ABSTRACT
While vesicles are usually associated with phospholipids [1] or double chain surfactants it is well known now that vesicles can also be formed from single chain surfactants. Actually, a whole variety of methods are known by which vesicles can be prepared. In general, vesicles are found in systems where one expects to find La-phases because vesicles can always be produced by shear from La-phases [2,3]. Vesicles are therefore observed in aqueous mixtures of cat-anionic surfactants at a certain mixing ratio, in mixtures of surfactants and cosurfactants and in ionic
surfactant solutions in combination with strongly hydrophobic counterions [4-6]. Furthermore, mixtures of Ca-salts of ionic surfactants and zwitterionic surfactants can also reveal vesicles under special conditions [7]. The size of the vesicles in these systems usually depends on the way the phases have been prepared and therefore on the history of the phases. While shear stress was involved in the preparation of many of these vesicle phases it is also true that vesicles are formed under conditions where no shear is involved. There actually is evidence that vesicles can be thermodynamically stable species. That means that the properties of these thermodynamically stable vesicle systems do not depend on the way they are prepared; the same phase composition will always lead to the same microscopic and macroscopic properties (e.g., size distribution of the vesicles, rheological behavior) if the system is given enough time for reaching equilibrium. So, on discussing surfactant vesicles we have to be careful in making general statements. What might be valid in one particular situation might not be true in another situation. The vesicle phases which can be prepared with the given methods have usually a broad size distribution including small unilamellar vesicles together with big multilamellar vesicles.
