Micellar Behaviour Of Ionic Liquid Surfactants In Aqueous Solution

As novel and green solvent, ionic liquids (ILs) have been received much attention of researchers in recent years. ILs with amphiphilicity named ionic liquid surfactants (IL-saa) can form self-assembled structures. This combination of ILs and surfactant brings unique properties of ILs into colloid field. To keep in pace of green chemistry progress, greener IL-saa will be the focus of study in the future. Based on this purpose, in this dissertation, we synthesized new series of halogen-free IL-saa and investigated their aggregation behavior in aqueous solution. There are three main parts in this dissertation as follows:1. We acquired1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12SO4]) by ion exchange reaction. Its aggregation behavior was studied by surface tension, conductivity, steady-state fluorescence quenching, and1H-NMR measurements. To evaluate effect of character of organic counterion on the surface activity of [C4mim][C12SO4], N-butyl-N-methylpyrrolidinium dodecylsulfate ([C4MP][C12SO4]) was synthesized and the contrast between the micellization behavior of [C4mim][Ci2SO4] and [C4MP][C12SO4] has been made in this dissertation. In addition, salts effect on the aggregation behavior of [C4mim][C12SO4] was examined. The following three conclusions have been summarized based on the experimental results.(1)[C4mim][C12SO4] and [C4MP][C12SO4] show micelle characteristics superior to traditional surfactant sodium dodecyl sulfate (SDS) and conventional imidazolium-based SAIL, C12mimBr and C12MPB, with the same hydrophobic chain length, e.g., lower cmc and higher Πcmc. values. The1H NMR results for [C4mim][C12SO4] verified the mechanism of micelle formation and the micellar structure. Dodecyl aggregates into micelle while imidazolium cation contacts to anion headgroup on the surface of micelle by electrostatic and hydrophobic interaction and butyl on imidazolium cation penetrates into the micelle.(2) Differences for [C4mim]+and [C4MP]+, such as hydrophobicity, size, and ability to form hydrogen-bond result in the surface activity distinctions between [C4mim][C12SO4] and [C4MP][C12SO4]. Compared with [C4MP][C12SO4], the smaller cmc and γcmc of [C4mim][C12SO4] can be attributed to the delocalized positive charge on the imidazolium ring and the ubiquitous hydrogen bonds among the imidazolium cations. (3) Both inorganic salts (LiCl, NaCl and MgCl2) and organic salts ((CH3)4NBr,(C2H5)4NBr,(C3H7)4NBr, and (C4H9)4NBr) investigated can promote the surface activity of [C4mim][C12SO4] in aqueous solution. For inorganic electrolytes, hydrated radius and valance of ions are the main factors which influence surface activity of surfactants. While for organic electrolytes, the condition is complex. Based on the experimental results, when alkyl chain changes to butyl, tetrabutylammonium cation adsorbs at the interface to reduce surface tension of water. Upon effect of ions concentration, we also did some work and achieved a conclusion that values of cmc and γcmc decrease with the increase of ions concentration. Beyond that, we found that the size and aggregation number of [C4mim][C12SO4] micelles were not affected obviously by the additional electrolytes.2. In this part, we investigated influence of active group characteristic on the micellar properties of IL-saa.1-Butyl-3-methylimidazolium bis(2-ethylhexyl) sulfosuccinate (C4mim-AOT) and1-butyl-3-methylimidazolium bis(2-ethylhexyl) phosphate (C4mim-DEHP) which having double alkyl chains.Experimental results show that properties of these IL-saa are different remarkably from that of [C4mim][C12SO4], indicating that imidazolium cation can only improve the surface activity of IL-saa which was dominated by the active group. Quantum chemistry calculation results show that the higher electronegativity of the polar head group DEHP anion and larger amount of molecular conformations for C4mim-DEHP can be responsible for the higher aggregation number and cmc values.3. Length of alkyl chain is another important factor which influences the aggregation behavior of IL-saa. It is especially meaningful to investigate effect of alkyl chain length on the aggregation behavior of IL-saa with surface active groups both on anion and cation. Several1-alkyl-3-methylimidazolim alkyl sulfates, designated by [Cnmim][CmS04], where n=6,8,10(fixed m=12) and m-10,14(fixed n=4), were synthesized and their self-assembly features at low concentration range dependent on the chain length of the catanionic IL-saa were investigated. It is found that either increasing the alkyl chain length on the cation or anion (with a fixed another alkyl chain on anion or cation) of IL-saa can lead to lower cmc values and higher effectiveness in surface tension reduction. Compared with the shorter alkyl chain, addition methylene group to the larger alkyl chain makes a greater influence on the surface activity of [Cnmim][CmOO4].