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The electrostatic complexation between DOTAP-DOPC unilamellar liposomes andan oppositely charged polyelectrolyte (NaPA) has been investigated in a widerange of the liposome surface charge density. We systematically characterizedthe "reentrant condensation" and the charge inversion ofpolyelectrolyte-decorated liposomes by means of dynamic light scattering andelectrophoresis. We explored the stability of this modelpolyelectrolyte/colloid system by fixing each time the charge of the bareliposomes and by changing two independent control parameters of thesuspensions: the polyelectrolyte/colloid charge ratio and the ionic strength ofthe aqueous suspending medium. The progressive addition of neutral DOPC lipidwithin the liposome membrane gave rise to a new intriguing phenomenon: thestability diagram of the suspensions showed a novel reentrance due to thecrossing of the desorption threshold of the polyelectrolyte. Indeed, at fixedcharge density of the bare DOTAP/DOPC liposomes and for a wide range of polyionconcentrations, we showed that the simple electrolyte addition first (low saltregime) destabilizes the suspensions because of the enhanced screening of theresidual repulsion between the complexes, and then (high salt regime)determines the onset of a new stable phase, originated by the absence ofpolyelectrolyte adsorption on the particle surfaces. We show that the observedphenomenology can be rationalized within the Velegol-Thwar model forheterogeneously charged particles and that the polyelectrolyte desorption fitswell the predictions of the adsorption theory of Winkler and Cherstvy. Ourfindings unambiguously support the picture of the reentrant condensation asdriven by the correlated adsorption of the polyelectrolyte chains on theparticle surface, providing interesting insights into possible mechanisms fortailoring complex colloids via salt-induced effects.

Salt-induced reentrant stability of polyion-decorated particles with tunable surface charge density