1-烷基-3-甲基咪唑氯化物焓变的热重分析

Thermogravimetric Analysis of Enthalpy Variation of 1-Alkyl-3-methylimidazole Chloride

The structures of ionic liquids (ILs) based on 1-alkyl-3-methylimidazolium chloride C_nmim]Cl (n = 2, 4, 6), (1-ethyl-3-methylimidazolium chloride C₂mim]Cl, 1-butyl-3-methylimidazolium chloride C₄mim]Cl, and 1-hexyl-3-methylimidazolium chloride C₆mim]Cl) were elucidated by ¹H NMR and ¹³C NMR experiments. The vaporization characteristics of these ILs were studied by thermogravimetric analysis. Dynamic and isothermal thermogravimetric experiments were conducted in this study. The purpose of the dynamic experiments was to determine the initial decomposition temperature of the experimental sample and the temperature range for the isothermal thermogravimetric experiments. The purpose of the isothermal experiments was to record the mass dependence of the sample on time in the experimental temperature range. The Langmuir equation and Clausius-Clapeyron equation were used to fit the experimental data and obtain the vaporization enthalpies of these ILs at the average temperature within the experimental temperature range. However, in order to expand the applicability of the estimated values and to compare them with the literature data, the vaporization enthalpy ΔH_vap(T_av) measured at the average temperature was converted into vaporization enthalpy ΔH_vap(298) at ambient temperature. The difference between the heat capacities of the ILs in the gaseous and liquid states at constant pressure, Δ_l^gC_pm^? proposed by Verevkin, was used in this conversion process. The experimental data for substance density and surface tension at other temperatures were obtained by referring to the literature. In addition, the data for density and surface tension at T = 298.15 K were obtained by applying the extrapolation method to the literature values for other temperatures. The vaporization enthalpy of the 1-octyl-3-methylimidazolium chloride IL C₈mim]Cl was estimated by using the new vaporization model we had proposed in our previous work and compared with the reference value. The estimated value for C₈mim]Cl was on the same order of magnitude as the reference value. We compared the vaporization enthalpies in the present study with those for the carboxylic acid imidazolium and amino acid imidazolium ILs (C_nmim]Pro (n = 2-6) and C_nmim]Thr (n = 2-6), respectively in our previous work. The results revealed that a change in the anion type affects the vaporization enthalpy of the ILs in the order amino acid imidazolium > carboxylic acid imidazolium > halogen imidazolium, when the cation is the same. Considering the structural differences between the three kinds of ILs, the abovementioned order may be related to the intermolecular hydrogen bonds. There were no intermolecular hydrogen bonds in the C_nmim]Cl (n = 2, 4, 6) ILs studied here. Therefore, the vaporization enthalpy of C_nmim]Cl (n = 2, 4, 6) was the lowest among the three kinds of ILs considered.