PH-Sensitive Supramolecular Assemblies of β-Cyclodextrin and 2-Aminodiphenylamine in Water Medium: Structure, Solubility and Stability

Abstract

The influence of β-cyclodextrin on the solubility, protolytic properties, and complex formation behavior of 2-aminodiphenylamine was studied using UV spectroscopy. In accordance with the results of the spectrophotometric titration that was carried out in phosphate buffer solutions, the presence of β-cyclodextrin does not change the value of the deprotonation constant of the primary amino group of 2-aminodiphenylamine. This fact confirms that the primary amino group is not involved in the complex formation and interaction between β-cyclodextrin and 2-aminodiphenylamine occurs via the phenylimine moiety. The phase solubility profiles of 2-aminodiphenylamine in the presence of β-cyclodextin, at pH = 1.00 and pH = 5.00, indicate formation of inclusion complexes with limited solubility and can be classified as Higuchi–Connors curves of BI and BS type, respectively. The stoichiometry of the complexes and the apparent stability constants that characterize the β-cyclodextrin complexes with both protonated and neutral forms of 2-aminodiphenylamine were estimated from the dependence of absorbance intensity of the guest molecule at increasing amounts of β-cyclodextrin by the Ketelar equation. The linearity of the plots in the coordinates of the Ketelar equation proves the formation of 1:1 inclusion complexes for both protolytic forms of 2-aminodiphenylamine. It was found that inclusion complex formation is preferable for the neutral form of 2-aminodiphenylamine.