Equilibrium vesicles: fact or fiction?

Molecular aggregates of surfactant molecules consisting of one or more bilayers arranged in a hollow, closed, usually spherical geometry are termed ''vesicles'' or ''liposomes''. Often vesicles are formed by the application of intense mechanical shear to coarse dispersions of water-insoluble surfactants, in which case the thermodynamic state of the mixture evidently consists of the aqueous liquid plus a coexisting lamellar phase. In recent years it has been found that in certain systems the vesicular structure forms spontaneously and is long-lived, and it has been suggested that these structures may in fact constitute the equilibrium state in these cases (as is true of micelles). It is here argued that vesicular mixtures are invariably biphasic, and that vesicular structure is to be regarded as a dimension of colloidal structure rather than of phase structure. If phase separation occurs when a cationic and an anionic surfactant are mixed in water, two new components result and diagnosis of the state of the mixture demands use of the rules governing five-component mixtures. By imposing the restriction that the surfactant in limiting quantity reacts irreversibly to form a catanionic surfactant salt, however, such mixtures may be rigorously treated as four-component systems. If the byproduct non-surfactant salt is removed, a manageable ternary system (catanionic surfactant, ionic surfactant, and water) results. By taking this approach the thermodynamic state of these mixtures may be analyzed more simply and accurately, and a useful view of the physical science of vesicular mixtures results. (C) 1997 Published by Elsevier Science B.V.