New Aspect of the Spontaneous Formation of a Bilayer Lipid Membrane

Abstract

An artificial lipid bilayer in planar form, well known as bilayer lipid membrane (BLM), spontaneously forms from a lipid droplet (diphytanoyl phosphatidylcholine in n‐decane and chloroform in this work) in an aperture of a thin partition in aqueous solution. The thinning dynamics of the lipid droplet or membrane has been studied by simultaneous capacitance and image recording. The simultaneous measurements have revealed the two‐step thinning of the lipid membrane from its specific capacitance value: first, the initial droplet thins to yield a membrane of 60 nm thickness (0.03 μF/cm2), and second, within this thin lipid membrane, a lipid bilayer of 4 nm thickness (0.45 μF/cm2) suddenly emerges and grows with keeping a bilayer structure. The revealed dynamics provides a quantitative support for a “zipper” mechanism proposed by Tien and Dawidowicz; in the mechanism, the first thinning results in a sandwich consisting of the organic solvent between two adsorbed lipid monolayers whose distance is the order of 100 nm, and then a chance contact of both monolayers initiates the formation and growth of a lipid bilayer in a zipper‐like manner. However, because of the existence of the two solvent‐water interfaces containing surface‐active molecules, phospholipids, this work claims that the zipper mechanism should be modified in view of the Marangoni effect. The present formation and growth of a lipid bilayer can be explained by the classic nucleation theory of two‐dimensional crystallization. BLM systems with the simultaneous measurements can be considered as a useful environment for the study of soft‐matter chemical physics.