Temperature and salt-induced micellization of dodecyltrimethylammonium chloride in aqueous solution: a thermodynamic study.

Thermodynamics of micelle formation of the cationic surfactant dodecyltrimethylammonium chloride (DTAC) in water and aqueous NaCl solutions were investigated. Isothermal titration calorimetry (ITC) has been used to study the effect of the added NaCl on the critical micelle concentration, cmc, and enthalpy of micellization, Delta(mic)H(o), between 278.15 and 318.15K. Gibbs free energy, Delta(mic)G(o), and entropy, Delta(mic)S(o), were deduced by taking into account the counterion binding. From the temperature dependence of Delta(mic)H(o) the heat capacities of micellization, Delta(mic)c(p)(o), were determined. NaCl shifts cmc strongly towards lower values, indicating the screening of the repulsions of the polar head groups by counterions; but it influences Delta(mic)H(o) at higher concentrations of salt only. Delta(mic)G(o) is always negative and slightly temperature dependent. The temperature dependence of Delta(mic)S(o) indicates that the process of micellization is entropically driven. Delta(mic)H(o) decreases strongly with increasing temperature and passes through zero (endothermic to exothermic processes). The temperature dependence of the critical micelle concentration exhibits a minimum characterized by Delta(mic)H(o)=0, where Delta(mic)G(o) is of purely entropic contribution in all solutions. Delta(mic)c(p)(o) are strongly negative in all solvents, relating directly to the removal of water accessible non-polar surface of DTAC in the presence of excess counterions also.