phase transition

A phase transition is the moment a material switches from one physical form to another such as ice melting into water, or water boiling into steam. It is the crossing of a boundary where the way molecules are arranged suddenly reorganises. In pharmacy these crossings happen constantly: when a wax suppository melts at body temperature, when a frozen biologic thaws, or when an amorphous drug slowly recrystallises on the shelf.

Common transitions include melting (solid to liquid), freezing, vaporisation (liquid to gas), condensation, and sublimation (solid straight to gas). Most transitions absorb or release heat at a fixed temperature for a pure substance for example, a pure crystal melts sharply at its melting point. Formulators exploit transitions deliberately, as when a suppository base is chosen to be solid in the box but to melt at 37 degrees Celsius once inserted.

Transitions can also be unwanted and damaging. A drug that converts from one crystal form to another (a polymorphic transition) may suddenly dissolve differently and change its absorption; an amorphous solid that crystallises loses the solubility advantage it was designed to provide. Monitoring and controlling these solid-state changes is a central task of preformulation and stability work.