Kinetic Studies of the Complex Formation in the System of Cyclodextrin, Iodine, and Iodide

Abstract

Abstract Kinetic studies of complex formation in the system consisting of cyclodextrin, iodine, and iodide have been made by the temperature-jump method. By addition of α-cyclodextrin to the iodine–iodide solution, a new relaxation in addition to that due to I62−, formation was observed, but not in the case of addition of β-cyclodextrin. The reciprocal relaxation time is independent of the iodine concentration, but increases with increase of both iodide and α-cyclodextrin concentration, with a tendency to reach a constant value at their high concentrations. From these concentration dependences of the relaxation time, the observed relaxation was assigned to the conformational change of the complex between α-cyclodextrin and iodide ion, and the rate constants were estimated to be 5.9×105 and 1.2×105s−1 for the forward and backward processes, respectively. A rationalization for the difference between α-cyclodextrin and β-cyclodextrin in this complex formation is proposed in terms of the molecular structure of cyclodextrin.