Determination of the Coverage of Phosphatidylcholine Monolayers Formed at Silicone Oil-Water Interfaces by Vesicle Fusion

Phospholipid monolayers at oil-water interfaces are used for various biological applications and often formed by vesicle adsorption. However, the adsorbed structures are not well characterized; therefore, fundamental investigation on vesicle adsorption behavior is necessary for correct understanding of the monolayer systems. Herein, we investigated the adsorption of phosphatidylcholine vesicles onto silicone oil-water interfaces using fluorescence microscopy and pendant drop tensiometry. The interfacial monolayer coverage, S, was determined by assuming S = 1 for tightly packed monolayers. Adsorption for 10 min with a lipid concentration of 0.2 mM resulted in S approximate to 0.4. An increase in lipid concentration (0.5-2 mM) and adsorption time (1 h) moderately increased monolayer coverage (S approximate to 0.6). However, extended adsorption for 24 h only slightly increased coverage (S approximate to 0.7). Monolayers with an S < 0.6 were homogeneous, while those with an S > 0.6 were associated with several vesicular structures. The surface density of these bound vesicles increased with increasing S. We conclude that vesicles readily fused with the interface to form monolayers at S < 0.6 and that their fusogenicity considerably decreased at S > 0.6. These results demonstrate that the ability of vesicles to form monolayers is determined by the interfacial coverage.