Charged lipid monolayers at the air-/solution interface: coupling to polyelectrolytes

The thermodynamic behaviour of mixed cationic-/zwitterionic lipid mixtures dioleoyltrimethylammoniumpropanedipalmitoylphosphatidylcholine (DOTAP-/DPPC) at different mole fractions have been investigated by means of surface pressure measurements (Langmuir isotherms). Excess Gibbs energies of mixing have been calculated as a function of layer composition and surface pressure. Deviation from ideality evidences different molecular organisation of the two lipids, yielding the formation of cationic lipid domains in a zwitterionic lipid matrix. The stability of the mixed monolayer in the presence of both a simple electrolyte solution (NaCl) and a polyelectrolyte solution (NaPAA of different molecular weights) in the same charge ratio interval has been investigated and the influence of the polymer conformation briefly discussed.