The interaction between a nonionic surfactant and acid dyes studied by measurements of cloud point

In order to clarify the interaction behaviors between a nonionic surfactant (S) and an acid dye (D), the cloud points (CP) were measured for the solution containing S and D with increasing amounts of electrolytes and are plotted as a function of the electrolyte concentration. A clear transition point Z was observed in the CP curve. The value of C^z, the electrolyte concentration at Z, depends upon the valence number of the cation of the added electrolyte rather than that of the anion. While the value of T^z, the temperature at Z, is independent of the kind of electrolyte and dye, but depends on the molar ratio of S to D in the solution. On the basis of the phase rule, the existence of Z was interpreted in terms of a triple point of three types of mixed micelles which are formed of S and D.