Conductometric investigation of dye-surfactant ion pair formation in aqueous solution

The formation of a dye-surfactant ion pair was studied using a conductometric method in which the conductance of aqueous solutions of C.I. Acid Orange 7 was measured in the presence of the cationic surfactants dodecylpyridinium chloride or hexadecylpyridinium chloride at four different temperatures. Two theoretical models to calculate the relevant equilibrium constants were derived. Both methods of calculation led to similar results that were in good agreement with other methods of investigation of dye-surfactant association. The results have shown that an increase in temperature lowers the tendency for ion pair formation as the equilibrium constants decrease with increasing temperature. A comparison of the behaviour of both surfactants revealed that the surfactant which contained the longer hydrophobic chain had a stronger tendency to associate with the dye and the corresponding equilibrium constants were considerably higher than those recorded for the surfactant with the shorter aliphatic chain. Such findings indicate that not only long range electrostatic forces but also short range, non-electrostatic interactions have a significant influence on dye-surfactant ion pair formation. (C) 2000 Elsevier Science Ltd. All rights reserved.