Preparation and Dehydration Kinetics of Complex Sulfadiazine Calcium Hydrate with Both Channel-Type and Coordinated Water

A new
hydrate of sulfadiazine calcium (Hydrate I) was discovered,
and the crystal structure was determined using single crystal X-ray
diffraction. Both channel-type water (9.23 wt %) and calcium-ion coordinated
water (11.87 wt %) existed in the unit cell. The thermal stability
and dehydration of Hydrate I were investigated by thermal gravimetric
analysis, hot stage microscopy, powder X-ray diffraction, Fourier
transform infrared spectroscopy, and scanning electron microscopy.
A two-step dehydration process was detected from Hydrate I to anhydrous
phase (AP) with the intermediate of the less-hydrated form (Hydrate
II). The dehydration kinetics of Hydrate I with both channel-type
and coordinated water was studied using model fitting method and model
free method in isothermal mode. The dehydration activation energy
was derived via the Friedman method. Further, the first-step dehydration
of Hydrate I was determined to be the 2D phase boundary reaction mechanism,
and the second-step dehydration was found to be 3D phase boundary
reaction mechanism via model fitting approach.