Study On The Micellization And Interfacial Activity Of Cationic Surfactants

Six cationic surfactants with hydroxyethyl substituent on the quaternary ammonium headgroup, N-dodecyl-N-2-hydroxyethyl-N,N-dimethyl ammonium bromide,N-dodecyl-N, N-dihydroxyehtyl-N-methyl ammonium bromide, N-tetradecyl-N-hydroxyethyl-N, N-dimethyl ammonium bromide, N-tetradecyl-N, N-dihydroxyehtyl-N-methyl ammonium bromide, N-cetyl-N-2-hydroxyethyl-N, N-dimethyl ammonium bromide,N-cetyl-N, N-dihydroxyehtyl-N-methyl ammonium bromide, and six Gemini surfactants, trimethylene-1,3-bis(N-dodecyl-N, N-dimethylammonium bromide), tetramethylene-1,4-bis(N-dodecyl-N, N-dimethylammonium bromide), hexamethylene-1,6-bis(N-dodecyl-N, N-dimethylammonium bromide), trimethylene-1,3-bis(N-tetradecyl-N, N-dimethylammonium bromide), tetramethylene-1,4-bis(N-tetradecyl-N, N-dimethylammonium bromide) and hexamethylene-1,6-bis(N-tetradecyl-N, N-dimethylammonium bromide)have been synthesized and characterized by using IR, NMR and elemental analysis (EA).The critical micelle concentration (CMC) and the degree of couterion dissociation (β) for all the twelve surfactants in aqueous solutions have been investigated by the measurements on conductivity at 298.15,303.15,308.15,313.15, 318.15 and 323.15 K. Then, the CMC values are also determined through the surface tension measurement and the isothermal titration calorimetry (ITC). The values show reasonable agreements.For the six cationic surfactants with hydroxyethyl substituent on the quaternary ammonium headgroup, it has been found that the values of both CMC andβfor each surfactant increase slightly with increasing the temperature. At a given temperature, the values of CMC,βdecrease obviously with lengthening the alkane chain. The increase of the number of hydroxyethyl substituents on the quaternary ammonium headgroup results in the slight decrease of CMC and the slight increase ofβ. For six Gemini surfactants, the CMC value becomes large with the increase of the temperature, while decreases with the increase of the alkyl chain length. The Gibbs free energy (ΔGmic), enthalpy (ΔHmic), and entropy (ΔSmic) of micellization are then calculated by using the mass action model. Among the thermodynamic functions, the values ofΔHmic andΔSmic decrease with the increase of the temperature, while the value ofΔGmic nearly keeps constant. The enthalpy-entropy compensation effect can be observed for the micellization of all surfactants studied.The interfacial tensions for the system of water/n-heptane/surfactant are measured by using the spinning-drop interfacial tensiometer. The interfacial activity of the surfactants has been studied. The results show that the Gemini surfactants have a better interfacial activity than the surfactants with hydroxyethyl substituents. With almost the same value of minimum interfacial tension, however, the systems consisted of Gemini surfactants reache the minimum interfacial tension at a much lower concentration than those containing surfactants with hydroxyethyl substituents does. It is also found that the additives can largely influenc the interfacial tension. Addition of an inorganic salt, NaCl, can make the interfacial tension minima occour at lower surfactant concentrations, while it cannot affect the minimum value of interfacial tension. Besides, addition of ethanol, propanol and n-butanol to the systerm has no obvious effects on the surfactant concentration which is needed to make system reach the minimum interfacial tension, but can reduce the minimum value of the interfacial tension.