Thermodynamics of micelle formation of alkyltrimethylammonium chlorides from high performance electric conductivity measurements.

Understanding micelle formation requires its complete thermodynamic characterization. In this work micellization of the model ionic surfactants decyltrimethylammonium chloride (DETAC), dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC) was investigated by high performance conductivity measurements, using instrumentation developed in our laboratory. The temperature dependence of the critical micelle concentration exhibits a minimum characterized by Delta(mic)H(0)=0 (endothermic to exothermic process) and the degree of micelle ionization increases slightly with increasing temperature. The temperature change of Delta(mic)S(0) indicates that the process of micellization is entropically driven. Delta(mic)G(0) is always negative and slightly temperature dependent. The temperature dependence of the thermodynamic parameters is discussed in terms of the alkyl chain length and nature of the counterion. The micellization process is more favourable for surfactants with longer alkyl chain length and larger (less hydrated) counterions.