Investigations of structure property relationships of highly hygroscopic Imidazolium and Choline based ILs for the prediction of thermodynamic trends

Abstract

Abstract:1,3-alkyl-imidazolium-based ionic liquids show a wide range of applications from use as a "designer solvent", in catalysis 1-3 up to material science [4][5][6] . The interaction between cation and anion can be investigated by analyzing the crystal structure.7 In our work, we synthesized various 1,3-functionalized imidazolium and choline based Ils for the use as drying agent in sorption systems. Here especially substances with organic anion such as acetate, lactate and formate were quiet interesting because they expected, among other beneficial properties, a low melting point and a strongly pronounced hygroscopic behavior. However, the purification of the synthesized compounds showed a number of previously unknown crystal structures, which allowed to examine the interaction of cations and anions could be examined. In addition, the crystal structure data provided the molecular volume of the ions, which is used to predict thermodynamic aspects. Synthesis: Scheme 1For the use in our systems, it is necessary to have liquid media, which can be pumped and preferably does not crystallize during regeneration. As a result, especially RTILs are an interesting approach to avoid this problem. The known requirements reduce the possible anions mainly to the organic ones, esp. formate, acetate, lactate and oxalate. It is understandable, that the choice of the cations results from the same considerations as for the anions. This means a relative short alkyl chain length or hydrophilic groups like alcohols or carboxylic acids attached to the 3-position in 1-methyimidazole (Scheme 1).Chart 1