Molecular-Level Investigation of Hydrate–Anhydrous Phase Transformations of the Dapsone Structurally Related Compound 3,3′-Diaminophenyl Sulfone

The solid state of a novel hydrate form of 3,3′-diaminophenyl sulfone (3APS_0.5H2Oo) as well as its temperature-induced phase transitions is reported. On increasing the temperature, first a partial dehydration occurs, leading to the partially hydrated 3APS_0.1H2Oo, which then transforms into the monoclinic anhydrous form, 3APS_Am. Finally, by recrystallization of the melt 3APS_Am, a second anhydrous orthorhombic phase is obtained (3APS_Ao). This last phase transforms in 3APS_Am at a temperature between 360 and 400 K. XRD, DSC, TGA, HSM, and in silico data have been used to better understand the dehydration mechanism. With the same aim, hydration/dehydration tests have been carried out. Finally, the dehydration behavior of 3APS_0.5H2O has been discussed in comparison with that of the dapsone hydrated phase 4APS_0.33H2Om.