Mesodynamics: watching vesicle formation in situ by small-angle neutron scattering

Self-aggregating systems form a large variety of different structures depending on type and concentration of the amphiphiles. Morphological changes can be triggered by mixing two surfactant solutions with the stopped-flow technique and observing by highly time-resolved SANS experiments, thereby allowing to obtain of structural information with 25-50 ms time resolution. Here, the formation of unilamellar vesicles by mixing zwitterionic tetradecyldimethylamine oxide (TDMAO) with the anionic lithium perfluorooctanesulfonate (LiPFOS) was studied as a function of the mixing ratio. SANS data were analyzed in terms of the structural changes and especially with respect to intermediate structures. Vesicle formation occurs via disk-like micelles as intermediates that close to form very monodisperse unilamellar vesicles after reaching a certain size. This process is controlled by the competition between bending energy of the bilayer and the line energy of the disk rim, and its characteristic time is controlled by the electrostatic repulsion that is determined by the TDMAO/LiPFOS ratio.