Aggregation and stability of polyelectrolyte-decorated liposome complexes in water–salt media Simona Sennato, * a Domenico Truzzolillo b and Federico Bordi a Received 13th March 2012, Accepted 24th April 2012 DOI: 10.1039/c2sm25576f 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. 1 Introduction In recent years, there has been a renewed interest in the study of the mechanisms governing the complexation of polyelectrolytes and oppositely charged colloidal particles. This is not only due to the their interesting, and still not completely understood, phenomenology, but also to the increased awareness of their potential for innovative applications in nano-medicine and nano- biotechnology, 1 particularly with the emergence of new poly- electrolyte architectures, such as spherical brushes 2 or multi- layers. 3 A key point for the technological development of practical devices and nano-structured materials based on these systems is the achievement of a fine control of the stability of the complexes. For example, keeping the complexes in solution is of primary importance in food or the pharmacological industry. Conversely, a precise tuning of the destabilization is required for protein separation via polyelectrolyte coacervation. 4 Being the result of a delicate balance of forces of different nature, the supramolecular self-assembly of polyelectrolytes and oppositely charged colloids show a complex phenomenology, where small variations in the physico-chemical parameters may induce large changes in the resulting structures. Due to the entropic and conformational effects related to the polymeric nature of the polyelectrolyte, polyelectrolyte adsorp- tion can occur in a highly correlated way. This correlation results from a balance of the electrostatic attraction of the poly- electrolytes to the oppositely charged surface, of the repulsion between the like-charged adsorbing chains and of their confine- ment entropy, depending on the chain flexibility and on the ‘‘mismatch’’ between the charge distribution on the surface and the chains. 5–8 As a consequence the particles surface appears ‘‘decorated’’ by statistically regular patterns, rather than uniformly coated. 7 It has been shown that, under appropriate conditions, corre- lation promotes the aggregation of the polyelectrolyte-decorated particles (pd-particles) in stable finite-size clusters. 7,9 Due to the presence of alternating domains of polyelectrolyte and bare surface of the colloid, the surface charge distribution of these pd- particles is highly non uniform. From such a ‘‘patchy’’ distribu- tion of the surface charge, and due to the screening effect of the small counterions in the solutions, a short range attraction arises between the pd-particles, 10–13 which may yield the formation of stable aggregates although the particles are (considering the net charge) like-charged (see for example ref. 7 and 13). Notably, the correlated adsorption is also responsible for the overcharging phenomenon, which occurs when the adsorption a Dipartimento di Fisica Sapienza Universit  a di Roma and CNR-IPCF UOS Roma, Piazzale A. Moro 2, I-00185 Rome, Italy. E-mail: simona.sennato@ roma1.infn.it; Fax: +39 06 4463158; Tel: +39 06 49913503 b FORTH, Institute of Electronic Structure and Laser, GR-71110 Heraklion, Crete, Greece 9384 | Soft Matter , 2012, 8, 9384–9395 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2012, 8, 9384 www.rsc.org/softmatter PAPER Downloaded by Universita' di Roma La Sapienza on 06 September 2012 Published on 01 June 2012 on http://pubs.rsc.org | doi:10.1039/C2SM25576F View Online / Journal Homepage / Table of Contents for this issue