带相反电荷胶体颗粒混和悬浮体的流变学性能

研究了带相反电荷的粘土颗粒和MMH(铝、镁混合金属氢氧化物)颗粒形成的混和悬浮体的流变学性能,考察了盐对混合体系流变学性能的影响.结果表明,当粘土质量分数为1%时,悬浮体为牛顿型流体;当质量分数升至2%时,悬浮体表现出固体的弹性响应;特定粘土含量的悬浮体中,随着MMH量的增加,混合体系的屈服值和弹性模量亦增加,表明凝胶结构增强.向混合体系中加入NaCl,弹性模量、屈服值和粘度等流变参数均降低.与单组分粘土悬浮体或MMH悬浮体相比,双组分混合体系的结构恢复很快.     

Improved dewatering behavior of clay minerals dispersions via interfacial chemistry and particle interactions optimization

Orthokinetic flocculation of clay dispersions at pH 7.5 and 22 degrees C has been investigated to determine the influence of interfacial chemistry and shear on dewatering and particle interactions behavior. Modification of pulp chemistry and behavior was achieved by using kaolinite and Na-exchanged (swelling) smectite clay minerals, divalent metal ions (Ca(II), Mn(II)) as coagulants and anionic polyacrylamide copolymer (PAM A) and non-ionic polyacrylamide homopolymer (PAM N) as flocculants. The pivotal role of shear, provided by a two-blade paddle impeller, was probed as a function of agitation rate (100-500 rpm) and time (15/60 s). Particle zeta potential and adsorption isotherms were measured to quantify the interfacial chemistry, whilst rheology and cryogenic SEM were used to investigate particle interactions and floc structure and aggregate network, respectively. Osmotic swelling, accompanied by the formation of "honeycomb" particle network structure and high yield stress, was produced by the Na-exchanged smectite, but not kaolinite, dispersions. Dispersion of the clay particles in 0.05 M Ca(II) or Mn(II) solution led to a marked reduction in particle zeta potential, complete suppression of swelling, honeycomb network structure collapse and a concomitant reduction in shear yield stress of smectite pulps. Optimum conditions for improved, orthokinetic flocculation performance of negatively charged clay particles, reflecting faster settling flocs comprised (i) coagulation, (ii) moderate agitation rate, (iii) shorter agitation time, and (iv) anionic rather than non-ionic PAM. The optimum dewatering rates were significantly higher than those produced by standard, manual-mixing flocculation techniques (plunging and cylinder inversion) commonly used in industry for flocculant trials. The optimum flocculation conditions did not, however, have a significant impact on the final sediment solid content of 20-22 wt%. Further application of shear to pre-sedimented pulps improved consolidation by 5-7 wt% solid. Higher shear yield stresses and greater settling rates were displayed by PAM A based than PAM N based pulps and this is attributed to the former's more expanded interfacial conformation and greater clay particles bridging ability. It appears that the intrinsic clay particles' physico-chemical properties and interactions limit compact pulp consolidation.     

Synchrotron SAXS/WAXD and rheological studies of clay suspensions in silicone fluid

Suspensions of two commercial smectite clays, montmorillonite KSF and montmorillonite K10, in a low-viscosity silicone oil (Dow Corning 245 Fluid) were studied by simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) techniques and rheological measurements. In the 0.5% (w/v) KSF clay suspension and two K10 clay suspensions (0.5% and 1.0%), WAXD profiles below 2theta=10.0 degrees did not display any characteristic reflection peaks associated with the chosen montmorillonite clays, while corresponding SAXS profiles exhibited distinct scattering maxima, indicating that both clays were delaminated by the silicone oil. In spite of the large increase in viscosity, the clay suspensions exhibited no gel characteristics. Dynamic rheological experiments indicated that the clay/silicone oil suspensions exhibited the behavior of viscoelasticity, which could be influenced by the type and the concentration of the clay. For the K10 clay suspensions, the frequency-dependent loss modulus (G") was greater in magnitude than the storage modulus (G') in the concentration range from 0.5 to 12.0%. The increase in the clay concentration shifted the crossover point between G' and G" into the accessible frequency range, indicating that the system became more elastic. In contrast, the KSF clay suspension exhibited lower G' and G" values, indicating a weaker viscoelastic response. The larger viscoelasticity response in the K10 clay suspension may be due to the acid treatment generating a higher concentration of silanol groups on the clay surface.     

Aqueous Dispersions of Positively Charged Al/Mg Mixed Metal-Hydroxide Particles:Surface Chemistry and Rheological Properties

The solids concentration, pH and NaCl were found to have a very significant effect on the rheological properties of aqueous dispersions of positively charged Al-Mg mixed metal hydroxide particles (Al-Mg MMH). At low solids concentrations the flow curves of the dispersions at natural pH followed the Newtonian model very well in the observed range of shear rate, while at high solids contents the dispersions developed a yielding type of response, with yield stress increasing rapidly as solids concentration increases. The variation in rheological properties with pH and NaCl contents correlated well with the change in surface properties of Al-Mg MMH particles. Al-Mg MMH dispersions with maximum yield stress occurred near the isoelectric point (IEP) where the Zeta potential (or electrophoretic mobility) is zero. At the IEP, the shear yield stress decreased monotonously in magnitude as a function of the increasing NaCl concentration, which is in contrast to the increment in the yield stress observed below the DEP at NaCl concentrations less than 0.1 mol dm^-3. At low NaCl concentrations, significant shear yield stress differences exist at and below the IEP. By contrast, at high NaCl concentrations, almost identical shear yield stress versus NaCl results are obtained in both cases.     

铝/镁混合金属氢氧化物正电胶体粒子体系的触变性

采用恒定低剪切速率方法和动态实验方法研究了铝/镁混合金属氢氧化物(MMH)悬浮体的流变性,着重考察了剪切历史和恢复时间等因素对悬浮体触变性的影响,发现MMH粒子深度的增加使得悬浮体从粘性流体变为“类固体”;恒定低剪切速率方法和动态实验方法研究悬浮体的结构恢复过程给出了不同的结果,唯象地解释了MMH悬浮体触变性产生的原因,认为触变性结构是由于粒子间的静电排斥作用而形成的,与粘土悬浮体相比,两者的流变性具有诸多相似之处,只是所带电荷符号相反。     

Effect of cationic surfactant adsorption on the rheological and surface properties of bentonite dispersions

In this study, the adsorption, bridging, and intercalation effects of a cationic surfactant, benzyldimethyltetradecyl ammonium chloride (BDTDACl), on bentonite clay suspensions was investigated. The adsorption, rheological behaviors, and colloidal properties of the clay dispersions were determined as a function surfactant concentration. Adsorption isotherms were obtained using the batch-equilibrium technique. The rheological behavior of the clay suspensions was obtained by shear stress-shear rate measurements within 0-350 s-1 shear rates. The structure of the composite particles was analyzed by using X-ray diffraction analysis and it was found that the expansions of basal d-spacings are less than 16.80 A, suggesting a monolayer structure.     

Relating clay yield stress to colloidal parameters

The ability to predict rheological behavior of clay dispersions would be useful in formulating systems rheologically modified by clays as well as designing equipment to handle clay based materials. Deriving predictive equations for these materials has been hindered due to the various mechanisms by which clay systems develop microstructure. This work seeks to observe the relationship between the yield stress, tauy, and the zeta potential, zeta, and determine the nature of the particle interactions for dispersions of laponite (a synthetic clay) and dispersions of a mixture of naturally occurring kaolinites and bentonites. It is found that the relationship of tauy to zeta2 for clay suspensions is opposite to that found for homogeneously charged spheroidal colloids. This result can be traced to the type of particle interactions occurring, which for the systems studied appear to be edge-to-face attractions.     

Evaluation of the organization of the homoionic smectite layers (Na(+) or Ca(2+)) in diluted dispersions using granulometry, microscopy and rheometry

Smectites are swelling clay materials with pronounced colloidal properties that are widely used in industry. These properties originate in the electrokinetic properties of the smectite layers and their linkage capacities. Thin layers may be dispersed or aggregated according to many parameters, such as concentration, particle size and morphology, exchangeable cation nature and chemical environment (pH, ionic strength). The literature usually provides general rules, like the sodium dispersion contains a lot of small units whereas the calcium dispersion contains a few large units. A volume of water molecules bound to the clay surface is considered as the immobile water phase that behaves like the solid phase obstructing the flow. The water immobilized around layers and trapped inside aggregates cannot participate to the flow. In this study, we evaluated the volume occupied by calcium and sodium units inside the dispersion containing the immobile water phase. First, the smectite was cautiously extracted from a raw bentonite and its physicochemical properties were determined. A large quantity of extracted and saturated smectite (Na-smectite and Ca-smectite) was obtained. Second, the unit size and a shape factor for each sample were evaluated using granulometry and scanning transmission electron microscopy on wet samples (Wet STEM) and some flow curves. Na-smectite dispersions contain 0.13 microm(2) surface units with a shape factor of 50. Ca-smectite dispersions contain 0.32 microm(2) surface units with a shape factor of 3.3. Finally, rheometry allowed us to evaluate the unit occupancy using an adaptation of the Krieger-Dougherty law. We used shape factors and evaluated the concentration from which the entire immobile volume was connected (6.4% for Na-smectite and 11.9% for Ca-smectite). This study explains the evolution of flow properties with increasing concentrations by the evolution of layer interactions at the microscopic scale for homoionic smectite particles in diluted dispersions.     

咪唑基表面活性离子液体在气/液界面的扩张粘弹性

合成了两种咪唑基表面活性离子液体,通过界面膨胀流变法研究了其在气/液界面的聚集行为,考察了咪唑基表面活性离子液体浓度、无机盐和温度对聚集行为的影响。研究发现,咪唑基表面活性离子液体在吸附过程中吸附控制占主导作用,而弛豫过程不是单一指数函数;加入无机盐或升高温度可以提高咪唑基表面活性离子液体的表面活性、增强其在界面的吸附能力、降低表面张力。扩张流变结果显示扩张模量、弹性模量和粘性模量随震荡频率增加而增加;随表面活性离子液体浓度增大,扩张模量先增大后减小。扩张模量随温度升高或无机盐(NaBr或CaBr₂)的加入而降低。表面活性离子液在气/液界面形成的吸附膜以弹性模量为主,而且C₁₄mimBr的界面膜弹性模量大于C₁₂mimBr的界面膜弹性模量。     

Interfacial rheological studies of gelatin-sodium dodecyl sulfate complexes adsorbed at the air-water interface

The dilational rheological behavior of gelatin molecules adsorbed at the air-water interface has been studied as a function of sodium dodecyl sulfate (SDS) concentration for a 7 wt % gelatin-SDS solution at 40 degrees C. Binding of SDS molecules to the gelatin strands disrupts the cross-linked network structure of adsorbed gelatin molecules and results in a reduction of the surface elastic modulus of the adsorbed layer that continues until the bulk SDS concentration reaches 1 mM. Beyond this SDS concentration, the dilational rheological properties of the adsorbed gelatin layer are indistinguishable from those of pure SDS adsorbed layers.     

Influence of the exchangeable cations on stability and rheological properties of montmorillonite suspensions

The effects of NaCl and CaCl₂ on the colloid stability and rheological properties of Na- and Ca-montmorillonite dispersions were studied. The distribution of cations between the surface and the bulk phase was determined. For both of monocationic montmorillonite, the critical coagulation concentration were 250 mmol NaCl/dm³ and 2 mmol CaCl₂/dm³. The changes in the Bingham yield stresses of Na- and Ca-montmorillonite dispersions as functions of the NaCl and CaCl₂ concentration could be explained in terms of the surface excess amount and equilibrium concentration of the cations, the second electroviscous effect and the formation of a gel structure.     

Phase separation behavior of whey protein isolate particle dispersions in the presence of xanthan

In this study, phase separation of colloidal whey protein isolate (WPI) particle dispersions was studied using a rod-like polysaccharide xanthan. Effects of different xanthan concentration, particle volume fraction, and temperature were analyzed by visual observations, turbidity measurements, and particle mobility tracking method. Particle mobility was determined using a diffusing wave spectroscopy (DWS) set up. Xanthan concentration was kept low in order not to increase the viscosity of dispersions, so that the phase separation could be observed easily. Visual observations showed that there was a minimum concentration of xanthan to induce phase separation at a constant particle volume fraction, and xanthan concentration was found to have an important effect on the degree of phase separation. The temperature was also found to have an effect on depletion mechanism. Phase separation was mainly a result of different sizes of WPI particles, and xanthan induced the depletion interaction between WPI particles, as supported by the data obtained from DWS. The results of this study explained both the mechanism and the stability range of particle dispersions in the presence of xanthan, which is important for the design of stable systems, including colloidal particles.     

Effect of high salt concentrations on the stabilization of bubbles by silica particles

The stabilization of air bubbles by hydrophobically modified silica particles has been investigated in detail. The silica particles used had a nominal primary particle size of 20 nm and were made hydrophobic by treatment with dichlorodimethylsilane to yield particles with varying percent grafting of alkyl chains ("% SiOR"). Contact-angle (theta) measurements of pure water droplets on flats made from compressed samples of the particles showed a steep increase in theta above ca. 20% SiOR. Other measurements also showed a significant increase in theta when the salt concentration was raised to 1-3 mol dm(-3). Bubbles were formed in a sonicated dispersion of particles by suddenly lowering the pressure. Maximum stability was obtained with 33% SiOR particles and 2-3 mol dm(-3) NaCl. Under these conditions, theta was around 40 degrees. Above a threshold size of around 70 microm, bubbles were extremely stable to disproportionation and coalescence and bubble stability increased significantly with an increase in the NaCl concentration from 0.5 to 3 mol dm(-3). Furthermore, rheological measurements showed that at NaCl concentrations in this range weak particle gels were formed with a finite yield stress. The strength of these gels increased with an increasing NaCl concentration between 0.5 and 3 mol dm(-3) and with an increasing time of aging the dispersions, implicating this as part of the mechanism leading to an increased bubble stability in these systems. Dispersions in the absence of NaCl showed little or no foamability at all. Use of CaCl2 and Al(NO)3 at similar ionic strengths showed that equivalent stability could not be obtained with these salts. Atomic force microscopy (AFM) measurements of the adhesion between a pure (0% SiOR) silica sphere and flat showed a significant increase in the adhesion between 0.5 and 3 mol dm(-3) NaCl, even though in this concentration range no significant change in the electrostatic repulsion might be expected. It is concluded that the increased particle-particle adhesion, effective hydrophobicity, and bubble-stabilization properties of the particles at high NaCl concentrations are probably due to the collapse of protruding polysilicic acid chains on the surface of the silica.     

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.     

Effect of thickener structure on paper-coating color properties

Physical and chemical properties of clay-based paper-coating colors have been characterized. The “surface potential” (zeta potential) of kaolin particles used in paper-coating formulations was determined as functions of pH and sodium polyacrylate (used as a dispersant) concentration. The optimal pH and dispersant concentration have been established. The effect of adsorption of two different thickeners on the kaolinite particle potential was also investigated. The rheological properties of coating colors, thickened with an associative polymer and a commonly used thickener, have been compared. The rheological behavior of all the coating colors studied was found to be similar, except for the magnitude of the elastic modulus, which was considerably larger for the more hydrophobic thickener. The water-retention properties of the colors could be qualitatively correlated with the molecular structure of the thickeners. An interaction mechanism (e.g. formation of hydrophobic micellar domains) between kaolinite particles and the associative polymer has been proposed. Received: 10 August 2000/Accepted: 2 January 2001     

Coating of silica sand with aluminosilicate clay

The objective of this work was to coat aluminosilicate clays on an inert silica support, and to characterize the properties and stability of the clay-silica coating. Two polymers, polyacrylamide (PAM) and polyvinyl alcohol (PVA), were used to bind kaolinite, illite, and smectite onto silica grains. The clay-polymer composites were studied by X-ray diffraction, FTIR, and electrophoretic mobility. Clay coatings on silica grains were characterized by mass coverage, scanning electron microscopy, specific surface area, and pH stability. Silica sand was successfully coated with clays by using the two polymers, but with PVA, the clay coating had a greater mass coverage and was more stable against pH variations. Less polymer was needed for the clay coating using PVA as compared to using PAM. Clay-polymer complexes and pure clay minerals had similar cation exchange capacities and electrophoretic mobilities, indicating that overall surface charge of the clays was little affected by the polymers. Some decrease in hydrophilicity was observed for illite and smectite when clays where coated with the polymers. The methodology reported here allows the generation of a clay-based porous matrix, with hydraulic properties that can be varied by adjusting the grain size of the inert silica support.     

Negative electrorheological behavior in suspensions of inorganic particles

An investigation is described on the electric-field-induced structures in colloidal dispersions. Both rheological determinations and direct microscopic observations are used with that aim. The starting point of this study is the so-called electrorheological (ER) effect, consisting of the mechanical reinforcing of a fluid or suspension due to formation of chains of molecules or particles after being polarized by the action of the field. One macroscopic manifestation of this phenomenon is the transformation of the fluid from a typically Newtonian behavior to a viscoelastic material, with finite yield stress and high elastic modulus. The systems investigated were suspensions of elongated goethite (β-FeOOH) particles in silicone oils with varying amounts of silica nanoparticles. The results showed the rather unusual behavior known as "negative ER effect", which can be best described by saying that the application of an electric field reduces the yield stress and the elastic modulus, that is, produces destruction of structures rather than their build up. The negative behavior is also found for suspensions of other inorganic powders, including hematite and quartz. On the contrary, the usual positive ER response is found for suspensions of cellulose and montmorillonite clay. The same happens if goethite suspensions are prepared in high volume fractions, high-viscosity fluids, or both. All of the results found are compatible with the so-called interfacial model of electrorheology: the reduction of the yield stress of goethite suspensions when the applied field is high enough is the consequence of particle migration toward the electrodes because of charge injection and subsequent electrophoresis. The migration leaves the gap between the electrodes devoid of particles and explains the decrease in yield stress. The addition of silica nanoparticles contributes to reduce the strength of this effect by hindering the charging and making it necessary to increase the field strength to observe the negative effect. The model appears to also be applicable to cellulose, although the positive response found for such particles is explained by their large size: larger diameters bring about larger attraction forces between particles, leading to a tendency to produce strong aggregates. This is likely to occur in suspensions of colloids which, because of their relatively high electrical conductivity, tend to acquire charge even in such nonpolar liquids as silicone oils.     

Contact angles of aluminosilicate clays as affected by relative humidity and exchangeable cations

Contact angles of aluminosilicate clays are difficult to determine. Not only does their small particle size present measurement difficulties, but contact angles may vary with relative humidity and cation composition. In this paper, we determined the effects of relative humidity and exchangeable cations on contact angles of three aluminosilicate clays (smectite, kaolinite, illite). Contact angles were measured on clay films with the sessile drop method under different relative humidity (19, 33, 75, 100%), and with clays saturated either with Na, K, Mg, or Ca. The results showed that the water contact angles on smectite increased with relative humidity between 19 and 75%, but for kaolinite and illite, little differences in water contact angles between 19 and 75% relative humidity were observed. For all three clays, however, the water contact angles decreased at 100% relative humidity as compared to the lower relative humidities. Cations affected not only the adsorption of water but also the surface charge, and both factors influenced the contact angles of the clays. Negligible effect of the different cations Na⁺, K⁺, Mg²⁺, or Ca²⁺ on contact angles was observed.     

Surface modifications of illite in concentrated lime solutions investigated by pyridine adsorption

The surface properties and functionality of an illite-containing clay mineral in alkaline solutions containing increasing quantities of calcium hydroxide were investigated using FTIR monitored pyridine adsorption at various temperatures. Results showed that the binding behavior of electron donor pyridine molecules to illite platelets is mostly governed by electron acceptor sites located at the edges of the clay particles. The binding of bulky hydrated calcium ions at the surface of the illite platelets decreases the surface area of illite. Moderate thermal treatments up to 450 °C do not affect the structure of the clay mineral but strongly decrease the number of Lewis and Brönsted sites available at the edges of the clay platelets.     

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.