Study On The Self-assembly Of Gemini Surfactants With Hydroxyl Groups In An Ionic Liquid

Gemini surfactants have unique molecular structures and novel self-assembly behavior.The ordered molecular aggregates formed by Gemini surfactants in aqueous solutions have received wide attention. Besides the surfactant molecular structure, the nature of the solvent in which surfactants interact is also important for the self-assembly of surfactants. As a green solvent, ionic liquid has the advantages of wide region of liquid state, good thermal stability and high ionic conductivity. Investigation on the self-assembly of Gemini surfactants in ionic liquids is helpful to understand their unique behavior and expand their applications. In this thesis, the quaternary ammonium surfactants containing hydroxyl groups near the head groups have been investigated. Their surface activities, lyotropic liquid crystals and microemulsions in protonic ionic liquid ethylammonium nitrate(EAN) have been explored. The main results are summarized as follows:(1) The surface activities of three Gemini surfactants 12-3-12, 12(2Et)-3-12(2Et),12(2OH)-3-12(2OH) and their corresponding monomeric ones, have been investigated by equilibrium surface tension and FT-IR methods. It is revealed that the slight change at the head groups of surfactants have tremendous effect on the surface activities of surfactants in ionic liquid. The hydrogen bonds generated by the hydroxyl groups at the head group region obviously promote the adsorption and aggregation of surfactants. Even at higher temperatures,the hydroxyl groups still make a difference.(2) The lyotropic liquid crystals formed by Gemini surfactants in EAN have been investigated by differential scanning calorimetry(DSC), X-ray diffractometer(XRD),polarizing microscope(POM) equipped with a hot stage and FT-IR. All surfactants form smectic A phase(Sm A) in liquid crystalline region. The EAN moleculars mainly exist in the polar sublayers of liquid crystals. The temperature ranges of liquid crystal phase increase with increasing surfactant concentration and alkyl chain length while shows maximum with the increase of spacer length. The hydroxyl groups decrease the interactions between the ionic head groups and counterions. The temperature range of liquid crystal state is thus narrowed.(3) The solubilization of EAN in microemulsion systems constructed by 12-3-12 and12-3(OH)-12 was first examined. The microscopic region of IL/O microemulsion was divided according to the electrical conductivity measurements. The formation of microemulsions was further identified by DLS measurements. The relationship between the particle size and temperature was discussed. The results showed that the microemulsions of ionic liquid show rather higher thermal stability. The single phase region of microemulsions containing 12-3-12 is larger than that of 12-3(OH)-12. The particle size of microemulsions, which increases with temperature, shows linear relationship with the molar ratio of EAN and surfactant. The presence of hydroxyl groups decreases the particle size of microemulsions and also the effects of temperatures on particle size.