Protonation Equilibrium of Azo Oil Dye in the Aqueous Solutions of Mixed Surfactant Systems

Abstract

The protonation equilibrium of 4-phenylazo-1-naphthylamine (4-NH2), which occurs between the azo and quinoidal forms, was studied in aqueous solutions of anionic-nonionic mixed surfactant systems; these systems are sodium dodecyl sulfate (SDS) -alkyl poly (oxyethylene) ethers (CmPOEn, m=12, 14, 16, and 18 at n=20; n=10, 20, 30, and 40 at m=16) The maximal absorption wavelength of the dye in the pure SDS micellar solutions is different from that in pure CmPOEn solutions. In the aqueous SDS solutions mixed with C12POE20, the maximal absorption wavelength shifted to 450nm with increasing molar ratio of C12POE20. Even when the molar ratio of C18POE20 increased to 0.5, the absorption spectra still remained at 535nm, due to the quinoidal form of 4-NH2. Furthermore, in the case of SDS-C16POE10 mixed surfactant system, the absorption spectra shifted to 535nm (except for the pure nonionic), and then the peak at 450nm vanished. In the neighborhood of the CMC region of mixed surfactant (SDS-C16POE10), the spectrum with a peak at 535nm changed with temperature into another one with a 450nm band. 4-NH2 exists in the azo form in aqueous micellar solutions of SDS and CmPOEn which have short alkyl chain lengths and/or a long polyoxyethylene chains, but is found in the quinoidal form in solutions of SDS and CmPOEn having long alkyl chain lengths and/or a short polyoxyethylene chains.