Study On The Thermodynamic Properties Of [Hemim][Ac]、[Hemim][Pro]、[Ch][Ac] And [Ch][Pro] Ionic Liquids

Ionic liquids have been extensively studied as green solvents to replace traditional organic solvents due to their unique and adjustable properties,such as wide liquid range,high ionic conductivity,and negligible vapor pressure.They have been applied in various fields such as batteries,fuel cells,additives,and chemical reaction catalysts.Among them,the thermal stability of ionic liquids plays a major role in the application of ionic liquids.However,the thermal stability of ionic liquids varies with the size of anions and the type of cations.Therefore,this paper synthesized two hydroxyl functionalized ionic liquids,1-（2-hydroxyethyl）-3-methylimidazole acetate[Hemim][Ac],1-（2-hydroxyethyl）-3-methylimidazole propionate[Hemim][Pro],and two choline ionic liquids,choline acetate[Ch][Ac]and choline propionate[Ch][Pro].Characterization of four ionic liquids[Hemim][Ac],[Hemim][Pro],[Ch][Ac],and[Ch][Pro]by ¹H NMR and ¹³C NMR using nuclear magnetic resonance technology.By analyzing the spectrum,it can be concluded that the products are the target ionic liquids.The mass fractions of C,H and N in four types of ionic liquids were determined using an elemental analyzer.The decomposition temperatures of four ionic liquids[Hemim][Ac],[Hemim][Pro],[Ch][Ac]and[Ch][Pro]were determined using dynamic thermogravimetric experiments,at 246℃,241℃,215℃and 212℃,respectively.Four types of ionic liquids[Hemim][Ac],[Hemim][Pro],[Ch][Ac]and[Ch][Pro]were tested through constant temperature thermogravimetric experiments.The Langmuir equation was reorganized to obtain a new equation expression,and then the vapor pressure changes of four ionic liquids[Hemim][Ac],[Hemim][Pro],[Ch][Ac],and[Ch][Pro]with temperature were calculated and analyzed using the new equation expression.By combining the Langmuir equation with the Clausius-Clapeyron equation,a new formula can be obtained to calculate the evaporation enthalpy.Based on the experimental data of evaporation enthalpy,we conclude that the value of evaporation enthalpy is related to the types and sizes of anions and cations.When the cations are the same,the smaller the relative molecular weight of carboxylate anions,the larger the（35）_l^gH_m^?of the ionic liquids,such as（35）_l^gH_m[Hemim][Ac]^?>（35）_l^gH_{m[Hemim][Pro]}^?,（35）_l^gH_m[Ch][Ac]^?>（35）_l^gH_m[Ch][Pro]^?.When the anions are the same,the hydrogen bonding and electrostatic attraction of hydroxyl functionalized ionic liquids are greater,the larger the（35）_l^gH_m^?of the ionic liquids,such as（35）_l^gH_m[Hemim][Ac]^?>（35）_l^gH_m[Ch][Ac]^?,（35）_l^gH_m^?[Hemim][Pro]>（35）_l^gH^?m[Ch][Pro].Clausius-Clapeyron equation can predict the normal boiling point T_b,and can also calculate the evaporation enthalpy（35）_l^gH_m^o（T_i）,Gibbs free energy（35）_l^gG_m^o（T_i）and the evaporation entropy（35）_l^gS_m^o（T_i）,and analyze the obtained data.In the temperature range from the ambient temperature T=298.15 K to the normal boiling point T_b,the vaporization enthalpy（35）_l^gH_m^o（T_i）,Gibbs free energy（35）_l^gG_m^o（T_i）and the vaporization entropy（35）_l^gS_m^o（T_i）of the four ionic liquids at any temperature T_i have the same trend of change with temperature,and all show a gradual decrease with the increase of temperature.