Aggregation and stability of polyelectrolyte-decorated liposome complexes in water-salt media

Despite the initial disparity of the early results that appeared in the literature, due to the variety of the experimental conditions and to the intrinsic complexity of these systems, the interesting phenomenology shown by the aqueous co-suspensions of oppositely charged polyelectrolytes and colloidal macroions can now be described within a unified picture. For a general consensus, within this picture screened electrostatic interactions are the main driving force, and the non uniformity of charge distribution, resulting from the correlated adsorption of the polyelectrolyte on the colloid surface, plays a pivotal role. However, there is still a lack of a complete theory able to describe, satisfactorily, all the different features of the observed complexity. In this article, we begin to explore systematically the phase diagram of a typical polyelectrolyte/colloid system, considering in addition to the effects of the polyelectrolyte/colloid charge ratio and temperature, also those of the ionic strength of the aqueous suspending medium. We show that a simple interparticle potential built up as the sum of two terms is able to account for the observed phase diagram very satisfactorily. The first term of this potential describes the screened electrostatic interaction, taking into account the surface charge inhomogeneity of the polyelectrolyte-decorated particles, while the second describes the ubiquitous van der Waals interactions.