 ) 1.0 ((0.5) × 10-6 cm2/s), where little change with microemulsion composition was also found.2 In both cases the independence of D0 on composition was taken as evidence for probe residence at the microemulsion interface. Highly charged probes are expected to be associated electrostatically with a membrane phase of opposite charge, and measured D0 values will reflect the mobility of the O/W microemulsion droplet when the probes are strongly bound. With the cobalt(III) complexes, similar charge considerations apply and strong electrostatic associations with the surfactant phase appear likely. In addition, the strongly hydrophobic phenanthroline ligands may give rise to some additional interaction with the microemulsion oil phase. Some hydrophobic contribution to micellar binding was apparent for [Co(phen)3]3+ in SDS micellar solutions,7 and similar effects would not be unexpected with SDS microemulsions. A microemulsion droplet structure is envisaged in which the metal center is located near the surface of the microemulsion to maximize ionic interactions between the cationic charge and the anionic surfactant head groups but is partly buried in the hydrocarbon region to provide favorable hydrophobic interactions between two of the hydrophobic ligands and the dodecane oil core. Similar structures have been proposed for polypyridine complexes of ruthenium and osmium in SDS micelles based on solvent accessibility data for bound photosensitizers.16 The somewhat smaller D0 values found for [Co(4,7-Me2phen)3]Cl3 (D0 ) 1.3 ((0.2) × 10-7 cm2/s) and [Co(4,7phen2-phen)3]Cl3 (1.5 ((0.3) × 10-7 cm2/s), than for [Co(phen)3]3+ (D0 ) 4.7 ((1.6) × 10-7 cm2/s), suggest a greater degree of partitioning into the microemulsion droplet, presumably as a result of increased hydrophobic interaction with the dodecane core. The strongly hydrophobic 4,7-dimethyl-1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline ligands would be expected to enhance the association of the methyl- and phenylsubstituted complexes with the droplet oil phase, while further limiting their solubility in water. Assuming complete partitioning into the droplet and using ηwater ) 0.0089 P(g‚cm-1 s-1) as the viscosity of the continuous aqueous phase, the D0 values for [Co(4,7-Me2-phen)3]3+ and [Co(4,7-phen2-phen)3]3+ yield microemulsion hydrodynamic droplet radii (rh) of 189 and 163 Å, respectively, through the Stokes-Einstein equation

D ) kT/6Πηrh

(4)

Droplet size increases with solubilization of the larger substituted phenanthroline probes in the hydrocarbon interior. The [Co(phen)3]3+ complex is partitioned between the microemulsion droplet and the bulk aqueous phase due to its appreciable solubility (ca. 5 × 10-3 M) in water. (16) Hauenstein, B. L.; Dressick, W. J.; Bull, S. L.; Demas, J. N.; Degraff, B. A. J. Am. Chem. Soc. 1983, 105, 4252-4255.

Figure 2. Effect of [SDS] on diffusion coefficient of TMAFc2+ (0), [Fc]PF6 (∇), and [Fc]BF4 (O) in 0.10 M NaBr.

Ferrocene Derivatives in SDS Micellar Solutions and SDS Microemulsions. Measurement of Association Constants for [TMAFc]Br, [Fc]BF4, and [Fc]PF6 with SDS Micelles. Because of their relevance to diffusion behavior in SDS microemulsions, diffusion coefficient measurements were first made on amphiphilic ferrocene derivatives in SDS micellar solutions. Both [TMAFc]Br and the tetrafluoroborate and hexafluorophosphate salts of the ferrocenium cation, [Fc]BF4 and [Fc]PF6, were employed. On adding SDS to a solution (0.50-0.78 mM) of the probe in 0.10 M NaBr, the measured diffusion coefficient decreased sharply from an initial value (ca. 6 × 10-6 cm2/ s) with no added surfactant to values of 1.12 × 10-7, 1.29 × 10-7, and 4.74 × 10-7 cm2/s for [TMAFc]Br, [Fc]BF4, and [Fc]PF6, respectively. Minimum values were obtained at SDS concentrations between 0.10 and 0.20 M, where association with the micellar phase appeared to be complete (Figure 2). This behavior is very similar to that reported previously for the cobalt complexes, [Co(phen)3](ClO4)3 and [Co(terpy)2](ClO4)2, which also exhibited ca. 20-fold decreases in D0 with added surfactant.7 The Dobs-[SDS] data were fitted using the same mass action model (eq 5) described previously for the cobalt

Dobs )

Df + K1(St - cmc)Db 1 + K1(St - cmc)

(5)

complexes,7 where Df and Db are diffusion coefficients of the free (obtained in the absence of SDS) and micellar bound probes, St is the total molar concentration of added surfactant, and K1 is the association constant per unit of SDS in the micelle. The diffusion coefficient values used are DR values and relate to the association of the oxidized probes, Fc+ and TMAFc2+, with SDS micelles. Values of K1 and Db, obtained from eq 5, are shown in Table 5. The estimated Db values presented in the table are indicative of the diffusion of completely micelle-bound species. The error associated with such values are generally large due to the interdependence of K1 and K1Db as adjustable fitting parameters. The unusually small Db value for TMAFc2+ we believe is a reflection of the fitting of the data to eq 5 rather than to the size of the TMAFc2+-bound micelle. Similar small Db values were obtained from Co(phen)33+/2+-SDS data reported earlier.7 The data in Table 5 indicate that the association of TMAFc2+ is an order of magnitude higher than that of the unsubstituted Fc+ cation, whether present as the tetrafluoroborate or hexafluorophosphate salt, and electrostatic effects clearly

Cobalt(III) Complexes and Ferrocene Derivatives Table 5. Least-Squares Fitting Parameters from Mass-Action Models for Ferrocenium Tetrafluoroborate, Ferrocenium Hexafluorophosphate, and (Ferrocenylmethyl)trimethylammonium Bromide in SDS with 0.1 M NaBra parameter

[Fc]BF4

[Fc]PF6

TMAFc2+

K1 cmc (mM) K1Db Df r2 Db

167 ( 50 3.9 ( 2.0 8.5 ((3.0) × 10-5 3.0 ((1.0) × 10-6 0.9896 5.1 ((2.3) × 10-7

120 ( 5 8.6 ( 0.2 2.6 ((0.6) × 10-5 3.0 ((0.4) × 10-6 0.9977 2.2 ((0.5) × 10-7

1180 ( 240 1.9 ( 0.03 7.7 ((1.5) × 10-5 5.1 × 10-6 0.9946 6.5 ((1.6) × 10-8

a K in L/mol; cmc, critical micelle concentration (molar) of SDS 1 in the presence of a complex; r2, squares of correlation coefficient. Df is considered as a fitting parameter for [Fc]BF4 and [Fc]PF6. For TMAFc2+, Df is fixed. Db values are estimated from parameter values. SDS concentration range: 0.94-63 mM for FcBF4; 7-100 mM for FcPF6; 0-68 mM for TMAFc2+. Errors are one standard deviation at 95% confidence level.

dominate. This suggests that the association of TMAFc2+ with the anionic micellar surface arises mostly from the positive charge on the quaternary nitrogen. This is possibly more accessible than that on the iron center, which is buried between and delocalized by the cyclopentadienyl rings. It is possible that substitution of the electronwithdrawing quaternary nitrogen group in the cyclopentadienyl ring may also serve to enhance the positive charge at the iron center by inhibiting its delocalization through the π-system of the organic ligands. The binding constants measured for the ferrocene derivatives may be compared with those previously reported for amphiphilic Co(II)/Co(III) redox probes.7 The magnitude of the binding constant measured for TMAFc2+ (K1 ) 1175 M-1) is comparable with that obtained for [Co(phen)3]2+ (K1 ) 1038 M-1). The larger K1 value obtained for association of [Co(phen)3]2+ than that for [Co(phen)3]3+ (K1 ) 550 M-1) has been attributed to greater hydrophobic solvation of the phenanthroline ligands, when attached to Co(II), offsetting the electrostatic advantage of the 3+ complex. Although some hydrophobic contribution to the micellar binding of TMAFc2+ is also possible, through partial solvation of the unsubstituted cycopentadienyl ring or the methyl groups of the quaternary nitrogen center, electrostatic effects arising from the compact metallocene structure and the high positive charge density of the quaternary nitrogen are assumed dominant. The smaller K1 values measured for the Fc+ cation (K1 ≈ 150 M-1) than those for the divalent and trivalent probes further support the dominance of electrostatic factors in the micellar binding of these cationic probes. We are not aware of any previous literature value for the association of the ferrocenium cation with SDS micelles. A ferrocene-SDS micellar association constant (K1 ) 340 M-1) has been reported.17 The higher value for the uncharged ferrocene molecule is not unexpected, due to its greater hydrophobic nature and enhanced solubilization in the micellar interior. Such findings illustrate the sensitive balance between electrostatic and specific hydrophobic factors which determines the micellar binding of amphiphilic redox probes of low charge. The limiting DR value of 1.29 × 10-7 cm2/s obtained for [Fc]BF4 in ca. 0.10 M SDS compares to that measured for the ferrocenium cation in 0.20 M Li2SO4/0.20 M C12TAB (D0 ) 6.7 × 10-6 cm2/s).18 The latter value was virtually unchanged from the value (6.0 × 10-6 cm2/s) obtained in 0.10 M NaBr in the absence of surfactant. The data are consistent with the Fc+ cation residing exclusively in the (17) Bunton, C. A.; Cerichelli, G. Int. J. Chem. Kinet. 1980, 12, 519. (18) Ohsawa Y.; Aoyagui, S. J. Electroanal. Chem. 1982, 136, 353360.

Langmuir, Vol. 13, No. 17, 1997 4733 Table 6. Table of Diffusion Coefficients, Micellar Volumes, Micelle-Bound Fractions, and Distribution Constants for FcBF4, FcPF6, and TMAFc2+ in 0.10 M NaBra

FcBF4 FcPF6

[TMAFc]2+

[SDS] (M)

D × 106 (cm2/s)

Vm (mL)

Fb

Kd ( s

0.00464 0.00920 0.0152 0.0196 0.0407 0.0762 0.106 0.0556 0.0116 0.0218 0.0683

2.9 1.5 1.83 1.29 0.811 0.515 0.474 1.44 0.390 0.283 0.112

0.00230 0.0166 0.0206 0.0345 0.102 0.219 0.316 0.0114 0.0304 0.0631 0.215

0.599 0.918 0.425 0.618 0.789 0.894 0.909 0.729 0.936 0.957 0.991

1450 ( 960 4060 ( 2200 570 ( 300 755 ( 280 600 ( 200 640 ( 200 520 ( 170 3790 ( 1400 7740 ( 2700 5740 ( 2000 8180 ( 2800

a V was calculated from the total [SDS] and the cmc obtained m from nonlinear least-squares parameters as shown in Table 3. Fb is the fraction of solutes bound to micelles. s is the standard deviation of Kd at 95% confidence interval.

bulk aqueous phase in CTAB solutions due to repulsions between the Fc+ cation and the positively charged CTAB micellar surface, whereas in SDS solutions they are strongly associated with the anionic micelles. The volume fraction partition coefficients of the probes were calculated using eq 6, where Fb is the micelle-bound

Kd ) [Fb/Vm]/[Ff/(Vs - Vm)]

(6)

fraction, Ff is the fraction of free solute, Vm is the volume of micelles, and Vs is the total solution volume. These values were calculated as described in ref 7 by using the best parameters in Table 5. The results are presented in Table 6. Kd values are reported only for solutions above the cmc and for Fb < 1. Kd is relatively constant for all probes within experimental error. The Kd values should be independent of the phase ratio for ideal dilute solutions. Although the experimental errors are large due to uncertainty in the estimated Db values, the partition coefficients do show little dependence on the concentration of SDS and the data support the distribution model reasonably well. This is particularly true for [Fc]PF6 and TMAFc2+. No firm conclusion can be made for the [Fc]BF4 salt due to limited data. The cmc values suggest that Fc+ and TMAFc2+ have no significant effect on the SDS micellar aggregation. They are in good agreement with the literature value of 1.4 mM in 0.1 M NaCl.19 The best fit cmc value obtained with [Fc]PF6 is unexpected and is difficult to explain. The cmc obtained with this probe is similar to that of pure SDS with no salt present. In the absence of SDS, the diffusion coefficients of all the ferrocene derivatives are similar and close to the literature value for ferrocene in 0.1 M NaCl (D ) 6.3 × 10-6 cm2/s).20 Diffusion Measurements for Ferrocene Derivatives in SDS Microemulsions. Diffusion coefficients obtained for the ferrocene derivatives PMFc and TMAFc in SDS microemulsions are summarized in Tables 3 and 4. In the highest water content microemulsions (87% H2O/ 2% dodecane/3.32% SDS/6.68% 1-pentanol), D0 values for TMAFc2+/+ and PMFc+ (ca. 2 × 10-7 cm2/s) were virtually the same as the lowest diffusion coefficients obtained in aqueous micellar solutions at high SDS concentrations. The SDS concentration (ca. 0.12 M) is comparable in the two media, and the microstructures are expected to be (19) Ohsawa, Y.; Shimazaki, Y.; Aoyagui, S. J. Electroanal. Chem. 1980, 114, 235. (20) Dayalan, E.; Qutubudin, S.; Hussam, A. Langmuir 1990, 6, 715721.

4734 Langmuir, Vol. 13, No. 17, 1997

Shah et al.

Table 7. Variation of Hydrodynamic Radiusa and Interfacial Area per Surfactant Moleculeb with Microemulsion Composition at Constant Surfactant Concentrationc wt % H2O/ wt % oil

φ0d

79.2/4.0 77.2/6.0 75.7/7.5 74.4/8.8 73.2/10.0

0.0549 0.0820 0.102 0.119 0.135

PMFc rha (Å) asb (Å2) 38.9 43.8 49.1 51.1 55.8

48.2 61.9 66.9 74.6 76.2

TMAFc rha (Å) asb (Å2) 44.6 53.3 66.3 70.1 72.1

40.3 48.3 46.4 51.1 56.1

a Calculated from diffusion data using the Stokes-Einstein equation. b Estimated from eq 7 assuming thickness of surfactant film ) 10 Å. c 5.5% SDS. d Volume fraction of oil calculated from φ0 ) volume dodecane/(volume dodecane + volume water + volume 1-pentanol), using known densities.

somewhat similar. The high water content 87/2 microemulsion is a “classical” dilute microemulsion, with a small proportion of the dispersed phase localized in isolated, spherical swollen micelles. The droplet concentrations are low, and their mutual interactions are expected to be very small. The D0 values observed for TMAFc2+ in waterrich SDS microemulsions are much lower than the value, 2.0 × 10-6 cm2/s, reported for the methylviologen cation, MV2+, in a similar composition SDS microemulsion.2,20 The higher D0 value of MV2+ has been attributed to appreciable partitioning of MV2+ into the aqueous phases and to only a small proportion residing in the nonaqueous phase and diffusing at a slower droplet diffusion rate. The weaker association of MV2+ with the nonaqueous pseudophase, than was found for TMAFc2+, presumably arises because of the respective charge distributions in the 2+ cations. In TMAFc2+, the positive charge on the quaternary nitrogen is buried within the four alkyl groups of the alkylammonium center, and the largely delocalized charge on the iron would appear to favor a greater degree of solvation by the hydrophobic oil core than is experienced by MV2+. In MV2+, the positively charged nitrogens reside at opposite ends of the bipyridinium cation, where they can be more strongly solvated by the polar water. After correction for the probe partitioning between phases, values of 106 and 120 Å were determined for the hydrodynamic radii of O/W SDS microemulsions using MV2+ as probe.20 These values compare with the value of 112 Å, which we calculate for the droplet radius using the PMFc value (D0 ) 2.2 × 10-7 cm2/s), assuming that the highly hydrophobic pentamethylferrocene is completely partitioning into the oil droplet. In microemulsions with 79.2-73.2% H2O and variable dodecane, diffusion coefficients obtained for PMFc and [TMAFc]Br decreased as the oil:water ratio was increased from 1:20 to 1:7. A ca. 36% decrease was observed with both probes. This is consistent with quasielastic light scattering21,22 measurements which have indicated that the droplet size increases with the oil content, giving rise to slightly slower diffusion rates for the larger oil droplet. The hydrodynamic radii of the droplets, calculated with eq 6, assuming that the probes are completely partitioned into the droplet phase, are shown in Table 7. The rh values show a reasonably linear dependence on both weight percent of dodecane and the droplet volume fraction, φ0, over the microemulsion composition range studied (Figure 3). The linearity is better for PMFc than for TMAFc. If the dodecane is completely solubilized, forming the microemulsion oil core, the droplet volume fractions are (21) Mackay, R. A.; Dixit, N. S.; Agarwal, R.; Seiders, R. P. J. Dispersion Sci. Technol. 1983, 4, 397-407. (22) Cheung, H. M.; Qutubuddin, S.; Edward, R. A.; Mann, J. A., Jr. Langmuir 1987, 3, 744-752.

Figure 3. Changes in the hydrodynamic radius of a O/W droplet with a dodecane volume fraction for a 5.5% SDS/10.3% 1-pentanol/1% NaBr microemulsion with PMFc+ (O) and TMAFc2+ (b) as probes.

expected to be proportional to the weight percent of dodecane. Extrapolation of the calculated hydrodynamic radii to φ0 ) 0 gives a mean value of 26.1 ( 1.2 Å, with the two probes, for the radius of the SDS/1-pentanol/NaBr micelle with no dodecane. The corresponding linear dependence of rh on weight percent of dodecane gives a mean value of 26.6 ( 1.1 Å at 0% dodecane. Estimates of the dodecane core radius can be obtained if one assumes that the hydrodynamic radius of the droplets, rh, is equal to rc + t, where rc is the oil core radius and t is the thickness of the SDS layer at the interface. Estimates of both the hydrodynamic radius and the thickness of the surfactant film of SDS/butanol/NaCl and SDS/butanol/water/NaCl microemulsions have been determined by neutron small-angle scattering studies.23 A value of about 10 Å was obtained for the thickness of the SDS layer. This value was found to be independent of the size of the microemulsion droplet. This compares to the approximate length of the surfactant molecules in SDS micelles, the value of which has been estimated to be between 15.5 and 21.0 Å.24-27 Penetration of the SDS surfactant tail into the oil core is expected to produce a somewhat lower value for the thickness of the surfactant shell in SDS microemulsions. The presence of the cosurfactant molecules reducing the electrostatic repulsive energy of the head groups may also contribute to a smaller SDS shell in microemulsions than that in simple micelles. Simple geometric considerations relate the size of the droplet core, rc, to the area, as, per surfactant molecule at the oil/water interface by eq 728,29 where R is the molar

rc ) 3RVm/as

(7)

ratio of dispersed phase to surfactant and Vm is the (23) Tabony, J.; De Geyer, A. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1986; Vol. 3, p 1287. (24) Tanford, C. The Hydrophobic Effect, 2nd ed.; Wiley: New York, 1980. (25) Almgren, M.; Swarup, S. In Surfactants in Solution; Mittal, L., Bothorel, P., Eds.; Plenum: New York, 1986; Vol. 1, p 613. (26) Mazer, N. A.; Carey, M. C.; Benedek, G. B. Micellization, Solubilization and Microemulsions. Proc. Int. Symp. 1977, 359. (27) Porte, G. In Micelles, Membranes, Microemulsions and Monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: New York, 1994; p 111. (28) Clint, J. H. Surfactant Aggregation; Blackie: London, 1992; p 231. (29) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 5, 12101217.

Cobalt(III) Complexes and Ferrocene Derivatives

molecular volume of the dispersed component. Values of as calculated by eq 7, using 10 Å for the thickness of the surfactant interface, are shown in Table 7. Equation 7 assumes that the droplets are all spherical and identical and that all of the surfactant molecules reside at the oilwater interface. For a given surfactant-cosurfactant mixture and for sufficiently large droplets (rh . t), as is expected to be independent of the size of the droplets. A typical value for as in SDS microemulsions is reported to be 60 Å2.30 Measurements of the surfactant film in SDS/ butanol/toluene/NaCl microemulsions using X-ray and neutron scattering techniques have produced a value of 70 ( 10 Å.31 Mean values of 65.6 ( 8.4 and 48.4 ( 4.1 Å2 respectively are obtained for as from the PMFc and TMAFc data in Table 7. A trend toward higher values with increasing φ0 is evident in our data. This may be a consequence of incomplete probe partitioning into the microemulsion droplet. Although 100% residence in the oil core was assumed in calculating rh, some partitioning into the aqueous phase is not unreasonable, particularly for TMAFc. Also, since the requirement for eq 6 that rh . t is not truly satisfied by our data, the values for as which they afford are in reasonable agreement with previously determined values. Qualitatively, at very low oil content the microemulsion droplets can be considered as hard spheres whose size increases with the infusion of oil into the core. In the presence of electrolytes, this increase in size reduces the charge density of the droplets and the repulsive interactions amongst them. At some point, with an increase in the oil volume fraction, the microemulsion droplets will coalesce, resulting in the formation of a bicontinuous phase. Quantitative estimates of such interactions (the osmotic second virial coefficient) require diffusion coefficient values along a constant dilution line.32,33 The present data are not sufficient to provide such information. The necessary experiments are planned for future studies. Diffusion Coefficients for Co(III) Complexes and Ferrocene Derivatives in Low Water Content Microemulsions. In low water content microemulsions (39-35% H2O), D0 values measured for the cobalt(III) complexes resemble those obtained at the higher water content (87-73% H2O) compositions. This finding contrasts with that found with the ferrocene derivatives, where a distinct trend toward larger D0 values was observed, particularly with the most hydrophobic probe, PMFc. The measured value for PMFc in the 39% H2O microemulsion (D0 ) 1.8 × 10-6 cm2/s) was almost an order of magnitude larger than the D0 value (2.0 × 10-7 cm2/s) found for PMFc associated with the O/W droplet existing at the high water content composition and much more closely resembles that of ferrocene in dodecane34 (D0 ) 4.4 × 10-6 cm2/s). It also resembles self-diffusion values (ca. 1 × 10-6 cm2/s) that have been measured by NMR for the oil phase in SDS microemulsions with oil/water composi(30) Shulman, J. P.; Riley, D. P. J. Colloid Sci. 1948, 3, 383. (31) Auvray, L. In Micelles, Membranes, Microemulsions and Monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: New York, 1994; p 351 and references therein. (32) Nicholson, J. D.; Clarke, J. H. Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York. 1984; Vol. 3, p 1663. (33) Cazabat, A. M.; Chatenay, D.; Langevin, D.; Meunier, J.; Leger, L. Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, p 1729. (34) Sharp, M. Electrochim Acta 1983, 28, 301-308. (35) Guerin, P.; Lindman, B. Langmuir 1985, 1, 464. (36) Clarkson, M. T.; Beaglehole, D.; Callaghan, M. T. Phys. Rev. Lett. 1985, 54, 1722. (37) Lindman, B.; Kamenka, N.; Kathopoulis, T. M.; Brun, B.; Nilsson, P. J. Phys. Chem. 1980, 84, 2485-2490. (38) We are grateful to referee 1 for convincing us on this point.

Langmuir, Vol. 13, No. 17, 1997 4735

tions which have been characterized as bicontinuous microemulsions.35,36 Generally, less well-defined bicontinuous microemulsions have been proposed for oil/water compositions in which both the hydrophilic water and hydrophobic oil components, still separated by the membrane interface, are simultaneously continuous, while mutually intertwined and maintaining the homogeneity characteristic of microemulsions. Under those conditions, the measured D0 value for a particular probe will reflect the bicontinuous phases with which the probe is associated. The much larger value measured for the highly hydrophobic PMF in the 39% H2O microemulsion therefore suggests that a bicontinuous microemulsion microstructure exists at this composition, with the hydrophobic PMFc dissolved in the dodecane oil phase and undergoing translational diffusion at oil diffusion rates. Similar transitions to bicontinuous microstructures have been suggested in earlier electrochemical investigations of ferrocene in SDS microemulsions.1,2 If the cobalt(III) complexes continue to associate with the charged surfactant in a bicontinuous microemulsion system, their diffusion coefficients will reflect the mobility of the surfactant phase existing there. Lindman and coworkers37 have shown that, although the self-diffusion of the oil and aqueous phases may vary considerably with microemulsion composition, surfactant diffusion is generally slower than the diffusion of the other components and has a roughly constant value over wide concentration ranges. The contrasting behavior of the cobalt complexes and the ferrocene derivatives in the low water content microemulsions could therefore be consistent with a bicontinuous microstructure existing at these compositions. The appreciable changes in half-wave potentials which were noted there further suggest a changed microstructure for these low water content systems. An alternative explanation for the constant D0 values found for the cobalt(III) complexes is that they reflect an O/W droplet microstructure persisting at all microemulsion compositions in the presence of the solubilized cobalt(III) probes. If this is so, the 3+ charge on the complex ions would appear to stabilize the droplet microstructure against coalescence and transition to a bicontinuous microemulsion under conditions where a bicontinuous microemulsion is formed in the presence of more hydrophobic probes. Although our limited data prevent a definitive distinction between these two possibilities, evidence favoring the existence of a bicontinuous microemulsion microstructure in both cases would appear to be more compelling.38 Effect of Microemulsion Composition on the HalfWave Potentials of Cobalt(III) Complexes and Ferrocene Derivatives. Half-wave potentials obtained for the ferrocene derivatives and cobalt phenanthroline complexes in various microemulsion compositions are summarized in Table 8. We have previously shown that the value E1/2 ) 0.129 V vs Ag/AgCl, determined for [Co(phen)3]3+/2+ in 0.20 M LiCF3SO3, undergoes a 83 mV positive shift, reaching 0.212 V at [SDS] ) 0.0101 M.7 In SDS microemulsions, a further increase in E1/2 was found, though E1/2 values measured for both the [Co(phen)3]3+/2+ (220 ( 4 mV) and TMAFc3+/2+ (450 ( 4 mV) couples were generally insensitive to the microemulsion compositions employed except for those having the lowest water content (39.0/6.0). Similar behavior was apparent for the methyland phenyl-substituted 1,10-phenanthroline complexes. The positive shift of the [Co(phen)3]3+/2+ couple in SDS micellar solution has been attributed to enhanced hydrophobic solvation of the 2+ state, arising from lower charge, larger radius, and greater polarizability of the

4736 Langmuir, Vol. 13, No. 17, 1997

Shah et al.

Table 8. Summary of Half-Wave Potentials for Ferrocene Derivatives and Cobalt Complexesa in SDS Microemulsions E1/2b (mV vs Ag/AgCl) water/ dodecane PMFc+/0 TMAFc2+/+ PHEN3+/2+ PPHEN3+/2+ MPHEN3+/2+ c87.0/2.0 d79.2/4.0 d77.2/6.0 d75.7/7.5 d74.4/8.8 d73.2/10.0 e39.0/6.0

-60 -49 -44 -39 -35 -35 -28

440 450 448 453 448 459 473

227 226 219 215 215 219 239

200 206

106 107

205 241

101 134

a PHEN ) [Co(phen) ]3+/2+, PPHEN ) [Co(4,7-phen -phen) ]3+/2+, 3 2 3 MPHEN ) [Co(4,7-Me2-phen)3]3+/2+. b E1/2 ) (Epa + Epc)/2. c 3.32% SDS; 6.68% 1-pentanol; 1% NaBr. d 5.50% SDS; 10.3% 1-pentanol; 1% NaBr. e 18.0% SDS; 36.0% 1-pentanol; 1% NaBr.

“softer” divalent complex dominating over electrostatic stabilization of the 3+ state. A continuation of the preferential stabilization of the divalent state is apparent in microemulsion solutions. Any electrostatic stabilization afforded to 3+ ions over 2+ ions in simple SDS micelles is also expected to be lowered in SDS microemulsions due to the smaller charge density at the droplet microemulsion surface arising from the 1-pentanol cosurfactant molecules interspersed amongst the negatively charged SDS head groups. The relative insensitivity of E1/2 of the cobalt complexes to the oil:water composition of the microemulsion complements the insensitivity in D0 values discussed above and is consistent with there being little change in the propor-

tion of membrane-bound complex with change in microemulsion composition. Due to their fairly high positive charges, both redox forms of the cobalt phenanthroline couple remain strongly associated with the membrane phase and as such probe similar microenvironments throughout most of the composition range studied. E1/2 values of membrane-bound probes do not appear to be sensitive to the relatively minor changes which occur, while the droplet radius increases slightly with added dodecane. Only in the low water (39.0% H2O/6.0% dodecane) microemulsion were significant changes in E1/2 observed. Positive shifts of 19, 29, 37, and 23 mV were noted for the [Co(phen)3]3+/2+, [Co(4,7-Me2-phen)3]3+/2+, [Co(4,7-phen2phen)3]3+/2+, and TMAFc2+/+ couples, respectively. This finding supports a changed environment for the redox centers such as that accompanying the transition from an O/W microemulsion to a bicontinuous microstructure, as discussed above. It was only with the strongly hydrophobic probe pentamethylferrocene that the measured half-wave potential appeared to undergo a gradual positive shift over the whole composition range studied. Since PMFc is expected to be strongly solubilized in the dodecane oil phase both in the high water content O/W droplet and in the low water bicontinuous microstructure, its environment is expected to experience least change with microemulsion composition, consistent with this finding. LA970181R