Preparation,Physicochemical Properties And Heat Transfer Application Of Triazolium Acetyl Amino Acid Ionic Liquids

Ionic liquids have been widely used in many fields because of their special physicochemical properties.The imidazolium cation is the most widely used cation in the synthesis of ionic liquids,and the amino acid anion is a natural green non-toxic anion.However,in some environments,the acidic protons on the carbon of the imidazole cation position 2 will be removed to form stable carbine,and the thermal stability of amino acid ionic liquids is generally relatively low.As an improvement of chemical stability and thermal stability of imidazolium amino acid ionic liquids,nine novel triazolium acetyl-amino acid ionic liquids were designed and synthesized in this work:1-ethyl-4-alkyl-1,2,4-triazolium acetyl glycine ionic liquids[Taz（2,n）][Acgly]（n=4,5,6）,1-ethyl-4-alkyl-1,2,4-triazolium acetyl alanine ionic liquids[Taz（2,n）][Acala]（n=4,5,6）,1-ethyl-4-alkyl-1,2,4-triazolium acetyl cysteine ionic liquids[Taz（2,n）][Accys]（n=4,5,6）.The ionic liquids were characterized by ¹H NMR,¹³C NMR,FT-IR and TG,and were determined to be the target products with purity greater than 99%,which met the requirements for determination of physicochemical properties.Based on the hydrophilic properties of amino acid ionic liquids,the density,surface tension and refractive index of ionic liquids[Taz（2,n）][Acgly],[Taz（2,n）][Acala]and[Taz（2,n）][Accys]（n=4,5,6）were measured by the standard addition method in the temperature range of 288.15 to 323.25 K.The contribution of methylene group and sulfur atom to the molecular volume,standard entropy and lattice energy in the volume properties were calculated and discussed respectively by using the measured data.Moreover,the change rules of surface entropy,surface energy,speed of sound,thermal expansion coefficient,molar polarization and molar susceptibility in the surface properties and polarization properties were calculated.The definition of the molar surface Gibbs free energy was substituted into the Lorentz-Lorenz relationship,an equation for predicting surface tensions of ionic liquids was obtained,and the predicted values are highly correlated with the experimental data.In addition,combining the vaporisation enthalpy model with Glasser’s lattice energy theory,a method of predicting the ion pair association enthalpy in the gas phase was proposed by using thermodynamic cycle,which helps to understand the interaction mode between anions and cations in ionic liquids.An equation of the scale for ionic liquids polarityδ_μwas calculated and compared.In the temperature range of 77-390 K,the ionic liquids[Taz（2,n）][Acgly],[Taz（2,n）][Acala]and[Taz（2,n）][Accys]（n=4,5,6）were measured with a high-accuracy automatic adiabatic calorimeter imported from Dalian Institute of Chemical Physics Chinese Academy of Sciences.Before the measurement,the calorimeter should be calibrated by reference standard materialα-Al₂O₃ and the accuracy of the calorimeter is±0.147%,which is in accordance with the measurement standard.Then,the least square method was used to fit the experimental heat capacities,and the polynomial fitting equation of nine ionic liquids were obtained,and the relative deviation between the fitting heat capacities and the experimental heat capacities was calculated.Finally,H_T-H_298.15,S_T-S_298.15 and G_T-G_298.15 of the nine ionic liquids in the temperature range of80-400 K were calculated,and the influence of the methylene group and sulfur atom on the heat capacity of the ionic liquids was discussed.In addition,combined with the density,surface tension,and molar heat capacity data of ionic liquids[Taz（2,n）][Acgly],[Taz（2,n）][Acala]and[Taz（2,n）][Accys]（n=4,5,6）,the derivative properties,namely isobaric thermal expansion coefficient,isothermal compression coefficient,isentropic compression coefficient,sound velocity,isobaric molar heat capacity and heat storage density,are compared and analyzed.The thermal conductivity values of nine ionic liquids at 293.15-323.15 K were measured by using the plane heat source method.Two different thermal conductivity prediction models were introduced and their applicability for triazolium acetyl amino acid ionic liquids was discussed.The kinematic viscosity values of[Taz（2,4）][Acgly],[Taz（2,4）][Acala]and[Taz（2,4）][Accys]ionic liquids with relatively small viscosity were measured at323.15-353.15 K using capillary viscometers.And the kinematic viscosity is converted into the corresponding dynamic viscosity value by density.Then,we selected a specific heat transfer process and calculated the heat transfer coefficient and the heat transfer area of ionic liquids[Taz（2,4）][Acgly],[Taz（2,4）][Acala],[Taz（2,4）][Accys]and[Taz（2,4）][Acgly]+1.768 wt%water based on the thermophysical properties in the temperature range of 323.15-353.15 K.Finally,the possibility of the abovementioned ionic liquids as heat transfer fluids was explored by comparing the measured and calculated results with ionic liquids widely studied as heat transfer fluids and commonly used commercial heat transfer fluids.