Ind. Eng. Chem. Res. 2007, 46, 4323-4329
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GENERAL RESEARCH Equation of State for the Vapor-Liquid Equilibria of Binary Systems Containing Imidazolium-Based Ionic Liquids Tengfang Wang, Changjun Peng,* Honglai Liu, and Ying Hu State Key Laboratory of Chemical Engineering and Department of Chemistry, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China
Jianwen Jiang* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576
The vapor-liquid equilibria of binary systems containing imidazolium-based ionic liquids (ILs) have been investigated using an equation of state (EOS) for the heteronuclear square-well chain fluids (hetero-SWCFs). ILs are represented by “diblock compounds”, with the alkyl group as one block and the imidazolium ringanion pair as the other block. Model parameters of the imidazolium ring-anion block are obtained by fitting the experimental pressure-volume-temperature (pVT) data of ILs, and they are combined with those of the alkyl block from our previous work to correlate and predict the molar volumes of homologous imidazolium ILs, with the average deviations of 0.62% and 3.24%, respectively. Vapor-liquid equilibria (VLE) of 22 binary systems that contain imidazolium ILs and solvents are correlated, with an average deviation of 5.38%. The results reveal that the imidazolium ILs can be regarded as diblock compounds fairly well and the heteroSWCF-EOS is well-suited for calculation of their thermophysical properties. 1. Introduction Ionic liquids (ILs) are typically composed of organic cations and organic/inorganic anions. At ambient room temperature, they exist as liquids and have a wide variety of unique properties (for instance, negligible vapor pressure, favorable solvation behavior, low viscosity, and high reactivity and selectivity1). These features ensure that ILs are good candidates for “green chemistry” applications. ILs have been suggested to replace the environmentally unfriendly volatile organic solvents. In the past decade, there has been considerable interest in the applications of ILs (e.g., chemical reactions, separation processes, and renewable batteries2,3). Particularly, by varying the cation, anion, and/or substituent groups, the properties of ILs could be readily tuned to fit into a specific requirement. The increasing utilization of ILs in chemical and industrial processes requires reliable and systematic thermophysical properties such as activity coefficients, heats of mixing, densities, solubilities, vapor-liquid equilibria (VLE), and liquid-liquid equilibria (LLE). In addition, the transport properties are also needed (viscosity, electric conductivity, mutual diffusion coefficients, etc.). Currently, a large number of experimental VLE4-11 and LLE11-14 data are available in the literature for pure ILs and their mixtures. However, experimental measurements are timeconsuming and usually are difficult to control; thus, it is highly desirable to have predictive methods. Several thermodynamic models have been proposed for IL systems. For instance, a new lattice model developed by us, based on the advanced statistical thermodynamic theories was used to correlate the VLE and LLE of binary IL systems.15 The conductor-like screening model (COSMO) supported by the common quantum chemical package of GAUSSIAN 03 was used to predict the VLE of 13 binary * To whom correspondence should be addressed. For C.P.: Tel.: 86-21-64252921. Fax: 86-21-64252921. E-mail:
[email protected]. ForJ.J.: Tel.: 65-65165083.Fax: 65-67791936.E-mail:
[email protected].
Figure 1. Molar volumes of pure ionic liquids (ILs) at 1 bar: (0) [Ph(CH2)-mim][Tf2N],33 (4) [C6mim][PF6],35 and (×) [C3mmim][Tf2N].31 Solid lines represent correlations, and dashed lines represent predictions.
Figure 2. Plotted p-x diagrams of benzene in different ILs at 353.15 K: (×) [C4mim][Tf2N], (4) [C2mim][Tf2N], and (0) [C1mim][Tf2N].9 Solid lines represent correlations.
systems that contained 5 imidazolium ILs.16 Wilson and UNIQUAC models were applied to correlate the VLE data.4,8,9
10.1021/ie061577d CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007
Figure 3. Plotted p-x diagrams of benzene in [C8mim][BF4] at different temperatures:10 (0) 298.15 K, (4) 303.15 K, (×) 308.15 K, and (]) 313.15 K. Solid lines represent correlations.
Figure 4. Plotted p-x diagrams of cyclohexane in different ILs at 353.15 K: (4) [C4mim][Tf2N], (0) [C2mim][Tf2N], and (×) [C1mim][Tf2N].9 Solid lines represent correlations.
Figure 5. Plotted p-x diagrams of water, 2-propanol, and acetone respectively in [C4mim][Tf2N] at 353.15 K: (0) water, (4) 2-propanol, and (×) acetone.8 Solid lines represent correlations.
NRTL theory was also used to estimate the VLE4,6,9 and LLE17,18 for systems that contained ILs. Original and modified UNIFAC group contribution methods were examined, using available data for the various systems of imidazolium bis(trifluoromethylsulfonyl)imides [RR’IM][BTI] with alkanes, alkenes, cyclic hydrocarbons, and alcohols.19 The predicted results were found in good agreement with experimental data. Nevertheless, the calculation of phase behavior using an equation of state (EOS) has been explored relatively less. Evidenced from the radial distribution functions of cationcation, anion-anion, and anion-cation obtained through molecular dynamics (MD) simulations, IL molecules exist
Figure 6. Plotted p-x diagrams of water, 2-propanol, and acetone respectively in [C2mim][Tf2N] at 353.15 K: (0) water, (4) 2-propanol, and (×) acetone.8 Solid lines represent correlations.
Figure 7. Plotted p-x diagrams of propanol in [C8mim][BF4] at different temperatures: (0) 298.15 K, (4) 303.15 K, (×) 308.15 K, and (]) 313.15 K.10 Solid lines represent correlations, and dashed lines represent predictions.
Figure 8. Plotted p-x diagrams of benzene in [C8mim][BF4] at different temperatures: (0) 298.15 K, (4) 303.15 K, (×) 308.15 K, and (]) 313.15 K.10 Solid lines represent correlations, and dashed lines represent predictions.
predominantly as ion pairs/clusters.20,21 Moreover, they are generally large and can be reasonably regarded as chainlike molecules. Consequently, ILs can be described by an equation of state (EOS) for chainlike fluids, as demonstrated recently.22-24 In our previous work, the pressurevolume-temperature (pVT) conditions of ILs and the gas solubilities in ILs were examined using an EOS.24 ILs were treated as a homonuclear square-well chain fluid (homoSWCF) characterized by three temperature-independent molecular parameters, namely, chain length (r), segment collision diameter (σ), and square-well depth (). Although, in Kroon et
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4325 Table 1. Model Parameters for the Imidazolium Ring-Anion Blocka
Table 2. Correlations and Predictions for Molar Volumes of Pure ILsa
block
/k (K)
r
σ3 (× 106 m3/mol)
κAB
∆ (%)
-mimPF6 -mimBF4 -mimTf2N -mmimTf2N
466.982 430.007 382.038 391.430
6.41247 6.01507 4.97972 4.89220
16.3581 14.3330 29.8894 32.8902
0.0169 -0.0306 0.0153 0.0102
0.13 0.02 0.05 0.04
Nm exp exp Note: ∆ (%) ) 100 × {∑i)1 |Vcal i - Vi |/Vi }/Nm, where Nm is the number of data.
a
al.’s work,22 parameters were obtained using the standard combining rules
(uk) ) x(uk) (uk) IL
anion
cation
and
V∞IL )
manionV∞anion + mcationV∞cation
Correlation
Prediction
system
κAB
∆ (%)
∆ (%)
Nm
ref
[C4mim][Tf2N] [C6mim][Tf2N] [Ph(CH2)mim][Tf2N] [C3mmim][Tf2N] [C6mim][PF6] [C8mim][PF6] [C8mim][BF4]
0.2237 -0.5204 0.1993 -0.036 0.2718 0.5922 0.5772
0.38 0.17 0.15 0.13 0.02 1.53 1.93
1.63 3.42 1.80 0.28 2.18 6.20 7.14
168 163 6 5 9 18 19
33 33 33 31 35 32 32
0.62
3.24
average deviation Note: ∆ (%) ) 100 × number of data. a
Nm {∑i)1
|Vcal i
-
exp Vexp i |/Vi }/Nm,
where Nm is the
that is composed of B segments. Note that if there are two or more alkyl groups, the one with the longer alkyl is treated as one block and the rest are treated as the other block. Taking [C8mim][BF4] as an example, the scheme is
m∞IL
with corresponding parameters available for cations and anions, in our work, the model parameters were obtained by fitting the experimental pVT data.24 We found that the relative deviation between the calculated molar volume and experimental data was
