Aggregation of an Anionic Azo Dye with Conventional Cationic ‎Surfactants in Premicellar‏ ‏Region

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 constants were ‎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. ‎