Application of calorimetry and thermal analysis to determine the liquid range and the environmental toxicity of ionic liquids

Abstract

This PhD Thesis is focused on the applicability of thermal analysis and calorimetry techniques to determine phase transitions, thermal stability and ecotoxicity of ionic liquids (ILs) to be used as lubricants and/or absorbents in heat pumps. The studied compounds are based on different anions (imide, triflate, sulphonate, phosphate) and cations (phosphonium, pyridinium, pyrrolidinium, imidazolium, choline) in order to determine possible dependences of these properties according to a selected ion. Phase transitions: glass transition (tg), cold crystallization (tcc), solid-solid transition (tss) melting (tm), and freezing (tf) temperatures were determined through Differential Scanning Calorimetry on the interval temperature of (-75 to 120) ºC. All the selected ILs presented an endothermic peak attributed to melting point, indicating crystalline phase presence for all of them. Thermal stability was measured by Thermogravimetric Analysis (TGA). Due to the extensive variability on previous bibliographic reports, first of all, a broad study on the experimental conditions dependence has been done, reporting that thermal stability is higher on Nitrogen atmosphere and the associated degradation temperature increase whenever the heating rate decreases. To determine the real thermal stability, dynamic and isothermal methods have been combined, due to the overestimation of dynamic scans on decomposition temperatures. Dynamic scans showed that [C4C1C1Im][OTf] has the highest (436 ºC) and [C2C1Im][C6SO4] the lowest tonset values for the selected ILs, meanwhile, on lubricant bases, DiPEC7 presents a tonset of 318 ºC and 3 of the lubricant bases show lower tonset values tan ILs. Nevertheless the maximum operation temperature of the selected ILs depends on the exposition time, and it is between 150 ºC and 200 ºC lower than the corresponding onset temperatures. Activation energy was determined by isothermal methods for most selected ILs. In order to validate the obtained these results, kinetic dynamic analysis was also applied to one of the aforementioned ILs ([C4C1C1Im][NTf2]), obtaining very similar values. ILs ageing effect was also analysed with this technique, showing that the successive repetition of heating and cooling cycles, does not affect the thermal stability of the sample, if the temperature limit of these cycles is below the maximum operation temperature previously calculated. From the results of both techniques, the liquid range of the selected ILs was estimated using as limits of this interval tm and tonset/t´0.10/10h. Liquid range temperature values were determined since 145 ºC for [C2py][C1SO3] to 299 ºC for [C4C1Pyrr][NTf2] for the selected ILs, and 208 ºC for Krytox GPL 105. With the aim to stablish a protocol of actuation, ecotoxicity of two of the most studied ILs was analysed; microbial activity of soils was studied by isothermal microcalorimetry, Thermal Activity Monitor (TAM), obtaining those doses of 10% of a water solution of these ILs provoked a big stress on microorganisms of the selected soils, followed by their death. These results were also compared with that of seed germination test after adding the same doses of both ILs.