Vesicle structures spontaneous curvature

Fig. 12. Sources of area difference curvature AA (insertion, flip-flop) and spontaneous curvature C (block and amphiphile asymmetry, polydispersity) for bilayer structures. The vesicle will adapt a curvature that is a compromise between the two, depending on the related energy.

YatciUa et al. investigated the conversion of a mixtime of cationic and anionic surfactants to form vesicles. In this case, the reaction was veiy slow and a kinetic phase was associated with the evolution and growth of vesicles over a period of weeks (see Figure 6.12) to a final vesicular system, which, as already mentioned, was thought by the authors to be thermodynamically stable. A subsequent study explored the system CTAB mixed with sodium perfiuorooctanoate. Cylinders, disks, and spherical uni-lamellar vesicles were found to coexist at equilibrium by cryo-TEM. This observation confirms the importance of structural confirmation by cryo-TEM when this technique can be applied. Erom their analysis of the data, the mean curvature modulus, the Gaussian curvature modulus, and the spontaneous curvature could all be evaluated. 

Polymer vesicles are considered to form in a two-step process. First, the polymer chains form a bilayer-type membrane, which then subsequently closes to form a hollow structure (Figure 9). This process involves an interfacial curvature change, which can correspond to a change in the packing parameter for the polymer and hence a change in the resultant morphology. However, theoretical calculations have revealed that some vesicle formation process may be more complicated than the above two-step procedure. These results can be summarized as two different proposed mechanisms for the spontaneous formation of vesicles from the homogeneous state. 

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