Polyelectrolyte Adsorption Research Articles

Theory and experiment of chain length effects on the adsorption of polyelectrolytes onto spherical particles: the long and the short of it.

We study here the role of polyelectrolyte chain length, that is number of repeat units (mers), in the competitive adsorption of a simple model polyanion, poly(acrylic acid), onto 85 nm spherical silica particles capped with a model polycation, poly(allylamine hydrochloride). Performing fluorescence spectroscopy experiments, we measured chain-length dependence of dilute aqueous polyelectrolyte adsorption, at full surface coverage, onto an oppositely charged polyelectrolyte overtop spherical silica nanoparticles (10-3 g L-1). Preferential adsorption was determined by comparing the characteristic fluorescence intensities of the two fluorophore-labeled and narrowly disperse polyacrylic acid samples (NMA-PAA450k and Dan-PAA2k) of 450k- and 2k-molecular weight (6250- and 28-mers), respectively. To compare and validate experimental results, a lattice model was developed for computing the probabilities of the different arrangements of two polymer chain lengths of polyacrylic acid on the surface of the silica nanosphere. We then determined which numbers of long and short adsorbed chains corresponded to the most configurations in our model. Both spectroscopic experiment results and the combinatorial model demonstrated that there is an entropic preference for complete adsorption of the longer 450k polyacrylic acid chain vs. 2k. This study provides insights on entropy driven chain-length dependence of polyelectrolyte adsorption onto spherical nanoparticle surfaces for directing and optimizing their layer-by-layer self-assembly in organic films.

Mathematical modelling of transport of single and double charged ions through layered ion exchange membranes

Monovalent selectivity of ion exchange membranes is important for such areas as hardness removal, stabilization of mining waters and fining of foods. The modern approach to the creation of monovalent selective ion exchange membranes is layer-by-layer adsorption of polyelectrolytes onto a membrane surface, which showed significant improvement in comparison with previously used approaches, most notably in comparison with the creation of a single thick layer of polyelectrolyte. Despite the practical importance, the mechanism of improvement in selectivity is unknown. Knowledge of the distribution of ions within the membrane and adjacent layers of solution would answer the question of the mechanism, but such information is unobtainable through the experiment. Hence we develop the 1D mathematical model based on Nernst-Planck-Poisson equations to describe the fluxes and the concentrations of single and double charged ions, as well as potential drop / current density through the ion exchange membrane. In this paper we formulate the problem and list experimentally determined parameters. Results of calculations will be compared with experimentally recorded I-V and V-t curves.

Polyelectrolyte Complexes of Chitosan with Sodium Carboxymethyl Cellulose in Water–Alcohol Media and Microcapsules Based on Them

The formation of polyelectrolyte complexes between natural polysaccharides, chitosan and carboxymethyl cellulose sodium salt, in water–ethanol media was studied. It was found that, with an increase in the content of ethanol in the solvent to 50 vol %, the intensity of the Coulomb interactions between oppositely charged groups of polyelectrolytes gradually weakens and, as a consequence, the complexing ability of polymers decreases. The layer-by-layer adsorption of polyelectrolytes onto calcium carbonate microparticles yielded microcapsules with a different number of layers. An approach is proposed to control the permeability of polyelectrolyte microcapsule walls by changing the solvent polarity upon addition of 50 vol % ethanol to an aqueous solution. Microcapsules with a high efficiency of inclusion of a model drug substance, acetylsalicylic acid, were obtained using this approach.

Non-monotonic behavior of weak-polyelectrolytes adsorption on a cationic surface: A Monte Carlo simulation study

In this work, the weak polyelectrolyte (PE) adsorption on a strong cationic surface is studied with constant pH Monte Carlo simulations using a coarse-grained model. When a large number of PE chains is added to the system, the PE adsorbed amount vs pH curve exhibits a non-monotonic behavior, with the appearance of a maximum close to the intrinsic pKa0-value of the PE titratable groups. The apparent pKa-value of the PE chains shows a non-trivial tendency depending on the pH-value and the surface coverage degree. In increasing the pH-value, the small anions that accompany the cationic surface are replaced by PE chains and small cations. For pH>pKa0+1, an evident charge reversion of surface is observed. These results are explained analyzing the interplay between the attractive and repulsion electrostatic interactions between the different components of the system (inter- and intra-charged monomers of PE chains, the strong cationic surface and small ions) and their effects on the PE chain ionization.

In-situ monitoring of polyelectrolytes adsorption kinetics by electrochemical impedance spectroscopy: Application in fabricating nanofiltration membranes via layer-by-layer deposition

We herein report a novel approach based on electrochemical impedance spectroscopy (EIS) for in-situ monitoring of the adsorption kinetics in preparing polyelectrolyte multilayer nanofiltration membranes using layer-by-layer (LbL) deposition. Unlike existing methods for monitoring adsorption kinetics, this new approach is non-destructive and applicable to various substrates (as it does not use the substrate as a sensor). The model nanofiltration membrane used in this study is prepared by alternate depositions of Poly(diallyldimethylammonium chloride) and Poly(sodium 4-styrenesulfonate) on a polyacrylonitrile ultrafiltration membrane as the support. In each deposition step, the EIS measurements yield two important parameters, including the interfacial layer solution resistance and the film resistance, for probing the extent of polyelectrolyte deposition and membrane performance. The extent of polyelectrolyte deposition as probed by the EIS measurements is well corroborated by independent measurements of the membrane surface potential and the nanofiltration performance including water permeability and Na2SO4 rejection. This EIS-based approach enables the optimization of membrane fabrication using LbL deposition by conveniently identifying the minimum deposition time required to attain surface saturation.

Influence of charge density and ionic strength on diallyldimethylammonium chloride (DADMAC)-based polyelectrolyte multilayer membrane formation

Although the layer-by-layer technique to produce nanofiltration membranes has been studied frequently, the influence of ionic strength of the coating solution in relation to the charge density lacks understanding. Here, we investigate the effect of both parameters systematically by comparing two strong polyelectrolyte sets: poly(acrylamide-co-diallyldimethylammoniumchloride) (P(AM-co-DADMAC))/ poly(sodium-4-styrenesulfonate) (PSS) and polydiallyldimethylammoniumchloride (PDADMAC)/PSS, with a polycation charge density of 32 and 100%, respectively. The influence of the charge density and the ionic strength during layer formation (5∙10−5 M – 1 M) has been researched in terms of polyelectrolyte adsorbance, membrane surface charge, and filtration performance. A low charge density is a limiting parameter for polyelectrolyte adsorption and retention, due to limited uptake of two bilayers. Consequently, it is impossible to form NF membranes with solely P(AM-co-DADMAC)/PSS layers. For high charge density polyelectrolytes, increasing the ionic strength is crucial to increase PE adsorption and MgSO4 retention. However, there is an opposite effect of ionic strength and charge density on membrane surface charge and pure water permeability. An increasing ionic strength results in a decreasing surface charge and increasing permeability for P(AM-co-DADMAC)-based membranes, and vice versa for PDADMAC. These opposite trends undoubtedly show the importance of simultaneously taking into account the charge density and ionic strength.

Surface charge density and diffuse layer properties of highly defined 2:1 layered silicate platelets

Clays are not only ubiquitous in nature, but they are also used in huge quantities in a broad range of industrial applications, such as thixotropic drilling fluids, ore pelletizers, waste disposal sealants, or fillers in polymer nanocomposites. In order to model environmental processes or to design new materials on a rational base, it is of prime importance to determine and possibly modify the interfacial properties of clay platelets at the solid/electrolyte interface. In this context, the fundamental question rises how far the stoichiometric interlayer charges as determined by the composition of the silicate layer correlates with the diffuse double-layer properties. Here, this question is addressed by means of a series of purposely synthesized sodium 2:1 layered silicates with defined composition and hence interlayer charge densities, respectively. Platelets of layered silicates of large enough diameter to perform AFM colloidal probe measurements were produced by melt synthesis. For comparison also, a natural muscovite mica has been included in this study. The diffuse layer properties in electrolyte solution have been determined by direct force measurements using the colloidal probe AFM technique and by electrokinetic measurements, respectively. We find that the diffuse layer potential decreases with increasing interlayer charge of the 2:1 layered silicates. This counterintuitive finding is attributed to ion adsorption and was further corroborated by determining the quantitative adsorption of polyelectrolytes, namely poly(amidoamine) dendrimers.Graphical abstract

Critical adsorption of multiple polyelectrolytes onto a nanosphere: splitting the adsorption-desorption transition boundary.

Employing extensive Monte Carlo computer simulations, we investigate in detail the properties of multichain adsorption of charged flexible polyelectrolytes (PEs) onto oppositely charged spherical nanoparticles (SNPs). We quantify the conditions of critical adsorption-the phase-separation curve between the adsorbed and desorbed states of the PEs-as a function of the SNP surface-charge density and the concentration of added salt. We study the degree of fluctuations of the PE-SNP electrostatic binding energy, which we use to quantify the emergence of the phase subtransitions, including a series of partially adsorbed PE configurations. We demonstrate how the phase-separation adsorption-desorption boundary shifts and splits into multiple subtransitions at low-salt conditions, thereby generalizing and extending the results for critical adsorption of a single PE onto the SNP. The current findings are relevant for finite concentrations of PEs around the attracting SNP, such as the conditions for PE adsorption onto globular proteins carrying opposite electric charges.

Influence of Charge Regulation and Charge Heterogeneity on Complexation between Polyelectrolytes and Proteins.

We employed a combination of single chain in mean field methodology and constant pH Monte Carlo framework to compare the influence of charge regulation and charge heterogeneity on the adsorption and bridging characteristics of polyelectrolytes in solution on proteins. By adopting a coarse-grained representation of the proteins as spherical particles and embedding a simple framework for probing charge heterogeneities, we probe the influence of charge patches, net charge of the particle, the ratio of positive to negative charges on the proteins on the net adsorption, and bridging probabilities of polyelectrolytes. Our results demonstrate that charge regulation increases the probability of bridging between two particles when compared to proteins with the same fixed charge. The influence of charge regulation first increases and then decreases with an increase in the number of charge patches of proteins. An increase in the ratio of positive to negative charges and the net charge of the protein are also seen to increase the propensity for polyelectrolyte adsorption and bridging.

Functionalized layer-by-layer assembled film with directional 5-fluorouracil release to target colon cancer.

The objective of this work was to prepare and characterize pH-sensitive capsule containing functionalized layer-by-layer (LbL) assembled polymeric film with directional drug release and evaluate its effectiveness against colon cancer. 5-Fluorouracil (5FU) loaded LbL film was prepared by sequential adsorption of chitosan and alginate polyelectrolytes. This LbL film was coated with polycaprolactone (PCL, 95% w/w) as a backing layer to restrict 5FU release on one-side. The other side constituted the folic acid conjugated chitosan layer for cancer targeting. This film was encapsulated into a gelatin capsule coated with pH-sensitive Eudragit S100. 5FU loaded LbL film was characterized for physical and mechanical properties. Mucoadhesion studies performed using excised rabbit colon showed that chitosan-side of LbL film adhered with significantly (p<0.05) greater strength compared with PCL-side. Non-everted rat colon-sac model and open colon membrane model studies showed greater permeation of 5FU across the colon wall when adhered to chitosan-side of LbL film compared with PCL-side of the film. Cell monolayer and 3D-spheroid model studies using Caco-2 and COLO 320DM colorectal cancer cells showed significant (p<0.05) growth inhibition by 5FU loaded LbL film compared with free 5FU solution. In conclusion, pH-sensitive capsule containing 5FU loaded LbL film can be developed to target colorectal cancer for regional drug delivery.

Probing Polyelectrolyte Adsorption in Charged Nanochannels by Streaming Potential Measurements.

It remains a great experimental challenge to obtain quantitative information on the polyelectrolyte (PE) behavior confined in charged nanoporous materials. Here, we propose an original approach using transverse streaming potential measurements (TSPMs), an efficient technique providing information on the electrical surface properties of nanoporous materials through the ζ-potential determination. We conduct TSPMs within the thin double-layer approximation on a model system composed of individual nanochannels, a nanoporous anodic aluminum oxide (AAO) membrane, filled with a well-known PE,sodium polystyrenesulfonate (NaPSS). We demonstrate that TSPMs can provide the AAO ζ-potential under different experimental conditions and monitor the PE penetration in AAO with positive or negative surface charge. On the positive surface, the PE irreversibly adsorbs, while it does not when the surface is negatively charged, indicating the electrostatic nature of the PE adsorption. In the context of experimental limitations to investigate PE behavior on concave surfaces, this study shows that the TSPM is suitable to extract quantitative information and can be exploited to gain an understanding of the PE adsorption and desorption in a confined medium.

NMR studies of self-organization behavior of hydrophobically functionalized poly(4-styrenosulfonic-co-maleic acid) in aqueous solution

Specially designed hydrophobically functionalized polyelectrolytes (HF-PEs) provide an important area of study, their unique properties being similar to those of biological systems, among them the strong tendency toward self-organization in aqueous solution caused by a combination of electrostatic and hydrophobic forces. A significant issue in this context is the comprehensive physicochemical characterization of adsorption and aggregation properties of these amphiphilic polyelectrolytes. The main aim of the present research has been to synthesize and study aggregation properties in an aqueous solution of the hydrophobically functionalized poly(4-styrenosulfonic-co-maleic acid) (PSS/MA) with a differing degree of hydrophobization, side-chain lengths and pH-labile moiety type (i.e., ester (PSS/MA-g-CnOH) or secondary amide (PSS/MA-g-CnNH2) groupings). Comprehensive analysis of self-assembly properties, including utilization of high-resolution NMR techniques (diffusion-ordered and nuclear Overhauser effect spectroscopies, T1 and T2 relaxometry) and dynamic light scattering, revealed a strong dependence on concentration and type of labile linking group (ester or amide). It has been noticed that more rigid amide bonds hinder the formation of highly organized structures in aqueous systems compared to polyelectrolytes with ester moieties. Our results were supported by molecular modeling. The performed studies enabled us to obtain crucial information about the hydrophobized PEs structure and its influence on aggregation properties that are crucial for their application in pH-sensitive nanosized thin films and nanocarriers.

Regulation of the stability of kaolin dispersion in the salt media by acrylamide copolymers

As a result of the study of the effect of acrylamide copolymers on the stability of the kaolin salt dispersion, it was shown that the aggregation of solid phase particles and the deposition rate of flocs increases when polymers with different adsorption activity are alternately dosed to a disperse system. The adsorption limit, adsorption capacity and adsorption constant are at maximum for alternate adsorption of anionic polyelectrolyte on a cationic polyelectrolyte layer. High adsorption capacity and interaction energy of polymers with the surface and with each other leads to an improvement in flocculation and to the increase in the deposition rate of the kaolin-salt dispersion by 1.3–1.5 times.

Effect of adsorbed polyelectrolytes on the interactions and elasticity of charged surfactant bilayers

We present studies on the structure of complexes of the cationic, bilayer-forming surfactant, didodecyldimethylammonium bromide (DDAB), and the anionic polyelectrolyte sodium polyacrylate (PAANa). In the presence of uncomplexed polyelectrolyte in the coexisting aqueous solution, these complexes are found to exhibit a swelling transition followed by a deswelling transition on increasing the salt concentration. Lamellar structures with low periodicities occur at both low and high salt concentrations, which are stabilized by polymer bridging and van der Waals attraction, respectively. The swollen complex found at intermediate salt concentrations forms the sponge phase. Our results reveal that polyelectrolyte adsorption on bilayers has a profound effect on inter-bilayer interactions. The polymer-induced interaction changes from being attractive to repulsive as the surface coverage increases on increasing the salt concentration. Our results also confirm that polymer adsorption alters the elastic moduli of the bilayer, in agreement with earlier theoretical predictions.

Direct Measurement of Charge Reversal on Lipid Bilayers Using Heterodyne-Detected Second Harmonic Generation Spectroscopy.

We report the detection of charge reversal induced by the adsorption of an aqueous cationic polyelectrolyte, poly(allylamine hydrochloride) (PAH), to supported lipid bilayers (SLBs) used as idealized model biological membranes. Through the use of an α-quartz reference crystal, we quantify the total interfacial potential at the interface in absolute units using heterodyne-detected second harmonic generation (HD-SHG) as an optical voltmeter. This quantification is made possible by isolating the phase-shifted potential-dependent third-order susceptibility from other contributions to the total SHG response. We detect the sign and magnitude of the surface potential and the point of charge reversal at buried interfaces without prior information or complementary data. Isolation of the second-order susceptibility contribution from the overall SHG response allows us to directly characterize the Stern and diffuse layers over single-component SLBs. We apply the method to SLBs formed from three different zwitterionic lipids having different gel-to-fluid phase transition temperatures (Tm's). We determine whether the surface potential changes with the physical phase state (gel, transitioning, or fluid) of the SLB. Furthermore, we incorporate 20% of negatively charged lipids to the zwitterionic SLB to investigate how the surface potential and the second-order nonlinear susceptibility χ(2) change with surface charge.

Overcharging of polyelectrolyte complexes: an entropic phenomenon.

Overcharging in complex coacervation, in which a polyelectrolyte complex coacervate (PEC) initially containing equal moles of the cationic and anionic monomers absorbs a large excess of one type of polyelectrolyte species, is predicted using a recently developed thermodynamic model describing complexation through a combination of reversible ion binding on the chains and long-range electrostatic correlations. We show that overcharging is favored roughly equally by the translational entropy of released counterions and the binding entropy of polyelectrolytes in the polyelectrolyte complex, thus helping resolve competing explanations for overcharging in the literature. We find that the extent of overcharging is non-monotonic in the concentration of added salt and increases with both strength of ion-pairing between polyions and chain hydrophobicity. The predicted extent of overcharging of the PEC is directly compared with that of multilayers made of poly(diallyldimethylammonium), PDADMA, and poly(styrene-sulfonate), PSS, overcompensated by the polycation in two different salts: KBr and NaCl. Accounting for the specificity of salt ion interactions with the polyelectrolytes, we find good qualitative agreement between theory and experiment.

Polyelectrolyte adsorption in single small nanochannel by layer-by-layer method

Nanochannel size control by using layer-by-layer (LBL) deposition of polyelectrolytes is experimentally studied in single hard Polydimethylsiloxane (PDMS) nanochannel. Electroosmotic flow (EOF) velocity is measured by using the current-slope method in the modified nanochannel with various numbers of the deposition layers. The results reveal that the growth of polyelectrolyte layer in nanochannels is quite different from that on flat substrates. In small nanochannels, the increment of the multilayer thickness will be limited unlike that on flat surfaces. As the number of the deposition layers increases, the nanochannel size will decrease initially and reach a saturation value finally where the nanochannel cannot be filled with the polyelectrolytes. It is observed that both the decrement and the saturation value of the nanochannel size strongly depend on the concentration of the salt addition in polyelectrolyte solutions. At low ionic concentration, increasing the salt addition increases the average layer thickness within the nanochannels and decreases the saturation value of nanochannel size. However, the effects of salt addition can demonstrate opposite behaviors at high ionic concentration. The EOF can be alternatively reversed in the modified nanochannels by repeatedly coating oppositely charged polyelectrolytes, and the EOF velocity will gradually decrease with the increasing deposition cycles.

Tensiometric and Rheological Characteristics of Fractions of Humic and Hymatomelanic Acids

The Du Nouy ring detachment and pendant drop methods have been employed to study the tensiometric (dynamic and equilibrium surface tensions) and surface rheological (viscoelasticity modulus and phase angle) characteristics of aqueous solutions of fractions of humic and hymatomelanic acids at a solution–air interface. It has been found that the fraction of low-molecular-weight hymatomelanic acids has high surface activity, while its surface layers are characterized by high viscoelasticity and storage elastisity moduli. The experimental dependences of the equilibrium surface tension and viscoelasticity modulus on the concentration of hymatomelanic acid salt solutions are adequately described in terms of the model of real two-dimensional solutions for polymolecular adsorption of polyelectrolytes.

Monte Carlo study of the effects of macroion charge distribution on the ionization and adsorption of weak polyelectrolytes and concurrent counterion release

HypothesisAdsorption of weak polyelectrolytes onto charged nanoparticles, and concurrent effects such as spatial partitioning of ions may be influenced by details of the polyelectrolyte structure (linear or star–like) and size, by the mobility of the nanoparticle surface charge, or the valence of the nanoparticle counterions. ExperimentsIonization and complexation of weak polyelectrolytes on spherical macroions with monovalent and divalent countrions has been studied with constant–pH Monte Carlo titrations and primitive electrolyte models for linear and star–like polymers capable, also, of forming charged hydrogen bonds. Nanoparticles surface charge has been represented either as a single colloid–centered total charge (CCTC) or as surface–tethered mobile monovalent spherical charges (SMMSC). FindingsDifferences in the average number of adsorbed polyelectrolyte arms and their average charge, and in the relative amount of macroion counterions (m–CI’s) released upon polymer adsorption are found between CCTC and SMMSC nanoparticles. The amount of the counterions released also depends on the polymer structure. As CCTC adsorbs a lower number of star–like species arms, the degree of condensation of polymer counterions (p–CI’s) onto the polyelectrolyte is also substantially higher for the CCTC colloid, with a concurrent decrease of the osmotic coefficient values.

Like-charge polymer-membrane complexation mediated by multivalent cations: One-loop-dressed strong coupling theory.

We probe the electrostatic mechanism driving adsorption of polyelectrolytes onto like-charged membranes upon the addition of tri- and tetravalent counterions to a bathing monovalent salt solution. We develop a one-loop-dressed strong coupling theory that treats the monovalent salt at the electrostatic one-loop level and the multivalent counterions within a strong-coupling approach. It is shown that the adhesive force of the multivalent counterions mediating the like-charge adsorption arises from their strong condensation at the charged membrane. The resulting interfacial counterion excess locally maximizes the screening ability of the electrolyte and minimizes the electrostatic polymer grand potential. This translates into an attractive force that pulls the polymer to the similarly charged membrane. We show that the high counterion valency enables this adsorption transition even at weakly charged membranes. Additionally, strongly charged membranes give rise to monovalent counterion-induced correlations and intensify the interfacial multivalent counterion condensation, strengthening the complexation of the polymer with the like-charged membrane, as well as triggering the orientational transition of the molecule prior to its adsorption. Finally, our theory provides two additional key features as evidenced by previous adsorption experiments: first, the critical counterion concentration for polymer adsorption decreases with the rise of the counterion valency and, second, the addition of monovalent salt enhances the screening of the membrane charges and suppresses monovalent counterion correlations close to the surface. This weakens the interfacial multivalent counterion condensation and results in the desorption of the polymer from the substrate.