ARTICLE pubs.acs.org/JPCB
Correlation between Hydrogen Bond Basicity and Acetylene Solubility in Room Temperature Ionic Liquids Jelliarko Palgunadi,† Sung Yun Hong,† Jin Kyu Lee,† Hyunjoo Lee,‡ Sang Deuk Lee,‡ Minserk Cheong,*,† and Hoon Sik Kim*,† †
Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Dongdaemun-gu, Seoul 130-701, Republic of Korea ‡ Clean Energy Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
bS Supporting Information ABSTRACT: Room temperature ionic liquids (RTILs) are proposed as the alternative solvents for the acetylene separation in ethylene generated from the naphtha cracking process. The solubility behavior of acetylene in RTILs was examined using a linear solvation energy relationship based on Kamlet-Taft solvent parameters including the hydrogen-bond acidity or donor ability (R), the hydrogen-bond basicity or acceptor ability (β), and the polarity/polarizability (π*). It is found that the solubility of acetylene linearly correlates with β value and is almost independent of R or π*. The solubility of acetylene in RTILs increases with increasing hydrogen-bond acceptor (HBA) ability of the anion, but is little affected by the nature of the cation. Quantum mechanical calculations demonstrate that the acidic proton of acetylene specifically forms hydrogen bond with a basic oxygen atom on the anion of a RTIL. On the other hand, although C-H 3 3 3 π interaction is plausible, all optimized structures indicate that the acidic protons on the cation do not specifically associate with the π cloud of acetylene. Thermodynamic analysis agrees well with the proposed correlation: the higher the β value of a RTIL is, the more negative the enthalpy of acetylene absorption in the RTIL is.
’ INTRODUCTION Ethylene, obtained from naphtha or natural gas cracking, contains a small amount of acetylene ( [OAc] > [MeHPO3] . [MeSO4] . [TFA] > [BF4] . [PF6] > [Tf2N]. Solubilities of CO29,16 and some light hydrocarbons3,4,9,17 in RTILs are known to increase with increasing chain length of the alkyl substituent on the cation. Camper et al.,17 using simplified regular solution theory (RST), pointed out that the solubility of hydrocarbon in a RTIL are strongly dependent on the solubility parameters (δi) of solute and solvent. Accordingly, if RST is applicable, a linear correlation between the logarithmic values of solubility of acetylene (expressed in Henry’s law constant) and the inverse molar volumes of various solvents could exist. However, the data in Table 2 indicate that no regular trend is found to correlate the solubility of acetylene with the molar volume of the RTILs under study, suggesting that the solubility parameter of RTILs could be a minor factor in determining the solubility of acetylene. Acetylene is known to weakly bind to the polar organic molecules containing a heteroatom through X 3 3 3 H-Cacetylene (hydrogen bond)8 and X-H 3 3 3 πacetylene (where X can be a proton acceptor such as O, N, or halogen atoms) interactions.18 Likewise, acetylene can be hydrogen bonded to the heteroatom or heteroatoms of the anion of RTILs. This is probably the reason why acetylene is highly soluble in RTILs with an oxygencontaining anion. The much lower solubility of acetylene in RTILs bearing a fluorinated anion can be attributed to the electron-withdrawing effect of fluorine atoms, which results in the reduced electron density on the oxygen atom or atoms. This can be clearly seen from the comparison of the acetylene solubility in [BMIm][OAc] with that in [BMIm][TFA]. Previously, we proposed that the solubility of acetylene in RTILs is strongly related to the basicity of the anions in water, i.e., the lower the pKaq b (the stronger the bacicity) of the anion, the greater the acetylene solubility.3 However, such pKaq b -solubility correlation was found to have some limitation to be applicable to a broader range of RTILs. For instance, the pKaq b value of the anion of [BMIm][OAc] (pKaq b ∼ 9.3) is much lower than that of [BMIm][Me2PO4] (pKaq b ∼ 12.7), but the acetylene solubility in the latter RTIL in fact is higher than that in the former. Moreover, the acetylene solubility in [BMIm][MeSO4] (pKaq b ∼ 17.4) is higher than that in [BMIm][BF4] (pKaq b ∼ 14.4) or in aq [BMIm][Tf2N] (pKaq b ∼ 18.0). Poor pKb -solubility relationaq ship shown in Figure 2 suggests that pKb is not an appropriate parameter to describe the solubility behavior of acetylene in RTILs. For better estimation of solvency power of RTILs to dissolve acetylene, we have attempted to employ Kamlet-Taft solvent parameters (R, β, and π*) to differentiate the hydrogen-bond 1069
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Figure 2. Correlation of the acetylene solubility at 313 K and pKaq b of various anions of RTILs: (f) [BMIM][OAc], () [BMIm][MeHPO3], (3) [BMIm][Me2PO4], (diamond dotted) [BMIm][TFA], (b) [BMIm][BF4], (~) [BMIm][MeSO4], and (0) [BMIm][Tf2N].
Table 3. Kamlet-Taft Parameters for Selected RTILs and Literature Sources R
RTIL
a
β
π*
Figure 3. Correlation of the acetylene solubility at 313 K and the HBA ability (β Kamlet-Taft) of various RTILs: (O) [EMIm][Tf2N], (2) [BMIm][PF6], (0) [BMIm][Tf2N], (þ) [BMPyrr][Tf2N], (b) [BMIm][BF4], ([) [EMIm][MeSO4], (4) [EMIm][EtSO4], (]) [EMIm][MeHPO3], (9) [EMIm][Me2PO4], (f) [BMIm][OAc], (~) [BMIm][MeSO4], and (3) [BMIm][Me2PO4]. Dashed lines represent the 95% lower and upper levels of confidence intervals of the correlation.
ref
[EMIm][MeHPO3]
0.520
1.000
1.060
25a
[EMIm][Me2PO4]
0.510
1.000
1.060
25a
[EMIm][MeSO4] [EMIm][EtSO4]
0.570 n.a.a
0.610 0.710
1.090 n.a.
25b 25c
[EMIm][Tf2N]
0.420
0.100
1.020
25d
[BMIm][Tf2N]
0.617
0.243
0.984
19 19
[BMPyrr][Tf2N]
0.427
0.252
0.954
[BMIm][OAc]
0.550
1.090
0.990
25e
[BMIm][BF4]
0.627
0.376
1.047
19
[BMIm][PF6]
0.634
0.207
1.032
19
Not available.
donor/acceptor ability and dipolarity/polarizability of RTILs19 because Kamlet-Taft parameters were proven to be effective for representing the bulk solvent strength.20-23 Kamlet-Taft parameters for RTILs determined using a single set of dyes, Reichardt’s dye 30 or 33, 4-nitroaniline, and N,N-diethyl-4-nitroaniline are available in the literature. The π* parameter is measured using N, N-diethyl-4-nitroaniline, β value is determined using the solvatochromic shift of 4-nitroaniline relative to N,N-diethyl-4-nitroaniline, and the R value is determined using the ET(30) (Reichardt’s dye) and π* values.24 Those values for 10 selected RTILs used in this investigation are tabulated in Table 3. Emphasizing the ability of acetylene to form hydrogen bonds with the anions of RTILs, the natural log values of Henry’s law constant for the acetylene solubility versus the β values of several selected RTILs are plotted in Figure 2 and the corresponding linear regression equation is given in eq 1. lnðKH =105 PaÞ ¼ 3:43 - 1:55β
ð1Þ
As shown in Figure 3, a fairly good linear correlation was attained (R2 = 0.96, n = 10, sd = 0.12, F < 0.0001) between the acetylene solubility and the β value. Negative value of the slope indicates that the greater the β value, the lower the Henry’s law constant (the higher the solubility of acetylene). The validity of the obtained correlation is tested by predicting the solubility of acetylene in, for examples [BMIm][MeSO4], [EMIm][MeSO3], and [BMIm][Me2PO4]. Given the β value26 of [BMIm][MeSO4]
Figure 4. Optimized structures showing the interactions of 1,3-dialkylimidazolium RTILs with acetylene: (a) [EMIm][MeHPO3] þ C2H2; (b) [EMIm][Me2PO4] þ C2H2, (c) [EMIm][MeSO4] þ C2H2, and (d) [EMIm][Tf2N] þ C2H2. Bond distances are given in Å.
is 0.672, the correlation equation produces ln(KH,calc/105 Pa) = 2.39 (KH,calc/105 Pa = 10.9). This calculated Henry’s constant only deviates within 1% from the experimental value, KH,exp/105 Pa = 10.8 (ln(KH,exp/105 Pa) = 2.38). The β value25a of 0.700 for [EMIm][MeSO3] results ln(KH,calc/105 Pa) = 2.35 or KH,calc/ 105 Pa =10.5 giving 9% relative deviation from the experimental values (KH,exp/105 Pa=11.5 or ln(KH,exp/105 Pa) = 2.44). Larger positive deviation of Henry’s law constant (12% within the 95% confidence intervals of the correlation) is obtained for [BMIM][Me2PO4] where its β value26 of 1.118 gives ln(KH,calc/105 Pa) = 1.70 or KH,calc/105 Pa = 5.5 (KH,exp/105 Pa = 4.9; ln(KH,exp/105 Pa) = 1.59). For illustration, ln(KH,exp/105 Pa) for [BMIm][MeSO4] and [BMIm][MePO4] are also plotted in Figure 3. Using the above correlation, higher acetylene solubility in relatively basic RTILs, [BMIm][OAc] (β=1.09), [EMIm][Me2PO4] (β = 1.00), and [EMIm][MeHPO3] (β = 1.00) than that in [EMIm][MeSO4] (β = 0.61) can be rationalized on the basis of 1070
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Table 4. Solubility of Acetylene in Several RTILs at Various Temperatures KH/105 Pa RTIL
303.1 K a
313.1 K
318.1 K
323.1 K
328.1 K
333.1 K
[DMIm][MeHPO3]
6.1 (0.1)
7.1 (0.1)
8.2 (0.4)
9.4 (0.1)
10.8 (0.2)
12.3 (0.1)
13.9 (0.2)
[EMIm][MeHPO3]
5.2 (0.1)
6.0 (0.1)
6.9 (0.1)
7.9 (0.3)
9.2 (0.6)
10.3 (0.6)
11.8 (0.2)
[BMIm][MeHPO3]
5.3b (0.4)
6.1 (0.4)
6.9 (0.7)
8.0 (0.4)
9.1 (0.1)
10.4 (0.6)
11.8 (0.4)
4.6 (0.4)
5.3 (0.2)
6.2 (0.7)
7.1 (0.3)
8.1 (0.1)
9.3 (0.3)
13.0 (0.1)
14.7 (0.1)
16.7 (0.6)
18.9 (1.1)
20.9 (0.7)
23.7 (0.6)
[EMIm][Me2PO4] [EMIm][MeSO4]
11.4 (0.5)
[EMIm][EtSO4]
a
308.1 K
8.9 (0.1)
10.2 (0.1)
11.6 (0.3)
13.2 (0.3)
14.9 (0.1)
16.9 (0.1)
18.6 (0.2)
[EMIm][BF4] [BMIm][BF4]
20.9c (0.6) 13.2 (0.2)
20.5 (0.8) 14.9 (0.1)
23.0 (0.7) 16.7 (0.5)
25.7 (0.6) 18.8 (0.3)
28.6 (0.4) 21.0 (0.4)
31.5 (1.1) 23.1 (0.3)
35.1 (0.4) 25.5 (0.1)
[BMIm][PF6]
18.8d (0.5)
20.2 (0.4)
22.2 (0.3)
24.6 (0.1)
27.1 (0.5)
29.1 (1.5)
32.5 (0.6)
[EMIm][Tf2N]
20.2 (0.8)
21.8 (0.5)
23.6 (0.8)
26.3 (1.3)
28.4 (1.0)
30.3 (0.6)
33.1 (0.3)
Values in parentheses are the percent absolute deviation from the fit given in Table 5. b 303.6 K. c 308.8 K. d 305.1 K.
Table 5. Coefficients of eq 2 and the Percent Average Absolute Deviation of the Fit (AAD %) RTIL
B0
B1
B2
-2.477 10
[EMIm][MeHPO3]
þ11.36
-3.123 103
þ5.247 104
0.3
[BMIm][MeHPO3] [EMIm][Me2PO4]
þ12.6 þ10.73
-3.971 103 þ1.371 105 -2.846 103 þ2.477 103
0.5 0.4
[EMIm][MeSO4]
þ11.87
-3.317 103
þ1.371 105
0.6
-1.35 10
-1.794 105
0.2
-2.822 103 þ1.012 105
0.7
[EMIm][EtSO4] [EMIm][BF4]
þ8.584 þ11.11
3
-4.791 10
AAD %
[DMIm][MeHPO3] þ10.49
3
4
0.2
[BMIm][BF4]
þ7.396 -6.304 102
-2.526 105
0.3
[BMIm][PF6]
þ9.161 -1.859 103
-1.321 104
0.6
-2.734 103 þ1.666 105
0.8
[EMIm][Tf2N]
þ10.19
HBA ability. The moderate to low acetylene solubilities observed in neutral RTILs27 having a fluorinated anion such as [BMIm][BF4], [BMIm][PF6], and [RMIm][Tf2N] can also be ascribed to their low HBA ability. To obtain a more precise correlation, correlations using combinations of R and β or R, β, and π* were also constructed employing regression of multiple variables. However, any significant improvement was not attained by the inclusion of two other parameters, R and π* (R2 = 0.96, sd = 0.12, F < 0.0001, for R and β; R2 = 0.95, sd = 0.13, F < 0.0003, for R, β, and π*). The strong HΒΑ dependency of acetylene solubility as demonstrated by eq 1 could be rationalized as follows: in RTILs, R and β values are largely determined by the nature of the anion and the cation, respectively.18 Such a close correlation implies that the acetylene solubility in RTILs is governed mainly by the HBA ability of the anion. The negligible contribution of R parameter to the correlation could be attributed to the lack of secondary interaction involving the acidic protons in the dialkylimidazolium ring and the π cloud in acetylene (a typical interaction of C-H 3 3 3 π). On the other hand, the negligible effect of π* in the correlation could arise from the similar values of π* (close to unity) for the whole RTILs under investigation. Although the solubility of acetylene in a RTIL is mostly determined by the type of anion, the effect of alkyl group on the cation on the solubility should not be completely neglected. As shown in Table 2, the Henry’s law constant decreases slightly with the increase of alkyl chain length on the cation of the homologue series of [RMIm][MeHPO3], [RMIm][Me2PO4], and [RMIm][MeSO4] (where R = methyl, ethyl, and n-butyl).
Figure 5. Temperature-dependent solubility of acetylene in 1,3-dialkylimidazolium-based RTILs at a standard state of pressure (p° = 0.1 MPa): (O) [EMIm][Tf2N], (2) [BMIm][PF6], (1) [EMIm][BF4], (b) [BMIm][BF4], ([) [EMIm][MeSO4], (4) [EMIm][EtSO4], () [BMIm][MeHPO3], (/) [DMIm][MeHPO3], (]) [EMIm][MeHPO3], and (9) [EMIm][Me2PO4]. Lines represent the smoothed data from eq 2.
The slightly higher acetylene solubility (smaller KH) in a RTIL with longer alkyl chain could be ascribed to the combined effects of increased HBA ability and van der Waals interactions between acetylene and RTIL. The latter effect seems to correspond to the increase of the molar volume of RTILs and is more pronounced when the anion possesses relatively lower basicity. To gain deeper insight on the nature of the interactions between acetylene and RTILs, quantum chemical calculations were carried out. The optimized geometries showing the interaction of acetylene with [EMIm][MeHPO3], [EMIm][Me2PO4], [EMIm][MeSO4], or [EMIm][Tf2N] are depicted in Figure 4. As seen in Figure 4, a, b, and c, acetylene is located at the vicinity of the ethyl substituent on the cation with one of the acidic protons heading for the oxygen atom in the anion, due to the strong interactions of acidic acetylene with basic anions, [MeHPO3]-, [Me2PO4]-, and [MeSO4]-. The BSSE-corrected interaction energies for C2H2-[MeHPO3]-, C2H2[Me2PO4]-, and C2H2-[MeSO4]- are -21.9, -19.8, and -16.7 kJ mol-1, respectively. BSSE corrections of about 9-12% are rather significant at the level of theory employed. It is interesting to notice that, as shown in Figure 4d, acetylene interacts with the sulfonyl oxygen in [Tf2N]- rather than with 1071
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Table 6. Gibbs Free Energy, Enthalpy, and Entropy of Solvation for Acetylene in Several 1,3-Dialkylimidazolium-Based RTILs at Different Temperatures (the Values Are Consistent with p° = 0.1 MPa) T/K
ΔsolvG/kJ 3 mol-1
303.1
4.5
308.1 313.1
5.0 5.4
ΔsolvH/kJ 3 mol-1
ΔsolvS/J 3 mol-1 3 K-1
T/K
ΔsolvG/kJ 3 mol-1
-23.2
-91.6
303.1
5.5
-21.1
-87.6
-23.2 -23.1
-91.4 -91.3
308.1 313.1
5.9 6.4
-20.9 -20.8
-87.1 -86.6
[DMIm][MeHPO3] þ C2H2
ΔsolvS/J 3 mol-1 3 K-1
[EMIm][EtSO4] þ C2H2
318.1
5.9
-23.1
-91.2
318.1
6.8
-20.6
-86.1
323.1
6.4
-23.1
-91.0
323.1
7.2
-20.5
-85.7
328.1
6.8
-23.0
-90.9
328.1
7.7
-20.3
-85.2
333.1
7.3
-23.0
-90.8
333.1
8.1
-20.2
-84.8
AAD %a
0.4
0.4
1.1
AAD %
0.4
0.3
1.1
[EMIm][MeHPO3] þ C2H2
[EMIm][BF4] þ C2H2
303.1
4.1
-23.1
-89.7
308.1
7.7
-18.0
-83.5
308.1 313.1
4.6 5.0
-23.1 -23.2
-89.9 -90.0
308.8 313.1
7.8 8.2
-18.0 -18.1
-83.6 -83.8
318.1
5.5
-23.2
-90.1
318.1
8.6
-18.2
-84.1
323.1
5.9
-23.3
-90.3
323.1
9.0
-18.3
-84.3
328.1
6.4
-23.3
-90.4
328.1
9.4
-18.3
-84.6
333.1
6.8
-23.4
-90.5
333.1
9.8
-18.4
-84.8
AAD %
0.7
0.8
2.6
AAD %
1.7
0.8
2.0
[BMIm][MeHPO3] þ C2H2 303.6 308.1
4.2 4.6
-22.2 -22.4
[BMIm][BF4] þ C2H2 -87.0 -87.5
303.1 308.1
6.5 6.9
-19.1 -18.9
-84.4 -83.6
313.1
5.0
-22.5
-88.0
313.1
7.3
-18.7
-82.9
318.1
5.5
-22.7
-88.5
318.1
7.7
-18.5
-82.3
323.1
5.9
-22.9
-89.0
323.1
8.1
-18.3
-81.6
328.1
6.4
-23.0
-89.5
328.1
8.5
-18.0
-81.0
333.1
6.8
-23.2
-90.0
333.1
8.9
-17.9
-80.4
AAD %
1.1
0.5
1.8
AAD %
0.7
0.7
2.3
[EMIm][Me2PO4] þ C2H2 308.1 313.1
3.9 4.3
-23.5 -23.5
[BMIm][PF6] þ C2H2 -89.0 -89.0
305.1 308.1
7.4 7.7
-16.2 -16.2
-77.4 -77.3
318.1
4.8
-23.5
-89.0
313.1
8.0
-16.2
-77.3
323.1
5.2
-23.5
-89.0
318.1
8.4
-16.2
-77.3
328.1
5.7
-23.5
-89.0
323.1
8.8
-16.1
-77.2
333.1
6.1
-23.5
-89.0
328.1
9.2
-16.1
-77.2
333.1
9.6
-16.1
-77.2
AAD %
0.9
0.7
2.3
AAD %
1.5
1.7
5.4
[EMIm][MeSO4] þ C2H2
a
ΔsolvH/kJ 3 mol-1
[EMIm][Tf2N] þ C2H2
303.1
6.1
-20.1
-86.3
303.1
7.5
-13.6
-69.7
308.1
6.5
-20.2
-86.7
308.1
7.9
-13.7
-70.1
313.1
7.0
-20.3
-87.1
313.1
8.2
-13.9
-70.6
318.1
7.4
-20.4
-87.4
318.1
8.6
-14.0
-71.0
323.1
7.8
-20.5
-87.8
323.1
8.9
-14.2
-71.5
328.1
8.3
-20.6
-88.1
328.1
9.3
-14.3
-71.9
333.1
8.7
-20.7
-88.4
333.1
9.7
-14.4
-72.2
AAD %
1.4
1.3
4.1
AAD %
2.0
1.7
5.2
Percent average absolute deviation of experiments from the fit (Table 5) estimated based on the literature procedure.29
the fluorine atom or atoms. A much smaller BSSE-corrected interaction enthalpy of -9.6 kJ mol-1 for C2H2-[Tf2N]- can be ascribed to the electron-withdrawing effect of the trifluoromethyl groups, resulting in the reduction of the proton accepting ability of oxygen. The calculated distance of Oanion 3 3 3 H-Cacetylene are
in the order of [EMIm][Tf2N] > [EMIm][MeSO4] > [EMIm][MeHPO3]>[EMIm][Me2PO4]. This order agrees well with the relative HBA ability and the acetylene capacity of the RTILs. All the optimized structures indicate that acetylene is not specifically associated with the cation via C-H 3 3 3 π bonding, confirming 1072
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Figure 6. Solubility-enthalpy of solvation relationship for acetylene in several 1,3-dialkylimidazolium-based RTILs at 313 K: (O) [EMIm][Tf2N], (2) [BMIm][PF6], (1) [EMIm][BF4], (b) [BMIm][BF4], (() [EMIm][MeSO4], (4) [EMIm][EtSO4], () [BMIm][MeHPO3], (/) [DMIm][MeHPO3], (]) [EMIm][MeHPO3], and (9) [EMIm][Me2PO4].
that Oanion 3 3 3 H-Cacetylene interaction is the dominant factor in the acetylene solubilization in RTILs as also suggested by the solubility correlation. To verify the ab initio calculations and the β-solubility relationship, thermodynamic properties for the solvation of acetylene in RTILs were determined based on the solubility measurements at various temperatures using the isochoric technique. [DMIm][MeHPO3], [EMIm][MeHPO3], [BMIm][MeHPO3], [EMIm][Me2PO4], [EMIm][MeSO4], [EMIm][EtSO4], [EMIm][BF4], [BMIm][BF4], [BMIm][PF6], and [EMIm][Tf2N] were selected as representative RTILs for the evaluation of the cation and anion effect and on the relative HBA ability. The Henry’s law constants at several temperatures are presented in Table 4 and the temperature dependency of acetylene solubility in the 10 RTILs is displayed in Figure 5. As expected, the solubilities of acetylene in those RTILs decrease with the temperature rise. The correlations displayed in Figure 5 (or data in Table 4) are mathematically expressed by an empirical formula from Krause and Benson.14 n X ln KH ðTÞ=105 Pa ¼ Bi ðT=KÞ - i ð2Þ i¼0
The optimized coefficients Bi (i = 0-2), as well as the average absolute deviation of the fit considered as the precision of the experimental data, are listed in Table 5. Based on the coefficient values obtained from eq 2, Gibbs free energy, enthalpy, and entropy of solvation at various temperatures including at the standard condition (298 K and 0.1 MPa) were calculated using the equations14 below and the analysis results are shown in Table 6. ! KH ðT, pÞ Δsol G ¼ RT ln ð3Þ po Δsol H ¼ R
D lnðKH ðT, pÞ=poÞ Dð1=TÞ
Δsol S ¼
Δsol H - Δsol G T
ð4Þ p
ð5Þ
More emphasis is given on the enthalpy values because they are more closely related to the crossed solute-solvent molecular interactions and the acetylene absorption. Although the enthalpy of solvation resulted from the combination of various types of interactions, the hydrogen bond interaction is considered only because it is the most dominant factor in determining the acetylene solubility. Table 6 shows that enthalpies and entropies of solvation in RTILs do not vary significantly within the temperature ranges investigated. All RTILs exhibit negative enthalpies, indicating that the solvation process is exothermic. Some information regarding the solubility behavior of acetylene can be obtained from these thermodynamic data. Similar enthalpies for [DMIm][MeHPO3], [EMIm][MeHPO3], and [BMIm][MeHPO3] may imply that the different acetylene solubilities could be due to the variations on the solubility parameter resulted from the different molar volume of the RTILs. Solubility parameter, also known to be a cohesive parameter, is inversely proportional to the molar volume of solvent. Thus, the greater the molar volume is, the smaller the solubility parameter (the lower the internal cohesion of the solvent) is, leading to higher solubility.28 On the other hand, the lower acetylene solubilities in the rest of the RTILs than those in [RMIm][MeHPO3] can simply be attributed to the weaker hydrogen-bonding interaction of acetylene with the anion of RTILs as indicated by the lower enthalpies of solvation. To show the solubility-enthalpy relationship, logarithmic values of Henry’s law constant versus enthalpies of acetylene solvation at 313 K are plotted in Figure 6. A fairly good linear relationship with a negative slope similar to Figure 3 is observed (R2 = 0.86, n = 10, sd = 0.22, F < 0.0002). It is worthy to note that the order of acetylene solubility in [EMIm][Tf2N], [BMIm][PF6], [BMIm][BF4], [EMIm][MeSO4], [EMIm][EtSO4], [EMIm][MeHPO3], and in [EMIm][Me2PO4] shown in Figure 6 is in excellent agreement with that found in the solubility-HBA relationship (Figure 3).
’ CONCLUSIONS The solubility behavior of acetylene in RTILs was evaluated using a linear solvation energy relationship on the basis of Kamlet-Taft parameters (R, β, and π*) as the polarity descriptors. It is shown that the solubility of acetylene in RTILs can be linearly correlated with the β value, hydrogen-bond acceptor ability of the anions. On the contrary, the acetylene solubility seems little affected by the other two parameters, R and π*. Although the correlation equation presented here is not perfect enough for the precise estimation of the solubility, such correlation could be used as guidance in the selection of RTILs for acetylene capture. Once β value for a RTIL is determined independently using a spectroscopic method, the relative solubility of acetylene in a RTIL at a specified temperature (in our case 313 K) can be estimated using the proposed correlation. However, one should note that the RTILs used in this study are limited to classical RTILs without bearing specific functionalities on the cation and/or the anion. Molecular modeling and thermodynamic calculations also support the correlation between the acetylene solubility and β value, resulting in more negative enthalpy of acetylene absorption for the RTIL with higher β value. In conclusion, higher acetylene absorption can be easily achieved by selecting a RTIL with higher β value. 1073
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’ ASSOCIATED CONTENT
bS
Supporting Information. Text describing details of data reduction for the solubility experiments. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: þ82-2-966-3701. E-mail:
[email protected] (M.C.);
[email protected] (H.S.K.).
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