Pathway of vesicle formation

The non-equilibrium behaviour of surfactant systems, such as the evolution of structures occurring during surfactant phase transitions, has not been much addressed. We investigated the transformation between a monolayer and bilayer topology as it for example occurs during vesicle formation. The spontaneous formation of lipid vesicles (liposomes) in aqueous lecithin/cholesterol/bile salt mixtures was studied using time-resolved light and neutron scattering. The data are in quantitative agreement with a kinetic model that we present. It identifies the key kinetic steps during vesicle formation: rapid transformation to disc-like intermediate micelles, growth of these micelles and closure to form vesicles. Our model furthermore reproduces the correct trend of vesicle size as a function of composition, which equilibrium calculations fail to predict. This indicates that the properties of our vesicles are controlled by kinetics rather than thermodynamics.