Evaporation thermodynamics of 1‐butyl‐3‐methylimidazolium tetrafluoroborate ionic liquid

Abstract

Thermodynamic properties of evaporation of 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMImBF4) ionic liquid (IL) were determined by Knudsen effusion mass spectrometry. Heating of this compound is accompanied by competing evaporation and decomposition. Interpretation of mass spectrum was carried out by the analysis of data obtained for different evaporation conditions: i) Knudsen cell with a small effusion orifice, ii) open Knudsen cell, and iii) open surface of IL. It was revealed that at non-equilibrium Langmuir conditions BMImBF4 IL evaporates congruently practically only in the form of neutral ion pairs (NIPs). In equilibrium conditions, decomposition products prevail in vapor with only about 7 % vapor fraction of NIPs. The main route of IL decomposition is the formation of substituted imidazole‐2‐ylidene (C8N2H14BF3) and hydrofluoric acid (HF). The mass spectrum of C8N2H14BF3 was obtained by the analysis of a distillate collected during evaporation in the Knudsen cell. The decomposition products, due to their high volatility, do not accumulate practically in the liquid phase and hence the activity of IL may be considered to be 1. This circumstance makes it possible to perform the thermodynamic calculations for the IL evaporation reaction. The temperature dependence of the NIPs’ vapor pressure, ln(p/Pa) = (–13.806 ± 0.252)∙103/T + (23.443 ± 0.519), was used to obtain the vaporization enthalpy, ΔvH°(298.15 K) = 138 ± 5 kJ·mol−1, by the methods of the second and third laws. The formation enthalpy of BMImBF4 NIPs, ΔfH°(298.15 K) = −1084 ± 15 kJ·mol−1, was found from combination of the vaporization enthalpy and the formation enthalpy of IL. The vapor composition over BMImBF4 under equilibrium conditions was computed. It was demonstrated that decomposition of IL is kinetically hindered so the studied system is characterized by a partial equilibrium.