The solubility of a gas is governed by a combination of factors namely; polarizability of the gas, the dispersion forces and the interactions between the gas and the solvent.
The relative influence of each of these factors differs with the nature of the gas and the solvent involved. Several studies reported that CO2 solubility in ILs depends primarily on the strength of interaction of CO2 with the anion [211]. In this study we made a comparison between different ILs with same cation and different anions to see their affinity to absorb CO2. Five ILs with same cation ([C2CN Him]) and five different anions (DOSS, DDS, TFMS, SBA and BS) were used to study the effect of these anions on CO2 solubility.
The CO2 solubility in [C2CN Him] cation incorporating DOSS, DDS, TFMS, SBA and BS anions at temperature 298.15 K and pressures 1, 5, 10, 15 and 20 bar are measured and the time-dependence of CO2 uptake by the studied ILs is reported in Table B- 1 to Table B- 4 in Appendix B and shown in Fig 5-1 to Fig 5-5 in terms of mol fraction (xCO2) versus time. The results showed that TFMS and BS anions based ILs reached equilibrium in shorter time compared to the other anions. This may be due to the lower viscosities of these anions. The gas diffusion in IL depends on the IL‘s viscosity; where the increase of the viscosity of IL results in greater diffusion
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0 50 100 150 200 250 300
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-1 CO2 solubility in [C2CN Him]DOSS as a function of time at 298.15 K
0 50 100 150 200 250
0.0 0.1 0.2 0.3 0.4 0.5 0.6
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-2 CO2 solubility in [C2CN Him]TFMS as a function of time at 298.15 K
0 50 100 150 200 250 300
0.0 0.1 0.2 0.3 0.4 0.5
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-3 CO2 solubility in [C2CN Him]DDS as a function of time at 298.15 K
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0 50 100 150 200 250 300
0.0 0.1 0.2 0.3 0.4 0.5
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-4 CO2 solubility in [C2CN Him]SBA as a function of time at 298.15 K
0 50 100 150 200 250
0.0 0.1 0.2 0.3 0.4
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-5 CO2 solubility in [C2CN Him]BS as a function of time at 298.15 K
versus time. This increased the time required to reach equilibrium [61]. The viscosities of the studied ILs are moderately high, so the diffusion of gas into these ILs can be slow.
The time required for the five ILs to reach the equilibrium is greater than that for the ILs incorporating the amine functionality (180 min) [61] and also greater than that reported for non functionalized imidazolium-based ILs (90 – 180 min) [165, 168] but lower that that reported for [emim]NTf2 (1800 – 2400 min) and [bmim]NTf2 (2160 - 2880 min) [61].
The solubility of CO2 increases with increasing pressure for all five ILs, as expected (increased pressure will force the gas into the IL). The solubility of a gas is
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defined as the concentration of the dissolved gas in equilibrium with the substance in the gaseous state. At equilibrium, the rate at which solute gas molecules escape the solution equals the rate at which the gas molecules reenter the solution. An increase in pressure results in more molecules of gas striking the surface of the liquid and entering the solution in a given time. The solution eventually reaches a new equilibrium when the concentration of gas dissolved in the solvent is high enough that the rate of gas molecules escaping the solution again equals the rate of gas molecules entering the solution [212].
The results showed that the CO2 solubility is dependent on the choice of the anion which is in agreement with reported literature (the effect of the anion on the solubility of CO2 in the ILs was investigated experimentally. The solubility of CO2 in the IL was higher for the ILs with lower anion polarity) [213]. The isotherms for the effect of the anion on the solubility of CO2 in [C2CN Him]-based ILs at 298.15 K are shown in Fig 5-6 and presented in Table 5-1. The gas solubilities appear linear as a function of the pressure but exhibited a nonlinear trend as the pressure increased for all the studied ILs.
0 5 10 15 20
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
CO2
P(bar)
[C2CN Him]DOSS [C2CN Him]DDS [C2CN Him]TFMS [C2CN Him]SBA [C2CN Him]BS
Fig 5-6 CO2 solubility in [C2CN Him]-based ILs incorporating DOSS, DDS, TFMS, SBA and BS anions at 298.15 K.
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The IL with the DOSS anion has a considerably higher affinity for CO2 compared with the ILs incorporating DDS, TFMS, SBA and BS anions. The DOSS anion has several features that are known to enhance a molecule‘s CO2-philicity and lead to good solubility of CO2; carbonyl and sulfonyl functionalities, long alkyl chains and branched alkyl chains [82]. In addition, another major factor that affects the capacity of CO2 solubility is the molar free volume of the IL. In general, the larger the molecular size of the anion, the larger the free volume in which CO2 can occupy [214]. The molar free volume of the IL exerted a more pronounced effect than the anion basicity which is one of the key factors in determining the CO2 solubility in ILs.
Table 5-1 Experimental solubility data for CO2 in [C2CN Cnim]X at 298 K Pressure
(bar)
CO2 mol fraction [C2CNHim]
DDS
[C2CNHim]
TFMS
[C2CNHim]
SBA
[C2CNHim]
BS
[C2CNHim]
DOSS 1±0.01 0.021673 0.035242 0.031718 0.025789 0.069501 5±0.01 0.159438 0.196824 0.177141 0.139046 0.253262 10±0.03 0.281268 0.340454 0.306288 0.261962 0.446553 15±0.04 0.378069 0.460321 0.414288 0.349617 0.598146 20±0.05 0.448793 0.546700 0.492030 0.410599 0.737900
g CO2 / g IL
1±0.01 0.002 0.005 0.004 0.003 0.005
5±0.01 0.018 0.030 0.023 0.019 0.024
10±0.03 0.036 0.064 0.048 0.043 0.056
15±0.04 0.057 0.106 0.076 0.065 0.104
20±0.05 0.076 0.149 0.104 0.084 0.197
Moreover, DOSS anion has a known low toxicity [215] and is not likely to be as environmentally persistent as fluorinated ILs, while maintaining good capacity for CO2 solubility. The [N4,4,4,4] IL incorporating DOSS anion showed a good capacity for CO2 and is higher than [Hmim]NTf2 at the high pressures [82].
The relatively high solubility of CO2 in the TFMS-based IL compared to the DDS, SBA, and BS-based ILs may be due to the greater interactions between CO2 and the fluoroalkyl substituents on the TFMS anion [211, 216]. Two types of interactions are
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known: one is the acid-base interaction between CO2 and the anion in ILs, and the other is between CO2 and fluorine in the anion [8]. In fact, CO2 solubility increases with increasing number of CF3 groups in the anion [8]. The fluorinated compounds has a high stability and low reactivity which gives them many excellent properties, but these also lead them to being poorly biodegradable and persistent in the environment [156].
The relatively high solubility of CO2 in IL with SBA anion compared to those with DDS and BS anions is associated with the presence of carbonyl functionality.
This enhances a molecule‘s CO2-philicity and lead to good solubility of CO2. Moreover the relatively high solubility capacity of CO2 in DDS-based IL compared to BS-based IL is due to the long alkyl chain of the DDS anion which results in the increases of the van der Waals-type interactions between the gas and the liquid.