Although fine particles can be produced directly by controlled precipitation, crystallization, or drying a fine spray of solution, in many cases the material is powdered in some kind of mill.
THE IMPORTANCE OF FINE PARTICLES
IN PHARMACY
Although
fine particles can be produced directly by controlled precipitation,
crystallization, or drying a fine spray of solution, in many cases the material
is powdered in some kind of mill. From our point of view, the most important
result of this operation is the increase in the surface area of a given weight
of the powder and its influence on diffusional processes. A cube of side 0.01 m
has a surface area of 6 x 10-4 m2. If, by some ideal size
reduction process, this cube was divided into cubes of side 0.001 m, we should
have a thousand particles each with a surface area of 6 x 10-6 m2
and a total surface area of 6 x 10-3 m2. A
tenfold increase in surface area has been given by a tenfold decrease in
particle size. Generalizing, we may say that the surface area is inversely
proportional to the particle size assuming that the shape of the particles
remains the same.
The
rate of most chemical and physical reactions involving solids and liquids is
greatly influenced by the area of interfacial contact. In chemical reactions, a
reagent must diffuse toward the surface of the solid and the reaction products
away, a procedure that depends, among other things, on the area between solid
and liquid. The effect of particle size on dissolution rate exem-plifies another
aspect of diffusion that is of importance to the pharmacist. Most commonly,
drugs are taken orally in the form of solid particles, and absorption, which is
usually rapid, must be preceded by dissolution. A full discussion of the role
of particle size in oral, parenteral, and topical therapies may be found in
reviews (Newman and Axon, 1961; Wagner, 1961).
The
rate at which fine chemicals or drugs are extracted from a vegetable source is
increased by an increase of surface area. Reduction of particle size increases
the area available for transfer of materials and decreases the distance over
which solvent and solute must diffuse and has a marked effect on the drying of
porous materials.
Other
effects, not based on diffusion and its dependence on surface area, are found
in mixing and various formulation requirements. If we withdraw a sample from a
mixture of powders, it is unlikely to contain exactly the correct proportion of
ingredients. However, the larger the number of particles in the sample, the
closer the sample will represent the overall proportions of the mixture. We can
therefore increase the accuracy of the sample, which might eventually form a
tablet or a capsule, by increasing the number of particles it contains, that
is, reducing the particle size of the components of the mix. Since difference
of particle size will promote segregation, the components should be produced
with a similar particle size distribution.
Formulation
requirements often dictate the use of fine particles. Impal-pability and spreading
are required of dusting and cosmetic powders. Particles of 3.5 x 10-5
to 4.0 x 10-5 m can be detected as single particles when applied to
the skin and may give the impression of grittiness. Such powders should, in
general, be finer than 3.0 x 10-5 m. When powders are tinted, the
particle size of powder and pigment affects the final color. In tabletting,
careful size reduction of imperfect tablets provides a material suitable for
compression. The flow properties of suspensions of high disperse phase concentration
are affected by particle size and size distribution. At a given disperse phase
concentration, decrease of particle size gives an increase in viscosity, while
a broadening of particle size distribution yields a decrease in viscosity.
Sedimentation is a function of particle size.
Numerous examples
have been quoted to stress the importance of fine particles in pharmacy.
Milling or grinding offers a method by which these particles may be produced,
size classification gives a means, where applicable, of selecting a desired
fraction or of removing oversize or undersize particles, and size analysis
provides the analytical tool by which these operations may be assessed and
controlled.
TH 2019 - 2023 pharmacy180.com; Developed by Therithal info.