Influence of the polymer charge density on lipid-polyelectrolyte complexes at the air/water interface

Coupling of a polyelectrolyte to a charged lipid monolayer changes the interplay between the lateral forces within the monolayer and the entropic forces in the polymer. This results in structure changes in the monolayer region. Charge dilution within the polymer can help to evaluate the role of electrostatic attraction. The structure of a negatively charged phosphatidic acid monolayer complexed with copolymers consisting of the positively charged diallyldimethylammoniumchloride and the neutral acrylamide in different ratios was studied by grazing incidence X-ray diffraction. Ellipsometry measurements were used to determine the thickness of the adsorption layers. Lipid-polyelectrolyte complexes exhibit rectangular lattices at low pressures. The tilt angles of the molecular chains relative to the surface normal decrease with decreasing charge of the polyelectrolyte. In a complex with 21% polymer charge, a transition to a hexagonal phase is observed at 35 mN/m. The oblique lattice of the DPPA monolayer on water is suppressed in all systems. The adsorption layer, consisting of similar to 85% water, gets thicker with decreasing polymer charge. This is a result of the formation of loops and tails.