Thermodynamics of anionic dye–cationic surfactant interactions in cationic–nonionic surfactant mixtures in comparison with binary systems

The thermodynamics of the interactions between the anionic dye Acid Red 88 (AR88) and the cationic surfactant dodecyltrimethylammonium bromide (DTA) in the aqueous solution of the nonionic surfactant Triton X-100 (TX100) at different temperatures has been investigated by using potentiometry. The standard free energy change, ΔGo1, the standard enthalpy change, ΔHo1, and the standard entropy change, ΔSo1, for the first association step of DTA–AR88 complex formation in the presence of TX100 micelles were determined and compared to the thermodynamic functions of DTA–AR88 and DTA–TX100 interactions in binary systems. From ΔGo1 results, it has been observed that irrespective of the studied temperature the binding free energy of DTA cation to AR88 anion as well as the complex stability is lower in TX100 solution than in binary mixture. The increase in the TX100 concentration causes the decrease in the tendency of DTA–AR88 complex formation. From this it can be seen that the mode and the strength of intermolecular interactions strongly depend on the presence of TX100 micelles. The values of ΔHo1 and ΔSo1 for DTA–AR88 complex formation in TX100 solution which are directly affected by the ratio of AR88 and TX100 concentrations are much higher compared to those obtained in binary mixtures without TX100. These results also lead to a conclusion that the binding of DTA cation to AR88 anion in the presence of TX100 micelles, which is an entropy-driven process, is much more different from the enthalpy-controlled formation process of DTA–AR88 complex in binary mixtures.