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

The association of C.I. Acid Orange 7 (D) and the surfactant cetylpyridiniu m chloride (S) was studied at 15, 25, 35 and 45 degrees C in the presence o f 0, 0.05 and 0.1 mol/kg NaCl, by potentiometric titration, using a surfact ant cation sensitive membrane electrode. By using the association constant (K-1) for the first step of the association [D- + S+ <->(DS)degrees], the s tandard 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 hydrophob ic 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)degree s <-> S-2)(+)], the unitary entropy change in the medium-level surfactant c oncentration domain was highly positive, indicating that water-structure co ntributions are appreciable in the stabilisation of the (DS2)+ species. It was shown that a further stepwise association is not very likely in the thi rd domain, and that a neutral quadruple species of type (SD)(2)degrees may be present in the solution. Further increases in surfactant concentration l ed to a very steep increase in bound surfactant as the concentration of fre e surfactant ions approached the c.m.c. point. It was also shown that the a ddition of a simple electrolyte (e.g. NaCl) decreased K-1, mainly due to in terference with the interaction of dye and surfactant ions. Since long rang e Coulombic interactions are absent in the second association step, the inc rease in K-2 with increasing NaCl concentration can be mainly attributed to short range hydrophobic interactions. (C) 2000 Elsevier Science Ltd. All r ights reserved.