pseudopolymorph

True polymorphs are the same molecules packed differently — like the same furniture arranged two ways in identical rooms. A pseudopolymorph is subtly different: it is a crystal of the drug that has invited extra guest molecules of solvent into the lattice. When that solvent is water it is a hydrate; when it is ethanol, acetone or another solvent it is a solvate.

Strictly, a pseudopolymorph is not a polymorph at all, because polymorphs must have identical chemical composition while a solvate contains additional solvent. Pharmacopoeias and crystallography purists often prefer the term solvate (or hydrate). Still, in practice pseudopolymorphs behave a lot like polymorphs: each has its own melting and dehydration behavior, density, X-ray powder diffraction pattern, solubility and stability.

Pseudopolymorphs matter because solvent can enter or leave the lattice during crystallization, drying, granulation or storage, switching the solid form and its properties. Many solvates are undesirable because the included organic solvent is toxic and must be removed; hydrates, by contrast, are often acceptable and sometimes the chosen stable form. As with polymorphs, the solid form must be identified and controlled throughout development.