Kinetics of Dye/β-Cyclodextrin Interactions

Abstract

The rate of association of β-cyclodextrin (β-CD) with each of three similar azo dyes, methyl orange (MO), methyl red (MR) and pyridine-2-p-dimethylaniline (PADA) was determined by the continuous flow method of integrating observation (CFMIO) at pH 8.4. For all three systems, the rate constants were similar and decreased with increasing dye concentration from 10⁷dm³mol^-1 S^-1 in l mmol dm^-3 of β-CD solution to k = 10⁴ dm³mol^-1 S^-1 in 7 mmol dm^-3 β-CD solution. Discrepancies with joule-heating temperature jump results in the literature disappeared when the rate constants were re-determined in 0.5 mol dm^-3 NaCl. The chloride ion was identified as the weakly binding species to β-CD that affected the temperature-jump results. Binding constants were obtained conductometrically for the β-CD/M0 and β-C /MR complex species and for β-CD/PADA by a competitive stopped-flow method. The principal inclusion complex with MO is 1:1 but with MR a considerable quantity of the 2:1 β-CD/MR complex is also formed.