Thermodynamic and kinetic study of the binding of azo-dyes to α-cyclodextrin

Abstract

A thermodynamic and kinetic study concerning the binding of azo-dyes to α-cyclodextrin is described. Joule-heating temperature-jump and stopped-flow techniques with optical detection have been used to elucidate the mechanism of inclusion-complex formation.

Azo-dyes form 1:1 inclusion complexes with α-cyclodextrin in which the dye is located inside the cyclodextrin cavity. The mechanism of insertion involves a fast pre-equilibrium step to form an intermediate. In a subsequent, slower, rate-determining step the intermediate is converted into the final stable complex.

Comparing rates of inclusion for a number of azo-dyes containing two substituted phenyl groups separated by an azo-linkage, it is possible to determine which aromatic ring of the dye preferentially enters the cyclodextrin cavity during the binding process.

The forward and reverse rate constants for the inclusion process are very dependent on the nature of the substituent groups on the phenyl rings. Charge and steric effects are found to be particularly important in influencing the kinetics. The overall equilibrium constant for complex formation does not vary much as the dye substituents are altered.

An attempt is made to interpret the proposed mechanism in terms of the molecular rearrangements which take place as the dye binds to the cyclodextrin.