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, ΔmicHo, between 278.15 and 318.15 K. Gibbs free energy, ΔmicGo, and entropy, ΔmicSo, were deduced by taking into account the counterion binding. From the temperature dependence of ΔmicHo   the heat capacities of micellization, Δmiccpo, 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 ΔmicHo at higher concentrations of salt only.ΔmicGo is always negative and slightly temperature dependent. The temperature dependence of ΔmicSo indicates that the process of micellization is entropically driven. ΔmicHo 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 ΔmicHo = 0, where ΔmicGo is of purely entropic contribution in all solutions.Δmiccpo 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.

Effect of NaCl on the enthalpic and self-association behavior of dodecyltrimethylammonium chloride at 298.15 K.Figure optionsDownload high-quality image (150 K)Download as PowerPoint slide