dissolution

Dissolution is the act of a solid melting away into liquid — not by heat, but by molecules peeling off the surface one by one and wandering off into the surrounding fluid. Watch a sugar cube fade in coffee and you are watching dissolution.

Mechanistically it happens in two steps: first solvent molecules attack the solid surface and free individual drug molecules (the actual surface reaction), then those molecules diffuse away across a thin stagnant layer of liquid into the bulk. For most drugs the slow, rate-limiting step is this diffusion across the boundary layer, which is why stirring and fine particles speed dissolution up.

Dissolution matters enormously in pharmacy because a drug taken as a tablet or capsule cannot be absorbed until it has dissolved in the gut fluids. For poorly soluble drugs, dissolution is often the slowest step on the way to the bloodstream and therefore controls how fast and how completely the drug works. This is why dissolution testing is a routine quality-control and product-development tool.