The formation of a dye-surfactant ion pair was studied using a conductometr
ic method in which the conductance of aqueous solutions of C.I. Acid Orange
7 was measured in the presence of the cationic surfactants dodecylpyridini
um chloride or hexadecylpyridinium chloride at four different temperatures.
Two theoretical models to calculate the relevant equilibrium constants wer
e derived. Both methods of calculation led to similar results that were in
good agreement with other methods of investigation of dye-surfactant associ
ation. The results have shown that an increase in temperature lowers the te
ndency for ion pair formation as the equilibrium constants decrease with in
creasing temperature. A comparison of the behaviour of both surfactants rev
ealed that the surfactant which contained the longer hydrophobic chain had
a stronger tendency to associate with the dye and the corresponding equilib
rium constants were considerably higher than those recorded for the surfact
ant with the shorter aliphatic chain. Such findings indicate that not only
long range electrostatic forces but also short range, non-electrostatic int
eractions have a significant influence on dye-surfactant ion pair formation
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