Surface Activity and Micellization Parameters of Quaternary Ammonium Surfactants Containing a Hydroxyethyl Group

Based on the measurements of conductivity and surface tension and our previously reported results of isothermal titration calorimetry (ITC), the surface activity and micellization parameters, such as the critical micellar concentration (CMC), the degree of counterion dissociation (beta), and thermodynamic functions of micellization, for six quaternary ammonium surfactants, dodecyl-(2-hydroxyethyl)-dimethylazanium bromide, tetradecyl-(2-hydroxyethyl)-dimethylazanium bromide, hexadecyl-(2-hydroxyethyl)-dimethylazanium bromide, dodecyl-di(2-hydroxyethyl)-methylazanium bromide, tetradecyl-di(2-hydroxyethyl)methylazanium bromide, and hexadecyl-di(2-hydroxyethyl)-methylazanium bromide in aqueous solutions have been investigated. The values of CMC determined from three different methods are compared, and they show reasonable agreement. From the CMC values obtained from the conductivity measurements in this work and the previously reported values of calorimetric enthalpy of micellization, the Gibbs free energy (Delta G(mic)) and entropy (Delta S-mic) of micellization are calculated through the mass-action model. The influences on the micellization parameters of the temperature, the length of attune chain, and the number of hydroxyethyl substituents on the surfactant headgroup are discussed.