A study of dye¯surfactant interactions. Part 3. Thermodynamics of the association of C.I. Acid Orange 7 and cetylpyridinium chloride in aqueous solutions

The association of C.I. Acid Orange 7 (D) and the surfactant cetylpyridinium chloride (S) was studied at 15, 25, 35 and 45°C in the presence of 0, 0.05 and 0.1 mol/kg NaCl, by potentiometric titration, using a surfactant cation sensitive membrane electrode. By using the association constant (K1) for the first step of the association [D-+S+ (DS)°], the standard free energy change, standard enthalpy change, and standard entropy change of the association were calculated at low surfactant concentrations. The unitary entropy of association was negative, suggesting that hydrophobic interactions do not play a major role in the initial interaction between dye and surfactant. On the other hand, in the second step [S++(DS)° (DS2)+], the unitary entropy change in the medium-level surfactant concentration domain was highly positive, indicating that water-structure contributions are appreciable in the stabilisation of the (DS2)+ species. It was shown that a further stepwise association is not very likely in the third domain, and that a neutral quadruple species of type (SD)2° may be present in the solution. Further increases in surfactant concentration led to a very steep increase in bound surfactant as the concentration of free surfactant ions approached the c.m.c. point. It was also shown that the addition of a simple electrolyte (e.g. NaCl) decreased K1, mainly due to interference with the interaction of dye and surfactant ions. Since long range Coulombic interactions are absent in the second association step, the increase in K2 with increasing NaCl concentration can be mainly attributed to short range hydrophobic interactions.