Polyelectrolyte Solutions Research Articles

Simple Coacervation of Guanidinium-Containing Polymers Induced by Monovalent Salt

Liquid–liquid phase separation (LLPS) of macromolecules, called coacervation, is induced by non-covalent intermolecular associations in aqueous solutions, which is frequently observed in biological processes including cellular compartmentation, signaling, and regulation. Recently, associative interactions between π-conjugated residues have emerged to induce LLPS, along with electrostatic interaction. An arginine residue having a positively charged guanidinium group plays a pivotal role in protein phase behavior because the orientational amphiphilicity of guanidinium groups allows face-to-face π–π stacking despite Coulombic repulsion. In this study, we investigate the simple coacervates of guanidinium-functionalized polyelectrolytes (i.e., poly-arginine) in aqueous media as a function of salt concentration, salt type, and temperature to understand the exclusive role of π-conjugated moieties. Contrary to ammonium-functionalized polyelectrolyte (i.e., poly-lysine) solutions, guanidinium-functionalized polyelectrolyte solutions become turbid by adding monovalent salts and exhibit the upper critical solution temperature (UCST) behavior; the critical temperature is harnessed by salt concentration and salt type. Although a π–π stacked guanidinium pair can exist at a lower salt concentration, LLPS occurs when the number of intermolecular guanidinium pairs goes beyond a critical value to produce the network structure. Furthermore, the enthalpically favored π–π interaction directly affects the bulk rheological behavior and the interfacial property of the coacervates. Our findings provide insights into the underlying interactions of protein phase separation and shed new light on the critical role of π–π stacking in the biological process and material design.

Responsive polymer thin films

Responsive polymer thin films

Entropic Origin of Ionic Interactions in Polar Solvents.

Implicit solvent models that reduce solvent degrees of freedom into effective interaction potentials are widely used in the study of soft materials and biophysical systems. For electrolyte and polyelectrolyte solutions, coarse-graining the solvent degrees of freedom into an effective dielectric constant embeds entropic contributions into the temperature dependence of the dielectric constant. Properly accounting for this electrostatic entropy is essential to discern whether a free energy change is enthalpically or entropically driven. We address the entropic origin of electrostatic interactions in a dipolar solvent and provide a clarified physical picture of the solvent dielectric response. We calculate the potential of mean force (PMF) between two oppositely charged ions in a dipolar solvent using molecular dynamics and dipolar self-consistent field theory. We find with both techniques that the PMF is dominated by the entropy gain from the dipole release, owing to the diminished orientational polarization of the solvent. We also find that the relative contribution of the entropy to the free energy change is nonmonotonic with temperature. We expect that our conclusions are applicable to a broad range of problems involving ionic interactions in polar solvents.

Environmentally benign few-bilayer intumescent nanocoating for flame retardant enzyme/plasma modified polyester fabric

The flammability of polyester textiles remains a significant challenge that must be overcome to allow their use in a variety of clothing and upholstery applications. In this work, water-based polyelectrolyte solutions composed of chitosan (CH) and pectin (P), as carbon-rich biomacromolecules, and mono ammonium phosphate (MAP) were deposited on enzyme/corona modified polyester via layer-by-layer (LbL) deposition. Four bilayers (BL) of 1% CH3/(1% P-30% MAP)4.2 renders the polyester fabric self-extinguishing in vertical flame testing. This recipe yields a 42.6% lower peak heat release rate and a nearly 40% reduction in the fire growth capacity, compared to uncoated polyester, as measured by micro cone calorimetry. The enzyme/corona modification, combined with the CH/(P-MAP) intumescent system, results in a very effective flame retardant treatment with few BL deposited.

Effect of isopropanol cosolvent on the rheology and spinnability of aqueous polyacrylic acid solutions

AbstractWe investigate the effect of alcohol fraction (isopropanol, IPA) in a binary water‐alcohol solvent mixture on the shear and extensional rheological properties, as well as the role of viscoelasticity on fiber formation of poly(acrylic acid) (PAA) in electrospinning. Comparison of the scaling of both specific viscosities ηsp and extensional relaxation times λE of PAA in water–IPA mixtures, showed stronger scaling compared to salt‐free aqueous polyelectrolyte solutions, except for the ηsp in the unentangled regime displaying a polyelectrolyte‐like scaling ηsp ~ c0.5 for all IPA%. Such deviation suggested IPA induces association/aggregation of PAA. However, the trends between ηsp and λE magnitudes as a function of IPA% differ for concentrations compared in the entangled regime. The ηsp as well as their elastic moduli exhibit a maximum, whereas λE increases monotonically with IPA%, suggesting a complex interplay of various interactions are dictating their structure in water‐IPA mixtures, affecting their shear and extensional response differently. Electrospinning experiments showed increasing IPA% reduces the onset of both beaded and uniform fibers. Analysis using dimensionless numbers indicated the enhancement of their elasticity by IPA, and the consequent stabilizing effect on their jets/filaments against break‐up during electrospinning, plays a role in the improvement of their fiber formation.

Research progress in thermodynamic properties of polyelectrolyte solutions at surfaces and interfaces

聚电解质的表界面行为调控(如表面吸附和粘附)在粘合剂设计、胶体稳定/聚沉、海水淡化和生物医药材 料研发等多个领域发挥着举足轻重的作用. 聚电解质的浓度、刚性、序列结构、盐浓度和表面性质等可设计参 数显著影响着聚电解质的表界面微观结构, 进而决定聚电解质溶液的表界面热力学性质. 为了加速高性能聚电解 质材料的发展, 利用理论方法(如自洽场理论和密度泛函理论等)和计算机模拟(如蒙特卡洛模拟和分子动力学模 拟等)等手段开展高通量、系统性的研究是非常必要的. 本文总结了近年来聚电解质溶液的表界面热力学性质的 相关理论和模拟研究进展, 同时也对该领域未来的发展趋势进行了总结与展望.

Polyelectrolyte nanoparticles based on poly[N-(2-hydroxypropyl)methacrylamide-block-poly(N-(3-aminopropyl)methacrylamide] copolymers for delivery of heparin-binding proteins

In addition to the delivery of therapeutic nucleic acids and low-molecular weight drugs, polyelectrolyte nanoparticles (NPs) have recently been studied as carriers for protein delivery. However, the stability of NPs during isolation steps, which ensures their easy redispersion, needs to be solved empirically for individual systems using cryoprotectants and stabilizers. To avoid the use of additives, we studied the formation of polyelectrolyte NPs consisting of a newly synthesized polycationic diblock copolymer based on poly(N-(2-hydroxypropyl)methacrylamide)-block-poly(N-(3-aminopropyl)methacrylamide) (p(HPMA-b-APMA) and heparin (Hep). The p(APMA) blocks electrostatically complexed with Hep, and the p(HPMA) blocks formed a neutral corona of NPs, limiting NP aggregation. Self-assembly was monitored through the changes in size and zeta potential of the formed NPs, which depended on the copolymer composition and the concentration of polyelectrolyte solutions. The interactions between the NP components were analysed by FTIR spectroscopy, and XPS analysis indicated the presence of p(HPMA) blocks on the surface of the NPs. The encapsulation of the model chemokine CXCL12 and basic fibroblast growth factor (FGF-2) was driven by their specific bioaffinity for Hep, resulting in a high 90 % entrapment efficiency and increased protein stability. CXCL12 was released over 48 h, while FGF-2 exhibited a sustained release of up to 38 % over four weeks. In addition, the released CXCL12 effectively stimulated the migration of macrophages and T-lymphocyte cells, indicating the preserved protein bioactivity. Considering the proven noncytotoxic performance of the NPs towards fibroblasts and mesenchymal stem cells, the polyelectrolyte NPs of p(HPMA-b-APMA) and Hep loaded with heparin-binding proteins can be considered as promising candidates for the controlled delivery of bioactive proteins in biomedical applications.

Concentrated Nonaqueous Polyelectrolyte Solutions: High Na-Ion Transference Number and Surface-Tethered Polyanion Layer for Sodium-Metal Batteries.

Na metal is a promising anode material for the preparation of next-generation high-energy-density sodium-ion batteries; however, the high reactivity of Na metal severely limits the choice of electrolyte. In addition, rapid charge-discharge battery systems require electrolytes with high Na-ion transport properties. Herein, we demonstrate a stable and high-rate sodium-metal battery enabled by a nonaqueous polyelectrolyte solution composed of a weakly coordinating polyanion-type Na salt, poly[(4-styrenesulfonyl)-(trifluoromethanesulfonyl)imide] (poly(NaSTFSI)) copolymerized with butyl acrylate, in a propylene carbonate solution. It was found that this concentrated polyelectrolyte solution exhibited a remarkably high Na-ion transference number (tNaPP = 0.9) and a high ionic conductivity (σ = 1.1 mS cm-1) at 60 °C. Furthermore, the surface of the Na electrode was modified with polyanion chains anchored via the partial decomposition of the electrolyte. The surface-tethered polyanion layer effectively suppressed the subsequent decomposition of the electrolyte, thereby enabling stable Na deposition/dissolution cycling. Finally, an assembled sodium-metal battery with a Na0.44MnO2 cathode demonstrated an outstanding charge/discharge reversibility (Coulombic efficiency >99.8%) over 200 cycles while also exhibiting a high discharge rate (i.e., 45% capacity retention at 10 mA cm-2).

Salt Effect on the Viscosity of Semidilute Polyelectrolyte Solutions: Sodium Polystyrenesulfonate

We studied the viscosity of semidilute aqueous solutions of sodium polystyrenesulfonate as a function of polymer and salt concentrations. The viscosity data were quantified by applying a scaling relationship between solution correlation length ξ = lgν/B and number of monomers g per correlation volume for chains with monomer projection length l. The specific values Bpe,Bg, and Bth of the B-parameter corresponding to exponents ν = 1, 0.588, and 0.5 were determined by the fraction of charged monomers and their degree of ionization, the effective solvent quality for the polymer backbone, the chain Kuhn length, and the type and strength of monomer–solvent interactions. The values of the B-parameters were obtained from the plateaus of normalized specific viscosity ηsp(c)/Nw(cl3)1/(3ν–1) as a function of the monomer concentration c for polyelectrolytes with weight-average degree of polymerization Nw. The extension of this approach to the entangled solution regime allowed us to determine the packing number of a chain of correlation blobs, P̃e, which completes the set of parameters {Bpe, Bg, Bth, P̃e} uniquely describing static and dynamic properties of polyelectrolyte solutions. This information was used to construct a diagram of states, calculate the fraction of free counterions and the energy of the electrostatic blobs, and establish a crossover concentration to the entangled solution regime.

Macroscopic forces in inhomogeneous polyelectrolyte solutions.

In this paper, we present a self-consistent field theory of macroscopic forces in spatially inhomogeneous flexible chain polyelectrolyte solutions. We derive an analytical expression for a stress tensor which consists of three terms: isotropic hydrostatic stress, electrostatic (Maxwell) stress, and stress rising from conformational entropy of polymer chains-conformational stress. We apply our theory to the description of polyelectrolyte solutions confined in a conductive slit nanopore and observe anomalous behavior of disjoining pressure and electric differential capacitance.

Amphiphilic Block-Random Copolymers: Shedding Light on Aqueous Self-Assembly Behavior

The aqueous solution and aggregation behavior of polystyrene-b-poly[styrene-r-(acrylic acid)] block-random copolymers have been examined. Previous work conducted by our group showed that these materials exhibit an unanticipated ease of dispersion in water compared to polystyrene-b-poly(acrylic acid) block copolymers of similar molecular weight and composition. Herein, fluorescence labeling experiments and the presence of a critical aggregation concentration suggest the self-assembly of these materials into multichain aggregates as opposed to the self-folding behavior that was previously hypothesized. In our analysis, we demonstrate that caution must be used when interpreting particle size data from dynamic light scattering with polyelectrolyte solutions and that other characterization methods should be used to confirm findings. The fundamental understanding of block-random copolymer solution properties enables the widespread application of these easily dispersible materials in fields where amphiphilic copolymers are of interest, including biomedicine, catalysis, and stabilizers in emulsion polymerization.

Single‐step polyelectrolyte complex coating on hollow fibers yields nanofiltration or biocatalytic properties

The modification of membranes with polyelectrolytes via the Layer-by-Layer (LBL) method has become state of the art in recent years. It is used to fabricate nanofiltration hollow fiber membranes or to immobilize biomolecules on a membrane surface. However, it still remains a time consuming process. In contrast, this work explores a single-step membrane modification with coating solutions containing both polyanions and polycations. High salt concentration in the coating solution suppresses the complexation of the polyelectrolytes prior to the coating. Then, the controlled reduction of the salt concentration during the coating triggers the formation of a polyelectrolyte complex layer on the membrane. Three coating methods are proposed: (1) In interfacial complexation (IC), the polyelectrolyte solution coats the membrane and is subsequently precipitated by flushing with water. (2) Diffusive desalination (DDS) uses the concentration difference between the coating solution in the lumen and a water stream in the shell side to remove salt ions continuously. (3) In polyelectrolyte concentration (PC), the polyelectrolyte solution is coated at a constant flux. Here, the membrane retains the polyelectrolyte while ions permeate through. First, we evaluate the coating methods regarding their ability to produce nanofiltration membranes, which varies depending on the coating method used. With PC, membranes with up to 79% MgCl2 rejection and a permeability of 30 LMH/bar are obtained. Moreover, in-situ functionalization of the membranes is investigated by the addition of enzymes. Here, with DDS enzymes are immobilized, mostly achieved through adsorption via electrostatic interactions.

Polyelectrolyte complexation of two oppositely charged symmetric polymers: A minimal theory.

Interplay of Coulomb interaction energy, free ion entropy, and conformational elasticity is a fascinating aspect in polyelectrolytes (PEs). We develop a theory for complexation of two oppositely charged PEs, a process known to be the precursor to the formation of complex coacervates in PE solutions, to explore the underlying thermodynamics of complex formation, at low salts. The theory considers general degrees of solvent polarity and dielectricity within an implicit solvent model, incorporating a varying Coulomb strength. Explicit calculation of the free energy of complexation and its components indicates that the entropy of free counterions and salt ions and the Coulomb enthalpy of bound ion-pairs dictate the equilibrium of PE complexation. This helps decouple the self-consistent dependency of charge and size of the uncomplexed parts of the polyions, derive an analytical expression for charge, and evaluate the free energy components as functions of chain overlap. Complexation is observed to be driven by enthalpy gain at low Coulomb strengths, driven by entropy gain of released counterions but opposed by enthalpy loss due to reduction of ion-pairs at moderate Coulomb strengths, and progressively less favorable due to enthalpy loss at even higher Coulomb strengths. The total free energy of the system is found to decrease linearly with an overlap of chains. Thermodynamic predictions from our model are in good quantitative agreement with simulations in literature.

Ion correlation and negative lithium transference in polyelectrolyte solutions.

Polyelectrolyte solutions (PESs) recently have been proposed as high conductivity, high lithium transference number (t+) electrolytes where the majority of the ionic current is carried by the electrochemically active Li-ion. While PESs are intuitively appealing because anchoring the anion to a polymer backbone selectively slows down anionic motion and therefore increases t+, increasing the anion charge will act as a competing effect, decreasing t+. In this work we directly measure ion mobilities in a model non-aqueous polyelectrolyte solution using electrophoretic Nuclear Magnetic Resonance Spectroscopy (eNMR) to probe these competing effects. While previous studies that rely on ideal assumptions predict that PESs will have higher t+ than monomeric solutions, we demonstrate that below the entanglement limit, both conductivity and t+ decrease with increasing degree of polymerization. For polyanions of 10 or more repeat units, at 0.5 m Li+ we directly observe Li+ move in the "wrong direction" in an electric field, evidence of a negative transference number due to correlated motion through ion clustering. This is the first experimental observation of negative transference in a non-aqueous polyelectrolyte solution. We also demonstrate that t+ increases with increasing Li+ concentration. Using Onsager transport coefficients calculated from experimental data, and insights from previously published molecular dynamics studies we demonstrate that despite selectively slowing anion motion using polyanions, distinct anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates reduce the t+ in non-entangled PESs. This leads us to conclude that short-chained polyelectrolyte solutions are not viable high transference number electrolytes. These results emphasize the importance of understanding the effects of ion-correlations when designing new concentrated electrolytes for improved battery performance.

Mechanical deformation affects the counterion condensation in highly-swollen polyelectrolyte hydrogels.

Polyelectrolyte gels can generate electric potentials under mechanical deformation. While the underlying mechanism of such a response is often attributed to changes in counterion-condensation levels or alterations in the ionic conditions in the pervaded volume of the hydrogel, the exact molecular origins are largely unknown. By using all-atom molecular dynamics simulations of a polyacrylic acid hydrogel in explicit water as a model system, we simulate the uniaxial compression and uniaxial stretching of weakly to highly swollen (i.e., between 60-90% solvent content) hydrogel networks and calculate the microscopic condensation levels of counterions around the hydrogel chains. The counterion condensation under deformation is highly non-monotonic. Ionic condensation around the constituting chains of the deformed hydrogel tends to increase as the chains are stretched. This increase reaches a maximum and decreases as the chains are strongly stretched. The condensation around the collapsed chains of the hydrogel is weakly affected by the deformation. As a result, both compressing and stretching the model hydrogel lead to an overall increase in the counterion condensation. The effect vanishes for weakly swollen hydrogels, for which most ions are already condensed. The simulations with single, stretched polyelectrolyte chains show a qualitatively similar response, suggesting the effect of chain elongation on the ionic distribution throughout the hydrogel. Notably, this deformation-induced counterion condensation phenomenon does not occur in a polyelectrolyte solution at its critical concentration, indicating the role of hydrogel topology constraining the chain ends. Our results indicate that counterion condensation in a deforming polyelectrolyte hydrogel can be highly heterogeneous and exhibit a rich behaviour of electrostatic responses.

Comment on "Simulations of ionization equilibria in weak polyelectrolyte solutions and gels" by J. Landsgesell, L. Nová, O. Rud, F. Uhlík, D. Sean, P. Hebbeker, C. Holm and P. Košovan, Soft Matter, 2019, 15, 1155-1185.

In a recent review Landsgesell et al., Soft Matter 2019, 15, 1155 stated that pH - pKa is a "universal parameter" for titrating systems. We show that this is not the case. This broken symmetry has important implications for constant pH (cpH) simulations. In particular, we show that for concentrated suspensions the error resulting from the use of cpH algorithm described by Landsgesell et al. is very significant, even for suspension containing 1 : 1 electrolyte. We show how to modify the cpH algorithm to account for the grand-canonical nature of the cpH simulations and for the charge neutrality requirement.

Reply to the 'Comment on "Simulations of ionization equilibria in weak polyelectrolyte solutions and gels"' by J. Landsgesell, L. Nová, O. Rud, F. Uhlík, D. Sean, P. Hebbeker, C. Holm and P. Košovan, Soft Matter, 2019, 15, 1155-1185.

Levin and Bakhshandeh suggested in their comment that (1), we stated in our recent review that pH-pKA is a universal parameter for titrating systems, that (2), we omitted to mention in our review the broken symmetry of the constant pH algorithm, and that (3), a constant pH simulation must include a grand-canonical exchange of ions with the reservoir. As a reply to (1), we point out that Levin and Bakhshandeh misquoted and hence invalidated our original statement. We therefore explain in detail under which circumstances pH-pKA can be a universal parameter, and also demonstrate why their numerical example is not in contradiction to our statement. Moreover, the fact that pH-pKA is not a universal parameter for titrating systems is well known in the pertinent literature. Regarding (2), we admit that the symmetry-breaking feature of the constant pH algorithm has escaped our attention at the time of writing the review. We added some clarifying remarks to this behavior. Concerning (3), we point out that the grand-canonical coupling and the resultant Donnan potential are not features of single-phase systems, but are essential for two-phase systems, as was shown in a recent paper by some of us, see J. Landsgesell et al., Macromolecules, 2020, 53, 3007-3020.

Interplay of Polymer Structure, Solvent Ordering, and Charge Fluctuations in Polyelectrolyte Solution Thermodynamics

We develop a polymer field-theoretic model that incorporates explicit treatment of semiflexibility and polar solvents, providing a framework to determine the impact of solvent ordering and concentration fluctuations on the thermodynamic behavior of polyelectrolyte solutions. We study the effect of dipole ordering on the electrostatic potential near a charged surface and the phase behavior of oppositely charged symmetric polyelectrolytes. The phase diagrams, incorporating quadratic-order concentration fluctuation corrections, are determined using different approximations of the chain structure factor for semiflexible polymers. While wormlike-chain statistics provides the most broadly accurate prediction for phase behavior, a Gaussian-rigid interpolation remains an adequate approximation for quadratic-order fluctuation corrections for many experimentally relevant systems. However, our theory provides the basis for predicting the impact of higher-order concentration fluctuations, which requires chain statistics that cannot be rendered from existing approximate methods.

Molecular Dynamics Simulations of Rhodamine B Zwitterion Diffusion in Polyelectrolyte Solutions.

Polyelectrolytes continue to find wide interest and application in science and engineering, including areas such as water purification, drug delivery, and multilayer thin films. We have been interested in the dynamics of small molecules in a variety of polyelectrolyte (PE) environments; in this paper, we report simulations and analysis of the small dye molecule rhodamine B (RB) in several very simple polyelectrolyte solutions. Translational diffusion of the RB zwitterion has been measured in fully atomistic, 2 μs long molecular dynamics simulations in four different polyelectrolyte solutions. Two solutions contain the common polyanion sodium poly(styrene sulfonate) (PSS), one with a 30-mer chain and the other with 10 trimers. The other two solutions contain the common polycation poly(allyldimethylammonium) chloride (PDDA), one with two 15-mers and the other with 10 trimers. RB diffusion was also simulated in several polymer-free solutions to verify its known experimental value for the translational diffusion coefficient, DRB, of 4.7 × 10-6 cm2/s at 300 K. RB diffusion was slowed in all four simulated PE solutions, but to varying degrees. DRB values of 3.07 × 10-6 and 3.22 × 10-6 cm2/s were found in PSS 30-mer and PSS trimer solutions, respectively, whereas PDDA 15-mer and trimer solutions yielded values of 2.19 × 10-6 and 3.34 × 10-6 cm2/s. Significant associations between RB and the PEs were analyzed and interpreted via a two-state diffusion model (bound and free diffusion) that describes the data well. Crowder size effects and anomalous diffusion were also analyzed. Finally, RB translation along the polyelectrolytes during association was characterized.

Dynamics of Filamentous Viruses in Polyelectrolyte Solutions

Dynamics of Filamentous Viruses in Polyelectrolyte Solutions