Phase Behavior of Water and Cyclodextrin Systems

Abstract: The effect of water on α-, β-, and γ-cyclodextrins (CD) was investigated using sorption calorimetry, differential scanning calorimetry and dynamic vapor sorption in order to understand the phase behavior and to create phase diagrams. By relating earlier X-ray diffraction patterns of cyclodextrins equilibrated over saturated salt solutions and literature regarding the cyclodextrin hydrates to the sorption calorimetry and vapor sorption results, it proved possible to determine several distinct cyclodextrin structures depending on the cyclodextrin/water composition. For α-cyclodextrin, a phase diagram was derived, containing four distinct hydrates with a possibly stoichiometric hydrate at α-CD∙6H2O and a hydrate of unknown stoichiometry close to α-CD∙11H2O. For β-cyclodextrin, the suggested phase diagram has three distinct hydrates that are non-stoichiometric in nature, with the most hydrated form being β-CD∙10-12H2O. The γ-cyclodextrin was investigated with sorption calorimetry and compared to X-ray diffraction patterns, which suggested up to four separate hydrates. The structures were less crystalline at a lower humidity than the α- and β-cyclodextrin counterparts.