Colloids, naturally occurring

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

An important technique for the qualitative and quantitative analysis of different macromolecular materiafs is based on the efectrophoretic separation of particfes having different transport vefocities (e.g., because they have different zeta potentiafs). This technique is used for the anafysis of proteins, pofysaccharides, and other naturally occurring substances whose molecular size approaches that of colloidal particles (for more details, see Section 30.3.4). It is an advantage of the electrophoretic method that mild experimental conditions can be used—dilute solutions with pH values around 7, room temperature, and so on—which are not destructive to the biological macromolecules. 

The lack of a method to determine the spatial distributions of permeability has severely limited our ability to understand and mathematically describe complex processes within permeable media. Even the degree of variation of intrinsic permeability that might be encountered in naturally occurring permeable media is unknown. Samples with permeability variations will exhibit spatial variations in fluid velocity. Such variations may significantly affect associated physical phenomena, such as biological activity, dispersion and colloidal transport. Spatial variations in the porosity and permeability, if not taken into account, can adversely affect the determination of any associated properties, including multiphase flow functions [16]. 

As pointed out by Sposito (1984) this equation initiated the surface chemistry of naturally occurring solids. Maarten van Bemmelen published this equation (now referred to as the Freundlich isotherm) more than 100 years ago and distilled from his results, that the adsorptive power of ordinary soils depends on the colloidal silicates, humus, silica, and iron oxides they contain. 

Synonyms Natural latex—milky fluid that consists of extremely small particles of rubber obtained from plants, principally from the Hevea brasiliensis (rubber) tree, dispersed in an aqueous medium. It contains a variety of naturally occurring substances in a colloidal suspension, including about 1% proteins (e.g., a-globulins, hevein), which are the allergenic fraction. ... 

Over thousands of years for writing, the ancient people used naturally occurring colloidal fine material from ash (mostly charcoal) dispersed in oil (olive oil). Modem inkjet printers employing color are based on much more sophisticated components. Inkjet printers have a number of nozzles that inject ink droplets on the surface of paper. Simultaneously, different colors are mixed to obtain the desired color shade (more than hundreds of thousands). In a typical printer, there may be 30,000 injections per second, and there may be more than 500 nozzles (each with a size less than a human hair (pm =10 6 m). (The ink has a shelf life of more than a year.) In this process, the surface and colloidal principles most obvious are... 

Celis, R. Cornejo, J. Hermosin, M.C. (1998) Textural properties of synthetic clay-ferrihy-drite associations. Clay Miner. 33 395-407 Celis, R. Hermosin, M.C. Cox, L. Cornejo, J. (1999) Sorption of 2,4-Dichlorophenoxyacetic acid by model particles simulating naturally occurring soil colloids. Environ. Sci. Techn. 31 1200-1206... 

It, of course, was not possible for Fischer to make a naturally occurring protein because at that time no structure of a protein was known. In fact, many believed that proteins were not even discrete single large molecules, but were actually colloidal mixtures of smaller compounds. It was, however, possible for Fischer to apply his new methods to the synthesis of large peptides that he felt would simulate the properties of natural proteins. 

Photonic crystals have only been studied in the laboratory for two decades, but naturally occurring examples exist, with the best known being the gemstone opal. Opals consist of tiny spheres of silica arranged in a face centred cubic structure. These are thought to have formed from colloidal silica solutions, and the colour depends on the size of the spheres. 

Periodically, these mixtures were centrifuged and an aliquot of the clay-free supemate taken for counting analysis. Two naturally-occurring clays were selected for the experiments one was labeled kaolin (for the mineral kaolinite) while the second was referred to as attapulgite (or polygorshite). Both were obtained from the Source Clay Mineral Repository (3) as standard clays representative of each class of clay and were used as received. Stable, colloidal suspensions of each were prepared by ultrasonically dispersing weighed quantities of each clay in triple-distilled water. 

Colloids are always present in natural waters containing the transuranium elements. (Colloids are defined as particles with sizes ranging from 1 to 450 nm. These particles form stable suspensions in natural waters.) Colloids of the transuranium elements can be formed by hydrolysis of transuranium ions, or by the sorption of transuranium elements on the naturally occurring colloids. The naturally occurring colloids include such species as metal hydroxides, silicate polymers, organics (such as humates), and the like. The mobility of the transuranium elements in an aquifer is determined largely by the mobility of its pseudocolloids, that is, those colloidal species formed by the adsorption of the transuranium ions upon the naturally occurring colloids. 

Lake Trawsfynydd, North Wales). The results, also given in Table 13.8, indicate that although a slightly higher retention value was observed they supported the view that complexation of 137Cs with naturally occurring colloidal material was not appreciable in this lake. 

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