The effects of α- and β-cyclodextrins (CD) on the acid dissociations and tautomeric equilibria of sodium 4-(4′-hydroxy-1′-naphthylazo)-1-naphthalenesulfonate (1) and of the conjugate acid (2) of Methyl Orange were spectrophotometrically examined and were compared with the effect of a nonpolar solvent (THF). The apparent pKa values for the dyes decreased with increasing CD concentration. The effect of CD is mainly explained in terms of the hydrophobic interactions between the azo dyes and CD, resulting in a shift in the tautomeric equilibria from less acidic azonium or hydrazone forms to more acidic azo forms of 1 and 2 in an aqueous solution.
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Matsui et al. (1978) studied this question.
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