Aggregation of an anionic azo dye with conventional cationic surfactants in premicellar region: A conductometric study

The interaction of Congo Red (CR) with a series of Alkyl trimethyl ammonium bromide (CnTAB), N-hexadecyl pyridinium bromide (CPB) and N-hexadecyl pyridinium chloride (CPC) were investigated using conductometry and UV-Vis spectroscopy technique. Job’s method was used to determine the stoichiometric ratio of dye and surfactant in ion pairs. The equilibrium constant and other thermodynamic parameters for the ion pair formation were calculated on the basis of a theoretical model using the data obtained by conductometry. It was found that an increase in temperature reduces the tendency for ion pair formation as equilibrium constants decrease with increasing temperature. The results showed that as the chain length of the CnTAB molecule in Dye/CnTAB systems was increased, the equilibrium constantswere significantly increased. Therefore, the short range as well as long range interactions is 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 energy change for the formation of the anionic dye–cationic surfactant ion pair. The reaction between the dye and the surfactants was revealed to be an enthalpy-driven reaction which is highly dependent on the temperature and the structures of both dye and the surfactant.