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الكلية كلية الصيدلة
القسم فرع الصيدلانيات
المرحلة 2
أستاذ المادة ظافر قحطان سعيد الامين الماشطة
5/27/2011 2:10:36 PM
air permeability method
the resistance to the flow of a fluid through a plug of compacted powder is the surface area of the powder. the greater is the surface area per gram of powder,sw, the greater is the resistance to flow. hence, for a given pressure droping across the plug, permeability is inversely proportional to specific surface measurement of the former provides a means of estimating this parameter.
because of the simple instrumentation and the speed with which determinations can be made, permeability methods are widely used pharmaceutically for specific-surface determinations, especially when the aim is to control batch-to-batch variations. when using this technique for more fundamental studies, it would seem prudent to calibrate the instrument.
pore size
materials of high specific area may have cracks and pores that adsorb gases and vapors, such as water, into their interstices. relatively insoluble powdered drugs may dissolve more or less rapidly in aqueous medium depending on their adsorption of moisture or air. other properties of pharmaceutical importance, such as the dissolution rate of drug from tablets, may also depend on the adsorption characteristics of drag powders.
the adsorption of water vapor, flavoring agents, and other volatile substances into films, containers, and polymeric materials used in pharmacy are important in product formulation and in the storage and use of drug products.
derived properties of powders
the preceding sections of this chapter have been concerned mainly with size distribution and surface areas of powders. these are the two fundamental properties of any collection of particles. there are, in addition, numerous derived properties that are based on these fundamental properties. those of particular relevance to pharmacy are discussed in the remainder of this chapter.
porosity
suppose a powder, such as zinc oxide, is placed in a graduated cylinder and the total volume is noted. the volume occupied is known as the bulk volume, vb. if the powder is nonporous, that is, has no internal pores or capillary spaces, the bulk volume of the powder consists of the true volume of the solid particles plus the volume of the spaces between the particles. the volume of the spaces, known as the void volume, v, is given by the equation
in which vp is the true volume of the particles. the method for determining the volume of the particles will be given later.
the porosity or voids e of the powder is defined as the ratio of the void volume to the bulk volume of the packing:
porosity is frequently expressed in percent, e × 100.
example: a sample of calcium oxide powder with a true density of 3.203 g/cm3 and weighing 131.3 g was found to have a bulk volume of 82 cm3 when placed in a 100-ml graduated cylinder. calculate the porosity.
the volume of the particles is
packing arrangements
powder beds of uniform-sized spheres can assume either of two ideal packing arrangements: (a) closest or rhombohedral, and (b) most open, loosest, or cubic packing. the theoretical porosity of a powder consisting of uniform spheres in closest packing is 26% and for loosest packing is 48%. the arrangements of spherical particles in closest and loosest packing are shown in figure 19-16.
the particles in real powders are neither spherical in shape nor uniform in size. it is to be expected that the particles of ordinary powders may have any arrangement intermediate between the two ideal packings of figure 19-16, and most powders in practice have porosities between 30% and 50%. if the particles are of greatly different sizes, however, the smaller ones may sift between the larger ones to give porosities below the theoretical minimum of 26%. in powders containing flocculates or aggregates, which lead to the formation of bridges and arches in the packing, the porosity may be above the theoretical maximum of 48%. in real powder systems, then, almost any degree of porosity is possible. crystalline materials compressed under a force of 100,000 lb/in.2 can have porosities of less than 1%.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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