3.3.6. CO2 Absorption Measurement

The CO2 absorption of the PILs was studied by using a magnetic suspension balance (MSB) from Rubotherm Präzisionsmesstechnik GmbH (Bochum, Germany). In this gravimetric method, the weight change of the PILs upon absorption of CO2 was measured and calculated in a range of pressure from 1 to 29 bar at room temperature. The sample absorption chamber linked to the microbalance, which has a precision of ±20 μg, via an electromagnet and a suspension magnet which keeps the balance at ambient conditions during the CO2 absorption experiments. In a typical CO2 absorption measurement, approximately 1g of PIL sample was loaded in the sample chamber and the absorption system was evacuated at 10−<sup>3</sup> mbar (Pfeiffer model DUO5) to remove any impurities until the weight remained constant. Then, the sample chamber was pressurized with CO2 at a constant temperature by means of an oil circulator (Julabo, model F25-ME, ±0.1 ◦C accuracy, Seelbach, Germany) and the weight change due to the absorption of the gas in the PIL was observed and recorded. Once a constant weight reading was recorded, the system was allowed to stand in the condition for an additional 3-4 h to ensure complete equilibration of the binary CO2–PIL system. The absorption measurement was repeated with different pressure values

of CO2 to yield a series of absorption isotherm. The weight of the CO2 dissolved in the PILs sample was calculated using Equation (9) available from literature [72,73].

$$\text{wrt CO}\_2 = \left[\text{wt} - (\text{wt}\_{\text{Sc}} + \text{wt}\_{\text{S}})\right] + \left[(\text{V}\_{\text{Sc}} + \text{V}\_{\text{S}})(\rho \text{CO}\_2)\right] \tag{9}$$

where wt (g) is the corrected weight of the balance, wtSc + wtS (g) are the weights of sample cell and sample, respectively, VSc + VS (cm<sup>−</sup>3) are the volumes of the sample cell and sample, respectively, and *ρ*CO2 (g.cm−3) is the density of CO2 at the pressure and temperature during the CO2 absorption. The results of CO2 absorption are presented in terms of mole fraction of CO2 (x) dissolved in the PIL, which was calculated using Equation (10):

$$\mathbf{x} = \mathbf{n}\_{\text{CO2}} / (\mathbf{n}\_{\text{liq}} + \mathbf{n}\_{\text{CO2}}) \tag{10}$$

where nCO2 is the mole of CO2 absorbed in the PIL and nliq is the mole of the PIL.
