MOLECULAR-THERMODYNAMIC MODELING OF MIXED CATIONIC ANIONIC VESICLES/

Vesicles are widely used as model cells in biology and medicine and ar e also potentially useful as drug carriers and other industrial encaps ulating devices. To facilitate the practical implementation of vesicle s, as well as to gain a fundamental understanding of the process of ve sicle formation, we have developed a molecular-thermodynamic theory to describe the formation of two-component mixed vesicles in aqueous sol utions. The central quantity in this theory is the free energy of vesi culation, which is calculated by carefully modeling the various free-e nergy contributions to vesiculation. In particular, we (i) estimate th e surfactant-tail packing free energy using a mean-field approach that accounts for the conformations of the surfactant tails in the vesicle hydrophobic region, (ii) adopt a more accurate equation of state in t he calculation of the surfactant-head steric repulsions, and (iii) uti lize the nonlinear Poisson-Boltzmann equation to calculate the electro static interactions in the case of mixed cationic/anionic charged vesi cles. Particular attention has also been paid to issues such as the lo cation of the outer and inner steric-repulsion surfaces in a vesicle a nd the curvature correction to the interfacial tensions at the outer a nd inner hydrocarbon/water vesicle interfaces. By knowing only the mol ecular structures of the surfactants involved in vesicle formation and the solution conditions, our theory can predict a wealth of vesicle p roperties, including vesicle size and composition distribution, surfac e potentials, surface charge densities, and compositions of vesicle le aflets. More importantly, this theory enables us to gain an understand ing of (i) the underlying mechanisms of stabilization in mixed cationi c/anionic vesicular systems, (ii) the effect of the interplay between the various intravesicular free-energy contributions on vesiculation, and (iii) the role of the distribution of surfactant molecules between the two vesicle leaflets in vesicle formation. As an illustration, th e theory has been applied to describe vesicle formation in an aqueous mixture of cetyltrimethylammonium bromide (CTAB) and sodium octyl sulf ate (SOS). In this system, the vesicles are found to be stabilized ent ropically, with a predicted mean radius of about 1200 Angstrom for a m ixture containing 2 wt% surfactant and a CTAB/SOS weight ratio of 3/7, a value which compares well with the experimentally measured value of 1300 Angstrom. In addition, the predicted outer surface potential of -72 mV is consistent with the measured zeta potential value. The effec t of added salt on vesicle properties has also been studied using this theory, and the predicted results conform well to experimental observ ations.