Study of interaction between cationic surfactants and cresol red dye by electrical conductivity and spectroscopy methods

Interaction of anionic dye, cresol red (CR), with cationic surfactants, dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB), in aqueous solution in the submicellar and micellar concentration ranges has been investigated by using thermodynamic and spectroscopic methods. The equilibrium model has been used to calculate the standard free energy (Delta G(M)(o)), enthalpy (Delta H-m(o)), and entropy (Delta S-m(o)) of micelle formation. The increase in the critical micelle concentration (cmc) with rise in temperature is attributed to the disruption of the structured water surrounding the hydrophobic groups of the surfactants. A marked decrease in the cmc is observed as the number of carbon atoms in the hydrophobic group increases from DTAB to CTAB. Higher values of the degree of ionization (alpha) of the micelles of DTAB and CTAB are obtained in the presence of CR than in its absence. Negative values of Delta G(m)(o) and Delta H-m(o), for the surfactants in aqueous and in aqueous CR solutions show that the process of micellization is thermodynamically spontaneous and exothermic. The values of -T Delta s(m)(o) are much higher than the Delta H-m(o), values, indicating that the micellization process is governed primarily by the entropy gain associated with it. The heat capacity of micellization (Delta C-p,m(o).) is negative which is mainly due to the change in the exposure of the hydrophobic groups to the water molecules. UV-visible spectra suggest interaction between dye and surfactant monomers forming ion-pair complex in the pre-micellar region whereas above the cmc solubilization of dye in the micelle dominates over ion-pair formation. (C) 2014 Elsevier B.V. All rights reserved.