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2.6 Particle Size Distribution

Most powders are polydisperse, meaning they contain a range of particle sizes.
Plotting the number or weight of particles versus size yields a frequency distribution curve,
which shows the mode (most frequent size).

Alternatively, cumulative plots show the percentage of particles above or below a given size,
producing a sigmoidal curve.
Understanding distribution is critical, since samples with identical average diameters can
have very different behaviors if distributions differ.

2.7 Derived Properties of Powders

1. Density

Density is defined as mass per unit volume.
However, powders contain pores and voids, so multiple definitions exist:

Type Includes Measured by

True density Solid material only Helium displacement

Granule (envelope) density Includes internal pores Mercury displacement

Bulk density Includes interparticulate voids Graduated cylinder

Bulkiness is the reciprocal of bulk density. Smaller particles → larger surface area → lower
bulk density → higher bulkiness.

Differences in particle packing cause significant variations; e.g., calcium carbonate bulk
density ranges from 0.1 to 1.3 g/mL.

2. Porosity and Packing

Porosity (ε) = (Void volume / Bulk volume) × 100
Most powders have porosities between 30–50%.
Polydisperse powders (mixed particle sizes) can achieve porosities lower than the theoretical
minimum, while aggregated powders may exceed 48%.

3. Applications of Density Data

• Predicting tablet hardness and disintegration time.
• Determining appropriate packaging and mixing vessel sizes.
• Ensuring uniformity of bulk chemicals.
• Understanding powder behavior during compression and flow.

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