2.1.1. IL–H2O

The effect of the addition of H2O in [DMAPAH][Formate] (0.5:1, 1.0:1, 2.0:1, 2.5:1) was studied by Vijayaraghavan et al. [30]. Except for the molar ratio of amine to acid of 0.5:1, CO2 solubility initially increased up to a certain water amount of 20 wt % (Figure 1); then, it steadily decreased as the H2O concentration increased. As shown in Figure 1, the best candidate for CO2 capture is [DMAPAH][Formate] (2.5:1) + H2O (20 wt %) (5.69 mol/kg). The CO2 solubility in [TMGH][Im] + H2O (1–25 wt %) was studied by Li et al. [12]. The result indicates that CO2 solubility first increased at a water content range of 1–7 wt %, and then, it decreased when the water content was larger than 7 wt % in [TMGH][Im], resulting in the best absorption capacity of 4.23 mol/kg for [TMGH][Im] + H2O (7 wt %). Huang et al. reported that a [P4442][Suc] structure with basic anion can improve the reaction of CO2 and [P4442][Suc] aqueous solution by forming bicarbonate and conjugate acid [31]. The result indicates that the CO2 solubility of [P4442][Suc] + H2O (3.3 wt %) (1.9 mol/mol) is slightly higher than [P4442][Suc] (1.87 mol/mol); however, the addition of 8.8 and 17.6 wt % of H2O to [P4442][Suc] has a negative effect on their CO2 solubilities compared with [P4442][Suc]. As shown in Table 1, various compositions of [P4444][HCOO] in water were measured at 0.1 MPa in the temperature range of 248.75–324.65 K for CO2 solubility and compared with [N2224][Ac] and [N2222][Ac] [32]. The results indicate that the CO2 solubility of [P4444][HCOO] + H2O first increased from 0.01 to 1 mol/mol at the water contents range of 29–66 mol% and then decreased with the increase of water contents of 70–91 mol%. Temperature also affects the absorption performance of [P4444][HCOO] + H2O. For example, at 273.15 K, the CO2 solubility of [P4444][HCOO] + H2O (<80 mol%) is higher than [N2224][Ac] + H2O and [N2222][Ac] + H2O, while at 298.15 K, it is higher than [N2224][Ac] + H2O at a water content less than 50 mol%, but it is lower at a water content high than 50 mol%. For 323.15 K, the CO2 solubility in [P4444][HCOO] + H2O (<80 mol%) is lower than [N2222][Ac] + H2O. Nathalia et al. [33] evidenced that there are three processes for CO2 capture in [BMMIM][Im] and [BMMIM][Ac] aqueous solutions, i.e., physical, CO2–imidazolium adduct generation, and bicarbonate formation, resulting in a maximum CO2 solubility of 8.15 mol/mol in [BMMIM][Im] + H2O (99.9 mol%). The CO2 solubility in [P4443][Gly] + H2O (59.9, 80.1, 90, 95 wt %) was measured at temperatures ranging from 278.14 to 348.05 K and pressures of 0.1–7.75 MPa [34]. A feature of physical absorption in these hybrid solvents was observed. The best CO2 solubility of 2.44 mol/kg was acquired for [P4443][Gly] + H2O (59.9 wt %) at 298.06 K and 4.6 MPa. Aghaie et al. tested the CO2 solubilities of [HMIM][Tf2N], [HMIM][FAP], and [BMIM][Ac] aqueous solutions [35]. The result indicates that the CO2 solubility reduced by 45% in these three ILs aqueous solutions compared to the solubility of CO2 in pure IL at 298 K and water content of 10 wt %.

**Figure 1.** Effects of water on CO2 solubility in [DMAPAH][Formate]. The values in Figure 1 are cited from Vijayaraghavan et al. [30]. Copyright 2018 Elsevier.

#### 2.1.2. IL–Organic/Organic Aqueous Solution

Huang et al. investigated the CO2 capture capacity in [TETAH][Lys] + ethanol + H2O [36], finding that the CO2 solubility first increases with the increase of volume ratio of ethanol in water from 10:0 to 5:5 *v*/*v*, and then, it decreases from 5:5 to 2:8 *v*/*v*. Compared with the reported results, the maximum CO2 solubility of 2.45 mol/mol for [TETAH][Lys] + ethanol + H2O (5:5 *v*/*v*) is higher than those of [P6444][Lys] [37], [C2NH2MIM][Lys] [38], and [TETAH][Lys] + H2O. Taheri et al. measured the CO2 solubility of [AMIM][Tf2N] + methanol at temperature and pressure ranges of 313.2–353.2 K and 0.98–6.19 MPa, respectively, indicating that the presence of methanol in [AMIM][Tf2N] enhances the CO2 solubility and results in a maximum of 3.89 mol/mol [39].

In order to overcome the drawbacks of high CO2 capture enthalpy and water volatilization in ILs aqueous solution, PEG200 was introduced in [Cho][Gly] + H2O [17]. CO2 solubility was measured in such solvent at 308.15–338.15 K and pressure lower than 0.68 MPa, and the CO2 desorption enthalpy was estimated. Owing to its physical–chemical properties, [Cho][Gly] (30 wt %) + PEG200 (30 wt %) + H2O (40 wt %) has a higher CO2 solubility (0.41–1.23 mol/kg) and regeneration efficiency (95%) compared with [Cho][Gly] (30 wt %) + H2O (70 wt %). Li et al. found that the addition of PEG200 to [Cho][Pro] not only improves the absorption rate but also enhances the desorption efficient, resulting in a maximum CO2 solubility of about 0.6 mol/mol for [Cho][Pro] + PEG200 with the mass ratios of 1:1, 1:2, 1:3, respectively [40]. PEG400 was introduced to [P4444][Gly], [P4444][Ala], and [P4444][Pro] [41], evidencing that [P4444][Pro] + PEG400 (70 wt %) (1.61 mol/kg) > [P4444][Gly] + PEG400 (70 wt %) (1.58 mol/kg) > [P4444][Ala] + PEG400 (70 wt %) (1.57 mol/kg). The effect of three types of PEG (i.e., PEG200, PEG300, and PEG400) and water content on the CO2 solubility in [DETAH][Br] and [DETAH][BF4] at 293.15 K and 0.1 MPa was investigated by Chen et al. [42]. The result evidenced that the CO2 solubility follows the order of [DETAH][Br] + PEG200 (1.18 mol/mol) > [DETAH][Br] + PEG300 (0.87 mol/mol) > [DETAH][BF4] + PEG200 (0.65 mol/mol) > [DETAH][Br] + PEG300 (0.32 mol/mol) at a mass ratio of [DETAH][Br]:PEG = 1:4. Additionally, the CO2 solubility in [DETAH][Br] + PEG200 + H2O (4.7 wt %) (1.18 mol/mol) is higher than that in [DETAH][Br] + PEG200 + H2O (1.3 wt %) (1.05 mol/mol), which may be because water weakens the interaction of the IL cation and anion and enhances the interaction with CO2. Due to the high CO2 solubility of PEO1000 (0.35 mol/mol, 323 K, 4.98 MPa), it is introduced to [N4111][Tf2N]. A maximum CO2 solubility of 1.16 mol/mol was acquired at 323 K, 4.99 MPa for [N4111][Tf2N] + PEO1000 (75 mol%), which is higher than

that of pure [N4111][Tf2N] (0.14 mol/mol, 323 K, 5 MPa) [43]. Additionally, a higher amount of PEO1000 in [N4111][Tf2N] corresponds to higher CO2 solubility, which is attributed to the strong interaction between CO2 and PEO. However, Jiang et al. [44] found that increasing the molar fraction of TEG in [BMIM][BF4]/[BMIM][BF4] + H2O results in a decrease of CO2 solubilities, which is on the contrary of the result from Lepre et al. [43]. Moreover, with increasing the [BMIM][BF4] contents in [BMIM][BF4] + TEG mixtures, the Henry's constant is increased (Table 2) [44]. The Henry's constant of [BMIM][BF4] + TEG is higher than [BMIM][BF4] but lower than TEG, indicating that CO2 is more soluble in [BMIM][BF4]. The CO2 solubilities of [P66614][3-Triz] + TG (30 mol%) and [P66614][4-Triz] + TG (30 mol%) at 313.15–353.6 K and pressure less than 3 MPa were measured by Fillion et al. [18]. The CO2 solubility of [P66614][4-Triz] + TG (30 mol%) is 2.23 mol/mol at 313.15 K and 3.03 MPa, which is higher than [P66614][3-Triz] + TG (30 mol%) (1.55 mol/mol, 313.15 K, and 2.68 MPa). [TEPAH][2-MI] combined with propan-1-ol (NPA) and EG was used for CO2 capture [45]. The result indicates that the CO2 solubility in [TEPAH][2-MI] + NPA + EG can reach up to 1.72 mol/mol, which was much higher than that of [C3OHmim]Cl + MEA (0.3 mol/mol) [46], AMP + MEA + H2O (0.5 mol/mol) [47], [P66614][Gly] (1.26 mol/mol) [48], and TETA + AMP + ethanol (1.03 mol/mol) [49].

#### 2.1.3. IL–Amine/Amine Aqueous Solution

Three base-rich diamino ILs of [DMAPAH][Formate], [DMEDAH][Formate], and [DMAPAH] [Octanoate] were synthesized with different molar ratios of base to acid (0.5:1, 1.0:1, 2.0:1, and 2.5:1, respectively) and hybrid with MEA for CO2 capture, respectively [30]. According to Table 1 and Figure 2, the hybrid solvents of the synthesized ILs with an additional MEA showed enhanced CO2 solubility, which agrees with the studies from Zeng et al. [50] and Meng et al. [51] that applied MDEA and DMEE as the hybrid solvents to [DMAPAH][Ac] and [N1111][Lys], respectively. Among them, [DMAPAH][Formate] (2.0:1) + MEA (30 wt %) with 6.24 mol/kg was identified to be the best one for CO2 capture. The CO2 capture performance in [BMPyrr][DCA] (5 wt %) + DEA (35 wt %) + H2O (60 wt %) and [BMPyrr][DCA] (10 wt %) + DEA (30 wt %) + H2O (60 wt %) were studied by Salleh et al. [52] and compared with DEA (40 wt %) + H2O (60 wt %). The result indicates that the CO2 solubility increases with increasing the [BMPyrr][DCA] amount in the hybrid solvent. However, the CO2 solubilities of these two hybrid solvents are lower than those in DEA (40 wt %) + H2O (60 wt %).

**Figure 2.** Effects of MEA on CO2 solubilities in [DMAPAH][Formate] and [DMAPAH][Octanoate] [30]. Copyright 2018 Elsevier.

In conclusion, (1) a certain amount of water in ILs (mainly for chemical-based ILs) can enhance the CO2 solubility, due to the decrease in viscosity and the formation of new products (e.g., carbamate and bicarbonate). However, excess water in ILs corresponds to a low ILs concentration and results in the decrease of CO2 solubility; (2) the IL–organic and IL–organic aqueous solution as absorbents exhibit remarkable CO2 capture performances, including high absorption capacity and low desorption enthalpy. The organic molecular weight, type, and water content in ILs can affect their CO2 capture performance. Based on the summarized result, the organic solvent with low molecular weight together with a certain amount of water is beneficial for capturing CO2; (3) IL–MEA shows better CO2 capture performance than that of IL–MDEA and IL–DMEE; additionally, the IL–amine based hybrid solvent has higher CO2 solubility than that of IL–H2O and IL–organic hybrid solvents. The best for each of them are [DMAPAH][Formate] (2.0:1) + MEA (30 wt %) (6.24 mol/kg, 298 K, 0.1 MPa), [DMAPAH][Formate] (2.5:1) + H2O (20 wt %) (4.61 mol/kg, 298 K, 0.1 MPa), and [P4444][Pro] + PEG400 (70 wt %) (1.61 mol/kg, 333.15 K, 1.68 MPa). Sometimes, the presence of water in IL–organic/amine hybrid solvents improves the CO2 solubility.

#### *2.2. Viscosity*
