CORRELATIONS BETWEEN SOLVENT POLARITY SCALES AND ELECTRON-TRANSFER KINETICS AND AN APPLICATION TO MICELLAR MEDIA

Correlations of a series of solvent polarity scales with apparent standard heterogeneous rate constants k' for electro-oxidation of ferrocene at Pt in 16 solvents covering a wide range of properties were evaluated. Single-parameter scales, including the dielectric continuum model, gave poor correlations. The best Correlation was found for the Taft model, which accounts for solvent polarizability and hydrogen bond donor and acceptor properties. Correlations with Taft parameters were used for comparison with electron transfer (ET) rates of ferrocene in micellar solutions. ET rates for ferrocene in cetyltrimethylammonium (CTAB) and sodium dodecylsulphate (SDS) micelles were slightly smaller than in alcohols and much smaller than predicted for water by using Taft coefficients. The ET rate in micellar SDS was somewhat smaller than in CTAB. If, as a first approximation, equal ET distances are assumed, results suggest apparent average microenvironments for ferrocene during ET in aqueous micellar CTAB and SDS that are more polar than in many apolar solvents, but less polar than in water.