Hindered diffusion of synthetic polyelectrolytes in charged microporous membranes

To examine electrostatic effects on the diffusion of macromolecules in membranes, diffusivities of narrow molecular-size fractions of the polyelectrolytes ficoll sulfate and dextran sulfate were measured in polycarbonate track-etch membranes. Radius, number density, and surface charge density of membrane pores were determined from a combination of hydraulic permeability, glucose diffusion, and streaming potential measurements. Molecular charge and Stokes—Einstein radius for each macromolecule fraction were determined from free-solution electrophoretic mobility and diffusivity in a large pore radius membrane, D∞, respectively. As ionic strength was increased from 0.005 to 0.1 M, D∞ for ficoll sulfate remained constant while D∞ for dextran sulfate increased slightly (15-18%). Macromolecule diffusivities in small pore membranes, D, were much more sensitive to ionic strength. For membranes where the ratio of Stokes—Einstein radius to pore radius ranged from 0.08 to 0.29, D/D∞ for ficoll sulfate and dextran sulfate increased by factors ranging from 2.5 to 14 for the same increase in ionic strength. Recent theoretical results for electrostatic double layer interactions in hindered diffusion are in good quantitative agreement with these findings.     

Charging and aggregation of positively charged latex particles in the presence of anionic polyelectrolytes

Charging behavior and colloidal stability of amidine latex particles are studied in the presence of poly(sodium styrene sulfonate) (PSS) and KCl. Detailed measurements of electrophoretic mobility, adsorbed layer thickness, and aggregation (or coagulation) rate constant on varying the polymer dose, molecular mass of the polymer, and ionic strength are reported. Polyelectrolyte adsorption leads to the characteristic charge reversal (or overcharging) of the colloidal particles at the isoelectric point (IEP). In accordance with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, uncharged particles tend to aggregate because of van der Waals attraction, whereas charged particles are stabilized by electrical double layer repulsion. Attractive patch-charge interactions originating from the laterally inhomogeneous structure of the adsorbed polymer substantially decrease the suspension stability or even accelerate the aggregation rate beyond diffusion control. These electrostatic non-DLVO forces become progressively important with increasing molecular mass of the polymer and the ionic strength of the solution. At higher polymer dose of typically 10 times the IEP, one observes the formation of a saturated layer of the adsorbed polymer with a thickness of several nanometers. Its thickness increases with increasing molecular mass, whereby the layer becomes increasingly porous. This layer does not seem to be involved in the suspension stabilization, since at such high polymer doses the double layer repulsion has attained sufficient strength to stabilize the suspension.     

Proteins and polyelectrolytes: A charged relationship

We review the interaction of charged polymeric systems with proteins. In solutions of low ionic strength there are many examples of proteins attracted to polyelectrolytes even if both systems carry the same overall charge. This attractive interaction is widespread, having been observed for single polyelectrolyte chains as well as for polyelectrolytes grafted to surfaces (polyelectrolyte brushes) and charged polymeric networks. In all cases, adding salt weakens the interaction considerably. We discuss the suggestion that the attractive force at low salinity originates from the asymmetry of interaction between charged polymer segments and charged patches on the surface of the protein globule. This can be explained if the attractive force is mainly due to a counterion release force, i.e., the polyelectrolyte chains become the multivalent counterions for the patches of opposite charge localized on the surface of the proteins. We review a selection of simple models that lead to semi-quantitative estimates of this force as the function of salt concentration.     

Fully charged: Maximizing the potential of cationic polyelectrolytes in applications ranging from membranes to gene delivery through rational design

Polyelectrolytes have found utility across various applications in materials science, from electrochemical devices to biotechnology, due to their facile processing and tunable properties. Through chemistry, rational manipulation of molecular structure enables individual families of polyelectrolytes to be used in emerging advanced applications. In this perspective, we focus on cationic systems, and we describe how various structural motifs influence macromolecular properties for anion exchange membranes and gene delivery vectors. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3167–3174     

Electrophoretic mobility of latex spheres in the presence of divalent ions: experiments and modeling

Electrophoretic mobilities (EPM) of negatively charged latex spheres were measured as a function of salt type and salt concentration. The measured values of EPM were analyzed using a standard electrokinetic model that includes double layer relaxation and the Poisson–Boltzmann model of diffuse double layer. Calculated values of EPM were in good agreement with experimental data taken in simple 1:1 (KCl) and 1:2 (Na₂SO₄) electrolyte solutions without using any fit parameters. For 2:1 electrolytes (CaCl₂ and MgCl₂), however, the magnitude of EPM calculated by the model was higher than the measured values of EPM at higher electrolyte concentrations. The difference between measured and calculated EPM was reduced by assuming the distance of slipping plane x _s?=?0.25 nm or by assuming the decrease of the magnitude of surface charge density from ?0.07 to ?0.025 C/m². These are probably due to the accumulation of divalent counterions in the vicinity of a particle’s surface.     

Precipitation of oppositely charged polyelectrolytes in salt solutions

We study phase separation in symmetric solutions of weakly charged flexible chains of opposite sign. Precipitation is caused by effective attractions due to charge fluctuations and by short-range attractions between monomers. The contribution from charge fluctuations is computed within the random phase approximation (RPA), which takes into account the connectivity of charges in the polyions. The impenetrability of the ions is accounted for by using a modified Coulomb potential in the RPA. In good solvent conditions the precipitate monotonically swells and eventually dissolves upon addition of salt. However, near the theta-solvent condition, but still in the good solvent, the precipitate can be stable at any salt concentration. Moreover, the density of the precipitate after initial decrease can increase with addition of salt. This effect is a result of redistribution of salt between the precipitate and the supernatant, which is due to an interplay of electrostatic and hardcore interactions. For not too weakly charged polyions the precipitate properties become strongly dependent on temperature even in good solvent conditions.     

Competing forces in the interaction of polyelectrolytes with charged interfaces

In this review, we discuss the competition of non-DLVO forces in the adsorption of polyelectrolytes onto charged surfaces. We consider two particularly illustrative problems, namely the adsorption of polyelectrolytes onto similarly charged surfaces and the reversal of surface charge by adsorption of polyelectrolytes. Emphasis is made on how simulation results help to understand relevant experimental situations.     

Interplay of ionic and nonionic interactions in weakly charged polyelectrolytes

We describe the results of theoretical and experimental studies of the regular heterogeneities on a nanometer scale which are formed in the systems containing weakly charged polyelectrolytes due to the competition of ionic and hydrophobic interactions. In particular, we consider the effect of microphase separation in poor solvent polyelectrolyte solutions and gels and nano-self-assemblies emerging in the complexes of polyelectrolyte gels with oppositely charged surfactants. The practically important application connected with metal nanoparticles formation in regular microstructures in polyelectrolyte systems is considered as well.     

Complexation and coacervation of polyelectrolytes with oppositely charged colloids

Polyelectrolyte-colloid coacervation could be viewed as a sub-category of complex coacervation, but is unique in (1) retaining the structure and properties of the colloid, and (2) reducing the heterogeneity and configurational complexity of polyelectrolyte-polyelectrolyte (PE-PE) systems. Interest in protein-polyelectrolyte coacervates arises from preservation of biofunctionality; in addition, the geometric and charge isotropy of micelles allows for better comparison with theory, taking into account the central role of colloid charge density. In the context of these two systems, we describe critical conditions for complex formation and for coacervation with regard to colloid and polyelectrolyte charge densities, ionic strength, PE molecular weight (MW), and stoichiometry; and effects of temperature and shear, which are unique to the PE-micelle systems. The coacervation process is discussed in terms of theoretical treatments and models, as supported by experimental findings. We point out how soluble aggregates, subject to various equilibria and disproportionation effects, can self-assemble leading to heterogeneity in macroscopically homogeneous coacervates, on multiple length scales.     

Interaction between charge-regulated surfaces in the presence of polyelectrolytes

This work gives a brief overview of the backgrounds of the self-consistent mean-field theory as applied to the problem of interaction between charge-regulated surfaces in the presence of polyelectrolytes. A general algorithm for obtaining a self-consistent solution is described, and the asymptotic properties of the solution at close separations are analyzed. Known limitations of the self-consistent field approximation are discussed, highlighting the role of local effects and system equilibration.

     

Ion transport and states of water in charged poly(allylamine) membranes

Cationically charged poly(allylamine) (PAA) membranes having various water contents [0.49 < H < 0.63 (g H₂O/g wet membrane)] were prepared. Sorption and permeation of simple salts (sodium chloride and sodium tetraphenylborate) were investigated, taking into account the state of the water in these membranes. The weight ratios of freezable water and free water to total water (W_fz/W_t, and W_f/W_t) in the membranes were estimated by means of DSC and pulsed ¹H-NMR measurements, respectively. Partition coefficients K for total water were converted into those in freezable and free water, K_fz and K_f, using W_fz/W_t and W_f/W_t. The permeability of both salts in the membranes could be interpreted satisfactorily by an equation derived from the Teorell-Meyer-Sievers theory using values of K_f. The free water is mainly involved in the permeation of simple salts through PAA membranes while bound water hardly takes part.     

Cationic amphiphilic polyelectrolytes and oppositely charged surfactants at the silica-aqueous interface

The influence of sodium dodecyl sulfate (SDS) on the interfacial behavior of two amphiphilic polyelectrolytes, which are copolymers of the cationic monomers triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride, at the silica-aqueous interface was studied. The fraction of amphiphilic monomers was varied, where 0DT, 40DT, and 80DT contained 0, 40, and 80 mol % monomers with dodecyl side chains, respectively. We used in situ ellipsometry to follow the kinetics of adsorption, in terms of adsorbed amount and adsorbed layer thickness, as well as the response of the adsorbed layers to changes in ionic strength and surfactant concentration. Different results were obtained when surfactant was added to the preadsorbed layers compared to the cases when complexes were preformed in the solution prior to the adsorption. In the whole range of concentrations studied, SDS interacts with 40DT and 80DT noncooperatively, whereas for 0DT cooperativity of binding is observed. The amount adsorbed increased significantly as the SDS concentration was close to the cmc. At high SDS concentrations, a lowering of the layer density was observed. For the amphiphilic polyelectrolytes, 40 DT and 80DT, no desorption from the interface was detected for the range of SDS concentrations studied, while 0DT features a maximum in adsorbed amount at concentrations close to the cmc of SDS. Adsorption of 40DT and 80DT from their mixtures with SDS is found to be path dependent with respect to the variation in SDS concentration, where the reversibility decreases with increasing SDS concentration above the expected charge neutralization point. The coadsorption of 80DT and SDS is highly irreversible with respect to changes in the ionic strength within the time scale of the experiment. In this study, we attempt to illustrate both general mechanisms and specific effects. With regard to the general behavior, it is important to note the charge regulation of both the silica surfaces and the polyion/surfactant complexes; an interplay between the two charge-regulating effects is the key to understanding our observations.     

Adsorption of linear and star-shaped polyelectrolytes to monolayers of charged amphiphiles

Adsorption of polyelectrolytes has been studied employing monolayers of ionic amphiphiles at the air water interface as model surfaces. The adsorption of polyelectrolytes from a solution brought into contact with the amphiphile monolayer results in changes of the monolayer structure and properties. Monitoring these changes can be done by recording the changes in surface pressure. The kinetics of the adsorption depends strongly on the nature of the polyelectrolyte. Depending on the structure of the polyelectrolyte a purely diffusion controlled adsorption or a sequence of diffusion controlled adsorption and ordering processes have been identified to determine the kinetics. The influence of the molecular architecture on the polyelectrolyte adsorption has been further studied employing linear and star shaped poly(acrylic acid) and poly(N-propyl-4-vinyl pyridinium bromides), respectively. An unexpected behavior with an induction period in the adsorption kinetics of both polymers has been found. Furthermore, the degree of branching has only very minor effects on the adsorption kinetics.     

Viscosity from ternary mixtures of two weakly charged polyelectrolytes in solution

The mode-mode coupling approximation (MMCA) is applied to describe the behavior of the reduced viscosity η_r from ternary mixtures of two weakly charged polyelectrolytes in solution. Similarly (ε = +1) as well as oppositely (ε = −1) weakly charged polyions are discussed in this work. The variation of η_r with the total polyelectrolyte concentration C_T exhibits a peak. The position and the height of this peak show for a given concentration of added salt C_S a sensitive variation with: the charge parameters (degrees of ionization) f₁ and f₂ of both polyelectrolytes, the proportion of polyelectrolyte 1 within the mixture (x = C₁/C_T) and the Flory interaction parameter χ_F between monomeric units of different species. This calculation is presented within the framework of the Rouse model where no hydrodynamic interactions are taken into account.     

Effective intramolecular interactions in weakly charged polyelectrolytes: Relation to structural behavior of solution

A self-consistent integral equation theory in the form of a hybrid Monte Carlo/PRISM computation scheme was used to study a polyelectrolyte solution. The static conformational and structural properties of polyions of different rigidities in a good solvent were studied with explicit allowance for counterions over a wide concentration range. An analysis of the calculated effective potentials and correlation functions confirms the presence of effective attraction between units of the charged polymer in semidilute and concentrated solutions; this attraction leads to the collapse of polyions under certain conditions. It was shown that the cause of effective attraction is the dipole-dipole interaction of ion pairs. For the region of polyelectrolyte transition from the semidilute to the concentrated state of solution, the results qualitatively agree with experimental data and theoretical predictions. Visualized images of conformations in the test range of parameters are given.     

Ionic micelles in solutions of polyelectrolytes and block copolymers with oppositely charged blocks: Computer simulation

Complexation in solutions of strongly charged polyelectrolytes and diblock copolymers composed of oppositely charged and neutral blocks were studied via the molecular dynamics method. Stoichiometric micellar complexes formed in a dilute solution represent cylindrical brushes whose conformation is determined by the linear charge density on the polyelectrolyte and by temperature. As the concentration of macromolecules increases, the orientational ordering of anisotropic ionic micelles takes place. The complexation can induce the stiffening of the polyelectrolyte chain.     

Intrinsic viscosities of polyelectrolytes: determination and modeling of the effects of extra salt

Based on early measurements of J. J. Hermans and co-workers (D. T. F. Pals, J. J. Hermans, Recl. Trav. Chim. Pays-Bas 1952, 71, 513-520; D. T. F. Pals, J. J. Hermans, J. Polym. Sci. 1950, 5, 733-734; D. T. F. Pals, J. J. Hermans, J. Polym. Sci. 1948, 3, 897-898), the present contribution demonstrates how primary data should be evaluated in order to obtain reliable intrinsic viscosities. This procedure yields detailed information on the changes of the intrinsic viscosities and of the corresponding viscometric interaction parameters caused by an increasing salinity of water. Both quantities decline from a maximum value in the pure solvent to a minimum value, which is approached in the limit of sufficiently high salt concentrations, and can be modeled quantitatively by means of a Boltzmann sigmoid. Particular attention is paid to the significance of results obtained by means of the method of isoionic dilution, proposed by J. J. Hermans and co-workers.     

Coagulation of polystyrene latex in the presence of a cation-active surfactant and polyelectrolytes

Coagulating effect of cation-active polyelectrolytes, a copolymer of N,N-dimethyl-N,N-diallylammonium chloride and maleic acid, a fraction of the homopolymer of N,N-dimethyl-N,N-diallylammonium chloride, and low-molecular cation-active surfactant (1-dodecylpyridinium bromide) on a diluted polystyrene latex at pH 7 and 3 was studied. The presence of macromolecules with different amounts of cation groups in the copolymer accounts for the appearance of several peaks in the curves describing the dependence of a “minute” optical density of the latex synthesized in the presence of a purified surfactant, sulfuric acid monododecyl ester sodium salt, on the copolymer additive.     

Phase diagram for the adsorption of weak polyelectrolytes at a soft charged surface

We have experimentally studied the adsorption of polyelectrolytes at oppositely charged surfaces. A weak flexible polyelectrolyte, poly(acrylic acid), was adsorbed from dilute solutions on a Langmuir film of a cationic amphiphile, dimethyldioctadecylammonium bromide. The polymer surface coverage, Gamma, at equilibrium was measured by two reflectivity techniques-ellipsometry and polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS)-as a function of the surface charge density, sigma, and of the polymer ionization degree, alpha. Different adsorption regimes were evidenced. For weakly charged surfaces, sigma < sigma sat, Gamma increases with sigma and with 1/alpha, as expected for a neutralization of the surface by the adsorbed polymers. For highly charged surfaces, sigma > sigma sat, the adsorption of polyelectrolytes saturates. The mean orientation of the adsorbed chains also depends on the value of sigma: it is parallel to the surface for sigma < sigma (< sigma sat) and orthogonal to the surface for sigma > sigma. We have measured the values of sigma sat and sigma as a function of alpha and compared the results with existing theories.     

Studies on polymeric nanofiltration-based water softening and the effect of anion properties on the softening process

The water softening capability of a polymeric NF-based membrane is investigated in a cross-flow mode. Also, the effects of the anion properties binding to the cation in the feed water are investigated. The softening process is monitored using parameters such as apparent rejection, softening factor, relative softened water recovery and the relative softened water purity to assess the effects of the operating conditions on the softening process. Moreover, a combined solution–diffusion/film theory (SDFT) model reduced to Gupta’s model (dilute solutions) is employed to extract three (3) parameters: the hydraulic permeability (L_p), the reflection coefficients (σ) and the solute permeability (ω) which characterizes the membrane. Preliminary, results show the membrane to possess good water softening properties while the charge, size, and the shape of the anions each play a vital role in the water softening process. Thus, the above protocol provides techniques for monitoring, characterizing and selecting potential polymeric membranes for water softening applications.