Polarizability and complex conductivity of dilute suspensions of spherical colloidal particles with charged (polyelectrolyte) coatings

The dielectric relaxation of polyelectrolyte-coated colloidal particles is examined via "exact" numerical solutions of the governing electrokinetic equations. The charged polymer coatings are characterized by a nominal charge density, thickness, and permeability. Brush-like segment density profiles are considered here, but more sophisticated segment and charge density profiles are accommodated by the model. The role of added counterions and nonspecific adsorption is considered briefly before examining how the experimentally measured conductivity and dielectric constant increments reflect the frequency of the applied electric field, the strength of the electrolyte, and characteristics of the polymer coatings, namely the charge, charge density, and permeability. Finally, a strategy is suggested by which dielectric spectroscopy and electrophoresis can be used to characterize polymer-coated particles. This approach complements experiments where electroviscous effects such as dynamic light scattering and sedimentation are weak.     

Dielectric spectroscopy and electrophoretic mobility measurements interpreted with the standard electrokinetic model

We report results from complementary electrokinetic measurements-dielectric relaxation and electrophoretic mobility-undertaken to test the applicability of the standard electrokinetic theory with a model system. Dielectric spectra were obtained at frequencies between 1 kHz and 40 MHz with a new, two-electrode cell design [Hollingsworth and Saville, J. Colloid Interface Sci. 257 (2003) 65-76]; mobility data were acquired with an electrophoretic light scattering instrument. Data from the two-electrode cell were collected at different electrode separations and interpreted with newly developed procedures to remove the influence of electrode polarization. Methodology A employs extrapolation to infinite electrode separation to compute the dielectric constant and conductivity as functions of frequency. The contributions from suspended particles are reported in terms of dielectric constant and conductivity increments. Methodology B uses a theoretical model of electrode polarization and the standard electrokinetic model in a nonlinear regression scheme. Results are presented in several forms: frequency-dependent dielectric constant and conductivity increments, frequency-dependent dielectric constants and conductivities, and the complex dipole coefficient. It is shown that the standard model provides a consistent methodology for interpreting particle behavior; zeta-potentials inferred from mobility and dielectric relaxation agree to within experimental error. Moreover, the cell design and interpretation are straightforward and provide relatively simple ways to obtain complementary measurements over a wide frequency range. The results unambiguously show that electrokinetic character of this dispersion follows the standard model.     

Electrorheological characterization of polyaniline-coated poly(methyl methacrylate) suspensions

Surface-conductive particles consisting of a poly(methyl methacrylate) (PMMA) core and a polyaniline (PA)-coated shell were synthesized and adopted as suspended particles for electrorheological (ER) fluids. The PA-PMMA composite particles synthesized were monodisperse and spherical in shape. The PA-PMMA suspensions in silicone oil showed typical ER characteristics under an applied electric field. The PA-PMMA composite particles possess a higher dielectric constant and conductivity than the pure PA particle, within an acceptable conductivity range for ER fluids, but the PA-based ER fluid showed larger shear-stress enhancement than the PA-PMMA-based systems. This phenomena can be explained by the interfacial polarizability of PA-based ER fluids, which is the difference between the ER fluid's dielectric constant and loss factor - this polarizability was higher than that of PA-PMMA-based ER fluids, as shown by the dielectric spectrum of each fluid. The insulating PMMA core suppressed the interfacial polarization in ER fluids, resulting in reduced interaction among particles under an imposed electric field.     

Synthesis and Electrorheological Properties of LTNO-PS Composites

Core-shell structural Li and Ti co-doped NiO/polystyrene samples (LTNO/PS) were synthesized by a sol-gel method and studied by TEM, X-ray diffraction, IR spectroscopy and LCR meter. It was found that the core-shell LTNO/PS particles are nearly smooth spheres with an average size of around 4.0 mm, while the LTNO particles alone have a size of around 3.5 mm. The electrorheological activity of LTNO particles with PS coating is larger than that of the bare ones with the same components. This research reveals that the high dielectric constant and corresponding dielectric loss of the LTNO is related to the Li and Ti contents. The increase of electrorheological activity of LTNO particles with PS coating is caused by the increase of dielectric permittivity, the surface structural change, and the reduction of leakage current of PS-coated samples due to the high resistivity and soft contacting of PS shell. By the preparation of core/shell structural materials and taking the advantage of the shell to reduce the leakage current between the particles, the electrorheological effect can be effectively increased.     

Electrokinetic effects of adsorbed neutral polymers

On the basis of a previously developed hydrodynamic model for adsorbed polymers the charge flow along a charged interface with adsorbed (uncharged) polymer is calculated. An effective electrokinetic layer thickness is defined and its dependence on the characteristics of the adsorbed polymer and the ionic strength is studied. It is found that tails are very important for the hydrodynamic effects considered because they effectively screen the solvent flow from inner parts of the absorbed layer. The electrokinetic layer thickness increases with decreasing ionic strength, and tends to a limit equal to the hydrodynamic thickness at very low ionic strength.     

Coil-to-globule-type transition of poly(N-isopropylacrylamide) adsorbed on colloidal silica particles

 The temperature dependence of the dimensions of poly(N-isopropylacrylamide) (PNIPAM) adsorbed on two different colloidal silica particles was studied with dynamic light scattering. The hydrodynamic diameter was measured when the temperature was varied stepwise from 10 to 60 °C. PNIPAM molecules free in solution undergo a conformational transition at the θ temperature. We have found that PNIPAM adsorbed onto silica particles also undergoes a transition below the θ temperature. When a small amount of polymer was adsorbed the coil-to-globule transition at the θ temperature did not occur. Potentiometric titrations showed that the surface charge of the silica particles was not affected by the polymer adsorption. Sodium dodecyl sulfate (SDS) (100–1200 mg/l) was added to improve the stability. The particles with a higher zeta potential required a smaller addition of SDS to prevent coagulation compared to the particles with a smaller surface potential. For low additions of SDS the transition curves of adsorbed PNIPAM were unaffected. For larger additions of SDS the collapse of PNIPAM was shifted to higher temperatures. When as much as 1200 mg/l SDS was added, two regions with weak transitions were observed before the collapse. It was also observed that the presence of SDS results in a smaller adsorption of PNIPAM onto the particles. The addition of SDS strongly increased the magnitude of the electrophoretic mobility of the polymer–particle unit. From the electrophoretic measurements an electrokinetic layer thickness was calculated and it was found to be smaller than the corresponding hydrodynamic layer thickness, as obtained by dynamic light scattering. Received: 14 December 1999/In revised form: 22 February 2000/Accepted: 6 March 2000     

Open-tubular capillary electrochromatography using a polymeric surfactant coating

A stable polyelectrolyte multilayer (PEM) coating was investigated for use in open-tubular capillary electrochromatography (o-CEC). In this approach, the PEM consisted of the cationic polymer of a quaternary ammonium salt, poly(diallyldimethylammonium chloride) and the anionic polymeric surfactant, poly(sodium undecylenic sulfate). Both the cationic and anionic polymers were physically adsorbed to the surface of a fused-silica capillary by use of a simple coating procedure. This procedure involved an alternate rinse of the positively and negatively charged polymers. The performance of the PEM coating as a dynamic stationary phase was evaluated by use of electrochromatographic experiments and showed good selectivity for both phenols and benzodiazepines. Reproducibility of the PEM coating was also evaluated by calculating the relative standard deviations (RSDs) of the electroosomotic flow (EOF). The run-to-run and capillary-to-capillary RSD values of the EOF were less than 1.5%. The endurance of the coating was more than 100 runs. The importance of the PEM coating was illustrated by comparing separations on a bare uncoated capillary with the coated capillary. In addition, the chromatographic performance using o-CEC and micellar electrokinetic chromatography (MEKC) was compared for the separation of benzodiazepines.     

The electric birefringence of polyelectrolytes: an electrokinetic approach

We discuss the Kerr constant of a polyelectrolyte solution in the dilute regime. We show that the birefringence induced in a suspension of nonspherical polyelectrolytes by an external electric field probes the electrokinetic properties of the suspension. This is because the Kerr constant is directly connected to the electric torque exerted on the particles, and therefore contains information on the induced dipole, similarly to the other electrokinetic techniques. The article is a guideline for the development of an electrokinetic theory of the electric birefringence of polyelectrolytes. We compare two different methods to derive the Kerr constant of the polyelectrolyte solution. The first method uses an expression for the electric torque which is obtained through electrostatics, and yields a Kerr constant which has the same frequency dependence as the anisotropy of the real part of the polarizability of the dressed particle (that is, of the particle plus surrounding ions). The second method assigns an effective value of the induced electric dipole per particle by using the theory of the dielectric enhancement, and gives a Kerr constant proportional to the anisotropy of the real part of the dielectric constant of the suspension. The two methods give a considerably different frequency dependence of the Kerr constant: we suggest that the expression obtained by the second method is more capable of correctly describing the main features of the experimental results obtained with polyelectrolytes having small shape anisotropy.     

Simulating polarizable molecular ionic liquids with Drude oscillators

The Drude oscillator model is applied to the molecular ionic liquid 1-ethyl-3-methyl-imidazolium triflate. The range of manageable Drude charges is tested. The strength of the polarizability is systematically varied from 0% to 100%. The influence on the structure, single particle dynamics, and collective dielectric properties is investigated. The generalized dielectric constant can be decomposed into a dielectric permittivity, a dielectric conductivity, and an optical dielectric constant ?(∞). The major part of the static generalized dielectric constant comes from the collective rotation of the ions, i.e., the dielectric permittivity. The translational contribution from the dielectric conductivity is about 58% of the dielectric permittivity. For the evaluation of the optical dielectric contribution, the computational dielectric theory was adapted to the case of heterogeneous polarizabilities. In case of 100% polarizability, it reaches a value of approximately 2.     

A novel particle separation method based on induced‐charge electro‐osmotic flow and polarizability of dielectric particles

A new microfluidic method of particle separation was proposed and studied theoretically in this paper. This method is based on the induced charge electro‐osmotic flow (ICEOF) and polarizability of dielectric particles. In this method, a pair of metal plates is embedded on the side channel walls to create a region of circulating flows under applied electric field. When a dielectric particle enters this region, the vortices produced by ICEOF around the particle will interact with the circulating flows produced by the metal plates. Such hydrodynamic interaction influences the particle's trajectory, and may result in the particle being trapped in the flow circulating zone or passing through this flow circulating zone. Because the hydrodynamic interaction is sensitive to the applied electric field, and the polarizability and the size of the particles, separation of different particles can be realized by controlling these parameters. Comparing with electrophoresis and dielectrophoresis methods, this strategy presented in this paper is simple and sensitive.     

Polymeric stabilized emulsions: steric effects and deformation in soft systems

Polymeric stabilizers are used in a broad range of processes and products, from pharmaceuticals and engine lubricants to formulated foods and shampoos. In rigid particulate systems, the stabilization mechanism is attributed to the repulsive force that arises from the compression of the polymer coating or "steric brush" on the interacting particles. This mechanism has dictated polymer design and selection for more than thirty years. Here we show, through direct measurement of the repulsive interactions between immobilized drops with adsorbed polymers layers in aqueous electrolyte solutions, that the interaction is a result of both steric stabilization and drop deformation. Drops driven together at slow collision speeds, where hydrodynamic drainage effects are negligible, show a strong dependence on drop deformation instead of brush compression. When drops are driven together at higher collision speeds where hydrodynamic drainage affects the interaction force, simple continuum modeling suggests that the film drainage is sensitive to flow through the polymer brush. These data suggest, for drop sizes where drop deformation is appreciable, that the stability of emulsion drops is less sensitive to the molecular weight or size of the adsorbed polymer layer than for rigid particulate systems.     

Electrokinetic study of adsorption of ionic surfactants on titania from organic solvents

The electrokinetic potential of titania was studied as a function of concentration of SDS, DOSS and CTMABr in a series of solvents. In water and 50–50 water–methanol mixture, which are the most polar studied solvents, the organic ion is adsorbed on titania and the small inorganic ion remains in solution. In hexane the adsorption behavior is reversed, that is, the organic ion remains in solution and the small inorganic ion is adsorbed on titania. The borderline between these two types of behavior corresponds to solvent dielectric constant of about 25. In solvents, which have a dielectric constant in this range (methanol and 1-propanol) the adsorption preferences vary from one surfactant to another. The affinities of the organic ion and of the small inorganic ion to the surface are often similar, and then none of the ionic components is preferentially adsorbed, and the electrokinetic potential is not affected. In such cases, ionic surfactants are not suitable as agents for regulation of zeta potential.     

Adsorption of cellulose derivatives on flat gold surfaces and on spherical gold particles

The adsorption of hydroxyethylcellulose (HEC), ethyl(hydroxyethyl)cellulose (EHEC), and their hydrophobically modified counterparts HM-HEC and HM-EHEC has been studied on planar gold and citrate-covered gold surfaces by means of quartz crystal microbalance with dissipation monitoring (QCM-D), and on citrate-covered gold particles with the aid of dynamic light scattering (DLS). The QCM-D results indicate that larger amounts of polymer are adsorbed from aqueous solutions of HM-HEC and HM-EHEC on both substrates than from solutions of their unmodified analogues. The adsorption affinity for all the polymers, except EHEC, is higher on the citrate-covered surfaces than on the bare gold substrate. This indicates that more adsorption sites are activated in the presence of the citrate layer. The experimental adsorption data for all the polymers can be described fairly well by the Langmuir adsorption isotherm. However, at very low polymer concentrations significant deviations from the model are observed. The value of the hydrodynamic thickness of the adsorbed polymer layer (delta h), determined from DLS, rises with increasing polymer concentration for all the cellulose derivatives; a Langmuir type of isotherm can be used to roughly describe the adsorption behavior. Because of good solvent conditions for HEC the chains extend far out in the bulk at higher concentrations and the value of delta h is much higher than that of HM-HEC. The adsorption of EHEC and HM-EHEC onto gold particles discloses that the values of delta h are considerably higher for the hydrophobically modified cellulose derivative, and this finding is compatible with the trend in layer thickness estimated from the QCM-D measurements.     

FTIR study of ion dissociation in PMMA based gel electrolytes containing ammonium triflate: Role of dielectric constant of solvent

The effect of dielectric constant of solvent on the presence of ion aggregates/undissociated salt and their dissociation with the addition of polymer has been studied by FTIR for polymethylmethacrylate (PMMA) based gel electrolytes containing ammonium triflate (NH₄CF₃SO₃). Salt is fully dissociated in electrolytes containing dimethylacetamide (DMA)—a high dielectric constant solvent and some ion aggregates are also present whereas in electrolytes containing diethylcarbonate (DEC)—a low dielectric constant solvent, some undissociated salt is present. The conductivity behaviour of polymer gel electrolytes has been found to depend upon the dielectric constant of the solvent used. PMMA plays the role of a stiffener in electrolytes containing DMA and results in a small decrease in conductivity whereas in electrolytes containing DEC, the addition of PMMA results in an increase in conductivity which has been explained to be due to an increase in free ion concentration by the dissociation of undissociated salt and ion aggregates. The presence of free ions, ion aggregates, undissociated salt has also been examined by FTIR spectroscopy.     

非导电球型粒子悬浮液宽频介电谱——薄双电层理论的计算机模拟

以Shilov等提出的带有紧密层表面电导率的非导电球型粒子悬浮液宽频介电弛豫的薄双电层理论为基础, 从电动力学角度解释了粒子分散系两种典型介电弛豫(高频和低频弛豫)的机制. 在此基础上, 利用Mathcad程序将该理论定量程序化并建立了粒子/水相分散系介电谱参数与体系内部相参数的关系. 进而利用该程序模拟了溶液浓度、Zeta电位以及分散粒子半径等内相参数对两种弛豫的影响, 结合该理论阐述了不同环境下这两种弛豫的变化规律, 从而为今后更好地利用这两种弛豫表征纳米至毫米级球形粒子分散系的各相电及界面性质提供了有价值的参考.     

Controlled clustering and enhanced stability of polymer-coated magnetic nanoparticles

The clustering and stability of magnetic nanoparticles coated with random copolymers of acrylic acid, styrenesulfonic acid, and vinylsulfonic acid has been studied. Clusters larger than 50 nm are formed when the coatings are made using too low or too high molecular weight polymers or using insufficient amounts of polymer. Low-molecular-weight polymers result in thin coatings that do not sufficiently screen van der Waals attractive forces, while high-molecular-weight polymers bridge between particles, and insufficient polymer results in bare patches on the magnetite surface. The stability of the resulting clusters is poor, but when an insufficient polymer is used as primary coating, and a secondary polymer is added to coat remaining bare magnetite, the clusters are stable in high salt concentrations (>5 M NaCl), while retaining the necessary cluster size for efficient magnetic recovery. The magnetite cores were characterized by TEM and vibrating sample magnetometry, while the clusters were characterized by dynamic light scattering. The clustering and stability are interpreted in terms of the particle-particle interaction forces, and the optimal polymer size can be predicted well on the basis of these forces and the solution structure and hydrophobicity of the polymer. The size of aggregates formed by limited polymer can be predicted with a diffusion-limited colloidal aggregation model modified with a sticking probability based on fractional coating of the magnetite cores.     

Note on nonaqueous electrokinetic transport in charged porous media

A model for electrokinetic transport in charged capillaries is compared with experiments using nonaqueous lithium chloride solutions. The electrokinetic parameters considered are the pore fluid conductivity and the concentration potential. Methanol/water mixtures were the solvent, and track-etched mica membranes with a well-characterized pore structure were the porous medium. The electrolyte concentrations used were such that the Debye lengths of solutions in pores ranged from much smaller to much larger than the radius of pores. The space-charge model is found to be capable of qualitatively describing the trend of the electrokinetic data, but as expected, at higher concentrations the model fails, probably because the assumption that ion—ion interactions are negligible no longer holds. The experimental results show that the pore fluid conductivity depends strongly on the dielectric constant of the solvent, that the absolute value of the pore wall charge tends to decrease with the lowering of the solvent dielectric constant, and that the wall charge tends to increase with the concentration of the chloride ion.     

Single step hybrid coating process to enhance the electrosteric stabilization of inorganic particles

We report on a single-step coating process and the resulting colloidal stability of silica-coated spindle-type hematite nanoparticles (NPs) decorated with a layer of poly(acrylic acid) (PAA) polyelectrolyte chains that are partially incorporated into the silica shell. The stability of PAA coated NPs as a function of pH and salt concentration in water was compared to bare hematite particles and simple silica-coated hematite NPs, studying their electrophoretic mobility and the hydrodynamic radius by dynamic light scattering. Particles coated with this method were found to be more stable upon the addition of salt at pH 7, and their aggregation at the pH of the isoelectric point is reversible. The hybrid coating appears to increase the colloidal stability in aqueous media due to the combination of the decrease of the isoelectric point and the electrosteric stabilization. This coating method is not limited to hematite particles but can easily be adapted to any silica-coatable particle.     

Electrokinetic potential of bentonite and kaolin particles in the presence of polymer mixtures

In order to elucidate the mechanisms of flocculation by polymer mixtures, the effect of adsorption of non-ionic poly(ethylene oxide) — PEO, two samples of strongly (SNF FO 4800) and medium charged (SNF FO 4350) cationic and two samples of medium (SNF AN 935) and weakly charged (SNF AN 905) anionic polyelectrolytes (PE) as well as their binary mixtures on the electrokinetic potential of bentonite and kaolin particles has been studied. It is shown that in the presence of PEO-anionic/cationic polymer mixture, the electrokinetic potential of particles is determined by the adsorption of the polyelectrolyte; neither cationic nor anionic segments can be displaced by the non-ionic polymer. In mixtures of cationic and anionic polyelectrolytes, the ζ-potential of particles is determined by the adsorbed amount of anionic polymer independently of the charge density of PE and way of addition of the mixture components to the suspension, i.e. (1) first adding the cationic polymer, then the anionic one, or (2) first adding the anionic polymer then the cationic one, or (3) adding an increasing amount of pre-prepared 1: 1 mixture. The highest absolute ζ-potential values are observed for pH 7.5 when the surface of bentonite or kaolin particles is “purely” negatively charged and the anionic PE layer is most extended because of few contacts to the surface. With decreasing the pH, the (negative) ζ-potential of particles decreases due to appearance of a small amount of positive charges on the surface that bond an increasing amount of negative segments and results in shrinking of the adsorbed layer of the anionic PE. It is shown also that the electrokinetic potential of particles in anionic and cationic PE mixtures at all studied pH (4, 5, and 7.5) depends on the spatial distribution of negatively charged segments near the surface. The regularities observed are explained by formation of long loops and tails of anionic segments on the surface because of the small number of contacts to the surface; the cationic polyelectrolyte forms on the surface a thin layer with a big number of contacts and which is hidden behind the more extended anionic polymer layer.