ELECTROCHEMICAL INVESTIGATION OF AMPHIPHILIC COBALT(III) COMPLEXES AND FERROCENE DERIVATIVES IN SODIUM DODECYL-SULFATE MICROEMULSIONS

Cyclic voltammetry and chronocoulometry have been used to measure diff usion coefficients (D-0) and half-wave potentials (E-1/2) Of a series of amphiphilic cobalt complexes and ferrocene derivatives in sodium do decyl sulfate (SDS)/dodecane/water/1-pentanol/NaBr microemulsions. The electrochemical behavior of the cobalt(III) complexes, [Co(phen)(3)]( ClO4)(3), [Co(4,7-Me-2-phen)(3)] Cl-3, and [Co(4,7-phen(2)-phen)(3)]Cl -3, has been compared with that of bis(pentamethylcyclopentadienyl)iro n, Fe[C-5(CH3)(5)](2) (PMFc), and (ferrocenylmethyl)trimethylammonium bromide, [C5H5FeC5H4CH2N(CH3)(3)]Br ([TMAFc]Br), in similar microemuls ion environments. The dependence of measured D-0 and E-1/2 values on m icroemulsion composition has been used to provide information on the d ynamic structure of the microemulsions and to examine the details of t he solubilization processes involved for the different redox probes. D -0 values obtained for [Co(phen)(3)](3+) (4.7 (+/-1.6) x 10(-7) cm(2)/ s), [Co(4,7-Me-2-phen)(3)](3+) (1.1(+/-0.2) x 10(-7) cm(2)/s), and [Co (4,7-phen(2)-phen)(3)](3+) (1.4 (+/-0.3) x 10(-7) cm(2)/s) are largely independent of microemulsion composition varying in water content fro m 87 to 35%. Such behavior is consistent with association of the compl exes with the charged surfactant phase of the microemulsion, though th e smaller D-0 values obtained with the hydrophobic methyl-and phenyl-s ubstituted complexes suggest some partitioning into the oil phase. For PMFc and [TMAFc]Br, the dependence of D-0 values on the oil volume fr actions has been related to changes in the hydrodynamic radii of the O /W droplets. Estimates of the interfacial area per surfactant head gro up at the droplet surface have also been made. In contrast to the coba lt(III) complexes, higher D-0 values are observed for PMFc(1.8 x 10(-6 ) cm(2)/s) and [TMAFc] Br(9.1 x 10(-7) cm(2)/s) in low water content m icroemulsions. Differences are discussed in terms of possible microstr uctures existing at these compositions. Binding constants, K-1, and pa rtition coefficients, K-d, have also been determined for the associati on of TMAFc(2+) and the ferrocenium cation, Fc(+), with SDS micelles f rom the dependence of measured D-0 values on the SDS concentration. K- 1 values obtained for Fc(+) (120 (+/-5) M-1) and TMAFc(2+) (1180 (+/-2 40) M-1) are compared with values previously determined for ferrocene itself and for amphiphilic cobalt complexes.