An equilibrium study of p-Methyl Red inclusion complexes with α- and β-cyclodextrins

Abstract

The effects of cyclomaltohexaose (α-cyclodextrin, αCD) and cylomaltoheptaose (β-cyclodextrin, βCD) on the UV-visible spectra of the anion and cation of p-Methyl Red 4-[4-(dimethylamino)phenylazo]benzoic acid in aqueous solution were analyzed according to 1:1 inclusion processes. The values of the thermodynamic parameters Δ H°, Δ S°, and Δ° were calculated from the effect of temperature on the thermodynamic stability constants of the inclusion complexes of the anion and cation with both cyclodextrins. The stability constants of the inclusion complexes of the tautomers of p-Methyl Red were evaluated at 25°C. The addition of a cyclodextrin shifts the position of the tautomeric equilibrium towards the side of the ammonium tautomer. Both α- and β-cyclodextrin bind the ammonium tautomer more strongly than the azonium tautomer. The most stable inclusion complex is that of the anion with αCD; the other complexes are of comparable stability.