Characterising ultrafine particles:\nFor ultrafine powders, which measure best expresses their size in a way that relates to process behavior?

Introduction / Context:
Ultrafine powders exhibit behavior (reactivity, dissolution rate, rheology) that correlates more with total surface area than with a single length dimension. Traditional sieve analysis fails below about 38 µm, so alternative metrics become useful. Specific surface area is therefore the preferred descriptor for very fine powders in catalysis, pharmaceuticals, and ceramics.

Given Data / Assumptions:

• Ultrafine range where sieving is impractical.
• Surface-driven phenomena are of interest (e.g., reaction or sintering rates).

Concept / Approach:
While micrometre-scale sizing (laser diffraction, DLS) provides number- or volume-based distributions, these still reduce complex polydispersity to a few moments. Specific surface area (e.g., via BET nitrogen adsorption) captures the cumulative surface of all particles, directly impacting adsorption, catalytic activity, and cohesive forces. Thus, for ultrafines, reporting m^2/g is often more informative than a nominal “size.”

Step-by-Step Solution:

Verification / Alternative check:
Materials data sheets for fumed silica, nano-oxides, and micronised APIs prominently list BET surface area instead of sieve size.

Why Other Options Are Wrong:

• Centimetre/micron alone: single length scales miss surface-driven effects.
• Screen size: not measurable for ultrafines.
• Particle count: difficult to determine accurately and not directly linked to surface phenomena.

Common Pitfalls:
Comparing powders only by “average size” can be misleading; different distributions can share the same mean yet have very different surface areas.

Final Answer:
Specific surface area (surface area per unit mass)