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

Abstract

The resonance Raman spectra of aqueous methanol solutions of a surface-active dye, p-t-octylphenol yellow amine poly(ethylene oxide), have been measured at different concentrations and at different methanol contents. In methanol + water solutions, the spectra of the dye excited by the 488.0 nm line of an argon-ion laser show that the dye molecules, both in monomeric form and in micelles, are characterised by a band at 1391 cm^–1, assignable to the NN stretching mode, and are therefore predominantly in the azo form. In the concentration range where the primary micelles are abundant, a band at ca. 1610 cm^–1, assigned to the CC stretching mode of the o-quinoid hydrazone tautomer superposed by the benzene ring vibration, is stronger, suggesting that the hydrazone tautomer is stable in the primary micelle.

The resonance Raman spectra excited by the 514.5 nm line for methanol +0.1 mol dm^–3 HCl solutions of the dye at 10^–5 mol dm^–3 exhibit bands at 1624 and 1277 cm^–1, assigned to the CC (quinoid) and C—N stretching modes of the resonance hybrid of the azonium form of the protonated dye. This is caused by the resonance Raman effect, which is stronger for the azonium form of the protonated dye than for its ammonium form. When the dye concentration exceeds the critical micelle concentration, the Raman bands assignable to the azo form of the dye become very strong, indicating that the dye molecules are deprotonated in the micelles.