Structure and stability of oligomer/$\alpha $-cyclodextrin inclusion complexes.

Cyclomaltohexaose ($\alpha $-cyclodextrin, $\alpha $-CD) can form inclusion complexes (ICs) with polymer molecules in the columnar crystal in which $\alpha $-CD molecules stack to form a molecular tube. Complementary water vapor sorption and wide-angle X-ray diffractomery (WAXD) were performed on oligomer/$\alpha $-CD ICs to probe their structures and stabilities. To discern the effect of guest molecule hydrophobicity on water adsorption isotherms, polyethylene glycol (PEG, MW = 600 g/mol) and hexatriacontane (HTC) guests were used. Sorption isotherms for PEG/$\alpha $-CD IC are similar to those obtained for pure $\alpha $-CD and PEG, suggesting the presence of dethreaded PEG in the sample. WAXD collected before and after water vapor sorption of PEG/$\alpha $-CD IC indicated a partial conversion from columnar to cage crystal structure, the thermodynamically preferred structure for pure $\alpha $-CD, due to dethreading of PEG. This behavior does not occur for HTC/$\alpha $-CD IC. Sorption isotherms collected at 20, 30, 40 and 50\r{ } C allowed the calculation of differential heats of adsorption and integral entropies of adsorbed water, while solid-state $^{13}$C NMR suggested a dramatic increase in HTC and $\alpha $-CD mobilities upon complexation.