AN EQUILIBRIUM STUDY OF P-METHYL RED INCLUSION COMPLEXES WITH ALPHA-CYCLODEXTRIN AND BETA-CYCLODEXTRIN

The effects of cyclomaltohexaose (alpha-cyclodextrin, alphaCD) and cyclomaltoheptaose (beta-cyclodextrin, betaCD) on the UV-visible spectra of the anion and cation of p-Methyl Red (4-[4-(dimethylamino)phenyl-azo]benzoic acid) in aqueous solution were analyzed according to 1:1 inclusion processes. The values of the thermodynamic parameters DELTAH-0, DELTAS0, and DELTAG0 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-degrees-C. The addition of a cyclodextrin shifts the position of the tautomeric equilibrium towards the side of the ammonium tautomer. Both alpha- and beta-cyclodextrin bind the ammonium tautomer more strongly than the azonium tautomer. The most stable inclusion complex is that of the anion with alphaCD; the other complexes are of comparable stability.