Size and charge modulation of surfactant-based vesicles.

Nonstoichimetric mixtures of two oppositely charged surfactants, such as sodium dodecylsulfate and hexadecyltrimethylammonium bromide or tetradecyltrimethylammonium bromide and tetraethylammonium perfluorooctanesulfonate, a fluorinated species, form vesicles in dilute concentration regimes of the corresponding phase diagrams. Vesicles size and charge density are tuned by changing the mole ratio between oppositely charged species, at fixed overall surfactant content. They are also modulated by adding neutral electrolytes, or raising T. In the investigated regions, mixtures made of sodium dodecylsulfate/hexadecyltrimethylammonium bromide show ideality of mixing, the other non ideality and phase separation. The formation of unilamellar vesicles occurs in the sodium dodecylsulfate/hexadecyltrimethylammonium bromide mixture, but not in the other. DLS, viscosity, and electrophoretic mobility quantified the above effects. Surface charge density, surface tension, elasticity, and osmotic pressure concur to the stability of unilamellar vesicles and a balance between the above contributions is demonstrated. The results are relevant for practical applications of vesicles as carriers in biomedicine.