Abstract
The electrostatic complexation between DOTAP/DOPC unilamellar liposomes and an oppositely charged polyelectrolyte (NaPA) has been investigated in a wide range of the liposome surface charge density. We systematically characterized the reentrant condensation and the charge inversion of polyelectrolyte-decorated liposomes by means of dynamic light scattering and electrophoresis. We explored the stability of this model polyelectrolyte/colloid system at different values of the surface charge of the bare liposomes and by changing two independent control parameters of the suspensions: the polyelectrolyte/colloid charge ratio and the ionic strength of the aqueous suspending medium. The progressive addition of neutral DOPC lipid within the liposome membrane gave rise to an interesting phenomenon which has not been observed previously: the stability diagram of the suspensions showed a novel reentrance due to the crossing of the desorption threshold of the polyelectrolyte. Indeed, at fixed charge density of the bare DOTAP/DOPC liposomes and for a wide range of polyion concentrations, we showed that the simple electrolyte addition first (low salt regime) destabilizes the suspensions because of the enhanced screening of the residual repulsion between the complexes, and then (high salt regime) determines the onset of a new stable phase, originated by the absence of polyelectrolyte adsorption on the particle surfaces. We show that the observed phenomenology can be rationalized within the modified Velegol–Thwar model for heterogeneously charged particles and that the polyelectrolyte desorption fits well the predictions of the adsorption theory of Winkler and Cherstvy [1]. Our findings unambiguously support the picture of the reentrant condensation as driven by the correlated adsorption of the polyelectrolyte chains on the particle surface, providing interesting insights into possible mechanisms for tailoring complex colloids via salt-induced effects.
Highlights
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], and by technology, as in industrial colloidal stabilization, water treatment and paper making [3]
The ionic strength due to the 1-valent counterions Na+ and Cl− in the pdliposome suspensions was varied from 0.005 M to 0.8 M by adding appropriate amounts of NaCl to the polyelectrolyte solution to be mixed with the liposome suspensions
We focused here on the effect of the surface charge density of the liposomes on the stability/instability diagram of pd-liposomes as a function of the polyelectrolyte/lipid charge ratio ξ and of the ionic strength of the solution
Summary
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], and by technology, as in industrial colloidal stabilization, water treatment and paper making [3]. Such a ”patchy” distribution of charge, together with the screening effect given by the small counterions, generates a short range attraction between these ”polyelectrolyte-decorated particles” (hereafter pd-particles), that has been widely studied in the last two decades [21, 22, 23, 24] The superposition of this attraction, the ubiquitous van der Waals interaction and the electrostatic repulsion due to the residual net charge of the complexes, results in a inter-particle potential characterized by a barrier that, in proper conditions, yields the formation of stable, finite size clusters [25], even if all the primary particles bear the same net charge [18], the adsorbed polyelectrolyte layer representing a sort of electrostatic glue that sticks together the particles [26]. A deeper understanding of the relation between polyion adsorption, pd-liposome interactions and stability of the resulting aggregates is key for further developments of functional colloidal particles with application in biotechnology and material science
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