MEASUREMENT OF FORCES BETWEEN SPONTANEOUS VESICLE-FORMING BILAYERS

The forces between bilayers self-assembled onto mica from dilute aqueous solutions containing vesicles of mixed cationic surfactant cetyltrimethylammoniun tosylate (CTAT) and anionic surfactant sodium dodecylbenzenesulfonate (SDBS) were measured using the surface forces apparatus. At large separations (D > 20 Angstrom) the forces can be described by the DLVO theory of repulsive electrostatic and attractive van der Waals forces; in dilute solution, the electrostatic repulsion is sufficient to make the net interaction of the mixed surfactant bilayers repulsive at all separations. The electrostatic force dominates any undulation or other long-range interactions and is sufficient to render the vesicles stable against aggregation in dilute solution. Although the net interactions between the mixed surfactant bilayers were monotonically repulsive, fusion of the bilayers could be induced at applied pressures orders of magnitude lower than for single component phospholipid or surfactant bilayers. At sufficiently high concentrations of added salt (> 0.5 M), the electrostatic interactions were screened sufficiently that multilayers were trapped between the surfaces. The measured forces correlate well with the ternary phase diagram and the vesicle microstructures observed using freeze-fracture and cryo-electron microscopy.