The effect of water and guest hydrophobicity on the complexation of oligomers with solid α-cyclodextrin

α-Cyclodextrin (α-CD), a cyclic oligosaccharide, can form inclusion complexes (ICs) with polymer molecules in which α-CD molecules stack in the columnar crystal to form a molecular tube. Physical mixtures of α-CD powder and oligomeric liquids such as poly(ethylene glycol) (PEG) have been shown to spontaneously form an IC, which is accompanied by a solid-state α-CD phase transformation from the cage to the columnar crystal structure. In this paper, the phase transformation is tracked with wide-angle X-ray diffraction as a function of temperature, atmospheric water vapor content and the type of guest molecule. A first-order kinetic model is used to describe the kinetics of complexation. The time required to completely complex PEG200 (200 g/mol) at low water activities is greater than 300 h, whereas only a few hours are necessary at high water activities. Solid-state complexation of α-CD with a hydrophobic guest molecule (hexatriacontane, HTC), is also reported here for the first time. Slower complexation kinetics are observed for α-CD with HTC compared to PEG600 (600 g/mol).