Many Facets of the Polyelectrolyte and Oppositely Charged Colloidal Particle Complexation: Counterion Release and Electrical Conductivity Behavior

Abstract

The lateral correlated adsorption of polyions onto oppositely charged vesicles, leading to the formation of stable equilibrium clusters of mesoscopic size, is associated to the release of a fraction of counterions, initially condensed on the polyion chains. This ulterior release of counterions provokes an increase of the number of free ions, besides the ones due to the partial ionization of both charged particles and polyions, that can be appropriately monitored by means of electrical conductivity measurements of the whole system. We have investigated this behavior in a suspension of cationic vesicles made up by dioleoyl trimethyl ammonium propane (DOTAP) liposomial vesicles interacting with an anionic polyelectrolyte composed by polyacrylate sodium salt. This system has been in the past extensively studied by us by means of different experimental techniques, and its behavior has been sufficiently characterized, as far as hydrodynamic and electrical properties are concerned. In this note, we report on the dc electrical conductivity behavior during the whole aggregation process, from the single polyion-coated liposomal particles, to polyion-induced liposome clusters, to finally polyion-fully covered liposomes, in polyion excess conditions. We have evaluated the excess of released counterions on the basis of the standard theory of the electrical properties of aqueous charged solutions and compared this quantity with the one predicted by the lateral correlation adsorption model. The agreement is quite good, offering strong experimental evidence of the role played by the release of counterions in the aggregation process. Finally, we have considered a similar liposomial system, where the lateral correlation adsorption was inhibited by structural reasons, having replaced the polyion by a simple electrolyte, whose dissociated ions will adsorb randomly at the particle surface, rather than in a correlated manner. In this case, no counterion release upon complexation occurs, and the electrical conductivity of the suspension approaches the one theoretically expected.

Thermo‐responsive hydrogels from cellulose‐based polyelectrolytes and catanionic vesicles for biomedical application

“…The cluster formed in this way arranges itself in three‐dimensional, randomly cross‐linked networks, with size distribution depending on the concentration of both polymer chains and vesicles and on their mutual‐ and self‐interactions. Recent experimental evidence, gained in the case study sodium polyacrylate‐unilamellar cationic liposomes, supports the validity of the lateral correlation adsorption model to explain the mechanism at the basis of this kind of meso‐structure. According to this model, the correlated adsorption of the polymer chains on the vesicle surface, as a consequence of counterion release, results in a short‐range attraction potential between the liposomes .…”

“…Recent experimental evidence, gained in the case study sodium polyacrylate‐unilamellar cationic liposomes, supports the validity of the lateral correlation adsorption model to explain the mechanism at the basis of this kind of meso‐structure. According to this model, the correlated adsorption of the polymer chains on the vesicle surface, as a consequence of counterion release, results in a short‐range attraction potential between the liposomes . Polyacrylate is quite different from the PQ‐67 considered here, possessing higher flexibility and, above all, a higher charge density .…”

“…Interestingly, the vesicles do not appear to be strongly perturbed by the presence of the polycations: they cannot be disrupted and maintain the thermal behaviour observed in solution. Nevertheless, they are affected by polymer addition; in fact, they exhibit release of sodium counterions, a decreased amount of free dodecylsufate in the bulk, larger size as unilamellar aggregates, polyhedral shape and electrostatic interactions, which favour gelation, probably originated by a nonrandom adsorption of polycations on the surface of oppositely charged vesicles . As confirmed both by PGSTE NMR analysis and release studies, the vesicular hydrogel demonstrated the effective hindering behavior relating to theophylline diffusion.…”

“…According to this model, the correlated adsorption of the polymer chains on the vesicle surface, as a consequence of counterion release, results in a short‐range attraction potential between the liposomes . Polyacrylate is quite different from the PQ‐67 considered here, possessing higher flexibility and, above all, a higher charge density . Nevertheless, as also applies at the present, the mutual arrangement of the polymer strands on vesicle surface might not be random but is assumed to be responsible for the stability of mesoscopic clusters made of polymer‐decorated vesicles.…”

Investigation of Effect of Adding Hydrophobically Modified Water Soluble Polymers on the Structure and Viscosity of Anionic Vesicle Dispersion

“…6, the specific conductivity for the vesiclespolymer system is shown as a function of the amount of added polymer. Without polymer, the conductivity values of the vesicles at higher concentration regions are higher due to the degree of counterion binding [29]. In the case of higher vesicle concentration, the conductivity increase upon addition of Pluronic L35 is comparable to Pluronic 10R5.…”