- Catanionic surfactant systems containing PEG− lipid molecules are studied at the air− solution interface and in bulk. It is found that, upon introduction of the optimum amount of PEG− lipid molecules, the region of vesicle stability in the bulk is increased. At the air− solution interface, the system forms an equimolar (salt) film upon compression. The transition to the salt structure in the presence of PEG− lipid took place at higher surface pressure, thus supporting the results of increased stability observed with the bulk samples. The presence of PEG− lipid molecules induces film buckling, resulting in significantly smaller areas per molecule. The combined results are discussed in terms of electrostatic and steric repulsion forces.