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

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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.

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