A PHASE AND MOLECULAR-ORIENTATION STUDY OF A SPIN-LABELED DIOCTADECYLMETHYLAMMONIUM CATIONIC SURFACTANT

The synthesis of the nitroxide-containing dialkyl quaternary ammonium chloride 4-(N,N-dioctadecyl-N-methyl) ammonium-2,2,6,6-tetramethylpipe ridine-1-oxyl, chloride, is described. This nitroxide is shown to have similar phase behaviour to dioctadecyldimethylammonium chloride, with the nitroxide undergoing the transition from gel (L(beta)) to lamella r (L(alpha)) phase about 2 degrees C higher than the unlabelled surfac tant. When low levels of the spin-labelled surfactant are solubilized in dioctadecyldimethylammonium chloride, the nitroxide moiety is locat ed above the hydrocarbon regions of the bilayer in the L(beta) phase. In the L(alpha) phase, the nitroxide protrudes into the hydrocarbon re gion. These data establish the suitability of the nitroxide surfactant for spin-probe studies of dialkyl cationic surfactant systems.