VESICLES VERSUS MEMBRANE-FRAGMENTS IN DODAC SUSPENSIONS

We report on the organization properties of DODAC molecules in water solutions. We have measured a value of 8.3 ± 0.5% for the encapsulation efficiency of DODAC for 5NS, a membrane adsorbed probe. This value, which is consistent with most published ones, implies that vesicles are only a minor fraction in DODAC suspensions. Light-scattering experiments show that most of the DODAC molecules are organized in small open structures: flat disklike bilayers. These membrane fragments are formed by destruction of the vesicles during the sonication process. Upon annealing or after addition of Fe(CN)64-, fragments gather to form a population of vesicles with a narrow size distribution. This size monodispersity has been rationalized by geometric models, assuming thermodynamic equilibrium. But our studies show that, as phospholipid vesicles, DODAC suspensions are not in thermodynamic equilibrium. Models that consider both the energetics and the kinetics of the vesicles formation have been proposed by Lasic and by Fromherz. The formation of a size paucidisperse population of vesicles from a size polydisperse population of membrane fragments, that we observe, is the first experimental evidence supporting such models. © 1990 American Chemical Society.