Synthesis And Performances Of A Co₂-Responsive Surfactant

Stimuli-responsive surfactants are one kind of amphiphiles whose surface(interface) activities can be smartly tuned by exposing to external minor variation in the environment, including p H, temperature, electronic, light, etc. As an inexpensive, non-toxic, biocompatible, and benign stimulus, CO2 has been recently found to be able to switch some surfactants. However, few efforts were devoted to the switchable behaviors in high salinity solution due to the lack of CO2-responsive surfactants with excellent salt-tolerance. Thus, a novel CO2-switchable surfactant, N-dodecyl polyoxyethylene(3)-N,N-dimethyl tertiary amine(C12EO3A) was synthesized, and studied in this dissertation. The CO2-responsive behaviors in brine solution was systematic evaluated, compared with the structure-related counterpart, N-dodecyl-N,N-dimethyl tertiary amine(C12A). Moreover, four CO2-switchable microemulsion systems were for the first constructed using C12EO3 A or C12 A. Based on these studies, the following conclusions can be drawn:(1) CO2-switchability and salt-tolerance of C12EO3 A.Compared with the structure-related counterpart(C12A), the introduction of EO segment can not only reduced the p Ka value and the critical micelle concentration(cmc) of the protonated C12EO3A(C12EO3A bicarbonate salt), but also imparts excellent salt tolerance, resulting in that C12EO3 A still exhibits fast and high effective CO2-responsiveness in saturated Na Cl solution without the liquid-liquid phase separation appearing in C12 A system.(2) Preparation and properties of oil-in-water(O/W) CO2-switchable microemulsions.O/W microemulsion regions were clearly obtained through pseudo-ternary phase diagrams of [H2O/SDS/C12EO3 A or C12A]-Heptane-Butanol. Upon alternative treatment of CO2 and N2, the designed O/W microemulsions can be reversibly switched “off”(microemulsion break, complete phase separation) and “on”(microemulsion formation, single phase), showing smart CO2-responsive behaviors. The results originating from dynamic light scattering(DLS) and freeze-fracture transmission electron microscopy(FF-TEM) reveal that: in absent of CO2, the system is an apparent and homogenous single phase and the diameter of O/W microemulsion drops is nano-sized and narrow-ranged; when exposed to CO2, two tertiary amines(C12EO3A or C12A) were protonated into cationic quaternary ammonium salt, which would bind with anionic SDS, losing mutual surface activities, forming oil substances, and then leading to completely phase separation; when CO2 was displaced with N2, the microemulsions can be obtanined again, and the size distributions of dispersed droplets are also essentially constant. Note that if HCl/Na OH was used as the succedaneum of CO2/N2, the reversible transition of microemulsions is almost impossible.(3) Preparation and properties of water-in-oil(W/O) CO2-switchable microemulsions.Two W/O microemulsion regions([C12EO3A or C12A/SDS/isoamylol]-CyclohexaneWater) were successfully constructed with the aid of pseudo-ternary phase diagrams. Both C12EO3A-based and C12A-based W/O microemulsions exhibit similar CO2-reversible switchabilities to that of C12EO3A-based and C12A-based O/W microemulsions. This suggests that the strategy of forming CO2-responsive microemulsions utilizing the effect of CO2 on the bind interaction of tertiary amine and SDS is universal. Furthermore, nano-sized and narrow-ranged silver nanoparticles with small diameter can be prepared in C12A-containing W/O microemulsion, and the existence of silver nanoparticles do not influence the switch of the microemulsion.