Reactivity of a lipophilic ingredient solubilized in anionic or cationic surfactant micelles

The aim of this work was to investigate the location and reactivity of a lipophilic spin probe, 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO) in anionic (sodium dodecyl sulfate, SDS) or cationic (dodecyl trimethylammonium bromide, DTAB) surfactant micelles. The analysis of electron paramagnetic resonance (EPR) spectra of PTMIO in micellar systems showed that probe molecules partitioned between two populations: a more mobile fraction in the aqueous phase and a less mobile fraction in the micelle. The fraction of PTMIO incorporated in surfactant micelles increased with surfactant concentration. The rate of the reduction of the nitroxide group of PTMIO by the negatively charged, water-soluble ascorbate decreased when the probe was solubilized in anionic SDS micelles and increased when it was solubilized in cationic DTAB micelles. Thus, both the surface charge as well as the solubilization capacity of the micelles controlled the reactivity of the lipophilic molecule.