Enhanced physical stability of positively charged catanionic vesicles: Role of cholesterol-adjusted molecular packing

A pseudodouble-chain ion pair amphiphile, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), and a double-chain cationic surfactant, ditetradecyldimethylammonium bromide (DTDAB), were used as the main materials to form positively charged catanionic vesicles with various cholesterol contents. The effects of cholesterol on physical stability of the charged catanionic vesicles were then investigated by size, zeta potential, and Fourier transform infrared analyses. With the presence of cholesterol, the physical stability of the vesicles was enhanced. The inclusion of cholesterol would increase the distance between the charged headgroups of the vesicle-forming molecules and reduce the tendency of the counterion binding onto the charged vesicle surfaces, leading to a more pronounced charge character of the vesicles. Furthermore, with the incorporation of cholesterol, the sterol ring of cholesterol tended to maximize the contact with neighboring hydrocarbon chains, thus improving the bilayer mechanical strength. In the meantime, the alkyl side-chain of cholesterol together with the segments of the hydrocarbon chains near the ends in the vesicle-forming molecules could form flexible regions within the vesicular bilayers. Thus the presence of cholesterol in the bilayer structures of HTMA-DS/DTDAB catanionic vesicles not only enhanced the inter-vesicular electrostatic repulsion but also adjusted the intra-vesicular molecular packing, resulting in improved physical stability of the vesicles.