Assessment of Defects and Amorphous Structure Produced in Raffinose Pentahydrate upon Dehydration

ABSTRACTThe progressive conversion of crystalline raffinose pentahydrate to its amorphous form by dehydration at 60°C, well below its melting temperature, was monitored by X-ray powder diffraction over a period of 72 h. The presence of defects within the crystal structure and any amorphous structure created was determined computationally by a total diffraction method where both coherent long-range crystalline order and incoherent short-range disorder components were modeled as a single system. The data were analyzed using Rietveld, pair distribution function (PDF), and Debye total diffraction methods. Throughout the dehydration process, when crystalline material was observed, the average long-range crystal structure remained isostructural with the original pentahydrate material. Although the space group symmetry remained unchanged by dehydration, the c-axis of the crystal unit cell exhibited an abrupt discontinuity after approximately 2 h of drying (loss of one to two water molecules). Analysis of diffuse X-ray scattering revealed an initial rapid build up of defects during the first 0.5 h with no evidence of any amorphous material. From 1–2 h of drying out to 8 h where the crystalline structure is last observed, the diffuse scattering has both amorphous and defect contributions. After 24 h of drying, there was no evidence of any crystalline material remaining. It is concluded that the removal of the first two waters from raffinose pentahydrate created defects, likely in the form of vacancies, that provided the thermodynamic driving force and disorder for subsequent conversion to the completely amorphous state.