Polyion Charge Density Research Articles

Polymer complexation: Partially ionizable asymmetric polyelectrolytes.

Theories of bulk coacervation of oppositely charged polyelectrolytes (PE) obscure single molecule level thermodynamic details, considered significant for coacervate equilibrium, whereas simulations account for only pairwise Coulomb interaction. Also, studies of effects of asymmetry on PE complexation are rare compared to symmetric PEs. We develop a theoretical model, accounting for all entropic and enthalpic contributions at the molecular level, and the mutual segmental screened Coulomb and excluded volume interactions between two asymmetric PEs, by constructing a Hamiltonian following Edwards and Muthukumar. Assuming maximal ion-pairing in the complex, the system free energy comprising configurational entropy of the polyions and free-ion entropy of the small ions is minimized. The effective charge and size of the complex, larger than sub-Gaussian globules as for symmetric chains, increase with asymmetry in polyion length and charge density. The thermodynamic drive for complexation is found to increase with ionizability of symmetric polyions and with a decrease in asymmetry in length for equally ionizable polyions. The crossover Coulomb strength demarcating the ion-pair enthalpy-driven (low strength) and counterion release entropy-driven (high strength) is marginally dependent on the charge density, because so is the degree of counterion condensation, and strongly dependent on the dielectric environment and salt. The key results match the trends in simulations. The framework may provide a direct way to calculate thermodynamic dependencies of complexation on experimental parameters such as electrostatic strength and salt, thus to better analyze and predict observed phenomena for different sets of polymer pairs.

Impact of polysulfone functionalization with N,N-dimethylbutylamine on conformational characteristics

The functionalized polysulfones with N,N-dimethylbutylamine (QPSF) were synthesized having as strategy the control of the chain flexibility in order to improve their solubility, functionality, and processability for future applications. Polymer processing starts from the solution phase which is the first step in assessing and controlling the structural parameters. In this context, the conformational properties of the functionalized polysulfone were evaluated by viscometry method, applying the Huggins and Rao models. According to the obtained viscometric data, as a function of the charge density of polyion, concentration, and temperature, it can be stated that the QPSF solutions dynamic is influenced by the balance between flexibility and structural stability determined by the chemical and physical responses of polymers to cumulative effects of the specific interactions and rearrangement of counterions in solution.

DNA-Polyelectrolyte Complexation Study: The Effect of Polyion Charge Density and Chemical Nature of the Counterions.

Complexes of polycations and DNA, also known as polyplexes, have been extensively studied in the past decade, as potential gene delivery systems. Their stability depends strongly on the characteristics of the polycations, as well as the nature of the added salt. We present here a study of the DNA ionene complexation in which we used fluorescence, UV, and CD spectroscopy, combined with molecular dynamics computer simuations, to systematically examine the influence of the polycation charge density, as well as the influence of the nature of the counterion, on the stability of these systems. Ionenes as polycations, depending on their structural characteristics, have previously been found to possess low cytotoxicity, and are therefore particularly interesting as potential gene delivery agents. The results show that the DNA solutions in the presence of the polycation are more stable in the case of very large or very small ionene charge density, suggesting different mechanism of complexation. The computer simulations show that the ionenes with high charge density bind to the minor groove of the DNA molecules, while the ionenes with lower charge density bind to the major groove of the DNA. The nature of the counterions play only a minor role: precipitation of the DNA molecules occurs at slightly lower ionene concentration when fluoride counterion are present, compared to the bromide counterions.

Effect of the Charge Regulation Behavior of Polyelectrolytes on Their Nonequilibrium Complexation with Oppositely Charged Surfactants.

The nonequilibrium features of oppositely charged macromolecule/surfactant mixtures have been the subject of intensive research recently. Although the nature and extent of polyion charge crucially affect the development of trapped states in these systems, their role in the nonequilibrium association is still poorly understood. In the present study, the impact of mixing on the complexation of hexadecyltrimethylammonium bromide (CTAB) with sodium poly[(vinyl alcohol)-co-(vinyl sulfate)] (PVAS) and poly(acrylic acid) (PAA) samples of similar charge densities has been compared using a variety of experimental methods. The results indicate largely different nonequilibrium behavior depending on the polyions. In the case of the weak polyacid PAA, the binding of CTAB increases its ionization degree, which leads to enhanced equilibrium two-phase concentration range, where kinetically arrested states can be observed upon rapid mixing of the solution components. In contrast, the charge density of the double-hydrophilic PVAS copolymer is fixed, and in addition to the binding of CTAB onto its vinyl sulfate groups, the surfactant molecules also bind onto the vinyl alcohol segments of the copolymer. These factors result in reduced precipitation concentration range and diminishing nonequilibrium effects with decreasing polyion charge density. Our study clearly demonstrates that the charge regulation behavior of various polyelectrolytes can be successfully used to tune the nonequilibrium characteristics of macromolecule/surfactant association.

Parallel Control over Surface Charge and Wettability Using Polyelectrolyte Architecture: Effect on Protein Adsorption and Cell Adhesion

Surface charge and wettability, the two prominent physical factors governing protein adsorption and cell adhesion, have been extensively investigated in the literature. However, a comparison between these driving forces in terms of their independent and cooperative effects in affecting adhesion is rarely explored on a systematic and quantitative level. Herein, we formulate a protocol that features two-dimensional control over both surface charge and wettability with limited cross-parameter influence. This strategy is implemented by controlling both the polyion charge density in the layer-by-layer (LbL) assembly process and the polyion side-chain chemical structures. The 2D property matrix spans surface isoelectric points ranging from 5 to 9 and water contact angles from 35 to 70°, with other interferential factors (e.g., roughness) eliminated. The interplay between these two surface variables influences protein (bovine serum albumin, lysozyme) adsorption and 3T3 fibroblast cell adhesion. For proteins, we observe the presence of thresholds for surface wettability and electrostatic driving forces necessary to affect adhesion. Beyond these thresholds, the individual effects of electrostatic forces and wettability are observed. For fibroblast, both surface charge and wettability have an effect on its adhesion. The combined effects of positive charge and hydrophilicity lead to the highest cell adhesion, whereas negative charge and hydrophobicity lead to the lowest cell adhesion. Our design strategy can potentially form the basis for studying the distinct behaviors of electrostatic force or wettability driven interfacial phenomena and serve as a reference in future studies assessing protein adsorption and cell adhesion to surfaces with known charge and wettability within the property range studied here.

Volumetric and calorimetric properties of aqueous ionene solutions

The volumetric (partial and apparent molar volumes) and calorimetric properties (apparent heat capacities) of aqueous cationic polyelectrolyte solutions - ionenes - were studied using the oscillating tube densitometer and differential scanning calorimeter. The polyion's charge density and the counterion properties were considered as variables. The special attention was put to evaluate the contribution of electrostatic and hydrophobic effects to the properties studied. The contribution of the CH2 group of the polyion's backbone to molar volumes and heat capacities was estimated. Synergistic effect between polyion and counterions was found.

Water-Insoluble Surface Coatings of Polyion-Surfactant Ion Complex Salts Respond to Additives in a Surrounding Aqueous Solution.

Hydrated, but water-insoluble, "complex salts" (CS) composed of alkyltrimethylammonium surfactant ions with polyacrylate counterions are known to exhibit a rich phase behavior in bulk mixtures with water and have recently been shown to act as water-responsive surface coatings. Here it is shown, by SAXS measurements, that surface coatings of CS also respond to various added solutes in a surrounding aqueous solution, by altering their liquid crystalline structure. The obtained results provide new information on the phase behavior of CS in contact with water and aqueous solutions. Solutes such as acids, salts, excess ionic surfactant, or water-soluble polymers act on the CS by altering the polyion charge density, screening the electrostatic interaction, changing the curvature of the surfactant aggregate, or increasing the osmotic pressuring in the surrounding solution, all of which may result in a phase transition in the film. In water, all studied CS surface coatings had a micellar cubic structure, which could change to 2D hexagonal, HCP, or disordered micellar structure, depending on the identity of the CS and the identity and concentration of the added solute. For some systems, even dissolved CO2 from the ambient air was sufficient to induce a structural change in the film. Especially the films containing the long polyions remained intact even for large concentrations of solutes in the contacting solutions, and extensive washing in water resulted, in most cases, in films with the "original" structure found in water.

Ionic polymers based on quaternized polysulfones: hydrodynamic properties of polymer mixtures in solution

Abstract Hydrodynamic properties developed in a series of mixtures, obtained from quaternized polysulfone and cellulose acetate phthalate or polyvinyl alcohol in N-methyl-2-pyrrolidone, were evaluated by viscometric investigations. Theoretical and experimental aspects concerning the viscometric data for binary polymer/solvent and ternary polymer/polymer/solvent mixtures have been discussed by the new Wolf model, as a function of the charge density of polyion, structural peculiarity of polymers, and polymer mixture composition. Intrinsic viscosity and also the hydrodynamic parameters obtained by the Wolf method offer new information on the competition between different types of interactions manifested in ternary polymer/polymer/solvent systems. The complex dependence of viscosity on polymer composition is influenced by conformational changes of constituent polymers from the mixture, as well as by cumulative effects of electrostatic interactions, hydrogen bonding or association phenomena. Additionally, the above-mentioned interactions indicate the compatibility of these polymers over a large composition domain. This study investigates the hydrodynamic functions from the perspective of some newly-issued theories and analyzes the choice of optimal polymer mixtures compositions for specific applications in biomedical domains.

Solution behavior of progressively quaternized poly(dimethylaminoethyl methacrylate) as a function of charge density

Hydrodynamic behavior of progressively quaternized poly(dimethylaminoethyl methacrylate) (PDMAEM) in dilute and semi-dilute salt–free aqueous solutions as a function of molar mass and charge density, which varied from 5 up to 100mol%, was studied. The role of long range electrostatic interactions on the viscosity and on the position and value of the reduced viscosity corresponding to the peak, ηred,p, as a function of the charge density of polycation has been evaluated. The overlap concentration of polyelectrolytes, c∗, has been evaluated as the inverse of the intrinsic viscosity determined by fitting the viscometric results on Rao equation. The values of c∗ have been discussed as a function of polyion molar mass and charge density. The c∗ values calculated in the frame of Odijk model for polyelectrolytes having quaternization degree ⩾50 were lower but comparable with those determined by viscometry, for the same molar mass.

Ionic Polymers Based on Dextran: Hydrodynamic Properties in Aqueous Solution and Solvent Mixtures

Hydrodynamic properties of a series of ionic polysaccharides with different charge density but the same molecular weight have been evaluated in salt-free aqueous solution and aqueous/organic solvent mixtures by means of capillary viscometry. The polyelectrolytes investigated contain quaternary ammonium salt groups, N-ethyl-N,N-dimethyl-2-hydroxypropylammonium chloride, attached to a dextran backbone. The experimental viscometric data have been plotted in terms of the Wolf method. The results show that the experimental data fit well with this model and allow the calculation of intrinsic viscosities and other hydrodynamic parameters, which provide new information about the dependence of the polyion conformation on its polyion charge density as well as on solvent composition.

Counterion-counterion correlation in the double layer around cylindrical polyions: Counterion size and valency effects

Monte Carlo simulation and Poisson-Boltzmann results on some aspects of structure and thermodynamics of aqueous polyelectrolyte solutions are presented. The polyelectrolyte solution is described by an infinitely long cylindrical polyion surrounded by counterions modeled as rigid ions moving in a continuum dielectric. Ion-ion correlations in the form of volume average of the counterion-counterion distribution function in the double layer surrounding the polyion are reported for mono- and divalent counterions and for a range of polyion concentrations and charge density parameters in each case. These results confirm again strong influence of the charge density parameter of polyions on properties of polyelectrolyte solutions. The structural information is supplemented by the calculated thermodynamic properties such as osmotic coefficients and heats of dilutions; the latter quantity has not been examined yet in detail by computer simulations. The results are discussed in view of the existing experimental data from the literature for these properties.

Controlling structure in associating polymer-surfactant mixtures

Abstract Structures of concentrated mixtures of associating polymer-surfactant mixtures are important in many technical applications. Yet an in-depth understanding of how molecular parameters affect these structures is lacking. We here summarize the results of ongoing work using a novel simplified approach to the study of associating oppositely charged polymer-surfactant mixtures, introducing a minimum number of components to the various mixtures. The results illustrate the relations between systems with and without polyions, as well as effects of systematic changes of parameters such as surfactant chain length, polyion length, polyion charge density, and the charge density of the surfactant aggregate.

Charge patch attraction and reentrant condensation in DNA–liposome complexes

We investigated the formation of complexes between cationic liposomes built up by DOTAP and three linear anionic polyions, with different charge density and flexibility, such as a single-stranded ssDNA, a double-stranded dsDNA and the polyacrylate sodium salt [NaPAA] of three different molecular weights. Our aim is to gain further insight into the formation mechanism of polyion–liposome aggregates of different sizes (lipoplexes), by comparing the behavior of DNA with a model polyelectrolyte, such as NaPAA, with approximately the same charge density but with a higher flexibility. We employed dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements, in order to explore both the hydrodynamic and structural properties of the aggregates resulting from polyion–liposome interaction and to present a comprehensive picture of the complexation process. The phenomenology can be summarized in a charge ratio-dependent scenario, where the main feature is the formation of large equilibrium clusters due to the aggregation of intact polyion-coated vesicles. At increasing polyion–liposome ratio, the size of the clusters continuously increases, reaching a maximum at a well-defined value of this ratio, and then decreases (“reentrant” condensation). The aggregation mechanism and the role of the polyion charge density in the complex formation are discussed in the light of the recent theories on the correlated adsorption of polyelectrolytes at charged interfaces. Within this framework, the phenomena of charge inversion and the reentrant condensation, peaked at the isoelectric point, finds a simple explanation.

Self‐assembly in ternary systems: cross‐linked polyelectrolyte, linear polyelectrolyte and surfactant

AbstractThe competitive interactions in ternary systems consisting of a slightly cross‐linked polyelectrolyte hydrogel and the mixture of linear polyelectrolyte and micelle forming surfactant both oppositely charged relative to the polyelectrolyte network were studied. It was shown that the equilibrium in the competitive reactions depends on the linear polyion charge density and the length of the surfactant aliphatic radical. Dependency on these characteristics the interpolyelectrolyte complex formed by cross‐linked and linear polyelectrolytes can uptake surfactant ions from water solution transforming into the cross‐linked polyelectrolyte‐surfactant complex and releasing the linear polyelectrolyte or vice versa. The ternary systems of this kind are perspective to design the novel family of delivery constructs.

Study of the Effect of Polyion Charge Density on Structural Properties of Complexes between Poly(acrylic acid) and Alkylpyridinium Surfactants

Structural properties of complexes of poly(acrylate) anion (PA) and the cationic surfactants dodecylpyridinium (DPC) and cetylpyridinium chloride (CPC) were investigated by synchrotron X-ray scattering at various degrees of neutralization αN of poly(acrylic acid) (HPA) and at a fixed surfactant:polymer molar ratio. In all scattering curves, one broad (PA−DPC complexes, all αN-values, and PA−CPC complexes, αN = 0) or several sharp peaks (PA−CPC complexes, αN > 0) were observed, pointing to a high degree of ordering in these systems. The fundamental type of organization of surfactant is in the form of polymer-induced micelles that are clustered around the polymer. The multiple reflections in the case of PA−CPC complexes point to a Pm3n cubic structure. The calculated values of structural parameters indicate a larger degree of ordering in complexes at low αN-values (i.e., at αN < 0.5) than that at higher ones. This suggests that the binding of the surfactant by the polyelectrolyte is influenced by factors ot...

Effect of Polyion Charge Density on the Morphology of Nanostructures in Polyelectrolyte−Surfactant Complexes

The poly(acrylate) (PA)/surfactant and poly(methacrylic acid) (PMA)/surfactant complexes were investigated by synchrotron X-ray scattering at various degrees of neutralization, α, of the polyion and at a fixed surfactant to polyion charge ratio. At low α-values, the surfactant organizes into polymer-induced micelles that are attracted by the polymer through hydrophobic interactions. This is followed by a formation of a Pm3n cubic structure at higher α-values as a consequence of very strong electrostatic attractions between the polyion and charged surfactant aggregates. From the position of the maxima in the scattering curves, the characteristic distance d was obtained. In PA/surfactant complexes, d varies linearly with α. This is an indication of a smooth expansion of the PA chain upon neutralization. In PA/dodecylpyridinium chloride (DPC) solutions, d increases from 30.2 A at α = 0 to 36.6 A at α = 1, and correspondingly d increases from 44.1 to 51 A in the PA/CPC case. The d-value at α = 0 almost matche...

Polyelectrolyte–surfactant complexes: an aqueous titration method to model ion-pairing within polymeric membranes of polyion-sensitive electrodes

A model for estimating the degree of cooperative ion-pairing that occurs between polyions and lipophilic ion-exchangers doped into the organic polymer membranes of polyion-sensitive electrodes has been developed. The binding of several polyanionic (e.g., heparins, carrageenans, polyphosphates, and polyglutamates) and polycationic species (e.g., protamine, polyarginine, and polylysines) with water soluble surfactants, quaternary ammonium and alkyl- and alkylbenzene sulfonates, respectively, which are structurally analogous to the ion-exchangers, is monitored in aqueous solutions with surfactant-selective electrodes. Analysis of the binding data reveals that the polyion structure, including polyion molecular weight, lipophilicity, and charge density, has a significant impact on the observed binding affinities, as does the surfactant structure. It is also shown that the measured aqueous phase binding affinities have a strong linear correlation with the equilibrium EMF changes observed with the respective polyion-sensitive electrodes, confirming the validity of using an aqueous system to model the binding behavior of ion-exchangers with polyions within the organic polymer membranes of polyion-sensitive electrodes.

Molecular Thermodynamics for Protein Precipitation with a Polyelectrolyte

A molecular−thermodynamic framework is developed for phase equilibria in an aqueous system containing a charged globular protein and an oppositely charged linear polyelectrolyte. The globular protein is represented by a spherical macroion with its coions; the linear polyelectrolyte is represented by a charged hard-sphere chain (polyion) with corresponding counterions. The potential of mean force contains Coulombic interactions between macroions, polyions, and small ions; long-range dispersion attractions between protein macroions; and hydrophobic macroion−polyion and macroion−macroion associations. Analytic expressions for thermodynamic properties are obtained, and liquid−liquid phase equilibria (precipitation) are calculated for model systems. Adding polyelectrolyte to a protein solution leads to precipitation, but further addition of polyelectrolyte leads to redissolution of the protein. This destabilization−restabilization phenomenon follows from electrostatic interactions with coupled polymer adsorption. The effects on phase equilibria of protein charge, protein size, association energy between protein−polyion, polyion chain length, and polyion charge density are investigated for model systems and compared with experimental data. Brief consideration is given to fractional precipitation for binary aqueous mixtures of proteins with different charge densities.

Structure and Charge Distribution in DNA and Poly(styrenesulfonate) Aqueous Solutions

DNA and synthetic poly(styrenesulfonate) (PSS) solutions without excess simple salt were investigated with small-angle neutron scattering. For both polyelectrolytes, the transition from the rod to the coil regime was covered by an appropriate choice of molecular weights. The polymer, polymer−counterion, and counterion partial structure functions were obtained using contrast variation. For PSS, the single-chain scattering (form function) was observed from samples with zero-average polyion scattering length density contrast. The PSS polymer structure can be described by a locally rodlike configuration, but the projected monomer repeat distance 0.17 nm is smaller than the value expected for a fully stretched (trans) conformation. The PSS persistence length is of order 10 nm and does not agree with any theoretical analysis based on either the bending rigidity of a wormlike chain or modern variational results. The interpolymer structure was derived and compared with results based on the random-phase approximation. Poor agreement was observed, due to the high linear polyion charge density and, hence, strong electrostatic coupling. For highly charged linear polyelectrolytes, it was shown that from the full set of partial structure functions information on the radial counterion profile can be obtained without resorting to a model describing chain correlations. For PSS and DNA, the data agree with the counterion distribution obtained from the classical Poisson−Boltzmann theory and the cylindrical cell model, if the momentum transfer is far greater than the inverse persistence length.

Extraction Thermodynamics of Polyanions into Plasticized Polymer Membranes Doped with Lipophilic Ion Exchangers: A Potentiometric Study

The equilibrium potentiometric (EMF) response of a previously reported polymer membrane-based heparin sensitive electrode toward heparin and other macromolecular polyanions is examined in detail. The precise structure of the lipophilic tetraalkylammonium anion-exchange sites doped within the plasticized poly(vinyl chloride) membrane phase of the sensor is found to play a critical role in the extraction of the polyanions into such membranes. The influence of polyion charge density and molecular weight (chain length) for a series of structurally similar polyanions (sulfonated/carboxylated polysaccharides) on the membrane's EMF response is also studied. It is found that the membrane potential becomes more negative (for given mass/volume concentration of polyion) with an increase of polyion molecular weight up to 3800 for heparin-like structures. In addition, a greater overall equilibrium response is observed as the charge density of the polyanion increases. The observed role of polyanion structure on the membrane's equilibrium EMF response can be explained by a favorable ion-pair formation between the lipophilic anion-exchanger sites in the membrane and the extracted polyanions. The free energy change for such ion-pair formation can be estimated by comparing the membrane's equilibrium EMF response toward a given polyion to that of a second polymeric membrane doped with anion-exchange sites that do not form strong ion pairs with polyanionic macromolecules. The influence of the sample background electrolyte concentration on the extraction behavior of different polyions is also examined.