Thermal studies of solvent exchange in isostructural solvates of a tetroxoprim-sulfametrole complex

Isostructural solvates of the 1: 1 molecular complex between the antibacterial drugs tetroxoprim (TXP) and sulfametrole (SMTR) with formulae TXP . SMTR . CH3OH ( I), TXP . SMTR.C2H5OH (II) and TXP . SMTR . H2O ( III), were investigated to establish their propensity for guest exchange. Separate exposure of powdered ( I), ( II) and ( III) to a saturated atmosphere of each solvent of the complementary solvate pair at ambient temperature resulted in reversible solvent exchange in all cases. DSC and TG were the methods of choice for monitoring the exchange processes since ( I) -( III) have distinct onset temperatures of desolvation and characteristic mass losses. Interpretation of the results in terms of the known locations of the solvent molecules in crystals of ( I) -( III) led to the conclusion that solvent exchange probably proceeds by a co-operative mechanism involving material transport through channels while the common host framework is maintained.