Suspensions and Sols

A suspension is a dispersion of solid particles in a liquid. A colloidal suspension is a sol. Colloidal properties become significant when the size of the parhcles is of the order of a few micrometer or less. In suspensions of large particles, for example, of some 10 pm or higher, hydrod5mamic interactions dominate the suspension flow properties emd particle packing behaviour. In colloidal suspensions interaction forces between the particles as well as hydro-dynamic interactions play a role in determining the flow and particle packing properties. 

A homogeneous particle packing can be obtained when the suspension flow units do not change on a time scale of that of the coating process. In industrial practice it is often also desirable that the suspension properties are stable on a longer time scale (shelf life) for economic reasons. This means that sedimentation and ageing cannot be permitted amd must accordingly be prevented. 

A dispersion of solids can be prepared by dispersing a powder in a liquid or by synthesising the solid particles in situ in a liquid. Examples of the first method are suspensions of submicrometer alumina or zirconia powders in water. Examples of the second method are boehmite sols and titania sols prepared from organo-metallic precursors.  

3 The dispersion of powders in liquids is a unit operation in colloidal processing which is very important and deserves a separate treatment. This is, however, beyond the scope of this chapter. (The reader is referred to Ref. [5] or [21] for more information.) The same is valid for the synthesis of sols (see for example Ref. [41]). 

6 — PREPARATION OF ASYMMETRIC CERAMIC MEMBRANE SUPPORTS BY DIP-COATING 

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Bheopectic Fluids. Rheopectic fluids are characterized by an increase in their viscosity with time at a constant shear rate and fixed temperature. As for a thixotropic fluid, a hysteresis loop is also formed with a rheopectic fluid if it is sheared from a low to a high shear rate and back to a low shear rate. However, a different rate is usually followed upon lowering the shear rate, as is shown in Figure 3. Bentonite clay suspensions and sols are typical examples of rheopectic fluids (3). 

In real suspensions and sols several interaction forces (or interaction free energies) play a role. Some are attractive and some are repulsive. The total interaction free energy is determined by the sum of all these contributions. The microstructure of the dispersion depends on the sum of the contributions. Because this sum is the total of large positive (repulsive) and large negative (attractive) contributions, the final result is a delicate balance. Prediction of the overall force between the particles requires that the separate contributions are accurately known which is often rather difficult in practice. Nevertheless, it is... 

Prediction of the layer growth kinetics of support layers to be prepared from (sub)micrometer suspensions and sols is, in principle, possible on the basis of Eq. (6.39). Usually, the average pore properties of the substrate, 8i and Rj are known or can be rather easily determined. Estimation of the effective volume fraction of sols, however, may be more difficult as the particles become smaller (see Section 6.2.4). 

The addition small quantities (often less than 0.01%) of some polymeric surfactants to suspensions and sols leads to flocculation, which in its appearance resembles coagulation. Polymeric surfactants capable of causing flocculation are referred to as flocculants. Typical flocculants used to flocculate aqueous dispersions are polyacrilamides, polyethyleneimines, etc. 

The formation of systems with solid backbones is often the result of aggregation processes taking place in suspensions and sols which lead to the development of spacial networks and final conversion of disperse systems into materials with valuable properties (Chapter IX, 2). In some cases, e.g. during solidification of metal alloys, the processes of structuring accompany formation of new phases. The systems with solid dispersion medium also form upon the solidification of a continuous phase in foams, emulsions, suspensions and sols. 

Time-dependent rheological properties reflect the nature of a system s structure and can be due to viscoelasticity, structural changes, or both (Cheng and Evans, 1965 Harris, 1972). Structure breakdown can result in a decrease in the viscosity of a substance. It occurs in emulsions, suspensions, and sols. The characterization of the time-dependent flow properties of food systems is important for process design and control, for product devel-... 

Several methods for the incorporation of catalysts into microreactors exist, which differ in the phase-contacting principle. The easiest way is to fill in the catalyst and create a packed-bed microreactor. If catalytic bed or catalytic wall microreactors are used, several techniques for catalyst deposition are possible. These techniques are divided into the following parts. For catalysts based on oxide supports, pretreatment of the substrate by anodic or thermal oxidation [93, 94] and chemical treatment is necessary. Subsequently, coating methods based on a Uquid phase such as a suspension, sol-gel [95], hybrid techniques between suspension and sol-gel [96], impregnation and electrochemical deposition methods can be used for catalyst deposition [97], in addition to chemical or physical vapor deposition [98] and flame spray deposition techniques [99]. A further method is the synthesis of zeoUtes on microstructures [100, 101]. Catalysts based on a carbon support can be deposited either on ceramic or on metallic surfaces, whereas carbon supports on metals have been little investigated so far [102]. 

The coefficient Tj is termed the modulus of rigidity. The viscosities of thixotropic fluids fall with time when subjected to a constant rate of strain, but recover upon standing. This behavior is associated with the reversible breakdown of stmctures within the fluid which are gradually reestabflshed upon cessation of shear. The smooth sprea ding of paint following the intense shear of a bmsh or spray is an example of thixotropic behavior. When viscosity rises with time at constant rate of strain, the fluid is termed rheopectic. This behavior is much less common but is found in some clay suspensions, gypsum suspensions, and certain sols. 

Sol-Gel Techniques. Sol-gel powders (2,13,15,17) are produced as a suspension or sol of coUoidal particles or polymer molecules mixed with a Hquid that polymerizes to form a gel (see Colloids SoL-GELtechnology). Typically, formation of a sol is foUowed by hydrolysis, polymerization, nucleation, and growth. Drying, low temperature calciaation, and light milling are subsequently required to produce a powder. Sol-gel synthesis yields fine, reactive, pseudo-crystalline powders that can be siatered at temperatures hundreds of degrees below conventionally prepared, crystalline powders. 

The synthesis of silica membranes has only recently been described. Silica forms sols and gels very easily both by the colloidal suspension and by the polymeric gel route. Its chemical resistance and its thermal stability in the presence of water vapor or metal impurities are not very good however. Larbot et al. (1989) have described the synthesis of silica membranes starting with a commercially available silica sol (Cecasol Sobret) in an aqueous solution at pH 8. 

The existence of surface charge prevents coagulation in a low ionic strength suspension or sol of one solid component and thereby slows sedimentation (86, 87). The rate of coagulation and sedimentation can be increased by adjusting chemical conditions so that the surface charge is zero. Many examples can be found in the extensive literature of colloid and soil chemistry. 

Using preformed sols instead of metal alkoxides as precursors is an attractive alternative in sol-gel preparation because recent advances in inorganic colloidal dispersions allow some control over the characteristics of the starting sols [11]. Often a colloidal suspension of sol particles is stabilized (i.e. prevented from flocculation) by pH adjustment. Thus, pH of the solution, which can be changed by the addition of either acid or base, is the single most important parameter in obtaining a gel from preformed sols. Other parameters that can impact on gel quality are the size and concentration of the starting sol particles. 

Fabrication methods have overwhelmingly focused on improving nanotube dispersion because better nanotube dispersion in polyurethane matrix has been found to improve the properties of the nanocomposites. The dispersion extent of CNTs in the polyurethane matrix plays an important role in the properties of the polymer nanocomposites. Similar to the case of nanotube/solvent suspensions, pristine nanotubes have not yet been shown to be soluble in polymers, illustrating the extreme difficulty of overcoming the inherent thermodynamic drive of nanotubes to bundle. Therefore, CNTs need to be surface modified before the composite fabrication process to improve the load transfer from the polyurethane matrix to the nanotubes. Usually, the polyurethane/CNT nanocomposites can be fabricated by using four techniques melt-mixing (15), solution casting (16-18), in-situ polymerization (19-21), and sol gel process (22). 

The earliest routes for forming ceramics from sol-gel solutions involved the precipitation of metal oxide particles from solutions. These form a true colloidal suspension a sol. Upon destabilization of this sol, aggregation takes place and a rigid network is formed a gel. A gel is intermediate between a solid and a liquid. The term sol—gel has since been used by the materials science community to describe, albeit erroneously, virtually all chemical processing of ceramics from solutions (e.g., metal oxide particle precipitation or metalorganic decomposition). This discussion focuses on the gel aspects of sol—gel synthesis and not on the sol aspects, which are treated separately in this book. 

Another method that has been developed for the preparation of thicker layers involves the use of diol solvents, such as 1,3-propanediol. Phillips et al. and Tu et al. have explored this route most thoroughly, and it has also been utilized by manufacturers. While the method has not been as intensely investigated as the 2-methoxyethanol process, a number of spectroscopic investigations have been carried out to study the details of the reaction chemistry. Coating thickness per deposited layer is 0.5 pm, compared to 0.1 pm for layers prepared by sol-gel methods using more common solvents, or by chelate or MOD processes. The manufacture of 1 pm films is therefore significantly less labor intensive. Lastly, others have prepared suspensions from sol-gel precursor solutions and powders... 

The chief advantage offered by this rule, proposed by Radushkevich is not only that it considerably reduces the number of systems being subject to statistical treatments but also simplifies the latter. For instance, by applying the reversal rule to closed-cell foamed plastics we convert the latter into their reciprocals in which the individual particles (formerly cells) are arranged in such a manner that they have very few contacts, if any. The latter systems formally all exhibit the characteristics of suspensions (or sols) which can at present readily be studied and described using the well-developed statistical apparatus, which is not possible for the original real system (foamed plastic), due to the insurmountable mathematical difficulties involved. 

Bentonite is a naturally occurring hydrated aluminum silicate used primarily in the formulation of suspensions, gels, and sols, for topical pharmaceutical applications. It is also used to suspend powders in aqueous preparations and to prepare cream bases containing oil-in-water emulsifying agents. 

In the reinforcing sector, a-Al203-fibers are the furthest in development. Low diameter fibers are obtained by the sol-gel process, the spinning of solutions or suspensions and from melts. 

In 2002, Chiang and coworkers[124] developed a new scheme for the confined synthesis of TPA-silicalite nanocrystals. The surfactant cetyltinmethylammonium bromide (CTAMeBr) (in ethanol solution) was added to the single- and double-heated TPA-silicalite precursor sols (SHPS and DHPS), and the mixture was flocculated at a certain pH value to collect the nano-size silicate species in the precursors, and then dried. The dried precursor/surfactant hybrid was pressed into pellets and then steamed in a stainless steel autoclave at 110 150°C for 7 36 h. Finally, the product was calcined to remove the surfactants and TPA. The particle sizes of silicalite-1 produced in this method are about 30 nm. The study indicates that the nanoparticles collected by surfactants already exhibit the structural features of MFI. They crystallize entirely to form silicalite-1 nanocrystals after steam treatment at 110 150 °C. This new solid-phase approach provides a way to synthesize MFI nanocrystals without the problem of separation and collecting nanocrystals from suspension, and it also avoids the large consumption and cost of special mesoporous templates used in the confined-synthesis methods. 

There is no principal difference in particle packing from suspensions and particulate sols. In both cases the densification of the dispersion may be accompanied by agglomeration of the particles cind the formation of a continuous... 

Smoke, dust, and some other aerosols are examples of solid-in-gas dispersions. The solid-in-liquid dispersions are termed suspensions or sols. The pastes and some glues are highly concentrated suspensions. The gels represent bicontinuous structures of solid and liquid. The pastes and some glues are highly concentrated suspensions. Solid-in-solid dispersions are some metal alloys, many kinds of rocks, some colored glasses, etc. 

Silica supports were prepared by a complexing-agent assisted sol-gel method from tetraethyl orthosilicate and calcined at 500°C for 2h. Titanium iso-propoxide was added to silica-2-propanol suspension and then water was added to the suspension. The resulting solid was dried at 110 C and then calcined at 500°C for 2h. 

For the preparation of washcoated monoliths the suspensions of sol-aluminium hydroxide with pseudoboehmite structure have been used. This sol formed during the reaction between the hydroxide and nitric acid serves both as a binder and a source of y-A Oa in the final product after calcination. Salts of additives were introduced into sol. The influence of the following parameters on the formation of thermostable washcoated layer have been studied concentration of anhydrous alumina in the sol amount of added HNO3 dipping time number of dippings drying and calcination duration. 

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