Flip-flop in adsorbed bilayers

There is very rapid exchange between molecules between an adsorbed (supported) bilayer and solution. Complete exchange of a bilayer occurs within seconds; the actual exchange process may be faster but is limited by the resolution of our technique. The exchange process was monitored by infrared spectroscopy, which can independently monitor deuterium-labeled and unlabeled surfactants, and surface selectivity was achieved by using an internal reflection configuration. Total exchange requires exchange of both the inner and outer layer. We demonstrate that the desorption of a monolayer into water is extremely slow, yet exchange of the inner layer into surfactant solution is extremely fast when a bilayer is present. This shows that flip-flop for an adsorbed bilayer is very fast. Rapid flip-flop at interfaces is in sharp contrast to the extremely slow rate of flip-flop in vesicles. We hypothesize that the rapid rate of flip-flop in adsorbed layers is a consequence of surface-induced defects; in free vesicles, the defects are closed by the film pressure.