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.