Kinetic study of the transformation of mefenamic acid polymorphs in various solvents and under high humidity conditions.

The transformation kinetics of mefenamic acid form II to form I in three kinds of solvents and under high humidity conditions were extensively investigated. Form II crystals were suspended in water, 50% ethanol and ethanol at 28, 33 and 37 degrees C, or stored at 50, 60 and 70 degrees C at 97% RH. Form II transformed to form I under all storage conditions and the rate of transformation depended on the kind of solvent. The transformation followed the three-dimensional nuclei growth mechanism, depending on temperature. The nuclei formation and growth processes were significantly accelerated in ethanol compared with water. The addition of seed crystals of the stable form I shortened the both nuclei formation and growth processes and therefore the transformation was accelerated.