2.2.1. IL–H2O

The viscosities of IL–H2O hybrid solvents are given in Table 3. Figure 3 displays the viscosities of [DEA][Bu] + H2O [14]; the result indicated that the [DEA][Bu] has a strong hygroscopic characteristic. The viscosity of [DEA][Bu] + H2O decreases with the increase of water content and temperature. Yasaka et al. found that the viscosities of [P4444][HCOO] + H2O decreased with water contents from 25 (356 mPa·s) to 91 mol% (14.4 mPa·s), corresponding to an increase of the CO2 solubility between the water conetnt of 25 and 50 mol%, and then, they decrease from 50 to 91 mol%, which is regarded as the typical property of carboxylate ILs [32]. Aghaie et al. measured the viscosities of [HMIM][Tf2N], [HMIM][FAP], and [BMIM][Ac] aqueous solutions at 298–333 K, 2 MPa, and water mass percentages of 0.1, 1, 2, 5, and 10 wt %, respectively [35]. The result indicates that water has a significant effect on [BMIM][Ac] viscosity, e.g., the viscosity of [BMIM][Ac] decreased from 47.64 (pure IL) to 3.77 (10 wt % H2O) mPa·s at 333 K. However, for [HMIM][Tf2N] and [HMIM][FAP], their viscosities only slightly decrease at 0.1–10 wt % water. For example, the viscosity of [HMIM][FAP] at 333 K is 20.72 mPa·s, while it is 20.47 mPa·s for [HMIM][FAP] + H2O (10 wt %). Additionally, increasing the water amount in these three ILs results in the decrease of CO2 solubility.

**Figure 3.** Viscosity of [DEA][Bu] + H2O, 283.15–343.15 K, 0.1 MPa [14]. Copyright 2018 Elsevier.

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

For [TETAH][Lys] + H2O + ethanol, it was observed that its viscosity increased with the decrease amount of ethanol, corresponding to a decreased CO2 solubility [36]. Liu et al. found that the viscosities of [Cho][Gly] (84.3–7.2 mPa·s), PEG200 (31.8–8.8 mPa·s), and [Cho][Gly] + PEG200 (70 wt %) (101.3–28.6 mPa·s) are much higher than that of [Cho][Gly]/H2O + PEG200 (30 wt %) (7.96–3.43 mPa·s) at 308.15–338.15 K [17]. To avoid the decrease of absorption rate by increasing PEG200, less than 30 wt % of PEG200 was recommended to [Cho][Gly] + PEG200 + H2O. The viscosities of [P4444][Gly], [P4444][Ala], and [P4444][Pro] hybrid with PEG400 were measured at 298.15–393.15 K and 0.1 MPa [41]. The result indicates that the viscosities of these amino-acid ILs + PEG400 are about half with respect to the pure amino acid ILs at 298.15 K. Chen et al. reported that the viscosity of the [DETAH][Br] + PEG200 (80 wt %) is 71.7 mPa·s at 293.15 K and 0.1 MPa [42]. Despite the viscosities' increases with the increasing long-chain polymers, i.e., [N4111][Tf2N] + PEO222 < [N4111][Tf2N] + PEO500 < [N4111][Tf2N] + PEO1000, [N4111][Tf2N] + PEO1000 was proposed for CO2 capture due to the higher CO2 solubility of the pure PEO1000 [43]. The viscosities for 18 kinds of ILs (imidazolium- and phosphonium-based) + TG at different of mole ratio of each of these ILs at 278.15–323.15 K and 0.1 MPa were studied [18], evidencing that the presence of TG can significantly decrease the viscosity, resulting in about 50 mPa·s for these hybrid solvents (Table 3).

#### 2.2.3. IL–Amine

As shown in Table 3, the viscosities of the [N1111][Lys] + DMEE and [BMIM][BF4] + DETA hybrid solvents increased with increasing the content of ILs and decreasing temperature [51,53]. Based on the study of Meng et al. [51], the viscosity of [N1111][Lys] + DMEE significantly decreased when the IL is <60 wt % compared to the pure [N1111][Lys].

From Table 3, it can be found that the viscosities of IL-based hybrid solvents are very sensitive to H2O, organic, and amine solvents. Their viscosities can significantly decrease with the increased amount of H2O, PEG, TEG, TG, DMEE, and DETA; however, it increases with the increased amount of methanol. The lowest viscosity obtained from Table 3 for IL–H2O, IL–organic, and IL–amine based hybrid solvents are [DEA][Bu] + H2O (98.78 mol%) (0.59 mPa·s, 343.15 K), [N1114][Tf2N] + PEO222 (75.06 mol%) (2.57 mPa·s, 353.15 K), and [BMIM][BF4] + DETA (94.9 mol%) (2.68 mPa·s, 353.15 K) at 0.1 MPa, respectively.


CO2solubility of ionic liquid (IL)-based hybrid solvents.

**Table**

**1.**


**Table 2.** Henry's constant of IL-based hybrid solvents.


[BMIM][BF4] + TEG (20, 50, 80 mol%)

273.15–353.15

4.84–20.02, 5.65–22, 6.48–27.63

 [44]

*Crystals* **2020** , *10*, 978



*Crystals* **2020**, *10*, 978


*Crystals* **2020** , *10*, 978

#### **3. DESs-Based Hybrid Solvents**

The CO2 solubility data for 33 kinds of DESs-based hybrid solvents, together with viscosities for six types of DESs-based hybrid solvents since 2016, and Henry's constants for 21 kinds of DES-based hybrid solvents since 2013 have been reported, as summarized in Tables 4 and 5. The full names of the studied components of DESs are given in Table S1.
