Dispersions, coarse colloidal

The defined size ranges and limits are somewhat arbitrary since there are no specific boundaries between the categories. The transition of size ranges, either from molecular dispersions to colloids or from colloids to coarse dispersions, is very gradual. For example, an emulsion may exhibit colloidal properties, and yet the average droplet size may be larger than 1 pm. This is due to the fact that most disperse systems are heterogeneous with respect to their particle size [1-2]. 

In colloid chemistry all kinds of foams, both liquid and solid, are classified as lower coarse-disperse systems, i.e. such in which the minimum pore diameter is not less than 1 mm. The grouping of plastic foams into coarse disperse systems has until recently, been justified, since the minimum size of the cells that could be observed did not exceed this value. However, microcells have been discovered in polymer foam structures, this requires a revision of the old concepts regarding the dispersity. The real minimum size of microcells is a few hundredth of a micron and their number is well above that of macrocells. These results support the conclusion that plastic foams belong to the group of finely dispersed or colloid systems... 

Along with the classification of disperse systems based on the phase state of the dispersed phase and the dispersion medium, and their classification as coarse dispersed or colloidal, structured or unstructured, dilute or concentrated, one can also subdivide disperse systems into lyophilic or lyophobic types. Systems belonging to these principally different classes differ in the nature of colloid stability and in the intensity of interfacial intermolecular interactions. High degree of similarity between the dispersed phase and the dispersion medium, and, consequently, compensation of the... 

In summary, ultrasonication facilitates the dispersion of colloidal suspensions at relatively high stress intensities. It relies on the presence of cavitation and on a deep penetration of the sound field into the suspension. The latter is guaranteed by low suspension viscosities (e.g. as for aqueous media with low solid content). A particular problem is the abrasion of the sonotrode tip, which depends on the solid phase, the particle size and shape, the sound intensity, as well as on the abrasion history of the tip. Abrasion particles from sonotrodes are coarse particles in the micrometre range and may spoil the quality of colloidal suspensions. 

The butter fat is a coarse dispersion readily removable on standing or by a centrifuging operation. The casein will be present in the skimmed milk as colloidally dispersed micelles of diameter of the order of 10 cm, and is associated with calcium and phosphate ions. 

In fact, even in pure block copolymer (say, diblock copolymer) solutions the self-association behavior of blocks of each type leads to very useful microstructures (see Fig. 1.7), analogous to association colloids formed by short-chain surfactants. The optical, electrical, and mechanical properties of such composites can be significantly different from those of conventional polymer blends (usually simple spherical dispersions). Conventional blends are formed by quenching processes and result in coarse composites in contrast, the above materials result from equilibrium structures and reversible phase transitions and therefore could lead to smart materials capable of responding to suitable external stimuli. 

Palit and Guha (110) drew further attention to the connection between polymerization rate and the colloidal nature of the precipitating polymer. They found that as the amount of redox initiator increased the polymerization rate first increased, then decreased and finally increased again. These regions corresponded to a fine sol, a milky dispersion and a coarse precipitate. Generally the rate of polymerization ran parallel to the amount of electrolyte required to precipitate the colloid. 

A coarse sulphur sol can be prepared by pouring a saturated solution of sulphur in alcohol or acetone into water just below boiling point. The alcohol or acetone vaporises, leaving the water-insoluble sulphur colloidally dispersed. This technique is convenient for dispersing wax-like material in an aqueous medium. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

Figure 6.17 Some of the phase separations that may occur during rheological measurements involving colloidal dispersions. The sample shown contains oil droplets ( ), coarse solids (O), and fine solids (-). From Schramm [95]. Copyright 1992, American Chemical Society.

The concise scientific definition of an aerosol refers specifically to a colloidal state of material suspended in a gas. However, the term has acquired an additional meaning in common household usage. In the commercial packaging field, the term aerosol now is synonymous with pressurized products that are released in a dispersed form from a can or a bottle. The discharge ranges from coarse fogs and mists to finely divided liquid or powder dispersions. 

Particle Size in Emulsions When a solid drug is suspended in an emulsion, the liquid dosage form is known as a coarse dispersion. In addition, a colloidal dispersion has solid particles as small as 10nm-5pm and is considered a liquid between a true solution and a coarse dispersion [44],... 

Sorption of radionuclides on particulates in solution is frequently observed. The particles may be coarsely or finely dispersed. Their surface properties (surface layer, charge, ion-exchange and sorption properties) play a major role. In general, they offer a great number of sorption sites on the surfaee, and microamounts of radionuclides may be found on the surface of these particles instead of in solution. Sorption of radionuclides on colloidal particles leads to formation of radioeolloids (carrier colloids, section 13.4). 

Aquatic sediments are formed in all surface waters by the settling of coarse and fine inorganic and organic particles. They are present in rivers, in lakes and in the oceans, and radionuclides deposited on the surface of the earth will sooner or later come into contact with these sediments. They may enter the sediments by sorption of molecularly-dispersed species (ions, molecules), by precipitation or coprecipitation, by coagulation of colloids (in particular carrier colloids) followed by sedimentation of the particles formed, or by sedimentation of coarse particles (suspended matter). By desorption, the radionuclides may be remobilized and released again into the water. 

In classical colloidal chemistry, solid foams were traditionally ass ed to low or coarse dispersion systems, i.e. to systems with minimal pore (cell) sizes of about 1 im or more and with a degree of dispersion (specific surface) of 10 - 10 cm". ... 

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