Recent trends in bilayer formation of synthetic amphiphiles

Properties of the amphiphilic films in mixed cationic/anionic vesicles: a comprehensive view from a literature analysis

The so-called 'catanionic' vesicles are made from mixtures of cationic and anionic surfactants. They are attracting much interest because they form spontaneously and they can be obtained from a variety of surfactants, either commercially available or issued from original synthesis. A distinction can be made between the properties of simple surfactant mixtures and of ion pair amphiphiles (IPA), in which the counterions have been removed. We have drawn up in this paper, an inventory of the different vesicular systems which have been described in the literature, insisting on the specific features associated with these two categories of systems. We have collected here especially, information concerning the phase behaviors, the microscopic composition of the vesicular particles, their structural and size determinations, the dynamic aspects (including the micelle-vesicle transition), the theoretical predictions from thermodynamic models and the entrapment of probe molecules. We discuss the potential of catanionic vesicles as delivery systems and we show that a full understanding of their entrapment/release properties will call for much more experimental work with well defined protocols. We also point out some unsolved questions concerning the role of the excess surfactant in the stabilization of the particles and the conditions required to obtain a favourable curvature of the surfactant film.