2004 Volume 77 Issue 12 Pages 2193-2198
Ultrasonic absorption method was applied to elucidate the dynamic properties of host–guest complexation between α-cyclodextrin or β-cyclodextrin (host) and butanol isomers (guest) at 25 °C. The aim of this work was to reveal the effect of a structural difference in the hydrophobic part of a guest on the kinetic parameters for an inclusion reaction with cyclodextrins. Moreover, it was expected to clarify the effect of the cavity size on the reaction. Only when the host and guest existed together in water, a clear single relaxational phenomenon was observed. The cause of the relaxation was responsible for the complexation reaction between the host and the guest. The rate and equilibrium constants were obtained from the concentration dependence of the relaxation frequency, and the standard volume changes of the complexation reaction were from the maximum absorption per wavelength. The backward rate constant, kb, was very dependent on the structure of the guest molecule. The kb value decreased in the next hydrophobic series, nor- < sec- < tert- in β-CD systems, although a smaller kb value was observed when the guest possessed a normal carbon chain in α-CD systems. The results of the volume change of the reaction implied that α-CD could only include a guest quite shallowly, and almost the entire guest molecule was included into the β-CD cavity.
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