Structure and Physical Stability of Hydrates and Thermotropic Mesophase of Calcium Benzoate

The aim of this study is to investigate the hydration and the dehydration processes of calcium benzoate hydrates (trihydrate and monohydrate), thermotropic mesophases (dehydrated mesophase and lyophilized mesophase) and amorphous state, and the influence of their molecular order on those processes. X-ray analysis revealed that trihydrate has a planar structure composed of two types of planes-one from benzoic acid, water, and calcium ion and another from benzoic acid and water-and that both planes are linked by three water molecules. It was found that calcium benzoate was able to exist as thermotropic mesophases by dehydration of trihydrate and lyophilization. These mesophases were characterized by polarizing-light microscopy (PLM), X-ray powder diffraction (XRPD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). Both mesophases prepared by two procedures showed some similar physical properties, but lyophilized mesophase seemed to have molecular structure with higher order than dehydrated mesophase. The mesophases exhibited different hydration behavior. The dehydrated mesophase showed a stepwise rehydration process where it became monohydrate first and then trihydrate. The lyophilized mesophase became trihydrate without appearance of monohydrate. An amorphous form could also be prepared and it rehydrated first to the monohydrate and then trihydrate. The results suggest that the more disordered dehydrated mesophase and amorphous state change to monohydrate whereas the more ordered lyophilized mesophase cannot change to monohydrate but only to trihydrate.