An ionic liquid as polar phase in low-temperature-stable microemulsions

The present work is divided in four main sections. The aim of the thesis was to formulate low-temperature-stable microemulsions. This comprises structured liquid systems that do not exclusively exist at temperatures around 25 °C but also at temperatures that are stable well below 0 °C. For this purpose water, the conventional polar phase in microemulsions was replaced by the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([emim][etSO4]) comprising a glass transition temperature of -80 °C. In addition, a natural oil namely limonene that exhibits a very low melting point of -96 °C was incorporated in the microemulsions. The polar and apolar phase were kept constant and the effect of surfactant nature was studied. Three non-ionic amphiphiles have been used, Triton X 100 (HLB = 13.5), Triton X 114 (HLB = 12.4) and C10E4 (HLB = 10.6). The first two surfactants are commercial available surfactants with a special EO distribution and the last one is a pure surfactant, synthesized in the lab. The first part of this work deals with the determination of critical aggregation concentrations (cac) of the used surfactants in water and the utilized ionic liquid, [emim][etSO4]. The obtained values in water and ionic liquid are compared to each other and to values that were found in literature. As the cacs in ionic liquids are in general significantly higher than in water, this effect is discussed and explained. The following part of the work comprises the establishment of a microemulsion system including the amphiphile Triton X 100. The system was characterized at ambient temperatures and temperatures down to -10 °C by means of small angle X-ray scattering, conductivity, viscosity and phase diagrams. The system shows a remarkable wide temperature range where microemulsions exist, for high IL-contents between (-10 and 40) °C. The hydrophilic lipophilic balance (HLB) is a measure of the degree of hydrophilicity and lipophilicity of a surfactant and results for Triton X 100 in a value of 13.5. The question arises which effect the hydrophobicity of the surfactant has on the formulation of low-temperature-stable microemulsions including an ionic liquid as polar phase. Consequently, the third part of this work deals with the effect of surfactant hydrophobicity of non-aqueous microemulsions that should be stable well below 0 °C. Therefore, the system composed of [emim][etSO4], limonene, and Triton X 114 (HLB = 12.4) was studied. The main difference between both surfactants is the number of ethyleneoxide groups. Triton X 100 comprises in average 9.5 EO groups and Triton X 114 in average 7.5 EO groups. Thus, this exchange is an increase in surfactant hydrophobicity (lower HLB value). The two microemulsion systems have been compared. The main difference is the increased temperature range in which Triton X 114 microemulsions can be applied. Microemulsions with high IL-content do not undergo a phase separation or freezing down to -35 °C. This value indeed is limited by the enormous viscosity at those temperatures. Finally, the next goal was to formulate systems that exhibit lower viscosities and can be applied therefore as reaction media at low temperatures. Further, the assumption is obvious that a lower HLB value of the surfactant would lead to even better temperature stabilities. The last part of this work describes the formulation and characterization of microemulsions containing the amphiphile C10E4. This surfactant was synthesized in the lab and is therefore of high purity in contrast to the previous used amphiphiles. This and the resulting low viscosities enable these microemulsions for their application as reaction media. Finally, this has been exemplarily shown for some selected reactions. The formation of catalytically active Ru-particles including a subsequent hydrogenation of the oil phase was reported as well as an epoxidation reaction of limonene in o/IL microemulsions. Altogether, this work comprises a detailed study of low-temperature-stable microemulsions with the aim to formulate systems that are suitable for future application, e.g. as reaction media. The studied systems represent some model systems that can be enlarged to other surfactants, ionic liquids or oils, respectively. An important step has been done towards future works on low-temperature-stable microemulsions