MEASUREMENT OF FORCES BETWEEN SPONTANEOUS VESICLE-FORMING BILAYERS

The forces between bilayers self-assembled onto mica from dilute aqueo us solutions containing vesicles of mixed cationic surfactant cetyltri methylammoniun tosylate (CTAT) and anionic surfactant sodium dodecylbe nzenesulfonate (SDBS) were measured using the surface forces apparatus . At large separations (D > 20 Angstrom) the forces can be described b y the DLVO theory of repulsive electrostatic and attractive van der Wa als forces; in dilute solution, the electrostatic repulsion is suffici ent to make the net interaction of the mixed surfactant bilayers repul sive at all separations. The electrostatic force dominates any undulat ion or other long-range interactions and is sufficient to render the v esicles stable against aggregation in dilute solution. Although the ne t interactions between the mixed surfactant bilayers were monotonicall y repulsive, fusion of the bilayers could be induced at applied pressu res orders of magnitude lower than for single component phospholipid o r surfactant bilayers. At sufficiently high concentrations of added sa lt (> 0.5 M), the electrostatic interactions were screened sufficientl y that multilayers were trapped between the surfaces. The measured for ces correlate well with the ternary phase diagram and the vesicle micr ostructures observed using freeze-fracture and cryo-electron microscop y.