Thermoanalytical characterization of pseudopolymorphs of sulphadimidine and sulphadimidine-trimethoprim molecular complexes

In recrystallization experiments of sulphadimidine from water, dioxane and their mixtures aimed at preparing the unstable
hydrate, a non-stoichiometric solvate with dioxane and a pseudopolymorph with similar X-ray powder diffraction and infrared absorption properties but different thermal behaviour (DSC, TG) were isolated. In co-crystallization experiments of trimethoprim and sulphadimidine, a monohydrate of the respective 1 : 2 molecular complex and a methanol solvate of the equimolecular complex were obtained from aqueous ethanol and anhydrous methanol, respectively. The thermal stability of both pseudopolymorphic complexes is re¯ected by the high DSC and TG onset temperatures of solvent escape from the crystal lattice (Te=163 °C and Te = 144 °C, respectively, for water and methanol) and is consistent with the important role of water in the hydrated crystal packing and the very strong hydrogen-bonding interaction of methanol with a sulphonamido oxygen atom in the methanol solvate crystal.