Suspensions subdivision/dispersion

Subdivision (dispersion) is also common in geologic systems. In this case dispersion involves weathering of parent rock (physical disintegration and/or chemical dissolution with re-precipitation) followed by suspension of the particles in liquid. The physical disintegration can be caused by a range of events [49] ... 

Another example of subdivision/dispersion is given by the classic disintegration of metals that occurs when two electrodes of the same metal are submersed and subjected to an electric potential that causes arcing (Bredig s arc). Here, metal melts at the electrode tips and becomes dispersed into suspension. Such suspensions can be stabilized by the presence of peptizing electrolyte. 

A classification by chemical type is given ia Table 1. It does not attempt to be either rigorous or complete. Clearly, some materials could appear ia more than one of these classifications, eg, polyethylene waxes [9002-88 ] can be classified ia both synthetic waxes and polyolefins, and fiuorosihcones ia sihcones and fiuoropolymers. The broad classes of release materials available are given ia the chemical class column, the principal types ia the chemical subdivision column, and one or two important selections ia the specific examples column. Many commercial products are difficult to place ia any classification scheme. Some are of proprietary composition and many are mixtures. For example, metallic soaps are often used ia combination with hydrocarbon waxes to produce finely dispersed suspensions. Many products also contain formulating aids such as solvents, emulsifiers, and biocides. 

In terms of the two-phase system which comprises dispersions of solids in liquids, the minimum energy requirement is met if the total interfacial energy of the system has been minimized. If this requirement has been met, chemically, the fine state of subdivision is the most stable state, and the dispersion will thus avoid changing physically with time, except for the tendency to settle manifest by all dispersions whose phases have different densities. A suspension can be stable and yet undergo sedimentation, if a true equilibrium exists at the solid-liquid interface. If sedimentation were to be cited as evidence of instability, no dispersion would fit the requirements except by accident—e.g., if densities of the phases were identical, or if the dispersed particles were sufficiently small to be buoyed up by Brownian movement. 

Suspensions can be formed either by nudeation or by subdivision and dispersion. The nudeation process requires a phase change, such as condensation of vapour to yield solid, or precipitation of a salt from a supersaturated solution. In the latter case a supersaturated solution must be formed. The supersaturation condition is then alleviated by condensation on nudei (which need not be composed of the same material) already present, or else by formation of nuclei with subsequent condensation. The nudei eventually grow to microscopic, or macroscopic, size. Additional details of this process are discussed elsewhere [49,320],... 

As mentioned before, the monomer has a low boiling point ( - 72.2°C) and a relatively low critical temperature (54.7°C). For comparison, vinyl chloride has a boiling point of - 13.8°C and a critical temperature of 147°C (cf. this series, Vol. 11, 2nd ed., p. 358). Therefore the polymerization of vinyl fluoride is usually carried out under high-pressure conditions which probably resemble the polymerization of ethylene more than that of the other vinyl halides. The monomer is soluble in a variety of solvents. At room temperature, the homopolymer is insoluble. Therefore the neat subdivision of polymerization techniques into bulk, solution, suspension, and emulsion procedures which we have used in many other sections in this series becomes blurred. Most procedures seem to lead to blocks of polymer or some sort of dispersions from which... 

Further subdivision of the colloids concerned many researchers. For example, studies of coagulation processes lead Miiller to connect suspensions with physical disintegration processes and large molecules with chemical precipitation methods. He designated as high molecular such substances as albumin and colloidal silica. The later classification by Staudinger into colloidal dispersions, micellar colloids (assocation colloids), and colloidal molecules (macromolecules) proved to be very suitable and forms the foundation of modern textbooks on colloid science. ... 

As stated earlier, colloids represent a state of subdivision of matter. The matter, finety divided, is uniformly distributed in a continuous medium. However, the dispersed particles are neither so large that they separate on standing, nor so small that they can be said to be in solution. This means that the colloidal state is an intermediate state between a suspension and a true solution. 

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