Conductance study of the binding of methyl orange, o-methyl red and p-methyl red anions by α-cyclodextrin in water

Abstract

A conductance study concerning the binding of methyl orange (MO, 4-[4-(dimethylamino)phenylaza]benzene-sulfonic acid), o-methyl red (o-MR, 2-[4-(dimethylamino)phenylaza]benzoic acid) and p-methyl red (p-MR) anions by α-cyclodextrin (α-CD) in water has been carried out at 25 °C. The observed molar conductances (Λ_s) were found to decrease significantly for mole ratios less than unity. A model involving 1 : 1 stoichiometry has been used to analyse the conductance data. The stability constant (K) and the molar conductance (Λ_c) for each 1 : 1 inclusion complex were determined from the conductance data by using a non-linear least-squares curve-fitting procedure. The binding sequence, based on the value of K at 25 °C, is found to be p-MR > MO > o-MR. The values of the ionic molar conductances of the complexed anions at infinite dilution are reported and the differences between such values and those of uncomplexed anions are in accord with the above binding sequence.