Dispersion and Agglomeration

The ubiquitous van der Waals forces ensure that particles attract each other, thus favouring agglomeration. The magnitude of this attractive force can be calculated if the Hamaker 

Once the particles have agglomerated, the surfaces come into intimate contact and other, short-range forces can become important. Thus, the force required to separate the particles will depend on the van der Waals force plus any specific surface interactions that have occurred such as water bridging, hydrogen bonding or Lewis acid-base interactions. The polymer melt competes to interact with the surface so that the overall energetics depend on whether it is more favourable for the filler surface to interact with another particle, or with the polymer phase. 

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A second principle applying to these model systems is derived from their colloidal nature. With the usual thermodynamic parameters fixed, the systems come to a steady state in which they are either agglomerated or dispersed. No dynamic equilibrium exists between dispersed and agglomerated states. In the solid-soil systems, the particles (provided they are monodisperse, i.e., all of the same size and shape) either adhere to the substrate or separate from it. In the liquid-soil systems, the soil assumes a definite contact angle with the substrate, which may be anywhere from 0° (complete coverage of the substrate) to 180° (complete detachment). The governing thermodynamic parameters include pressure, temperature, concentration of dissolved... 

In general, flocculants are used in solid-liquid separation processes such as thickening and filtration. Inorganic salts are also used sometimes to aggregate fine particles. Flocculation technique has been developed further for special applications of selective flocculation, selective dispersion and agglomeration flotation. 

Stabilization and partial destabilization of emulsions and foams by controlling the state of dispersion and agglomeration of oil droplets or fat globules... 

The fifth chapter presents detailed review of technological aspects of ferroic nanoparticles fabrication. We present the detailed information about methods of chemical synthesis of above nanoparticles. Among them are hydrothermal, sol-gel and coprecipitation methods. We also present the method of unstable compounds decomposition. The combined synthesis methods have also been discussed. Namely, we consider mechanochemical, sonochemical and template synthesis methods. The main idea of these methods is to control the dispersity and agglomeration degree of nanoparticles by inspection of nucleation and growth of a new phase. Self-assembly and self-organization of ferroic nanoparticles as well as composites formation on their base by means of colloidal processes have also been considered. 

The stability of colloids is a prime consideration in determining their behavior. It is involved in important aquatic chemical phenomena, including the formation of sediments, the dispersion and agglomeration of bacterial cells, and the dispersion and removal of pollutants (e.g., crude oil from an oil spill). 

Matsuoka M, Tatami J, Wakihara T (2014) Control of dispersion and agglomeration of CNTs for their networking—mechanical and electrical properties of CNT/alumina composites. Ceram Trans 243 117-120... 

Wilkie and co-workers [57] utilized the anions 2-ethylhexyl sulfate, bis(2-ethylhexyl) phosphate, and dodecyl benzenesulfonate as intercalated anions to synthesize organo-LDHs. Nanocomposites of PMMA and PS with organo-LDHs were prepared both by meltblending and by bulk polymerization. XRD and TEM results revealed that the phosphate and sulfonate LDHs in PMMA show fairly good dispersion at the nanometer scale, whereas sulfa LDH is poorly dispersed. Eor PS, the LDHs are poorly dispersed and agglomerated LDH particles are observed. The reductions in PHRR for nanocomposites containing sulfate, phosphate, and sulfonate LDH are 27, 37, and 45% in PMMA and 32, 33, and 49% in PS, respectively. Both PMMA and PS samples obtained from bulk polymerization show poorer dispersion and less reduction in PHRR than samples obtained from melt-blending. 

Fig. 2. Tlie models of both well dispersed and agglomerated nano-sized powder.

In the post-dispersion process, the soHd phenoHc resin is added to a mixture of water, cosolvent, and dispersant at high shear mixing, possibly with heating. The cosolvent, frequently an alcohol or glycol ether, and heat soften the resin and permit small particles to form. On cooling, the resin particles, stabilized by dispersant and perhaps thickener, harden and resist settling and agglomeration. Both resole and novolak resins have been made by this process (25). 

For products intended to remain stable dispersions for an extended period, a particle size of 2 p.m or less is desirable. A thickening agent is usuaUy added after the reaction has been completed and the mixture is cooled in order to prevent settling and agglomeration. Examples of thickeners are guar gum, xanthan gum, and hydroxyethylceUulose. The final products are generaUy between 40 and 50% soUds, with a viscosity of 1500 5000 mPa-s(=cP). 

The value of pigments results from their physical—optical properties. These ate primarily deterrniaed by the pigments physical characteristics (crystal stmcture, particle size and distribution, particle shape, agglomeration, etc) and chemical properties (chemical composition, purity, stabiUty, etc). The two most important physical—optical assets of pigments are the abiUty to color the environment in which they ate dispersed and to make it opaque. 

Nomenclature. Colloidal systems necessarily consist of at least two phases, the coUoid and the continuous medium or environment in which it resides, and their properties gready depend on the composition and stmcture of each phase. Therefore, it is useful to classify coUoids according to their states of subdivision and agglomeration, and with respect to the dispersing medium. The possible classifications of colloidal systems are given in Table 2. The variety of systems represented in this table underscores the idea that the problems associated with coUoids are usuaUy interdisciplinary in nature and that a broad scientific base is required to understand them completely. 

The degree of agglomeration of fillers affects the dispersion and the rheological properties of adhesives [41], and disagglomeration of fillers during adhesives manufacturing should be produced to obtain acceptable properties. 

The usual problems encountered with any sedimentation procedure are present. It is necessary to use a dispersing liq compatible with the material being tested. Also, dispersing agents and technique must insure complete dispersion and prevent agglomeration. These factors may vary with materials, and therefore will require special attention when new materials are to be analyzed... 

Many of the crucial problems for researchers in this area are the same as the ones encountered in other areas of surface and interfacial science. The research of chemical engineers on high-performance ceramic materials, field-induced bioseparations, and fouling also addresses phenomena such as agglomeration and clustering in dispersions and rheology of dispersions. For EPIDs,... 

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