Pseudopolymorphism, solvates, and hydrates

McCrone (1965) and Haleblian and McCrone (1969) pointed out that pseudopolymorphism has been used to describe a number of phenomena that are related to polymorphism among them are desolvation, second-order transitions (some of which may be considered examples of polymorphism), dynamic isomerism, mesomorphism. 

In spite of the objections to the use of pseudopolymorphism to describe solvated structures of a material, the term seems to have gained quite a general acceptance in this context, especially in the pharmaceutical industry, both in the characterization (Kitamura et al. 1994 Nguyen et al. 1994 Kiaoka and Ohya 1995 Brittain et al. 1995 Kitaoka et al. 1995 Caira et al. 1996 Gao 1996 Kalinkova and Hristov 1996 Kritl et al. 1996 de Ilarduya et al. 1997 Ito et al. 1997 deMatas et al. 1998) and production/processing aspects (Adyeeye et al. 1995 Hendrickson et al. 1995 Joachim etal. 1995). 

In light of the variety of behaviour exhibited by solvates, Byrn (1982) has suggested a classification scheme for crystal solvates based on that behaviour, rather than on stability. He proposed that the solvates for which the solvent can be removed from the crystal and added back to the crystal reversibly without greatly changing the X-ray powder diffraction pattern (Section 4.4) would be considered pseudopoly-morphic solvates. Those which undergo a change in structure, as evidenced by a different powder diffraction pattern, would be described as polymorphic solvates. The appellation does not seem to have been adopted by many other workers. 

McCrone (1965) also noted that second-order phase transitions have been termed as pseudopolymorphic. Such transitions are difficult to detect by optical methods, because of the small structural changes that occur hence, the origin of the prefix pseudo sometimes used to describe them. However, the birefringence of the crystals changes during such phase changes (see Section 4.2), so the use of crossed polarizers makes the phase change readily detectable. 

While this system falls somewhere on the fuzzy line between polymorphism and dynamic isomerism we agree with McCrone (1965) and Threlfall (1995) that this phenomenon should not be described as pseudopolymorphism. 

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