Phase behaviour and heat capacities of selected 1-ethyl-3-methylimidazolium-based ionic liquids

Ionic liquids start to leave the position of novel and astonishing compounds and become commercially available in a relatively high purity. Consequently, their thermodynamic properties should be described in a more thorough way than in the times of first pioneering studies revealing the importance of the water content. Available thermodynamic data, however, contradict this assumption keeping large discrepancies even among the data published in the most recent papers. Eight common ionic liquids based on the 1-ethyl-3-methylimidazolium cation, [C1C2Im], which are coupled with simple organic or inorganic anions are selected in this work and their phase behaviour, heat capacities, and crystal structures are studied. Special attention is paid to drying of the samples and evaluation of the effect of water content on the melting temperature. Resulting melting temperatures, fusion enthalpies, and heat capacities are compared to the available literature data. For the compounds, which melt above the room temperature, crystal structures were determined using the X-ray powder diffraction experiments to identify the studied crystalline phases. The thermodynamic and crystallographic data were used simultaneously to interpret the polymorphism of the investigated ionic liquids, where it occurred. Moreover, a crystal structure for the dimethyl phosphate salt [C1C2Im][Me2PO4] is reported for the first time.