Thermodynamics of micellization of a non-ionic surfactant Myrj 45: Effect of additives

Critical micelle concentrations of the non-ionic surfactant Myrj 45 in aqueous solution at various temperatures (35, 40, 45 and 50 degrees C) were determined by the iodine solubilization method. A few systems were also studied by direct surface tension measurement. The effect of additives, e.g. PEG 400, sucrose, acetamide and urea, on the CMC was studied. The free energy, enthalpy and entropy of micellization were determined. A negative free energy and a positive entropy of micellization were observed for all the systems. The micellization process was exothermic in all cases except in aqueous solution without additives, where the process was endothermic. An enthalpy-entropy compensation effect was observed for all the systems. Transfer enthalpies and heat capacities were evaluated from enthalpy of micellization data. The transfer enthalpy Delta H-m,H-tr was negative throughout, indicating the transfer of hydrophilic groups from water to the aqueous additive solutions. A maximum observed in the plot of Delta C-p,C-m,C-tr-concentration of additives was ascribed to a micellar structural transition.