SURFACE FORCES IN SUPPORTED SPHERICAL LIPID-MEMBRANES

Spherical lipid membranes (SLMs) were formed in aqueous 0. 10 M KCl solution from decane solutions of glyceryl monooleate (GMO) at the tip of glass capillaries. Thinning of SLMs was monitored by a videorecorder system and capacitance measurements. Experimental arrangements permitted the direct measurement of transmembrane pressure (DELTAP12), the radius of the SLM (R1), as well as the radii of the exterior surface of the reservoir droplet (R2) and the curvature of the Plateau-Gibbs border-SLM interface (R4). These measurements, in turn, led to the determination of the bifacial surface tension of the SLM (sigma(L)) and the surface tension of the exterior curvature of the Plateau-Gibbs border (gamma14) at different SLM thickness. At each DELTAP12, the SLM adjusted to an equilibrium thickness (h(e)) such that its surface-area-to-thickness ratio remained constant at 20 m (+/- 5%). Sigma(L) linearly increased with R1 and, at R1 = 0, a value (0.22 +/- 0.02 mN m-1) for the interfacial surface tension for a flat GMO BLM was obtained, in good agreement with previous data. Similarly, extrapolation of the plot of gamma14 vs 1/R1 to 0 (ie., to R1 = infinity) provided a value for the bulk GMO surface tension (3.82 +/- 0.27 mN m-1) which agreed well with the bulk surface tension of GMO. Excess tensions of the interacting interfaces of the SLM (DELTAsigma), Hamaker coefficients (A(h)), and disjoining pressure (PI(h)) were assessed from DELTAsigma = sigma(L) -2gamma14 congruent-to DELTAF congruent-to A(h)/12PIh(e)2 congruent-to 1/2h(e)PI(h) at different SLM thicknesses. Excess tensions (DELTAsigma(vdW), Hamaker coefficients (A(h)(vdW), and disjoining pressure (PI(h)vdW) were calculated by the numerical solution (by the Mathcad program, version 2.5) of the Lifshitz expression for a two-layer interacting system. Satisfactory agreement between the experimentally determined (DELTAsigma, A(h), and PI(h)) and simulated (DELTAsigma(vdW),A(h)vdW) and PI(h)vdW) parameters led to the proposition that the experimentally determined excess tension of the interacting SLM represents the free energy of SLM thinning (ie., DELTAsigma congruent-to DELTAF). Conversely, using a single thickness invariant, gamma(B) = 3.84 mN m-1, instead of gamma14 for SLMs led to DELTAF, A'(h), and PI'(h) values which were approximately 1000-fold smaller than those obtained for DELTAF, A(h), and PI(h) by the use of the experimentally determined and thickness-varient gamma14 values. Thickness-dependent DELTAF and DELTAPI(h) Values were rationalized by assuming two minima in the potential energy diagram. Only the second minimum was observed for SLMs. Thicknesses corresponding to the first minimum (3-5 nm) could not be experimentally realized for the SLMs investigated here.