The Thermodynamic Properties Of Mixed Ionic Surfactant Solutions

The aggregation behavior and thermodynamic properties of mixed ionic surfactant solutions 12-2-12/C12mimBr,12-6-12/C12mimBr, DTAB/C12mimBr and 12-2-12/DTAB at various temperatures have been investigated by isothermal titration calorimetry (ITC) and measurements of conductometry. The critical micelle concentration (cmc), the enthalpy of micellization and the degree of dissociation of conterion have been measured and the experimental results were used to determine the micelle compositions, the thermodynamic functions of micellization, the excess enthalpies, the excess entropies and the excess free energies based on the phase separation model and the two-parameter Margules equation. It was shown that the variations of excess enthalpies, the excess entropies and the average degree of dissociation of counterion of mixed micelles with the overall composition of surfactant αi were in consistence, which indicated that the non-ideal mixing behavior derives from the non-ideal degree of dissociation. In a certain range of lower overall composition αi, the formation of the second micelle phase in the mixed surfactant solutions was observed. A hypothesis of the coexistence of two micelle phases and a solution phase has been proposed to analyze the thermodynamic conditions for emerging the second micelle phase and to reasonably explain the experimental results.