Electron Spin Echo Modulation Studies of Doxylstearic Acid Nitroxide Spin Probe Locations in Micelles

Abstract

Electron spin echo modulation spectrometry has been used to probe the weak electron-nuclear hyperfine interactions of x-doxylstearic acid spin probes in frozen micellar systems prepared in D₂O or with specifically deuterated surfactant molecules. Spin probes with x = 5, 7, 10, 12, and 16 were used. The surfactant molecules were synthesized with deuterium labels in the polar headgroup or counterion and at the terminal methyl group of the nonpolar alkyl tail. Anionic micelles were prepared from sodium and tetramethylammonium dodecylsulfate surfactants. Cationic micelles were prepared from dodecyltrimethylammonium bromide and hexadecyltrimethyl- ammonium bromide. It is concluded that the sodium dodecylsulfate micelles have a reasonably compact headgroup structure with a low degree of water penetration and relatively well organized alkyl chains with the terminal alkyl groups concentrated near the center region of the micelle. In contrast, the tetramethylammonium dodecylsulfate micelles show a more open headgroup structure with significant water penetration into the Stern region and a locus of terminal alkyl groups that is broadly distributed throughout the micelle. Nitroxide spin probe molecules can have a folded conformation in both anionic and cationic micelles.