Swelling and Inflation in Polyelectrolyte Complexes

Polyelectrolyte complex-based materials for separations: progress, challenges and opportunities

Polyelectrolyte complex (PEC) based materials could provide a sustainable alternative to conventional materials, especially for separation applications. However, reproducible production remains a challenge due to the many parameters influencing the polyelectrolyte complexation process, eventually affecting the properties and performance of the final material. Here, we provide an overview of how different parameters affect polyelectrolyte complexation and discuss promising PEC-based materials for separation applications, i.e., porous membranes, functional and barrier coatings, adhesives, saloplastics, and extraction media. Additionally, we highlight the challenges and opportunities and discuss what is needed to get to the next level. We envision that collaboration between experimentalists and theoreticians can leverage experimental datasets with accurate descriptions of all the parameters for multiscale modelling, machine learning and artificial intelligence approaches that can be used to design PEC materials and predict their properties.

Impacts of Natural Organic Matter and Dissolved Solids on Fluoride Retention of Polyelectrolyte Multilayer-Based Hollow Fiber Nanofiltration Membranes

This study examines the effects of natural organic matter (NOM) and dissolved solids on fluoride (F-) retention in polyelectrolyte multilayer-based hollow-fiber nanofiltration membranes (dNF40). Lab-scale filtration experiments were conducted under varying operating conditions (initial salt concentration, NOM concentration, permeate flux, crossflow velocity, and recovery rate). dNF40 membranes exhibited F- retention above 70% ± 1.2 in the absence of NOM and competing ions. However, when filtering synthetic model water (SMW) designed to simulate groundwater contaminated with high total dissolved solids (TDSs) and NOM, F- retention decreased to approximately 60% ± 0.7, which was generally attributed to ion competition. Furthermore, despite limited declines in normalized permeability, the addition of NOM to SMW notably deceased F- retention in the steady state to~20% due to fouling effects. The facilitated transport of the divalent cations Ca2+ and Mg2+ could be observed, as they accumulated in the organic fouling layer. While SO42- retention remained relatively stable, the retention of monovalent anions (NO3-, Cl-, and F-) decreased substantially due to drag effects. Na+ retention improved slightly to maintain electroneutrality. Feed salinity was shown to significantly affect separation efficiency, with PEC layers undergoing swelling and certain structural changes as the ionic strength increased. During batch filtration experiments at varying recovery rates, the retention of monovalent anions further decreased, with F- retention reducing to just ~10% at a 90% recovery rate. This study provides valuable insights into better understanding and optimizing the performance of PEC-based NF membranes across diverse water treatment scenarios.

Optimization of Polyelectrolyte Coacervate Membranes via Aqueous Phase Separation

Polymeric membranes fabricated via the nonsolvent-induced phase separation process rely heavily on toxic aprotic organic solvents, like N-methyl-pyrrolidine (NMP) and dimethylformamide. We suggest that the "saloplastic" nature of polyelectrolyte complexes (PECs) makes them an excellent candidate for fabricating next-generation water purification membranes that use a more sustainable aqueous phase separation process. In this study, we investigate how the properties of PECs and their interactions with salt can form pore-containing membranes from the strong polyelectrolytes poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) in the presence of potassium bromide (KBr). How the single-phase polymer-rich (coacervate) dope solution and coagulation bath impacted the formation, morphology, and pure water permeance (PWP) of the membranes was systematically evaluated by using scanning electron microscopy and dead-end filtration tests. The impact of a salt annealing post-treatment process was also tested; these treated PEC membranes exhibited a PWP of 6.2 L m-2 h-1 bar-1 and a dye removal of 91.7% and 80.5% for methyl orange and methylene blue, respectively, which are on par with commercial poly(ether sulfone) nanofiltration membranes. For the first time, we have demonstrated the ability of the PEC membranes to repel Escherichia coli bacteria under static conditions. Our fundamental study of polyelectrolyte membrane pore-forming mechanisms and separation performance could help drive the future development of sustainable materials for membrane-based separations.

Segregative phase separation of strong polyelectrolyte complexes at high salt and high polymer concentrations

The phase behavior of polyelectrolyte complexes and coacervates (PECs) at low salt concentrations has been well characterized, but their behavior at concentrations well above the binodal is not well understood. Here, we investigate the phase behavior of stoichiometric poly(styrene sulfonate)/poly(diallyldimethylammonium) mixtures at high salt and high polymer concentrations. Samples were prepared by direct mixing of PSS/PDADMA PECs, water, and salt (KBr). Phase separation was observed at salt concentrations approximately 1 M above the binodal. Characterization by thermogravimetric analysis, FTIR, and NMR revealed that both phases contained significant amounts of polymer, and that the polymer-rich phase was enriched in PSS, while the polymer-poor phase was enriched in PDADMA. These results suggest that high salt concentrations drive salting out of the more hydrophobic polyelectrolyte (PSS), consistent with behavior observed in weak polyelectrolyte systems. Interestingly, at the highest salt and polymer concentrations studied, the polymer-rich phase contained both PSS and PDADMA, suggesting that high salt concentrations can drive salting out of partially-neutralized complexes as well. Characterization of the behavior of PECs in the high concentration limit appears to be a fruitful avenue for deepening fundamental understanding of the molecular-scale factors driving phase separation in these systems.

Achieving lysozyme functionalization in PDADMAC-NaPSS saloplastics through salt annealing

Hot-pressed saloplastics are dense and transparent polyelectrolyte complex materials governed by ionic crosslinking. Such plastics have several advantages, for example, salt water processibility and recyclability. Here, we demonstrate a simple but effective post-treatment method to incorporate lysozyme as a biocatalytic component into the hot-pressed saloplastics. Changes in salt concentration can be used for annealing and curing the saloplastics, where the temporary opening allows for lysozyme loading. This process was carefully examined by two different routes and the salt concentrations and incubation times were varied systematically. Optimised saloplastics showed an enzymatic activity against Micrococcus lysodeikticus of 4.44 ± 0.37 U cm-2 and remained partially active (∼72% activity preserved) after 7 days. This approach opens new routes to incorporate enzymes or other biological functionality into saloplastics which is difficult to achieve for conventional plastics.

A Perspective on the Glass Transition and the Dynamics of Polyelectrolyte Multilayers and Complexes

Polyelectrolyte multilayers (PEMs) or polyelectrolyte complexes (PECs), formed by layer-by-layer assembly or the mixing of oppositely charged polyelectrolytes (PEs) in aqueous solution, respectively, have potential applications in health, energy, and the environment. PEMs and PECs are very tunable because their structure and properties are influenced by factors such as pH, ionic strength, salt type, humidity, and temperature. Therefore, it is increasingly important to understand how these factors affect PECs and PEMs on a molecular level. In this Feature Article, we summarize our contributions to the field in the development of approaches to quantify the swelling, thermal properties, and dynamic mechanical properties of PEMs and PECs. First, the role of water as a plasticizer and in the glass-transition temperature (Tg) in both strong poly(diallyldimethylammonium)/poly(sodium 4-styrenesulfonate) (PDADMA/PSS) and weak poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) systems is presented. Then, factors influencing the dynamics of PECs and PEMs are discussed. We also reflect on the swelling of PEMs in response to different salts and solvent additives. Last, the nature of water's microenvironment in PEMs/PECs is discussed. A special emphasis is placed on experimental techniques, along with molecular simulations. Taken together, this review presents an outlook and offers recommendations for future research directions, such as studying the additional effects of hydrogen-bonding hydrophobic interactions.

A Molecular Thermodynamic Model of Coacervation in Solutions of Polycations and Oppositely Charged Micelles

To guide the rational design of personal care formulations, we formulate a molecular thermodynamic model that predicts coacervation from cationic polymers and mixed micelles containing neutral and anionic surfactants and added salt. These coacervates, which form as a result of dilution of conditioning shampoos during use, deposit conditioning agents and other actives to the scalp or skin and also provide lubrication benefits. Our model accounts for mixing entropy, hydrophobic interactions of polycation with water, free energies of bindings of oppositely charged groups to micelles and polycations, and electrostatic interactions that capture connectivity of charged groups on the polycation chain and the micelle. The model outputs are the compositions of surfactants, polycation, salt, and water in the coacervate and in its coexisting dilute phase, along with the binding fractions and coacervate volume fraction. We study the effects of overall composition (of surfactant, polycation, and added salt), charge fractions on micelles and polycations, and binding free energies on the phase diagram of coacervates. Then, we perform coacervation experiments for three systems: sodium dodecyl sulfate (SDS)-JR30M, sodium methyl cocoyl taurate (Taurate)-JR30M, and sodium lauryl alaninate (Alaninate)-JR30M, where JR30M is a cationic derivative of hydroxyethylcellulose (cat-HEC), and rationalize their coacervation data using our model. For comparison with experiment, we also develop a parametrization scheme to obtain the requisite binding energies and Flory-Huggins χ parameter. We find that our model predictions agree reasonably well with the experimental data, and that the sulfate-free surfactants of Taurate and Alaninate display much larger 2-phase regions compared to SDS with JR30M.

Electrostatically Complexed Natural Polysaccharides as Aqueous Barrier Coatings for Sustainable and Recyclable Fiber-Based Packaging

Driven by the ever-growing awareness of sustainability and circular economy, renewable, biodegradable, and recyclable fiber-based packaging materials are emerging as alternatives to fossil-derived, nonbiodegradable single-use plastics for the packaging industry. However, without functional barrier coatings, the water/moisture vulnerability and high permeability of fiber-based packaging significantly restrain its broader application as primary packaging for food, beverages, and drugs. Herein, we develop waterborne complex dispersion barrier coatings consisting of natural, biodegradable polysaccharides (i.e., chitosan and carboxymethyl cellulose) through a scalable, one-pot mechanochemical pathway. By tailoring the electrostatic complexation, the key element to form a highly crosslinked and interpenetrated polymer network structure, we formulate complex dispersion barrier coatings with excellent film-forming property and adaptable solid-viscosity profiles suitable for paperboard and molded pulp substrates. Our complex dispersions enable the formation of a uniform, defect-free, and integrated coating layer, leading to a remarkable oil and grease barrier and efficient water/moisture sensitivity reduction while still exhibiting excellent recyclability profile of the resulting fiber-based substrates. This natural, biorenewable, and repulpable barrier coating is a promising candidate to serve as a sustainable option for fiber-based packaging intended for the food and food service packaging industry.

Polyelectrolyte Complexes as Desiccants: Thirsty Saloplastics

Desiccants or drying agents are used extensively to remove water from liquids and gases. Many organic reactions, from the laboratory to the industrial scale, are sensitive to even trace amounts of water. A new class of desiccants made from complexed polyelectrolytes, PECs, is described here, exploiting the affinity of charged polymer repeat units for water. The enthalpy of hydration of dry PECs was used for the first time as a quantitative measure of PEC water affinity. Several combinations of positive, Pol+, and negative, Pol-, polymers were used to prepare PECs. All of these displayed significant exothermic (favorable) enthalpies of hydration, measured at room temperature using solution calorimetry. A PEC made from poly(diallyldimethylammonium) and poly(styrene sulfonate) was extruded into convenient shapes. This PEC was used to dry three common solvents, acetonitrile, tetrahydrofuran, and toluene, representing a range of polarities. Added water was radiolabeled with tritium to provide accurate and sensitive detection of residual water after treatment. This PEC was almost as efficient as the comparison desiccants, molecular sieve 3A and calcium sulfate, after 3 days of static drying but could be regenerated at a lower temperature (120 °C) and shed far fewer dust particles.

Accelerating Payload Release from Complex Coacervates through Mechanical Stimulation

Complex coacervates formed through the association of charged polymers with oppositely charged species are often investigated for controlled release applications and can provide highly sustained (multi-day, -week or -month) release of both small-molecule and macromolecular actives. This release, however, can sometimes be too slow to deliver the active molecules in the doses needed to achieve the desired effect. Here, we explore how the slow release of small molecules from coacervate matrices can be accelerated through mechanical stimulation. Using coacervates formed through the association of poly(allylamine hydrochloride) (PAH) with pentavalent tripolyphosphate (TPP) ions and Rhodamine B dye as the model coacervate and payload, we demonstrate that slow payload release from complex coacervates can be accelerated severalfold through mechanical stimulation (akin to flavor release from a chewed piece of gum). The stimulation leading to this effect can be readily achieved through either perforation (with needles) or compression of the coacervates and, besides accelerating the release, can result in a deswelling of the coacervate phases. The mechanical activation effect evidently reflects the rupture and collapse of solvent-filled pores, which form due to osmotic swelling of the solute-charged coacervate pellets and is most pronounced in release media that favor swelling. This stimulation effect is therefore strong in deionized water (where the swelling is substantial) and only subtle and shorter-lived in phosphate buffered saline (where the PAH/TPP coacervate swelling is inhibited). Taken together, these findings suggest that mechanical activation could be useful in extending the complex coacervate matrix efficacy in highly sustained release applications where the slowly releasing coacervate-based sustained release vehicles undergo significant osmotic swelling.

Covalent adaptable networks using boronate linkages by incorporating TetraAzaADamantanes

Boronic esters prepared by condensation of boronic acids and diols have been widely used as dynamic covalent bonds in the synthesis of both discrete assemblies and polymer networks. In this study we investigate the potential of a new dynamic-covalent motif, derived from TetraAzaADamantanes (TAADs), with their adamantane-like triol structure, in boronic ester-based covalent adaptable networks (CANs). The TetraAzaADamantane-boronic ester linkage has recently been reported as a more hydrolytically stable boronic ester variant, while still having a dynamic pH response: small-molecule studies found little exchange at neutral pH, while fast exchange occurred at pH 3.8. In this work, bi- and trifunctional TetraAzaADamantane linkers were synthesised and crosslinked with boronic acids to form rubber-like materials, with a Young's modulus of 1.75 MPa. The dynamic nature of the TetraAzaADamantane networks was confirmed by stress relaxation experiments, revealing Arrhenius-like behaviour, with a corresponding activation energy of 142 ± 10 kJ/mol. Increasing the crosslinking density of the material from 10% to 33% resulted in reduced relaxation times, as is consistent with a higher degree of crosslinking within the dynamic networks. In contrast to the reported accelerating effect of acid addition to small-molecule TetraAzaADamantane complexes, within the polymer network the addition of acid increased relaxation times, suggesting unanticipated interactions between the acid and the polymer that cannot occur in the corresponding small-molecules analogues. The obtained boronate-TetraAzaADamantane materials were thermally stable up to 150°C. This thermal stability, in combination with the intrinsically dynamic bonds inside the polymer network, allowed these materials to be reprocessed and healed after damage by hot-pressing.

Liquid-liquid and liquid-solid separation in self-assembled chitosan-alginate and chitosan-pectin complexes

The complexation between two oppositely charged polyelectrolytes (PE) can lead liquid-liquid (complex coacervates, CC) or liquid-solid (solid precipitates, SP) phase separations. Herein, the effect of pH (2-11) and ionic strength (I, 0.05-1.0 M KCl) on the associative interactions between chitosan (QL)-alginate (SA) and QL-Pectin (Pec), polysaccharides widely used in biotechnology field, is described. pH and I, exhibited significant effect on the structure and phase transitions by modifying the ionization degree (α), pk_a, and associative interactions between PE. Onset of binding was established at pH_c 9, while continued acidification (pH_τ 5.8) led to simultaneous CC and SP exhibiting a maximum turbidity in both systems. At pH_δ 4.0, QL-Pec showed preferably CC structures whereas QL-SA maintained the CC and SP structures. At pH_ω 2, the associative interactions were suppressed due to the low ionization of Pec and SA. I (1.0 M) significantly diminished the interactions in QL-Pec due to charge screening. Molecular weight, second virial coefficient, hydrodynamic size, ionizable groups, and persistence length of polyion, influenced on the phase behavior of QL-Pec and QL-SA systems. Therefore, CC and SP are found simultaneously in both systems, their transitions can be modulated by intrinsic and environmental conditions, expanding the functional properties of complexed polysaccharides.

Ionic Strength-Dependent Assembly of Polyelectrolyte-Nanoparticle Membranes via Interfacial Complexation at a Water-Water Interface

Complexation between oppositely charged nanoparticles (NPs) and polyelectrolytes (PEs) is a scalable approach to assemble functional, stimuli-responsive membranes. Complexation at interfaces of aqueous two-phase systems (ATPSs) has emerged as a powerful method to assemble these functional structures. Membranes formed at these interfaces can grow continuously to thicknesses approaching several millimeters and display a high degree of tunability via modification of solution properties such as ionic strength. To identify the membrane assembly mechanism, we study interfacial assembly in a prototypical dextran/PEG ATPS, in which silica (SiO₂) NPs suspended in the PEG phase undergo interfacial complexation with poly(diallyldimethylammonium chloride) (PDADMAC) supplied in the dextran phase. Using a microfluidic device that facilitates sequential insertion of fluorescent and nonfluorescent PDADMAC, we observe a transition in the membrane growth mechanism with ionic strength. In the absence of added salt ([NaCl] = 0 mM) PDADMAC chains permeate through the existing membrane to complex with NPs on the PEG side of the membrane, leading to the formation of well-stratified structures. At elevated ionic strength ([NaCl] = 500 mM), this permeation mechanism is lost. Rather, the complexing species incorporate uniformly across the membrane. We attribute this transition to a rapid exchange of PE-counterion, NP-counterion, and PE/NP binding sites facilitated by an increase in extrinsically compensated charged groups on the NPs and PEs at high salinity. These PDADMAC/SiO₂ NP membranes have tremendous potential for the formation of functional membranes, offering control over the internal structure and serving as an ideal system for the generation of targeted release systems.

Film Instability Induced by Swelling and Drying

Poly(2-vinyl pyridine), P2VP, films display a surface pattern of craters in a dried state after being immersed in aqueous solutions containing HAuCl₄ and its mixtures with low contents of K₂CO₃. The morphologies of craters indicate that the formation of craters involves three stages through film blistering and drying: (i) the permeability of water and solutes to swell P2VP films, (ii) partial wetting of liquid droplets near the substrate interface in the presence of the P2VP film, and (iii) evaporation-driven flows. The three stages produce the swelling pressure, Laplace pressure, and interplays among capillary flows, Marangoni flows, and pinning effects, respectively, by which craters of different dimensions and morphologies are obtained. The first stage softens the P2VP films and produces swelling pressure. This stage relies on interactions between AuCl₄^- ions, water, and protonated P2VP chains. The second stage produces liquid droplets inside the film and near the substrate interface. The surface tensions of those liquid droplets at contact lines deform swollen P2VP films. Changing film thicknesses or substrate types alters craters' lateral dimension and depth. The results indicate that film thicknesses and substrate interface energies influence the shape and dimension of liquid droplets on the substrate interface. The third stage determines morphologies of craters through interplays among capillary flows, Marangoni flows, and pinning/depinning events.

Quantification of Water-Ion Pair Interactions in Polyelectrolyte Multilayers Using a Quartz Crystal Microbalance Method

Water existing within thin polyelectrolyte multilayer (PEM) films has significant influence on their physical, chemical, and thermal properties, having implications for applications including energy storage, smart coatings, and biomedical systems. Ionic strength, salt type, and terminating layer are known to influence PEM swelling. However, knowledge of water’s microenvironment within a PEM, whether that water is affiliated with intrinsic or extrinsic ion pairs, remains lacking. Here, we examine the influence of both assembly and post-assembly conditions on the water–ion pair interactions of poly(styrene sulfonate)/poly(diallyldimethylammonium) (PSS/PDADMA) PEMs in NaCl and KBr. This is accomplished by developing a methodology in which quartz crystal microbalance with dissipation monitoring is applied to estimate the number of water molecules affiliated with an ion pair (i), as well as the hydration coefficient, π_salt^H₂O. PSS/PDADMA PEMs are assembled in varying ionic strengths of either NaCl and KBr and then exposed post-assembly to increasing ionic strengths of matching salt type. A linear relationship between the total amount of water per intrinsic ion pair and the post-assembly salt concentration was obtained at post-assembly salt concentrations >0.5 M, yielding estimates for both i and π_salt^H₂O. We observe higher values of i and π_salt^H₂O in KBr-assembled PEMs due to KBr being more effective in doping the assembly because of KBr’s more chaotropic nature as compared to NaCl. Lastly, when PSS is the terminating layer, i decreases in value due to PSS’s hydrophobic nature. Classical and ab initio molecular dynamics provide a microstructural view as to how NaCl and KBr interact with individual polyelectrolytes and the involved water shells. Put together, this study provides further insight into the understanding of existing water microenvironments in PEMs and the effects of both assembly and post-assembly conditions.

The critical Transition from Soluble Complexes to Colloidal Aggregates of Polyelectrolyte Complexes at Non-stoichiometric Charge Ratios

The transition from soluble to colloidal polyelectrolyte complex normally occurs at a critical non-stoichiometric charge ratio. Here, we demonstrated that the conventional batch mixing produces heterogeneous binding and complexation, which could easily mask this soluble-colloidal complex transition (sol-col transition) even for weakly binding polyelectrolytes like polyacrylic acid (PAA) and poly(diallyldimethylammonium chloride) (PDADMAC). When mixed efficiently using multi-inlet vortex mixer (MIVM), the sol-col transition occurs beyond a critical charge ratio (n-/n+) and the large colloidal complexes are formed through the aggregation of small primary complexes (as revealed by atomic force microscopy). Moreover, the sol-col transition occurs at a constant charge ratio below the overlapping concentration (c*) of the long host polyelectrolyte, but at lower charge ratios above c* due to chain entanglement. When adding NaCl to the solution, the sol-col transition charge ratio first decreases, then remained stable for a period and finally increased and vanished at high ionic strength. When replacing NaCl with chaotropic salts, the sol-col transition occurs at lower charge ratios, while kosmotropes had little impact. The solvent quality and polymer hydrophobicity effects are also discussed. With the assistance of rapid mixing, this study provides a more reliable way of studying the sol-col transition of polyelectrolyte complexes. This article is protected by copyright. All rights reserved.

Processing Polyelectrolyte Complexes with Deep Eutectic Solvents

Polyelectrolyte complexes (PECs) formed from mixtures of polycations and polyanions are useful in a variety of applications and can be processed by the addition of salt. Salt mediates the ionic interactions within the polyelectrolyte complexes, with appropriately chosen salts enabling complete dissolution of solid PEC in aqueous media. Substantial complications arise from the crystallization of the salt during subsequent processing steps. Here we show that appropriately chosen noncrystallizing deep eutectic solvents can be used to process solid PECs. Mixtures of ethylene glycol and guanidine thiocyanate are used for a particularly effective deep eutectic solvent. The phase behaviors of this deep eutectic system and of its mixtures with a model polyelectrolyte complex were quantified.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

Decoupling salt- and polymer-dependent dynamics in polyelectrolyte complex coacervates via salt addition

In polyelectrolyte complex coacervates, changes in salt concentration and changes in polymer concentration are typically strongly coupled, complicating interpretation of the salt- and polymer-concentration-dependent dynamics of these materials. To address this problem, we developed a "salt addition" method for preparation of complex coacervates that allows the salt concentration of a coacervate sample to be varied without changing its polymer concentration. This method was used to prepare coacervates of poly(styrene sulfonate) (PSS) with poly(diallyldimethylammonium chloride) (PDADMAC) with salt concentrations between 1.2 and 2 M and volume fractions of polymer between 0.1 and 0.25. Characterization of these samples by small-amplitude oscillatory shear rheology revealed that the relaxation times scale significantly more strongly with polymer volume fraction than has been previously assumed, highlighting the need to account for both salt and polymer-dependent contributions to the dynamics of these complex materials.

Ion Pairing and the Structure of Gel Coacervates

A scaling model for the structure of coacervates is presented for mixtures of oppositely-charged polyelectrolytes of both symmetric and asymmetric charge-densities for different degrees of electrostatic strength and levels of added salt. At low electrostatic strengths, weak coacervates, with the energy of electrostatic interactions between charges less than the thermal energy, k B T, are liquid. At higher electrostatic strengths, strong coacervates are gels with crosslinks formed by ion pairs of opposite charges bound to each other with energy higher than k B T. Charge-symmetric coacervates are formed for mixtures of oppositely-charged polyelectrolytes with equal and opposite charge-densities. While charge-symmetric weak coacervates form a semidilute polymer solution with a correlation length equal to the electrostatic blob size, charge-symmetric strong coacervates form reversible gels with a correlation length on the order of the distance between bound ion pairs. Charge-asymmetric coacervates are formed from mixtures of oppositely-charged polyelectrolytes with different charge-densities. While charge-asymmetric weak coacervates form double solutions with two correlation lengths and qualitatively different chain conformations of polycations and polyanions, charge-asymmetric strong coacervates form bottlebrush and star-like gels. Unlike liquid coacervates, for which an increase in the concentration of added salt screens electrostatic interactions, causing structural rearrangement and eventually leads to their dissolution, the salt does not affect the structure of strong coacervates until ion pairs dissociate and the gel disperses.

Phosphonium versus Ammonium Compact Polyelectrolyte Complex Networks with Alginate-Comparing Their Properties and Cargo Encapsulation

Phosphonium and ammonium polymers can be combined with polyanions to form polyelectrolyte complex (PEC) networks, with potential application in self-healing materials and drug delivery vehicles. While various structures and compositions have been explored, to the best of our knowledge, analogous ammonium and phosphonium networks have not been directly compared to evaluate the effects of phosphorus versus nitrogen cations on the network properties. In this study, we prepared PECs from sodium alginate and poly[triethyl(4-vinylbenzyl)phosphonium chloride], poly[triethyl(4-vinylbenzyl)ammonium chloride], poly[tri(n-butyl)(4-vinylbenzyl)phosphonium chloride], poly[tri(n-butyl)(4-vinylbenzyl)ammonium chloride], and poly[tris(hydroxypropyl)(4-vinylbenzyl)phosphonium chloride]. These networks were ultracentrifuged to form compact PECs (CoPECs), and their physical properties, chemical composition, and self-healing abilities were studied. In phosphate-buffered saline, the phosphonium polymer networks swelled to a higher degree than their ammonium salt-containing counterparts. However, the viscous and elastic moduli, along with their relaxation times, were quite similar for analogous phosphoniums and ammoniums. The CoPEC networks were loaded with anions including fluorescein, etodolac, and methotrexate, resulting in loading capacities ranging from 5 to 14 w/w % and encapsulation efficiencies from 29 to 93%. Anion release occurred over a period of several days to weeks, with the rate depending largely on the anion structure and polycation substituent groups. Whether the cation was an ammonium or a phosphonium had a smaller effect on the release rates. The cytotoxicities of the networks and polycations were investigated and found to depend on both the network and polycation structure.

Polyelectrolyte Complex Membranes via Salinity Change Induced Aqueous Phase Separation

Polymeric membranes are used on very large scales for drinking water production and kidney dialysis, but they are nearly always prepared by using large quantities of unsustainable and toxic aprotic solvents. In this study, a water-based, sustainable, and simple way of making polymeric membranes is presented without the need for harmful solvents or extreme pH conditions. Membranes were prepared from water-insoluble polyelectrolyte complexes (PECs) via aqueous phase separation (APS). Strong polyelectrolytes (PEs), poly(sodium 4-styrenesulfonate) (PSS), and poly(diallyldimethylammonium chloride) (PDADMAC) were mixed in the presence of excess of salt, thereby preventing complexation. Immersing a thin film of this mixture into a low-salinity bath induces complexation and consequently the precipitation of a solid PEC-based membrane. This approach leads to asymmetric nanofiltration membranes, with thin dense top layers and porous, macrovoid-free support layers. While the PSS molecular weight and the total polymer concentrations of the casting mixture did not significantly affect the membrane structure, they did affect the film formation process, the resulting mechanical stability of the films, and the membrane separation properties. The salt concentration of the coagulation bath has a large effect on membrane structure and allows for control over the thickness of the separation layer. The nanofiltration membranes prepared by APS have a low molecular weight cutoff (<300 Da), a high MgSO4 retention (∼80%), and good stability even at high pressures (10 bar). PE complexation induced APS is a simple and sustainable way to prepare membranes where membrane structure and performance can be tuned with molecular weight, polymer concentration, and ionic strength.

Polyelectrolytes Assembly: A Powerful Tool for Electrochemical Sensing Application

The development of sensing coatings, as important sensor elements that integrate functionality, simplicity, chemical stability, and physical stability, has been shown to play a major role in electrochemical sensing system development trends. Simple and versatile assembling procedures and scalability make polyelectrolytes highly convenient for use in electrochemical sensing applications. Polyelectrolytes are mainly used in electrochemical sensor architectures for entrapping (incorporation, immobilization, etc.) various materials into sensing layers. These materials can often increase sensitivity, selectivity, and electronic communications with the electrode substrate, and they can mediate electron transfer between an analyte and transducer. Analytical performance can be significantly improved by the synergistic effect of materials (sensing material, transducer, and mediator) present in these composites. As most reported methods for the preparation of polyelectrolyte-based sensing layers are layer-by-layer and casting/coating methods, this review focuses on the use of the latter methods in the development of electrochemical sensors within the last decade. In contrast to many reviews related to electrochemical sensors that feature polyelectrolytes, this review is focused on architectures of sensing layers and the role of polyelectrolytes in the development of sensing systems. Additionally, the role of polyelectrolytes in the preparation and modification of various nanoparticles, nanoprobes, reporter probes, nanobeads, etc. that are used in electrochemical sensing systems is also reviewed.

Comparing water-mediated hydrogen-bonding in different polyelectrolyte complexes

All-atom molecular dynamics simulations are used to investigate the polyelectrolyte-specific influence of hydration and temperature on water diffusion in hydrated polyelectrolyte complexes (PECs). Two model PECs were compared: poly(allylamine hydrochloride) (PAH)-poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA)-poly(acrylic acid) (PAA). The findings show that the strength of the hydrogen bonding i.e. polyelectrolyte water interaction has enormous influence on the water mobility, which has implications for PEC structure and properties. A 10-fold difference in the average water diffusion coefficient between PAH-PSS and PDADMA-PAA PECs at the same hydration level is observed. The vast majority of the water molecules hydrating the PDADMA-PAA PECs, for hydrations in the range of 26-38 wt%, are effectively immobilized, whereas for PAH-PSS PECs the amount of immobilized water decreases with hydration. This points to the polyelectrolyte-specific character of the PE-water hydrogen bonding relationship with temperature. PAA-water hydrogen bonds are found to be significantly less sensitive to temperature than for PSS-water. The polyelectrolyte-water interactions, investigated via radial distribution function, hydrogen bond distance and angle distributions, are connected with resulting structure of the PECs. The PDADMA-PAA and PAH-PSS PECs are prepared experimentally and the states of water at different hydration levels is determined using differential scanning calorimetry (DSC). Experiments confirm the differences between PDADMA-PAA and PAH-PSS PECs observed in the theoretical modelling. The results suggest that the initial predictions of the PEC's bonding with water can be based on simple molecular-level considerations.

Versatile and High-Throughput Polyelectrolyte Complex Membranes via Phase Inversion

High-flux filtration membranes constructed through scalable and sustainable methods are desirable for energy-efficient separations. Often, these criteria are difficult to be reconciled with one another. Polymeric membranes can provide high flux but frequently involve organic solvents in processing steps. Solubility of many polymeric membranes in organic media also restricts their implementation in solvent filtration. In the present work, we report a simple and high-throughput aqueous processing approach for polyelectrolyte complex (PEC) membranes with controllable porosity and stability in various aqueous and organic environments. PECs are materials composed of oppositely charged polymer chains that can form solids in aqueous environments, yet which can be dissolved in very specific salt solutions capable of breaking the interpolymer ion pairs. By exploiting the salt-induced dissolution and subsequent reformation of the complex, nano- to microporous films are rapidly synthesized which resemble membranes obtained through conventional solvent-phase inversion techniques. PECs remain stable in organic solvents because of the low dielectric constant of the environment, which enhances electrostatic interactions, making them suitable for a wide range of water and solvent filtration applications. Here, we elucidate how the polymer-phase behavior can be manipulated to exercise morphological control, test membrane performance for water and solvent filtration, and quantify the mechanical stability of PECs in relevant conditions.

Guanidinium Can Break and Form Strongly Associating Ion Complexes

Guanidinium is one of nature's strongest denaturants and is also a motif that appears in several interfacial contexts such as the RGD sequence involved in cell adhesion, cell penetrating peptides, and antimicrobial molecules. It is important to quantify the origin of guanidinium's ion-specific interactions so that its unique behavior may be exploited in synthetic applications. The present work demonstrates that guanidinium ions can both break and form strongly associating ion complexes in a context-dependent way. These insights into guanidinium's behavior are elucidated using polyelectrolyte complexes (PECs), where interpolymer ion pairs between oppositely charged polymers play an important role in determining material stability. Different polycation-polyanion combinations can span a large range of association affinities, where more strongly associating complexes can remain insoluble in concentrated salt solutions and in extreme pH conditions. This high stability is desirable in several application contexts for PECs, but also renders them challenging to process and, therefore, to study since they cannot be dissolved into polymer solutions. Here we demonstrate that guanidinium salts are very effective in dissolving the poly(styrenesulfonate)/poly(allylamine) (PSS:PAH) complex, which has one of the highest reported polycation-polyanion association affinities. We also demonstrate the importance of charge identity in complexation phenomena by functionalizing guanidinium directly into poly(allylamine), resulting in a complex that remains stable under highly denaturing conditions. The model system of PSS:PAH is used to glean insights into guanidinium's denaturing activity, as well as to broadly comment on the nature of ion-specific interactions in charged macromolecules.