Diffusion analysis of cosurfactant-induced current transitions in reverse microemulsions

Abstract

The cosurfactant-induced current transition observed in water/AOT/toluene reverse microemulsions is analyzed in terms of particle diffusion, for the oxidation of K4FE(CN)6 in the water pools of swollen micelles. Slow-scan diffusion-limited currents at ultramicroelectrodes are analyzed in terms of apparent particle diffusion and effective redox-active concentration. Apparent diffusion parallels that of the current, below and above the transition threshold. However, this apparent diffussion appears retarded relative to particle-limited diffusion determined by quasi-elastic light scattering. This independent measure of particle diffusion enables the overall phenomenon to be recast in terms of effective concentrations and concentration gradients at the electrode surface. Hemimicellar or surface partitioning alone cannot account for the observed currents. In this near-steady-state situation, the concentration gradient of redox species at the electrode/microemulsion interface results in a modulated flux retardation flux retardation relative to the flux obtained by particle-diffusion-limited mass-transfer from the bulk. This modulation can be assigned to increased exit kinetics from the micellar interior and/or through any hemimicelles on the electrode. A packing (disorder) transition of surfactant tail groups that makes the AOT interfatial region effectively more permeable to charged electroactive species appears to coincide with this modulation as well as with a related percolation threshold. © 1995.