A thermodynamic study of the inclusion processes of α- andβ-cyclodextrins with the acid forms of methyl orange and methyl yellow

Abstract

The effect of temperature on the UV-visible spectra of the acid forms of methyl orange and methyl yellow in the presence of α- andβ-cyclodextrins was analysed in terms of 1 : 1 inclusion processes. Three inclusion processes were considered for each azo dye/cyclodextrin system. The processes are the apparent inclusion process, the ammonium inclusion process, and the azonium inclusion process. The values of ΔH ⁰ and ΔS ⁰ are reported for each process. The inclusion complexes of the ammonium tautomer are more stable than those of the azonium tautomer and α-cyclodextrin inclusion complexes are more stable than those ofβ-cyclodextrin. The inclusion processes resulted in a shift in the position of the tautomeric equilibrium of an azo dye. A significant color fading was observed in α-cyclodextrin/methyl yellow solutions. No measurable changes in the absorbances were observed in the case of azo dye solutions containing γ-cyclodextrin.