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, a_cp. This study aims to develop a method to obtain the γ-a relationship from the γ-a/a_cp 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/a_cp. Furthermore, by calculating the surface pressure, Π, from γ and multiplying a/a_cp with an estimated a_cp value, the dependence of Π on a was obtained. When a value approximately equal to the a of POPC bilayers was assigned to a_cp, 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 Ų, 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.