Effect of surfactant tail-length asymmetry on the formation of mixed surfactant vesicles

A fundamental understanding of vesicle formation and stability in mixed surfactant systems is important for the description of their phase behavior, for the application of vesicles as encapsulating devices, and for the elucidation of cholesterol gallstone formation in bile. With this in mind, we have utilized our recently developed molecular-thermodynamic theory to study the formation of vesicles in mixtures containing cetyltrimethylammonium bromide (CTAB) and sodium alkyl sulfates of various tail lengths. The theory accounts for the essential free-energy contributions to the free energy of vesiculation, g(ves), with particular emphasis on their relative importance and interplay in the process of vesicle formation. We found that mixed surfactant vesicles can be stabilized energetically in highly asymmetric surfactant mixtures, such as those consisting of CTAB and sodium pentyl sulfate (SPS). These vesicles are characterized by small sizes and a narrow size distribution. In contrast, in mixtures consisting of CTAB and sodium pentadecyl sulfate (SPDS), where the tail-length asymmetry is small, vesicles are stabilized entropically and are characterized by large sizes and a wide size distribution. Small vesicles are formed by placing more molecules in the outer vesicle leaflet to relieve the outer interfacial free-energy penalty. The SPS molecules, having a short hydrophobic tail, can cover the outer hydrocarbon/water interface without incurring a high packing free-energy penalty, thus making g(ves) of small CTAB/SPS vesicles lower than that corresponding to a planar bilayer. In contrast, a high packing free-energy penalty is incurred in small CTAB/SPDS vesicles, due to the existence of a more crowded hydrophobic region. In this case, therefore, g(ves) of finite-sized vesicles is always higher than that corresponding to a planar bilayer, and the formation of vesicles in such systems is driven by the more favorable entropy of mixing. Surfactant tail-length asymmetry also affects the optimum composition of the vesicles by altering the tail transfer free-energy contribution, g(tr). Decreasing surfactant tail-length asymmetry reduces g(tr), which, in turn, decreases the influence of the energetics of vesicle formation, as compared to that of the entropy associated with localizing the surfactant molecules. In a mixture containing CTAB and SPDS (weight ratio = 3/7), therefore, the entropic penalty dominates and drives the vesicle composition toward that of the bulk solution. In contrast, in highly asymmetric mixtures such as those consisting of CTAB and SPS (weight ratio = 3/7), the optimum vesicle composition reflects a compromise between the entropic and energetic factors. The effect of decreasing surfactant tail-length asymmetry on the optimum vesicle composition is therefore similar to that of adding salt to the vesicle solution. Specifically, decreasing tail-length asymmetry reduces the energetic influence by decreasing g(tr), while adding salt produces the same effect through a reduction in the electrostatic free-energy contribution, g(elec).