Polyelectrolyte Solutions Research Articles

Charge Regulation Effects in Polyelectrolyte Adsorption.

Polyelectrolyte (PE) adsorption plays a pivotal role in tailoring surface properties, finding diverse applications across scientific and industrial domains. In aqueous environments, these polymers gain charge through the dissociation equilibrium of ionizable groups. Consequently, accurately representing their electrostatic interactions necessitates considering many-body couplings between charge and configuration-a phenomenon known as charge regulation (CR)-instead of relying on the constant charge approximation. Here, we show that CR effects can significantly enhance PE adsorption. In the vicinity of a like-charged interface, where short-range PE-surface attraction drives adsorption, both PEs and the surface undergo a reduction in charges during the adsorption process, especially in a semidilute PE solution. This reduction in charges leads to a noticeable decrease in the electrostatic repulsion barrier, facilitating the adsorption process. Conversely, near an oppositely charged surface, CR effects enhance the electrostatic charges of both PEs and surfaces coherently, suppressing the charge reversal of surface charges and ultimately promoting adsorption. These findings underscore the crucial CR effects in PE adsorption and challenge the direct use of electrostatic charges, typically estimated in bulk systems, for an accurate understanding of adsorption phenomena.

Origin of Anomalously Large Depletion Zones in Like-Charged Colloid-Polyelectrolyte Mixtures.

Depletion zones in polyelectrolyte solutions in contact with like-charged flat surfaces are investigated. Using a coupled self-consistent field and Debye-Hückel approach, an explicit expression for the thickness δ of the depletion layer is derived. It is found that δ∼δ_{n}+cκ^{-1}, where δ_{n} is the depletion thickness at a neutral surface, c is a function of the electrostatic characteristics of the system, and κ^{-1} is the Debye length. It is argued that the theory still holds beyond the mean-field approximation, which is confirmed by quantitative agreement between our theoretical results and experiments.

Effect of ion distribution on stress relaxation in polyelectrolyte complex gels.

Solutions of polyelectrolytes consisting of polycations and polyanions in equal proportions were studied in the present work. Due to the physical cross-links formed by the charged groups, physical gels were formed in such systems. The mechanical properties and structure of the obtained gels depending on the charge arrangement along the backbone and the dimensionless Bjerrum length λ were investigated. The response of the systems to a uniaxial affine deformation was studied first. It was found that the systems can be divided into three groups depending on the charge arrangement: showing an almost elastic response; showing a viscoelastic response with a very long relaxation time; and showing a weak viscoelastic response with a short relaxation time. Interestingly, no stable aggregates were formed in the systems with the charges located on spacers, probably because of the increased mobility of the charges in such systems. The obtained stress relaxation curves had different functional forms, indicating that the relaxation has at least two characteristic times, which are different for different systems. In order to understand the molecular nature of the observed mechanical response, the temporal evolution of the network structure of a system showing a viscoelastic response with a very long relaxation time was studied; the aggregates were found to be dynamic, which leads to the relaxation of the "subchains" conformation.

Dilute polyelectrolyte solutions: recent progress and open questions.

Polyelectrolytes are a class of polymers possessing ionic groups on their repeating units. Since counterions can dissociate from the polymer backbone, polyelectrolyte chains are strongly influenced by electrostatic interactions. As a result, the physical properties of polyelectrolyte solutions are significantly different from those of electrically neutral polymers. The aim of this article is to highlight key results and some outstanding questions in the polyelectrolyte research from recent literature. We focus on the influence of electrostatics on conformational and hydrodynamic properties of polyelectrolyte chains. A compilation of experimental results from the literature reveals significant disparities with theoretical predictions. We also discuss a new class of polyelectrolytes called poly(ionic liquid)s that exhibit unique physical properties in comparison to ordinary polyelectrolytes. We conclude this review by listing some key research challenges in order to fully understand the conformation and dynamics of polyelectrolytes in solutions.

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.

Charge transport in electrospinning of polyelectrolyte solutions.

This study elucidates the electrical charge transport during electrospinning of weak polyelectrolyte (poly(acrylic acid) (PAA)) solutions by employing current emission measurements. With pH variation, the PAA ionization degree could be controlled from uncharged at low pH to weakly charged at intermediate solution pH. Electrospinning neutral poly(vinylpyrrolidone) (PVP) as a reference polymer solution confirmed established current-flow rate scaling relationships as shown by De La Mora and Loscertales (1994), I ∼ (γKQ)ν, independent of the applied electric field polarity, where ν = 0.5, K is the conductivity, γ is the surface tension, Q is the flow rate, and I is the current. Similarly, the uncharged PAA did not display any polarity dependence, yet ν ≈ 0.8. Negatively charged PAA, however, showed a marked deviation in the current-flow rate behavior, which was affected by the applied electric field polarity. In the case of negative polarity, ν = 0.99, whereas for a positive polarity ν = 0.68. Similarly, the voltage required for stable cone-jet electrospinning of charged PAA was significantly higher in the negative polarity configuration for all tested flow rates (300-1600 μL h-1). As opposed to merely surface charges typically considered when electrospinning leaky dielectric fluids, as suggested by Melcher and Taylor (1969), our results suggest that the measured current is also affected by volumetric charges from charged PAA in the bulk of the jet. The proposed additional charge transport might affect the orientational order within PE-based nanofibers and their diameter.

Viscosity scaling and entangled solution rheology in aqueous and salt solutions of polyelectrolytes containing diallyl dimethylammonium groups

Viscosity scaling and entangled solution rheology in aqueous and salt solutions of polyelectrolytes containing diallyl dimethylammonium groups

3D printed polylactic acid-based nanocomposite scaffold stuffed with microporous simvastatin-loaded polyelectrolyte for craniofacial reconstruction

Critical sized craniofacial defects are among the most challenging bone defects to repair, due to the anatomical complexity and aesthetic importance. In this study, a polylactic acid/hardystonite-graphene oxide (PLA/HTGO) scaffold was fabricated through 3D printing. In order to upgrade the 3D printed scaffold to a highly porous scaffold, its channels were filled with pectin-quaternized chitosan (Pec-QCs) polyelectrolyte solution containing 0 or 20 mg/mL of simvastatin (Sim) and then freeze-dried. These scaffolds were named FD and FD-Sim, respectively. Also, similar PLA/HTGO scaffolds were prepared and dip coated with Pec-QCs solution containing 0 or 20 mg/mL of Sim and were named DC and DC-Sim, respectively. The formation of macro/microporous structure was confirmed by morphological investigations. The release of Sim from DC-Sim and FD-Sim scaffolds after 28 days was measured as 77.40 ± 5.25 and 86.02 ± 3.63 %, respectively. Cytocompatibility assessments showed that MG-63 cells had the highest proliferation, attachment and spread on the Sim containing scaffolds, especially FD-Sim. In vivo studies on a rat calvarial defect model revealed that an almost complete recovery occurred in the group treated with FD-Sim scaffold after 8 weeks and the defect was filled with newly formed bone. The results of this study acknowledge that the FD-Sim scaffold can be a perfect candidate for calvarial defect repair.

Parametric Study of Colloidal Particle Confinement near a Surface in the Presence of DLVO and Structural Interactions Using Brownian Dynamic Simulations.

Total internal reflection microscopy (TIRM) has become a crucial technique for understanding the surface interactions and dynamics of Brownian colloidal particles near a surface. However, for select colloidal systems, experimental limitations associated with TIRM can occlude exploration of nano- and submicrometer colloids dispersed in complex or structured fluids. It should be possible to use Brownian dynamic simulations to quantify, explore, or circumvent these limitations to extend the TIRM technique further. A Brownian dynamics algorithm based on the Langevin equation was utilized to identify favorable colloidal systems for conducting TIRM experiments in electrolyte and nonadsorbing polyelectrolyte solutions. In electrolyte solution, the motion of polystyrene and silica particles of nanometer- and micrometer-sized radii was simulated near a glass slide in the presence of retarded van der Waals and electric double-layer forces to develop potential energy profiles. In the case of nonadsorbing polyelectrolyte solutions, a structural force was also implemented into the simulation, and the influence of structural interactions on particle confinement was explored as a function of particle size, particle density, and polyelectrolyte concentration. In electrolyte solutions, our results were able to identify the minimum particle size required for TIRM experiments as well as insight into particle selection based on material density. For structural or oscillatory forces, our results show that prior to conducting TIRM experiments, Brownian dynamics simulation can be used to select the appropriate particle size, material, and polyelectrolyte concentration range where the colloidal particle can sample multiple structural energy wells without confinement. These results provide insight into the colloidal system suitable to experimentally study near-surface particle diffusion dynamics for a range of separations in the presence of structural interactions.

Nutritionally enhanced fibrous scaffolds by rolling droplet-interfacial polyelectrolyte complexation (RD-IPC)

The challenge of producing protein in a more sustainable manner than what is currently afforded by meat from livestock has led to the advent of cell-based or cultured meat. One method of obtaining whole-cuts of meat would be to culture the relevant cells on fibrous scaffolds to obtain meat-like textures. In the present work, a novel process of rolling droplet interfacial polyelectrolyte complexation (RD-IPC) is used to make aligned fibrous constructs that can be incorporated with nutritional components. Fiber is drawn by a falling droplet consisting of two oppositely charged polyelectrolytes that are continuously delivered by means of a syringe pump. The fibers are collected individually as they form and accumulate on a rotating collecting device to form planar constructs. Scaffolds incorporating gelatin, soybean flour, potato starch and garlic could be made by dispersing the respective powders into the polyelectrolyte solutions. Following the formation of the constructs, a heating step was found to improve their mechanical properties, in particular the potato starch-incorporated constructs, which exhibited higher tensile moduli due to a gelatinization process. Scaffolds imbued with gelatin and RGD moieties supported the adhesion, alignment, and differentiation of mouse myoblasts, while scaffolds incorporating gelatin alone sufficed for the differentiation of preadipocytes. RD-IPC provides for a scalable, continuous process that allows the incorporation of nutritional components into scaffolds possessing good mechanical and cell-adhesive properties. These fibrous scaffolds are constructed using edible polyelectrolyte components and aqueous solutions at relatively mild process conditions, underlining their promise for the nutritional enhancement of cultured meat.

Coarse-grained explicit-solvent molecular dynamics simulations of semidilute unentangled polyelectrolyte solutions.

We present results from explicit-solvent coarse-grained molecular dynamics (MD) simulations of fully charged, salt-free, and unentangled polyelectrolytes in semidilute solutions. The inclusion of a polar solvent in the model allows for a more physical representation of these solutions at concentrations, where the assumptions of a continuum dielectric medium and screened hydrodynamics break down. The collective dynamic structure factor of polyelectrolytes, S(q,t), showed that at [Formula: see text], where [Formula: see text] is the polyelectrolyte peak in the structure factor S(q) and [Formula: see text] is the correlation length, the relaxation time obtained from fits to stretched exponential was [Formula: see text], which describes unscreened Zimm-like dynamics. This is in contrast to implicit-solvent simulations using a Langevin thermostat where [Formula: see text]. At [Formula: see text], a crossover region was observed that eventually transitions to another inflection point [Formula: see text] at length scales larger than [Formula: see text] for both implicit- and explicit-solvent simulations. The simulation results were also compared to scaling predictions for correlation length, [Formula: see text], specific viscosity, [Formula: see text], and diffusion coefficient, [Formula: see text], where [Formula: see text] is the polyelectrolyte concentration. The scaling prediction for [Formula: see text] holds; however, deviations from the predictions for [Formula: see text] and D were observed for systems at higher [Formula: see text], which are in qualitative agreements with recent experimental results. This study highlights the importance of explicit-solvent effects in molecular dynamics simulations, particularly in semidilute solutions, for a better understanding of polyelectrolyte solution behavior.

Dielectric Mismatch Effects on Polyelectrolyte Solutions in Electrified Nanopores: Insights from Mean-Field Theory

We utilize the self-consistent field theory to explore the mechanical and electrical properties of charged surfaces immersed in polyelectrolyte solutions that could be potentially useful for electrochemical applications. Our research focuses on how the dielectric heterogeneity of the solution could affect the disjoining pressure and differential capacitance of the electric double layer. Relying on the developed theoretical framework, based on the Noether’s theorem, we calculate the stress tensor, containing the term, arising from the conformational entropy of the polymer chains. With its help we compute the disjoining pressure in polyelectrolyte solution confined between two parallel charged surfaces and analyze its behavior as a function of separation between the surfaces for different values of dielectric mismatch parameter. We also calculate the differential capacitance of the electric double layer and discuss how dielectric heterogeneity of the polyelectrolyte solution influences its values.

Solving the constitutive equation of dilute polymeric flows: A general Fokker–Planck approach for linear elastic dumbbell models

We present a mesoscopic numerical solver for the constitutive equation of dilute polymer solutions, as described by the Fokker–Planck equation of bead-spring dumbbell models with linear spring forces, such as Oldroyd-B, FENE-P (finitely extensible nonlinear elastic with Peterlin closure), and C-FENE-P (charged FENE-P). The configuration density function of the Fokker–Planck equation is expanded into a series of Hermite polynomials, and we show that only a second order in the expansion is needed to describe the evolution of the polymer-induced stresses. The polymer-induced stress tensor is given exactly through a discrete representation of the configuration distribution function, which is obtained from a finite set of Gauss–Hermite quadrature points. In addition, we derive the conformation tensor equation, which serves as a mathematical quality check for the method. We solve the time evolution of the extra stress tensor for homogeneous flows of dilute polyelectrolyte solutions, conceptualized by the C-FENE-P model, which is a generalization of the FENE-P dumbbell model. The results are shown to be in excellent agreement with analytical and semi-analytical reference results for simple shear and extensional flows. While in this paper the focus is on linear connector forces, we believe that the principles of the derivation are extendable to other force laws.

Dynamic Properties of Pulmonary Lipid Monolayers on the Surfaces of Sodium Polystyrene Sulfonate and Polydiallyldimethylammonium Chloride Solutions

Pulmonary surfactant, which is a complex mixture of lipids and proteins, plays a key role in the functional properties of the respiratory system. Lipids form complexes with proteins to maintain low values of surface tension at continuous compression/expansion deformations. However, interactions that lead to the complexation still remain to be unknown thus significantly hampering the development of synthetic analogs of the natural pulmonary surfactant. In this work, the methods of surface rheology and ellipsometry have been employed to study the dynamic properties of model phospholipid monolayers applied onto the surfaces of synthetic polyelectrolyte solutions. It has been shown, that electrostatic or hydrophobic interactions between lipids and macromolecules are insufficient for the efficient complexation and maintenance of low surface tension values.

Chain Dynamics in a Polyelectrolyte Solution Under Shear: A Rheological NMR Investigation

Rheological nuclear magnetic resonance (NMR) has been applied to study the effect of shear on the chain dynamics in a solution of a linear poly(sodium 4-styrenesulfonate). Information on the chain dynamics of poly(sodium 4-styrenesulfonate) sheared in a narrow-gap concentric double cylinder cell is inferred from the NMR transverse relaxation T2 for a concentration in the semidilute-entangled and in the concentrated regime. In the semidilute-entangled concentration enhanced relaxation is observed. In the concentrated regime signatures of a loss of entanglements are seen with increasing T2 and a decreasing fraction of the polymer showing restricted motion. The effect is enhanced when the electrostatic interaction along the polymer chain is reduced by the addition of NaCl.

Self-Solidifying Active Droplets Showing Memory-Induced Chirality.

Most synthetic microswimmers do not reach the autonomy of their biological counterparts in terms of energy supply and diversity of motions. Here, this work reports the first all-aqueous droplet swimmer powered by self-generated polyelectrolyte gradients, which shows memory-induced chirality while self-solidifying. An aqueous solution of surface tension-lowering polyelectrolytes self-solidifies on the surface of acidic water, during which polyelectrolytes are gradually emitted into the surrounding water and induce linear self-propulsion via spontaneous symmetry breaking. The low diffusion coefficient of the polyelectrolytes leads to long-lived chemical trails which cause memory effects that drive a transition from linear to chiral motion without requiring any imposed symmetry breaking. The droplet swimmer is capable of highly efficient removal (up to 85%) of uranium from aqueous solutions within 90min, benefiting from self-propulsion and flow-induced mixing. These results provide a route to fueling self-propelled agents which can autonomously perform chiral motion and collect toxins.

Viscosity of Sodium Polystyrenesulfonate with Cetyltrimethylammonium Bromide in the Mixture of Methanol and Water.

Viscosity of cetyltrimethylammonium bromide (CTAB) in the presence of sodium polystyrene sulfonate (NaPSS) in methanol-water mixed solvent media has been reported at 308.15 K. The results have been analyzed on the basis of a simple model based on the scaling theory of the viscosity of polyelectrolyte solutions put forward by Plucktaveesak et al. (J. Phys. Chem. B, 2003, 107, 8166-8171). The specific viscosity data obtained here for the CTAB-NaPSS couple in mixed solvent media agree very well with the predictions of the above model. It has been established that the fraction of free counterions of the polyelectrolyte can be conveniently recovered in these complex fluids.

Electrically Mediated Static Contact Angle and Hysteresis of Polyelectrolyte Solutions.

Manipulating a droplet by electrowetting-on-dielectric (EWOD) is vital in various fields ranging from industrial applications to life sciences. As of now, EWOD research has focused primarily on aqueous electrolytes and ionic liquids. This paper investigates the electrowetting behavior of weak polyelectrolyte solutions containing poly(acrylic acid) (PAA). The study reveals distinct wetting behavior of weakly and fully charged PAA droplets controlled by their solution pH. Under an applied electric field, strongly ionized PAA wets more effectively than weakly charged PAA. The electrowetting hysteresis of fully ionized PAA droplets was also higher than that of weakly charged droplets. The reason may be the suppression of retraction flow near the contact line. In this thin region, the electric field aligns the stretched polymer chains perpendicular to the dielectric surface, thus affecting the bulk rheological properties. The results reveal how charge-connectivity and polyelectrolyte conformation under an external electric field can control the electrowetting gain and the hysteresis. This previously unexplored electrowetting mechanism of polyelectrolyte solutions might help order and manipulate biological polyelectrolytes, such as deoxyribonucleic acid (DNA), polypeptides, and glycosaminoglycans.

Parametric study of the silver nanoparticles deposition on polyelectrolyte supporting layers

Colloidal nanoparticles deposition on polyelectrolyte (PE) supporting layers is a widespread technique to achieve high density nanoparticle coatings, however the choice of the parameters is often not consistent and varies in different reports. Frequently the obtained films suffer from the aggregation and non-reproducibility. Here, we addressed the main variables in silver nanoparticles deposition—the immobilization time; concentration of the PE in the solution; the thickness of PE underlayer and overlayer; the concentration of salt in the PE solution during the formation of a underlayer. We report on the formation of the high-density silver nanoparticle films and ways to tune their optical density in the wide range using the immobilization time as well as the thickness of PE overlayer. Silver colloidal films with maximum reproducibility were obtained by adsorption of nanoparticles on 1 underlayer of 5 g l−1 polydiallyldimethylammonium chloride with 0.5 M NaCl concentration. The results are promising for the fabrication of the reproducible colloidal silver films for multiple applications, such as plasmon enhanced fluorescent immunoassays and surface enhanced Raman scattering sensors.

Effect of Polypeptide Complex Coacervate Microenvironment on Protonation of a Guest Molecule.

Complex coacervate droplets formed by the liquid-liquid phase separation of polyelectrolyte solutions capture several important features of membraneless organelles including their ability to accumulate guest molecules and to provide distinct microenvironments. Here, we examine how polyions in complex coacervates can influence localized guest molecules, leading to a shifted protonation state of the guest molecule in response to its electrostatic environment. A fluorescent ratiometric pH indicator dye was used as a model guest molecule able to report its protonation state in the coacervate phase. Experimentally observed differences in dye-reported local apparent pH inside versus outside of coacervate droplets were largest for polyion pairs having lower salt stabilities and/or larger polyion length mismatch, which we attribute to the relative concentration of open sites on polyions within the coacervates based on theoretical calculations. Using the transfer matrix method, we confirmed that theoretical phase diagrams and critical salt stabilities generated for each polyion pair were consistent with experimental turbidity measurements and estimated the amount of available binding sites on polyions for guest molecules. We conclude that dye molecules likely experience an effective pKa shift due to interactions with coacervate polyions rather than reporting directly on local proton concentrations. Such a local pKa shift can also be anticipated for other guest molecules having protonatable groups, including, for example, many metabolites, ligands, and/or drug molecules that partition into coacervates or membraneless organelles based on ion pairing interactions.