The surface pressure(Π) and surface area/molecule(A) isotherms of DMPC & PEG-DMPE system were measured at various compositions to study the 2-dimensional interfacial properties of the binary mixture of a lipid and a lipid-polymer system at an air/water interface by using the Langmuir surface balance technique.
The finite surface pressure was observed in the range of a surface area that is much larger than the cross section of lipid molecules, and the isotherm in this concentration regime strongly depended on the composition. All the isotherms in this region become identical if the surface area is rescaled in terms of the surface area per PEG-molecule. The isotherm agrees with the two dimensional scaling law in a good solvent, and it is very similar to that of the PEG adsorbed monolayer.
Furthermore the mixing Gibbs free energy was evaluated at various compositions from the integration of the Π-A isotherm. The results indicate that the system is miscible in all composition and there exists very strong attractive interaction between them.