The interactions between an anionic dye and two cationic surfactants were s
tudied by conductometry. The specific conductance of dye-surfactant mixture
s was measured at three different temperatures in water-ethanol mixed solve
nt, containing 5, 10, 15 or 20 wt.% of ethanol. The equilibrium constants a
nd other thermodynamic functions for the process of dye-surfactant ion pair
formation were calculated on the basis of two theoretical models. The resu
lts showed that the presence of ethanol decreases the tendency for ion pair
formation. According to the results, long range as well as short range int
eractions are responsible for the formation of the ion pair. The importance
of long range electrical forces is basically to bring the dye anion and th
e surfactant cation close enough to enable the action of short range intera
ctions whose contribution represents the major part of the standard free en
thalpy change for the formation of the anionic dye-cationic surfactant ion
pair. (C) 2001 Elsevier Science Ltd. All rights reserved.