3.3 Thermodynamics and Enthalpy Change Estimation.
The enthalpy change, ΔH , is investigated from the effect of temperature on the absorption of CO2 in [N2222][CH(CN)2]-Eim and [N2222][CH(CN)2]. The solubility of CO2 in the DES or in the IL is determined and found to decrease with the increasing temperature (Figure 4a, b, the detailed solubility data are given in Table S6-7 ), in consistence with the fact that the chemical absorption of CO2 is exothermic 43,44. With the assumption of equimolar reaction of CO2 with the DES or IL (supported by the NMR and FTIR results), a reactive phase equilibrium model (RPEM) is developed to fit the experimental solubility of CO2 (see Supporting Information for model derivation). The model represents the experimental data quite well (see lines in Figure 4a, b ), and ΔH is calculated to be -39.6 kJ/mol in the DES and -41.8 kJ/mol in the IL (Figure S6 ). The experimental enthalpy change of -41.8 kJ/mol in [N2222][CH(CN)2] is slightly more negative than the theoretical value of -35.33 kJ/mol in [P66614][CH(CN)2] calculated by Dai’s group [14], due to the differences in the cation of IL and also in the methodology of calculation. Nevertheless, the enthalpy changes both in the DES and IL are moderate, favoring both the absorption and desorption of CO2.