UC Irvine

1.1. The structure of planar bilayer membranes formed from glycerylmonooleate in n-hexadecane has been investigated using precise measurements of specific geometric capacitance (Cg). Films were formed at 25–30 °C in 0.1 M NaCl and the temperature lowered at a rate of 0.1–0.2 °C/min. At about 16 °C the Plateau border (annulus) freezes but the film does not rupture. The freezing is accompanied by a large increase in Cg. Cg = 0.625 μF/cm 2 at 25 °C and 0.735 μF/cm2 at 10 °C.2.2. Equations for Cg were derived which take into account the temperature-dependent changes in density and dielectric coefficient of the films above the melting point of the n-hexadecane. The density of n-hexadecane and 1-heptadecene (taken as equivalent to the glycerylmonooleate acyl chain) were measured as a function of temperature. Calculations using these equations and data indicate that the observed changes in Cg above the melting point cannot be accounted for unless there are significant changes in the composition of the bilayer.3.3. The data are consistent with the following hypothesis: At 25 °C there are 7–8 n-hexadecane molecules per 100 glycerylmonooleate molecules in the true bilayer portions of the films. Additional solvent is trapped in microlenses. As the hexadecane freezes, the solvent in the true bilayer is disproportionated (i.e. it “freezes-out”) into additional microlenses leaving extensive regions of bilayer which are largely free of solvent.