(257ba) Isobaric Heat Capacity of 1-Hexyl-3-Methylimidazolium Acetate

Due to the unique physical and chemical properties, such as negligible volatility, high thermal conductivity, chemical and physical stability, ionic liquids (ILs) are considered to be a new kind of green solvents which can replace traditional solvents in reaction chemistry and separation technology. Isobaric heat capacity is related to several other thermodynamic properties, such as entropy, enthalpy and Gibbs free energy. Knowledge of heat capacity and its relationship to thermodynamic properties enables the study of other thermophysical properties. The c_p data are also used routinely for heat-transfer calculations for chemical engineering unit operations such as heat exchangers and reactors. Accurate heat capacities of ILs are required for applying ILs in reaction and separation process. However, for most ILs, the c_p data are lacking, especially in high pressure regions.

In this work, a flow calorimeter was developed to measure the isobaric heat capacities of ILs in temperature range from 293-473K and pressure up to 20MPa. The combined measurement uncertainties of temperature and pressure were 0.025K and 9kPa, respectively. The expended combined relative uncertainty of the isobaric specific heat capacity was estimated to be 0.98% with the coverage factor of k=2. In order to verify the accuracy of the experimental system, isobaric heat capacities of pure water in the temperature range from 296K to 373K and pressure up to 16MPa were investigated. The maximum deviation of the comparison result of experimental data and the calculated values from the IAPWS-95 Formulation is 0.73%, which demonstrates the accuracy of the experimental system. And then, experimental isobaric heat capacity of 1-hexyl-3-methylimidazolium acetate was measured in the temperature range from 303K to 393K along five isobars (0.1, 4, 8, 12 and 16 MPa). Based on the experimental results, a correlation equation is proposed for predicting the isobaric heat capacity of 1-hexyl-3-methylimidazolium acetate.