Physicochemical properties of salts of p-aminosalicylic acid. I. Correlation of crystal structure and hydrate stability.

The potassium (K), sodium (NA), calcium (CA), and magnesium (MG) salts of p-aminosalicylic acid were obtained, and their thermal behavior was characterized by means of differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). Their crystal and molecular structures were determined by single-crystal X-ray diffraction after powder patterns had shown them to be nonisomorphous. Different degrees of hydration were observed for the solid salts, and an assessment of hydrate stability to dehydration was made from thermogravimetric studies. The onset temperature of dehydration (Tt) of each salt varied within the series and exhibited correlation with X-ray determined structure. The observed onset of dehydration of MG and CA was higher than that of NA and is consistent with stronger ion-dipole interactions for the divalent salts. Crystallographic determination of the bond lengths between the metal ion and the water oxygens were 2.4 and 2.9 A for NA, between 2.0 and 2.1 A for MG, and 2.4 A for CA. The open nature and presence of a channel feature in the structure of the sodium salt may have facilitated escape of water molecules from the crystal. Particle presentation (e.g., size, crystallinity) was also shown to affect dehydration behavior.