For an alumina nanopowder, the primary particle diameter observed by TEM was about 30 nm, and the particle size obtained from the BET specific surface area assuming spheres was also about 30 nm. However, the mean particle size measured by dynamic light scattering after dispersing it in water was about 400 nm. Which interpretation of these results is most appropriate?

Answer: C. The primary particles are about 30 nm, but in the liquid many primary particles move together as agglomerates (secondary particles) of about 400 nm

The BET method counts surface that gas molecules can reach, so even when primary particles are gathered with point contacts, nearly all their surface is measured. Since the TEM primary particle size and the BET particle size (d = 6/(ρS)) agree, the primary particles can be taken to be about 30 nm, with few internal pores. Dynamic light scattering, on the other hand, measures the size of particle groups that move together in the liquid, so the value of about 400 nm represents the size of secondary particles formed by agglomeration of primary particles. Gas cannot enter closed pores, so they cannot cause the BET method to overestimate the surface area. Since primary particles are visible in TEM, the interpretation of 400 nm single crystals does not fit either. Breaking up the agglomerates with a dispersant or ultrasonication reduces the particle size measured in the liquid.