Thermodynamic Investigation on Micellization of Cationic Gemini Surfactants with Nitrophenoxy Groups in Hydrophobic Chains

Isothermal titration microcalorimetry (ITC) is a powerful technique for acquiring thermodynamic information on the micellization process of surfactants. In typical ITC experiments, consecutive injections of small volume of a surfactant micellar solution into water contained in a sample cell are performed. The sample cell is maintained at a constant temperature and the heat of processes occurring during dilution is monitored for each injection and plotted as a function of surfactant concentration in the cell. The interpretation of experimental data obtained by ITC is necessarily based on models describing micellization process. So far, there are two main models to the thermodynamic analysis of the micellization process: the mass action model, considering micelles and unassociated monomers to be in an association-dissociation equilibrium and the phase separation model, which regards micelles as a separate phase at critical micelle concentration (CMC) and assumes that the micellization process is strongly cooperative. The phase separation model is the simplest model for describing micelle formation assuming that this process is akin to a separate phase-precipitation. Based on these two models, an ITC curve of observed enthalpy change versus surfactant concentration allows the determination of CMC and the enthalpy of micellization (Delta H-mic) of a surfactant. Other thermodynamic parameters related to micellization, namely the free energy (Delta G(mic)), the entropy (Delta S-mic) and the heat capacity of micellization (Delta C-p,C-mic) can be calculated from the experimentally determined CMC and Delta H-mic. In this paper, ITC and electrical conductivity were employed to investigate the micellization process of cationic gemini surfactants, N,N,N,N'-tetramethyl- N,N'-bis[10-(4-nitrophenoxy)alkyl]-1,6-hexanediammonium dibromide (Nm-6-mN, with m=8, 10 and 12, which are the numbers of carbon in the hydrocarbon chains), in aqueous solutions. Both phase separation model and mass action model were used to obtain a series of thermodynamic parameters. The results show that the obtained Delta H-mic values based on the two models are very close, however, the obtained Delta G(mic) values based on the two models are not consistent. In addition, the Delta C-p,C-mic of the micellization process is mainly from the dehydration contribution of hydrophobic alkyl chains of the surfactants, which means the nitrophenoxy group located in the hydrophobic chain still contacts with water after the micellization. Furthermore, the micellar aggregation number n can be obtained by employing the mass action model. The micellar aggregation number n decreases with the increase of the hydrophobic chain length. The reason is that the surfactant with longer hydrophobic chains prefers to form premicelles, leading to the decrease of the average aggregation number.