Properties Of Polyelectrolyte Solutions Research Articles

Influence of counterion type on the scattering of a semiflexible polyelectrolyte.

Understanding the influence of counterion and backbone solvation on the conformational and thermodynamic properties of polyelectrolytes in solution is one of the main open challenges in polyelectrolyte science. To address this problem, we study the scattering from semidilute solutions of a semiflexible polyelectrolyte, carboxymethyl cellulose (CMC) with alkaline and tetra-alkyl-ammonium (TAA) counterions in aqueous media using small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS), which allow us to probe concentration fluctuations of the polymer backbone and counterions. In SAXS, the calculated contrast arises primarily from the polymer backbone for both alkaline and TAA salts of CMC. In SANS, however, the contrast is dominated by the counterions for the TAA salts and the polymer backbone for the alkaline salts. Solutions are found to display a correlation peak in their scattering function, which at low concentrations is independent of counterion type. At moderate salt concentrations (c ≳ 0.1 M), the peak positions obtained from SANS and SAXS for the CMC salts with the TAA counterions differ. This divergence suggests a decoupling in the lengthscale over which the couterions and the polymer fluctuate. Upturns in the scattering intensity in the low-q region signal the presence of long-ranged compositional inhomogeneities in the solutions. The strength of these decreases with increasing counterion-solvent interaction strength, as measured by the viscosity B coefficient, and are strongest for the corresponding sodium salt of CMC.

Solution Properties of Polyelectrolytes with Divalent Counterions

Divalent ions are known to strongly influence the phase behavior, conformation, and flow properties of polyelectrolytes in solution. Here, we report small-angle neutron scattering and viscosity data for the magnesium salt of polystyrene sulfonate (MgPSS) in salt-free water and compare these results with analogous measurements for NaPSS. In a dilute solution, both salts are found to exhibit a rod-like conformation for high degrees of polymerization, but the chain dimensions of MgPSS are significantly smaller than for NaPSS, which is consistent with the lower charge density of MgPSS found from conductivity data. In the semidilute regime, the correlation length scales as ξ ≃ 3.7c–1/2 nm for NaPSS and ξ ≃ 5.4c–1/2 nm for MgPSS. At high concentrations, deviations to weaker power laws are observed for both salts. The variation of the viscosity with the degree of polymerization (ηsp ∼ N) is consistent with unentangled, Rouse-like dynamics. Fuoss-law-type behavior (ηsp ∼ c1/2) is observed at low polymer concentrations, which turns into a stretched exponential dependence at high c, perhaps due to an increase in the local friction experienced by polymer segments.

Influence of divalent counterions on the solution rheology and supramolecular aggregation of carboxymethyl cellulose

Divalent counterions promote attractive forces between polyelectrolyte chains via electrostatic bridging, which play an important role in the conformation of ionic biopolymers. Further, counterion valence is known to affect the flexibility and aggregation properties of polyelectrolytes in solution. The present study seeks to resolve the effect of counterion valence and type on the structure and flow properties of a model semiflexible polyelectrolyte. We report rheology and light scattering data for the Na $$^+$$ , Mg $$^{2+}$$ , Ca $$^{2+}$$ , Mn $$^{2+}$$ , Co $$^{2+}$$ , Ba $$^{2+}$$ salts of carboxymethyl cellulose in aqueous solutions. The Na $$^+$$ and Mg $$^{2+}$$ counterions do not interact specifically with the carboxylate groups, and their CMC salts form clear solutions in the concentration (c) range studied (0.001 M 0.2 M. Compared NaCMC, divalent salts display a lower viscosities at low concentrations (in the non-entangled regime), suggesting less expanded chains, in agreement with earlier experimental results on flexible polyelectrolytes. Above the entanglement crossover ( $$c \simeq$$ 0.07 M), solutions with divalent counterions display viscosities up to an order of magnitude larger than NaCMC, possibly because interchain crosslinks form by electrostatic bridging. Dynamic light scattering measurements on semidilute non-entangled solutions reveal a bimodal decay function, where the relative amplitudes of the two modes vary with counterion valence, size as well as with the filter size employed and the time after filtration. These variables (except for counterion valency) do not strongly affect the solution viscosity, indicating that polyelectrolyte clusters only contain a small fraction of the total number of chains in solution.

Transport Properties of Polyelectrolyte Solutions. Effect of Confinement in Thin Liquid Films

Abstract The role of condensed counterions in transport properties, such as electrical conductivity and viscosity, has been investigated with solutions of a flexible polyelectrolyte. Comparisons with existing theories are proposed. Viscosity is strongly affected by confinement in thin films, depending whether polyelectrolyte chains are adsorbed or not at the film surfaces. The role of counterion mobility is however difficult to assess because the measurements are not accurate enough. It is proposed that this role could be tested by electrical conductivity measurements.

Effects of dielectric inhomogeneity in polyelectrolyte solutions

We illustrate the effects of dielectric inhomogeneity on the statistical properties of polyelectrolytes in solution, by a lattice Monte Carlo simulation that combines the bond fluctuation model with a local algorithm for computing the electrostatic interactions. Our model accounts for the difference in the dielectric properties between the polymer backbone and the solvent. Taking the coil–globule transition of a single polyelectrolyte chain in solvent as an example, we show that the chain conformation and the degree of counterion condensation are substantially affected by the dielectric contrast.

Ion-specific and charge effects in counterion binding to poly(styrenesulfonate) anions

In order to obtain a deeper insight into effects occurring when an electrolyte solution is added to a solution of a strong polyelectrolyte, the microcalorimetric and potentiometric titrations of poly(sodium 4-styrenesulfonate) (Na(+)PSS(-)) solution with different alkali, earth-alkali and tetraalkylammonium nitrate, perchlorate and chloride solutions were performed. From the calorimetric titrations the differences in sign and magnitude of enthalpy change upon addition of various electrolytes were observed depending on the salt used. Potentiometric titrations using a sodium ion selective electrode have revealed that addition of an electrolyte is accompanied by the increase in sodium activity until a certain critical value is reached, which seems to be the consequence of counterion substitution on the polyelectrolyte chain. In the case of addition of lithium and sodium salts the experimental results for ΔH of mixing can be qualitatively correctly explained by the Poisson-Boltzmann and Monte Carlo calculations based on the continuum solvent models. This is not the case for the mixtures with KNO(3), RbNO(3) and CsNO(3) salts. The results suggest that the ion-specific effects, associated with the changes in the water structure, have to be taken into account when thermodynamic properties of polyelectrolytes in solution are concerned. The calorimetric results imply that the enthalpically observed cation specificity for binding to a poly(styrenesulfonate) group could be correlated with corresponding cation hydration enthalpies. The counterion substitution of sodium with divalent cations was found to be endothermic, which is in qualitative agreement with the electrostatic theory.

Study of artemisinin nanocapsules as anticancer drug delivery systems

Artemisinin (ART) crystals were encapsulated with chitosan, gelatin, and alginate for the purpose of controlled release. These polyelectrolytes were alternatively deposited on ART crystals sized about 766 nm through the layer-by-layer technique. Size distribution, zeta potential, and swelling property of ART nanocapsules were measured. The morphology of ART nanocapsules was observed by transmission electron microscopy after dissolution. Release property of ART nanocapsules was measured in phosphate buffer solution (pH 7.4). It was proved that the ART nanocapsules dispersed well in aqueous solutions. Hydrophilicity of ART crystals was improved after encapsulation. The ART drug release rate was tuned by the properties of polyelectrolyte solutions, such as polyelectrolyte type, number of polyelectrolyte multilayers, sodium chloride concentration, and ethanol concentration in polyelectrolyte solution, and so on. Our research provides a feasible method of achieving prolonged drug release through self-assembly of polyelectrolytes on natural drug crystals. From the Clinical Editor Artemisinin (ART) crystals were encapsulated with chitosan, gelatin, and alginate for the purpose of controlled release of this anticancer drug. The reported method is feasible for achieving prolonged drug release through self-assembly of polyelectroytes on natural drug crystals.

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.

Linear polyelectrolytes in tetravalent salt solutions

The effect of adding tetravalent salt of different sizes to a solution of linear and flexible polyelectrolytes is investigated by molecular dynamics simulations. Upon the addition of salt, a chain reexpansion takes place, following a well-known collapsed conformation. The degrees of collapse and reexpansion increase with ion size. In the solution, tetravalent counterions replace monovalent ones and condense onto the chains. The condensation for small ions displays a profile different from that for large ones. In a high-salt region, ions can form layering orders around a polyelectrolyte and locally overcompensate the charge inside. Consequently, the integrated charge distribution reveals an oscillatory behavior away from a chain. By studying the radial distribution function between monomers on different polyelectrolytes, like-charge attraction between chains is demonstrated. This attraction is a prerequisite to chain aggregation or precipitation. The results show a strong dependence of salt concentration and ion size on the properties of polyelectrolyte solutions.

Colligative properties of polyelectrolyte solutions

The solution properties of polyelectrolytes are not well understood despite increasing theoretical and experimental effort, particularly during the last 10 years. Some of the current models for polyelectrolyte solutions are valid only at infinite dilution because they account only for long-range interactions. Nagvekar and Danner [M. Nagvekar, R.P. Danner, An Excess Gibbs Free Energy Model for Polyelectrolyte Solutions, Fluid Phase Equilibria 53 (1989) 219.] have developed an excess Gibbs free energy model for polyelectrolyte solutions. The key feature of the model is to express the excess Gibbs free energy as the sum of the contributions from long-range and short-range interactions. The `limiting laws' of Manning [G.S. Manning, Limiting Laws and Counterion Condensation in Polyelectrolyte Solutions. I. Colligative Properties, J. Chem. Phys. 51 (1969a) 924; G.S. Manning, Limiting Laws and Counterion Condensation in Polyelectrolyte Solutions. II. Self-diffusion of Small Ions, J. Chem. Phys. 51 (1969b) 934.] were used to account for the long-range interactions while a local composition model of the Non-Random Two Liquid (NRTL) type was used for short-range interactions. In this work, we present the expressions for counterion activity coefficients and the motic coefficients in polyelectrolyte solutions with and without added salts at finite concentrations. Data for a number of polyelectrolyte systems with univalent and divalent counterions are analyzed using Nagvekar's proposed model. In the low concentration regime, the osmotic coefficient data are practically independent of concentration, and are in reasonable agreement with the Manning model. With increasing polyelectrolyte concentration, the osmotic coefficient data are a strong function of concentration, and the local composition model is shown to successfully represent the data.

Some properties and morphology of poly(2-vinyl pyridine)- g-polyoxyethylene

Emulsifying properties, phase-transfer catalysis properties, crystalline properties, dilute solution properties and conductive properties of poly(2-vinyl pyridine) with uniform polyoxyethylene grafts (P2-VP- g-PEO) were studied. The results showed that the graft copolymers can emulsify the benzene/water system and increase the conversion of potassium phenolate during the Williamson reaction of solid potassium phenolate with n-butyl chloride or n-butyl bromide. The crystallinity of the graft copolymers increased with both content and molecular weight of PEO grafts, but diminished after the copolymer was complexed with LiClO 4. After neutralization with HC1 solution, the copolymers exhibited properties of polyelectrolytes in solution. Intramolecular phase separation phenomenon was indicated for dilute solutions of the graft copolymer. After the PEO grafts were doped with LiCIO 4 or after the P2-VP main chains were doped with TCNE, the copolymers showed ionic or electronic conductivity, respectively. Transmission electron microscopy (TEM) indicated that these graft copolymers exhibit microphase separation.

Solution properties of polyelectrolytes: X. Influence of ionic strength on the electrostatic secondary effects in aqueous size-exclusion chromatography

The retention behaviour of polyelectrolytes in aqueous size-exclusion chromatography (SEC), where electrostatic repulsion is the main secondary effect affecting to the separation mechanism, was investigated. A theoretical treatment was developed in order to establish the influence of the mobile phase ionic strength on calibration graphs, often used for the characterization of polyions by SEC. A master equation, derived in closed form, involving two terms related to the net charge of polyion and the residual surface charge on the gel packing, was derived. The formalism provides the basis for a more detailed analysis of chromatographic retention data in electrostatic interaction systems. Moreover, the introduction of some approximations in the original equation served to obtain an equivalent expression that is easier to use, and in which the functionality with respect to the ionic strength, I, remains unaltered. Reported data on the elution of sodium polystyrene sulphonate and poly( l-glutamic acid) from both organic and silica-based packings were used to test the goodness of the predictions carried out with the above-mentioned equations.

Solution properties of polyelectrolytes: VIII. A comparative study of the elution behaviour on two organic-based packings

Aqueous size-exclusion chromatography was used to analyse the elution behaviour of several standard ionic polymers, including poly( l-glutamic acid), sodium poly(styrene sulphonate) and poly(acrylic acid), different in nature and chain flexibility, as a function of the pH and ionic strength of the eluent. Two organic-based hydrophilic packings, Spherogel TSK PW4000 and Ultrahydrogel 250, were tested in order to select the optimal conditions of macromolecular separation, and the results obtained for each column were compared. A set of calibration graphs for the above polyions as a function of eluent pH and ionic strength were obtained and compared with those obtained for uncharged standards (dextran and polyethylene oxide, PEO) under the same experimental conditions. The divergence between both charged and uncharged plots served to interpret the separation mechanisms for the polyions, other than pure size exclusion. Deviations from ideal elution behaviour have been attributed to ion-exclusion and hydr ophobic effects, as a consequence of the repulsive or attractive interactions between the ionizable groups of the polyelectrolyte and the residual surface charge of the support.

Solution properties of polyelectrolytes: VII. Non-ideal mechanisms in size-exclusion chromatography of synthetic polyions, peptides and proteins

Chromatographic data for sodium polystyrene sulphonate were obtained on both silica- and polymer-based size-exclusion supports using mobile phases of various pH and ionic strength. Deviations of the elution volume were observed towards both lower and higher values relative to the reference calibration graph obtained with uncharged standards. An empirical correlation is proposed in order to account for all the secondary effects observed. The general applicability of this correlation was further tested for chromatographic data obtained for a series of peptides and proteins on a silica-based support under very different eluent conditions. Deviations from ideal elution behaviour such as ion-exclusion and hydrophobic effects were analysed in the light of this approach.

Solution properties of polyelectrolytes—V. Viscometric study of mixed polyanions in pure water

A viscometric study of sodium polystyrene sulphonate (NaPSS) and sodium poly( l-glutamate) (NaPLG) in salt-free water has been carried out as a function of the total concentration, c m, and the concentration of one polyelectrolyte at fixed concentration of the other. The influences of the molecular weight and concentration are considered in order to establish the degree of compatibility. For the first time, an equation is proposed to define the reduced viscosity of the NaPSS + NaPLG mixture in water. The viscometric interaction parameter, B m, taking into account the electrostatic and thermodynamic interactions between both polyions, is defined as a geometrical mean of the contribution of each one. Finally, the compatibility of NaPSS + NaPLG mixtures as a function of individual polyion concentration and molecular weight is analysed in the light of B m.

Solution properties of polyelectrolytes : IV. Use of a new hydrophilic size-exclusion chromatographic packing for the separation of anionic and cationic polyions

The chromatographic behaviour of sodium polystyrenesulphonate in Ultra-hydrogel aqueous size-exclusion chromatographic packings was analysed under different experimental conditions. Three types of mobile phases (salt-free water, sodium nitrate solution and buffers of different pH) were investigated in order to characterize the elution mechanisms and to optimize the separation. Several parameters, such as sample concentration, injection volume, eluent pH and ionic strength were considered. Finally, a “universal” calibration graph was obtained under simple, mild optimized conditions of the mobile phase (0.2 M acetate buffer, pH 4.0), which is congruent for both polyanions and polycations and also for uncharged polymers.

Solution properties of polyelectrolytes. III. Effect of sodium polystyrene sulfonate concentration on viscometric and size exclusion chromatographic behavior at different ionic strengths

AbstractThe effect of sodium polystyrene sulfonate concentration on the shape of its molecules in aqueous solution at different NaNO3 ionic strengths has been analyzed by viscometry and size‐exclusion chromatography (s.e.c). An equation has been developed which predicts the intrinsic viscosity, [η] p.cp.cs, at finite concentration of both polyion (cp) and electrolyte (cs). The experimental results obtained by both techniques can be accounted for in terms of the theory. Several factors involved in the elution mechanism have been considered and the variation of the slopes of s.e.c. calibration curves with cp and cs has been discussed in terms of polyion conformation changes.

Solution properties of polyelectrolytes. I. Exclusion chromatography of sodium polystyrene sulphonate in salt-free water as eluent

The effect of concentration of sodium polystyrene sulphonate on the elution volume in exclusion chromatography has been investigated using salt-free water as eluent. A non-linear increase in the elution volume has been observed with increasing injected sample concentration. Concentration-dependent calibrations have been obtained which approach the universal calibration for uncharged polymers (dextrans) as polyelectrolyte concentration increases. The separation mechanisms are interpreted in terms of electrostatic and hydrodynamic interactions. A semi-empirical model is proposed to predict concentration effects of linear polyions in exclusion chromatography and good agreement has been found between experimental and calculated elution volumes.

Solution properties of polyelectrolytes. II. Comparison of the effects of both polyion and eluent salt concentrations on the calibrations in aqueous exclusion chromatography

The elution behaviour of charged water-soluble polymers (sodium polystyrene sulphonate) in both salt-free and NaCl-containing aqueous mobile phases has been investigated by size-exclusion chromatography. A comparative analysis has been established between the results as a function of polyion and salt concentrations, which demonstrates that the chromatographic behaviour is similar in both cases. The variation of the slopes of the different concentration-dependent calibration curves is discussed in terms of polyelectrolyte conformational changes. Finally, a semi-empirical model has been developed to predict the dependence of the elution volume in either polyion or salt concentration, and its validity has been checked in a range from very dilute to moderate concentrations.

Thermodynamic properties of polyelectrolyte solutions determined from studies of Donnan equilibria

A new rapid potentiometric method for the determination of Donnan equilibria in polyelectrolyte solutions is described. The method has good accuracy down to the lowest concentrations of the permeable electrolyte. It has been used to study solutions of sodium polyacrylate and sodium carboxymethylcellulose in the presence of sodium chloride. The measurements are in accord with the additivity rule down to concentrations of ≈ 10–4 mol dm–3 of the permeable salt. At still lower concentrations deviations from this rule occur, the activity of the salt apparently increasing in this region.