Studies On Aggregation Behaviors Of Quaternary Ammonium Gemini Surfactants In Protic Solvents

The aggregation behaviors of cationic quaternary ammonium salt-type Gemini surfactants in protic solvents have been investigated in this dissertation. Not only the molecular structure of Gemini surfactants, like headgroups, spacer and alkyl chain, but also the type and structure of protic solvents do have influence on the aggregation behavior. Various characterizations have been carried out, such as surface tension, cryogenic-transmission electron microscopy (cryo-TEM), freeze-fracture transmission electron microscopy (FF-TEM), polarized optical microscopy (POM), small angle X-ray scattering (SAXS), rheological measurements, infrared spectroscopy and so on. Main results are as follows:1. The aggregation behaviors of Gemini surfactants with hydroxyl in headgroups, butane-1,4-bis(hydroxylethyl methyl alkyl ammonium) bromide, abbreviated as m-4-m MEA (m= 12,14,16), have been investigated in aqueous solutions. In solutions of 14-4-14 MEA, the formation of highly viscoelastic wormlike micelles can be detected by steady and dynamic rheological measurements. The formation of such wormlike micelles has also been confirmed by cryo-TEM and SAXS. Compared with the conventional bis(dimethyl alkyl ammonium) bromide Gemini molecules with the same spacer,14-4-14, a higher ability to fabricate wormlike micelles by 14-4-14 MEA could be concluded because of the structure change in headgroups. As for 16-4-16 MEA, which has longer alkyl chains, its aqueous solutions behave more like elastic gels. The unique viscoelastic behaviors of m-4-m MEA in water can be attributed to the synergistic interactions of hydrophobic attraction and hydrogen bonding. The obtained results are believed to be an important supplement to the headgroup effect of Gemini surfactants on their aggregation behavior in the dilute region.2. The aggregation behaviors of a cationic Gemini surfactant (12-3OH-12) with the hydroxyl group in the spacer in a protic ionic liquid ethylammonium nitrate (EAN) have been investigated. Here, the effects of the hydroxyl group on micellization and lyotropic liquid crystal formation were investigated. With the hydroxyl group in the spacer, the critical micellization concentration (CMC) of 12-3OH-12 was found to be lower than that of the homologue without hydroxyl (12-3-12) and the 12-3OH-12 molecules packed more densely at the air/EAN interface. It was then interesting to observe a coexistence of two separated phases at wide concentration and temperature ranges in this 12-3OH-12/EAN system. Such a micellar phase separation was rarely observed in the ionic surfactant binary system. With the increase of surfactant concentration, the reverse hexagonal and bicontinuous cubic phases appeared in sequence, while only a reverse hexagonal phase was found in 12-3-12/EAN system. But, the hexagonal phases formed with 12-3OH-12 exhibited lower viscoelasticity and thermostability than those observed in 12-3-12/EAN system. Such unique changes in phase behaviors of 12-3OH-12 were ascribed to their enhanced solvophilic interactions of 12-3OH-12 and relatively weak solvophobic interactions in EAN.3. The aggregation behaviors of a Gemini surfactant (12-2-12) in three protic ionic liquids (PILs), propylarnrnonium nitrate (PAN) and butylammonium nitrate (BAN), ethanolammonium nitrate (EOAN) have been investigated. Compared with EAN, the minor structural changes with only one or two methylene units increase in cationic chain length of PILs, resulted in a dramatic phase transition of formed aggregates. The CMC increased in PAN, while no micelle formation was detected in BAN. A normal hexagonal phase was observed in the 12-2-12/PAN system, while normal hexagonal, bicontinuous cubic, and lamellar phases were mapped in the 12-2-12/BAN system. Such aggregation behavior changes could be ascribed to weaker solvophobic interactions of 12-2-12 in PAN and BAN. The unique molecular structure of 12-2-12 was also an important factor to highlight such a dramatic phase transition.Unlike in water and EAN, only the lamellar phase was observed in the 12-2-12/EOAN system. With the increase of spacer chain length (s= 3,4,5,6), the hexagonal phases were obtained. This was because the increase of spacer chain length caused an increase in the area occupied by the headgroup, resulting in the formation of high curved aggregates. Such aggregation behaviors would illustrate the effect of spacer chain length.4. The phase behavior in a mixture of 12-2-12 and an anionic sodium deoxylcholate (SDC) in the aqueous solution was reported here. Mediated by the hydrophobic and electrostatic interactions of 12-2-12 and SDC, the wormlike micelles and vesicles were formed successively with increasing the SDC concentration. At low SDC concentrations, the wormlike micelles could be transformed into vesicles at lower pH. While at high SDC concentrations, the vesicles would be transformed into wormlike micelles at elevated temperatures. The obtained results suggest the possibility of smart aggregates’ formation by utilizing simple molecules with certain functional groups.Thanks for the financial supports from the National Natural Science Foundation of China (21373127) and the Specialized Research Fund for the Doctoral Program of Higher Education (20130131110010).