Thermal Properties of α-Cyclodextrin Crystal Forms

Abstract

Differences in thennal properties of two a-cyclodextrin hexahydrate polymorphs appear in differential scanning calorimetry (DSC) (pattern of the dehydration endothenns, endothennal effect due to a phase transition of a dehydration product peculiar of a single crystal fonn) and in thennogravimetric (fGA) and thennomechanical erMA) analyses. 1. INTRODUCTION In a previous paper the physicochemical characterization of anhydrous and hydrated (6 and 7.57 ~O) crystal fonns of a-cyclodextrin was carried out on the basis of X-ray diffraction and thennal properties [1]. Since two a-cyclodextrin hexahydrate polymorphs, fonn I and fonn II, with different crystal packing schemes are described [2,3], a thennoanalytical study was undertaken to establish whether and how the structural differences were reflected by the thermal behaviour. The basic thennoanalytical techniques i.e. differential scanning calorimetry (DSC), thennogravimetric analysis (fGA) and thennomechanical analysis erMA) were used. Simultaneous DSC-TGA runs were also done to complement the results of DSC and TGA experiments. Moreover, since the analysis of gaseous products during a thennal scan of a hydrate gives valuable information on its stability, the evolved gas analysis (EGA) curves of the a-cyclodextrin polymorphs were recorded. 2. MATERIALS AND METHODS 2.1. Materials Connnercial a-cyclodextrin i (purity grade >99%, aCd) was used. The hexahydrate polymorphs aCd6-1 and aCd6-II were prepared by recrystallization from water [2] and from amorphous aCd under 71.5% relative humidity at 40 °c for 3 days [4], respectively. 29