New aspect of the spontaneous formation of a bilayer lipid membrane

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 muF/cm(2)), and second, within this thin lipid membrane, a lipid bilayer of 4 nm thickness (0.45 muF/cm(2)) 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.