The thickness of the hydrophobic and polar regions of glycerol monooleate bilayers determined from the frequency-dependence of bilayer capacitance

Abstract

The frequency dependence of the equivalent parallel capacitance and conductance of glycerol monooleate bilayers formed with n-hexadecane and squalene solvents has been measured. The capacitance of such bilayers was always found to decrease by 3–5% with increasing frequency over the range 0.1 Hz to 5000 Hz. For KCI concentrations equal to or above 0.1 M it was found that the capacitance of the hydrophobic region was in the range 6.5–7.0 mF/m 2 for bilayers formed with n-hexadecane and 7.6–7.9 mF/m 2 for squalene. By fitting a model of a multilaminar dielectric sandwich to the experimental data it was possible to determine the equivalent parallel capacitance and conductance of 4 or 5 distinct layers within the bilayer. Using their electrical time constants as a guide, the dielectric constant within each polar region was estimated and consequently the thickness of each substructural region could be calculated from its measured capacitance. In this fashion the overall thickness of each polar region was estimated to be in the range 0.5–0.75 nm. We compare our results with those calculated previously from a combination of optical and high-frequency capacitance measurements, and a discussion is given of the magnitude of the systematic errors that will ensue as a consequence of the neglect of the frequency dependence of the bilayer capacitance and how these errors can be reduced.