INVITED REVIEW STABILITY OF COLLOIDAL DISPERSIONS

The stability of colloidal dispersions has traditionally attracted considerable attention in the chemical literature because of its origins in the physical chemistry of electrostatic and steric interactions between surfaces immersed in liquids. Moreover, progress in the physics and chemistry of stability phenomena has a direct impact on chemical engineering research, since stability of dispersions plays an important role in the rheology and transport properties of charged particles in liquids, separation processes (such as membrane filtration and solid/liquid separation), deposition and particulale fouling phenomena and numerous other processing operations. This paper presents a discussion of physical and chemical factors that affect colloidal stability and, in addition, presents an overview of the status of current research needs in this area. In addition to electrostatic and steric effects, some recent studies on the role of bulk motion of the supporting liquid on stability are also discussed. The emphasis throughout is on the stability of dilute dispersions, in which simultaneous interactions between more than two particles are negligible. Some major research needs in the above areas are also identified.     

Colloidal stability of surfactant-free radiation curable polyurethane dispersions

Radiation-curable polyurethane dispersions (UV-PUDs) are colloidal dispersions whose stability is mainly ensured by the electrostatic repulsion between the negatively charged polymer particles. In this article, particle stabilization is presented in terms of the physico-chemical characteristics of the polymer dispersion and its microstructure. The phenomenon of the colloidal destabilization at higher temperature is studied by multiple light scattering, then correlated with the evolution of the particle size distributions and the measurement of the apparent critical coagulation concentration of a salt as an indication of the energy barrier at the surface of the particles. The investigation of selected chemical parameters of the polymer on the colloidal stability aims to identify the most relevant ones with an understanding of the underlying mechanism. The study underlines that UV-PUDs constitute a waterborne polymer family with its own identity, adding complexity to the traditional radiation curing chemistry. Finally, it highlights the new perspectives offered for novel environmental-friendly products with high-end performance and extended stability and robustness.     

Colloidal stability of halloysite clay nanotubes

The colloidal stability of halloysite clay nanotubes dispersion is reviewed showing the strategy and the mechanism to obtain stable systems in water and apolar solvents. The selective modification of halloysite inner/outer surfaces can be achieved by exploiting electrostatic interactions. The adsorption of anionic surfactants onto the halloysite cavity allows generating inorganic cylindrical micelles that can be separated from the solvent. On the other hand, the functionalization of halloysite shell by positively charged surfactants drives to obtain stable water-in-oil emulsions. The interactions with ionic and nonionic polymers alters the dispersability of halloysite due to electrostatic and steric effects that are strongly dependent on the nanoarchitecture of the hybrid systems.Modified nanotubes by selective interactions lead to the formation of colloidal systems with tuneable surface properties and controlled colloidal stability adjusted to the solvent polarity. These dispersions are perspectives nanocarriers for substances such as antioxidants, biocides, drugs and corrosion inhibitors, to be released in response to external stimuli.     

Role of Interfacial Interactions in the Deposition of Colloidal Clay Particles in Porous Media

Colloidal clay particle transport under saturated conditions is believed to be controlled by its interactions with the surrounding environment. The dominating forces among these interactions are electrostatic forces that are determined by colloidal clay particle and porous medium surface charge density and Lifshitz–van der Waals forces that are determined by colloidal clay particle and porous medium surface thermodynamic properties. Electrostatic forces are greatly affected by solution chemistry in terms of solution ionic strength and pH. In this research, electrostatic and Lifshitz–van der Waals forces of natural colloidal clay particles with a model porous medium of silica sand were quantified at different ionic strength and pH conditions. At the same time, colloidal clay particle transport in the model medium of silica sand was conducted in a laboratory column. The maximum electrostatic forces, F ^EL (max), which occurred when the separation distance between colloidal clay particles and the porous medium was in the range of the sum of the double layer thicknesses of the colloidal clay particles and the porous medium, was found to be the determinant factor for colloidal clay particle deposition in the porous medium. Colloidal clay particle desorption in the porous media was related to the net effect of attractive Lifshitz–van der Waals forces and repulsive electrostatic forces, evaluated at the equilibrium distance where physical contact between the colloidal clay particle and silica sand actually occurred (i.e., affix force). Higher colloidal clay particle desorption was found to coincide with smaller affix force values.     

Rheological properties of alumina-polyacrylate gel dispersions

Rheological properties of aqueous alumina-polyacrylate gel dispersions have been investigated with a controlled stress rheometer as functions of both cross-linked polyacrylate and alumina concentrations at pH 13. These studies were conducted to understand the role of alumina-polyacrylate interactions in building the rheology of complex liquids that contain surfactants, sodium hypochlorite, alkali, and inorganic salts. Viscosity and viscoelasticity of polyacrylate gel dispersions can be enhanced by the addition of colloidal alumina. The trends in viscosity and viscoelasticity are explained on the basis of changes in the conformation of polyacrylate and repulsive interactions of negatively charged polyacrylate and alumina particles.     

Depletion Flocculation Strategy for the Fabrication of Ceramic Powder Coatings on Ceramic Fibers

A colloidal processing approach combining the phenomena of depletion flocculation and steric stabilization has been used in the preparation of high-quality coatings (monolayer and thicker) of silicon particles on silicon carbide fibers. Grafting procedures for the steric additive and the use of the depletion flocculation method to achieve dispersions of relatively high solids concentration are described. The impact of processing parameters on coating quality is demonstrated through electron microscopic images. The relationship between optimal processing parameters and the size and shape characteristics of the solid particles used is also discussed.     

Redispersible dried hydroxyapatite particles with grafted pH-sensitivity polymer brushes of poly(styrene-co-4-vinylpyridine)

Reversible aggregation-redispersion transition of colloidal particles is of considerable importance in colloidal science and various industrial fields. In the present study, stimuli-responsive hybrid hydroxyapatite (HA) particles were prepared with redispersible ability in aqueous solutions even after separation from the “as synthesized” state and subsequent dry heat treatment. We firstly modified HA particles with sodium oleate as chelating agent, then introduced bromoalkyl functional groups through an addition reaction of unsaturated oleate molecule with bromine, and finally grafted polymer brushes of polystyrene-co-4-vinylpyridine onto HA. It was the pH-sensitive polymer shell that had rendered HA particles the redispersible ability, which was attributed to the protonation of the pyridine blocks in acidic solutions. The reversible dispersion-aggregation transition of HA could be controlled by alternating the pH value of aqueous solution. Protonated pyridine blocks had provided enough electrostatic and steric repulsions for colloidal HA to avoid effective collision.     

Optical microscope observations of electrokinetic separation in solvent-refined coal

The response of the particulate matter in solvent-refined coal dispersions to electric perturbations was observed by optical microscopy, as a function of frequency (0–10⁴ Hz), voltage (0–3000 v), and time of application of the field. Cells giving uniform and radial electric field geometries, and having both all-metal and polymer-insulated electrode configurations, were used. Initially the field induces dipoles and orients and aggregates the particles into chains parallel to the field direction. Electrokinetic migration then leads to separation. A steady-field electrophoretic effect is seen, consistent with a net positive, low density, surface charge on the particles. Dielectrophoretic migration of induced-dipole structures towards the most intense field also occurs. It is strongest at steady and low frequency voltages and falls off with frequency in general agreement with the permittivity-frequency characteristics of solvent-refined coal dispersions. The microscopic mechanism is based on the surface polarization of elastically-bound ions in the electric double layer of the colloidal particles. The intrinsic electrophoretic and dielectrophoretic phenomena are complicated by electronic charging and discharging at the electrodes and the concomitant attraction and repulsion effects. These extrinsic phenomena become more important with increasing voltage and decreasing frequency but are significantly reduced by insulating one electrode. The results suggest that electrokinetic separation (as distinct from ‘electrostatic’ separation) is potentially useful for removing residual carbonaceous solids and mineral matter from the crude products of coal liquefaction. It is worthy of quantitative investigation on a larger scale.     

Effect of Ionic Strength on the Stability of Binary Ceramic Suspensions

The colloidal stability of aqueous Al₂O₃/ZrO₂ and Al₂O₃/SiC suspensions in the presence of electrolyte was investigated by rheological measurement. It was observed that the stability of binary systems improved with increasing ionic strength, where the solution pH value was maintained in the middle range of the two isoelectric points of the constituent powders. In this case, the electrostatic attractive interaction between dissimilar particles became more prominent when their number ratio approached unity. In addition, the salt-dependent stability was studied over the entire range of pH and relative component fraction. The results showed that the role of ionic strength in promoting flocculation or stabilization of colloidal dispersions was mainly determined by the predominant electrostatic interaction between particles, and the experimental stability maps were used to distinguish the stable and unstable suspensions with the addition of an electrolyte.     

First Studies on the Rheological Behavior of Suspensions in Ionic Liquids

The suspension rheology of hematite in the ionic liquid Ecoeng^TM.212 was studied in detail and compared to the pure ionic liquid. This is the first report on the rheological behavior of suspensions in ionic liquids, and it is postulated that colloidal stability and rheology must be considered to understand these results, and to overcome limitations on the production of nanosized particles in industrial applications. Concentrated suspensions of particles in the nanometer range show non‐Newtonian flow behavior including shear thinning and shear thickening. These phenomena are mainly caused by particle‐particle interactions in the suspension, and control of these interactions is critical. The influences of temperature and solid concentration on flow behavior were shown for the pure liquid and the suspensions. It is seen that the ionic liquid follows the Arrhenius equation for non‐associating electrolytes. It is possible to shift all hematite suspension curves to a master curve according to the model of Gleißle and Baloch. Furthermore, the flow behavior of the suspensions can be modeled with the well‐known Herschel‐Bulkley plot. A 10 wt % suspension of Fe₂O₃ follows Newtonian behavior over the entire range, similar to the pure ionic liquid. It is believed that the ionic liquid has an influence on the stability of the particles, leading to a decrease of attractive particle‐particle forces.     

Electrostatic and steric stabilization of PVC primary particles

Problems related to the formation and stability of primary particles in VCM suspension and bulk polymerization were theoretically analyzed, based on the DLVO (Deryaguin-Landau-Verway-Overbeek) and HVO (Hesselink-Vrij-Overbeek) theories of stability of colloidal systems. In the absence of secondary stabilizers, PVC primary particles were solely stabilized by negative electric charges, as a result of the presence of HCl formed by a chain transfer reaction of chloride redicals to PVC polymer chains. The dependence of the stability of primary particles on the ionic strength of the medium, temperature, and total particle charge was quantitatively investigated in terms of the stability ratio and the particle coalescence rate constant. The steric stabilization of primary particles, arising from the addition of surfactants or polymers to the monomer phase, was examined, using the HVO theory. It was shown that PMMA improved the stability of primary particles and, thus, could alter the particle agglomeration kinetics. The effects of molar mass and the amount of the absorbed PMMA, as well as the influence of solvent quality on the steric stabilization of primary particles, were also investigated. The experimental results, published on the electrostatic and steric stabilization of PVC primary particles, were in good agreement with the theoretical predictions obtained in this study. © 1993 John Wiley & Sons, Inc.     

离子液体支撑液膜的研究及应用进展

回顾了目前为止离子液体支撑液膜的制备方法、离子液体结构、支撑膜结构对离子液体支撑液膜稳定性的影响因素,介绍了其在有机物分离、气体分离、分离反应耦合方面的应用。由于传统的单元操作很难满足污染和对过程集成的要求,对离子液体支撑液膜在未来实现清洁生产的发展方向进行了展望。     

SELF-DIFFUSION IN POTENTIALS OF MEAN FORCE IN WEAKLY- AND STRONGLY-INTERACTING DISPERSIONS

Studies of interactions in dispersions have traditionally focused on the stability and transport properties of such systems at extreme dilution, that is, at concentrations for which colloidal and hydrodynamic interactions are significant between at most two particles at a given time. In practice, however, dispersions of interest often do not satisfy this restriction, and, consequently, many-body colloidal interactions and hydrodynamic coupling have important roles in the observed macroscopic behavior. This paper presents an analysis of the effects of many-body interactions on the self-diffusion coefficient in interacting dispersions. Self-diffusion coefficients have been determined for dilute-gas dispersions and for Yukawa dispersions interacting through appropriate potentials of mean force. It is shown that the diffusion coefficients change negligibly with attraction for typical magnitudes of the Hamaker constant in an otherwise repulsion-dominated (i.e., stable) dispersion. The results show that, for thin electrical double layers, dilute dispersions can be approximated by hard-sphere dispersions, even for large values of surface potentials. However, for thick double layers (i.e., thickness comparable to particle radius), while the short-time diffusion coefficient is affected only negligibly, the long-time coefficient can decrease considerably because of the 'memory' effects—even for moderate or low values of the surface potentials. Corresponding results are presented for both dilute-gas dispersions and dispersions with significant local structures. The long-time diffusion coefficients in the latter are effectively linear in volume fraction (up to about 0.2)for thin double layers and are given with reasonable accuracy by the dilute-gas approximation.     

Separation of Ionic Liquid Dispersions in Centrifugal Solvent Extraction Contactors

Abstract

Separations of dispersions formed by mixing immiscible organic room‐temperature ionic liquids (IL)/hydrocarbon/and aqueous systems using a centrifugal solvent‐extraction contactor have been successfully demonstrated in proof‐of‐concept testing. This accomplishment is significant in that physical property factors that are typical of ionic liquid systems (e.g., similar densities of the bulk phases, low interfacial tensions, and high viscosities) are typically unfavorable for dispersion separation, particularly in continuous processes. Efficient separation of dispersions containing ionic liquid solvents is essential for utilization of these compounds in liquid‐liquid extraction applications to maximize both solute transfer efficiency and solvent recovery. Efficient solvent recovery is of particular concern in IL applications because of the high cost of most IL solvents.

This paper presents the results of initial experiments with three hydrophobic ionic liquids to determine how their physical properties affect phase mixing and phase disengagement in contact with an aqueous solution using a centrifugal contactor. While the results of the reported work are promising, additional work is needed to optimize existing mathematical models of contactor hydraulics to address special considerations involved in IL‐based processes and to optimize the equipment itself for IL applications.     

Anwendung von Flockungsmitteln bei der mechanischen Flüssigkeitsabtrennung

Use of flocculants in the mechanical separation of liquids . The mechanical separation of liquids from turbid water or sewage sludges depends primarily on the size and density of the suspended particles. The process of separation by sedimentation, filtration, or centrifugation becomes increasingly difficult with decreasing size of the suspended particles and decreasing density difference between solid and liquid. The addition of flocculants in ppm amounts destabilizes the dispersion and flocculation sets in. Thus flocculants agglomerate suspended particles, improve the separation, and accelerate the overal separation process. The synthetic organic high polymers used in the treatment of municipal and industrial waste water are characterized by their chemical composition, their molar mass, and their charge density.     

Enhanced flocculation of colloidal dispersions by polymer mixtures

Bridging flocculation and electrolyte coagulation of negatively charged colloidal dispersions in the presence and absence, respectively, of uncharged polymers and polymer mixtures were studied. The relative coagulation and flocculation rates of particles in the presence of electrolyte and small polymer amounts were measured and the stability ratios have been calculated at various ionic strengths. Also, the structure of polymer layers formed in individual adsorption of polymers and in simultaneous competitive adsorption from binary polymer mixtures at particle/solution interfaces was investigated. The electrophoretic mobility and the diffusion coefficient of particles with and without adsorbed polymer were measured by laser Doppler-electrophoresis and photon correlation spectroscopy, respectively, and the electrophoretic and the hydrodynamic thickness of adsorbed polymer layers have been calculated. It was found that the adsorbed polymers may enhance or diminish the rate of successsful encounters between particles, even at low surface coverages, depending on the magnitude of the interparticle electrostatic repulsion. In addition, competitive adsorption of chemically different polymers for particle surfaces may result in considerable alteration in the conformation of macromolecules in the mixed adsorption layer. Close correlation was found between the effectiveness of polymers as flocculants and the thickness of adsorbed polymer layers formed at optimum polymer dosages on the particle surfaces. Binary mixtures of suitable polymers proved to be very efficient flocculants for the dispersions. The enhanced flocculating effect of some mixtures can be ascribed to extended polymer layers formed in competitive adsorption of chemically different macromolecules at particle/solution interfaces. These findings have relevance in many environmental technologies and offer a way of improving the effectiveness of solid–liquid separation processes.     

Theoretical Aspects of Ionic Liquids for Soft‐Matter Sciences

The thermodynamic and electrochemical properties of ionic liquids produce a broad spectrum of unconventional phenomena both microscopically and macroscopically. However, despite numerous theoretical and experimental studies, the fundamental roles of the relevant interactions such as electrostatic interactions and hydrogen bonding often remain unclear at the molecular level. The complexity of the molecular interactions typically increases when ionic liquids dissolve polymers or polar substances such as water. Accordingly, recent studies have revealed new features of ionic liquids. Further insights into the role of the molecular polarity of ionic liquids are required. This article presents an overview of the important phenomena of ionic liquids concerning soft‐matter sciences based on selected experimental and theoretical studies. We focus on the effect of the dielectric response of ionic liquids to distinguish ionic liquids from common inorganic salts, such as alkali metal halides.     

晶体硅切割废料回收的研究现状

晶体硅棒在加工成硅片的切割过程中产生了大量的切割废料。本文回顾了废料回收主要局限于回收聚乙二醇和碳化硅的现状,然而废料中的硅具有更大的回收价值。介绍了目前硅粉回收技术中物理方法、化学方法和间接回收的研究进展,重点阐述了物理沉降、重液分离、泡沫浮选、电泳分离、电选分离、高温处理等物理方法和利用碳化硅与硅化学稳定性的差异进行的化学分离的研究现状,评述和比较了各种方法的特点和优缺点。指出在现有的回收技术中要实现规模回收硅粉还存在较大的困难,需要通过深入广泛的研究,提高硅粉的回收率和纯度,降低成本,改善工作环境。     

Colloidal Fouling during Ultrafiltration

Abstract

Colloidal fouling causes serious problems in many membrane plants. Two different kinds of flux-reducing phenomena occur when treating colloidal dispersions. When treating stable dispersions the flux is reversible and can be restored after changing the operating parameters, such as the transmembrane pressure or the cross flow velocity. The flux reduction experienced when treating unstable colloidal dispersions is irreversible. In this study the DLVO theory, well-known from colloid chemistry, has been used to illustrate the mechanisms underlying the difference in filtration characteristics between stable and unstable colloidal dispersions.     

A universal cohesive energy estimation equation based on COSMO

A universal cohesive energy estimation equation for neutral molecules based on COSMO (COSMO-UCE) was developed in this work; which comprises contributions from dispersive (nonpolar), polar (electrostatic) and hydrogen-bonding interactions, and takes into account the influence of local segment surface charge density profile and entire molecule stereo charge distribution (dipole and quadrupole moment) on coulombic and hydrogen-bonding interaction energy calculations. We have examined this model for the molar cohesive energy of pure liquids, gases, and ionic liquids (ILs) with successful results, which indicates that this model is reliable for calculating the molar cohesive energy of conventional fluids (nonpolar, polar, and hydrogen-bonding liquids and gases) and ILs. This model can be used for solubility parameter calculations and solvent selections for chemical separation process (gas absorption and liquid–liquid extraction).