Ind. Eng. Chem. Res. 2009, 48, 4607–4610
4607
RESEARCH NOTES Effect of “Free” Cation Substituent on Gas Separation Performance of Polymer-Room-Temperature Ionic Liquid Composite Membranes Jason E. Bara,†,‡ Richard D. Noble,*,† and Douglas L. Gin*,†,‡ Departments of Chemical & Biological Engineering and Chemistry & Biochemistry, UniVersity of Colorado at Boulder, Boulder, Colorado 80309-0215
Room-temperature ionic liquid (RTIL) based monomers were photopolymerized in the presence of nonpolymerizable RTILs with various types of organic functional groups attached to the imidazolium cation. Groups employed include alkyl, ether, nitrile, fluoroalkyl, and siloxane functionalities. This straightforward method allows for a broad range of functional groups to be incorporated into poly(RTIL) matrices without the need to develop new monomers. The resultant poly(RTIL)-RTIL composites were homogeneous, optically transparent solids that exhibited no signs of phase separation, even after many months of storage. As thin films, poly(RTIL)-RTIL composites were utilized as gas separation membranes and tested for their permeabilities to CO2, N2, and CH4. The presence of 20 mol % “free” RTIL within the poly(RTIL) matrix was shown to increase CO2 permeability by ∼100-250% relative to the neat poly(RTIL) membrane with no free RTIL component. The nature of the organic functional group associated with the free RTIL cation can influence both gas permeability and CO2/N2 and CO2/CH4 selectivity. Polymerized room-temperature ionic liquids (poly(RTILs))1-8 are materials arising from the homopolymerization of RTILbased monomers (Figure 1). Upon polymerization of monomers of the types 1a or 1b, imidazolium cations are bound to the polymer backbone while anions remain “free”, that is not covalently bound to the polymer. A simple illustration of a poly(RTIL) framework with polymer-bound cations and free anions is displayed in Figure 2a. Formation of poly(RTIL) materials through polymerizable anions is also possible,3 and self-cross-linking systems can be synthesized using “gemini” RTIL monomers9,10 or when both cation and anion are polymerizable.3 We have previously synthesized RTIL monomers of the type 1a with alkyl, ether, and nitrile substituents, each with bis(trifluoromethane)sulfonimide (Tf2N) anion.1,2 Each substituent type has imparted unique behaviors with respect to CO2 separations in poly(RTIL) membranes when the parent monomer was polymerized in a thin film.1,2 Poly(RTIL) gas separation membranes were initially developed in an effort to overcome the limitations of supported ionic liquid membranes (SILMs).11-13 SILMs are a class of supported liquid membranes (SLMs) composed of RTILs encapsulated within a polymeric or inorganic support. Certain imidazolium-based RTILs have shown CO2 permeabilities near 1000 barrers and selectivities for CO2/N2 greater than 50 in the SILM configuration.11 These performance data, coupled with the nonvolatility of RTILs, appear to give SILMs much promise as a membrane platform. However, SILMs lack robust stability and are subject to failure under moderate pressures.11-13Poly(RTIL) membranes are more stable than SILMs and possess CO2/N2 and CO2/CH4 separation selectivities on par or greater than those observed in * To whom correspondence should be addressed. E-mail: NobleR@ Colorado.edu (R.D.N.);
[email protected] (D.L.G.). † Department of Chemical & Biological Engineering. ‡ Department of Chemistry & Biochemistry.
SILMs.1,2,11,12,14 Separation of these gas pairs in SILMs is primarily driven by the solubility selectivity of the RTIL.11,14-16 Separation selectivity in poly(RTIL) membranes is attributed to both solubility and diffusion contributions.1 Solubility selectivities in the initial poly(RTIL) membranes were near 40 for CO2/N2 and ranged from 7-19 for CO2/CH4.1 CO2/N2 separation was slightly disfavored by a diffusion selectivity of 0.7, while CO2/CH4 was favored by diffusion selectivity values of 2-3.1 However, neat poly(RTIL) membranes are much less permeable to gases, at levels only 1-5% of the most permeable SILMs.1,2,10,14 This major drop in gas permeability is attributed to large decrease in gas diffusivities through dense polymers relative to that through polymer-supported liquids.1,14 A mechanism we have employed for enhancing gas permeability in poly(RTIL)-based membranes has been to form a composite material (Figure 2b) by polymerizing an RTIL monomer in the presence of a nonpolymerizable, free RTIL.17,18 We have shown that free cation content of 20 mol % can
Figure 1. Examples of RTIL-based monomers with (a) styrene and (b) acrylate groups attached to imidazolium cations.
Figure 2. Representations of (a) poly(RTIL) framework with polymer-bound cations and (b) poly(RTIL)-RTIL composite containing 20 mol % free cations.
10.1021/ie801897r CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
4608 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 Table 1. CO2 Selectivity Data for RTILs 3b-f
Figure 3. Structures of (a) RTIL monomer used to form the poly(RTIL) component and (b-f) RTILs with various functional groups present as the free component in poly(RTIL)-RTIL composites.
improve CO2 permeability by up to 400% relative to the neat poly(RTIL) membrane with no free RTIL component.17,18 Poly(RTIL)-RTIL composites provide increased gas permeability with little or no sacrifice to CO2/N2 and CO2/CH4 selectivity17,18 and may have the potential to minimize the “fluxselectivity tradeoff” long associated with polymer gas separation membranes.19-21 Gas permeability can be increased in polymer membranes through opening the spaces between polymer chains and/or plasticization of the matrix.19-26 Both serve to increase the diffusivity of all gases, typically with detrimental impacts to selectivity, as diffusion differences have primarily been used as the mechanism for separating gas pairs such as CO2/CH4.19-26 However, in poly(RTIL)-RTIL composites, the polymer matrix is made more permeable through the presence of a free RTIL component not bound to the polymer backbone. “Free” RTILs serve as nonvolatile plasticizers and fill the spaces between polymer chains with a CO2-selective medium (Figure 2b), allowing for increased permeability while maintaining selectivity for CO2.17,18 Poly(RTIL)-RTIL composites may be considered as a type of SILM configuration, where the polymer phase is tailored for maximum compatibility with the RTIL component.27 It has been proposed that very strong ionic interactions hold the free RTIL within the poly(RTIL) matrix, preventing the nonpolymerizable component from escaping the membrane under applied pressure.17,18,27 In our experience, several atmospheres of pressure are insufficient to cause membrane failure through “blow out” of the free RTIL from the poly(RTIL)-RTIL composite,17,18 while we have observed SILM failures at applied pressures less than 2 atm.15 It is not currently clear at what applied pressure (if any) such a material failure of this nature might occur in poly(RTIL)-RTIL composites.27 Our previous report on poly(RTIL)-RTIL composite gas separation membranes focused on the effect of anion on CO2 permeability and selectivity.18 An ether-functionalized RTIL monomer (Figure 3a) was polymerized in the presence of 20 mol % RTILs comprising 1-ethyl-3-methylimidazolium ([C2mim]) cations with various anions (Tf2N, OTf, dca, and SbF6).18 We found that the poly(RTIL)-RTIL composite containing solely Tf2N anions formed the most permeable membrane, with a CO2 permeability of 60 barrers.18 Mixedanion composites (those containing both Tf2N and another anion) displayed CO2 permeabilities of about only 70% of that value.18 The nature of the anion originally associated with the nonpolymerizable RTIL had minimal impact on CO2/N2 and CO2/ CH4 selectivity, which ranged from 36 to 39 and 24 to 27, respectively.18 As poly(RTIL)-RTIL composites present nearly infinite possible configurations the design of gas separation membranes (and many other functional materials), the screening of anions provided an initial, yet crucial, guideline for engineeringpoly(RTIL)-RTILcompositeswithenhancedCO2 permeability.
RTIL
functionality
CO2/N2
CO2/CH4
T (K)
ref
3b 3c 3d 3e 3f
alkyl ether nitrile fluoroalkyl siloxane
23 30 35 27 19
11 13 16 19 10
313 313 313 295 295
29 29 28 15 16
Anion nature is but one variable that can be tuned to achieve the most desirable properties. The substituent on the “free’ imidazolium cation and the amount of free RTIL in the composite should also play roles in membrane performance. Thiscommunicationexplorestheperformanceofpoly(RTIL)-RTIL composites where the free cation has been appended with a variety of functional groups. Our work on CO2 separations in functionalized imidazoliumbased RTILs and neat poly(RTIL) membranes has shown that the nature of the substituent attached to the cation has significant effects on CO2 selectivity.1,2,15,16,28,29 Through analogy to organic solvents such as acetonitrile (CH3CN) and polymers such as polyethylene (PE), poly(ethylene glycol) (PEG), poly(acrylonitrile) (PAN), poly(dimethylsiloxane) (PDMS), and poly(tetrafluoroethylene) (PTFE), we have synthesized RTILs with a wide range of functionalities for use in the study of CO2 separations in RTILs (Figure 3b-f).15,16,28,29 Our previous publications have reported that RTILs and poly(RTILs) containing polar groups such as ethers28,29 (3c) and nitriles28 (3d) show improved CO2/N2 and CO2/CH4 separation selectivities compared to nonpolar alkyl analogues (3b). We have found that attachment of a fluoroalkyl group to an imidazolium cation (3e) results in decreased CO2/N2 selectivity, yet increased CO2/CH4 selectivity relative to those same alkyl analogues.15 An RTIL featuring a siloxane (3f) functionality was found to exhibit greatly diminished selectivities relative to the others.16 Table 1 details the CO2/N2 and CO2/CH4 selectivities of RTILs 3b-f. While the effects of each of these types of functionalities on CO2 separations in the bulk RTIL fluid have been studied,15,16,28,29 we have yet to explore what influence each might have on CO2 separations when included in a poly(RTIL)-RTIL composite membrane. These tailored imidazolium cations allow for the fabrication of poly(RTIL)-RTIL composites with a specific combinations of functionalities through simple addition of the free RTIL to the monomer solution rather than designing individually tailored RTIL monomers (which often require synthesis of a noncommercially available imidazole derivative as a starting material).1,2 The ability to tune the properties of polymers in this manner is a powerful tool for materials design and optimization.27 Poly(RTIL)-RTIL composite membranes containing various imidazolium-based free cations were fabricated by polymerizing monomer 3a in the presence of 20 mol % of a nonpolymerizable RTIL (3b-f), in a manner similar to those presented in our prior works in this area.17,18 The results of ideal (i.e., single gas) membrane experiments for CO2, as well ideal CO2/N2 and CO2/CH4 separation selectivities for each of the composites and neat poly(3a) are reported in Table 2. The changes in permeability and selectivity in the poly(RTIL)-RTIL composites relative to neat poly(3a) are reported alongside the respective data. Experiments were performed using a “time-lag” method at ambient temperature (295 K) and an initial driving force of 2 atm. More thorough details on these experiments can be found in our prior works.1,2,10,17,18,30 Table 2 shows that the presence of 20 mol % free cations increases CO2 permeability by 100-250% relative to neat
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4609 Table 2. Ideal Permeability of CO2 and Ideal Selectivities of CO2/N2 and CO2/CH4 in Poly(RTIL)-RTIL Composites and Neat Poly(3a) along with the Magnitude of Property Change Relative to Neat Poly(3a)a free cation
PCO2
change
CO2/N2
change
CO2/CH4
change
3b alkyl 3c ether 3d nitrile 3e fluoroalkyl 3f siloxane neat poly(3a)
51 ( 1 53 ( 1 33 ( 1 47 ( 1 55 ( 1 16
219% 231% 106% 194% 244%
37 38 40 36 33 41
-10% -7% -2% -12% -20%
24 26 28 27 20 33
-27% -21% -15% -18% -39%
Table 3. CO2 Solubility and Diffusivity Data for Poly(RTIL) Membranes along with the Magnitude of Property Change Relative to Neat Poly(3a)a free cation
SCO2
change
DCO2 (× 107)
change
3b alkyl 3c ether 3d nitrile 3e fluoroalkyl 3f siloxane neat poly(3a)
4.09 (0.17 3.71 (0.05 4.85 (0.10 4.39 (0.11 3.94 (0.17 3.37 (0.21
24% 10% 44% 30% 17%
0.95 (0.04 1.08 (0.02 0.52 (0.02 0.81 (0.03 1.06 (0.05 0.36 (0.02
164% 200% 44% 125% 194%
Data obtained at 295 K. PCO2 [)] barrers. Error bars represent one standard deviation obtained from at least three replicates.
a Data obtained at 295 K. SCO2 [)] cm3 (STP) cm-3 atm-1. DCO2 [)] cm2 s-1. Error bars represent one standard deviation obtained from at least three replicates.
poly(3a). Overall, composites containing ether, alkyl, fluoroalkyl, or siloxane groups on the free cation exhibit very similar CO2 permeabilities; all within a range of less than (10% of an average value of 52 barrers. This indicates that these types of functional groups on the free cation do not play a major role in determining CO2 permeability in poly(RTIL)-RTIL composites when the free RTIL is included at 20 mol%. The presence of a free RTIL is primarily responsible for the permeability gains, regardless of functionality type. While we have observed that pendant functional groups on imidazolium cations influence CO2/N2 and CO2/CH4 solubility selectivities in bulk RTIL solvents,15,16,28,29 the poly(RTIL) matrix appears to dominate CO2 selectivity in composite membranes when the free RTIL is present at 20 mol %. Each of the poly(RTIL)-RTIL composites is more selective for CO2/N2 and CO2/CH4 than the free RTIL component is as a bulk fluid,15,16,28,29 yet less selective than neat poly(3a). The loss in ideal selectivity for the poly(RTIL)-RTIL composites relative to neat poly(3a) is more pronounced for CO2/CH4 than CO2/N2. However, the beneficial gains in CO2 permeability in poly(RTIL)-RTIL composites relative to neat poly(3a) are offset by only minor decreases in CO2/N2. As has been observed previously, CO2/ CH4 selectivity in poly(RTIL)-RTIL is impacted more significantly, as the formation of a poly(RTIL)-RTIL composite appears to ease restrictions on CH4 diffusion observed in neat poly(RTIL) membranes, in turn diminishing the diffusion component of CO2/CH4 separation selectivity.1 Such behavior is not unexpected as the inherent selectivities of poly(RTIL)based membranes are similar to the solubility selectivities of bulk RTILs for CO2/N2, while poly(RTIL)-based membranes are proportionally much more selective for CO2/CH4 than their bulk RTIL counterparts.1,2,14-18,28,29 An increase in free RTIL content (i.e., formation of a composite) will result in a greater decrease CO2/CH4 separation than CO2/N2.17,18 The composite containing 20 mol % cations with nitrile functional groups is notable as it has a CO2 permeability of only about 60% of the other composites. This behavior is similar to what we have observed with poly(RTIL) membranes formed from nitrile-functionalized RTIL monomers analogous to 3a.2 The decreased gas permeability of nitrile-containing poly(RTIL) membranes relative to other neat poly(RTIL) materials was explained through analogy to the behavior of other nitrilecontaining polymers, such as PAN, which are well-known barrier materials.31 While poly(RTIL)-RTIL composites containing 20 mol % of 3d exhibited an increase in CO2 permeability relative to neat poly(RTIL) 3a, this increase was much smaller than each of the other composite membranes. Thus, while the free cation-anion pair serve to improve permeability in all cases, the nitrile functionality is detrimental to the magnitude of this effect. We speculate that the domains within the poly(RTIL)-RTIL composite that contain free cations are
rendered less permeable (i.e., slower gas diffusion) by the presence of the nitrile group. Table 3 displays solubility and diffusivity data for CO2 in each of the poly(RTIL)-RTIL composites examined in this work, as well as data for the neat poly(RTIL) membrane. It can be seen that formation of a composite has positive, though modest, effects on CO2 solubility relative to the neat poly(RTIL), which in turn favors slight increases in permeability. CO2 solubility values are similar those presented in our initial work with neat poly(RTILs) and the formation of a poly(RTIL)-RTIL composite does not seem to have any noteworthy impact on CO2 solubility. However, the major effect of composite formation is to enhance CO2 diffusivity, which is responsible for most of the gains in CO2 permeability reported in Table 2. With the exception of the composite containing nitrile groups, the inclusion of a free RTIL component enhanced CO2 diffusion between 125-200%. In these cases, the mere presence of any type of free RTIL allows CO2 to diffuse much more rapidly than in the neat poly(RTIL). The poly(RTIL)-RTIL composite with the nitrile group experienced the smallest diffusivity gain, indicating that this group acts to retard diffusion,2,31 even though all other composites experienced gains in CO2 diffusivity of 125% or more, regardless of the nature of the free RTIL. Reduced permeability due to the presence of a nitrile functionality is consistent with data we previously reported for neat poly(RTILs),2 and the barrier properties associated with conventional polymers based around nitriles.31 In summary, five poly(RTIL)-RTIL composites featuring free imidazolium cations with various functional groups were fabricated and their performances as gas separation membranes were examined. All materials exhibited increased gas permeabilities relative to a neat poly(RTIL) without any free component. Among these, the poly(RTIL)-RTIL composite containing ether-functionalized cations possesses the most promising combination of permeability and selectivity properties. A poly(RTIL)-RTIL composite containing nitrile groups exhibit the greatest selectivities for CO2/N2 and CO2/CH4 yet suffer from the lowest gas permeabilities. Conversely, poly(RTIL)-RTIL composites containing siloxane groups exhibit the greatest gas permeabilities, but lowest selectivities. We have now explored the performance of poly(RTIL)-RTIL composite gas separation membranes through several iterative generations of materials. The effects of RTIL monomer/ poly(RTIL), anion type, and free cation functionality have been examined with the free RTIL present at 20 mol %. Ample opportunities for further improvements exist. A forthcoming report will focus on the effect of the amount of free RTIL within the poly(RTIL) matrix on CO2 permeability and selectivity. We anticipate that the most dramatic changes in membrane performance will be achieved as the ratio of free cations to polymerbound cations is varied.
a
4610 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009
Acknowledgment Primary funding from the U.S. Army Research Office (AB07CBT010 and HDTRA1-08-1-0028) is gratefully acknowledged. Partial funding from the NSF is also gratefully acknowledged (DMR-0552399 to D.L.G.). Literature Cited (1) Bara, J. E.; Lessmann, S.; Gabriel, C. J.; Hatakeyama, E. S.; Noble, R. D.; Gin, D. L. Synthesis and Performance of Polymerizable Room Temperature Ionic Liquid Gas Separation Membranes. Ind. Eng. Chem. Res. 2007, 46, 5397. (2) Bara, J. E.; Gabriel, C. J.; Hatakeyama, E. S.; Carlisle, T. K.; Lessmann, S.; Noble, R. D.; Gin, D. L. Improving CO2 Selectivity in Polymerized Room-Temperature Ionic Liquid Gas Separation Membranes through Incorporation of Polar Substituents. J. Membr. Sci. 2008, 321, 3. (3) Ohno, H. Design of Ion Conductive Polymers Based on Ionic Liquids. Macromol. Symp. 2007, 249/250, 551. (4) Ogihara, W.; Washiro, S.; Nakajima, H.; Ohno, H. Effect of Cation Structure on the Electrochemical and Thermal Properties of Ion Conductive Polymers Obtained from Polymerizable Ionic Liquids. Electrochim. Acta 2006, 51, 2614. (5) Washiro, S.; Yoshizawa, M.; Nakajima, H.; Ohno, H. Highly Ion Conductive Flexible Films Composed of Network Polymers Based on Polymerizable Ionic Liquids. Polymer 2004, 45, 1577. (6) Tang, J.; Tang, H.; Sun, W.; Plancher, H.; Radosz, M.; Shen, Y. Poly(ionic liquid)s: A New Material with Enhanced and Fast CO2 Absorption. Chem. Commun. 2005, 26, 3325. (7) Ding, S.; Tang, H.; Radosz, M.; Shen, Y. Atom Transfer Radical Polymerization of Ionic Liquid 2-(1-butylimidazolium-3-yl)ethyl Methacrylate Tetrafluoroborate. J. Polym Sci. A Pol. Chem. 2004, 42, 5794. (8) Tang, J.; Sun, W.; Tang, H.; Radosz, M.; Shen, Y. Enhanced CO2 Absorption of Poly(ionic liquid)s. Macromolecules 2005, 38, 2037. (9) Nakajima, H.; Ohno, H. Preparation of Thermally Stable Polymer Electrolytes from Imidazolium-type Ionic Liquid Derivatives. Polymer 2005, 46, 11499. (10) Bara, J. E.; Hatakeyama, E. S.; Gabriel, C. J.; Zeng, X.; Lessmann, S.; Gin, D. L.; Noble, R. D. Synthesis and Light Gas Separations in Crosslinked Gemini Room Temperature Ionic Liquid Polymer Membranes. J. Membr. Sci. 2008, 316, 186. (11) Scovazzo, P.; Kieft, J.; Finan, D. A.; Koval, C.; DuBois, D.; Noble, R. Gas Separations Using Non-hexafluorophosphate [PF6]- Anion Supported Ionic Liquid Membranes. J. Membr. Sci. 2004, 238, 57. (12) Jiang, Y.-Y.; Zhou, Z.; Jiao, Z.; Li, L.; Wu, Y.-T.; Zhang, Z.-B. SO2 Gas Separation Using Supported Ionic Liquid Membranes. J. Phys. Chem. B. 2007, 111, 5058. (13) Ilconich, J.; Myers, C.; Pennline, H.; Luebke, D. Experimental Investigation of the Permeability and Selectivity of Supported Ionic Liquid Membranes for CO2/He Separation at Temperatures up to 125C. J. Membr. Sci. 2007, 298, 41. (14) Camper, D.; Bara, J.; Koval, C.; Noble, R. Bulk Fluid Solubility and Membrane Feasibility of Rmim-based Room Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2006, 45, 6279.
(15) Bara, J. E.; Gabriel, C. J.; Carlisle, T. K.; Camper, D. E.; Finotello, A.; Gin, D. L.; Noble, R. D. Gas Separations in Fluoroalkyl-functionalized Room-Temperature Ionic Liquids Using Supported Liquid Membranes. Chem. Eng. J. 2009, 147, 43. (16) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Guide to CO2 Separations in Imidazoliumbased Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48, 2739. (17) Bara, J. E.; Hatakeyama, E. S.; Gin, D. L.; Noble, R. D. Improving CO2 Permeability in Polymerized Room-Temperature Ionic Liquid Gas Separation Membranes through the Formation of a Solid Composite with a Room-Temperature Ionic Liquid. Polym. AdVan. Technol. 2008, 19, 1415. (18) Bara, J. E.; Gin, D. L.; Noble, R. D. Effect of Anion on Gas Separation Performance of Polymer - Room-Temperature Ionic Liquid Composite Membranes. Ind. Eng. Chem. Res. 2008, 47, 9919. (19) Robeson, L. M. The Upper Bound Revisited. J. Membr. Sci. 2008, 320, 390. (20) Robeson, L. M. Correlation of Separation Factor Versus Permeability for Polymeric Membranes. J. Membr. Sci. 1991, 62, 165. (21) Freeman, B. D. Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes. Macromolecules 1999, 32, 375. (22) Lin, H.; Van Wagner, E.; Raharjo, R.; Freeman, B. D.; Roman, I. High-Performance Polymer Membranes for Natural-Gas Sweetening. AdV. Mater. 2006, 18, 39. (23) Lin, H.; Van Wagner, E.; Freeman, B. D.; Toy, L. G.; Gupta, R. P. Plasticization-Enhanced Hydrogen Purification Using Polymeric Membranes. Science 2006, 311, 639. (24) Wessling, M.; Schoeman, S.; Van der Boomgaard, T.; Smolders, C. A. Plasticization of Gas Separation Membranes. Gas Sep. Purif. 1991, 5, 222. (25) Wind, J. D.; Staudt-Bickel, C.; Paul, D. R.; Koros, W. J. The Effects of Crosslinking Chemistry on CO2 Plasticization of Polyimide Gas Separation Membranes. Ind. Eng. Chem. Res. 2002, 41, 6139. (26) Baker, R. W.; Lokhandwala, K. Natural Gas Processing with Membranes: An Overview. Ind. Eng. Chem. Res. 2008, 47, 2109. (27) Lodge, T. P. A Unique Platform for Materials Design. Science 2008, 321, 50. (28) Carlisle, T. K.; Bara, J. E.; Gabriel, C. J.; Noble, R. D.; Gin, D. L. Interpretation of CO2 Solubility and Selectivity in Nitrile-Functionalized Room-Temperature Ionic Liquids Using a Group Contribution Approach. Ind. Eng. Chem. Res. 2008, 47, 7005. (29) Bara, J. E.; Gabriel, C. J.; Lessmann, S.; Carlisle, T. K.; Finotello, A.; Gin, D. L.; Noble, R. D. Enhanced CO2 Separation Selectivity in Oligo(Ethylene Glycol) Functionalized Room Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2007, 46, 5380. (30) Bara, J. E.; Kaminski, A. K.; Noble, R. D.; Gin, D. L. Influence of Nanostructure on Light Gas Separations in Cross-Linked Lyotropic Liquid Crystal Membranes. J. Membr. Sci. 2007, 288, 13. (31) Allen, S. M.; Fujii, M.; Stannett, V.; Hopfenberg, H. B.; Williams, J. L. The Barrier Properties of Polyacrylonitrile. J. Membr. Sci. 1977, 2, 153.
ReceiVed for reView December 09, 2008 ReVised manuscript receiVed March 18, 2009 Accepted April 01, 2009 IE801897R
