Hydrocarbon Penetration into Phospholipid Monolayers Formed at Hydrocarbon-Water Interfaces.

Phospholipid monolayers formed at oil-water interfaces are used for various biological applications. However, monolayer structures are not well understood. Herein, we investigated hydrocarbon partitioning in 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine monolayers formed at hydrocarbon-water interfaces using fluorescence microscopy and pendant drop tensiometry. The monolayers strongly interacted with squalene, n -hexadecane, n -tetradecane, n -dodecane, n -decane, and n -butylcyclohexane. These alkane and alkylcyclohexane molecules remained within the monolayers during area compression. In contrast, the monolayers interacted weakly with n -pentylbenzene and n -butylbenzene. These alkylbenzenes were gradually removed from the monolayers upon area compression and were completely expelled at an area per lipid of ∼70 Å 2 . Surface pressure analysis indicated that the ability of hydrocarbons to penetrate the monolayers was enhanced in the order of n -butylbenzene < n -pentylbenzene < n -butylcyclohexane < n -hexadecane.