porosity

Porosity is the fraction of empty space inside a powder bed or a solid compact, the proportion that is air rather than substance. Think of a sponge or a loosely filled jar of marbles: a lot of what you see is actually the gaps. Porosity puts a number on those gaps, usually as a fraction or a percentage of the total volume.

It is calculated from the difference between the bulk volume a material occupies and the true volume of solid it contains, or equivalently from one minus the ratio of bulk density to true density. The voids can be between particles (interparticle) or inside porous particles themselves (intraparticle), and both count toward the total.

Porosity shapes many pharmaceutical behaviours. In a tablet, the network of pores lets water wick in so a disintegrant can swell and break the tablet apart, which controls how fast it dissolves; too little porosity gives a hard, slow-dissolving tablet, while too much weakens it. In a powder bed, high porosity means many voids that can collapse on tapping, signalling poor flow.

It is measured by methods such as mercury intrusion porosimetry, which forces mercury into pores under rising pressure to map their sizes, or gas pycnometry to get true density. A useful caveat is that total porosity says nothing about pore size or connectivity, yet it is the size and connectedness of pores, not just their volume, that often govern water uptake and dissolution.