Tautomeric Equilibrium in Sulfo Derivatives of 4-(Phenylazo)-1-Naphthol in Micellar Solutions of Nonionic Surfactants

Two reagents of the 4-(phenylazo)-1-naphthol series were synthesized: 4-(4-sulfobenzeneazo)-1-naphthol-2-sulfonic acid (SBANS) and 4-(benzeneazo)-1-naphthol-2-sulfonic acid (BANS). These reagents contain the sulfo group at the ortho position with respect to the hydroxy group in the naphthol ring and differ by the presence or the absence of one more sulfo group at the para position in the benzene ring. The effect of ethanol and micelles of three types of nonionic surfactants (Proxamine-385, Brij-35, and Triton X-100) on the azo–quinonehydrazone tautomerism of BANS, SBANS, and their analogue Tropaeolin 000 was studied. It was demonstrated that an increase in the concentration of ethanol or the solubilization of the reagents in the micelles of nonionic surfactants shifts the tautomeric equilibrium to the less polar azo form. It was found that the sulfo group in the naphthol ring controls the lability of the tautomeric equilibrium. Tautomeric constants in aqueous–ethanolic solutions and nonionic surfactant micelles were calculated, and the effective dielectric constant of the medium at the position of localization of the reagents in the hydroxyethylene layer of the micelles was estimated using a model approach.