General trends in the desolvation behavior of calcium salts

Purpose. This paper is concerned with the solid-state characterization of dehydrated calcium salts as well as the effect of dehydration on the physical properties of these salts. Methods. The salts were analyzed by X-ray powder diffraction (XRPD), single crystal X-ray and polarized light microscopy (PLM). Results. Our research was able to identify three general behaviors of the desolvation of calcium salts of carboxylic acids. Upon desolvation, ethanolate and pentahydrate calcium indomethacin (CaIndo and CaInd2, respectively) stayed crystalline, fenoprofen calcium (CaFEN), calcium ketoprofen (CaKTN), calcium salicylate (CaSAL), calcium mefenamate (CaMEF) and calcium tolfenamate (CaTOLF) became mesophases, while calcium diflunisal became partially crystalline. On the other hand, the solubility studies of CaFEN, CaKTN and CaSAL showed that all dehydrated calcium salts had higher solubility than their crystalline counterparts and amorphous CaKTN had higher solubility than mesomorphous CaKTN. Conclusions. Several factors influence the desolvation behavior of calcium salts. We believe the flexibility of the benzene rings in CaKTN, CaFEN, CaMEF and CaTOLF was important for these products to become mesomorphous when they loose their crystalline water; meanwhile, CaDIF where the two benzene rings are coplanar remained crystalline when heated. Additionally, the existence of water channels and the hydrogen bonding networks in the crystals is hypothesized to play an important role in the desolvation behavior of these materials.