Dye-surfactant interactions studied using Job's method

interactions between the anionic dye Acid Orange 7 and two cationic surfactants N-cetylpyridinium chloride (CPCl) and cetyltrimethylammonium bromide (CTMABr) in aqueous solutions far below the CMC are studied using the method of continuous variations, also called Job's method. Light absorbance in the visible spectral range is measured. Both surfactants form stable associates with the acid dye. Job's method is suitable for determining the stoichiometric composition and equilibrium association constants of the associates studied. The molar binding ratio of both surfactant/dye associates is 1:1. The calculated equilibrium constants are high (about 10(6)), indicating strong interactions. From the constants calculated at different temperatures (15, 25, and 35 degrees C), the corresponding thermodynamic functions Delta G(0), Delta H-0, and Delta S-0 are obtained. We have concluded that strong interactions between cationic surfactants and anionic dyes are the result of attractions between oppositely charged ions combined with hydrophobic and dispersive forces.