Acta Phys. -Chim. Sin. ›› 2018, Vol. 34 ›› Issue (2): 194-200.doi: 10.3866/PKU.WHXB201707262

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Measurement of Vapor Pressure and Vaporization Enthalpy for Ionic Liquids 1-Hexyl-3-methylimidazolium Threonine Salt[C6mim][Thr]by Isothermogravimetric Analysis

Jing TONG*(),Ye QU,Liqiang JING,Lu LIU,Chunhui LIU   

  • Received:2017-06-16 Published:2017-11-13
  • Contact: Jing TONG E-mail:tongjinglnu@sina.com
  • Supported by:
    the National Natural Science Foundation of China(21273003)

Abstract:

The amino acid ionic liquid (AAIL) 1-hexyl-3-methylimidazolium threonine salt, [C6mim][Thr], was prepared by the neutralization method and its structure was confirmed by 1H and 13C NMR spectroscopy. Using benzoic acid as the reference material, the vapor pressure and evaporation enthalpy ΔglHm? (Tav) of [C6mim][Thr] were determined by isothermogravimetric analysis at the average temperature (Tav = 438.15 K) and DglHm? (Tav)was found to be (128.5 ± 6.0) kJ·mol-1. Using Verevkin's method, the difference between the heat capacities of the vapor and liquid phases, ΔglCpm?, was calculated to be -70.8 J·K-1·mol-1. Subsequently, the enthalpy of vaporization for AAIL [C6mim][Thr] at different temperatures was determined based on the reference enthalpy of vaporization at 298.15 K, ΔglHm? (298.15 K) = 138.4 kJ·mol-1. This value is 1.6 kJ·mol-1 higher than that predicted by our theoretical model and less than the experimental error (±3.0 kJ·mol-1) of the isothermogravimetric method. These results show that our theoretical model for determining the evaporation enthalpy of ILs is reasonable. In terms of the Clausius-Clapeyron equation, the hypothetical normal boiling point, Tb, was estimated to be 522.07 K. Thus, the evaporation entropy, ΔglSm? (T), and the evaporation Gibbs free energy, ΔglGm? (T), of [C6mim][Thr] could be determined for different temperatures. These results showed that DglGm? (T) decreases as the temperature increases, the evaporation entropy increases with increaseing temperature. Furthermore, the latter is the driving force in the evaporation process of the AAIL [C6mim][Thr].

Key words: Ionic liquid, Isothermogravimetrical analysis, Enthalpy of vaporization, Vapor pressure, Threonine

MSC2000: 

  • O642