Study On Synthesis And Aggregation Behaviors Of Quaternary Ammonium Salt-Type Gemini Surfactants In Ionic Liquids

Ordered molecular assemblies, such as micelles, vesicles, lyotropic liquid crystal (LLC), and microemulsion, fabricated by amphiphilic molecules in suitable solvents are playing active roles in areas of materials, life, pharmaceutical, separation, dispersion, information, and energy. Exploration on new aggregation systems may not only advance the better understanding of amphiphile self-assembly, but also extend their applications. Recently, much interest has been generated for Gemini surfactant, a new amphiphilic molecule, for their novel structures and excellent properties. Much study has been focused on their aggregation behaviors in water. However, they should be very carefully manipulated under high temperature or electric field for the evaporation and electrolysis of water or other solvents. As a novel solvent, the ionic liquids (ILs), so-called neoteric solvents and materials, have now not only changed our knowledge on solvent, but also provided opportunities for new self-assemblies in solvents other than water.This dissertation is focused on the aggregation behaviors of cationic quaternary ammonium salt-type Gemini surfactants in ILs, including micelle and lyotropic phases. There are four main experimental studies in this dissertation.1. The aggregation behaviors of symmetric Gemini surfactants in ethylammonium nitrate (EAN). The three symmetric Gemini molecules have been obtained by synthesis. Techniques of the polarized optical microscopy (POM), small-angle X-ray scattering (SAXS) and the rheological measurements are used to explore the formation of lyotropic liquid crystal (LLC). The effects of concentration of Gemini surfactants, length of alkyl chain, and temperature on the lyotropic phases are investigated. Following results are obtained.(1) With increasing the Gemini surfactants concentration, a reverse hexagonal phase (HⅡ) can be mapped over a large temperature range except for other ordered aggregates including the isotropic solution phase (L1) and a two-phase coexistence region. EAN molecules can form a three-dimensional hydrogen-bond network, which was seemed to be an essential feature in the self-assembly with it. And the solvophobic effect is a dominant driving force for lyotropic phase.(2) The concentration effect on the LLCs. The radius of EAN solvent core inside the cylinder of hexagonal phase becomes contracted in the case of more12-2-12, while the hydrocarbon layer of the cylinder becomes expanded. Furthermore, the area per molecule of12-2-12at the hydrophilic/hydrophobic interface in the reverse hexagonal phase, SⅡ, decreases with the12-2-12amount increased, reflecting a more dense packing. This can be supported by the fanlike texture images under the POM, which become more and more perfect with adding more12-2-12. Such a concentration effect is obviously due to the enhanced solvophobic effect.(3) The alkyl length effect on the phase behaviors of Gemini surfactants. The highest m-12-m concentration boundary of the micelle solution decreases with longer alkyl length. Meanwhile, the thermal stabilities of LLCs are enhanced with increasing hydrocarbon chains because of the strengthened solvophobic effect.(4) The temperature effect on the LLCs. The increased fluctuations at higher temperatures may shorten the average length of the alkyl chain and lead to the decrease of repeat lattice spacing. The EAN channel radius, decreases with increasing temperature, which means a larger curvature aggregate should be obtained.(5) Compared to water systems, Gemini molecules can pack densely in EAN due to its ionic nature, which causes a large degree of charge screening. Meanwhile, the higher affinity to EAN than to water might increase the volume of the solvophobic part, which also leads to a higher CPP and then results in the formation of a reverse phase.Above results enrich studies on Gemini surfactants in nonaqueous medium and provides further understanding on aggregation of amphiphilic molecules in ionic environment.2. The aggregation behaviors of dissymmetric Gemini surfactants in EAN. The work can be divided into two parts:the phase behaviors of m-2-n (m+n=24, m=16,14, and12) in EAN, including micelle and lyotropic phases; the lyotropic behaviors of16-2-m (m=8,10,12,14, and16) in EAN. Following results can be obtained.(1) In EAN, for m-2-n (m+n=24), with enhanced dissymmetry, larger hydrophobic interaction was obtained, and thus leading to lower CMC and more densely packing at air/EAN or the hydrophilic/hydrophobic interface because of the strengthened solvophobic effect. For the same reason, the thermal stabilities of corresponding LLCs are also enhanced, which are demonstrated by POM and rheological measurement. But a lower packing density of m-2-n type Gemini molecules presents with more dissymmetry, which can be attributed to the decreased order of molecule arrangement. Compared to water systems, these amphiphilic molecules can pack more densely in EAN at lower concentration for the charge screening and more densely in water at higher concentration for stronger solvophobic effect. Furthermore, the solvophobic interaction in EAN system behaves compatible to the weak driving force from solvent and hence exhibits large effect of dissymmetry on phase behaviors.(2) In EAN, the aggregation behaviors of16-2-m are closely related to the values of m. When m is between8-14, the isotropic solution phase, the two-phase coexistence region and the reverse hexagonal phase were obtained. With larger value of m, the micelle regions decrease and the LLCs region increase due to the enhanced solvophobic effect. When m increases to16, both intermolecular and intramolecular interactions are enhanced, which lead to a larger CPP, and the micelle region disappears.Above results enrich studies about the dissymmetry effect on Gemini surfactants in different mediums.3. The aggregation behaviors of Gemini surfactants with different spacer lengths in EAN. A series of Gemini surfactants with different spacer lengths,12-s-12(s=2,3,4,6,8,10,12), have been obtained by synthesis. Their phase behaviors in EAN have been investigated with the techniques of POM and SAXS. The aggregation behavior of corresponding monomeric counterpart——dodecyl trimethyl ammonium bromide (DTAB) in EAN has also been studied. Following results can be obtained.(1) The length of spacer, s, has a major influence on the phase behaviors of Gemini surfactants. When the spacer length is between2-8, micelle and reverse hexagonal phases were obtained in the phase diagrams. Increasing the value of s could expand the area per molecule of Gemini molecules at the hydrophilic/hydrophobic interface in the reverse hexagonal phase. This result was proved by comparing phase behaviors of16-2-16/EAN and16-3-16/EAN. In addition, the thickness of solvophilic domains, dⅡ, also becomes larger as s increases. When s reaches10or12, the lyotropic phase disappears. The reason can be due to that the longer spacer may bend toward the solvophilic domains and the increasing of dⅡ overtakes the expanding of SⅡ, which causes a higher CPP.(2) Compared to micellization process of DTAB in EAN,12-2-12shows higher surface activity and more negative standard Gibbs free energy (ΔGm). Because of the presence of spacer, the area per molecule of12-2-12molecules is smaller than twice of DTAB, which leads to a higher CPP and thus the reverse lyotropic phase.4. The aggregation behaviors of Gemini molecules in different ILs. The LLCs formed by12-2-12in three ILs:EAN, propylammonium nitrate (PAN) and butylammonium nitrate (BAN), have been investigated by POM and SAXS. Following results can be obtained.Both in EAN and PAN,12-2-12molecules only formed micelle and reverse hexagonal phases. But in BAN, a richer phase behavior was identified:micelle, normal hexagonal (HⅠ), bicontinuous cubic (VⅠ) and lamellar (La) phases. Because of the charge screening from the nature of ILs, the ILs molecules can easily partition into the interface regions with the12-2-12molecules and take part into aggregation process. When alkyl-chains of ILs are short (<4), increasing the length of alkyl may expand the area per molecule of12-2-12molecules which causes a smaller CPP and a richer phase behaviors.