Anionic dye-cationic surfactant interactions in water-ethanol mixed solvent

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