Solid-state studies on the hemihydrate and the anhydrous forms of flunisolide

Abstract Flunisolide exists in at least two different anhydrous crystalline forms (I and II) and in a hemihydrate form with distinctly different physico-chemical properties. Modification II and the hemihydrate form are the commercial products. Form I was obtained by heating all other forms above 230°C. The different crystalline forms of flunisolide were investigated by FTIR spectroscopy, X-ray powder diffractometry, differential scanning calorimetry (DSC), thermogravimetric analysis and thermomicroscopy both coupled with FTIR spectroscopy (TG-FTIR and FTIR thermomicroscopy). The three forms were easily differentiated by their IR spectra, X-ray powder diffraction patterns and thermal behaviour. Their stability was investigated under different experimental conditions to verify the tendency to solid–solid transition and to study the existence range of the three forms. The relationship among the two anhydrous polymorphs and the hemihydrate form and their equilibrium solubilities in water at 20°C were also investigated.