Search Results (49)

Biopolymeric films based on chitosan and starch offer an ecological alternative for food protection. Nevertheless, their practical application is often limited by their low mechanical properties and high solubility in aqueous solutions, due to weak interactions between the chains of the biopolymers. One approach to resolve this problem is to obtain biopolymeric films based on (bio)polyelectrolyte complex ((bio)PEC). These films exhibit stronger electrostatic interactions and homogeneous biopolymeric structure. In this study, films based on (bio)PEC were obtained by the casting method, using chitosan and carboxymethyl cassava starch with different degrees of substitution with a biopolymer concentration of 2.5 wt.% at pH = 6. The obtained films were analyzed using the optical and scanning microscopy, color method, ATR-FTIR spectroscopy, thermogravimetry, mechanical analysis under tension, solubility in water, simulated gastric fluid (SGF), and phosphate-buffered saline (PBS) solutions, and contact angle of water. The results demonstrated that the tensile strength and Young’s modulus of films based on (bio)PEC increased by 2–4 times, and the elongation at break by 20% compared to films based on a mixture chitosan and starch. This is due to the increase in the attraction between oppositely charged polyelectrolytes in (bio)PEC films. Additionally, the solubility of (bio)PEC films was reduced by ~40%, 35% and 70% in water, SGF and PBS solutions, respectively, when the carboxymethyl starch with highest degree of substitution was used, and z was near to 1. Full article

The self-consistent field Poisson–Boltzmann framework is applied for analysis of equilibrium partitioning of ampholytic protein-like nanocolloids between buffer solution and weak (pH-sensitive) versus strong polyelectrolyte (polyanionic) brushes with the same net charge per unit area. The position-dependent nanocolloid net charge and the insertion freeenergy profiles are derived as a function of pH and ionic strength in the solution. It is demonstrated that, similar to strong polyelectrolyte brushes, pH-sensitive brushes are capable of the uptake of nanocolloids in the vicinity of the isoelectric point, that is, when the net charge of the colloid in the buffer has either the opposite or the same sign as the ionized monomer units of the brush. At $pI\ge p{K}_{brush}$ and $pH\ge pI$, the particle absorption patterns by similarly (negatively) charged brushes are qualitatively similar in the cases of strong and weak polyelectrolyte brushes, but the freeenergy barrier at the brush periphery is wider for weak than for strong polyelectrolyte brushes, which may cause stronger kinetic hindrance for the nanocolloid uptake by the brush. A decrease in $pH$ below the IEP leads to a monotonic increase in the depth of the insertion freeenergy minimum inside a strong polyelectrolyte brush, whereas for weak polyelectrolyte brushes, a more peculiar trend is predicted: due to competition between the increasing positive charge of the nanocolloid and the decreasing magnitude of the negative charge of the brush, the absorption is weakened at low $pH$. Full article

We study the microstructural properties and state of hydration of aqueous low-molecular-weight poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) solutions and their dependence on polymer concentration and pH by means of detailed atomistic Molecular Dynamics (MD) simulations and experiments. For infinitely dilute solutions with a degree of polymerization of N = 30 at basic pH conditions, no temperature dependence is observed on the overall shape and state of hydration of the polyelectrolyte. This is supported by the experimental component of our work according to which the hydrodynamic radius, R_h, does not change dramatically with temperature. Small, but not drastic, changes are observed for solutions with longer PDMAEMA chains (N = 50, 70, and 110). Although the MD simulations demonstrate that temperature and salt do affect the strength of hydrophobic interactions between PDMAEMA and water, apparently these effects are not strong enough to cause drastic changes to the overall shape of the polymer. MD simulations also reveal that Na⁺ salt ions strongly interact with the oxygen atoms located at the side chain of the polyelectrolyte. While no significant changes in the global shape or state of hydration of the PDMAEMA chain are found, a strong dependence is revealed for the aggregation behavior of the polymer on temperature and salt in slightly more concentrated solutions. A structural transition from a collapsed coil to a stretched conformation is also observed as we move from basic to acidic pH conditions, which is strongly correlated with the degree of chain rigidity as a function of pH. Full article

Polyzwitterion (PZW) hydrogel has excellent marine anti-biofouling performance, but it is difficult to effectively work for a long time in natural seawater due to its weak mechanical strength. In this study, a new natural rubber (NR)-PZW composite hydrogel has been reported for long-term anti-biofouling by simply dispersing NR latex into the poly(sulfobetaine methacrylate) (PSBMA) hydrogel network. First of all, owing to the PZW hydrogel network having an anti-polyelectrolyte effect, this NR-PZW hydrogel can provide outstanding anti-biofouling performance, including broad-spectrum anti-bacteria, anti-algae, and anti-protein properties in marine environments. Furthermore, it has a composited natural rubber nanoparticle with a hydrophilic negatively charged outer protein membrane, which can uniformly disperse in the hydrogel to significantly improve its mechanical properties. Therefore, this composited hydrogel can provide not only highly enhanced tensile strength (0.52 MPa) but also ultra-high breaking elongation (738%), which can effectually resist harsh seawater environments. As a result, the NR-PZW composite hydrogel can achieve excellent anti-biofouling performance for more than 3 months within a real marine environment. This work can provide an excellent, robust polyzwitterionic hydrogel for long-term marine anti-biofouling, which will also inspire new strategies for anti-biofouling materials. Full article

Anionic thermo- and pH-responsive copolymers were synthesized by photoiniferter reversible addition–fragmentation chain transfer polymerization (PI-RAFT). The thermo-responsive properties were provided by oligo(ethylene glycol)-based macromonomer units containing hydrophilic and hydrophobic moieties. The pH-responsive properties were enabled by the addition of 5–20 mol% of strong (2-acrylamido-2-methylpropanesulfonic) and weak (methacrylic) acids. Upon initiation by visible light at 470 nm and in the absence of radical initiators, yields from the ternary copolymers reached 94% in 2.5 h when the process was carried out in continuous flow mode using 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid as a light-sensitive RAFT agent. The polymers were characterized using size exclusion chromatography, IR and NMR spectroscopy, and differential scanning calorimetry. The copolymers featured a sufficiently high molecular weight (93–146 kDa) consistent with theoretical values and satisfactory dispersities in the range of 1.18–1.45. The pH-responsive properties were studied in deionized water, saline, and buffer solutions. Dramatic differences in LCST behavior were observed in strong and weak acid-based polyelectrolytes. The introduction of sulfonic acid units, even in very small amounts, completely suppressed the LCST transition in deionized water while maintaining it in the saline and buffer solutions, with a negligible LCST dependence on the pH. In contrast, the incorporation of weak methacrylic acid demonstrated a pronounced pH dependence. The peculiarities of micelle formation in aqueous solutions were investigated and critical micelle concentrations and their ability to retain pyrene, a hydrophobic drug model, were determined. It was observed that anionic molecular brushes formed small micelles with aggregation numbers of 1–2 at concentrations in the order of 10⁻⁴ mg/mL. These micelles have a high ability to entrap pyrene, which makes them a promising tool for targeted drug delivery. Full article

Weak polyelectrolyte chains are versatile polymeric materials due to the large number of functional groups that can be used in different environmental applications. Herein, one weak polycation (polyethyleneimine, PEI) and two polyanions (poly(acrylic acid), PAA, and poly(sodium methacrylate), PMAA) were directly deposited through precipitation of an inter-polyelectrolyte coacervate onto the silica surface (IS), followed by glutaraldehyde (GA) crosslinking and extraction of polyanions chains. Four core–shell composites based on silica were synthesized and tested for adsorption of lead (Pb²⁺) and nickel (Ni²⁺) as model pollutants in batch sorption experiments on the laboratory scale. The sorbed/desorbed amounts depended on the crosslinking degree of the composite shell, as well as on the type of anionic polyelectrolyte. After multiple loading/release cycles of the heavy metal ions, the maximum sorption capacities were situated between 5–10 mg Pb²⁺/g composite and 1–6 mg Ni²⁺/g composite. The strong crosslinked composites (r = 1.0) exhibited higher amounts of heavy metal ions (Me²⁺) sorbed than the less crosslinked ones, with less PEI on the surface but with more flexible chains being more efficient than more PEI with less flexible chains. Core–shell composites based on silica and weak polyelectrolytes could act as sorbent materials, which may be used in water or wastewater treatment. Full article

Weak bonds usually make macromolecules stronger; therefore, they are often used to enhance the mechanical strength of polymers. Not enough studies have been reported on the use of weak bonds in flame retardants. A water-soluble polyelectrolyte complex composed of polyethyleneimine (PEI), sodium tripolyphosphate (STPP) and melamine (MEL) was designed and utilized to treat bio-based polyamide 56 (PA56) by a simple three-step process. It was found that weak bonds cross-linked the three compounds to a 3D network structure with MEL on the surface of the coating under mild conditions. The thermal stability and flame retardancy of PA56 fabrics were improved by the controlled coating without losing their mechanical properties. After washing 50 times, PA56 still kept good flame retardancy. The cross-linking network structure of the flame retardant enhanced both the thermal stability and durability of the fabric. STPP acted as a catalyst for the breakage of the PA56 molecular chain, PEI facilitated the char formation and MEL released non-combustible gases. The synergistic effect of all compounds was exploited by using weak bonds. This simple method of developing structures with 3D cross-linking using weak bonds provides a new strategy for the preparation of low-cost and environmentally friendly flame retardants. Full article

Silk fibroin (SF) is a protein with many outstanding properties (superior biocompatibility, mechanical strength, etc.) and is often used in many advanced applications (epidermal sensors, tissue engineering, etc.). The properties of SF-based biomaterials may additionally be tuned by SF interactions with other (bio)polymers. Being a weak amphoteric polyelectrolyte, SF may form polyelectrolyte complexes (PECs) with other polyelectrolytes of opposite charge, such as poly(acrylic acid) (PAA). PAA is a widely used, biocompatible, synthetic polyanion. Here, we investigate PEC formation between SF and PAA of two different molecular weights (MWs), low and high, using various techniques (turbidimetry, zeta potential measurements, capillary viscometry, and tensiometry). The colloidal properties of SF isolated from Bombyx mori and of PAAs (MW, overlap concentration, the influence of pH on zeta potential, adsorption at air/water interface) were determined to identify conditions for the SF-PAA electrostatic interaction. It was shown that SF-PAA PEC formation takes place at different SF:PAA ratios, at pH 3, for both high and low MW PAA. SF-PAA PEC’s properties (phase separation, charge, and surface activity) are influenced by the SF:PAA mass ratio and/or the MW of PAA. The findings on the interactions contribute to the future development of SP-PAA PEC-based films and bioadhesives with tailored properties. Full article

A coarse-grained model of linear polyfunctional weak charged biopolymers was implemented, formed of different proportions of acid-base groups resembling the composition of humic substances. These substances are mainly present in dissolved organic matter in natural water. The influence of electrostatic interactions computing methods, factors concerning the structure of the chain, different functional groups, and the ionic strength on polyelectrolytes were studied. Langevin dynamics with constant pH simulations were performed using the ESPResSO package and the Python-based Molecule Builder for ESPResSo (pyMBE) library. The coverage was fitted to a polyfunctional Frumkin isotherm, with a mean-field interaction between charged beads. The composition of the chain affects the charge while ionic strength affects both the charge and the radius of gyration. Additionally, the parameters intrinsic to the polyelectrolyte model were well reproduced by fitting the polyfunctional Frumkin isotherm. In contrast, the non-intrinsic parameters depended on the ionic strength. The method developed and applied to a polyfunctional polypeptide model, that resembles a humic acid, will be very useful for characterizing biopolymers with several acid-base functional groups, where their structure, the composition of the different functional groups, and the determination of the main intrinsic proton binding constants and their proportion are not exactly known. Full article

A large number of experimental studies have demonstrated that globular proteins can be absorbed from the solution by both polycationic and polyanionic brushes when the net charge of protein globules is of the same or of the opposite sign with respect to that of brush-forming polyelectrolyte chains. Here, we overview the results of experimental studies on interactions between globular proteins and polycationic or polyanionic brushes, and present a self-consistent field theoretical model that allows us to account for the asymmetry of interactions of protein-like nanocolloid particles comprising weak (pH-sensitive) cationic and anionic groups with a positively or negatively charged polyelectrolyte brush. The position-dependent insertion free energy and the net charge of the particle are calculated. The theoretical model predicts that if the numbers of cationic and anionic ionizable groups of the protein are approximately equal, then the interaction patterns for both cationic and anionic brushes at equal offset on the “wrong side” from the isoelectric point (IEP), i.e., when the particle and the brush charge are of the same sign, are similar. An essential asymmetry in interactions of particles with polycationic and polyanionic brushes is predicted when fractions of cationic and anionic groups differ significantly. That is, at a pH above IEP, the anionic brush better absorbs negatively charged particles with a larger fraction of ionizable cationic groups and vice versa. Full article

The entropy and charge distributions have been calculated for a simple model of polyelectrolytes attached to the surface of DNA using a field-theoretic method that includes fluctuations to the lowest one-loop order beyond mean-field theory. Experiments have revealed correlation-driven behavior of DNA in charged solutions, including charge inversion and condensation. In our model, the condensed polyelectrolytes are taken to be doubly charged dimers of length comparable to the distance between sites along the phosphate chains. Within this lattice gas model, each adsorption site is assumed to have either a vacancy or a positively charged dimer attached with the dimer oriented either parallel or perpendicular to the double-helix DNA chain. We find that the inclusion of the fluctuation terms decreases the entropy by ∼50% in the weak-binding regime. There, the bound dimer concentration is low because the dimers are repelled from the DNA molecule, which competes with the chemical potential driving them from the solution to the DNA surface. Surprisingly, this decrease in entropy due to correlations is so significant that it overcompensates for the entropy increase at the mean-field level, so that the total entropy is even lower than in the absence of interactions between lattice sites. As a bonus, we present a transparent exposition of the methods used that could be useful to students and others wishing to use this formulation to extend this calculation to more realistic models. Full article

We report the rheological behavior of aqueous solutions of a graft copolymer polyampholyte, constituted of polyacrylic acid (PAA) backbone grafted by Poly(L-lysine) (PAA-b-PLL). The graft copolymer self-assembles in aqueous media, forming a three-dimensional (3D) network through polyelectrolyte complexation of the oppositely charged PAA and PLL segments. Rheological investigations showed that the hydrogel exhibits interesting properties, namely, relatively low critical gel concentration, elastic response with slow dynamics, remarkable extended critical strain to flow, shear responsiveness, injectability, 3D printability and self-healing. Due to the weak nature of the involved polyelectrolyte segments, the hydrogel properties display pH-dependency, and they are affected by the presence of salt. Especially upon varying pH, the PLL secondary structure changes from random coil to α-helix, affecting the crosslinking structural mode and, in turn, the overall network structure as reflected in the rheological properties. Thanks to the biocompatibility of the copolymer constituents and the biodegradability of PLL, the designed gelator seems to exhibit potential for bioapplications. Full article

This article reviews the state of the art of the studies on charge regulation (CR) effects in flexible weak polyelectrolytes (FWPE). The characteristic of FWPE is the strong coupling of ionization and conformational degrees of freedom. After introducing the necessary fundamental concepts, some unconventional aspects of the the physical chemistry of FWPE are discussed. These aspects are: (i) the extension of statistical mechanics techniques to include ionization equilibria and, in particular, the use of the recently proposed Site Binding-Rotational Isomeric State (SBRIS) model, which allows the calculation of ionization and conformational properties on the same foot; (ii) the recent progresses in the inclusion of proton equilibria in computer simulations; (iii) the possibility of mechanically induced CR in the stretching of FWPE; (iv) the non-trivial adsorption of FWPE on ionized surfaces with the same charge sign as the PE (the so-called “wrong side” of the isoelectric point); (v) the influence of macromolecular crowding on CR. Full article

The modification of inorganic surfaces with weak cationic polyelectrolytes by direct deposition through precipitation is a fast approach to generating composites with high numbers of functional groups. The core/shell composites present very good sorption capacity for heavy metal ions and negatively charged organic molecules from aqueous media. The sorbed amount of lead ions, used as a model for priority pollutants such as heavy metals, and diclofenac sodium salt, as an organic contaminant model for emerging pollutants, depended strongly on the organic content of the composite and less on the nature of contaminants, due to the different retention mechanisms (complexation vs. electrostatics/hydrophobics). Two experimental approaches were considered: (i) simultaneous adsorption of the two pollutants from a binary mixture and (ii) the sequential retention of each pollutant from monocomponent solutions. The simultaneous adsorption also considered process optimization by using the central composite design methodology to study the univariate effects of contact time and initial solution acidity with the purpose of enabling further practical applications in water/wastewater treatment. Sorbent regeneration after multiple sorption-desorption cycles was also investigated to assess its feasibility. Based on different non-linear regressions, the fitting of four isotherms (Langmuir, Freundlich, Hill, and Redlich–Peterson models) and three kinetics models (pseudo-first order (PFO), pseudo-second order (PSO), and two-compartment first order (TC)) has been carried out. The best agreement with experiments was found for the Langmuir isotherm and the PFO kinetic model. Silica/polyelectrolytes with a high number of functional groups may be considered efficient and versatile sorbents that can be used in wastewater treatment processes. Full article

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 $\mathrm{pH}$ 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. Full article