Electrochemical Investigation of Amphiphilic Cobalt(III) Complexes and Ferrocene Derivatives in Sodium Dodecyl Sulfate Microemulsions

Abstract

Cyclic voltammetry and chronocoulometry have been used to measure diffusion coefficients (D 0 ) and half-wave potentials (E 1/2 ) 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 ](C1O 4 ) 3 , [Co(4,7-Me2-phen) 3 ]Cl 3 , and [Co(4,7-phen2-phen) 3 ]Cl 3 , has been compared with that of bis(pentamethylcyclopentadienyl)iron, Fe[C 5 (CH 3 ) 5 ] 2 (PMFc), and (ferroce-nylmethyl)trimethylammonium bromide, [C 5 H 5 FeC 5 H 4 CH 2 N(CH 3 ) 3 ] 3 Br ([TMAFc]Br), in similar microemulsion environments. The dependence of measured D 0 and E 1/2 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. D 0 values obtained for [Co(phen) 3 ] 3+ (4.7(±1.6) x 10 -7 cm 2 /s), [Co(4,7-Me2-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 from 87 to 35%. Such behavior is consistent with association of the complexes with the charged surfactant phase of the microemulsion, though the smaller D 0 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 D 0 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 D o 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 microemulsions. Differences are discussed in terms of possible microstructures existing at these compositions. Binding constants, K 1 , and partition coefficients, K d , have also been determined for the association of TMAFc 2+ and the ferrocenium cation, Fc + , with SDS micelles from 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 (±240) M -1 ) are compared with values previously determined for ferrocene itself and for amphiphilic cobalt complexes.