Supramolecular complexes of azocellulose and α-cyclodextrin: Isothermal titration calorimetric and spectroscopic studies

Isothermal titration calorimetry (ITC) and UV-visible spectroscopy were used to study the supramolecular complexes of α-cyclodextrin (α-CD) and azocellulose (azobenzene functionalized hydroxypropyl methylcellulose, AZO-HPMC) in aqueous solutions. The equilibrium constant K, enthalpy change ΔH, Gibbs free energy ΔG, entropy change ΔS, and the stoichiometric number n for the inclusion complexation were determined. α-CD forms stable inclusion complexes with AZO-HPMC with the stoichiometry of 1:1. The inclusion complexation is exothermic. Both negative enthalpy and entropy changes are observed, suggesting that the inclusion complexation is enthalpy driven and entropy opposed. AZO-HPMC with higher azobenzene content (DS _azo) is less favored to form inclusion complexes with α-CD due to strong H-aggregation of azobenzene groups. ITC studies confirmed that the cis azobenzene groups is unable to form an inclusion complex with α-CD. The red-shift in the absorption spectra of AZO-HPMC aqueous solution upon addition of α-CD also confirmed the formation of 1: 1 complex.