Polyelectrolyte Chains In Solution Research Articles

Coarse-grained molecular dynamics simulations study of the conformational properties of single polyelectrolyte diblock copolymers

We use coarse-grained molecular dynamics simulations to study a single di block polyelectrolyte chain in solution. We analyze the conformational properties of the chain and localization of counterions as a function of the charge fraction, backbone stiffness, Bjerrum length, and counterion valence. The interplay between the excluded-volume effects and the electrostatic interactions among charged residues leads to variation in block−polyelectrolyte architecture. Our computational findings indicate that varying such system properties lead to nontrivial effects and can be a powerful mechanism to tune the conformational properties of block polyelectrolytes.

Size, shape, and diffusivity of a single Debye-Hückel polyelectrolyte chain in solution.

Brownian dynamics simulations of a coarse-grained bead-spring chain model, with Debye-Hückel electrostatic interactions between the beads, are used to determine the root-mean-square end-to-end vector, the radius of gyration, and various shape functions (defined in terms of eigenvalues of the radius of gyration tensor) of a weakly charged polyelectrolyte chain in solution, in the limit of low polymer concentration. The long-time diffusivity is calculated from the mean square displacement of the centre of mass of the chain, with hydrodynamic interactions taken into account through the incorporation of the Rotne-Prager-Yamakawa tensor. Simulation results are interpreted in the light of the Odjik, Skolnick, Fixman, Khokhlov, and Khachaturian blob scaling theory (Everaers et al., Eur. Phys. J. E 8, 3 (2002)) which predicts that all solution properties are determined by just two scaling variables-the number of electrostatic blobs X and the reduced Debye screening length, Y. We identify three broad regimes, the ideal chain regime at small values of Y, the blob-pole regime at large values of Y, and the crossover regime at intermediate values of Y, within which the mean size, shape, and diffusivity exhibit characteristic behaviours. In particular, when simulation results are recast in terms of blob scaling variables, universal behaviour independent of the choice of bead-spring chain parameters, and the number of blobs X, is observed in the ideal chain regime and in much of the crossover regime, while the existence of logarithmic corrections to scaling in the blob-pole regime leads to non-universal behaviour.

Coarse-grained model of glycosaminoglycans in aqueous salt solutions. A field-theoretical approach.

We present results of self-consistent field calculations of thermodynamic and structural properties of glycosaminoglycans (chondroitin sulfate, hyaluronic acid, and heparin) in aqueous solutions with added monovalent and divalent salts. A semiphenomenological coarse-grained model for semiflexible polyelectrolyte chains in solution is proposed. The coarse-grained model permits one to focus on the essential features of these systems and provides significant computational advantages with respect to more detailed models. Our approach relies on the method of Gaussian equivalent representation for the calculation of the partition functions in the form of functional integrals. This method provides reliable thermodynamic information for polyelectrolyte solutions over wide ranges of monomer concentrations. In the present work, we use the comparison and fitting of the experimental osmotic pressure with a theoretical equation of state within the Gaussian equivalent representation. The degrees of ionization, radii of gyration, persistence lengths, and structure factors of chondroitin sulfate, hyaluronic acid, and heparin in aqueous solutions with added monovalent and divalent salts are calculated and discussed.

Sedimentation of polyelectrolyte chains in solutions: From dilute to semidilute

Sedimentation of poly(sodium styrenesulfonate) (PSSNa) in aqueous solutions with or without added salt (NaCl) has been investigated by use of analytical ultracentrifuge (AUC) via sedimentation velocity (SV). In a solution without added salt or a low salt solution, as PSSNa concentration ( C p) increases, the sedimentation coefficient ( s) decreases and exhibits two transitions corresponding to the overlap concentration ( C*) and entanglement concentration ( C e). On the other hand, either C ∗ or C e increases with salt concentration ( C s). When the salt concentration is high enough, the interchain electrostatic interactions are screened, and s slightly varies with C p at C p < C ∗. However, s decreases with C p at C p > C ∗ with s ∼ C p - α , where the scaling index α has dependence on the salt concentration but not on PSSNa molar mass.

Lysozyme binding to poly(4-vinyl- N-alkylpyridinium bromide)

The adsorption behavior of polycations at ionic strengths ( I ) ranging from 0.001 to 0.1 onto silicon wafers was studied by means of ellipsometry, contact angle measurements and atomic force microscopy (AFM). Polycations chosen were bromide salts of poly(4-vinylpyridine) N-alkyl quaternized with linear aliphatic chains of 2 and 5 carbon atoms, QPVP-C2 and QPVP-C5, respectively. Under I = 0.001 the reduction of screening effects led to low adsorbed amounts of QPVP-C2 or QPVP-C5 ( 1.0 ± 0.1 mg / m 2 ) , arising from the adsorption of extended chains. Upon increasing I to 0.1, screening effects led to conformational changes of polyelectrolyte chains in solution and to higher adsorbed amount values ( 1.9 ± 0.2 mg / m 2 ) . Advancing contact angle θ a measurements performed with water drops onto QPVP-C2 and QPVP-C5 adsorbed layers varied from ( 45 ± 2 ) ° to ( 50 ± 5 ) ° , evidencing the exposure of both hydrophobic alkyl groups and charged moieties. The adsorption of lysozyme (LYZ) molecules to QPVP-C5 layers was more pronounced than to QPVP-C2 films. Antimicrobial effect of LYZ bound to QPVP-C2 or QPVP-C5 layers or to Si wafers was evaluated with enzymatic assays using Micrococcus luteus as substrates. The adsorption behavior of QPVP-C2 and QPVP-C5 at the water–air interface was studied by means of surface tension measurements. Only QPVP-C5 was able to reduce water surface tension. Mixtures of LYZ and QPVP-C5 were more efficient in reducing surface tension than pure LYZ solution, evidencing co-adsorption at liquid–air interface. Moreover, antimicrobial action observed for mixtures of LYZ and QPVP-C5 was more pronounced than that measured for pure LYZ. Hydrophobic interaction between LYZ and QPVP-C5 in solution seems to drive the binding and to preserve LYZ secondary structure.

Effect of Molecular Interactions on the Photophysics of Rose Bengal in Polyelectrolyte Solutions and Self-Assembled Thin Films

Solutions and layer-by-layer self-assembled thin films containing Rose Bengal and poly(diallyldimethylammonium chloride) are studied with the aim of understanding the interactions controlling their structures and the photophysics of the dye in both media. A detailed spectroscopic and theoretical analysis shows that hydrophobic interactions among dye molecules contribute to the coiling of the polyelectrolyte chain in solution at low polyelectrolyte/dye ( P/ D) ratios, whereas extensive aggregation of the dye takes place even at ratios as high as 10(4) (expressed in monomeric units). A polyelectrolyte elongated form prevails in self-assembled thin films, providing an environment that reduces hydrophobic interactions and lowers the aggregation tendency. Self-assembled films with a roughly estimated overall dye concentration around 1 M at a P/ D ratio in the order of seven are fluorescent and photogenerate singlet molecular oxygen. This contrasts with the behavior of polyelectrolyte solutions, which are almost nonfluorescent and do not evidence triplet state generation at the same P/ D ratio.

Polyelectrolyte Multilayer Assembly as a Function of pH and Ionic Strength Using the Polysaccharides Chitosan and Heparin

The goal of this work is to explore the effects of solution ionic strength and pH on polyelectrolyte multilayer (PEM) assembly, using biologically derived polysaccharides as the polyelectrolytes. We used the layer-by-layer (LBL) technique to assemble PEM of the polysaccharides heparin (a strong polyanion) and chitosan (a weak polycation) and characterized the sensitivity of the PEM composition and layer thickness to changes in processing parameters. Fourier-transform surface plasmon resonance (FT-SPR) and spectroscopic ellipsometry provided in situ and ex situ measurements of the PEM thickness, respectively. Vibrational spectroscopy and X-ray photoelectron spectroscopy (XPS) provided details of the chemistry (i.e., composition, electrostatic interactions) of the PEM. We found that when PEM were assembled from 0.2 M buffer, the PEM thickness could be increased from less than 2 nm per bilayer to greater than 4 nm per bilayer by changing the solution pH; higher and lower ionic strength buffer solutions resulted in narrower ranges of accessible thickness. Molar composition of the PEM was not very sensitive to solution pH or ionic strength, but pH did affect the interactions between the sulfonates in heparin and amines in chitosan when PEM were assembled from 0.2 M buffer. Changes in the PEM thickness with pH and ionic strength can be interpreted through descriptions of the charge density and conformation of the polyelectrolyte chains in solution.

Layer-by-Layer Assembly of Polyelectrolytes in Nanopores

We study the layer-by-layer (LbL) deposition of a pair of strong polyelectrolytes within the nanopores of track-etched membranes, for pore diameters ranging from similar to 50 to 850 nm. The end-to-end distance of the polyelectrolyte chains in solution was varied from 10 to 50 nm by selecting polyelectrolytes of low and high molar mass and by playing with ionic strength. On flat model surfaces, a linear growth is obtained for all probed conditions and the growth increment is independent of molar mass and substrate nature. When LbL assembly is performed within nanopores, a very different picture of growth emerges, with increments of thickness per cycle of deposition being much larger than on flat surfaces (by factors as large as 100 in some cases), and no significant dependence on molar mass or ionic strength. These observations indicate that polyelectrolyte complexation occurs within a dense gel filling the whole nanopore, resulting from entanglement of the chains which are in a concentrated regime when passing in the confined space of the pore; upon drying, the gel collapses in a tube of wall thickness roughly proportional to its diameter. By using polyelectrolytes of lower molar mass, pore diameters as small as 50 nm could be filled, opening opportunities for the facile fabrication of LbL nanowires of very large aspect ratio.

Mineralization of Monodispersed CdS Nanoparticles on Polyelectrolyte Superstructure Forming an Electroluminescent “Necklace-of-Beads”

We report a nonmicellar method to synthesize monodisperse semiconducting nanoparticles templated on polymer chains dissolved in solution at high yield. The monodispersity is achieved due to the beaded necklace morphology of the polyelectrolyte chains in solution where the beads are nanometer-scale nodules in the polymer chain. The resultant structure is a nanoparticles studded necklace where the particles are imbedded in the beads. Multiple cycles of synthesis on the polymer template yield nanoparticles of identical size, resulting in a nanocomposite with high particle fraction. The resultant nanocomposite has beaded-fibrilar morphology with imbedded nanoparticles and can be solution-casted to make electroluminescent thin film device.

The effect of added salt on polyelectrolyte structure

We present results of molecular dynamics simulations of strong, flexible polyelectrolyte chains in solution with added salt. The effect of added salt on the polyelectrolyte chain structure is fully treated for the first time as a function of polymer density. Systems above and below the Manning condensation limit are studied. The chain structure is intimately tied to the ion density near the chain even for chains in the counterion condensation (CC) regime. The end-to-end distance is demonstrated to be a function of the inverse Debye screening length and the Bjerrum length. The ion density near the polymer chain depends on the amount of added salt, and above the condensation limit the chains significantly contract due to added salt.

Chain Structure in Polyelectrolyte Solutions at Nonzero Concentrations

We present an analytic theory for the structure of intrinsically flexible polyelectrolyte chains in solution at arbitrary concentrations, with or without added salt. We use a self-consistent approach based on an idea suggested by generalized Debye−Hückel theory, namely, that the screening length from polyions is wavevector-dependent, due to chain connectivity. At high wavevectors (short length scales), only counterions contribute to screening, but at lower wavevectors, both polyions and counterions contribute. The results of the theory are in quantitative agreement with molecular dynamics simulations, with no adjustable parameters.

Molecular weight fractionation in the adsorption of polyacrylic acid salts onto BaTiO 3

The molecular weight fractionation resulting from the adsorption of polyacrylic acid (PAA) salts onto BaTiO 3 depends on the molecular weight distribution (MWD) of the PAA salt. Preferential adsorption of the longest chains is found if the entire MWD is below a certain chain length. If the MWD exceeds this limit while still including a sufficient amount of smaller chains, preferential adsorption of an intermediate molecular weight fraction occurs. The effect is quite pronounced for PAA salts of relatively low molecular weight (<20 000). Adsorption over a wide molecular weight range but with a higher proportion of short chains is observed if the MWD of the PAA salt is mainly above the aforementioned limit. The molecular weight fractionation can be understood using a sequential adsorption process in which the smallest chains of the PAA salt tend to adsorb first. These initially adsorbed chains overcompensate for the positive surface charge and thereby generate an electrostatic barrier to the PAA salt chains in the solution. The height of this barrier depends (at a constant charge of the PAA salt covered surface) on the total charge on the polyelectrolyte chain in solution, which is directly related to the chain length at high pH. The length of the chains that displace the shorter ones is limited by the electrostatic barrier. With an increased salt concentration the barrier is lower, allowing longer chains to reach the surface and displace the shorter ones. Consequently, the preferential adsorption from solutions with higher salt concentration is shifted to higher molecular weights.

The Polyelectrolyte Brush: Implications from Simulation Results on Polyelectrolytes in Solution

Simulation results as well as experimental data indicate that full stretching of flexible polyelectrolytes will not occur for experimentally realizable solution conditions. Using density-dependent swelling exponents ν(O) as suggested by Stevens and Kremer (SK) from the results of a MD simulation study of fully ionized polyelectrolyte chains in solution without added salt, we present an Alexander-de Gennes-like scaling picture for the behavior of charged brushes. The brush height is found to become dependent on the grafting density as soon as internal stretching is incomplete, i.e., at ν(O)<1. Due to SK the maximum stretching occurring below a characteristic saturation density is given by ν s =0.90

The electric polarizability and the kerr constant of a gaussian charged polyelectrolyte chain in solution.

Abstract With help of a very simple model an expression is derived for the electrical polarizability arising from the perturbation by an electric field of the distribution of counterions for a Gaussian charged macromolecule in solution. With this electric polarizability a corresponding Kerr constant is found for the same kind of chain, neglecting contributions of permanent dipole moments and atomic polarizabilities. Whereas the electric polarizability thus derived is proportional to the square of the molar mass M of the chain, the Kerr constant increases linearly with M.

Size of a polyelectrolyte chain in solution containing counterions

Using the method of Edwards and Singh we calculate the mean square radius 〈R2〉 of the polyelectrolyte chain in solution containing counterions for the temperature greater than the Flory’s θ temperature. For the counterion density n we obtain: (a) 〈R2〉∝L2 when n→0, and (b) 〈R2〉∝L6/5 when n→∞. These results are in agreement with the Flory-type calculations and recent Monte Carlo simulations performed by Baumgärtner.

Configuration of a Polyelectrolyte Chain in Solution

Starting with the Gibbs-Boltzmann relation between the probability of a configuration and its energy, an expression is derived for the probability of the configuration of a random chain having central interactions among its groups. By an approximate integration method suggested by Debye, a distribution function is derived for the vector between the chain ends. The result is applied to find the distribution and the mean square end-to-end distance of a polyelectrolyte ion.