The surface tension of the aqueous solution of anionic dye and nonionic surfactant mixture was measured as a function of total concentration at various compositions and at 298.15 K under atmospheric pressure. The dye used is sodium 4'-(butylamino)azobenzene-4-sulfonate (SBAS) and the nonionic surfactant is 2-(octylsulfinyl)ethanol (OSE). By use of thermodynamic relations developed previously, the phase diagrams of adsorption and micelle formation were constructed. It was found that SBAS and OSE are completely miscible with each other in the adsorbed film and that the adsorbed film is enriched in OSE compared with the solution except for a composition close to pure OSE. It was concluded that SBAS and OSE interact attractively with each other in the adsorbed and micellar states and, furthermore, that the interaction is more attractive in the micelle than in the adsorbed film. The miscibility of dye and nonionic surfactant is suggested to play an important role in the working of nonionic surfactant as a levelling agent in dyeing process.
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