Article pubs.acs.org/jced
CO2 Solubilities in Ammonium Bis(trifluoromethanesulfonyl)amide Ionic Liquids: Effects of Ester and Ether Groups Takashi Makino,*,† Mitsuhiro Kanakubo,*,† and Tatsuya Umecky‡ †
National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan ‡ Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan ABSTRACT: The pressure−volume−temperature−composition relations in CO2 + ammonium bis(trifluoromethanesulfonyl)amide ionic liquid (IL) systems were measured over the pressure range up to 6 MPa at T = (298.15, 313.15, and 333.15) K. We focused on the effects of an ester or ether group in the cation of the IL on the CO2 solubilities on the mole fraction and molarity scales. N-Acetoxyethyl-N,Ndimethyl-N-ethylammonium ([N112,2OCO1]+) and N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium ([N112,2O2O1]+) were the ester and ether functionalized cations, respectively. Their nonfunctionalized analogues N,N-dimethyl-N-ethyl-N-pentylammonium ([N1125]+) and N,N-dimethyl-Nethyl-N-heptylammonium ([N1127]+), respectively, were also studied. Each IL shows the typical phase behavior as a physical absorbent. [N112,2O2O1][Tf2N] and [N1127][Tf2N] have the higher mole-fraction-scale solubilities of CO2 at certain temperatures and pressures, followed by [N1125][Tf2N] and [N112,2OCO1][Tf2N]. On the other hand, the molarity-scale solubilities of CO2 under the conditions investigated increase in the order [N112,2OCO1][Tf2N] < [N1125][Tf2N] ≈ [N1127][Tf2N] < [N112,2O2O1][Tf2N]. The ether functionalization in the ammonium cation is effective in the enhancement of physical absorption of CO2, in particular volumetrically, whereas the ester functionalization is negative.
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N,N-dimethyl-N-ethyl-N-heptylammonium ([N1127]+). It has been reported that oxygen functional groups in polymers can increase CO2-philicity, leading to higher solubilities of CO2 in such polymers.9−12 This is probably due to Lewis acid−base interactions between the acidic CO2 and basic oxygen atoms. Some investigations of CO2 solubilities in ILs with oxygen functional groups have been made,13−17 but CO2 solubilities per unit volume (molarity-scale solubilities) have not been reported to date for ILs with ether- or ester-functionalized cations. In the present study, the pressure−volume−temperature−composition (p−V−T−x) relations for the four ammonium IL + CO2 systems have been investigated over the pressure range up to 6 MPa at T = (298.15, 313.15, and 333.15) K. Comparisons among the present CO2 + IL systems are made to discuss the effects of the ester and ether groups on the CO2 solubilities on the mole fraction and molarity scales. The chemical structures and purities of the present ILs are summarized in Table 1.
INTRODUCTION Room-temperature ionic liquids (ILs) are generally defined as salts that melt at temperatures lower than 100 °C. One of their important features is the structural diversity, allowing a variety of combinations of cations and anions with chemical modifications. Therefore, it is possible to optimize the physicochemical properties for individual applications. In general, they have extremely low vapor pressures, good flame resistance, and high thermal and chemical stability. Thus, ILs are not eluted from the liquid phase into the vapor phase, although some gases are highly dissolved in the IL phase.1 Because of these features, a variety of chemical processes using CO2 + IL systems have been proposed, for example, CO2 separation, CO2 fixation and reduction, supercritical CO2 extraction, and chemical and material syntheses.1−7 A large number of investigations have also been performed on twophase equilibria in CO2 + IL systems. However, very limited information is available on the volumetric properties of mixtures of CO2 and ILs, which are important data for engineering design. We recently reported the densities, viscosities, and electrical conductivities of four quaternary ammonium salts coupled with bis(trifluoromethanesulfonyl)amide anion ([Tf2N]−).8 Two of the cations, N-acetoxyethyl-N,N-dimethyl-N-ethylammonium ([N112,2OCO1]+) and N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium ([N112,2O2O1]+), were functionalized with ester and ether groups, respectively. The others were the corresponding analogues with similar alkyl moieties, namely, N,N-dimethyl-N-ethyl-N-pentylammonium ([N1125]+) and © 2014 American Chemical Society
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EXPERIMENTAL SECTION Materials. CO2 (mole-fraction purity > 0.9999, Iwatani Industrial Gases Co.) was used without further purification. The ammonium ILs were synthesized as reported previously.8 The mole-fraction purity of each IL was ≥ 0.99. The halogen residues in aqueous solutions contacted with the present ILs Received: November 6, 2013 Accepted: March 25, 2014 Published: April 2, 2014 1435
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Table 1. Chemicals Used in the Present Study
a
N-Acetoxyethyl-N,N-dimethyl-N-ethylammonium bis(trifluoromethanesulfonyl)amide. bN,N-Dimethyl-N-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)amide. cN,N-Dimethyl-N-ethyl-N-pentylammonium bis(trifluoromethanesulfonyl)amide. dN,N-Dimethyl-N-ethyl-Nheptylammonium bis(trifluoromethanesulfonyl)amide.
assumption that the concentration of the IL in the CO2 phase was negligibly small:
were smaller than the detection limit of AgNO3 testing. Any excess water in each IL was further removed by evacuation at 343 K for approximately 30 h just prior to measurements. The water contents of [N112,2OCO1][Tf2N], [N112,2O2O1][Tf2N], [N1125][Tf2N], and [N1127][Tf2N] were mass fractions of (46, 45, 37, and 49)·10−6 as determined by Karl Fischer coulometric titration (KEM, MKC-510). The masses of IL loaded were determined using an electrical balance (Mettler Toledo AB204S). Volume Expansion Measurement. Detailed information on the experimental apparatus and procedures was described in our previous work.18 The volume of the IL phase was calculated from the calibration curve between the volume and the height of the liquid phase, which was measured visually using a cathetometer. The volume expansion ΔVL(T, p) under a certain set of conditions was determined as ΔV L(T , p) =
n1L(T , p) = n1i −
(1)
where VL(T, p) is the volume of the IL phase at temperature T and pressure p. The volume of the IL phase at atmospheric pressure p0 and temperature T, VL(T, p0), was calculated from the density at p0 (ρ0) and the mass of IL loaded. The temperature dependences of ρ0 for the present ammonium ILs were determined in our previous study.8 Finally, we obtained a quadratic equation for ΔVL(T, p) as a function of pressure at the temperatures investigated: L
ΔV (T , p) = a(T ) + b(T )p + c(T )p
2
V mG(T , p)
(3)
where VL(T, p) and VGm(T, p) are the volume of the IL phase and the molar volume of CO2 in gas phase, respectively, under the given conditions, ni1 is the molar amount of CO2 loaded in the experimental apparatus, and Vcell is the total inner volume of the experimental apparatus. VL(T, p) and VmG(T, p) were obtained from the quadratic expression in eq 2 and NIST REFPROP version 9.0,19 respectively. The density of CO2 was calculated using a Helmholtz-type equation of state,20 and its uncertainty was less than 0.05 %. From nL1 , the molar amount of IL loaded in the high-pressure cell (n2), and VL, the mole fraction of CO2 in IL phase (x1), the molar volume of the IL phase (VLm), and the molarity of CO2 in the IL phase (c1) can be obtained as follows:
V L(T , p) − V L(T , p0 ) V L(T , p0 )
Vcell − V L(T , p)
x1(T , p) = VmL(T , p) =
c1(T , p) =
(2)
n1L(T , p) n1L(T , p) + n2
(4)
V L(T , p) n1L(T ,
p) + n 2
n1L(T , p) L
V (T , p)
=
(5)
x1(T , p) VmL(T , p)
(6)
The uncertainties of T, p, and x1 are ± 0.02 K, ± 0.002 MPa, and ± 0.002, respectively. The relative uncertainties of VLm and c1 are ± 0.7 % and ± 0.8 %.
The uncertainties in T, p, and ΔVL are ± 0.02 K, ± 0.002 MPa, and ± 0.01, respectively. CO2 Absorption Measurement. Detailed information on the experimental apparatus and procedures was described in the recent study.18 The molar amount of CO2 dissolved in the IL, nL1 (T, p), was calculated using the following equation with the
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RESULTS AND DISCUSSION Tables 2 to 5 summarize the mole fractions of CO2 in the IL phase (x1), the molar volumes of the IL phase (VLm), and the 1436
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Table 2. Mole Fractions of CO2 (x1), Molarities of CO2 (c1), and Molar Volumes of the Liquid Phase (VLm) for the CO2 (1) + [N112,2OCO1][Tf2N] (2) Systema p
VLm
c1
MPa
x1
0.1 0.6580 1.1336 1.5074 1.9060 2.4829 3.0702 3.5528 4.0543 4.5757
− 0.166 0.259 0.320 0.378 0.446 0.504 0.545 0.581 0.614
0.1 0.5873 0.9752 1.2798 1.5937 2.0664 2.5391 3.2247 3.8830 4.1389 4.8450
− 0.119 0.184 0.230 0.272 0.328 0.378 0.438 0.485 0.501 0.540
mol·dm
Table 3. Mole Fractions of CO2 (x1), Molarities of CO2 (c1), and Molar Volumes of the Liquid Phase (VLm) for the CO2 (1) + [N112,2O2O1][Tf2N] (2) Systema
−3
p
cm ·mol 3
−1
MPa
x1
298.15 K 301.9b 258.7 234.4 218.4 203.2 185.5 170.1 159.3 149.6 140.9
0.1 0.5468 0.9901 1.4159 1.7810 2.3687 2.9950 3.7019 4.0782 4.5914
− 0.162 0.264 0.342 0.400 0.477 0.544 0.605 0.632 0.666
− 0.4377 0.7222 0.9457 1.174 1.515 1.862 2.342 2.778 2.940 3.372
304.8b 272.5 255.1 242.8 231.5 216.4 203.0 187.0 174.5 170.3 160.1
0.1 0.6059 1.2777 1.5358 1.8785 2.4761 2.9536 3.5044 3.9952 4.9788
− 0.139 0.262 0.300 0.345 0.413 0.460 0.506 0.541 0.602
0.1 0.5203 1.1391 1.5443 1.8538 2.7432 2.9201 3.4710 3.9312 4.8291
− 0.094 0.185 0.236 0.272 0.358 0.374 0.417 0.449 0.502
− 0.5768 1.051 1.501 1.894 2.522 3.196 3.959 4.361 4.925
328.1b 280.6 250.8 228.0 211.3 189.2 170.1 152.8 145.0 135.3
− 0.4744 1.020 1.220 1.485 1.947 2.312 2.732 3.098 3.832
331.4b 292.1 256.4 245.5 232.3 212.3 198.8 185.2 174.8 157.0
− 0.3015 0.6505 0.8774 1.050 1.532 1.630 1.925 2.172 2.630
335.7b 310.3 284.3 269.4 259.1 233.9 229.3 216.4 206.7 190.7
333.15 K
− 0.4033 0.6403 0.9910 1.231 1.499 1.828 2.181 2.554 2.712
308.6 277.5 262.1 242.4 230.5 218.7 205.7 193.4 182.2 177.9
b
a u(T) = 0.02 K, u(p) = 0.002 MPa, u(x1) = 0.002, ur(c1) = 0.008, and ur(VLm) = 0.007. bur(VLm) = 0.001 (densities at atmospheric pressure were measured with an Anton Paar DMM 5000 M densimeter).8
a
u(T) = 0.02 K, u(p) = 0.002 MPa, u(x1) = 0.002, ur(c1) = 0.008, and ur(VLm) = 0.007. bur(VLm) = 0.001 (densities at atmospheric pressure were measured with an Anton Paar DMM 5000 M densimeter).8
as the value for [N112,2OCO1][Tf2N]. kH for [N112,2OCO1][Tf2N] is approximately 10 % larger than that for [N1125][Tf2N]. The mole-fraction-scale solubility decreases upon introduction of the ester group. Similar decrements have been reported for CO2 + ammonium and pyridinium salts systems,16 although the negative effects of ester functionalization in the present and reported ammonium ILs are weaker than that in the pyridinium IL. On the other hand, the ether-functionalized ammonium shows almost the same kH as the unmodified counterpart. We observed the same behavior in the pyrrolidinium ILs with [Tf2N]−.17 In contrast, it was reported that ether-functionalized imidazolium salts have (15 to 20) % larger Henry’s constants for CO2 than their nonfunctionalized analogues.13 Thus, the effect of ester and ether functionalization on the CO2 solubility is not straightforward and is dependent on the cation structure. To discuss the influence of ester and ether functionalization in detail, the standard enthalpies and entropies of solvation of CO2 at infinite dilution (ΔsolH∞ and ΔsolS∞, respectively) were obtained from kH using eqs 8 and 9:21
molarities of CO2 in the IL phase (c1) at (298.15, 313.15, and 333.15) K for the four ILs. The p−x1 relations for the present binary systems are given in Figure 1. Each IL shows typical phase behavior as a physical absorbent: x1 increases linearly at low pressures, and the increment decreases at high pressures. As shown in Figure 1, the values of x1 under a particular set of conditions increase in the following order: [N112,2OCO1][Tf2N] < [N1125][Tf2N] < [N1127][Tf2N] ≈ [N112,2O2O1][Tf2N]. The Henry’s constants for CO2 (kH) were calculated as ⎡ f (T , p) ⎤ ⎥ kH(T ) = lim ⎢ 1 p → 0⎣ x1 ⎦
cm ·mol−1 3
313.15 K
333.15 K − 0.112 0.168 0.240 0.284 0.328 0.376 0.422 0.465 0.482
mol·dm
−3
298.15 K − 0.6431 1.107 1.468 1.859 2.403 2.962 3.419 3.882 4.358
313.15 K
0.1 0.7517 1.1948 1.8763 2.3410 2.8855 3.5495 4.2997 5.1698 5.5717
VLm
c1
(7)
where f1(T, p) is the fugacity of CO2 obtained from NIST REFPROP version 9.0.19 The values of kH at 313.15 K are summarized in Table 6. Deng et al.16 investigated the solubility of CO2 in N-acetoxyethyl-N,N,N-trimethylammonium bis(trifluoromethanesulfonyl)amide ([N111,2OCO1][Tf2N]), and kH for that IL (4.48 MPa at 313.15 K) is virtually the same 1437
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Table 4. Mole Fractions of CO2 (x1), Molarities of CO2 (c1), and Molar Volumes of the Liquid Phase (VLm) for the CO2 (1) + [N1125][Tf2N] (2) Systema p
VLm
c1
MPa
x1
0.1 0.5528 0.9083 1.4694 2.3176 2.6835 3.0049 3.5039 4.0283 4.6024
− 0.153 0.230 0.330 0.446 0.487 0.518 0.563 0.603 0.642
0.1 0.4260 0.7373 1.1762 1.8492 2.3306 2.8372 3.5592 4.1872 4.4994
− 0.096 0.156 0.230 0.321 0.376 0.426 0.486 0.530 0.550
0.1 0.4918 0.9431 1.4697 2.2582 2.9206 3.3956 3.9368 4.3611 4.9442
− 0.084 0.150 0.215 0.296 0.353 0.389 0.426 0.452 0.484
mol·dm
Table 5. Mole Fractions of CO2 (x1), Molarities of CO2 (c1), and Molar Volumes of the Liquid Phase (VLm) for the CO2 (1) + [N1127][Tf2N] (2) Systema
−3
cm ·mol 3
p −1
MPa
x1
298.15 K 316.4b 275.1 253.7 226.1 194.1 183.1 174.3 162.1 151.0 140.0
0.1 0.3911 0.7235 1.1769 1.8784 2.2965 2.9022 3.7941 4.2439 4.8023
− 0.119 0.199 0.292 0.405 0.460 0.525 0.605 0.638 0.673
− 0.3301 0.5670 0.9042 1.409 1.770 2.139 2.669 3.119 3.339
319.5b 292.0 275.0 254.0 228.1 212.6 199.0 182.1 169.9 164.6
0.1 0.4220 0.8924 1.4118 2.1969 2.5067 3.3235 4.1958 4.8464 5.3850
− 0.101 0.191 0.277 0.379 0.409 0.488 0.550 0.593 0.618
cm ·mol−1 3
− 0.2832 0.5377 0.8259 1.247 1.603 1.847 2.129 2.342 2.626
323.6b 297.9 278.8 260.0 236.9 220.5 210.6 200.1 193.0 184.3
0.1 0.4592 0.8250 1.2859 1.9812 2.7676 2.9979 3.8757 4.6987 6.1211
− 0.084 0.140 0.204 0.285 0.354 0.380 0.446 0.490 0.557
− 0.3782 0.6918 1.129 1.808 2.223 2.805 3.718 4.177 4.743
350.5b 313.8 288.2 258.9 223.9 206.9 187.2 162.7 152.7 142.0
− 0.3121 0.6489 1.040 1.613 1.815 2.435 3.032 3.531 3.842
353.9b 322.4 294.0 266.6 234.8 225.5 200.5 181.5 167.9 160.8
− 0.2527 0.4461 0.6924 1.063 1.432 1.595 2.053 2.407 3.044
358.5b 332.4 314.5 294.4 268.5 247.2 238.3 217.3 203.7 182.9
313.15 K
333.15 K
333.15 K
a
a u(T) = 0.02 K, u(p) = 0.002 MPa, u(x1) = 0.002, ur(c1) = 0.008, and ur(VLm) = 0.007. bur(VLm) = 0.001 (densities at atmospheric pressure were measured with an Anton Paar DMM 5000 M densimeter).8
u(T) = 0.02 K, u(p) = 0.002 MPa, u(x1) = 0.002, ur(c1) = 0.008, and ur(VLm) = 0.007. bur(VLm) = 0.001(densities at atmospheric pressure were measured with an Anton Paar DMM 5000 M densimeter).8
Δsol S∞(T ) = −RT
mol·dm
−3
298.15 K − 0.5555 0.9073 1.460 2.298 2.658 2.974 3.470 3.995 4.586
313.15 K
Δsol H ∞(T ) = −RT 2
VLm
c1
∂ ⎡ ⎛ kH(T ) ⎞⎤ ⎢ln⎜ ⎟⎥ ∂T ⎢⎣ ⎝ p° ⎠⎥⎦
have stronger affinities for CO2 than the nonfunctionalized analogues, which might be due to Lewis acid−base interactions between the oxygens in the cations and the carbon in CO2. On the other hand, the functionalized cations show more negative values of T·ΔsolS∞ than their aliphatic counterparts. Such an unfavorable entropic contribution is compensated by the favorable enthalpic contribution in the ether-functionalized ammonium but not in the ester-functionalized ammonium, which results in the decrement of the mole-fraction-scale solubility. The molar volume of the liquid phase, VLm, almost linearly decreases with increasing x1 at each temperature (figure not shown). The partial molar volume of CO2 (V̅ m,1) is determined by
(8)
⎡ ⎛ k ( T ) ⎞⎤ ∂ ⎡ ⎛ kH(T ) ⎞⎤ ⎢ln⎜ ⎟⎥ − R ⎢ln⎜ H ⎟⎥ ⎢⎣ ⎝ p° ⎠⎥⎦ ∂T ⎢⎣ ⎝ p° ⎠⎥⎦ (9)
where p° and R stand for the standard pressure (0.1 MPa) and the gas constant (8.3144621 J·K−1·mol−1), respectively. The values of ΔsolH∞ and T·ΔsolS∞ at 313.15 K are also given in Table 6. Since the lengths of the side chains in the cations are very similar for the functionalized and analogous pairs (i.e., [N112,2OCO1]+ vs [N1125]+ and [N112,2O2O1]+ vs [N1127]+), the differences in ΔsolH∞ and T·ΔsolS∞ can be mainly interpreted in terms of the effects of ester and ether functionalization. The ΔsolH∞ values for [N112,2OCO1][Tf2N] and [N112,2O2O1][Tf2N] are approximately 1 kJ·mol−1 more negative than those for [N1125][Tf2N] and [N1127][Tf2N]. The more negative enthalpies indicate that the ester- and ether-functionalized ILs
Vm,1 ̅ (T ) =
∂V L(T , p) ∂n1L(T , p)
(10)
V̅ ∞ m,1
The partial molar volumes of CO2 at infinite dilution, (c1 → 0), were obtained from the slopes of the plots of the volume 1438
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Figure 1. Mole-fraction-scale solubility of CO2 (x1) as a function of pressure (p). Circles, [N112,2O2O1][Tf2N]; squares, [N1127][Tf2N]; diamonds, [N1125][Tf2N]; triangles, [N112,2OCO1][Tf2N]. Solid symbols, 298.15 K; gray symbols, 313.15 K; open symbols, 333.15 K. The solid and dashed curves are eye guides.
Figure 2. Molarity-scale solubilities of CO2 (c1) as functions of pressure (p). Circles, [N112,2O2O1][Tf2N]; squares, [N1127][Tf2N]; diamonds, [N1125][Tf2N]; triangles, [N112,2OCO1][Tf2N]. Solid symbols, 298.15 K; gray symbols, 313.15 K; open symbols, 333.15 K. The solid and dashed curves are eye guides.
Table 6. Henry’s Constants of CO2 (kH) and Standard Thermodynamic Parameters of Solvation (ΔsolH∞ and T·ΔsolS∞) for the CO2 + IL Systems at 313.15 Ka
mol−1 at 298 K) has a much smaller molar volume than the nonfunctionalized analogue [N1127][Tf2N] (350.4 cm3 mol−1 at 298 K).8 This also leads to a smaller molar volume of the IL phase, VLm, for [N112,2O2O1][Tf2N] than for [N1127][Tf2N]. According to eq 6, when the mole-fraction-scale solubilities in absorbents are similar, the smaller molar volume of the absorbent increases the molarity-scale solubility. On the other hand, c1 in [N112,2OCO1][Tf2N] is slightly lower than that in [N1125][Tf2N], although VLm for [N112,2OCO1][Tf2N] is smaller than that for [N1125][Tf2N]. This is due to the lower x1 in [N112,2OCO1][Tf2N]. In addition, [N1125][Tf2N] has the almost same pressure dependence of c1 as [N1127][Tf2N], which means that the length of alkyl side chain has less effect on the molarityscale solubility of CO2.
[N112,2OCO1][Tf2N] [N112,2O2O1][Tf2N] [N1125][Tf2N] [N1127][Tf2N] a
kH
ΔsolH∞
T·ΔsolS∞
MPa
kJ·mol−1
kJ·mol−1
4.42 3.83 4.03 3.82
−13.7 −13.6 −12.8 −12.8
−23.6 −23.1 −22.4 −22.3
∞ ∞ ur(k∞ H ) = 0.02, ur(ΔsolH ) = 0.02, and ur(ΔsolS ) = 0.02.
of the liquid phase versus the molar amount of CO2. The V̅ ∞ m,1 values were found to be (38 ± 1, 39 ± 3, 42 ± 2, and 42 ± 1) cm3·mol−1 for [N112,2OCO1][Tf2N], [N1125][Tf2N], [N112,2O2O1][Tf2N], and [N1127][Tf2N], respectively, and were almost independent of temperature within the experimental errors. V̅ ∞ m,1 should be the sum of the van der Waals volume of CO2 and the volumetric change caused by CO2 dissolution, of which the former remains constant independent of solvent. In general, V̅ ∞ m,1 in ILs is much smaller than those in organic solvents because of the smaller volume expansions in the presence of the strong Coulombic forces in ILs. The slight differences in V̅ ∞ m,1 among the four ILs would be attributed to many-body interactions among cations, anions, and CO2 molecules but cannot be explicitly rationalized at present. Anyway, it is noted that V̅ ∞ m,1 is one of the useful measures for understanding the solvation environments around CO2 in ILs. Figure 2 presents the molarities of CO2 in the IL phase (c1) as functions of p. c1 almost linearly increases with increasing p at each T under the conditions studied in this work. The slopes at 298.15 K are 1.07 mol·dm−3·MPa−1 for [N112,2O2O1][Tf2N], 0.99 mol·dm−3·MPa−1 for [N1125][Tf2N], 0.98 mol·dm−3· MPa−1 for [N1127][Tf2N], and 0.96 mol·dm−3·MPa−1 for [N112,2OCO1][Tf2N]. [N112,2O2O1][Tf2N] has the highest c1 under given conditions among the present ammonium salts. The ether-functionalized IL [N112,2O2O1][Tf2N] (328.1 cm3·
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CONCLUSION We have measured the p−V−T−x relations for four ammonium bis(trifluoromethanesulfonyl)amide IL + CO2 systems to investigate the effects of ester and ether groups on the physical solubilities of CO2. The standard enthalpy of solvation suggests that the oxygen-containing functional groups enhance the intermolecular interactions between the CO2 and ammonium IL molecules. However, the mole-fraction-scale solubility in the ester-functionalized ammonium is smaller than that in the nonfunctionalized analogue. The ether modification in the ammonium cation does not significantly influence the CO2 solubility on the mole fraction scale. These results are due to the negative entropic contribution caused by the introduction of functional groups. On the other hand, the molarity-scale solubility in the ammonium IL with the ether group is higher than that in the aliphatic analogue because of the smaller molar volume of the former IL. The introduction of the ester group also reduces the molar volume of IL phase, but the ester-modified ammonium shows a slightly smaller CO2 solubility even on the molarity scale than the aliphatic counterpart, which is attributed to the poorer solubility of CO2 on the mole fraction scale as mentioned above. 1439
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Fax: +81-22-232-7002. *E-mail:
[email protected]. Fax: +81-22-232-7002. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Ms. Eriko Niitsuma and Mr. Atsuhiro Oguni for their assistance with the measurements in the present study.
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