Spectroscopic study of salicylate–cyclodextrin systems in the presence and absence of alcohols

Abstract

The inclusional complexation between cyclodextrins (CDs) and salicylate (SA) in aqueous solution is studied. While no changes in both the absorbance and fluorescence spectra are detected when α- and γ-CDs are added to aqueous solutions of SA, spectral variations are produced by the presence of β-CD. An 1:1 SA–β–CD complex is detected and its equilibrium constant is determined by both absorptimetric and spectrofluorometric methods. The structure of this complex is proposed on the basis of MO calculations. When different alcohols are added to the SA–β–CD solution, a decrease in the fluorescence is observed, which is ascribed to the formation of ternary complexes. A thorough analysis of all equilibria involved in the interaction of SA, β-CD and alcohols is carried out and discussed. The values of the overall conditional equilibrium constants for the ternary complexes are calculated and compared. The present study demonstrates that the presence of alcohol increases the binding between SA and β-CD, and that the quenching produced by the third guest on the SA–β–CD fluorescence does not exhibit a direct correlation with the equilibrium constants for the ternary complexes.