Thermodynamic Studies of the Interactions of Cetyltrimethyl Ammonium Bromide (Ctab) and Methyl Orange Dye in Ethanol-Water Mixed Solvents

Abstract

The interaction between Cetyltrimethylammonium bromide (CTAB) and Methyl Orange dye in water and ethanol-water mixed solvent media at varying temperatures was studied by conductometry. The critical micelle concentrations and thermodynamic parameters of micelle formation such as enthalpy, entropy and Gibbs free energy were determined at different temperatures and volume fractions of ethanol. Results show that the CMC of CTAB increased with increasing volume fraction of ethanol indicating that the solubility of CTAB in aqueous solution may have decreased with increasing volume fraction of ethanol. The CMC of the surfactant solution also increased in the presence of the dye which may be attributed to steric hindrance due to the bulkiness of the azo dye. The increased CMC observed with temperature may suggest breakdown of structured water molecules surrounding the hydrophobic alkyl chain as well as increased molecular freedom of the surfactant molecules due to higher kinetic energy. The decreasing negative values of Gibbs free energy may indicate that micellization was thermodynamically feasible but less spontaneous on increasing volume fraction of ethanol, in the presence of the dye and with increasing temperature. However, temperature effect was not large. The enthalpy of the process shows that the interaction between the dye and surfactant was exothermic (?H 0m ? ? ) and the observed positive values of ?Sm is suggestive of entropy driven micellization process.