Tensions and free energies of formation of "solventless" lipid bilayers. Measurement of high contact angles.

A method is described for the accurate measurement of the interfacial tension of lipid bilayer membranes containing little or no solvent. The tensions were obtained from the interfacial tensions of the equilibrium film-forming solution in the Plateau-Gibbs border, measured by conventional techniques, and the contact angle between the border and the bilayer. The contact angles in these systems are large (greater than 10 degrees) and were estimated by a new method that involved the injection of small known volumes of lipid solution into the bilayer so as to form a lens. Results have been obtained for monoolein-triolein, monoolein-squalene, and monoolein-squalene-decane systems. Half bilayer tensions in these systems were up to approximately 1 mN m-1 less than the single interface tensions. Although bilayer tension tended to increase with bilayer thickness, the interdependence of these quantities varied with the alkane solvents present. In the monoolein-squalene-decane systems, small concentrations of decane have a larger effect on tension than on thickness. Free energies of formation of the near-solventless bilayers were much greater than estimated from the simple application of Lifshitz theory.