Thermodynamic investigation of room temperature ionic liquid: The heat capacity and standard enthalpy of formation of EMIES

The molar heat capacities of the room temperature ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES) were measured by an adiabatic calorimeter in temperature range from 78 to 390 K. The dependence of the molar heat capacity on temperature was given as a function of the reduced temperature X by polynomial equations, Cp,m (J K−1 mol−1) = 178.6 + 50.28X + 2.886X2 − 1.362X3 + 0.6616X4 + 7.155X5 [X = (T − 132.5)/54.5] for the solid phase (78–187 K) and Cp,m (J K−1 mol−1) = 376.2 + 25.94X − 3.397X2 − 0.6407X3 + 0.8091X4 + 0.9869X5 [X = (T − 292.5)/97.5] for the liquid phase (195–390 K), respectively. According to the polynomial equations and thermodynamic relationship, the values of thermodynamic function of the EMIES relative to 298.15 K were calculated in temperature range from 80 to 390 K with an interval of 5 K. The glass translation of EMIES was observed at 192.85 K. Using oxygen-bomb combustion calorimeter, the molar enthalpy of combustion of EMIES was determined to be ΔcHm°=−5152.6±4.6 kJ mol−1. The standard molar enthalpy of formation of EMIES was evaluated to be ΔfHm°=−579.13±0.51 kJ mol−1 at T = 298.150 ± 0.001 K.