Kinetics of complexation between beta-cyclodextrin and 1-propanol in aqueous solution by ultrasonic relaxation method

Ultrasonic absorption coefficients in aqueous solutions containing beta-cyclodextrin and alcohols (ethanol and 1-propanol) have been measured in the frequency range from 3 to 220 MHz at 25 degrees C. Although a single relaxational absorption has been observed in solution with beta-cyclodextrin and 1-propanol, it has not been found when the alcohol is ethanol. From the concentration dependence of the ultrasonic relaxation parameters, the cause of the relaxational absorption has been attributed to the perturbation of an equilibrium associated with the 1 : 1 complex formation (complexation) between beta-cyclodextrin and 1-propanol. The rate and equilibrium constants as well as the standard volume change were determined for the complexation reaction of beta-cyclodextrin and 1-propanol. The equilibrium constant determined in this study was found to be very consistent with the reported value for beta-cyclodextrin and 1-propanol. The kinetic process for complexation between beta-cyclodextrin and popular alcohols is discussed in relation to the sizes of the guest molecules.