Polyelectrolyte Surface Research Articles

Adsorption of Hydrophobic Polyelectrolytes at Oppositely Charged Surfaces

We develop a scaling theory of adsorption of necklace-like hydrophobic polyelectrolytes at an oppositely charged hydrophilic and hydrophobic surfaces. At low surface charge densities we predict a two-dimensional adsorbed layer with thickness determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities we expect a 3-dimensional layer with polymer density profile determined either by the balance between two-body monomer−monomer attraction or by electrostatic attraction to the surface and three-body monomer−monomer repulsion. These different stabilizing mechanisms result in the nonmonotonic dependence of the layer thickness on the surface charge density. For adsorption of polyelectrolyte chains from salt solutions the screening of the electrostatic repulsion between adsorbed polyelectrolyte chains results in large overcompensation of the surface charge for two-dimensional adsorbed layers. The polymer surface coverage for this regime increases with increasing salt concentration. The opposite trend is predicted for 3-D adsorbed layers where the polyelectrolyte surface excess decreases with increasing salt concentration.

Adsorption of Polyelectrolytes at Oppositely Charged Surfaces

We have developed a scaling theory of polyelectrolyte adsorption at an oppositely charged surface. At low surface charge densities, we predict two-dimensional adsorbed layers with thickness determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities, we expect a 3-dimensional layer with a density profile determined by the balance between electrostatic attraction and short-range monomer-monomer repulsion. These different stabilizing mechanisms result in a nonmonotonic dependence of the layer thickness on the surface charge density. For adsorption of polyelectrolyte chains from salt solutions, the screening of the electrostatic repulsion between adsorbed polyelectrolyte chains results in large overcompensation of the surface charge for two-dimensional adsorbed layers. At higher salt concentrations this overcompensation of the surface charge by the 2-d adsorbed layer is independent of the original surface charge and depends only on the fraction of the charged monomers on the polyelectrolyte chains and increases with ionic strength. The polyelectrolyte surface excess in 3-d adsorbed layers increases at low ionic strength and decreases at higher ionic strength.

Selective Deposition of Two-Dimensional Colloidal Arrays on Patterned Polyelectrolyte Multilayer Templates

AbstractA new method for the patterning of colloidal particles on polymer templates is presented. A polyelectrolyte multilayer film is deposited onto a chemically patterned monolayer surface, which is further used as a functional template to direct the deposition of charged particles through electrostatic and secondary interactions. After modified by an opposite charged surfactant, the particles change their deposition selectivity from the polyelectrolyte surface to a neutral ethylene glycol monolayer surface. Based on the different selectivity, we have first demonstrated a new approach to direct two component colloidal arrays onto a patterned polyelectrolyte multilayer template.

Selective Self-Organization of Colloids on Patterned Polyelectrolyte Templates

Submicron-sized colloidal particles have been self-organized into patterned arrangements on a substrate using a novel technique. At the substrate, a polyelectrolyte multilayer film has been deposited onto a chemically patterned surface; subsequently, the polyelectrolyte surface is immersed in an aqueous colloidal suspension of bare SiO2 microspheres or functionalized polystyrene latex particles. The colloids self-organize at the surface, driven by the spatially varied electrostatic and secondary interactions between the colloid and the substrate. The polyelectrolyte platform provides a strong bond to the colloids, imparting mechanical robustness which enables postprocessing of the patterned assemblies. An important advantage to this approach is that the use of a polyelectrolyte multilayer platform opens up the possibility of introducing functionality into the underlying layers. We have demonstrated control over the density and selectivity of particle adsorption. Three mechanisms have been used to control adsorption: (i) pH of the colloid suspension, which determines the ionization of the uppermost surface of the polyelectrolyte multilayer; (ii) ionic strength of the suspension, which determines the extent of charge screening about the colloid and polyelectrolyte; and (iii) concentration of added surfactant, which causes charge screening and introduces hydrophobic interactions between the surfactant and polyelectrolyte. Finally, an energy adsorption model is presented.

Divalent cations and the electrostatic potential around DNA: Monte Carlo and Poisson-Boltzmann calculations.

The predictions of counterion condensation theory for divalent ions were tested by comparison with the results of Monte Carlo calculations on an all-atom model of DNA. Monovalent-divalent competition at the polyelectrolyte surface was investigated by varying the partial molar volume of divalent ions. To assess the viability of using Poisson-Boltzmann (PB) calculations for determining divalent ion concentrations at DNA surfaces, Monte Carlo (MC) calculations were compared with PB calculations using different models of the dielectric continuum. It was determined that, while standard PB calculations of divalent ion surface densities are about 25-30% below those predicted by MC techniques, and somewhat larger than errors previously determined for monovalent ions, errors due to the use of the mean-field approximation of PB theory are smaller than those arising from common assumptions regarding the dielectric continuum.

Short Range Order of Rodlike Polyelectrolytes in the Isotropic Phase

The local order of aqueous solutions of rodlike polyelectrolytes is analyzed. It is shown that the scaling of the full width at half-height of the first peak of the static structure factor with the concentration C of polyelectrolyte is similar to the scaling of the peak position of the first maximum: it changes from C 1/3 to C 1/2 as C varies from the dilute to the semidilute regime. This result enables us to estimate the number of ordered next neighbor shells, N, arranged locally around a central particle. It is shown, in good agreement with Monte Carlo results, that N ∼ 1 when the ionic strength of the solution, I, is ∼10 -7 -10 -5 M and the polyelectrolyte surface charge in the solution, Q, is on the order of hundreds of electrons. This order decreases when I goes to ∼10 -3 -10 -2 M and Q decreases by 1 order of magnitude.

The Effect of Salt Concentration on Adsorption of Low-Charge-Density Polyelectrolytes and Interactions between Polyelectrolyte-Coated Surfaces

In this investigation surface force, X-ray photoelectron spectroscopy and ellipsometry techniques have been used to study the adsorption of a low-charge-density cationic polyelectrolyte on negatively charged surfaces. It is shown that the low cationicity of this polyelectrolyte induces an adsorption behavior which is limited by steric factors rather than by the substrate surface charge or potential. It is also established that an increase in ionic strength of the solution results in desorption of the polyelectrolyte accompanied by an increase in layer thickness. This phenomenon is typical of a screening-reduced adsorption regime where electrostatic interactions predominate in the adsorption process. An increase in layer thickness most often occurs as a result of an increased adsorbed amount. Here, however, the increase in layer thickness occurs despite a reduction in the adsorbed amount. This can be understood as resulting from a reduced polyelectrolyte–surface affinity and a swelling of the adsorbed layer. Finally, it is demonstrated that the employed techniques complement each other and reveal new information on the interaction forces and conformation of polyelectrolytes at the solid–liquid interface.

Adsorption of Weak Polyelectrolytes on Metal Oxide Surfaces: A Hybrid SC/SF Approach

A hybrid SC/SF model to describe the adsorption of weak polyelectrolytes on metal oxide surfaces is presented. This model couples the surface complexation (SC) model for the effects of surface and solute speciation on the adsorption of monomers on oxides with the Scheutjens−Fleer (SF) model for the effects of macromolecular properties on the adsorption of polymers and polyelectrolytes on surfaces. The average magnitude of the specific interactions between polyelectrolyte functional groups and surface sites, expressed by the parameter χs(hyb), varies systematically with surface and polyelectrolyte speciation and hence with solution chemistry. Measurements of the adsorption of polygalacturonic acid (PGUA) on hematite over wide ranges of pH and ionic strength are used to test the hybrid model. Good agreement of model simulations and adsorption measurements is achieved. A reaction represented by >MeOH + L- = >MeOHL- provides the best representation of the experimental data.

Influence of Polyelectrolyte Multilayer Coatings on Förster Resonance Energy Transfer between 6-Carboxyfluorescein and Rhodamine B-Labeled Particles in Aqueous Solution

The Förster resonance energy transfer (FRET) between 6-carboxyfluorescein (6-CF) (donor) and rhodamine B-labeled melamine formaldehyde (RhB-MF) particles (acceptor) in aqueous solution was exploited to investigate the layer properties of polyelectrolyte (PE) multilayers preadsorbed on the particle surface. The formation of poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) multilayers on the RhB-MF particles was confirmed by electrophoretic mobility measurements. The FRET process was found to proceed via adsorption of 6-CF onto the RhB-MF particles and was thus dependent on the degree of surface coverage of the PE on the surface. The PE surface coverage could be altered by depositing the layers with or without added electrolyte. The extent of FRET was also influenced by the number of PE layers (and hence layer thickness) surrounding the RhB-MF particles. Increasing the number of PE layers resulted in less energy transfer, reflecting less accessible sites on the RhB-MF particles for 6-CF adsorption. The PE layers were found to be permeable to 6-CF, with no diffusion effects evident on the time scale of the steady-state fluorescence measurements. Further, 6-CF was found to interact with PAH when the outer PE layer on the particles was PAH. This interaction presents a novel way of detecting amino sites of PAH not interacting with PSS in the PE multilayer films.

Local dielectric constants and Poisson-Boltzmann calculations of DNA counterion distributions

Recent work by the authors on the calculation of local solvent dielectric constants around polyelectrolytes using the Poisson–Boltzmann approach is analyzed in terms of the effect on surface potentials and counterion concentrations. Polyelectrolyte surface geometry, local electric fields, and counterion distributions contribute to the self-consistent prediction of local solvent dielectric constants. For an all-atom cell model of DNA with added monovalent salt varying from 0 to 0.5M, the Poisson–Boltzmann-determined electrostatic potential increases (negatively) by 50–100% upon the inclusion of local dielectric constants. This, in turn, implies that hydronium ion concentrations in the major and minor grooves increase by about 0.65 and 0.35 pH units, respectively. While counterion concentrations in the major groove change only slightly, those in the minor groove increase by 60–90%. It is also noted that while the local dielectric constant in the major groove monotonically increases away from the surface toward the bulk value of water the dielectric constant in the minor groove has a minimum about 2 Å from the surface due primarily to the local electric field. Certain other properties, such as ionic and dipole first passage times, are affected little by local dielectric constants (less than about 3%). © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1087–1093, 1997

Local dielectric constants and Poisson–Boltzmann calculations of DNA counterion distributions

Recent work by the authors on the calculation of local solvent dielectric constants around polyelectrolytes using the Poisson–Boltzmann approach is analyzed in terms of the effect on surface potentials and counterion concentrations. Polyelectrolyte surface geometry, local electric fields, and counterion distributions contribute to the self-consistent prediction of local solvent dielectric constants. For an all-atom cell model of DNA with added monovalent salt varying from 0 to 0.5M, the Poisson–Boltzmann-determined electrostatic potential increases (negatively) by 50–100% upon the inclusion of local dielectric constants. This, in turn, implies that hydronium ion concentrations in the major and minor grooves increase by about 0.65 and 0.35 pH units, respectively. While counterion concentrations in the major groove change only slightly, those in the minor groove increase by 60–90%. It is also noted that while the local dielectric constant in the major groove monotonically increases away from the surface toward the bulk value of water the dielectric constant in the minor groove has a minimum about 2 Å from the surface due primarily to the local electric field. Certain other properties, such as ionic and dipole first passage times, are affected little by local dielectric constants (less than about 3%). © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1087–1093, 1997

Kinetics of complexation of heavy metal ions with polyelectrolytes in aqueous solution

AbstractThe kinetics of the binding of Ni2+ to polycarboxylate anions have been studied using pressure‐ and temperature‐jump relaxation techniques. The reaction proceeds via the two‐step Eigen mechanisms: free ions form the outer‐sphere and in the second step the inner‐sphere complex. The outer‐sphere complex is exceptionally stable due to the polyelectrolyte effect, and the electrical potential at the polyelectrolyte surface is estimated to be ψ ≈−86 mV. The rate constant for the formation of the inner‐sphere complex is in the same range as that obtained for Ni2+ complexes with small anions. The formation of bidentate complexes does not contribute significantly to the stability of the complexes. A parallel study of the reactions of Ni2+ with malonate and succinate has been performed.

THE EFFECT OF THE LIQUID-SOLID INTERFACE ON THE PREPARATION AND PROPERTIES OF COAL-WATER SLURRIES

ABSTRACT Considering coal-water slurries (CWS) as dispersed systems with a large separation surface, we have studied the adsorption of same species at the solid-liquid interface; these species have been necessary for the preparation and stabilization of coal-water-slurries (CWS). In this paper, we report the results of the study on the adsorption of poly-naphthalene sulphonates (PNS) and poly-styrene sulphonates (PSS) with controlled molecular weight and steric purity, at model coal/water interfaces and at the real coal/water once The adsorption is Langmuir's type in both cases, but the interfacial distribution is different: in the case of PNS the aromatic rings are co-planar with die surface and the interactions with the pure aromatic regions of the solid surface are very strong. On the other hand, in the case of PSS the adsorption layer is constituted by a region directly bound to the surface, and another that is made up of loops and tails immersed in the aqueous solution We report also the results obtained for the adsorption of steam onto coal; in this case water molecules result arranged in clusters on hydrophilic sites of the surface. Finally, we report a possible model for the rheological degradation under stress for concentrated slurries. This degradation involves the interface and shows that the presence at the surface of ions and polyelectrolytes used as additives is the most important factor.

Electrooptic Study of Colloid Dispersions Stabilized by Polyelectrolyte Adsorption

Several electrooptic parameters can be used to follow the stages of polyelectrolyte adsorption on colloid particle surfaces. The process of electrostatic stabilization, achieved by polyelectrolyte surface recharging, is investigated for a series of samples and the results are compared to those obtained for low molecular surfactant stabilization. The results indicate a significant decrease of the relaxation frequency of particle electric polarizability in the presence of polyelectrolyte coverage. Evidence is given for the direct relation of the electrooptic relaxation frequency to the type of the adsorbed polymer molecule.

Measurements of conformational changes during adhesion of lipid and protein (polylysine and S‐layer) surfaces

The adhesion forces between various surfaces were measured using the "surface forces apparatus" technique. This technique allows for the thickness of surface layers and the adhesion force between them to be directly measured in controlled vapor or liquid environments. Three types of biological surfaces were prepared by depositing various lipid-protein monolayers (with thicknesses ranging from 1 to 4 nm) on the inert, molecularly smooth mica surface: (i) hydrophobic lipid monolayers; (ii) amphiphilic polyelectrolyte surfaces of adsorbed polylysine; and (iii) deposited bacterial S-layer proteins. The adhesion, swelling, and wetting properties of these surfaces was measured as a function of relative humidity and time. Initial adhesion is due mainly to the van der Waals forces arising from nonpolar (hydrophobic) contacts. Following adhesive contact, significant molecular rearrangements can occur which alter their hydrophobic-hydrophilic balance and increase their adhesion with time. Increased adhesion is generally enhanced by (i) increased relative humidity (or degree of hydration); (ii) increased contact time; and (iii) increased rates of separation. The results are likely to be applicable to the adhesion of many other biosurfaces, and show that the hydrophobicity of a lipid or protein surface is not an intrinsic property of that surface but depends on its environment (e.g., on whether it is in aqueous solution or exposed to the atmosphere), and on the relative humidity of the atmosphere. It also depends on whether the surface is in adhesive contact with another surface and-when considering dynamic (nonequilibrium) conditions-on the time and previous history of its interaction with that surface.

Interfacial behavior of block polyelectrolytes. 4. Polymorphism of (quasi) two-dimensional micelles

Langmuir films of block polyelectrolyte surface aggregates formed at the air-water interface have been visualized as Langmuir-Blodgett films using transmission electron microscopy. The morphologies adopted by these (quasi) 2-D aggregates are critically dependent upon the ratio of the hydrophobic and hydrophilic block sizes. Three morphologies are observable (starfish, rod, and planar) in polystyrene/decylated poly(vinylpyridinium) block polyelectrolytes

Electrostatic potential and polarity at the molecular surface of polyelectrolytes as probed by pH-sensitive chromophores covalently attached to the main chain

Mesures pour le poly(acrylamido-2 methyl-2 propanesulfonate de sodium) et le chlorure de poly([methacryloylamino-3] propyl) trimethylammonium

Adsorption of weak polyelectrolytes from aqueous solution

A theoretical model for the adsorption of weak polyelectrolytes from aqueous solution is developed by extending recent theories for uncharged polymers at interfaces with electrostatic contributions. The segment concentration profile is found from the optimal compromise of several factors, such as the conformational entropy of the chains, the configurational entropy of the solvent, the adsorption energy, the solvent quality, the electrostatic repulsion between the segments, and electrostatic interactions due to the surface charge. The degree of dissociation of the polyelectrolyte charges and the dielectric permittivity are allowed to vary with the distance from the surface. Some typical results are presented for the adsorption as a function of polyelectrolyte concentration, solvent quality, pH, and surface charge. A simple equation is derived that relates the critical value of the surface charge σ cr 0 above (or below) which no adsorption takes place, to the nonelectrostatic adsorption energy χ s. As far as comparison with experimental data is possible, the predictions of the model are at least semiquantitatively in agreement with experiment.

Dielectric behavior of polyelectrolytes

The role of the Coulomb forces between the counterions on the surface of polyelectrolytes on the dielectric response is analyzed. An estimate of the maximum dielectric increment (as a function of the number of counterions) is found as a function of the molecular length. The minimum-energy configuration of the counterions on a cylinder is found to be a double helix, suggesting the fundamental importance of electrostatic interactions in determining structure. Solutions of the dynamical equations for a few counterions indicate that a single mode dominates the relaxation which is enhanced by the inter-ion repulsions. A lower bound is found for this mode based on analysis of the system response for short lengths. Sum rules for the rates and amplitudes of the dipolar correlation function are derived and lead to an upper bound for the rate of the dominant mode. These bounds approach one another for the parameters characteristic of restriction fragments of DNA. This permits a prediction of the magnitude and time scale of the dielectric response.

Polyelectrolyte solutions without added salt. Concentration effects and condensation

Concentration effects in salt-free polyelectrolyte solutions are discussed in terms of scaling relations as proposed by Odijk. No explicit use is made however of the condensation approach and all important quantities are expressed in terms of an effective charge density parameter. It is shown that, in the concentration range most accessible to experimental investigation, the exact value of this parameter may have strong bearings on the applicability of the theory and its conclusions. The condensation approach is discussed in terms of the analytical solution of the Poisson-Boltzmann equation for salt-free polyelectrolyte solutions and its validity questioned in the same concentration range. Comparison of the Poisson-Boltzmann solution for the potential at the surface of the charged polyelectrolyte with values derived from filtration data of a weak polyacid shows qualitative agreement but suggests that the effective charge density parameter may be different from its structural value.