EFFECT OF SURFACTANT TAIL-LENGTH ASYMMETRY ON THE FORMATION OF MIXED SURFACTANT VESICLES

A fundamental understanding of vesicle formation and stability in mixe d surfactant systems is important for the description of their phase b ehavior, 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-thermo dynamic theory to study the formation of vesicles in mixtures containi ng cetyltrimethylammonium bromide (CTAB) and sodium alkyl sulfates of various tail lengths. The theory accounts for the essential free-energ y contributions to the free energy of vesiculation, g(ves), with parti cular emphasis on their relative importance and interplay in the proce ss of vesicle formation. We found that mixed surfactant vesicles can b e stabilized energetically in highly asymmetric surfactant mixtures, s uch as those consisting of CTAB and sodium pentyl sulfate (SPS). These vesicles are characterized by small sizes and a narrow size distribut ion. In contrast, in mixtures consisting of CTAB and sodium pentadecyl sulfate (SPDS), where the tail-length asymmetry is small, vesicles ar e stabilized entropically and are characterized by large sizes and a w ide size distribution. Small vesicles are formed by placing more molec ules in the outer vesicle leaflet to relieve the outer interfacial fre e-energy penalty. The SPS molecules, having a short hydrophobic tail, can cover the outer hydrocarbon/water interface without incurring a hi gh packing free-energy penalty, thus making g(ves) of small CTAB/SPS v esicles lower than that corresponding to a planar bilayer. In contrast , a high packing free-energy penalty is incurred in small CTAB/SPDS ve sicles, 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 ves icles in such systems is driven by the more favorable entropy of mixin g. Surfactant tail-length asymmetry also affects the optimum compositi on of the vesicles by altering the tail transfer free-energy contribut ion, 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 localizi ng the surfactant molecules. In a mixture containing CTAB and SPDS (we ight ratio = 3/7), therefore, the entropic penalty dominates and drive s the vesicle composition toward that of the bulk solution. In contras t, in highly asymmetric mixtures such as those consisting of CTAB and SPS (weight ratio = 3/7), the optimum vesicle composition reflects a c ompromise between the entropic and energetic factors. The effect of de creasing surfactant tail-length asymmetry on the optimum vesicle compo sition is therefore similar to that of adding salt to the vesicle solu tion. Specifically, decreasing tail-length asymmetry reduces the energ etic influence by decreasing g(tr), while adding salt produces the sam e effect through a reduction in the electrostatic free-energy contribu tion, g(elec).