Estimation Of Thermodynamic Properties Of Ionic Liquids [C_nmim]Cl(n=2,4,6) Using The Isothermogravimetrical Analysis Method

Ionic liquids have been in existence for more than a century since they were first discovered in 1914.With further research,the development of ionic liquids have become faster and the types have become more complex.Ionic liquids have unique physical and chemical properties and have attracted wide attention in various fields.In order to better meet the practical application,it is necessary to accurately describe the thermophysical properties of the ionic liquids.Therefore,it is a very meaningful job to study the physicochemical properties of ionic liquids and to supplement the basic data of thermodynamic properties of ionic liquids.In this paper,the experimentally synthesized ionic liquids[C,mim]Cl(n = 2,4,6)were characterized by 1H NMR,13N NMR and thermogravimetric analysis to confirm the target products.Using benzoic acid as the reference material,the vapor pressure and the molar evaporation enthalpy at the average temperature of the ionic liquids[Cnmim]Cl(n = 2,4,6)were determined by isothermogravimetrical analysis method.The molar evaporation enthalpy at average temperature of was converted to the molar evaporation at 298.15 K according to the method proposed by Verevkin et al.The calculation results show that the contribution of each methylene group(-CH2-)in the alkyl chain of the imidazole-based ionic liquids is 4.55 kJ·mol-1,which is not much different from the values in the literature.In this paper,the assumed normal boiling point Tb of the ionic liquids[C,mim]Cl(n = 2,4,6)were also estimated by using the Clausius-Clapeyron equation,and the molar evaporation entropy at the normal boiling point was obtained.It is assumed that the molar evaporation entropy at normal boiling point can also be converted to the molar evaporation entropy at any temperature by means of the difference in heat capacity of the ionic liquids,thereby the molar evaporation Gibbs free energy at any temperature can be calculated.The molar energy of the Gibbs evaporation decreases with increasing temperature,reaching zero at the assumed boiling point.The molar evaporation enthalpy decreases with the increase of temperature,but the molar evaporation entropy increases with the increase of temperature.This fact indicates that the molar evaporation entropy is the driving force for the evaporation process of the ionic liquids[Cnmim]Cl(n = 2,4,6).