Mixed monolayers of DOPC and palmitic acid at the liquid-air interface.

The interactions between 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and palmitic acid (PA) in mixed monolayers were investigated through surface pressure measurements, fluorescence microscopy (FM) and atomic force microscopy (AFM). The miscibility of two components and the interactions between DOPC and PA in the mixed monolayers were accessed by analyzing surface pressure (π)-area (A) isotherms. It was found that there were both positive deviations for excess molecular area and excess Gibbs energy, which suggests the interaction is repulsive in all case and the strongest repulsive occurs at XPA=0.5 ratio. The FM and AFM images showed that there were discernible phase separations at selected surface pressures. The increase of surface pressure and the phenomenon that the PA-enriched domains have been connected to form much larger platforms are in agreement with the thermodynamic results.