Nonadsorbing Polyelectrolytes Research Articles

Parametric Study of Colloidal Particle Confinement near a Surface in the Presence of DLVO and Structural Interactions Using Brownian Dynamic Simulations.

Total internal reflection microscopy (TIRM) has become a crucial technique for understanding the surface interactions and dynamics of Brownian colloidal particles near a surface. However, for select colloidal systems, experimental limitations associated with TIRM can occlude exploration of nano- and submicrometer colloids dispersed in complex or structured fluids. It should be possible to use Brownian dynamic simulations to quantify, explore, or circumvent these limitations to extend the TIRM technique further. A Brownian dynamics algorithm based on the Langevin equation was utilized to identify favorable colloidal systems for conducting TIRM experiments in electrolyte and nonadsorbing polyelectrolyte solutions. In electrolyte solution, the motion of polystyrene and silica particles of nanometer- and micrometer-sized radii was simulated near a glass slide in the presence of retarded van der Waals and electric double-layer forces to develop potential energy profiles. In the case of nonadsorbing polyelectrolyte solutions, a structural force was also implemented into the simulation, and the influence of structural interactions on particle confinement was explored as a function of particle size, particle density, and polyelectrolyte concentration. In electrolyte solutions, our results were able to identify the minimum particle size required for TIRM experiments as well as insight into particle selection based on material density. For structural or oscillatory forces, our results show that prior to conducting TIRM experiments, Brownian dynamics simulation can be used to select the appropriate particle size, material, and polyelectrolyte concentration range where the colloidal particle can sample multiple structural energy wells without confinement. These results provide insight into the colloidal system suitable to experimentally study near-surface particle diffusion dynamics for a range of separations in the presence of structural interactions.

Repulsive and attractive depletion forces mediated by nonadsorbing polyelectrolytes in the Donnan limit.

In mixtures of colloids and nonadsorbing polyelectrolytes, a Donnan potential arises across the region between surfaces that are depleted of the polyelectrolyte and the rest of the system. This Donnan potential tends to shift the polyelectrolyte density profile toward the colloidal surface and leads to the local accumulation of polyelectrolytes. We derive a zero-field theory for the disjoining pressure between two parallel flat plates. The polyelectrolyte is allowed to enter the confined interplate region at the cost of a conformational free energy penalty. The resulting disjoining pressure shows a crossover to a repulsive regime when the interplate separation gets smaller than the size of the polyelectrolyte chain, followed by an attractive part. We find a quantitative match between the model and self-consistent field computations that take into account the full Poisson-Boltzmann electrostatics.

On the origin of oscillatory interactions between surfaces mediated by polyelectrolyte solution.

We use a numerical implementation of polymer classical density functional theory with an incompressibility condition to study the system consisting of nonadsorbing polyelectrolytes confined by two planar surfaces and quantify the effective interaction between the two planar surfaces as a function of the polyelectrolyte and salt concentrations. Our results indicate that for the uncharged surfaces (and weakly charged surfaces), the effective interaction primarily consists of a short-range attraction due to the depletion followed by repulsion due to the electric double layer overlapping and electrostatic correlations. For salt-free and low salt concentration systems, the magnitude of the repulsion barrier is determined by the overlap between the electric double layers, while at relatively high salt concentrations, the magnitude of the repulsion barrier is determined by the electrostatic correlations. Due to the competition between the electric double layer and the electrostatic correlations, the magnitude of the repulsion barrier varies nonmonotonically. In contrast, a mean-field Poisson-Boltzmann treatment of the electrostatics predicts a monotonically decreasing repulsion barrier with increasing salt concentration. At moderate salt concentrations, our theory predicts oscillatory interaction profiles. A comparison with the mean-field Poisson-Boltzmann treatment of electrostatics suggests that the oscillations are due primarily to electrostatic correlations.

Oscillatory Packing and Depletion of Polyelectrolyte Molecules at an Oxide−Water Interface

Total internal reflection microscopy (TIRM) has been used to study the interactions between a 5 μm borosilicate glass sphere and a silica slide in the presence of a nonadsorbing polyelectrolyte, sodium (polystyrene sulfonate) (NaPSS). The effect of the polymer concentration, within the dilute solution regime, on the observed interactions was investigated. In all cases, the interactions displayed a short-range electrostatic repulsion followed immediately, at larger separations, by a decaying oscillatory interaction that is attributed to structuring of the polyelectrolyte in solution. The periodicity of the oscillations, as a function of concentration, indicates that at large surface separations the polymer chains are ordered as a nonintermixing, space-filling, latex. At polymer concentrations of between 200 and 1000 ppm, a transition to a system of ordered rods, parallel to the interface was seen for the final layer of polymer molecules.

Debye Length and Double-Layer Forces in Polyelectrolyte Solutions

We report experimental results and a theoretical analysis of the Debye length in aqueous solutions of nonadsorbing polyelectrolytes. The measurements were done using a surface forces apparatus, in which the normal forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured as a function of surface separation (to ±1 Å). HA is negatively charged and does not adsorb to the negatively charged surface of mica, as was established by optical and viscosity measurements and in agreement with the measured force−distance curves. From these measurements it appears that the multivalent polyelectrolyte is “depleted” from the gap between the surfaces. We use the mean-field Poisson−Boltzmann theory to theoretically predict the effective Debye length and double-layer force under such conditions and compare the predictions with the experimental results. The comparison gives excellent agreement and shows that the effective Debye length is determined solely by the monovalent ions in the solution. Specifically, the effective Debye length κeff-1 for the double-layer interaction is determined by an effective ionic concentration given by , where ns and nc are the bulk negative and positive monovalent ion concentrations, respectively.

The importance of adsorbed cationic surfactant structure in dictating the subsequent interaction of anionic surfactants and polyelectrolytes with pigment surfaces

Abstract The adsorption dynamics of the cationic surfactant cetyltrimethylammonium bromide (C16TAB) at the TiO2–water interface was investigated using Fourier transform infrared–attenuated total reflection spectroscopy (FTIR–ATR). Specifically, the response of the adsorbed C16TAB to the anionic surfactant sodium dodecyl sulfate (SDS) is used to discern the structure of the adsorbed C16TAB at the hemimicelle concentration (HMC). In this low concentration regime C16TAB is shown to adsorb as isolated clusters with a ‘defective’ bilayer structure. However, these bilayer structures can be effectively transformed into monolayer structures through the interaction with the non-adsorbing polyelectrolyte sodium polyacrylate (NaPA). It is shown that the structure of the adsorbed C16TAB layer, and not the amount of adsorbed surfactant, dictates the subsequent adsorption of anionic species on negatively charged TiO2 particles.

Measurement of Colloidal Stability in Solutions of Simple, Nonadsorbing Polyelectrolytes

The stability of a solution of charged polystyrene particles in the presence of nonadsorbing polyelectrolyte macromolecules is measured using optical light scattering. The particles were negatively charged polystyrene latex spheres (0.5–1 μm diameter) while the macromolecules were simulated using negatively charged colloidal silica spheres (5–7 nm diameter). Because of the electrostatic repulsion between the particles, the solution is found to be stable against primary flocculation (irreversible flocculation into a primary energy minima). However, because of long-range attractive depletion forces, reversible secondary flocculation of the particles occurs into a local potential energy minimum. As observed with uncharged macromolecules, the polyelectrolyte first induces flocculation at a critical flocculation concentration (v*), but later restabilizes the system at a critical restabilization concentration (v**). These critical concentrations are found to decrease with decreasing macromolecule size and increasing particle size. The restabilized solutions are found to remain suspended for periods greater than 20 days. Comparison of the measured flocculation and restabilization results to predictions made using a recently developed force-balance model show qualitative agreement.

Phase Separation of Concentrated Aqueous Silica Dispersions in the Presence of Nonadsorbed Polyelectrolytes

The stability of silica dispersions (Ludox HS) has been studied in the presence of two nonadsorbing polyelectrolytes, i.e., sodium carboxymethyl cellulose (CMC) and sodium polystyrene sulfonate (SPSS). For CMC, phase segregation occurred and polymer- and particle-rich phases were formed. The particle phase retained its liquidlike properties. Increasing the polymer charge density reduced the tendency for phase separation to occur while varying the molecular mass had little effect. For SPSS, particle flocculation/gelation occurred rather than polymer/particle segregation and increased polymer concentrations were required. The concentration of CMC required to induce separation increased in the presence of 0.1 M NH4NO3 while that of SPSS decreased. Particle interaction in the presence of CMC was monitored by turbidity measurements and the second virial coefficients obtained were in accordance with the trends observed in the phase behavior. Osmotic pressure and sodium ion activity measurements indicated that additional counterion condensation occurred on mixing of the polymer and particles. Potential energy calculations incorporating van der Waals, electrostatic, and depletion terms indicated that CMC will induce phase separation more readily than SPSS due to its increased depletion force and hence could explain the differences in behavior observed between the two polymers. The fact that CMC molecules are relatively stiff compared to SPSS molecules and thus can form concentrated anisotropic phases is also believed to be an important factor. Although depletion theory predicted that phase separation would occur more readily as the CMC charge density increased, the reverse trend was observed and was attributed to increased intermolecular repulsion between the CMC chains.

Flocculation of polymer latex dispersion induced by a polyelectrolyte

The stability of high surface charge latex dispersions containing non-adsorbing polyelectrolyte was investigated. Poly-(styrene/sodium p-vinylphenylsulphonate) copolymer latices and sodium polystyrenesulphonate systems were used. Flocculation was confirmed using three observational methods — transmission measurement, microscopy and dynamic laser light scattering particle analysis. It was found that in flocculation the cluster size of the flocculated particles split into two peaks — a small peak consisting almost entirely of single particles and a larger peak which increased with time. Moreover, the floc which formed was in an ordered state and changed into a crystalline phase after sedimentation.

Depletion layer measurements using nuclear magnetic resonance for silica dispersions with non-adsorbing poly(styrene sulphonate)

Nuclear magnetic resonance relaxation times have been used to estimate the depletion layer thickness for non-adsorbing polyelectrolytes in silica dispersions. The layer thicknesses have been estimated using a simple square-well model for the depletion zone. The results show an increase in the depletion layer thickness both with increasing molecular weight and decreasing equilibrium polymer concentration.