Structural changes of monolayers at the air/water interface contacted with n-alkanes

Monolayers of the phospholipids 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-dipalmitoylphosphatidylethanolamine (DPPE) at the air/water interface in contact with hexadecane and dodecane are studied by Synchrotron X-ray diffraction. The results show the existence of ordered phases with a reduction of the tilt angle of the aliphatic tails compared to films at the air/water interface. For DPPC, where due to the large head group, a nontilted phase does not exist at all at the air/water interface, insertion of oil yields a phase with zero tilt angle. In addition a phase sequence is found where with increasing pressure the tilt azimuth changes from next-nearest neighbor to nearest neighbor orientation. For DPPE the oil can be completely squeezed out of the film. For the two different alkanes the effects differ quantitatively as regards the reduction of tilt angle or projected area. They differ qualitatively, as for dodecane, in contrast to hexadecane, only untilted phases and a hexagonal lattice are formed.