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-sn-glycero-3-phosphocholine (POPC) monolayers at silicone oil–water interfaces to determine the dependence of interfacial tension, γ, on the area per lipid, a, compared to that of the closely packed monolayers, acp. This study aims to develop a method to obtain the γ–a relationship from the γ–a/acp 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 a/acp. Furthermore, by calculating the surface pressure, Π, from γ and multiplying a/acp with an estimated acp value, the dependence of Π on a was obtained. When a value approximately equal to the a of POPC bilayers was assigned to acp, the resultant Π–a profile partially or approximately completely overlapped with the Π–a isotherms obtained for the monolayers at the air–water interface using a Langmuir trough. The overlap for the silicone oil–water interface occurred at a ≤ 77 Å2, while that for the tricaprylin–water interface occurred in approximately the entire a region. The results indicate that the Π of the condensed monolayers is little affected by bulk oil. Thus, the γ–a relationship for the oil–water interface can be determined by comparing the compression isotherm with the one obtained for the air–water interface.