Abstract
Solvates and hydrates occur commonly for a large number of pharmaceutical materials. Pharmaceutical hydrates are viable forms for drug products because there is no safety concern about water as a crystal adduct. This chapter discusses pseudopolymorphism, and focuses primarily on crystal hydrates. Hydrates can be classified as three categories based on different structural aspects: Class I represents hydrates where the water molecules exist at isolated sites; Class II hydrates are generally referred to as channel hydrates; and Class III hydrates are generally referred to as ion-coordinated site hydrates. In such systems, water molecules form ion-water bonds that are usually much stronger than hydrogen bonds. A stoichiometric hydrate has a well-defined water content, exhibiting a unique crystal structure when compared to those of the anhydrous forms and other hydrates. Nonstoichiometric hydrates display very different thermodynamic properties compared to stoichiometric hydrates. Discovering hydrates and solvates is part of the polymorph screening process.
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