Role of water in the polymorphic transitions of small carbohydrates

Abstract

Crystallisation of small carbohydrates (hexoses, alditols, and disaccharides) in solution or from the melt is known to provide different polymorphs mainly due to their affinity with water and their molecular flexibility. Food and pharmaceutical applications sometimes require the use of a specific polymorph especially for its compaction and compression properties.Different factors affect the process of crystallisation and make the selection of a polymorph in a straightforward step difficult. As a general rule, the crystallisation of the most stable form or the easiest to crystallize is achieved and a polymorphic transition follows. To achieve the transition and obtain the desired crystal form, the control of water activity is needed.Examples of polymorph transitions are described. It is particularly the case for α-d-glucose monohydrate–anhydrous transition and the conversion of β sorbitol to the most stable polymorph γ.Polymorphic transitions were controlled by recording the XRPD patterns of the polymorph submitted to controlled water activity and temperature conditions. The morphology of crystals was also observed under microscope. FTIR spectra of the polymorphs obtained at different aw values were recorded as well as DSC thermograms. Water vapour sorption isotherms were established and the kinetics of water intake determined.An attempt of interpretation of the role of water in polymorphic transitions is made. The flexibility of sorbitol enhanced in presence of water vapour when samples are submitted to humid air seems at the origin of conformation change which induces conformational polymorphism. A difference is noticed between α-d-glucose monohydrate and sorbitol polymorphic transitions due to the difference in their solubility curves (enantiotropic for glucose, monotropic for sorbitol).