disperse system
Picture sand stirred into a glass of water, or oil whisked into vinegar for a salad dressing. In each case, one material is broken up into tiny pieces and scattered throughout another. That scattered-into-something arrangement is what pharmacists call a disperse system: one substance distributed as small particles or droplets throughout a surrounding medium.
Every disperse system has two parts. The discontinuous part — the bits that are spread out — is the dispersed phase. The surrounding material that holds them is the continuous phase. The size of the dispersed particles defines the type of system: molecular dispersions (true solutions) below about one nanometre, colloids from roughly one nanometre to one micrometre, and coarse dispersions (suspensions and emulsions) above about one micrometre.
Disperse systems matter in pharmacy because many medicines simply cannot be made as clear solutions. A drug that will not dissolve can be suspended as fine solid particles; an oily drug can be emulsified into droplets so it pours and tastes acceptably. The trade-off is that, unlike a true solution, a disperse system is not fully stable on its own — the dispersed phase tends to settle, cluster, or merge over time, so formulators must work to keep it evenly distributed.
Calamine lotion (a suspension of solid calamine in water) and milk (an emulsion of fat droplets in water) are both familiar disperse systems found in everyday life.
One label, two very different dispersions.
The dividing sizes (1 nm and 1 µm) are conventional guidelines, not sharp physical boundaries; real formulations often span a range and the categories blur at the edges.