Résumé
The phenomenon of complexation between oppositely charged colloids or macromolecules in aqueous solution is a fundamental process, widely exploited by nature, as for example in DNA packaging within biological cells [2], but also by technology, as in industrial colloidal stabilization, water treatment and paper making [3].Being the result of a delicate balance of forces of different nature, small variations in the physico-chemical parameters may induce large changes in the resulting complexes [4]. Electrostatics is clearly the relevant interaction in driving the aggregation, although non-electrostatic interactions can be important in modulating the process and can deeply affect the characteristics of the resulting self-assembled structures. However, despite the intense experimental, theoretical and computational research aimed to understand the mechanisms driving the formation and the stabilization of the complexes [5], [6] and [7] in the physical context of macroion-multivalent counterion interactions [8], due to the great complexity of these systems, also the details of purely electrostatic interactions remain to be completely clarified.The possibility to get a fine control of the stability of the complexes by tuning the competition between attractive and repulsive electrostatic interactions justifies the actual enduring attention on polyion–colloid complexation, being key to the technological development of nano-structured materials or nano-devices to be used for example in drug delivery or gene-therapy [9] and [10].In particular, a rich literature exists on the variety of structures formed by charged liposomes interacting via electrostatic forces with oppositely charged linear polyions, whether they are synthetic polymers, polypeptides or DNA [11], [12] and [13]. A feature characterizing the phase behavior of these systems is the presence of a “reentrant condensation” accompanied by a marked “overcharging”, or charge inversion [14] and [15]. Indeed, when a given volume of a liposome suspension is mixed with the same volume of a solution containing oppositely charged polyelectrolytes, the complexation is systematically observed due to a rapid adsorption of polyelectrolytes chains at the liposome surface, forming what we have defined as “polyelectrolyte-decorated particles” (hereafter pd-particles), followed by their aggregation in pd–liposome clusters, as it is schematically represented in panel A of Fig. 1. For low enough polyelectrolyte concentration the clusters are small (they are formed by a few liposomes), after their rapid formation they are very stable, their measured ζ-potential being only slightly lower, in absolute value, than that measured for the liposomes in the absence of adsorbed polyelectrolytes. As the polyion concentration (and hence the ratio, ξ, between the number of stoichiometric charges on the polymer and on the particles) is increased, larger but still stable clusters are formed, with a ζ-potential which is further reduced in absolute value. At a sufficiently high polyelectrolyte concentration, the liposome suspension is completely destabilized, the large clusters that form are not stable, their ζ-potential is close to zero and they rapidly coalesce in macroscopic “flocs”. Going beyond the isoelectric point, hence further increasing the polyelectrolyte concentration, stable clusters form again, but now their size decreases upon increasing the polyelectrolyte concentration in the mixture: the polyelectrolyte-induced condensation of the liposomes is hence “reentrant”. At the same time, the ζ-potential after having passed through the zero, increases again in absolute value, and the sign of the excess charge of the clusters is that of the polyelectrolyte, i.e. opposite to that of the original liposomes (charge inversion).