Electrochemical Estimation of Diffusion Anisotropy ofN,N,N′,N′-Tetramethyl-para-phenylenediamine within the Normal Hexagonal Lyotropic Mesophase of Triton X 100/Light Water: When Can the Effects of Cross-Pseudophase Electron Transfer be Neglected for Partitioned Reagents?

Abstract

The H(1) lyotropic liquid crystalline phase of Triton X 100 with aqueous 0.1 M potassium chloride is examined as a medium in which to determine the axiosymmetric anisotropy in the diffusion flux of N,N,N',N'-tetramethyl-para-phenylenediamine using electrochemical methods (voltammetry and potential step chronoamperometry) at both planar electrodes and two-dimensional flux microdisk electrodes. Comparison of experiment with theory suggests the ratio of anisotropic diffusion coefficients in the directions tangential and perpendicular to the electrode surface varies over two orders of magnitude (from 0.04 to 3.3) with increasing concentration of the redox analyte. This is understood through the occurrence of a long-range charge transfer across the pseudophase | pseudophase boundary interface, occurring as a result of differential diffusivities of the redox probe within the surfactant and aqueous subphases. These data and their dependence on the analyte concentration empower, in a proof-of-concept, the estimation of the partition equilibrium constant (K(P)); the value estimated for the small electroactive-drug mimetic considered is log K(P) = 2.01 ± 0.05 (at 294 ± 2 K) and is in agreement with that envisaged for its partition between n-octanol and water. It is suggested that only measurements at low analyte loadings allow for interphase electron transfers to be neglected, since then percolation effects appear to dominate the Faradaic current.