Thermodynamic study on salt effects on complex formation of α-, β- and γ-cyclodextrins with p-aminobenzoic acid

The aim of this work was to gain a deeper understanding of salt effects in the inclusion complex formation of cyclodextrins. For this purpose, thermodynamic study of complex formation of α-, β- and γ-cyclodextrins with p-aminobenzoic acid was carried out in water and solutions of KCl, KBr, KH2PO4 and K2SO4 (0.2 mol/kg). Stability constants were calculated from the binding isotherms obtained on the basis of 1H NMR measurements. Enthalpy and entropy of complex formation were estimated from the van't Hoff plots. It was found that effects of KCl, KH2PO4 and K2SO4 are insignificant, while the influence of KBr on complex formation of cyclodextrins with p-aminobenzoic acid is more pronounced and results in a decrease of the stability constants. Specific action of Br− is caused by the ability of these anions to penetrate into macrocyclic cavity. Coexistence of two complexation equilibria in KBr solution is accompanied by significant solvent reorganization originated from more intensive dehydration of the interacting species. This results in an increase of the enthalpy and entropy of complex formation. Manifestation of Br− effect was found to be the same in the binding of p-aminobenzoic acid with α-, β- and γ-cyclodextrins.