Study On Temperature-Induced Phase Inversion Of Cationic-Anionic Surfactants Two-Phase Systems

On the basis of the former work in this laboratory, an optimum synthesis route has been used to prepare six quaternary ammonium surfactants, which were characterized from the techniques of elemental analysis (EA), FTIR and1H-NMR. Meanwhile, the critical micellar concentrations (CMC) of these surfactants in water have been determined from measurements on the conductivity, surface tension and isothermal titration calorimetry (ITC). The optimum synthesis conditions for these surfactants are that:reaction solvent is acetone, reaction temperature is around60℃and reaction time is5～6h. The surfactants can be purified by recrystallization method. They should be dried and kept under vaccumm.Triangular phase diagrams with N-alkyl-N,N-2-dihydroxyethyl-N-methyl ammonium bromide (CnDHAB,n=12,14, or16), sodium dodecyl sulfate (SDS), and sodium chloride as three apexes have been drawn from the phase equilibrium experiments at40.0℃, in which the regions of aqueous two-phase systems of cationic-anionic surfactant mixtures with cationic surfactant in excess (ATPS-C) have been determined. Temperature-induced phase inversion has been observed in several ATPS-C with certain compositions. Three specific ATPS-C have been chosen to systematically investigate the influences of temperature on the phase properties, composition and microstructures before and after phase inversion. As the temperature increases, the density of the surfactant-concentrated phase in ATPS-C decreases faster than that of the dilute phase, which is the directviewing reason for phase inversion. The decrease rate of viscosity of the concentrated phase is much more obvious than that of the dilute phase. Ultralow interfacial tensions between the separated phases have been observed and the plot of interfacial tension versus temperature exhibits a minimum around the phase inversion temperature, which accords with the lowest density and viscosity differences between the separated phases. Composition analyses manifest that ions in the separated phases are redistributed with the increase of temperature. Evidently, inorganic ions transfer from the concentrated phase to the dilute phase and surfactant bulk ions still almost stay in the concentrated phase. These transformations make main contributions to the change of density difference between the separate phases. The results of viscosity measurement, dynamic light scattering and transmission electron microscopy indicate that network structures and large aggregates are destroyed as the temperature rises with the formation of smaller spherical micelles or vesicles in the concentrated phase. Vesicles and the fusion of vesicles can be observed in the dilute phase where most of the aggregates are irregular micelles at the temperatures before phase inversion. The breakdown of the compact network structures in the concentrated phase not only contributes to the fast density decreasing rate with temperature, but also corresponds to the large viscosity decreasing rate.For the ATPS-C of CnDHAB-SDS-NaCl, keeping two of the three variables, the concentration of NaCl (CNaCl), the total concentration of surfactants (CT) and molar ratio of cationic surfactant to anionic surfactant (MR), the changes of the phase inversion temperature were investigated. The results indicate that the phase inversion temperature increases as CT or MR increases, while decreases as CNaCl increases. In addition, the effects of the length of hydrophobic tail and hydrophilic block of the cationic surfactants on the phase inversion temperature have also been studied. The results show that the sequence of the phase inversion temperature follows that C14DHAB-SDS-NaCl>C12DHAB-SDS-NaCl> C16DHAB-SDS-NaCl. The reduction of ethoxy of the hydrophilic headgroup of cationic surfactant leads to the decrease of the phase inversion temperature dramatically.UV-Vis spectrometry has been employed to study the extraction of crystal violet in ATPS-C of C12DHAB-SDS-NaCl. The factors such as molar ratio of cationic surfactant to anionic surfactant (MR), the concentration of crystal violet (CJ), and temperature (T) are discussed. The results illustrate that as MR increases, the partition coefficient and extraction rate increase initially and then decrease. With increasing Cj or temperature, both partition coefficient and extraction rate decline.