The unit of particle size used most frequently in Micromeritics is micrometer (μm), also called as micron (μ).
Physical, chemical and pharmacological properties of a drug depend on particle size and hence surface area of particle.
Impact of Particle size
Surface area: Particle size is inversely proportional to surface area, ie. Decrease in particle size increases surface area. Increased Surface area enhances therapeutic effect of antacid suspensions.
Rate of solution: Decrease in particle size increases effective surface area which in turn increases rate of solution.
Bioavailability: Decrease in particle size increases bioavailability by improving solubility of drug.
Mixedness: Particle size of powders in a narrow size range provides better mixedness in mixing operation.
Stability of emulsion and suspensions: fine particles with narrow particle size range produced more stable emulsions and suspensions.
Size separation: It is a process of separation of mixture in to various fractions on the basis of particle size range.
Methods of Size separation:
Sieving
Sedimentation
Elutriation
Cyclone separator
Sieving method:
This method uses a series of standard sieves calibrated by the National Bureau of Standards
Sieve shaker machine
Principle: the powdered drug is separated according its particle size using a number of sieves in a nest which are then agitated for rapid size separation.
Construction: Standard sieves of different mesh numbers are fixed in a mechanical shaker apparatus with coarsest sieve at the top & finest sieve in the bottom.
Working: A powder sample ( 50 g) is placed on top sieve in a sieve set. This sieve nest is fixed to mechanical shaker apparatus & shaken for a certain period of time. The powder retained on each sieve is weighed
Advantages:
Sieving method provide rapid and reproducible results.
It is simple and inexpensive method.
Disadvantages:
It is useful for dry solids only.
size reduction of test material may take place due to vibration of sieve shaker which introduces error in estimation.
Disadvantages:
If powder is not dry , apertures get clogged
Lower limit of particle size is 50 μm
During shaking , attrition occurs causing size reduction of particles. This results in error in estimation.