KINETIC-STUDY FOR COMPLEXATION BETWEEN ALPHA-CYCLODEXTRIN AND ALCOHOLS IN WATER BY THE ULTRASONIC RELAXATION METHOD

Ultrasonic absorption measurements in the frequency range from 1 to 95 MHz were carried out in aqueous a-cyclodextrin solutions with l-propa nol or ethanol at 25 degrees C. A single relaxational absorption was o bserved, and the cause of the relaxation was attributed to a perturbat ion of a chemical equilibrium associated with a complexation reaction between alpha-cyclodextrin (host) and alcohols (guest). Rate and equil ibrium constants for the complexation reaction were determined from th e concentration dependence of the relaxation frequency for the solutio ns. A standard volume change of the reaction was also obtained from a maximum absorption per wavelength. These results were compared with th ose for the complexation between beta-cyclodextrin and some alcohols a nd were discussed in relation to cyclodextrin and alcohol molecular st ructures. It was found (1) that the formation rate of the complex was slightly dependent on the cavity size of the cyclodextrins, but the ra te of departure of alcohols from the cavity of the cyclodextrins was v ery dependent on the structure of the cyclodextrins and alcohols, and (2) that a part of the hydrophobic group in alcohols incorpoarated int o the cavity of cyclodextrins even if water molecules also participate d in the complexation reaction between cyclodextrins and alcohols.