1,721,096 research outputs found
Dielectric properties of differently flexible polyions: a scaling approach
No full textThe dielectric relaxations associated with counterion polarization along some typical polyion lengths have been measured in an extended frequency range ( from 10 kHz to 2 GHz) for four different polyelectrolyte solutions, differing for the polyion molecular weight and the backbone stiffness. Here, we deal with the so-called intermediate dielectric relaxation, falling between the polarization process concerning the whole polyion chain and the polarization process associated with the field-induced re-orientation of the water molecule dipoles. These observed intermediate relaxations have been characterized by means of two parameters, i.e., the dielectric strength De and the relaxation frequency nu(0), and their dependence on the polyion concentration has been described according to the scaling model of a polyelectrolyte solution. These dependencies follow the expected exponents of the scaling laws, both for the dilute and semidilute (unentangled and entangled) regimes. The different concentration regimes evidenced from dielectric relaxation measurements are in very good agreement with the ones determined by means of the zero-shear viscosity measurements. Our results confirm that the intermediate dielectric relaxation in polyelectrolyte solutions should be attributed to counterion fluctuations along some segments (e.g. the subunits of the Mandel model) of the polyion chain, independently of its overall stiffness. This counterion polarization effect is rather confined to the local structure of the polyion chain instead of the whole chain and it is largely independent of the polyion conformation
Does a cluster phase in polyion-liposome colloidal suspensions exist? An integrated experimental overview
No full textAlthough the properties of mesoscopic charged complexes, originating by the self-assembling of polyions onto oppositely charged particles, are been extensively investigated, both experimentally and theoretically, less attention has been addressed towards the existence of an equilibrium cluster phase in low-density colloidal suspensions, where particles maintain their integrity and polyions act as an electrostatic glue. The self-assembling of lipidic vesicles into mesoscopic aggregates induced by polyions is a hierarchical process where, at different basic levels, aggregates arrange themselves to form superstructures, giving rise to multi-lamellar complexes or honeycomb structures in which polyions are more or less intercalated between the lipid bilayers. This reorganization becomes relevant in many biological important processes such as drug delivery. However, besides these rather complex structural rearrangements, at a lower hierarchy level, clusters formed by intact vesicles, stuck together by oppositely charged polyions, may exist. These equilibrium clusters, which we call "intermediate" aggregates, can be considered as a new class of colloids with a rich and not yet completely understood phenomenology. In this review, we will refer to a specific example and will present a well-documented experimental evidence of the formation of equilibrium clusters composed by positively charged liposomes built up by DOTAP, stuck together by a simple highly charged linear polyion (polyacrylate sodium salt). These structures, governed by a balance between long-range electrostatic repulsion and short-range attraction, give rise to the formation of relatively large, equilibrium clusters whose size and overall charge can be continuously tunable by simple environmental parameters. Under appropriate experimental conditions, in a long time limit, these liposome clusters may further evolve towards a different structural arrangement, giving account for the supramolecular structures of polyion-liposome complexes, such as those observed in DNA-lipid complexes. (C) 2006 Elsevier B.V. All rights reserved
Polyelectrolyte-induced aggregation of liposomes: a new cluster phase with interesting applications
No full textDifferent charged colloidal particles have been shown to be able to self-assemble, when mixed in an aqueous solvent with oppositely charged linear polyelectrolytes, forming long-lived finite-size mesoscopic aggregates. On increasing the polyelectrolyte content, with the progressive reduction of the net charge of the primary polyelectrolyte-decorated particles, larger and larger clusters are observed. Close to the isoelectric point, where the charge of the adsorbed polyelectrolytes neutralizes the original charge of the particles' surface, the aggregates reach their maximum size, while beyond this point any further increase of the polyelectrolyte-particle charge ratio causes the formation of aggregates whose size is progressively reduced. This re-entrant condensation behavior is accompanied by a significant overcharging. Overcharging, or charge inversion, occurs when more polyelectrolyte chains adsorb on a particle than are needed to neutralize its original charge so that, eventually, the sign of the net charge of the polymer-decorated particle is inverted. The stability of the finite-size long-lived clusters that this aggregation process yields results from a fine balance between long-range repulsive and short-range attractive interactions, both of electrostatic nature. For the latter, besides the ubiquitous dispersion forces, whose supply becomes relevant only at high ionic strength, the main contribution appears due to the non-uniform correlated distribution of the charge on the surface of the polyelectrolyte-decorated particles('charge-patch' attraction). The interesting phenomenology shown by these system has a high potential for biotechnological applications, particularly when the primary colloidal particles are bio-compatible lipid vesicles. Possible applications of these systems as multi-compartment vectors for the simultaneous intra-cellular delivery of different pharmacologically active substances will be briefly discussed
Hybrid niosome complexation in the presence of oppositely charged polyions
No full textWe have investigated the formation of complexes between negatively charged niosomal vesicles (hybrid niosomes), built up by dicethylphosphate [DCP], Tween 20 and Cholesterol, and three linear differently charged cationic polyions, such as alpha-polylysine, epsilon-polylysine, and polyethylvinylpyridinium bromide [PEVP], with two different substitution degrees. Our aim is to investigate the interaction mechanism between anionic-nonionic vesicles (hybrid niosomes) and linear polycations, characterizing the resulting aggregates in view of possible applications of these composite colloidal particles as vectors for multidrug delivery. In order to explore the aggregation behavior of the complexes and to gain information on the stability of the single niosomal vesicles within the aggregates, we employed dynamic light scattering (DLS), laser Doppler electrophoretic measurements, and fluorescence measurement techniques. The overall phenomenology is well described in terms of the re-entrant condensation and charge inversion behavior, observed in different colloidal systems. The aggregate size and overall charge depend on the charge ratio between vesicles and polyions, and the aggregates reach their maximum size at the point of charge inversion (re-entrant condensation). While the overall phenomenology is similar for all three polycations investigated, the stability and the integrity of the hybrid niosomal vesicles forming the aggregates strongly depend on the chemical structure of the polycations. The role of the polycations in the aggregation process is discussed by identifying specific interactions with the niosomal membrane, pointing out their importance for possible applications as drug delivery vectors
Aggregation and stability of polyelectrolyte-decorated liposome complexes in water–salt media
No full textDespite 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. This journal is © 2012 The Royal Society of Chemistry
Phase Behavior of DNA-Stabilized Carbon Nanotubes Dispersions: Association with Oppositely-Charged Additives
No full textThe formation of liquid crystalline phases or isotropic clusters is observed in carbon nanotubes systems experiencing repulsive and attractive interactions, respectively. ssDNA-stabilized nanotubes act as strongly repulsive charged rods, showing nematic phases in (pseudo)-binary and ternary systems, in the presence of a nonadsorbing polymer. Switching between purely repulsive and attractive regime has not been investigated yet. For this reason, dispersions of ssDNA-stabilized nanotubes were added with an oppositely charged additive (i.e., protein or surfactant), and the resulting systems were investigated. In both phase diagrams a strong associative behavior was observed. At additive charge excess, a redispersion of the complex was obtained. The phenomenon was substantial in the case of surfactant system, where a charge inversion was also observed. A fine-tuning of attractive and repulsive interactions promoted aggregation and redispersion of carbon nanotube complexes. The introduction of weak attractive forces may promote the formation a cluster phase of ssDNA-stabilized nanotubes, with possible application as "multicompartimental" delivery systems
Large equilibrium clusters in low-density aqueous suspensions of polyelectrolyte-liposome complexes: A phenomenological model
No full textIn this paper, we revisit some of our previous results together with further experimental evidences for the existence of large equilibrium clusters in low-density aqueous colloidal suspensions, resulting from the screening of charged spherical macroions by oppositely charged linear polyelectrolytes. The aggregation process is described by a simple phenomenological model where aggregates interact via a long-range repulsion Yukawa potential and some supports to the equilibrium properties of the resulting aggregates, on the basis of dynamic light scattering, zeta potential, and transmission electron microscopy techniques, have been provided
Strong repulsive interactions in polyelectrolyte-liposome clusters close to the isoelectric point: a sign of an arrested state
No full textPolyion-Induced Cluster Formation in Different Colloidal Polyparticle Aqueous Suspensions
No full textThe formation of aggregates in polyion-induced charged colloidal particles in aqueous suspension is characterized, under appropriate conditions, by two complementary effects, known as re-entrant condensation and charge inversion, which are considered as proof for the existence of a cluster phase. In this paper, we extend our previous investigation to a set of aqueous colloidal particle suspensions, such as polystyrene spheres, colloidal gold particles, and polylactic acid particles. These systems are characterized by the evolution of the average size of the aggregates and their surface electrical charge (charge inversion) by means of dynamic light-scattering measurements and laser Doppler electrophoretic techniques. The results, together with the previous ones concerning liposome particles, support the notion of a common behavior of this group of complex colloids characterized by short-ranged attractive interactions. The study provides some insights into these structures, which are potentially useful in biotechnological applications, such as multicompartmental carriers in nonviral drug delivery
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