Phase Transformation between Anhydrate and Monohydrate of Sodium Dehydroacetate

The
crystal structures of monohydrate and anhydrous substance were determined
from the single crystals for the first time. The phase transformation
between anhydrate and monohydrate of sodium dehydroacetate was in
situ investigated by using Raman spectroscopy. The mechanism of the
phase transformation was proposed. The results showed that the monohydrate
crystalline phase of sodium dehydroacetate can transform to anhydrous
phase through solid–solid transformation upon heating or solution-mediated
phase transformation. From powder X-ray diffraction (PXRD) patterns
and thermal gravimetric analysis (TGA) data, it was found that the
anhydrous crystals obtained by these two methods are the same in structure.
However, the scanning electron microscopy (SEM) results revealed that
the surface of the anhydrous sodium dehydroacetate crystals obtained
by high-temperature dehydration was much rougher than that obtained
by solution-mediated phase transformation. Furthermore, the dynamic
vapor sorption (DVS) results showed that the anhydrous crystals with
rough surface had faster hydration rate than the anhydrous crystals
with smooth surface when increasing humidity. The reasons behind these
phenomena were discussed.