The thickness of monoolein lipid bilayers as determined from reflectance measurements

Measurements of the reflectance of monoolein $\text{n-}\text{alkane}$ and monoolein/squalene lipid bilayers have been made. The total thickness of the bilayer was calculated from the dependence of reflectance on the refractive index of the aqueous salt or sucrose solution surrounding the bilayer. The total thickness was then compared to the thickness of the hydrocarbon chain region as determined from capacitance measurements. From this comparison, we found that the thickness of each polar region of the bilayers in salt solutions was $\text{0.5 ± 0.1}\text{nm}$, independent of the hydrocarbon solvent used. When the aqueous solutions contained sucrose, each polar region was approx. 0.9 nm thick. When $\text{n-}\text{tetradecane}$ and $\text{n-}\text{hexadecane}$ were used as solvents, microlenses of solvent trapped in the monoolein bilayer increased the reflectance. After about one hour, the coalescence of microlenses into larger lenses allowed the reflectance of the bilayer alone to be measured. The use of reflectance to measure the thickness of monoolein bilayers appears to be consistent with other methods and to give useful information about the structure of lipid bilayers.