Investigation of interactions between some anionic dyes and cationic surfactants by conductometric method

The interactions of cationic surfactants with anionic dyes were studied by conductometric method. Benzyltrimethylammonium chloride (BTMACl), benzyltriethylammonium chloride (BTEACl) and benzyltributylammonium chloride (BTBACl) were used as cationic surfactants and indigo carmine (IC) and amaranth (Amr) were chosen as anionic dyes. The specific conductance of dye–surfactant mixtures was measured at 25, 35 and 45 °C. A decrease in measured specific conductance values of dye–surfactant mixture was caused by the formation of non-conducting or less-conducting dye–surfactant complex. The equilibrium constants, K₁, the standard free energy changes, ΔG₁°$\mathrm{\Delta }{G}_1°$, the standard enthalpy changes, ΔH₁°$\mathrm{\Delta }{H}_1°$ and the standard entropy changes, ΔS₁°$\mathrm{\Delta }{S}_1°$ for the first association step of dye–surfactant complex formation were calculated by a theoretical model. The results showed that the equilibrium constants and the negative standard free energy change values for all systems decreased as temperature increased. Also these values decreased for all systems studied with increasing alkyl chains of surfactants due to the steric effect. When the equilibrium constant values, K₁, for the first association step of IC–surfactant and Amr–surfactant systems with the same surfactant were compared, the values of K₁ for IC–surfactant system were higher than that of Amr–surfactant system.