A study of dye-surfactant interactions. Part 1. Effect of chemical structure of acid dyes and surfactants on the complex formation

Abstract

The study of the interactions between anionic azo dyes C.I. Acid Orange 7 (AO7) and C.I. Acid Red 88 (AR88), with different surfactants, i.e., the cationic surfactants dodecylpyridinium chloride (DPC) and cetylpyridinium chloride (CPC) and the anionic surfactant sodium dodecylsulphate (SDS), in submicellar concentration ranges has been researched by a potentiometric technique using a surfactant-cation-sensitive membrane electrode. The e.m.f. measurements versus surfactant concentration have been performed for the systems CPC-AO7, DPC-AO7, DPC-AR88 and SDS-AO7 in pure water at 25°C. Experimental measurements give the option to determine the concentration of free and of bound surfactant cations at a given stoichiometric concentration and calculation of the dye-surfactant complex formation constant, K, and the standard free enthalpy change, ΔG°. The results show that under the same experimental conditios the complex formation between CPC and AO7 takes place at much lower surfactant concentration than between DPC and AO7, accompanied by much higher values of K. It can be seen from the shapes of the binding isotherms that the binding of DPC to AO7 is most likely cooperative. The results indicate that in the DPC-AR88 system, where more hydrophobic dye AR88 is used, the dye-surfactant interactions are stronger compared to DPC-AO7 system. Such observations enable the conclusion, to be made that, beside the electrostatic attractive interactions, the noncoulonic interactions are also very important for the formation of the complex between the oppositely-charged dyes and surfactants. The study of the interactions between the same-charged dye AO7 and surfactant SDS shows that they do not interact with one another in the whole measured concentration range. The most logical explanation for this is that the repulsive electrostatic interactions prevent the formation of the complex between the same charged AO7 and SDS.