Interactions between Anionic Surfactants and Oil Dyes in the Aqueous Solutions. III. Thermochromism in the Presence of Surfactant Micelle and Dye–Surfactant Complexes

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Abstract
The interactions between 4 anionic surfactants and 3 azo oil dyes have been studied by a spectrophotometer. The spectrographic behavior of these solutions was dependent on the kinds of hydrophilic groups in each surfactant and the kind of auxochrome in the azo oil dyes. In particular, a sulfate and two sulfonates including strongly hydrophilic groups interacted strongly with the oil dyes (4-NH2) which have an amino group at 4-position of naphthalene. At lower concentrations, an isosbestic point was recognized; this occurred because of the formation of the hydrophobic dye–surfactant complex (1:1). In the neighborhood of the cmc region, the spectrum with a peak at 520 nm changed with temperature into another one with a 440 nm band; this is the so-called thermochromism. This may be attributed to the fact that, in the presence of surfactant micelles, the 4-NH2 exhibits an azo form at higher temperature, but a resonance stabilized azonium form is found at lower temperature.