Interaction between anionic surfactants and oil dyes in the aqueous solutions: V. The effects of the oxyethylene chain length and/or the alkyl chain length in surfactant molecule on the tautomerism of an azo dye

Abstract

The tautomerism of 4-phenylazo-1-naphthol (4-OH), which occurs between the hydrazo form and the azo form, has been studied in the aqueous solution of nonionic surfactant [alkyl poly(oxyethylene) ethers]. The tautomerism of 4-OH is dependent on the surfactant concentration and molecular properties in the solubilized solution. 4-OH exhibits only a hydrazo form and its maximal absorption appears at the 470-nm band below the CMC, while above the CMC it exhibits both hydrazo and azo forms (tautomerism) and the absorption shows two bands at 470 and 415 nm. Below the limit of solubilization, the equilibrium constant of tautomerism is independent of the increase of the surfactant concentration. It shows, however, remarkable dependence on the alkyl chain length and/or the polyoxyethylene chain length in a nonionic surfactant molecule. The determination of limits of solubilization of two chemical species showed that the hydrazo form of 4-OH is solubilized into the polyoxyethylene part of a nonionic surfactant micelle and the amount does not change with increase of the polyoxyethylene chain length, while the azo form appears to be solubilized in its alkyl part and the amount increases with increase of the alkyl chain length. Therefore, the effect on the tautomerism of 4-OH is larger for the alkyl chain length than for the oxyethylene chain length in a surfactant molecule.