Influence of anion and cation on the vapor pressure of binary mixtures of water + ionic liquid and on the thermal stability of the ionic liquid

Abstract

Since two decades, water+ionic liquids (ILs) are considered as new working pairs for absorption cycles. To estimate the performance of the cycles, thermophysical properties, and especially the vapor liquid equilibrium (VLE) behavior has to be known. Therefore, the vapor pressure of the possible working pairs water+1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][OMs]), water+1-ethyl-3-methylpyridinium methanesulfonate ([E3MPy][OMs]) and water+1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf]) have been measured over the whole concentration range and in the temperature range of T=(293.15–353.15)K. The vapor pressure data have been compared with the ones of the three binary mixtures water+diethylmethylammonium methanesulfonate ([DEMA][OMs]), water+diethylmethylammonium trifluoromethanesulfonate ([DEMA][OTf]) and water+1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]). The results indicate that the anion is essential for the vapor pressure reduction of an IL. The water affinity decreases in the following order [OAc]−>[OMs]−>[OTf]−. As the performance of an absorption cycle is mainly affected by a high vapor pressure reduction, this result leads to a first selection of possible absorbents. The vapor pressure measurements were used to estimate temperature-dependent NRTL coefficients. These parameters have been employed to predict the excess molar enthalpies of the binary mixtures. A comparison with experimental data found in literature shows a good agreement. Besides the thermodynamic suitability, the thermal stability of the working pair is essential for the possible use in absorption cycles. Therefore thermogravimetric analyses (TGA) are also presented and indicate that water+[EMIM][OMs] and water+[E3MPy][OMs] could be considered as new working pairs in absorption heat transformers.