Numerical study of the interplay of monomer-surface electrostatic and non-electrostatic interactions in the adsorption of weak polyelectrolytes on oppositely charged surfaces

The adsorption of weak polybase on oppositely charged planar surfaces has been investigated numerically by using the self-consistent field theory (SCFT). Particular attention was paid to the interplay of monomer-surface electrostatic and non-electrostatic interactions in the adsorption behaviors of weak polybase. In this study, the strength of monomer-surface non-electrostatic interactions was set to be no more than the thermal energy k _B T. It was found from the numerical study that in the regime of low surface charge density of the substrate and low pH or high bulk degree of ionization, both the screening-enhanced and screening-reduced salt effects emerge. On the contrary, in the opposite regime, only the screening-reduced salt effect was observed. Moreover, the overall charge neutrality inside the adsorption layer was analyzed. The underlying mechanism governing the adsorption behaviors of weak polybase on oppositely charged surfaces was elucidated.     

Effect of introduced charge in cellulose gels on surface interactions and the adsorption of highly charged cationic polyelectrolytes

The interaction between cellulose surfaces in aqueous solution has been measured using colloidal probe microscopy. Cellulose thin films with varying charge through carboxyl group substitution were used in this study with the surface forces fit to DLVO theory. It was found that the surface potential increased, as expected, with increasing carboxyl substitution. Furthermore, for a given degree of substitution, the surface potential increased as a function of increasing pH. At low pH, the surface forces interaction were attractive and could be fit to the non-retarded Hamaker equation using a constant of 3 x 10(-21) J. At pH greater than 5, the force interactions were monotonically repulsive, regardless of the ionic strength of the solution for all charge densities of the cellulose thin films. The adsorption of polyDADMAC to these charged cellulose films was also investigated using the quartz crystal microbalance. It was found that for the low charge film, a low surface excess of PDADMAC was sensed and that the adsorbed conformation was essentially flat. However for the higher charged cellulose film, a spontaneous de-swelling was observed resulting in no possibility of quantitatively determining the sensed mass using QCM.     

Behavior of rodlike polyelectrolytes near an oppositely charged surface

The behavior of highly charged short rodlike polyelectrolytes near oppositely charged planar surfaces is investigated by means of Monte Carlo simulations. A detailed microstructural study, including monomer and fluid charge distributions and chain orientation, is provided. The influence of chain length, substrate's surface-charge density, and image forces is considered. Due to the lower chain entropy (compared to flexible chains), our simulation data show that rodlike polyelectrolytes can, in general, better adsorb than flexible ones do. Nonetheless, at low substrate-dielectric constant, it is found that repulsive image forces tend to significantly reduce this discrepancy.     

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.     

Determination of the surface potential for adsorption layers of polyelectrolytes by the streaming potential method

A physicochemical model is proposed to describe electrokinetic phenomena in capillaries and pores the surface of which is coated with a charged porous adsorption layer. The use of this model makes it possible to explain experimental data on the surface potentials of polyelectrolyte adsorption layers upon their deformation resulting from solution flow. The commonly used Smoluchowski equation is shown to lead to large errors in the determination of the potential and charge of the surface of an adsorption layer.     

A modified Poisson-Boltzmann model including charge regulation for the adsorption of ionizable polyelectrolytes to charged interfaces, applied to lysozyme adsorption on silica

The equilibrium adsorption of polyelectrolytes with multiple types of ionizable groups is described using a modified Poisson-Boltzmann equation including charge regulation of both the polymer and the interface. A one-dimensional mean-field model is used in which the electrostatic potential is assumed constant in the lateral direction parallel to the surface. The electrostatic potential and ionization degrees of the different ionizable groups are calculated as function of the distance from the surface after which the electric and chemical contributions to the free energy are obtained. The various interactions between small ions, surface and polyelectrolyte are self-consistently considered in the model, such as the increase in charge of polyelectrolyte and surface upon adsorption as well as the displacement of small ions and the decrease of permittivity. These interactions may lead to complex dependencies of the adsorbed amount of polyelectrolyte on pH, ionic strength, and properties of the polymer (volume, permittivity, number, and type of ionizable groups) and of the surface (number of ionizable groups, pK, Stern capacity). For the adsorption of lysozyme on silica, the model qualitatively describes the gradual increase of adsorbed amount with pH up to a maximum value at pHc, which is below the iso-electric point, as well as the sharp decrease of adsorbed amount beyond pHc. With increasing ionic strength the adsorbed amount decreases (for pH > pHc), and pHc shifts to lower values.     

Theory of inhomogeneous weakly charged polyelectrolytes

A theory of inhomogeneous multicomponent systems containing weakly charged polyelectrolytes is developed. The theory treats the polymer conformation and the electrostatics simultaneously using a functional integral representation of the partition function. A mean‐field approximation to the theory leads to two sets of coupled mean‐field equations: a Poisson‐Boltzmann type equation describing the electrostatic potential, and a set of self‐consistent field equations describing the equilibrium densities. Asymptotic forms of the theory at weak and strong segregation limits are derived. The theory can be used to study the interfacial properties, microphase structures, and adsorptions of a variety of weakly charged polyelectrolyte systems. As a simple example, the interface between the polymer‐rich and polymer‐poor phases of a polyelectrolyte solution is studied.     

Adsorption of weakly charged polyelectrolytes onto oppositely charged spherical colloids

The adsorption of a long weakly charged flexible polyelectrolyte in a salt solution onto an oppositely charged spherical surface is investigated. An analytical solution for Green's function is derived, which is valid for any sphere radius and consistently recovers the result of a planar surface in the limit of large sphere radii, by substituting the Debye-Hückel potential via the Hulthén potential. Expressions for critical quantities like the critical radius and the critical surface charge density are provided. In particular, we find a universal critical line for the sphere radius as a function of the screening length separating adsorbed from desorbed states. Moreover, results for the monomer density distribution, adsorbed layer thickness, and the radius of gyration are presented. A comparison of our theoretical results with experiments and computer simulations yields remarkably good agreement.     

Adsorption of highly charged polyelectrolytes onto an oppositely charged porous substrate

The adsorption behavior of highly charged cationic polyelectrolytes onto porous substrates is electrostatic in nature and has been shown to be highly dependent on the polyelectrolyte properties. Copolymers of acrylamide (AM) and diallyldimethylammonium chloride (DADMAC) were synthesized to have a range of macromolecular properties (i.e., charge density and molecular mass). Traditional titration methods have been complemented by fluorescence labeling techniques that were developed to directly observe the extent that fluorescently labeled poly(AM- co-DADMAC) adsorbs into the pore structure of a cellulosic substrate. Although contributing to the electrostatic driving force, the charge density acts to limit adsorption to the outermost surface under electrolyte-free conditions. However, adsorption into the pores can occur if both the molecular mass and charge density of poly(AM- co-DADMAC) are sufficiently low. Adsorption initially increases as the electrolyte concentration is increased. However, the electrostatic persistence length of poly(AM- co-DADMAC) restricts the polyelectrolyte from entering the pores. Therefore, changes in the adsorption behavior at moderate electrolyte concentrations have been attributed to swelling of the polyelectrolyte layer at the fiber exterior. The adsorption behavior changes again at high electrolyte concentrations such that poly(AM- co-DADMAC) could adsorb into the pore structure. This occurred when the electrolyte concentration was sufficient to screen the electrostatic persistence length of poly(AM- co-DADMAC), provided that the entropic driving force for adsorption still existed. It is suggested that adsorption into the pore structure is a kinetic process that is governed by localized electrostatic interactions between poly(AM- co-DADMAC) and the charges located within the pores.     

Phase diagram for a model of urate oxidase

Urate oxidase from Asperigillus flavus has been shown to be a model protein for studying the effects of polyethylene glycol (PEG) on the crystallization of large proteins. Extensive experimental studies based on small angle x-ray scattering [Vivares and Bonnete, J. Phys. Chem. B 108, 6498 (2004)] have determined the effects of salt, pH, temperature, and most importantly PEG on the crystallization of this protein. Recently, some aspects of the phase diagram have also been determined experimentally. In this paper, we use Monte Carlo techniques to predict the phase diagram for urate oxidase in solution with PEG, including the liquid-liquid and liquid-solid coexistence curves. The model used includes an electrostatic interaction, van der Waals attraction, and a polymer-induced depletion interaction [Vivares et al., Eur. Phys. J. E 9, 15 (2002)]. Results from the simulation are compared with experimental results.     

Nanoparticle modification by weak polyelectrolytes for pH-sensitive pickering emulsions

The affinity of weak polyelectrolyte coated oxide particles to the oil-water interface can be controlled by the degree of dissociation and the thickness of the weak polyelectrolyte layer. Thereby the oil in water (o/w) emulsification ability of the particles can be enabled. We selected the weak polyacid poly(methacrylic acid sodium salt) and the weak polybase poly(allylamine hydrochloride) for the surface modification of oppositely charged alumina and silica colloids, respectively. The isoelectric point and the pH range of colloidal stability of both particle-polyelectrolyte composites depend on the thickness of the weak polyelectrolyte layer. The pH-dependent wettability of a weak polyelectrolyte-coated oxide surface is characterized by contact angle measurements. The o/w emulsification properties of both particles for the nonpolar oil dodecane and the more polar oil diethylphthalate are investigated by measurements of the droplet size distributions. Highly stable emulsions can be obtained when the degree of dissociation of the weak polyelectrolyte is below 80%. Here the average droplet size depends on the degree of dissociation, and a minimum can be found when 15 to 45% of the monomer units are dissociated. The thickness of the adsorbed polyelectrolyte layer strongly influences the droplet size of dodecane/water emulsion droplets but has a less pronounced impact on the diethylphthalate/water droplets. We explain the dependency of the droplet size on the emulsion pH value and the polyelectrolyte coating thickness with arguments based on the particle-wetting properties, the particle aggregation state, and the oil phase polarity. Cryo-SEM visualization shows that the regularity of the densely packed particles on the oil-water interface correlates with the degree of dissociation of the corresponding polyelectrolyte.     

Water distribution in multilayers of weak polyelectrolytes

The water localization in thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) was investigated with neutron reflectivity in an atmosphere of controlled humidity and with bulk water. Water was found to be distributed asymmetrically within the multilayer and to localize preferentially at the polymer surface. The diffusion of water into the multilayer did not completely penetrate to the substrate, but instead there appeared to be an exclusion zone near the Si substrate. These results help to explain previous observations of anomalous water transport kinetics in weak polyelectrolyte systems.     

UV-cross-linkable multilayer microcapsules made of weak polyelectrolytes

Microcapsules composed of weak polyelectrolytes modified with UV-responsive benzophenone (BP) groups were fabricated by the layer-by-layer (LbL) technique. Being exposed to UV lights, capsules shrunk in the time course of minutes at irradiation intensity of 5 mW/cm(2). The shrinkage adjusted the capsule permeability, providing a novel way to encapsulate fluorescence-labeled dextran molecules without heating. Cross-linking within the capsule shells based on hydrogen abstraction via excited benzophenone units by UV showed a reliable and swift approach to tighten and stabilize the capsule shell without losing the pH-responsive properties of the weak polyelectrolyte multilayers.     

Layer-to-layer adsorption of oppositely charged polyelectrolytes: The effect of molecular mass: 2. Bilayer systems

The effect of molecular mass on the formation of a bilayer structure upon the layer-to-layer adsorption of a cationic polyelectrolyte (poly(dimethyldiallylammonium chloride), molecular mass M = 500000 and 100000-200000 Da) and an anionic polyelectrolyte sodium (poly(acrylate), M = 30000 and 2100 Da) on the surface of fused quartz is studied by the capillary electrokinetic method. The time required to reach constant adsorption values and the structure of bilayer systems depend on the ratio between molecular masses of the cationic and anionic polyelectrolytes. The deformability of the bilayer system significantly exceeds that of the first layer in the case when the second layer is formed from an anionic polyelectrolyte with a lower molecular mass, thus suggesting the loosening of the first adsorption layer of the cationic polyelectrolyte. The adsorption of the anionic polyelectrolyte with higher molecular mass insignificantly affects the density of the first layer. Variation in the deformability of the layer with time (its aging) depends on the ratio between molecular masses of the polyelectrolytes.     

Multilayers by adsorption of functional polyelectrolytes

The synthesis of polycations bearing reactive groups, namely tertiary anilines located in the side chain or in the main chain, is described. These polyions are employed in conventional alternating polyelectrolyte multilayer assembly or in a modified build-up process including an azo coupling reaction on the topmost layer after every polycation adsorption step, which offers the potential to create multilayer structures with a non-centrosymmetric structure. The characterization of these modified assemblies by UV/VIS spectroscopy reveals significant differences in the spectral data in dependence of the polycation structure.     

A two-state solvent model for rigid rods adsorption at charged interfaces

A simple two-state solvent model is developed to describe the adsorption of rigid rods at polarizable interfaces. The adsorption isotherm and the equation of state are determined by means of the lattice theory of strictly regular solutions, assuming that the solvent at the interface exists in the form of monomers with two possible orientations, and the adsorbed rods are oriented normal to the electrode surface. In the case of the mercury/aqueous inert electrolyte interface in the presence of small, polar, aliphatic hydrocarbon derivatives, the model predicts that the variation of the isotherm interaction parameter is determined mainly by the dipole-dipole interactions between the permanent dipoles of the adsorbed molecules. This variation becomes more pronounced as the dipolar interactions between adsorbate-solvent molecules become more intense. The Marshall-Conway treatment for the polarization catastrophe and the approach of Levine et al. in incorporating polarizability effects are also taken into consideration and examined critically.     

Layer-to-layer adsorption of oppositely charged polyelectrolytes: The effect of molecular mass: 1. Formation of the first layer

The formation and properties of adsorption layers of poly(dimethyldiallylammonium chloride) with different molecular masses on the surface of fused quartz are studied by the capillary electrokinetics method. It is shown that the value of ζ potential depends on the flow rate of liquid determined by the pressure drop. Such behavior can be explained by the deformation of the adsorption layer. At low rates of liquid flow, constant values of adsorption and time needed to achieve these values decrease for the samples of lower molecular masses, which is probably related to the more closely packed structure and, hence, to the lower deformability of the adsorption layers, as well as to the shortest times during which conformational rearrangements proceed in the layer. The time of conformational changes in the adsorption layer significantly exceeds the time of adsorption. The adsorption of cationic polyelectrolyte is irreversible. It is found that the compaction of adsorption layers increases with time; the rate of compaction of layers of a low-molecular-mass polyelectrolyte is higher and the layers of a high-molecular-mass polyelectrolyte retain the residual deformability even for six days. The measurements of the filtration of polyelectrolyte solutions through thin quartz capillaries allow the thickness of adsorption layers and their deformation under pressure to be estimated.     

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.     

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.