ABSTRACT
The formation of vesicles has been observed in a great number of surfactant systems. They are formed in aqueous solutions of double-chained surfactants, in mixtures of various surfactants with cosurfactant, in mixtures of cationic surfactants with large hydrophobic counterions, and in mixtures of cationic and anionic surfactants [1-6]. It is generally assumed that the vesicle formation occurs spontaneously in these systems. However, there is evidence that the microstructures formed can be strongly affected by the preparation process. It has been found that the size distribution of the vesicles depends on the formation pathway as well as on preformed structures that are already present in the solution when the vesicles are built (matrix effect) [7-9]. In addition, vesicular solutions usually have to be mixed during the preparation process to homogenize the components. Therefore, they are exposed to shear during the preparation process. It is well known, however, that shear forces may have a great impact on the vesicle formation. To avoid the application of shear during the vesicle formation, chemical reactions, e.g., the hydrolysis of organic esters, can be used. With ester hydrolysis the composition of the surfactant aggregates changes in a way that enables the formation of vesicular phases. The hydrolysis of MF within phases containing TDMAO, for example, leads to protonation of the zwitterionic surfactant TDMAO, the degree of charging is altered, and transformations of one structural type to another can be triggered. If shear is avoided by doing so, it has been shown that the resulting microstructures in some systems are classical Lα phases of stacked bilayers instead of vesicular phases of multilamellar vesicles. Only if the Lα phases are exposed to shear are the stacked bilayers transformed to multilamellar vesicles [10-12].
