Browsing by Author "Hunt, Marcus Andrew"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • No Thumbnail Available
    The Role of Water in the Formation and Structure of Oligomer/alpha-Cyclodextrin Inclusion Complexes
    (2007-07-05) Hunt, Marcus Andrew; Samuel Hudson, Committee Member; Alan E. Tonelli, Committee Co-Chair; C. Maurice Balik, Committee Co-Chair; Keith Dawes, Committee Member; Saad Khan, Committee Member
    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.
  • No Thumbnail Available
    Structure and Stability of Columner alpha- and gamma-Cyclodextrin Hydrates
    (2004-08-25) Hunt, Marcus Andrew; Dr. Alan Tonelli, Committee Co-Chair; Dr. Samuel Hudson, Committee Member; Dr. C. Maurice Balik, Committee Co-Chair
    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.