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 H-mic degrees, between 278.15 and 318.15 K. Gibbs free energy, Delta(mic)G degrees, and entropy, Delta S-mic degrees, were deduced by taking into account the counterion binding. From the temperature dependence of Delta H-mic degrees the heat capacities of micellization, Delta C-mic(p)degrees, 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 H-mic degrees at higher concentrations of salt only. Delta(mic)G degrees is always negative and slightly temperature dependent. The temperature dependence of Delta S-mic degrees indicates that the process of micellization is entropically driven. Delta H-mic degrees 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 H-mic degrees = 0, where Delta(mic)G degrees is of purely entropic contribution in all solutions. Delta C-mic(p)degrees 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. (C) 2009 Elsevier Inc. All rights reserved.