The evolution of vesicles from bulk lamellar gels

The remarkable ability of phospholipids to generate vesicles and lamellar morphologies has been mimicked by amphiphilic block copolymers and both classes of amphiphiles have been extensively studied in bulk and in dilute solution. The most common vesicle preparation method involves diffusion of water into a lamellar gel. The transformation proceeds from a collection of water-poor planar lamellae, to a water-rich interconnected layer-phase and then a collection of closed lamellae (vesicles) that form a close-packed gel. This is demonstrated by scattering and microscopy with the evolution of the gel structure being driven by increasing curvature. Vesicles have been observed to form when the copolymer-water system still has long-range order, with a discrete, clustered-vesicle structure leading to a turbid dispersion. Eventually, at very low concentration, the packed vesicles separate, generating very stable isotropic dispersions that are transparent. This phase sequence, involving a dispersion of close-packed vesicles, accounts for the formation of a narrow size distribution in these systems.