Molecular Recognition Kinetics of β-Cyclodextrin for Several Alcohols by Ultrasonic Relaxation Method

Ultrasonic absorption coefficients in aqueous solutions of 2-propanol, 2-butanol, and 2-methyl-1-propanol (guest) in concentrations less than 0.40 mol dm^-3 with β-cyclodextrin (host) in the range below 0.011 mol dm^-3 were measured in the frequency range 0.8−95 MHz at 25 °C. A single Debye-type relaxational absorption was found only when both solutes coexisted. From the concentration dependences of the relaxation frequency and the amplitude of the relaxation, the cause of the observed relaxation was attributed to a perturbation of a chemical equilibrium associated with a dynamic interaction between β-cyclodextrin and the alcohols. The rate and thermodynamic parameters for the dynamic interaction were determined, and the results were compared with those for other alcoholic systems with β-cyclodextrin, which were reported previously. The isomeric effect of the guests on the dynamic interaction with β-cyclodextrin was considered and it is deduced from the experimental results that the hydrophobicity of the guests is very sensitive for recognition by the host. Also, an anchor mechanism was proposed for the complexation between the host and guest.