Structural and thermal characterization of dioctadecyldimethylammonium bromide dispersions by spin labels.

Dioctadecyldimethylammonium bromide (DODAB) dispersions obtained by simply mixing the amphiphile in water, and by bath-sonication, were investigated by electron spin resonance (ESR) of stearic acids and their methyl ester derivatives, labeled at the 5th and 16th carbons of the acyl chain. The ESR spectra indicate that the non-sonicated dispersions are formed mainly by one population of DODAB vesicles, either in the gel (T<T(m)) or in the liquid-crystalline (T>T(m)) state. Around T(m) there is a co-existence of the two phases, with a thermal hysteresis of about 3.2 degrees C. In sonicated DODAB dispersions, spin labels indicate two different environments even for temperatures far below T(m): one similar to that obtained with non-sonicated samples, a gel phase, and another one in the liquid-crystalline state. The fluid phase domain present below T(m) could correspond to either the periphery of bilayer fragments, reported to be present in sonicated DODAB dispersions, or to high curvature vesicles.