FROM VESICLES TO MICELLES

On the system soybean lecithin (Epicuron 200)/tetradecyldimethylaminoxide C14DMAO/water the aggregation behavior of mixtures composed of surfactant and phospholipid in a dilute solution was studied. The lipid was dispersed into water as vesicles, which were prepared by sonication. By varying the surfactant-to-lipid-ratio at a constant total concentration the whole spectrum of properties from vesicles to micelles with its transition properties could be investigated. Most of the measurements were carried out at a total concentration of 10 mM. Three different regions exist in this system: a micellar region on the surfactant rich side, a vesicular region on the lipid rich side and a two-phase-region between these two phases. In the micellar region there are spherical micelles, which grow to rodlike micelles with increasing molefraction of lipid X(L). This region was investigated by electric birefringence, by rheology and by measuring the interfacial tension. In the lipid rich solution vesicles are the dominant structures. Vesicles in this region can exist with a large molefraction of surfactant. These vesicles were examined by electron microscopy, dynamic light scattering, electric birefringence and conductivity measurements. We found that the vesicles in the region X(L) = 1-0.27 will grow only a small amount with added surfactant. For X(L) < 0.27 the vesicles grow rapidly. A two-phase-region exists between the micellar and the vesicular region. One of these phases contains nearly the whole amount of lipid and surfactant in a very small volume of the solution. In this phase the lipid and surfactant molecules are present in the form of rodlike micelles. The other phase contains almost pure water. The two-phase-region shows a very strong temperature dependence.