micromertics part1

Micromeritics
The science and technology of small particles was given the name micromeritics. Knowledge and control of the size and the size range of particles are of profound importance in pharmacy. Thus, size, and hence surface area, of a particle can be related in a sig­nificant way to the physical, chemical, and pharmacologic properties of a drug. Clinically, the particle size of a drug can affect its release from dosage forms that are adminis­tered orally, parenterally, rectally, and topically. The success­ful formulation of suspensions, emulsions, and tablets, from the viewpoints of both physical stability and pharmacologic response, also depends on the particle size achieved in the product. In the area of tablet and capsule manufacture, control of the particle size is essential in achieving the necessary flow properties and proper mixing of granules arid powders.
Particle size and size distribution
In a collection of particles of more than one size (i.e., in a polydisperse sample), two properties are important, namely
(a) the shape and surface area of the individual particles, and (b) the size range and number or weight of particles present and, hence, the total surface area.
The size of a sphere is readily expressed in terms of its diameter, however, as the degree of asymmetry of particles increases, so does the difficulty of expressing size in terms of a meaningful diameter. Under these conditions, there is no one unique diameter for a particle. Recourse must be made to the use of an equivalent spherical diameter, which relates the size of the particle to the diameter of a sphere having the same surface area, volume, or diameter. Thus, the surface diameter, ds, is the diameter of a sphere having the same sur­face area as the particle in question. The diameter of a sphere having the same volume as the particle is the volume diame­ter, dv, whereas the projected diameter, dp, is the diameter of a sphere having the same observed area as the particle when viewed normal to its most stable plane. The size can also be expressed as the Stokes diameter, dst, which describes an equivalent sphere undergoing sedimentation at the same rate as the asymmetric particle. Invariably, the type of diameter used reflects the method employed to obtain the diameter. For example, the projected diameter is obtained by micro­scopic techniques, whereas the Stokes diameter is determined from sedimentation studies on the suspended particles.
Any collection of particles is usually polydisperse. It is therefore necessary to know not only the size of a certain particle, but also how many particles of the same size exist in the sample. Thus, we need an estimate of the size range present and the number or weight fraction of each particle size. This is the particle-size distribution, and from it we can calculate an average particle size for the sample.
Average particle size
A general equation for the average particle size, whether it be an arithmetic, a geometric, or a harmonic mean diameter was derived.