Particle size reduction suspensions

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

Disperse systems often necessitate particle size reduction, whether it is an integral part of product processing, as in the process of liquid-liquid emulsification, or an additional requirement insofar as solid particle suspensions are concerned. (It should be noted that solid particles suspended in liquids often tend to agglomerate. Although milling of such suspensions tends to disrupt such agglomerates and produce a more homogeneous suspension. 

Particle size reduction of a material suspended in a liquid medium can be affected by a Microi uidizeP processor (MicroLuidics Corp., Newton, MA, http //www.microLuidicscorp.com/ processors.html). A schematic of a MicroLuidizer processor is presented in Figure 17.6. In a Micro-Luidizer processor, a liquid stream is bifurcated and the two streams are directed upon each other under pressures as high as 40,000 psi. Particles suspended in the liquid are reduced in size by forces of shear and cavitation. Suspensions can be prepared with the MicroLuidizer processor with mean particle sizes in the micron to submicron range. 

Variations in assay results can be avoided by the preparation of homogeneous, well-mixed, or non-settling fine particle suspensions (size 1-10 pm). Particle size reduction results in slow, more uniform settling rates. The bioavailability of drugs is improved by reducing the size of suspension particles. Furthermore, drug particles smaller than 20 pm produce less pain and tissue irritation when injected parenterally. However, fine particles may have a deleterious effect on chemical stability because of their high dissolution rate. 

In the preparation of suspension suppositories it is important that the used particles of the active substance are small and remain small (don t reagglomerate). Small particles being essential for a correct content and a sufficient content uniformity of the suppositories, dispersion of the active substance in the suppository base will usually be preceded by or combined with particle size reduction, see Sects. 29.2 and 29.3). Large primary particles should be ground and agglomerates should be broken up. If an active substance is not available in the required particle size, the coarse powder must be ground in a rough stone or porcelain mortar. Active substances kept in stock in the required primary particle size... 

In sohd—sohd separation, the soHds are separated iato fractions according to size, density, shape, or other particle property (see Size reduction). Sedimentation is also used for size separation, ie, classification of soHds (see Separation, size separation). One of the simplest ways to remove the coarse or dense soHds from a feed suspension is by sedimentation. Successive decantation ia a batch system produces closely controUed size fractions of the product. Generally, however, particle classification by sedimentation does not give sharp separation (see Size MEASUREMENT OF PARTICLES). 

Pharmacists should also take a dim view of changes in the particle size, size distribution, or particulate nature of semisolid suspensions. They are the consequence of crystal growth, changes in crystalline habit, or the reversion of the crystalline materials to a more stable polymorphic form. Any crystalline alteration can lead to a pronounced reduction in the drug-delivery capabilities and therapeutic utility of a formulation. Thus, products exhibiting such changes are seriously physically unstable and unusable. 

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