Low-temperature Heat Capacity Research Of Four Natural Organic Anionic Imidazolium Ionic Liquids

The raw material selection and synthesis methods of natural organic anion ionic liquids all meet the requirements of green chemistry,in recent years,such ionic liquids have been extensively studied and applied in many fields such as bio-enzymatic catalysis,separation and purification,and chemical reaction.Studying their properties is the basis for their application,and exploring their thermochemical stability is a top priority.In this paper,the low-temperature heat capacity of four natural organic anion imidazole-based ionic liquids were measured using a high-accuracy automatic adiabatic calorimeter,and the relevant thermodynamic functions were calculated based on the heat capacity.Firstly,four kinds of natural organic anion imidazole-based ionic liquids were synthesized:1-butyl-3-methylimidazolium lactate ionic liquid[Bmim][Lact],1-butyl-3-methylimidazoliumglutamateionicliquid[Bmim][Glu],1-pentyl-3-methylimidazoliumglycinateionicliquid[Pmim][Gly],1-hexyl-3-methylimidazolium glycinate ionic liquid[Hmim][Gly].And the products were characterized by ¹H NMR,DSC and TG.Before measuring the heat capacity of the sample,the cryogenic thermal calorimeter is first calibrated with null calibration and?-Al₂O₃ calibration to ensure the accuracy and reliability of the measured data.Then the low-temperature heat capacity of the 1-butyl-3-methylimidazolium lactate ionic liquid in the temperature range of 80 K to 400 K,the 1-butyl-3-methylimidazolium glutamate ionic liquid in the temperature range of 86 K to 400 K,the 1-pentyl-3-methylimidazolium glycinate ionic liquid in the temperature range of 87 K to 400 K and the1-hexyl-3-methylimidazolium glycinate ionic liquid in the temperature range of 86 K to 400 K were measured with a high accurate automatic adiabatic calorimeter.The least square method was used to fit the measured values to the polynomial fitting equations of these ionic liquids,and the relative deviation between the measured values and the fitted values were obtained.Based on the heat capacity data,H_T-H_298.15,S_T-S_298.15 and G_T-G_298.15 were calculated for the four ionic liquids in the 298.15 K to400 K temperature range.The influence of the anion volume on the heat capacity was discussed and also the effect of the same anion on the chain length of the cation substituent on the heat capacity.