Determining the Dependence of Interfacial Tension on Molecular Area for Phospholipid Monolayers Formed at Silicone Oil-Water and Tricaprylin-Water Interfaces by Vesicle Fusion.

Phospholipid monolayers formed at oil-water interfaces have been used to explore biological interface properties. Thus, monolayer systems need to be quantitatively understood. Previously, we investigated the formation of 1-palmitoyl-2-oleoyl--glycero-3-phosphocholine (POPC) monolayers at silicone oil-water interfaces to determine the dependence of interfacial tension, γ, on the area per lipid, , compared to that of the closely packed monolayers, . This study aims to develop a method to obtain the γ- relationship from the γ-/ data by investigating POPC monolayers at the silicone oil-water and tricaprylin-water interfaces. Pendant drop tensiometry was used to obtain the dependence of γ on /. Furthermore, by calculating the surface pressure, Π, from γ and multiplying / with an estimated value, the dependence of Π on was obtained. When a value approximately equal to the of POPC bilayers was assigned to , the resultant Π- profile partially or approximately completely overlapped with the Π- isotherms obtained for the monolayers at the air-water interface using a Langmuir trough. The overlap for the silicone oil-water interface occurred at ≤ 77 Å, while that for the tricaprylin-water interface occurred in approximately the entire region. The results indicate that the Π of the condensed monolayers is little affected by bulk oil. Thus, the γ- relationship for the oil-water interface can be determined by comparing the compression isotherm with the one obtained for the air-water interface.