Cooperative Micelle Binding and Matrix Effect in Oleate Vesicle Formation

Kinetics and particle size distributions in the spontaneous micelle−vesicle transformation of oleate vesicles are investigated by dynamic light scattering and optical density measurements. It is shown that, depending upon the conditions, there are two different kinetic processes for vesicles formation, a cooperative and a uncooperative one. In particular, the time progress of vesicle formation follows a sigmoid behavior and it is accelerated by the presence of preformed vesicles, confirming that a cooperative micelle−vesicle interaction takes place. This effect is investigated by adding fresh oleate surfactant to preformed, extruded (50 and 100 nm) oleate vesicles as well as to 50 and 100 nm extruded POPC (POPC is 1-palmitoyil-2-oleyl-sn-glycero-3-phosphocholine) liposomes. It is shown that both in the case of oleate and POPC the rate process of vesicle formation is remarkably accelerated with respect to the control (no preformed vesicles); and the final size distribution is very close to the narrowly distribution curve of the preformed vesicles, whereas in the control experiment (no preadded vesicles) a very broad distribution is obtained. We called this phenomenon the matrix effect to stress that preexisting vesicles or liposomes act as a matrix on the formation of the new ones. The time evolution of the particles average radius after addition of fresh surfactant is followed by means of the dynamic light scattering cumulant analysis, and thus when oleate micelles are added to oleate vesicles, a slight increase of the average dimensions is observed, whereas in the case of oleate added to POPC liposomes, the final radius is actually smaller than the initial radius. This confirms that under certain conditions, fission of vesicles is possible. Finally, some theoretical considerations on the kinetics of the micelle−vesicle cooperative transformation are drawn.