A THERMODYNAMIC STUDY OF THE INCLUSION PROCESSES OF ALPHA-CYCLODEXTRINS AND BETA-CYCLODEXTRINS WITH THE ACID FORMS OF METHYL-ORANGE AND METHYL YELLOW

The effect of temperature on the UV-visible spectra of the acid forms of methyl orange and methyl yellow in the presence of alpha- and beta-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 DELTAH0 and DELTAS0 are reported for each process. The inclusion complexes of the ammonium tautomer are more stable than those of the azonium tautomer and alpha-cyclodextrin inclusion complexes are more stable than those of beta-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 alpha-cyclodextrin/methyl yellow solutions. No measurable changes in the absorbances were observed in the case of azo dye solutions containing gamma-cyclodextrin.