Figure 2 shows the amount of captured nCO₂ as a function of propanol concentration in the [N4111][TFSI]- propanol system at a temperature of 25˚C. The molar fraction of nCO₂ was calculated as,

where n_IL and are the moles of RTILs and nCO₂, respectively. We did not consider the amount of propanol in this study, as propanol is relatively inexpensive compared with the RTILs. The results show that the addition of 2-propanol can promote nCO₂ capture in the propanol-rich region. In contrast, 1-propanol did not enhance nCO₂ capture. The value of η for the 1-propanol-based mixtures remained almost constant with changing propanol concentration; this is a typical isomer effect of propanol, as indicated by the phase diagrams [15] . The isomer effect of nCO₂ capture is discussed in the next section along with liquid stability. The above tendency is also seen in other systems. The molar fractions at the points of maximum nCO₂ sorption for all RTILs-propanol systems studied herein are indicated in Figure 3. Here, temperature was fixed at 25˚C. The nCO₂ sorption of the 1-propanol-based mixture was larger than that of the 2-propanol one only for the [DEME][TFSI] system. Relative large values of η were obtained in the quaternary ammonium cation-based systems. In contrast, the phosphonium and imidazolium systems exhibited lower nCO₂ capture abilities. The high efficiency of nCO₂ capture in the quaternary ammonium cation-based systems can be attributed to the syntheses of these cations. An example of synthesis using the Halogen-free carbonate ester method [21] can be written as follows:

The scheme in Equation (2) leads to the coexistence of quaternary ammonium cation, alcohol and CO₂, and provides a clue to explain the high nCO₂ capture obtained using quaternary ammonium cations. We predict that the cation, CO₂ and alcohol are affirmative each other. Among the RTILs used in this study, the [N4111][TFSI]- 2-propanol system provided the best nCO₂ storage.

To investigate the thermal characteristics of nCO₂ capture, the η value of the [N4111][TFSI]-80 mol% 2-propanol

system is plotted as a function of temperature in Figure 4(a). Upon cooling down to 15˚C, almost monotonic increase of η was observed. According to conventional thermodynamics, η decreases with increasing temperature. At lower temperature, CO₂ absorption efficiency was elevated. In [N4111][TFSI]-propanol system, however, there is a problem of phase separation in the propanol-rich region. Figure 4(b) reveals the phase diagram of [N4111][TFSI]-1-propanol and -2-propanol. The phase diagram of [N4111][TFSI]-propanol system was constructed based on visual cloud-point determinations [14] [15] [17] . The cloud-points of 1- and 2-based mixtures are represented by red and blue closed circles, respectively. On the phase diagrams, different molecular interactions of 1- and 2-propanol was predominant, since phase separation curves are calculated using the UNIQUAC interaction parameters [15] . The UNIQUAC model has the nearest neighbor correlation. The UCST of the [N4111][TFSI]-80 mol% 2-propanol mixture was approximately 15˚C. Therefore, below 15˚C, it is impossible to use nCO₂ capture in the [N4111][TFSI]-2-propanol system for industrial applications.

Phase diagram including phase instability is connected with nCO₂ capture ability in the propanol-rich region

(Figure 2). At the UCST, the phase separation behavior could significantly influence the nCO₂ capture. Generally, liquid becomes unstable as a precursor phenomenon close to phase separation. Fluctuations in the propanol concentration in the vicinity of the clouding point were not ignored. Furthermore, UCST and the critical concentration (x_c = 85 mol%) in the phase diagram have significant meaning on the UCST [15] . In thermodynamics, fluctuations are critical phenomena that are enhanced at UCST and x_c. In this study, the intrinsic instability of the liquid phase in the RTILs-propanol mixtures became distinct at around x_c. Thus, we deduce that the unstable liquid phase in the propanol-rich region was stabilized by nCO₂ sorption.

TFSI⁻ conformation stabilities in the RTIL-propanol mixtures were estimated by Raman spectroscopy [15] [17] - [19] [22] . The observed Raman bands were assigned as C₁/C₂ conformers by DFT calculations [18] [22] . As an example, the Raman spectrum of pure [N4111][TFSI] is shown in Figure 5(a). The experimental C₂/C₁ ratio was calculated by decomposing the Raman peaks, and the asymmetric pseudo-Voigt function was used to separate peaks. The highest level of nCO₂ capture for [N4111][TFSI] was obtained at C₂/C₁ = 0.539. In contrast to [N4111][TFSI], the C₂/C₁ values of [N1123][TFSI] and [P2225][TFSI], which exhibited poor nCO₂ capture abilities, were 0.853 and 0.869, respectively. For instance, the Raman spectrum of [P2225][TFSI] is displayed in Figure 5(b). Despite pure systems, the C₂/C₁ ratio of TFSI⁻ anion is regarded as a good indicator of nCO₂ capture ability, and C₂/C₁ has been used to determine the mixing states of [C_nmim][TFSI]-propanol [15] and -bu- tanol [17] . In both the pure and mixed systems, the TFSI^- anion conformer indicates energetically stable/unstable states in the liquid state.

DFT calculations are indispensable to interpret experimental results, although DFT calculations provide the molecular-level details on the gas phase. DFT calculations were performed using the Lee-Yang-Peer correlation (B3LYP) with the 6-31++G(d,p) basis set [23] [24] in the PC-GAMESS package [25] . In the DFT simulation box, we introduced the torsion angle (α) of TFSI⁻ anion (Figure 6(a)); the geometrical definition of α was provided

by the C-S-S-C angle. DFT calculations of the [N4111][TFSI] system were used to examine a relation between a of TFSI⁻ and molecular configurations of propanol isomer and CO₂ additive (Table 1). The calculated torsion

angle is sensitive to the presence of propanol and its conformation. In pure [N4111][TFSI], α = 30.756˚; the addition of 2-propanol increased the torsion angle to 72.517˚. The effect of 2-propanol on torsion angle was larger than that of 1-propanol. These results are in agreement with previous DFT calculations. The experimentally obtained C₂/C₁ ratio of TFSI⁻ anion is known to reflect the stabilities of the liquid, glass, and solid phases [14] [15] [17] [26] . Thus, increased α in the [N4111][TFSI]-2-propanol mixture has significant implications for liquid state stabilization. The torsional potential of TFSI⁻ anion has two local minima at 80˚ and 280˚ [27] . Although the potential calculated between the two minima is relatively low, TFSI⁻ has a higher torsional barrier at approximately 0˚ (C₁). Hence, in case of the [N4111][TFSI]-2-propanol mixture, 2-propanol causes the TFSI^- anion to twist, and stabilizes energetically. In the [N4111][TFSI]-1-propanol system, α cannot reached to 70˚. The difference in α between the 1- and 2-propanol-based RTILs is directly connected to the propanol isomer effect on nCO₂ capture. To clarify the nCO₂-driven stabilization in the RTIL-2-propanol systems, we replotted the observed CO₂ capture against the calculated α angle (Figure 6(b)). The most nCO₂ capture in the [N4111]- [TFSI]-2-propanol was observed at α = 70˚. In contrast, the minimum nCO₂ capture in the [N1123]-[TFSI]-2- propanol system, which is fully stabilized without the addition of 2-propanol, was shifted to lower value of α. The α dependence of η in Figure 6(b) can be explained by assuming that CO₂ compensates for geometrically mismatching of TFSI⁻ conformer and additives in 2-propanol based mixtures.

For actual applications, the RTIL-propanol mixtures must be gas selective. Figure 7 shows CO₂, O₂, and N₂ sorption in the [N4111][TFSI]-80 mol% 2-propanol system at 25˚C. N₂ cannot contribute to energetic stabilization in an unstable liquid system in the propanol-rich region. N₂ mostly occupied in the air has the lowest sorption.

The gas selectivity in the [N4111][TFSI]-80 mol% 2-propanol system has an advantage for industrial applications. The results clearly show that CO₂ is preferred in the [N4111][TFSI]-2-propanol system. CO₂ selectivity was realized at ambient pressure. CO₂ plays an important role for the high efficient CO₂ capture system, although the mechanism remains unclear.