Structure and Stability of Columner alpha- and gamma-Cyclodextrin Hydrates

Abstract

α- and γ-Cyclodextrin (CD) recrystallized rapidly from aqueous solution can form columnar structures with only water as the guest molecule. Complementary water vapor sorption and wide-angle x-ray diffractometry (WAXD) experiments were performed on columnar α-CD to elucidate the crystal structure present at varying sorption levels. Equilibrium isothermal water vapor sorption experiments at 40° C revealed that columnar α-CD is unstable above a water activity of approximately 0.67. This was confirmed by WAXD patterns collected over time, which further revealed that columnar α-CD undergoes a phase transformation from columnar to cage structure after approximately 0.25 hours at 40° C and a water activity of 1.0.

Upon vacuum-drying at 90° C for 15 hours, γ-CD in the columnar structure undergoes a phase change to an amorphous structure. Complementary water vapor sorption and WAXD experiments were performed on columnar γ-CD in its vacuum-dried and as-precipitated states to elucidate its stability in humid environments and the crystal structure present at varying sorption levels. Isothermal water vapor sorption at 40° C revealed that vacuum-dried γ-CD reverts to cage structure sometime during a sorption experiment at a water activity of 1.0. As-precipitated columnar γ-CD converts to cage structure after 164 hours at a = 1.0 and 40° C.

The crystal morphology of as-received CD in the cage polymorph observed by scanning electron microscopy is indicative of its crystal structure, while the crystal morphology of columnar CD is not discernable due to the rapid precipitation method employed.