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Molecular Mechanism of CO2 and SO2 Molecules Binding to the Air/Liquid Interface of 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid: A Molecular Dynamics Study with Polarizable Potential Models Collin D. Wick,† Tsun-Mei Chang,‡ and Liem X. Dang*,§ Louisiana Tech UniVersity, Ruston, Louisiana 71270, United States, UniVersity of Wisconsin, Parkside, Wisconsin 53141, United States, and Pacific Northwest National Laboratory, Richland, Washington 99352, United States ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: September 9, 2010
Molecular dynamics simulations with many-body interactions were carried out to understand the bulk and interfacial absorption of gases in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). A new polarizable molecular model was developed for BMIMBF4, which was found to give the correct liquid density but which also had good agreement with experiment for its surface tension and X-ray reflectivity. The potential of mean force of CO2 and SO2 was calculated across the air-BMIMBF4 interface, and the bulk free energies were calculated with the free-energy perturbation method. A new polarizable model was also developed for CO2. The air-BMIMBF4 interface had enhanced BMIM density, which was mostly related to its butyl group, followed by enhanced BF4 density a few angstroms toward the liquid bulk. The density profiles were observed to exhibit oscillations between high BMIM and BF4 density indicating the presence of surface layering induced by the interface. The potential of mean force for CO2 and SO2 showed more negative free energies in regions of enhanced BF4 density, while more positive free energies were found in regions of high BMIM density. Moreover, these gases showed free-energy minimums at the interface, where the BMIM alkyl groups were found to be most prevalent. Our results show the importance of ionic liquid interfacial ordering for understanding gas solvation in them. I. Introduction Room-temperature ionic liquids (RTILs) have great potential to serve as new solvents for a variety of processes because of their unique properties and the ability to customize their properties by changing their substituents.1-4 One of their most useful properties is their ability to preferentially absorb certain gases, such as CO2 and SO2, while having near zero vapor pressures and very high stabilities. This makes them good candidates for physical solvents for CO2 and SO2 capture from flue gas streams.5,6 Current CO2 capture technology often relies on passing the flue gas through alkanolamine based aqueous solutions, which create a reaction with CO2 to create ammonium carbamate.7-10 After this, the solvent can then be recycled. However, this process requires high energy, has problems with loss of solvent through evaporation and degradation, and can cause corrosion.11-13 On the other hand, physical absorption has the benefit of not requiring high energy input as pressure can serve as the mechanism for absorption and desorption. Since coal gasifiers operate at fairly high CO2 partial pressure, this creates conditions favorable for their physical solvation.5 Some desirable properties for a physical solvent include low vapor pressures, high selectivity, low viscosity, and noncorrosive behavior.5,14 Common RTILs hold many of these properties, including a very low volatility and high stability.15 Also, while RTILs have fairly high viscosities near room temperature,16 many gasifiers operate at fairly high temperatures (>450 K)17 in which RTIL viscosities can be fairly low.18 * To whom correspondence should be addressed. † Louisiana Tech University. ‡ University of Wisconsin. § Pacific Northwest National Laboratory.
Because of the aforementioned promising properties, RTILs have been investigated extensively as a neat liquid,19-24 and their gas-sorption properties have been investigated both computationally25-28 and experimentally.29-34 Computational methods, specifically, have the benefit of being able to extract molecular level information from simulations, which can then be related to thermodynamic or dynamic properties. One factor that is increasingly becoming of interest with regard to RTILs is their interfacial properties with air35-46 and with supercritical CO2.47 Interfaces, in general, are of interest because they hold unique properties that can be exploited for specific tasks, for example, phase-transfer catalysis.48 Experiments can add to our understanding through surface-sensitive spectroscopic techniques, such as X-ray reflectivity,45 and surface tension measurements.43,44 These methods can bring significant insight into the average interfacial structure and orientation, but they lack a way to investigate the microscopic detail of molecular structures and geometries. Other methods, such as sumfrequency generation (SFG) spectroscopy, can bring more detailed information of the structure and orientation of RTILs but often can be difficult to analyze.49-52 This is where computational methods can greatly improve the understanding of surface structures by direct observation but rely on molecular models that need to be verified to provide confidence in their predictions and analysis. One factor that has been found to be important for the development of accurate molecular models for RTILs is many-body interactions. These forces have been found to be essential for modeling bulk RTIL structures and properties22,53 and become of increased importance when an interface is present.37 The reason for this is that an interface breaks the symmetry of a system in which the polarity on one
10.1021/jp106768y 2010 American Chemical Society Published on Web 09/30/2010
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side is significantly different from the other. For air-water interfaces, many-body interactions have been found to be critical for the observation of certain properties.54-56 Another important aspect of RTILs is the mechanism in which they solvate gases that are to be accommodated. Any gas molecule that is solvated would first come in contact with the surface of an RTIL, and its behavior near the interface will affect the overall sorption process. The interfacial structure of alkylimidazoliums, specifically, butyl-methylimidazolium borate (BMIMBF4), is known to preferentially have its butyl chain oriented toward the air at the air-BMIMBF4 interface, which induces oscillating BMIM and BF4 densities toward the bulk.45 This surface layering would be expected to significantly influence the interfacial solvation of different species. Other aspects, such as the interplay between BMIM and BF4 at the surface (i.e., which is preferentially found on the outer edge of the interface), will also affect CO2 and SO2 sorption, which has been found to more strongly interact with RTIL anions especially fluorinated ones.33 To address these issues, we carried out molecular dynamics (MD) simulations with many-body interactions to determine the mechanism for CO2 solvation in BMIMBF4. The paper is organized as follows. Section II presents the methodology and procedure for developing the polarizable molecular model for BMIMBF4, CO2, and SO2. Section III gives the simulation details. Results and discussion are given in section IV, and section V outlines our conclusions.
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Figure 1. Schematic of the charges, bond, and angle parameters used for the molecular models described along with the labels for each of the atomic sites.
TABLE 1: Parameters for the Molecular Models Used in this Worka
II. Molecular Models We developed many-body molecular models for BMIMBF4 along with CO2 in this work. For all atomic positions, we placed Lennard-Jones (LJ), coulomb charges, and point polarizable interactions on them. All atomic polarizabilities for the BMIM cation were taken from the work of Applequist et al.,57 and all BMIM point charges and bonded intramolecular interactions were taken from previous work.23,58 The LJ parameters for BMIM were taken from a combination of our 1-3-dimethylimidazolium force field46 and a previous BMIM force field,58 and some were modified to get the correct BMIMBF4 density. The BF4 geometry was kept completely rigid with its geometry set to be the same as the minimum-energy configuration from MP2 calculations with the aug-cc-pvtz basis set. The electrostatic potential was used to derive the point charges for BF4 at the same level of theory. These calculations were performed with the NWChem computational package.59,60 The BF4 polarizabilities were taken from a previous force field,22 and the LJ interactions were parametrized to get the correct BMIMBF4 liquid density at 298 K. The CO2 geometry was kept rigid as well, and its charges and geometry were taken from a previously developed force field.61 Its polarizabilities were taken from Applequist et al.,57 and the LJ parameters were originally taken from another force field developed for phase equilibria.62 The LJ parameters were then modified slightly to get good agreement for its solvation free energy in BMIMBF4. A schematic of BMIM, BF4, and CO2 is given in Figure 1 showing the geometry for BF4 and CO2 along with atom types and electrostatic charges. For hydrogens, only one alkyl hydrogen is shown per alkyl carbon for clarity. Table 1 gives the remaining nonbonded parameters, and all BMIM geometry parameters are given in previous work.23 For SO2, its molecular model was developed previously for interactions with water, and its details are given in a recent publication.63 III. Simulation Details A. Bulk Liquid. For the simulations in bulk liquid, 400 ion pairs were placed in a periodic cubic box and were equilibrated
a
atom type
σ (Å)
ε (kcal/mol)
R (Å3)
CT C1 CR HC HR HA NA B F CO O
1.908 1.850 1.870 1.487 1.150 1.150 1.760 1.964 1.924 1.571 1.712
0.1094 0.1150 0.110 0.0157 0.0260 0.0300 0.190 0.20 0.10 0.054 0.157
0.878 0.878 0.878 0.135 0.167 0.167 0.530 0.500 0.610 0.616 0.465
Figure 1 gives the charges and atom types.
at constant volume at a temperature of 500 K. These simulations were carried out for 5 ns to ensure equilibration. After this “hot” equilibration, the system was slowly cooled to final temperatures used for the simulations (298, 323, 350, and 450 K) at a rate of 10 K per 100 ps. After the systems were cooled to their respective temperatures, they were equilibrated for another 5 ns in which volume fluctuations were allowed in the NpT ensemble with an external pressure bath of 1 atm. Temperature and pressure were controlled with the Berendsen thermostat and barostat,64 and the BF4 and CO2 geometries were kept fixed with the SHAKE algorithm.65 A potential truncation of 11 Å was enforced with analytic tail corrections, while the particle mesh Ewald summation technique was used to handle longranged electrostatics.66 After equilibration in the NpT ensemble, the free energy of solvation for CO2 and SO2 was calculated in BMIMBF4 utilizing the free energy perturbation (FEP) method.67 For the FEP method, a simulation is run in one state, while the energy of the system is calculated for two states, and the free energy is calculated by
〈 (
∆G ) -RT ln exp -
Ei+1 - Ei kBT
)〉
(1) i
where Ei is the system in which the sampling is taking place, Ei+1 is the energy of the system with which the free-energy difference is being calculated, and kB is the Boltzmann constant. The FEP method was carried out in different stages each
Molecular Dynamics Study representing a different strength of interactions between the solute and the solvent defined as λi. For i ) 1, λi ) 0, and for the final stage, λi+1 ) 1. There is an additional challenge when dealing with a polarizable model. If the ith system has no solute-solvent interactions and λi+1 is even slightly positive, there is a small probability that the polarization catastrophe could occur if a polarizable site locates very close to a charge location. To remedy this, the i ) 0 and the i ) 1 stages had no electrostatic interactions but only LJ, while the remaining λi stages had both LJ and electrostatic interactions with the same λi. For CO2, there were four stages (to integrate between five points) with λi ) [0, 0.001, 0.1, 0.4, 1.0], and for SO2, five stages were used with λi ) [0, 1.0 × 10-5, 0.001, 0.1, 0.4, 1.0]. The temperature for the CO2 calculation was 323 K, while it was 348 K for SO2, to coincide with temperatures in which experimental Henry’s laws constants are available. For each stage, a total of 2 ns of simulation time was carried out, and all errors were calculated from 200 ps blocks. B. Interfacial Systems. The BMIMBF4 system described in the previous section was duplicated after the “hot” run (at 500 K) doubling the size of the system in the z-direction and only modifying the xy directions to agree with the density achieved with the bulk NpT calculations. The system was then equilibrated at 450 K for another 1 ns followed by the elongation of the simulation box to approximately 2 times its original size in the z-direction. This elongation created two air-RTIL interfaces bisecting the z-axis that were parallel to the xy-plane. As a result, the liquid occupied approximately half of the simulation box. This system was equilibrated for another 5 ns followed by cooling to the desired temperature for each system investigated as described later. Then, another 10 ns of equilibration was carried out.
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Figure 2. (a) BMIM-BMIM, BMIM-BF4, and BF4-BF4 RDFs at 298 K. (b) The calculated structure factor for the ionic liquid BMIMBF4.
I(q) )
∑ ∑ fifjxixj i
j
sin(qrij) qrij
(2)
IV. Results and Discussion A. Bulk Liquid. The density calculated for the BMIMBF4 system at 298 K was 1.208 ( 0.002 g/cm3 from simulations with 5 ns of production, which compares with the experimental value of 1.211 g/cm3 showing good agreement. The heat of vaporization calculated from our simulations is 34.0 ( 2 kcal/ mol. It is more difficult to compare heats of vaporization with experiment as many different values are available on the basis of the method used.69 Recent surface tension measurements were used to extrapolate a value of 30.6 kcal/mol,70 which is somewhat lower than our results, but there is often a broad range of values available in the literature. To gauge the structure of BMIMBF4, we calculated the radial distribution functions (RDFs) between the center of mass of the ions at 298 K. Figure 2a gives the RDFs calculated for this work. The comparison of these RDFs with previous work using nonpolarizable models finds that the BMIM-BMIM RDF has a similar first peak position and a height of 4.0 Å and 0.9, respectively,58 while the second peak is at a slightly shorter distance in our work at 7 Å versus 8 Å. For the BMIM-BF4 RDF, the peak height and position from our work is similar to previous study,58 while the BF4-BF4 RDF shows two peaks at 7 and 9 Å again similar to previous work.58 One thing that is well-known but that should be further emphasized is that the RDFs show significant structure even at fairly large interatomic distances. We also calculated the atomic RDFs (not shown), but we used them to calculate the structure factor following the procedure outlined in recent work using Debye’s scattering equation71
where fi represents the atomic scattering factors, xi represents the atomic number density of atom type i, and rij is the distances between atoms i and j. This function is then multiplied by a q-dependent sharpening factor, M(q)
M(q) )
fN2(0) fN2(q)
exp(-0.01q2)
(3)
where fN is the atomic scattering factor for atomic nitrogen. Multiplying I(q) with M(q) and q gives the structure factor S(q) ) qM(q)I(q), which is plotted in Figure 2b as a function of q. Unfortunately, we are unable to make comparisons with experiment for the BMIMBF4 system as we cannot find an experimental structure factor with which to compare. However, this value could potentially provide a blind comparison of our model with future experimental work if one were to measure this. As stated earlier, the CO2 and SO2 free energies of solvation were calculated with the free energy perturbation method.67 Free energies of solvation are generally not reported for gas solvation in RTILs, but they can be calculated from measured Henry’s law coefficients (Kh) as follows
(
∆G ) -RT ln
RTFliquid Kh
)
(4)
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Figure 3. Density distribution of the BMIM and BF4 center of masses (COM) along with the density distribution of the BMIM alkyl groups at 350 K (bottom) and 450 K (top). Zero represents the Gibbs dividing surface of BMIMBF4.
where Fliquid is the liquid density in mol/m3. We carried out simulations at 323 K for CO2 and at 298 K for SO2. The free energy extracted for CO2 at 323 K was -0.34 ( 0.5 kcal/mol, and it was -3.0 ( 0.5 kcal/mol for SO2 at 298 K. These are compared with the experimentally derived values of -0.24 kcal/ mol for CO2 at 323 K33 and -2.99 kcal/mol for SO2 at 298 K.34 The agreement is fairly close for CO2, which is expected as the free energy of solvation was used to help parametrize the CO2 model. For the SO2 potential model, which was developed on the basis of its interactions with water,63 the agreement is also very good showing a degree of transferability for our models. B. Neat Air-BMIMBF4 Interface. For the neat airBMIMBF4 interface simulations, two different temperatures were simulated, 350 K and 450 K. The distribution of the BMIM and BF4 center of masses with respect to the liquid center of mass as a function of z-position (which is perpendicular to the air-RTIL interfaces) was calculated for the interfacial systems at 350 K and 450 K and is given in Figure 3. These distributions were calculated with respect to their average bulk density, which is set to 1, and zero represents the Gibbs dividing surface (GDS) of BMIMBF4. The density distribution at 350 K shows strong structural ordering that is pervasive throughout the whole liquid slab. At 450 K, this structuring is greatly reduced. At 350 K, there is a significant enhancement of BMIM density toward the outer edge of the air-RTIL interface with respect to its bulk value at 350 K being around 1.75 times higher. The BF4 density, on the other hand, has a similar interfacial density as its bulk value, but it has a region of enhanced density a few angstroms from the interface creating an electrical double layer. At 450 K, the same behavior can be observed but to a lesser degree than at 350 K. To better gauge the structure of BMIMBF4 at the surface, the density profile for the BMIM alkyl groups was calculated at 350 K and 450 K, which is given in Figure 3 also. What sticks out is that the BMIM alkyl group has a significantly enhanced density near the interface toward the air. This is consistent with previous studies stating that the alkyl tails of imidazolium cations prefer to orient toward the air.45 Toward the bulk, the BMIM alkyl groups closely follow the BMIM center of mass density distribution showing no significant preference for the region of enhanced BF4 density. A snapshot of the air-RTIL interface is given in Figure 4. It can be observed that there are some alkyl groups dangling toward the air in agreement with what can be seen in Figure 3. These should have some effect on the overall surface tension and may be the
Figure 4. Snapshot of the BMIMBF4 air/liquid structure at 350 K.
Figure 5. Surface tension average with respect to simulation time.
reason why longer alkyl groups decrease it.43,44 This may also have a significant effect on the behavior of solutes at the interface as the alkyl groups may preferentially solvate particular species, which will be described later. One way to validate our characterization of the air-RTIL surface is to compare our surface tension with experiment. We calculated the surface tension in the usual way on the basis of the pressure tensor.72 Figure 5 gives the time-averaged surface tension as a function of time from a 20 ns production simulation at 350 K. The fluctuations in the surface tension are rather slow, which is expected because of the high viscosity and bulky size of the RTILs. The value that is reached is around 42.5 ( 5 dyn/cm with the uncertainty determined by dividing the system into four 5 ns blocks. The experimental surface tension at 350 K is estimated to be 37.6 dyn/cm in one report,44 while a value of near 41 dyn/cm can be estimated from the temperature
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Figure 7. Potential of mean force W(z) as a function of z-position with zero representing the GDS. Figure 6. Electron density as a function of position (bottom), and structure factor as a function of frequency (top).
dependence in another study.45 Both of these show reasonable agreement with our simulation results. While surface tension provides a good experimental comparison, different interfacial structures can often give rise to similar surface tensions. A better comparison to determine if our simulations capture interfacial structures is to compare our results with X-ray reflectivity measurements.45 To compare with these measurements, we first have to calculate the electron density as a function of position, which can be simply evaluated by adding up the number of electrons present in each atom. Basically, the density for each atom type is multiplied by the number of electrons for that atom. For instance, each carbon has 12 electrons, each fluorine has 9, and so forth. Adding the total number of electrons for each atom type together gives a total electron density distribution. The bottom panel of Figure 6 gives the electron-density distribution as a function of z-position for BMIM4 at 350 K. There is enhanced electron density near the interface with respect to the bulk, which coincides with the position where the BMIM ring atoms have a large enhanced density, followed by a small reduction in electrons a few angstroms in. Farther toward the liquid bulk, the electron density only has minor oscillations. The structure factor can be calculated from this
φ(qz) )
1 Fb
exp(iqzz)dz ∫-∞∞ [ dF(z) dz ]
(5)
where Fb is the bulk electron density, and F(z) is the electron density as a function of position. To make a connection of this with the reflectivity, its complex conjugate square has to be scaled by a factor 2 |Φ(qz)| 2 ) |φ(qz)| 2exp(σsim qz2)
(6)
where σsim is often taken from capillary wave theory,73 but in practice, we fit this value to provide the best agreement with the experimental reflectivity. Our fitted value, σsim, is significantly smaller than the value calculated from the capillary wave theory (i.e., σsim ) 1.64 Å vs σcwt ) 2.29 Å). The top panel of Figure 6 gives a comparison of our results with experiment. Our results are somewhat noisy because of the slow movement of BMIMBF4 at 350 K, but the agreement between simulation and experiment is very close in overall shape and intensities. This, along with our agreement with the experimental surface
tension, gives strong support in our ability to represent the experimental interfacial structure. C. Potential of Mean Force. The potential of mean force (PMF) was calculated for the transfer of CO2 and SO2 across the air-BMIMBF4 interface. To get the PMF, we fixed the BMIMBF4 liquid and gas solute center of masses at different z-positions spaced in 1 Å increments. At each position, the average force acting along the z-axis was calculated, and this force is integrated along the z-axis to obtain the PMF.74 The PMF was calculated for CO2 at 323 K and for SO2 at 350 K. The reason for using 350 K instead of 298 K for SO2 is to improve sampling, which is very difficult at 298 K. For each position, between 800 ps and 2 ns of simulation time was carried out with the 800 ps times for positions in which the gas molecule is not fully solvated and at least 1.6 ns for the fully solvated species. Figure 7 gives the PMF for CO2 and SO2 across the air-RTIL interface with zero representing the GDS. The PMF is most negative at the air-RTIL interface for both gases at a region just outside the GDS toward the air. This result clearly reveals that there is a preference for both solutes to be at the interface. At the interface, the most prevalent BMIMBF4 functional group is the BMIM alkyl group, which is known to enhance CO2 solubility.29 The interfacial region, though, will have a different structure than in the bulk as these dangling alkyl groups will have a lower density than in the bulk. Another interesting feature for the solvation of both species is that their free energy clearly oscillates up and down as it crosses the interface. This behavior is not observed for solvation in water for many gas species, such as the hydroxyl radical,75,76 but is clearly present here. The reason for this can be understood for CO2 in that it is known to have favorable interactions with the fluorines on RTIL anions.33 The density profile in Figure 3 shows that there is strong ordering between the BMIM and BF4 ions with the BMIM having enhanced density at the -3 and -11 Å positions. These directly coincide with regions of higher PMF pointing to repulsion between the solutes and BMIM with respect to other regions. In contrast, for regions of enhanced BF4 density, at the -7 and -15 Å positions, the PMF appears to be more negative for both solutes. It is well-known that CO2 and SO2 prefer to be solvated by the fluorinated anion making this behavior somewhat expected. This also shows that the nanostructured ordering in the RTIL creates pockets of enhanced CO2 and SO2 solubility. This effect is less for SO2 than for CO2 probably because of the higher temperature at which SO2 is being simulated but also possibly because of different interactions between BMIMBF4 and SO2 as SO2 has a more negative free energy of solvation. The average PMF in the bulk region can be used to estimate the free energy of solvation in which CO2 gives a value around -0.7 kcal/mol compared with the FEP value of -0.34 ( 0.5 kcal/mol showing agreement within
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the error bar. The free energy of solvation for SO2 in BMIMBF4 is around -2.65 kcal/mol at 350 K, which is slightly higher than -3.0 kcal/mol calculated at 300 K, which is expected at the higher temperature. V. Conclusions We carried out MD simulations to investigate the interfacial structure of BMIMBF4 and to determine how its interfacial structure affects the solvation of CO2 and SO2. New polarizable force fields were developed for BMIM, BF4, and CO2 and gave good agreement with experiment for the BMIMBF4 liquid density and reasonable agreement with the experimental BMIMBF4 heat of vaporization. Comparisons with the experimental surface tension and X-ray reflectivity at the airBMIMBF4 interface showed good agreement for our model. The structure of BMIMBF showed significant layering at lower temperatures with BMIM having enhanced density at the air-BMIMBF4 interface and BF4 having enhanced density a few angstroms toward the bulk followed by an oscillation between enhanced BMIM and BF4 densities. The PMF of CO2 and SO2 showed that they have a more negative free energy at the surface than in the bulk and that their PMFs oscillated between more negative free energy in regions of enhanced BF4 density and more positive for the regions of enhanced BMIM density. This shows the importance of the nanostructured interfacial structure on gas solvation. Interestingly, this effect was not found in a previous study of BMIMPF6 in contact with supercritical CO2,47 and it would be interesting to better understand how higher concentrations, different interfaces, and different pressure conditions affect this. Acknowledgment. This work was funded by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy and by the Pacific Northwest National Laboratory’s (PNNL) Energy Conversion Initiative (ECI), Internal Laboratory Directed Research and Development (LDRD). Battelle operates the Pacific Northwest National Laboratory for the U.S. Department of Energy. In addition, some of the research was funded by the Louisiana Board of Regents Research Competitiveness Subprogram contract number 3LEQSF(2008-11)-RD-A-21. The calculations were carried out using the resources from the Louisiana Optical Network Initiative (LONI) and from the computer resources provided by the Office of Basic Energy Sciences, U.S. Department of Energy. References and Notes (1) Shirota, H.; Castner, E. W.; Chung, S. H.; Greenbaum, S. G.; Wishart, J. F. Abstr. Pap. Am. Chem. Soc. 2006, 231, 8. (2) Welton, T. Chem. ReV. 1999, 99, 2071. (3) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (4) Wasserscheid, P. Nature 2006, 439, 797. (5) Heintz, Y. J.; Sehabiague, L.; Morsi, B. I.; Jones, K. L.; Pennline, H. W. Energy Fuels 2008, 22, 3824. (6) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Ind. Eng. Chem. Res. 2009, 48, 2739. (7) Shen, K. P.; Li, M. H. J. Chem. Eng. Data 1992, 37, 96. (8) Choung, Y. H.; Cho, K. C.; Choi, W. J.; Kim, S. G.; Han, Y. S.; Oh, K. J. Korean J. Chem. Eng. 2007, 24, 660. (9) Alejandre, J.; Rivera, J. L.; Mora, M. A.; de la Garza, V. J. Phys. Chem. B 2000, 104, 1332. (10) Kidnay, A. J.; Parrish, W. R. Fundamentals of Natural Gas Processing; Taylor & Francis: Boca Raton, FL, 2006. (11) da Silva, E. F.; Svendsen, H. F. Ind. Eng. Chem. Res. 2006, 45, 2497. (12) McCann, N.; Phan, D.; Wang, X. G.; Conway, W.; Burns, R.; Attalla, M.; Puxty, G.; Maeder, M. J. Phys. Chem. A 2009, 113, 5022. (13) Hammond, G. P.; Akwe, S. S. O. Int. J. Energy Res. 2007, 31, 1180.
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