The Role of Water in the Formation and Structure of Oligomer/alpha-Cyclodextrin Inclusion Complexes
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Date
2007-07-05
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Abstract
α-Cyclodextrin (α-CD), a cyclic oligosaccharide, can form inclusion complexes (ICs) with polymer molecules in the columnar crystal in which α-CD molecules stack to form a molecular tube. As-received α-CD in the cage crystal structure can form an IC with neat poly(ethylene glycol) (PEG). The transformation of α-CD from cage to columnar structure as a result of IC formation is tracked with wide-angle X-ray diffraction as a function of temperature, atmospheric water vapor content and guest molecular weight and hydrophobicity. A first-order kinetic model is used to describe the kinetics of the complexation. The time required to complex PEG(200) (MW = 200 g⁄mol) at low water activities is greater than 300 hours whereas a few hours are necessary at high water activities. Additionally, the complexation kinetics of the linear alkane, hexatriacontane (HTC), mixed with solid α-CD are slower than PEG(600) (MW = 600 g⁄mol), which has a similar molecular weight and all-trans end-to-end length as HTC.
Complementary water vapor sorption and wide-angle X-ray diffractomery (WAXD) were performed on oligomer⁄α-CD ICs to determine their structures and stabilities. To discern the effect of guest molecule hydrophobicity on water adsorption isotherms, PEG(600) and HTC guests were used. Sorption isotherms for PEG(600)⁄α-CD IC are similar to those obtained for pure α-CD and PEG(600), suggesting the presence of dethreaded PEG(600) in the sample. WAXD collected before and after water vapor sorption of PEG(600)⁄α-CD IC indicated a partial conversion from columnar to cage crystal structure, the thermodynamically preferred structure for pure α-CD, due to dethreading of PEG600. This behavior does not occur for HTC⁄α-CD IC. Sorption isotherms collected at 20, 30, 40 and 50 °C allowed the calculation of the isosteric heats of adsorption and the integral entropies of adsorbed water which are characterized by minima that indicate the monolayer concentration of water in the ICs. Solid-state 13C NMR suggests a dramatic increase in HTC and α-CD molecular motion upon complexation.
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Keywords
hexatriacontane, poly (ethylene glycol) (PEG), inclusion complex, alpha-cyclodextrin, water vapor sorption
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Degree
PhD
Discipline
Fiber and Polymer Science
