Polyelectrolyte Gels Research Articles

Waste refractory brick material added chitosan/oxidized pullulan complex gel production and removal of heavy metals from waste water

Wastewater is a by-product of numerous industrial processes that have been demonstrated to have adverse effects on human and natural health due to the pollutants it contains. The pollutants in these substances are organic or inorganic molecules and heavy metal ions that significantly harm the environment and human health. A variety of techniques have been devised for the removal of heavy metal ions from wastewater. The adsorption process has attracted significant attention due to its straightforward implementation, cost-effectiveness, and the environmentally friendly production of adsorbent materials using biocompatible substances. In this study, the removal of Cu2+ ions from wastewater was conducted using chitosan pullulan, a biocompatible and biodegradable polymer. In addition to chitosan and pullulan, waste refractory materials from a furnace used in iron and steel production were added to these polymer materials to increase the adsorption capacity. The initial step involved grinding the waste refractory brick material. Subsequently, chitosan was dissolved in acetic acid. After that, the refractory material was suspended in this solution, facilitating the formation of hydrogel beads using a NaOH solution. The obtained hydrogels were coated with pullulan to produce polyelectrolyte gel. Pullulan was oxidized to 6-carboxypullulan by the TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) oxidation method and the negatively charged groups in its structure interacted with the positively charged groups in the chitosan structure to produce a complex gel. The chemical structure, morphological analysis, thermal analysis, and water release analysis of the produced waste refractory brick material added chitosan/oxidized pullulan complex gels were examined. The impact of the 6-carboxypullulan coating on the gels’ properties was elucidated. Furthermore, the adsorption of Cu2⁺ was conducted using solutions containing 100, 500, and 1000 ppm Cu2⁺ ions. It has been observed that the material can clean water with over 98% efficiency, even in solutions that exceed the standards set for wastewater. The material’s efficacy in cleaning solutions with concentrations above the standard for wastewater cleaning is evidence of its high performance. Furthermore, the kinetics and isotherm of the adsorption reaction were examined. The kinetics were determined to be consistent with the Pseudo Second Order (chemical reaction controlled) and aligned with the Langmuir and Freundlich Isotherm (mixed adsorption occurred on homogeneous and heterogeneous surfaces).

Preparation and Characterization of Preformed Polyelectrolyte and Polyampholyte Gel Particles for Plugging of High-Permeability Porous Media.

Excessive reservoir water poses significant challenges in the oil and gas industry by diminishing hydrocarbon recovery efficiency and generating environmental and economic complications. Conventional polymer flooding techniques, although beneficial, often prove inadequate under conditions of elevated temperature and salinity, highlighting the need for more resilient materials. In this research, two types of acrylamide-based preformed particle gels (PPGs) were synthesized, as follows: polyelectrolyte and polyampholyte. These PPGs were engineered to improve plugging efficiency and endure extreme reservoir environments. The polyelectrolyte gels were synthesized using acrylamide (AAm) and sodium acrylate (SA), while the polyampholyte gels incorporated AAm, AMPS, and APTAC, with crosslinking achieved through MBAA. The swelling properties, modulated by temperature, salinity, and pH, were evaluated using the Ritger-Peppas and Yavari-Azizian models. The mechanical characteristics and surface morphology of the gels were analyzed using SEM and BET techniques. In sand pack experiments designed to mimic high-permeability reservoirs, the inclusion of 0.5 wt.% of fine PPGs substantially reduced water permeability, outperforming traditional hydrogels. Notably, the polyampholyte PPGs demonstrated superior resilience and efficacy in plugging. However, the experiments were limited by the low test temperature (25 °C) and brine salinity (26.6 g/L). Future investigations will aim to apply these PPGs in high-temperature, fractured carbonate reservoirs.

Fabrication, characterization, and antibacterial properties of sodium alginate/chito-oligosaccharide gel beads

Developing sodium alginate (SA)/chito-oligosaccharide (COS) beads with antibacterial properties is highly appealing for food industrial applications. Herein, in this paper, the different viscosity-average molecular weight of SA/COS polyelectrolyte (SCP) gel beads were prepared by adding a low concentration of calcium ions (Ca2+) as crosslinker and their size, mechanical property, microscopic morphology, elemental composition, and thermal stability of the gel beads were characterized. Furthermore, the morphological changes of gel beads in in vitro simulated digestion were investigated. A time-dependent co-culture assay was used to explore the antibacterial effect of gel beads on Staphylococcus aureus and Escherichia coli. The results showed that as the viscosity-average molecular weight (Mv) of SA decreased, the bursting strength faded, the thermal stability decreased, and the swelling index increased. Similarly, increasing the Ca2+ content could also regulate the characteristics of the gel beads, such as diminishing size, increasing the bursting strength, making the surfaces rough, and producing the network structure in the cross-section. In addition, Ca2+ competes with COS in cross-linking with the carboxyl groups of SA, which leads to a decrease in the content of N element on the surfaces of the gel beads and to a weakening of the antibacterial effect. After the addition of COS, the gel beads obtained antimicrobial effects and a denser cross-sectional structure. The present study could provide a theoretical basis for the design and development of novel gel foods in the food industry.

Geometrically Scalable Iontronic Memristors: Employing Bipolar Polyelectrolyte Gels for Neuromorphic Systems.

Iontronics that are capable of mimicking the functionality of biological systems within an artificial fluidic network have long been pursued for biomedical applications and ion-based intelligence systems. Here, we report on facile and robust realization of iontronic bipolar memristors featuring a three-layer polyelectrolyte gel structure. Significant memristive hysteresis of ion currents was successfully accomplished, and the memory time proved geometrically scalable from 200 to 4000 s. These characteristics were enabled by the ion concentration polarization-induced rectification ratio within the polyelectrolyte gels. The memristors exhibited memory dynamics akin to those observed in unipolar devices, while the bipolar structure notably enabled prolonged memory time and enhanced the ion conductance switching ratio with mesoscale (10-1000 μm) geometry precision. These properties endow the devices with the capability of effective neuromorphic processing with pulse-based input voltage signals. Owing to their simple fabrication process and superior memristive performance, the presented iontronic bipolar memristors are versatile and can be easily integrated into small-scale iontronic circuits, thereby facilitating advanced neuromorphic computing functionalities.

Control of phase transition temperature of thermoresponsive poly(ionic liquid) gels and application to a water purification system using these gels with polydopamine

Demand for purified water is increasing as the population grows. Water purification systems combining thermoresponsive materials and adsorbents that can be used in areas without electricity or other energy infrastructure have begun to be reported. The thermoresponsive hydrogel used in this system must be able to adsorb and desorb a large amount of water, react drastically to temperature, and have a phase transition temperature that can be easily changed depending on the environment in which it is used. In this study, thermoresponsive ionic liquid-derived polyelectrolyte gels (poly(IL) gels) were prepared as lower critical solution temperature (LCST)-type thermoresponsive materials. As a result, poly(IL) gel, which exhibits drastic swelling/shrinking and reversible adsorption/desorption of a large amount of water (44 times the amount of water to its own weight), was successfully fabricated. The phase transition temperature can be controlled between 20 °C and 35 °C by copolymerizing ILs with different structures. Then, polydopamine (PDA), which can serve as a photothermal conversion material as well as an adsorbent for contaminants, was polymerized on the surface of the poly(IL) gel. In addition to its role as an adsorbent, PDA can also serve as a photothermal conversion material. First, a poly(IL)-based material with PDA is swollen in contaminated water. Subsequent irradiation with sunlight induces an LCST-type phase transition behavior by converting light energy into heat. Finally, the contaminants remain adsorbed, and only the water is discharged from the material to obtain purified water. Using this material, we were able to show that organic dyes are removed from aqueous solutions with high efficiency using sunlight.

Ion Partition in Polyelectrolyte Gels and Nanogels.

Polyelectrolyte gels provide a load-bearing structural framework for many macroscopic biological tissues, along with the organelles within the cells composing tissues and the extracellular matrices linking the cells at a larger length scale than the cells. In addition, they also provide a medium for the selective transportation and sequestration of ions and molecules necessary for life. Motivated by these diverse problems, we focus on modeling ion partitioning in polyelectrolyte gels immersed in a solution with a single type of ionic valence, i.e., monovalent or divalent salts. Specifically, we investigate the distribution of ions inside the gel structure and compare it with the bulk, i.e., away from the gel structure. In this first exploratory study, we neglect solvation effects in our gel by modeling the gels without an explicit solvent description, with the understanding that such an approach may be inadequate for describing ion partitioning in real polyelectrolyte gels. We see that this type of model is nonetheless a natural reference point for considering gels with solvation. Based on our idealized polymer network model without explicit solvent, we find that the ion partition coefficients scale with the salt concentration, and the ion partition coefficient for divalent ions is higher than for monovalent ions over a wide range of Bjerrum length (lB) values. For gels having both monovalent and divalent salts, we find that divalent ions exhibit higher ion partition coefficients than monovalent salt for low divalent salt concentrations and low lB. However, we also find evidence that the neglect of an explicit solvent, and thus solvation, provides an inadequate description when compared to experimental observations. Thus, in future work, we must consider both ion and polymer solvation to obtain a more realistic description of ion partitioning in polyelectrolyte gels.

Breakdown of electroneutrality in polyelectrolyte gels

AbstractMathematical models of polyelectrolyte gels are often simplified by assuming the gel is electrically neutral. The rationale behind this assumption is that the thickness of the electric double layer (EDL) at the free surface of the gel is small compared to the size of the gel. Hence, the thin-EDL limit is taken, in which the thickness of the EDL is set to zero. Despite the widespread use of the thin-EDL limit, the solutions in the EDL are rarely computed and shown to match to the solutions for the electrically neutral bulk. The aims of this paper are to study the structure of the EDL and establish the validity of the thin-EDL limit. The model for the gel accounts for phase separation, which gives rise to diffuse interfaces with a thickness described by the Kuhn length. We show that the solutions in the EDL can only be asymptotically matched to the solutions for an electrically neutral bulk, in general, when the Debye length is much smaller than the Kuhn length. If the Debye length is similar to or larger than the Kuhn length, then phase separation can be initiated in the EDL. This phase separation spreads into the bulk of the gel and gives rise to electrically charged layers with different degrees of swelling. Thus, the thin-EDL limit and the assumption of electroneutrality only generally apply when the Debye length is much smaller than the Kuhn length.

Polyelectrolyte Gels: Swelling and Deswelling upon Nonlinear Deformations

Polyelectrolyte Gels: Swelling and Deswelling upon Nonlinear Deformations

Charge Regulation of Polyelectrolyte Gels: Swelling Transition

We study the effects of charge-regulated acid/base equilibrium on the swelling of polyelectrolyte gels by considering a combination of the Poisson–Boltzmann theory and a two-site charge-regulation model based on the Langmuir adsorption isotherm. By exploring the volume change as a function of salt concentration for both nanogels and microgels, we identify conditions where the gel volume exhibits a discontinuous swelling transition. This transition is driven exclusively by the charge-regulation mechanism and is characterized by a closed-loop phase diagram. Our predictions can be tested experimentally for polypeptide gels.

The Dynamics of a Collapsing Polyelectrolyte Gel

The Dynamics of a Collapsing Polyelectrolyte Gel

Nanoscale imaging of tumor cell exosomes by expansion single molecule localization microscopy (ExSMLM)

Tumor cell exosomes play a very important role in the process of tumor cell proliferation and metastasis. However, due to the nanoscale size and high heterogeneity of exosomes, in-depth understanding of their appearance and biological characteristics is still lacking. Expansion microscopy (ExM) is a method that embeds biological samples in a swellable gel to physically magnify the samples to improve the imaging resolution. Before the emergence of ExM, scientists had invented several super-resolution imaging techniques that could break the diffraction limit. Among them, single molecule localization microscopy (SMLM) usually has the best spatial resolution (20–50 nm). However, considering the small size of exosomes (30–150 nm), the resolution of SMLM is still not high enough for detailed imaging of exosomes. Hence, we propose a tumor cell exosomes imaging method that combines ExM and SMLM (i.e. Expansion SMLM, denoted as ExSMLM), which can realize the expansion and super-resolution imaging of tumor cell exosomes. In this technique, immunofluorescence was first performed to fluorescently label the protein markers on the exosomes, then the exosomes were polymerized into a swellable polyelectrolyte gel. The electrolytic nature of the gel made the fluorescently labeled exosomes undergo isotropic linear physical expansion. The expansion factor obtained in the experiment was about 4.6. Finally, SMLM imaging of the expanded exosomes was performed. Owing to the improved resolution of ExSMLM, nanoscale substructures of closely packed proteins were observed on single exosomes, which has never been achieved before. With such a high resolution, ExSMLM would have a great potential in detailed investigation of exosomes and exosome-related biological processes.

Modeling the Phase Transition in Hydrophobic Weak Polyelectrolyte Gels under Compression.

One of the emerging water desalination techniques relies on the compression of a polyelectrolyte gel. The pressures needed reach tens of bars, which are too high for many applications, damage the gel and prevent its reuse. Here, we study the process by means of coarse-grained simulations of hydrophobic weak polyelectrolyte gels and show that the necessary pressures can be lowered to only a few bars. We show that the dependence of applied pressure on the gel density contains a plateau indicating a phase separation. The phase separation was also confirmed by an analytical mean-field theory. The results of our study show that changes in the pH or salinity can induce the phase transition in the gel. We also found that ionization of the gel enhances its ion capacity, whereas increasing the gel hydrophobicity lowers the pressure required for gel compression. Therefore, combining both strategies enables the optimization of polyelectrolyte gel compression for water desalination purposes.

A computational framework for the swelling dynamics of mucin-like polyelectrolyte gels

A computational framework for the swelling dynamics of mucin-like polyelectrolyte gels

Nonreciprocal oscillations of polyelectrolyte gel filaments subject to a steady and uniform electric field

Soft actuators typically require time-varying or spatially modulated control to be operationally effective. The scope of the present paper is to show, theoretically and experimentally, that a natural way to overcome this limitation is to exploit mechanical instabilities. We report experiments on active filaments of polyelectrolyte (PE) gels subject to a steady and uniform electric field. A large enough intensity of the field initiates the motion of the active filaments, leading to periodic oscillations. We develop a mathematical model based on morphoelasticity theory for PE gel filaments beating in a viscous fluid, and carry out the stability analysis of the governing equations to show the emergence of flutter and divergence instabilities for suitable values of the system’s parameters. We confirm the results of the stability analysis with numerical simulations for the nonlinear equations of motion to show that such instabilities may lead to periodic self-sustained oscillations, in agreement with experiments. The key mechanism that underlies such behaviour is the capability of the filament to undergo active shape changes depending on its local orientation relative to the external electric field, in striking similarity with gravitropism, the mechanism that drives shape changes in plants via differential growth induced by gravity. Interestingly, the resulting oscillations are nonreciprocal in nature, and hence able to generate thrust and directed flow at low Reynolds number. The exploitation of mechanical instabilities in soft actuators represents a new avenue for the advancement in engineering design in fields such as micro-robotics and micro-fluidics.

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.

Simulations Explain the Swelling Behavior of Hydrogels with Alternating Neutral and Weakly Acidic Blocks

We used computer simulations to study the equilibrium swelling of weak (pH-responsive) polyelectrolyte hydrogels as a function of pH and salt concentration in a supernatant solution. Our simulations were designed to represent recently synthesized gels composed of tetrapoly(acrylic acid) and tetrapoly(ethylene glycol) stars. To model the ionization equilibrium of the weak acid groups and the exchange of small ions with the reservoir, we applied the recently developed grand-reaction Monte Carlo method. We showed that the ionization of these gels as a function of the pH significantly deviates from the ideal Henderson–Hasselbalch equation due to two main contributions: (1) electrostatic interactions and (2) Donnan partitioning of small ions. The first contribution dominates in the gels composed of alternating neutral and acidic blocks, contrasting with our previous observations that both contributions are comparably strong in hydrogels composed of homogeneously distributed weak acid groups. We also critically examined the counterion condensation argument, previously invoked to explain why the experimentally observed swelling was lower than predicted by theory. Thus, a detailed analysis of the simulations allowed us to understand which of the above effects dominates in different systems and why, thereby allowing us to identify the correct physical origin of the deviations from ideal swelling. Such an understanding is important not only for correctly interpreting experimental measurements but also for designing polyelectrolyte gels tailored to exhibit specific swelling response to pH and salt concentration.

Hyperbranched polyamidoamine-chitosan polyelectrolyte gels crosslinking by polyacrylic acid and alginate for removal of anionic dyes

Difficult-to-treat wastewater discharged from printing and dyeing industries has posed an environmental risk. Three-dimensional porous structures exhibit excellent dye adsorption properties. In this study, chitosan (CTS) was introduced in situ on polyamidoamine (PAMAM). Then, the composite gels containing sodium alginate (SA)/CTS in situ grown on PAMAM/polyacrylic acid (SCPP-Gn (n = 0,1,2,3)) were prepared with a controllable structure using a semi-dissolution acidification sol-gel transition method (SD-A-SGT). Owing to the stronger mechanical strength and larger specific surface area, the SCPP-G2 composite gel has the higher stability and the greater adsorption capacity for dyes, with maximum adsorption capacities of 325.21 mg/g for rose bengal (RB) and 222.40 mg/g for sunset yellow (SY). The mechanism study showed that the adsorption toward RB and SY was dominated by chemical adsorption. After five adsorption-desorption cycles, it was found that 70 % of the initial adsorption capacity could be retained. Our study demonstrates the great potential of the SCPP-G2 composite gel as an adsorbent in water treatment.

Semi-unzipping of chitosan-sodium alginate polyelectrolyte gel for efficient capture of metallic mineral ions from tannery effluent

Recently, a rigorous discharge limit for metallic mineral ions containing effluent has been established to ensure the leather-making industry’s long-term sustainability. Nevertheless, the existing intelligent biomass sodium alginate (SA) and chitosan (CS) adsorbents in remediating metal ions-contaminated effluent have been plagued by their compact structures as well as more carboxyl groups being occupied and masked. Herein, chitosan-sodium alginate polyelectrolyte gel with a semi-unzipping system (CS-SA(GTA0.3)) was rationally constructed with the help of unzipper glutaraldehyde (GTA) intruding. Half of the electrostatic interaction and half of the Schiff base reaction were the essence of the semi-unzipping effect. Such character endowed the CS-SA(GTA0.3) composite with a loose, porous, and amorphous structure, thus improving its swelling behavior and exposition of more functional carboxyl groups, which demonstrated an efficient capture ability of commonly metallic mineral ions (Cr3+, Fe3+, Al3+, and Zr4+) compared with the traditional CS-SA gel bearing a compact structure. Based on pseudo-second-order kinetic model fitting results, the equilibrium capture capacity of CS-SA(GTA0.3) for Cr3+, Fe3+, Al3+, and Zr4+ was 96.42, 95.95, 85.49, and 151.57 mg/g. Even in a large number of suspended solids, the semi-unzipping of chitosan-sodium alginate polyelectrolyte gel was still capable of capturing 149.69 mg/g of Cr3+ from tannery effluent, demonstrating the excellent potential for practical application.