Critical Micellar Concentrations of Quaternary Ammonium Surfactants with Hydroxyethyl Substituents on Headgroups Determined by Isothermal Titration Calorimetry

The micelle formations in aqueous solutions of a set of quaternary ammonium surfactants, N-alkyl-N-2-hydroxyethyl-N,N-dimethyl ammonium bromide (C(n)HDAB, n = 12, 14, and 16), N-alkyl-N,N-2-dihydroxyethyl-N-methyl ammonium bromide (C(n)DHAB, n = 12, 14, and 16), along with N-dodecyl-N,N,N-trimethyl ammonium bromide (DTAB), and N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB), were investigated by the isothermal titration calorimetry (ITC). The critical micellar concentrations (CMCs) and the enthalpies of micellization at six different temperatures were determined. The entropies and Gibbs free energies of micellization were then calculated by using the pseudophase separation model. The effects on the thermodynamic data of the temperature, the length of the alkane chain, and the number of hydroxyethyl substituents on the surfactant headgroup were discussed. The CMC value of each surfactant increases slightly with the temperature increase. At a given temperature, both the increases of the length of the alkane chain and the number of hydroxyethyl substituents lead to the decreases of the CMC values.