Al-Mg MMH正电胶体粒子体系的流变学

研究了铝／镁混合金属氢氧化物（MMH）粒子含量,PH值,电解质等因素对MMH悬浮液流变学的影响,发现MMH粒子浓度的增加使悬浮体从牛顿型流体变为带有屈服值的假塑性流体;在相对很低的粒子浓度（2％）时,通过改变粒子表面化学状态和粒子周围的介质环境可以改变悬浮体的流动形态;MMH悬浮体触变性结构与高PH值时合成锂皂石悬浮体的结构极为相似,是粒子间的静电排斥作用所致。     

Electrophoretic mobility of poly(acrylic acid)-coated alumina particles

The effect of poly (acrylic acid) (PAA) adsorption on the electrokinetic behavior of alumina dispersions under high pH conditions was investigated as a function of polymer concentration and molecular weight as well as the presence, concentration and ion type of background electrolyte. Systems of this type are relevant to nuclear waste treatment, in which PAA is known to be an effective rheology modifier. The presence of all but the lowest molecular weight PAA studied (1800) led to decreases in dynamic electrophoretic mobility at low polymer concentrations, attributable to bridging flocculation, as verified by measurements of particle size distribution. Bridging effects increased with polymer molecular weight, and decreased with polymer concentration. Increases in background electrolyte concentration enhanced dynamic electrophoretic mobility as the polymer layers were compressed and bridging was reduced. Such enhancements were reduced as the cation was changed from K(+) to Na(+) to Cs(+).     

SUSPENSION PROPERTY OF ALUMINA IN LAPPING MEDIA

In order to understand the suspension properties of alumina grit in lapping liquids, with particular interest on their flocculation behavior both electro kinetic and sedimentation experiments were carried out on three different alumina powders, that were either wet or air classified in two commercial additives ( Mon - 60 and Rust Lick - 900 ). The results suggest that all three alumina powders have some acidic surface impurity and both Mon - 60 and Rust Lick - 900 are adsorbed on the alumina surface below pH 9. The sedimentation behavior of these alumina powders from their dispersions is independent of the classification of the alumina and nature of the additive but is dependent on the pH of the dispersion.     

THE FLOCCULATION OF AQUEOUS SUSPENSIONS OF TITANIA INDUCED BY SURFACE NUCLEATION OF CALCIUM PHOSPHATES#

ABSTRACT

Titanium oxide aqueous suspensions have been destabilized by a novel mechanism; the nucleation of octacalcium phosphate on the particle surfaces, using the constant composition technique. The results suggest that the potential energies resulting from the electric double layers around the titania particles play only secondary roles in the flocculation, in that a decrease in ζ-potential of titania colloids from ?47 mV to ?13 mV does not in itself cause the titania suipension to flocculate, in contrast to the predictions of the classical DLVO theory. However, in the process of hucleation of the moderately hydrophobic octacalcium phosphate phase (γ^- _sequals;19.7 mJ m^-2) onto titania particle surfaces, the latter become more hydrophobic with values ranging from 52 to 23.4 mJ m-2, This change is sufficient to cause the particles to flocculate since the energies of attraction result from hydrophobic interactions (90%) and van der Waals forces (10%).     

Dispersion of alumina suspension using comb-like and diblock copolymers produced by RAFT polymerization of AMPS and MPEG

Different copolymers of 2-acrylamido-2-methylpropanesulfonic acid sodium salt (AMPS) methoxypolyethyleneglycol methacrylate (MPEG) with statistical and diblock distributions were prepared using RAFT-controlled radical polymerization. The effect of polymer architecture and monomer ratio on the adsorption behavior, electrokinetic, and stability properties of the alumina suspensions was investigated. Adsorption isotherms showed that copolymer interaction depended on both the ratio of the monomers and their distribution within the macromolecular backbone. Changes in the electrokinetic properties of the alumina suspension after addition of the copolymers were investigated by monitoring the particle zeta-potential as a function of pH. A continuous shift in the isoelectric point IEP to a more acidic value was observed and particle charges were reversed when the amount of copolymer added exceeded a critical level.     

Influence of surfactant addition on the stability of concentrated alumina dispersions in water

The goal of this study was to assess the effects of surfactant addition on the stability and viscosity of concentrated alumina dispersions. The stabilizing effects of several candidate surfactants were investigated for concentrated dispersions of two different pseudoboehmite aluminas at pH 4 and 7. The stabilities of concentrated alumina dispersions treated by pH adjustment alone and by pH adjustment combined with surfactant addition were compared to assess the degree to which the surfactant enhanced stability. The initial rate of mass removal from a sedimenting alumina dispersion was used as a measure of stability.

The anionic surfactants Surfine WNT-A and DOWFAX 3B2 were identified as effective in enhancing the stability of concentrated alumina dispersions. The optimal doses of these surfactants for stabilizing 15% by weight VERSAL™ 250 alumina dispersions at pH 4 were determined to be about 4.6 × 10⁻⁵ mol g⁻¹ for both surfactants. On the basis of the initial rate of mass removal, surfactant-stabilized 15 wt.% suspensions were found to be approximately 2.5 and 10.6 times more stable than similar dispersions stabilized electrostatically by pH adjustment alone. These more stable dispersions exhibited lower viscosities than observed for the alumina dispersions not subjected to surfactant addition. The results indicate that the stability of concentrated alumina dispersions can be enhanced by anionic surfactant addition, and that such surfactants may therefore help to control the rheology of concentrated dispersions of alumina in water.     

Characterization of Fumed Alumina/Silica/Titania in the Gas Phase and in Aqueous Suspension

Fumed oxide alumina/silica/titania was studied in comparison with fumed alumina, silica, titania, alumina/silica, and titania/silica by means of XRD, ¹H NMR, IR, optical, dielectric relaxation, and photon correlation spectroscopies, electrophoresis, and quantum chemical methods. The explored Al₂O₃/SiO₂/TiO₂ consists of amorphous alumina (22 wt%), amorphous silica (28 wt%), and crystalline titania (50 wt%, with a blend of anatase (88%) and rutile (12%)) and has a wide assortment of Brønsted and Lewis acid sites, which provide a greater acidity than that of individual fumed alumina, silica, or titania and an acidity close to that of fumed alumina/silica or titania/silica. The changes in the Gibbs free energy (ΔG) of interfacial water in an aqueous suspension of Al₂O₃/SiO₂/TiO₂ are close to the ΔG values of the dispersions of pure rutile but markedly lower than those of alumina, anatase, or rutile covered by alumina and silica. The zeta potential of Al₂O₃/SiO₂/TiO₂ (pH of the isoelectric point (IEP) equals ≈3.3) is akin to that of fumed titania (pH(IEP_TiO2) ≈ 6) at pH > 6, but it significantly differs from the ζ of fumed alumina (pH(IEP_Al2O3) ≈ 9.8) at any pH value as well as those of fumed silica, titania/silica, and alumina/silica at pH < 6. The particle size distribution in the diluted aqueous suspensions of Al₂O₃/SiO₂/TiO₂ studied by means of photon correlation spectroscopy depends relatively slightly on pH in contrast to the titania/silica or alumina/silica dispersions. Theoretical calculations of oxide cluster interaction with water show a high probability of hydrolysis of Al–O–Ti and Si–O–Ti bonds strained at the interface of alumina/titania or silica/titania due to structural differences in the lattices of the corresponding individual oxides. Ab initio calculated chemical shift δ_H values of H atoms in different hydroxyl groups at the oxide clusters and in bound water molecules are in agreement with the ¹H NMR data and show a significant impact of charged particles (H₃O⁺ or OH⁻) on the average δ_H values of water droplets with (H₂O)_n at n between 2 and 48.     

Aggregation behavior of poly(N-isopropylacrylamide) microspheres

Electrophoretic mobility and aggregation in suspensions of three types of microspheres (Ms 1, Ms 2 and Ms 3) are studied at different pH, ionic strengths and temperatures of the medium. Here Ms 1 is a core particle composed of poly(N-isopropylacrylamide-co-styrene). Ms 2 is a core-shell microsphere consisting of Ms 1 as the particle core covered with a surface layer of poly(N-isopropylacrylamide) hydrogel. Ms 3 is also a core-shell microsphere composed of MS-1 covered with a surface layer of poly(N-isopropylacrylamide-co-acrylic acid) hydrogel. The charge density zN and the softness parameter 1/λ of the microspheres were obtained from the electrophoretic mobility data on the basis of an electrokinetic theory of soft particles. It is shown that when zN is large, suspensions of microspheres are always stable, showing no aggregation. When zN is small, the suspensions are stable for large 1/λ but show strong aggregation for small 1/λ.     

Stability of mixtures of charged silica, silica-alumina, and magnetite colloids

We report experiments on the stability of aqueous mixtures of charged colloidal magnetite and charged silica and silica covered with alumina particles of similar size. First, positively charged magnetite dispersions were mixed with negatively charged silica dispersions at pH 4, at different volume ratios and low colloid volume fractions, producing mixtures which were stable over a period of weeks despite the expected electrostatic attraction between the oppositely charged particles. When magnetite particles were mixed with positively charged silica covered with alumina at pH 4 under exactly the same conditions, some of the systems separated to form a magnetite sediment. When the volume fraction of the initial dispersions was increased, the behavior of the mixtures was the opposite: positive magnetite/negative silica mixtures were unstable at intermediate volume ratios. The unexpected behavior of the mixtures was investigated by means of electrophoretic mobility, initial susceptibility, and dynamic light scattering measurements as well as sedimentation experiments.     

A prediction method for the isoelectric point of binary protein mixtures of bovine serum albumin and lysozyme adsorbed on colloidal titania and alumina particles

Bovine serum albumin and lysozyme mixtures of different mole fractions were adsorbed to colloidal alumina (116 nm) and titania particle (271 nm) suspensions of 2 vol % solid content for 16 h at pH 7.5. The total protein amount normalized to the powder surface area was 1000 ng/cm2. The zeta potential of the protein-treated suspensions was measured as a function of pH and the isoelectric point (IEP) obtained. A simple prediction model in two refinement steps was derived and evaluated for the obtained IEPs. The best model fit which takes into account moles of protein and surface fractions yielded an average prediction error of 7.5% and a maximum error of 16.7%.     

Cell model of the direct current electrokinetics in salt-free concentrated suspensions: the role of boundary conditions

In this paper, a general electrokinetic theory for concentrated suspensions in salt-free media is derived. Our model predicts the electrical conductivity and the electrophoretic mobility of spherical particles in salt-free suspensions for arbitrary conditions regarding particle charge, volume fraction, counterion properties, and overlapping of double layers of adjacent particles. For brevity, hydrolysis effects and parasitic effects from dissolved carbon dioxide, which are present to some extent in more "realistic" salt-free suspensions, will not be addressed in this paper. These issues will be analyzed in a forthcoming extension. However, previous models are revised, and different sets of boundary conditions, frequently found in the literature, are extensively analyzed. Our results confirm the so-called counterion condensation effect and clearly display its influence on electrokinetic properties such as electrical conductivity and electrophoretic mobility for different theoretical conditions. We show that the electrophoretic mobility increases as particle charge increases for a given particle volume fraction until the charge region where counterion condensation takes place is attained, for the above-mentioned sets of boundary conditions. However, it decreases as particle volume fraction increases for a given particle charge. Instead, the electrical conductivity always increases with either particle charge for fixed particle volume fraction or volume fraction for fixed particle charge, whatever the set of boundary conditions previously referred. In addition, the influence of the electric permittivity of the particles on their electrokinetic properties in salt-free media is examined for those frames of boundary conditions.     

Surface electric and titration behaviour of fumed oxides

Adsorption of Pb(II), Sr(II), and Cs(I) on fumed silica, alumina, titania, silica/titania (ST), silica/alumina (SA), and alumina/silica/titania (AST) reveals that mixed oxides containing titania have a greater adsorptive capability in respect to metal cations than individual and SA oxides. Pyrocarbon deposits on fumed oxides enhance the adsorption of metal ions. Calculations of electrophoretic potential (ζ) with consideration for the porosity of aggregates of primary particles of AST show a significant influence of surface alumina (at pH<8) and titania and silica (at pH>8) on the ζ values. The effective diameter of particles (D_ef) of fumed oxides in aqueous media depends on pH for AST stronger than for ST (between isoelectric points (IEPs) of titania and alumina). A significant difference in the pH values of IEP and point of zero charge is observed for AST samples. A pyrocarbon influence on the ζ potential depends on the type of oxide matrix, since ζ increases for certain samples but for others it decreases. These changes depend nonlinearly on pH as well as the secondary particle size distributions (SPSDs) and D_ef.     

Rheology of aqueous carbon black dispersions

The interaction of carbon black with an acrylic resin has been investigated by rheology. Two carbon blacks, with similar particle size and surface characteristics but quite different particle morphologies, have been examined. These are somewhat arbitrarily denoted as "spherical" and "fractal" as shown by small-angle neutron scattering (SANS) and ultrasonic spectroscopy studies. In the absence of polymer, stable aqueous dispersions could not be obtained. Stable dispersions could be obtained, however, upon addition of polymer to a level corresponding to a ratio of 50 mg of polymer per 13 m2 (+/- m2) of surface area (i.e., 15 wt% particles). These stable dispersions exhibit flow typical of concentrated dispersions-Newtonian behavior up to some apparent "yield" or critical value, above which pronounced shear thinning is observed. The critical stress increases with increasing polymer concentration. When a significant amount of nonadsorbed polymer is also present, a second Newtonian plateau is superimposed on the shear-thinning behavior. This feature is observed for both particle types but is more pronounced for the fractal particle. When there is little or no nonadsorbed polymer, the viscosity of the fractal particle dispersions is greater than the viscosity of the spherical particle dispersions. At low polymer concentrations, the dispersions are predominantly viscous at low shear stresses. The phase angle decreases significantly over a narrow shear stress range and the rheology tends to more elastic behavior. At higher shear stresses, the dependence on particle morphology is weak.     

Granulating titania powder by colloidal route using polyelectrolytes

A new, convenient, and inexpensive approach to process and granulate titania powders by a chemical route is proposed. It is based on the use of a formulation that includes a polyanion such as poly(sodium 4-styrenesulfonate) (PSS). Such a polyelectrolyte is most often considered to achieve dispersion of oxide powders in water. Basically, it adsorbs onto the surface of particles and induces electrical and/or steric interactions between particles in the suspension, which prevents agglomeration and rapid sedimentation. The advantages of polyelectrolytes in ceramic processing is well documented in the literature to produce low viscosity suspensions that are further used to form ceramic parts. In the case of TiO2 powders, such aqueous dispersions were obtained by adding small quantities of PSS. However, when exploring the behavior of mixtures containing lower contents of dispersant, we have discovered that, well below the optimum concentration required to get stable dispersions, the polyelectrolyte can act as a binder for titania particles. This can confer cohesion to the agglomerates, which can be processed to form large size (e.g., millimeter size) spheres. This phenomenon takes place when the oxide surface carries both positive and negative electrical charges and can be explained on a simple basis involving surface chemistry. For the optimum concentration of PSS that disperses titania, a polycation such as chitosan should be added to get spheres. This simple technique is expected to receive increasing attention due its potentialities and strong advantages with respect to other granulation techniques, such as spray-drying, which are energy consuming.     

Colloid Vibration Potential in a Concentrated Suspension of Spherical Colloidal Particles

A relation between the dynamic electrophoretic mobility of spherical colloidal particles in a concentrated suspension and the colloid vibration potential (CVP) generated in the suspension by a sound wave is obtained from the analogy with the corresponding Onsager relation between electrophoretic mobility and sedimentation potential in concentrated suspensions previously derived on the basis of Kuwabara's cell model. The obtained expression for CVP is applicable to the case where the particle zeta potential is low, the particle relative permittivity is very small, and the overlapping of the electrical double layers of adjacent particles is negligible. It is found that CVP shows much stronger dependence on the particle volume fraction φ than predicted from the φ dependence of the dynamic electrophoretic mobility. It is also suggested that the same relation holds between the electrokinetic sonic amplitude of a concentrated suspension of spherical colloidal particles and the dynamic electrophoretic mobility. Copyright 1999 Academic Press.     

Investigation On the Behavior of Poly(acrylic acid) in Water Solution and Its Effect on the Rheology of α- Al2O3 Suspension

The influence of pH, relative molecular weight and concentration of poly(acrylic acid) (PAA) on the viscosity of PAA solution was investigated. It was found that the rheology of PAA solution was closely related to the ionization and conformation of PAA chains. The increasing rigidity and the stretching of polymer chain resulted in a maximum viscosity at pH=8. The viscosity change of PAAsolution with addition of alumina powder was studied. It was found that the suspension viscosity reached its minimum when the amount of ceramic powder and PAA was at a certain ratio. At the same time, the viscosity minimum was also influenced by the particle size of powder.     

Hydrodynamic and Colloidal Interactions in Concentrated Charge-Stabilized Polymer Dispersions

Hydrodynamic and colloidal interactions are explored in concentrated, charge-stabilized colloidal dispersions by measuring the dependence of rheology (e.g., low and high-shear viscosity, high-frequency viscosity, and modulus) and self-diffusivity on salt content, particle size, and concentration. Model, sulfonated polystyrene lactices of varying diameter are prepared and investigated by shear rheology, high-frequency torsional resonance, electrophoresis, titration, and dynamic light scattering. The high-frequency and high-shear viscosity both are dominated by hydrodynamic interactions, but are shown not to be identical, due to the microstructure distortion resulting from high shear rates. The short-time self-diffusion is also shown to be insensitive to direct particle interactions, but has a different concentration dependence than the high-frequency viscosity, further illustrating a predicted violation of a generalized Stokes-Einstein relationship for these properties. The apparent colloidal surface charge is extracted from the high-frequency elastic modulus measurements on concentrated dispersions. The surface charge is in good agreement with results from critical coagulation concentration measurements and perturbation theories, but disagrees with electrophoretic mobility experiments. This indicates that the effective surface charge determined by torsional high-frequency measurements is a more reliable predicter of the salt stability of charge-stabilized dispersions, in comparison to zeta-potentials determined from electrophoretic mobilities. Further, we demonstrate by direct comparison that measurements of the apparent plateau modulus by rotational rheometry underestimate the true, high-frequency modulus and provide unreliable estimates for the surface charge. Copyright 2000 Academic Press.     

Properties of aluminated silica sols

The properties of colloidal dispersions of “Positive Sol 130M” (E. I. du Pont de Nemours and Co.), which contains silica particles coated with alumina, have been studied in detail. Drifts of pH, leaching of aluminum species, and changes of electrokinetic mobility occurred during aging at room temperature for periods of up to a month, and were attributed to surface reactions. Despite its name, the sol has a point of zero charge (PZC) at pH 9, and consequently the particles carry a positive charge only below this pH. The alumina of the coating dissolves in acidic environments; the extent of leaching of aluminum species is dependent upon pH and length of aging at room temperature. The effect of different anions on this process was examined by using various inorganic acids; it was shown that phosphate and sulfate ions exhibit the strongest effects. The removal of alumina changed the surface characteristics as well as the stability of the sol 130M. The mobilities of the particles were found: to be directly related to the composition of the particle surface, regardless of the time of aging or the pH.     

Interface modification and characterization of silicon carbide platelets coated with alumina particles

A colloidal-chemistry-based technique was developed for interface modification of silicon carbide platelets (SiCp) by coating with alumina particles. The coating process utilizes electrostatic and/or electrosteric interactions between the particles to promote heterocoagulation. Since an understanding of the chemical interface between the SiCp and alumina is important for improving the coating process, the powders were characterized by the measurement of the electrokinetic sonic amplitude (ESA). The major factors studied were particle size, type of alumina, presence of polyacrylate surfactant, and concentration of alumina in the coating slip. The surface coverage and morphology of the alumina coating on SiCp was determined quantitatively by scanning electron microscopy (SEM). The results indicate that around pH 5.8, the surface charge difference between SiCp and alumina was the largest and the coating of alumina on SiCp was feasible. The SEM data showed that surface coverage was uniform, and the highest surface coverage on SiCp was 66% when the surface area ratio of alumina to SiCp was 250% in the suspension. No significant improvement of the coating rate was observed by the addition of a polyacrylate for dispersion of SiCp. The concentration of alumina in the slip exhibited a strong influence on the amount of surface coverage on SiCp.