Influence of water on pure sorbitol polymorphism

Abstract

The behaviour of very pure sorbitol and sorbitol–water mixtures has been examined in relation to water content by both differential scanning calorimetry (d.s.c.) and a new thermal and dielectric analyser operating at microwave frequencies. A new hydrate has been isolated. Four different crystalline forms have been obtained from this hydrate and characterized by d.s.c. and X-ray diffraction. The influence of both temperature and water content on transitions from hydrate to other polymorphs has also been studied. Different types of water interactions have been evidenced from dielectric measurements for sorbitol and glucose, and comparisons of these, according to the physical states of the mixtures and hydrate properties have been made. For both molecules, the influence of water content on dielectric relaxation maximum temperatures has been studied and compared with the viscosity or the entropy. Non-congruency and possible deviation from stoichiometry are suggested for the new sorbitol hydrate. The dielectric properties of mannitol and sorbitol diastereoisomers have been measured at room temperature and compared. The glass transitions of sorbitol and sorbitol solutions have also been investigated down to 170 K by calorimetry measurements. The lowering of T_g with respect to water content is compared with isoviscosity curves.