Effect of Guest Hydrophobicity on Water Sorption Behavior of Oligomer/α-Cyclodextrin Inclusion Complexes

• Published in: The journal of physical chemistry. B 2007-04, Vol.111 (15), p.3853-3858
• Main Authors: Hunt, Marcus A, Tonelli, Alan E, Balik, C. Maurice
• Format: Article
• Language: English
• Subjects: Adsorption; alpha-Cyclodextrins - chemistry; Hydrophobic and Hydrophilic Interactions; Polymers - chemical synthesis; Polymers - chemistry; Thermodynamics; Water - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp070145t

Cyclomaltohexaose (α-cyclodextrin, α-CD) can form inclusion complexes (ICs) with polymer molecules in the columnar crystal structure in which α-CD molecules stack to form a molecular tube. 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, polyethylene glycol (PEG, MW = 600 g/mol) and hexatriacontane (HTC) guests were used. Sorption isotherms for PEG/α-CD IC are similar to those obtained for pure α-CD and PEG, suggesting the presence of dethreaded PEG in the sample. WAXD collected before and after water vapor sorption of PEG/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 PEG. 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.