Crystal hydrates
In some compounds when they crystallise they entrap the solvent inside the crystal.
Crystals that contain water as entrapped solvent inside them are known as crystal solvates, or crystal hydrates. Crystals that donot contain water of crystallisation are known as anhydrates.
There are two types of crystal solvates:
1. Polymorphic solvates are very stable and is difficult to desolvate them because the solvent plays a key role in holding the crystal together. When these crystals lose their entrapped solvent they collapse and undergo recrystallise in to a new crystal form.
2. Pseudopolymorphic solvates lose their solvent more easily and desolvation does not destroy the crystal lattice. In these solvates the entrapped solvent is not part of the crystal bonding and it only occupies spaces in the crystal.
The solvate formed by a drug depends on the conditions of crystallisation, especially the solvent used.
The solvated forms of a drug have different physicochemical properties when compared to the anhydrous form:
•The melting point of the crystal hydrate is usually lower than that of the anhydrous form.
•Crystal hydrates have lower aqueous solubilities than anhydrous crystals
•The rates of dissolution of different solvated forms of a drug differ with each other but are generally higher when compared to that of the anhydrous forms.
•Bioavailabilities of the solvates of a particular drug are different; for example, the monoethanol solvate of prednisolone tertiary butyl acetate has an absorption rate in vivo which is nearly five times greater than that of the anhydrous form of same drug.
Authored by :
S.M.Imroz , M.Pharm(Pharmaceutics)
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