Polyelectrolyte–liposome complexes: An equilibrium cluster phase close to the isoelectric condition

We have investigated the complexation process between cationic liposomes built up by dioleoyltrimethylammoniumpropane [DOTAP] and linear polyelectrolytes, employing three different polyions with different charge density and flexibility, such as a single-stranded (ss)DNA, a double-stranded (ds)DNA and the sodium polyacrylate [NaPAA]. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements support a “reentrant condensation” scenario and suggest the presence of an arrested state occurring in polyelectrolyte–liposome suspensions, in the presence of high salt concentration. A simple phenomenological model is presented, describing the formation of large “equilibrium clusters”, close to the isoelectric condition.