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DrX Whiz Niraj
Unit 4: Micromeritics
Science of Small Particles & Powder Flow
1. Introduction to Micromeritics
Micromeritics is the science and technology of small particles and their behavior. It deals with particle size, shape, surface area, porosity, density, and flow properties.
Pharmaceutical Importance:
- Tablet and capsule formulation (Powder flow).
- Drug dissolution and absorption (Finer particles dissolve faster).
- Stability of suspensions and emulsions.
- Mixing and granulation process.
2. Methods for Determining Particle Size
Depending on the particle size, different methods are used:
- Microscopy Method: Directly measuring particles using a microscope. Gives exact shape but is time-consuming.
- Sedimentation Method: Based on Stoke's Law. Velocity of settling particles is measured.
- Coulter Counter Method: Measures change in electrical resistance as particles pass through an orifice. Highly accurate.
- Sieve Analysis: The most common method for larger particles. Powder is passed through a stack of sieves with decreasing mesh sizes.
3. Derived Properties of Powders
Properties that depend upon particle size, shape, and their arrangement in a powder bed are called Derived Properties.
Porosity (Epsilon, ε):
It is the ratio of void volume (empty spaces) to the total bulk volume of the powder.Packing Arrangement of Particles
The way particles arrange themselves directly affects the porosity of the powder bed.
| Packing Type | Porosity | Particle Alignment |
|---|---|---|
| 1. Cubical Packing | High (~47%) | Loosely packed (One exactly over another) |
| 2. Rhombohedral (Closest) | Low (~26%) | Densely packed (Particles fit into the gaps of the lower layer) |
4. Types of Densities
- True Density: Mass of powder divided by true volume (excluding pores).
- Bulk Density (BD): Mass of powder divided by the bulk volume (including empty spaces between particles). Formula:
ρb = Mass / Bulk Volume. - Tapped Density (TBD): Density obtained after mechanical tapping of the powder cylinder. Tapping removes voids, decreasing volume and increasing density.
Note: Bulkiness is the reciprocal of Bulk Density.
5. Flow Properties of Powders
Ability of a powder to flow freely is crucial for tablet compression, capsule filling, and uniform mixing. Poor flow causes weight variation in tablets.
1. Angle of Repose (θ)
It is the maximum angle possible between the surface of a powder pile and the horizontal plane.
Interpretation: Smaller Angle (< 30°) = Excellent Flow. Larger Angle (> 40°) = Poor Flow.
2. Carr’s Index & Hausner Ratio
Both are determined using Bulk Density (BD) and Tapped Bulk Density (TBD).
| Parameter | Formula | Interpretation (Good Flow) |
|---|---|---|
| Carr's Index (%) | [(TBD - BD) / TBD] × 100 | Lower value (5 - 15%) indicates Excellent Flow. |
| Hausner Ratio | Tapped Density / Bulk Density | Value less than 1.25 indicates Good Flow. |
🔥 Exam Corner: Important Questions
- 2 Marks: Define Micromeritics, Porosity, Angle of Repose, Hausner Ratio.
- 5 Marks: Explain the derived properties of powders (Densities and Porosity).
- 10 Marks: Discuss flow properties of powders and methods of their evaluation in detail.