Values of formal potential in microheterogeneous systems were related to the probe aqueous formal potential and the diffusion coefficients and partitioning constants of the oxidized and reduced forms of the probe. Ferrocene (Fc0/+), methyl viologen (MV2+/·+/0), and ferricyanide (Fe(CN)63-/4- were used to demonstrate limiting cases of the relationship between these parameters in the microheterogeneous system of a sodium dodecyl sulfate/l-pentanol/dodecane/0.1 M NaCl(aq) microemulsion (μE). The Fc0/+ couple illustrates the limiting case where one form of the probe is water soluble and the other is oil soluble. The MV2+/·+ and MV·+/0 couples demonstrated the limiting case where both forms of the probe are associated with oil phase (or interface) of the μE. The Fe(CN)63-/4- couple represents the limiting case where neither form associates with the oil or interfacial layer. This work shows that shifts in formal potential with μE composition are due to changes in both partitioning constants and diffusion coefficients. It was also shown that hydrophobic effects have a larger effect on partitioning constants than electrostatic effects. However, Fe(CN)63-/4- results were obscured by slow electron-transfer kinetics. With relationships which were developed, most shifts in formal potential can be predicted for μEs and other microheterogeneous systems.
ASJC Scopus subject areas
- Analytical Chemistry