# Ionic Liquids Hybridization for Carbon Dioxide Capture: A Review
## 2 ILs Utilization for CO\({}_{2}\) Capture
For instance, ether-functionalized choline tethered amino acid, [N\({}_{1,1,6,204}\)][Lys] was synthesized and has a viscosity value of 184 mpa-s at 80 \({}^{\circ}\)C, while [N\({}_{6,6,6,14}\)][Lys] was reported to have a viscosity value of \(>\)200 mpa-s at the same temperature. CO\({}_{2}\) absorption measurement by using low viscosity [N\({}_{1,1,6,204}\)][Lys] was found to reach equilibrium conditions within 3.5 h with the absorption value of 1.62 mol of CO\({}_{2}\)/mol ILs [46]. In comparison, ether-free ILs such as [N\({}_{6,6,6,14}\)][Lys] and P\({}_{6,6,6,14}\)][Lys] recorded significantly longer equilibration time, between 24 and 48 h, during CO\({}_{2}\) absorption. One of the purposes of designing AAILs is to overcome the high viscosity problem of ILs. However, the viscosity of AAILs have drastically increased after CO\({}_{2}\) absorption due to formation of hydrogen bond [47]. This finding has prompted researchers to develop a new class of ILs known as protic ILs (PILs). PILs are synthesized through proton transfer between a Bronsted acid and base. In addition to their low viscosity, PILs require a simple synthesis procedure without complicated purification steps. One of early examples of PILs for CO\({}_{2}\) is demonstrated by [PERSON] and co-workers in 2010 [48]. The study demonstrated that their superbase PILs [MTBDH\({}^{+}\)]\({}_{2}\) [HFPD\({}^{2-}\)] exhibited a remarkable CO\({}_{2}\) absorption capacity of 2.04 mol CO\({}_{2}\)/mol ILs, despite being in a gel form. This can be attributed to the presence of two CO\({}_{2}\)-reactive sites in the PIL. On the other hand, [PERSON] et al. prepared three low-viscous PILs for CO\({}_{2}\) absorption [49]. It was determined that [TMGH][Pyrr] with the largest basicity recorded the highest CO\({}_{2}\) absorption capacity of 0.66 mol CO\({}_{2}\)/mol IL and took 60 min for [TMGH][Pyrr] to reach equilibrium condition. However, [TMGH][Im] that possessed the second highest basicity exhibited rapid absorption rate as its reaching equilibrium within 20 min with CO\({}_{2}\) absorption capacity of 0.64 mol CO\({}_{2}\)/mol IL. Based on this section, it can be summarized that numerous efforts have been made by focusing on optimizing IL properties to achieve higher CO\({}_{2}\) absorption capacities, improved selectivity and enhanced stability. Researchers have explored various strategies such as modifying ILs structure by incorporating numerous functional groups into the anion or cation with the main goal of improving the CO\({}_{2}\) capture performance. Meanwhile, regardless of the class of ILs, the anion has been observed to play a significant role in CO\({}_{2}\) absorption. Table 1 lists several ILs according to ILs classes studied for CO\({}_{2}\) capture. In general, ILs could become a new alternative in CO\({}_{2}\) capture technology. In addition to ILs low volatility and thermal stability, the ability to customize their properties for specific applications has emerged as a significant factor that attracts researchers' attention to further explore their potential, particularly in the field of CO\({}_{2}\) capture. _Challenges in ILs Viscosity towards CO\({}_{2}\) Capture_  
As reported by [PERSON], an increase in solvent viscosity has resulted in a 30% reduction in CO\({}_{2}\) recovery [51]. Thus, viscosity remains a critical factor influencing the effectiveness of ILs in capturing CO\({}_{2}\). Though ILs offer promising solutions in terms of low volatility and recyclability, their viscosity issue poses several challenges when considering realistic operating conditions within industrial scales. In general, highly viscous ILs will lead to slow diffusion of CO\({}_{2}\) into the sorbent which, in turn, will cause reduction in mass transfer and, consequently, will increase energy consumption. Additionally, regenerating ILs necessitates the provision of heat to facilitate the desorption process. In the case of highly viscous ILs which have low heat transfer, additional energy input may be required thereby impacting process efficiency.