The interplay of the rate of water removal in the dehydration of α,α-trehalose

The removal of water from α,α-trehalose dihydrate, Th, has been studied both isothermally, at 130 °C, as a function of time, and as a function of the temperature at different scan rates by using differential scanning calorimetry. The protocols were chosen to discriminate the conditions that give rise to the formation of the anhydrous crystal form (Tβ) and those that, according to previous findings, generate another form, Tγ, characterised by a different X-ray powder diffraction pattern. Scrutiny of the thermal properties of Tγ and of the carefully determined powder diffraction patterns discloses that Tγ is actually a mixture of the dihydrate and of the Tβ forms. On the basis of the scanning rate under which the formation of the Tγ species occurs, the hypothesis is made that Tγ contains a Th hydrated core encapsulated by a Tβ anhydrous layer.