Electrochemical behaviour of (protoporphyrinato IX)iron(III) encapsulated in aqueous surfactant micelles

Abstract

The electrochemical behaviour of iron protoporphyrinate IX (hemin) [(3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropanoato)iron(III)] in aqueous sodium dodecyl sulfate (sds), hexadecyltrimethylammonium bromide or Triton X-100 surfactant micellar solution was investigated by cyclic voltammetry (CV) and Osteryoung square-wave voltammetry (OSWV) techniques. The dependence of the midpoint potential on the concentration and on the nature of the surfactant shows that the potentials shift anodically with respect to water or aqueous ethanol. The midpoint potentials of hemin at pH 7.0 vs. normal hydrogen electrode are –52 mV in NMe₃(C₁₆H₃₃)Br, –112 mV in Triton X-100, –152 mV in sds and –190 mV in ethanol–water (1∶1, v/v) solutions. Thus the hydrophobic effect of the micelle gives a positive shift of the midpoint potential. The maximum positive shift in surfactants (with respect to water) of ca. +120 mV was found in the micelles. The trend in the anodic shift is EtOH–water < sds < Triton X-100 < NMe₃(C₁₆H₃₃)Br. The diffusion coefficients of the hemin complex in the micelles are an order of magnitude smaller than that of monomeric hemin in aqueous ethanolic media. The rates of heterogeneous electron transfer at the glassy carbon electrode were found to be smaller in the micelles as compared to those in aqueous ethanolic media. The midpoint potential of hemin monomers encapsulated in aqueous surfactant micelles shows a pH dependence with ΔE/ΔpH ca. –59 mV indicating that electron transfer at the iron site is influenced by the uptake of protons at the axial ligand.