Absorption spectra and micellisation of a surface-active dye in aqueous methanol solutions

Abstract

The absorption spectra of aqueous methanol solutions of a surface-active dye, p-t-octylphenol yellow amine poly(ethylene oxide), have been measured at different dye concentrations, and the absorption bands are assigned to tautomeric forms of the 2′-hydroxy-4-dimethylaminoazobenzene derivative. The K band of the dye appears at 394 nm in methanol + water solutions when the dye concentration is lower than 10^–5 mol dm^–3. Hypochromism occurs and the shoulder band (B band) at 460–480 nm becomes manifest when the primary micelle is formed: the primary micelle is stabilised by the stacking interaction of dye molecules and the increased amount of their o-quinoid hydrazone tautomer. At higher dye concentrations the K band appears at 413 or 406 nm and its intensity increases. This step can be attributed to the second micellisation of dye molecules due to hydrophobic interaction. The methanol + 0.1 mol dm^–3 HCl solutions of the dye exhibit two absorption bands (K′ and Q bands) at 320 and 528 nm, which can be assigned to the ammonium form and the resonance-stabilised azonium tautomer, respectively, of the protonated dye, when the dye concentration is dilute. Above the critical micelle concentration the absorption spectra have the K band at 405 nm, indicating that the dye is deprotonated in the micelle.