Conventional Aspects of Unconventional Solvents: Room

Chapter 41

Downloaded by STANFORD UNIV GREEN LIBR on July 26, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch041

Conventional Aspects of Unconventional Solvents: Room Temperature Ionic Liquids as Ion-Exchangers and Ionic Surfactants 1

1

1

Mark L. Dietz , Julie A, Dzielawa , Mark P. Jensen , and Millicent A. Firestone 2

1

2

Divisions of Chemistry and Materials Science, Argonne National Laboratory, Argonne, IL 60439

With few exceptions, research in the field of room-temperature ionic liquids (RTILs) has sought to understand and exploit the "unconventional" aspects of these solvents, among them their wide electrochemical window, ionicity, and near absence of vapor pressure. The resemblance of certain families of RTILs to various well-known liquid ion-exchangers or ionic surfactants, however, raises a question as to the extent to which the behavior of these neoteric solvents can be understood on the basis of the known properties of these more conventional compounds. In this report, we examine this question as it relates to the development of novel, nanostructured media comprising ionic liquids and RTIL-based methods for metal ion separation and preconcentration. The results presented demonstrate that in certain respects, RTILs can be regarded as conventional chemical reagents.

Introduction Room-temperature ionic liquids (RTILs) have recently garnered intense interest as potential "green" alternatives to conventional organic solvents in a wide range of synthetic (1-4), catalytic (4-6), and electrochemical (7, 8) applications. Unlike ordinary {i.e., molecular) solvents, and in analogy to classical molten salts, ionic liquids exist in a fully ionized state and as a consequence, typically exhibit little or no vapor pressure (5). In addition, they

526

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


527 are charcterized by a wide electrochemical window (4) and an extraordinary degree of tunability, with relatively minor changes in cation and anion structure leading to significant variations in physicochemical properties (4). With few exceptions, research in the field of RTILs has sought to understand and exploit these "unconventional" properties. In certain instances (e.g., the application of ionic liquids in process-scale separations), however, an emphasis on the unconventionality of RTILs may actually serve as an impediment to their adoption as replacements for ordinary solvents. For this reason, our recent work has sought to determine the extent to which RTILs can be regarded as conventional chemical reagents, and the manner in which these more mundane aspects of ionic liquids might be used to advantage in either chemical separations or the design and synthesis of novel materials. While this may not initially appear to be an especially fruitful line of investigation, its potential becomes more evident when one considers the structural similarities between certain families of RTILs and various classical liquid anion-exchangers. The RTILs recently described by MacFarlane et al. (9), for example, differ little from the well-known liquid anion-exchanger, Aliquat 336™. The resemblance of the cationic constituent of these RTILs to certain cationic surfactants (e.g., CTAB) is also readily apparent (Figure 1). These similarities raise a question as to the degree to which the behavior of RTILs can be understood on the basis of the known properties of these two classes of compounds. In this chapter, we examine this question as it relates to the development of IL-based methods for the separation of strontium ion for potential application in large-scale nuclear waste processing or chemical analysis and to the preparation of nanostructured media comprising RTILs.

RTILs as Liquid Ion-Exchangers Strontium Partitioning into l-Alkyi-3-methylimidazoIium-Based Ionic Liquids Containing a Crown Ether. Over the last two decades, work in a number of laboratories has been directed at the development of improved processes for the removal of actinides and fission product radionuclides (particularly Sr-90 and Cs-137) from nuclear waste streams and for their separation and preconcentration from environmental and biological samples for subsequent determination (10-15). These separations are quite demanding; in each case, the matrix is a complex, multi-component mixture, often strongly acidic or alkaline, in which the concentration of the radionuclides present is far exceeded by the levels of any number of other sample constituents. Such separations require chemical systems (i.e., combinations of extradant and organic solvent) exhibiting both high efficiency and extraordinary selectivity. In the case of strontium extraction, the choice of extractant is relatively straightforward. That is, it is well known that certain crown ethers (CEs), in particular, those based upon 18-crown-6 (whose cavity size corresponds closely to that of the diameter of strontium ion), are capable of strong and selective complexation and/or extraction of S r (16-20). When the need to minimize the loss of the crown ether via solubilization in the aqueous phase and to reduce the 2+

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


§28

Tetraalkylammonium bis(trifluoromethylsulfonyl)imide salts

ÇlO-12 21-25

CI®

H

©|sj-^10-12^21-25

0-12H21-25

Aliquat 336™ CH

3

ΒΓ

Θ

Hexadecyitrimethylammonium bromide (CTAB) Figure 1.

The RTILs of MacFarlane (9), a conventional liquid anionexchanger ("Aliquat 336"), and a cationic surfactant ("CTAB").


529


expense and difficulty associated with its preparation are taken into consideration, the most appropriate choices of crown quickly become apparent: dicyclohexano-18-crown-6 (DCH18C6, shown below) or its di-i-butyl derivative (DtBuCH18C6) (21,22).

Selection of an appropriate solvent is more problematic. Among classical (i.e., molecular) solvents, chlorinated hydrocarbons would initially appear to offer much promise; solutions of various crown ethers in certain of these solvents have been found to provide relatively large distribution ratios (Ds , defined as [Sr] /[Sr] at equilibrium) (23-25). As a result of their toxicity and potential for adverse environmental impact, however, chlorinated hydrocarbons have long been regarded as unacceptable for any large-scale application. The lower toxicity of paraffinic hydrocarbons has made them the solvent of choice in many process-scale metal ion separations (11). Solutions of crown ethers alone in these solvents, however, yield extremely poor extraction of strontium ion from acidic media (23). In the early 1990's, workers at Argonne National Laboratory demonstrated the utility of various oxygenated, aliphatic solvents (in particular, 1-octanol) as diluents for crown ethers in the extraction of strontium (21, 22). Unlike many conventional solvents, 1-octanol exhibits a number of desirable physicochemical properties, most notably, the ability to dissolve significant concentrations of water, which by facilitating the transfer of incompletely dehydrated anions (e.g., nitrate) and cationic metal-crown ether complexes, greatly improves the efficiency of strontium extraction (26). The advantages over previously described approaches to the separation and preconcentration of strontium from aqueous solution afforded by crown ether-octanol systems are numerous (e.g., excellent selectivity, high extraction efficiency, facile recovery of extracted strontium) and have led to their emergence as a benchmark against which the performance of any proposed alternative must be measured (27). Recently, Dai et al. (28) examined the extraction of strontium by DCH18C6 into a series of N,N -dialkylimidazolium-based RTILs, such as are depicted here: r

org

aq

X=PF \Tf N 6

2


530 In several instances, extraction into l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (hereafter abbreviated as C2mim Tf2N~), for example, remarkably large Ds values were obtained, far exceeding those observed for any conventional solvent. That these results were achieved using an aqueous phase containing only millimolar concentrations of nitrate ion (as Sr(N03>2) is all the more remarkable. Interestingly, none of the solvents was found to dissolve any measurable quantity of water, thus indicating that unlike oxygenated aliphatic diluents, the presence of dissolved water is not an important factor in determining the efficiency of strontium partitioning into RTILs. Our own results using acidic, nitrate-containing aqueous phases (systems of greater potential practical value than those employed in Dai's studies) support this conclusion. As shown in Figure 2, which compares the effect of solvent water content on strontium partitioning for solutions of DCH18C6 in 1-octanol and C2mim Tf2N", although the water concentration in both solvents rises with increasing acidity, this increase is accompanied by a significant (4-fold) decrease in strontium partitioning in the RTIL system, in contrast to 1-octanol, for which a 50-fold increase in D$ is observed. This difference in response to changes in water content is not the only way in which strontium partitioning behavior into this solvent (and related RTILs) contrasts with that observed for 1-octanol. As shown in Figure 3, which depicts the dependency of Ds on DCH18C6 concentration (panel A), the relationship between strontium partitioning and crown ether concentration is decidedly nonlinear in 1-octanol, a result of significant aqueous phase Sr-CE complex formation (29). In the RTIL, however, the relationship between Ds and [DCH18C6] is a line of unit slope, consistent with partitioning of a 1:1 SrDCH18C6 complex and the absence of an appreciable equilibrium concentration of this complex in the aqueous phase. More noteworthy is the difference in the nitric acid dependencies observed for the two solvents (Figure 3, panel B). In 1-octanol, increasing nitric acid concentration is accompanied by an increase in Ds , as would be expected for extraction of a strontium-nitrato-DCH18C6 complex: +


r

+

T

r

r

r

Sr

2+

+ 2 N 0 - + DCH18C6 rg 12) alkyl chains will exhibit structural order (i.e., liquid crystallinity) (45-47). Shorter-chain (n\2) C mim salts suggest that some degree of alkyl chain interdigitation might reasonably be expected (45), this thickness may well be significantly greater. In an effort to obtain some insight into the molecular basis for gelation in 1

+

+

+

n

Cjomim Br", infrared spectroscopic studies of its fluid and gel phases have recently been undertaken. While our analysis is not complete at present, initial



538



Figure 5.

Two-dimensional small-angle X-ray scattering (SAXS) patterns and azimuthaily-averaged intensity as a function of scattering vector for the fluid (1.6% w/w water) and gel (16% w/w water) states of 1 -decy 1-3-methyiimidazolium bromide (T = 23 °C).


SO

540 results indicate that gelation has no significant effect on the shape or position of the symmetric (i.e., υ ( C H 2 ) ) or asymmetric ( υ ^ ( C H 2 ) ) methylene stretching bands for the C10 moiety, modes known to be sensitive probes of alkyl chain packing (49). Thus, gelation is not induced by two-dimensional alkyl chain packing or ordering effects. Gelation is, however, accompanied by a diminution in the bands associated with hydrogen bonding between the imidazolium ring (in particular, the proton on C-2) and the bromide ion (e.g., the "Sheppard effect" bands (50)). This observation is consistent with recent results obtained by Cammarata et al. (51) in infrared studies of the state of water added to Downloaded by STANFORD UNIV GREEN LIBR on July 26, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch041

5

C4mim Tf^N" and related RTILs, for which evidence of competition between the imidazolium cation and water for Η-bonding with the anion has been observed. This result, taken together with the fact that ionogel formation is not observed for samples containing less than 5% w/w or more than 40% w/w water +e

suggests that gelation relies on a proper balance between C|omim "Br and Br" •••H-O-H interactions, and that this balance, which apparently permits formation of a network comprising Ciomim , Br", and water molecules, can be achieved only over a specific range of RTIL water contents. Additional work to elucidate

the mechanism of gelation of RTILs, to devise means by which to "tune" their gelation (i.e., to adjust the lattice spacing of the ionogel), and to exploit these novel, nanostructured systems in chemical separations and materials synthesis is now underway in this laboratory.

Conclusions Although room-temperature ionic liquids are normally regarded as "unconventional", the results presented here demonstrate that efforts to understand their behavior by drawing analogies to conventional ion-exchangers and cationic surfactants can yield important and useful insights. In the area of separations, for example, these efforts have led to results that call into question


541


the "greenness" of RTILs as solvents in the extraction of metal ions by neutral ligands. Similarly, in the area of materials chemistry, this approach has led to the development of a simple means by which to induce the formation of a supramolecular assembly in an ionic liquid and thus, to the development of a system expected to provide both a novel medium in which to carry out chemical reactions and a versatile platform for the design and fabrication of new materials. We expect that the utility of this approach will become increasingly evident as investigations of A^W-dialkylimidazolium salts and other families of RTILs progress. .

Acknowledgements The authors thank Larry Curtiss and Peter Zapol (ANL-CHM) for electronic +

structure calculations on Cjomim Br", Soenke Seifert (ANL-CHM) for assistance with SAXS measurements, Paul G. Rickert (ANL-CHM) for N NMR analyses, and Urs Geiser (ANL-MSD) for X-ray crystallographic analysis of Sr(N03)2'18C6. This work was performed under the auspices of the Office of Basic Energy Sciences, Divisions of Chemical (MLD, JAD, MPJ) and Materials (MAF) Sciences, U.S. Department of Energy, under contract number W-31-109-ENG-38. 1 5

References 1. Seddon, K. R. J. Chem. Tech. Biotechnol. 1997, 68, 351-356. 2. Holbrey, J. D.; Seddon, K. R. Clean Prod. Processes 1999, 1, 223-236. 3. Earle, M . J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391-1398. 4. Olivier-Bourbigou, H.; Magna, L. J. Mol. Catal. A 2002, 182-3, 419-437. 5. Seddon, K. R. Kinet. Catal. 1996, 37, 693-697. 6. Zhao, D.; Wu, M . ; Kou, Y.; Min, E. Catal. Today 2002, 74, 157-189. 7. Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881-3886. 8. Carlin, R. T.; Fuller, J. In Proceedings of the 12th Annual Battery Conference on Applications and Advances-1997; Frank, Η. Α.; Seo, E. T., Eds.; Institute of Electrical and Electronics Engineers, New York, 1997; pp.261-266. 9. Sun, J.; Forsyth, M . ; MacFarlane, D. R. J. Phys. Chem. Β 1998, 102, 88588864. 10. Nash, K. L.; Barrans, R. E.; Chiarizia, R.; Dietz, M . L.; Jensen, M . P.; Rickert, P. G. Solv. Extr. Ion Exch. 2000, 18, 605-631. 11. Horwitz, E. P.; Schulz, W. W. In Metal-Ion Separation and Preconcentration: Progress and Opportunities; Bond, A. H.; Dietz, M . L.; Rogers, R. D., Eds.; ACS Symposium Series 716; American Chemical Society: Washington, DC, 1999, pp 20-50.



542 12. Science and Technology for Disposal of Radioactive Tank Wastes; Schulz, W. W.; Lombardo, Ν. J., Eds.; Plenum: New York, 1998. 13. Chemical Pretreatment of Nuclear Waste for Disposal, Schulz, W. W.; Horwitz, E. P., Eds.; Plenum: New York, 1994. 14. Dietz, M . L.; Horwitz, E. P.; Bond, A. H. In Metal-Ion Separation and Preconcentration: Progress and Opportunities; Bond, A. H., Dietz, M . L., Rogers, R. D., Eds.; ACS Symposium Series 716; American Chemical Society: Washington, DC, 1999, pp 234-250. 15. Dietz, M . L.; Horwitz, E. P. LC•GC 1993, 11,424-436. 16. Gokel, G. Crown Ethers and Cryptands; Royal Society of Chemistry: Cambridge, England, 1991. 17. Hiraoka, M . Crown Compounds: Their Characteristics and Applications; Elsevier: New York, 1982. 18. Izatt, R. M . ; Pawlak, K.; Bradshaw, J. S.; Breuning, R. L. Chem. Rev. 1995, 95, 2529-2586. 19. Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Breuning, R. L. Chem. Rev. 1991, 91, 1721-2085. 20. Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. Α.; Lamb, J. D.; Christensen, J. J. Chem. Rev. 1985, 85, 271-339. 21. Horwitz, E. P.; Dietz, M. L.; Fisher, D. E. Solvent Extr. Ion Exch. 1990, 8, 557-572. 22. Horwitz, E. P.; Dietz, M . L.; Fisher, D. E. Solvent Extr. Ion Exch. 1991, 9, 1-25. 23. Blasius, E.; Klein, W.; Schõn, U. J. Radioanal. Nucl. Chem. 1985, 89, 389398. 24. Gloe, K.; Muehl, P.; Kholkin, A. I.; Meerbote, M . ; Beger, J. Isotopenpraxis 1982, 18, 170-175. 25. Filippov, Ε. Α.; Yakshin, V . V.; Abashkin, V . M . ; Fomenkov, V . G.; Serebryakov, I. S. Radiokhimiya 1982, 24, 214-218. 26. Horwitz, E. P.; Dietz, M. L.; Fisher, D. E. Solvent Extr. Ion Exch. 1990, 8, 199-208. 27. Standard Test Method for Strontium-90 in Water, ASTM-D5811-95, American Society for Testing and Materials: West Conshohocken, PA, 1995. 28. Dai, S.; Ju, Y. H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1999, 12011202. 29. Dietz, M . L.; Bond, A. H.; Clapper, M.; Finch, J. W. Radiochim. Acta 1999, 85, 119-129. 30. Junk, P. C.; Steed, J. W. J. Chem. Soc., Dalton Trans. 1999, 407-414. 31. Dietz, M . L.; Dzielawa, J. A. Chem. Commun. 2001, 2124-2125. 32. Hamley, I. W. Introduction to Soft Matter: Polymers, Colloids, Amphiphiles, and Liquid Crystals, John Wiley: New York, 2000. 33. Firestone, Μ. Α.; Tiede, D. M . ; Seifert, S. J. Phys. Chem. Β 2000, 104, 2433-2438. 34. Rethwisch, D. G.; Chen, X.; Martin, B. D.; Dordick, J. S. In Biomolecular Materials by Design; Alper, M . ; Bayley, H.; Kaplan, D.; Navia, M., Eds.; Materials Research Society: Pittsburgh, PA, 1994, pp 225-230. 35. Tess, M . E.; Cox, J. A. J. Pharm. Biomed. Anal. 1999, 19, 55-68.



543 36. Fuller, J.; Breda, A. C.; Carlin, R. T. J. Electrochem. Soc. 1997, 144, L67L70. 37. Ikeda, Α.; Sonoda, K.; Ayabe, M . ; Tamaru, S.; Nakashima, T.; Kimizuka, N.; Shinkai, S. Chem. Lett. 2001, 1154-1155. 38. Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759-6761. 39. Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. Adv. Mater. 2002, 14, 351-354. 40. Yoshio, M . ; Mukai, T.; Kanie, K.; Yoshizawa, M . ; Ohno, H.; Kato, T, Chem. Lett. 2002, 320-321. 41. Firestone, Μ. Α.; Thiyagarajan, P.; Tiede, D. M . Langmuir 1998, 14, 46884698. 42. Firestone, Μ. Α.; Williams, D. E.; Seifert, S.; Csencsits, R. Nano Lett. 2001, 1, 129-135. 43. Nagamine, S.; Kurumada, K.; Tanigaki, M . Adv. Powd.Technol.2001, 12, 145-156. 44. Guinier, A . Crystals, Imperfect Crystals, and Amorphous Bodies, Dover: New York, 1994. 45. Bowlas, C. J.; Bruce, D. W.; Seddon, K. R. Chem. Commun. 1996, 16251626. 46. Gordon, C. M.; Holbrey, J. D.; Kennedy, A . R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627-2636. 47. Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 21332139. 48. Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A . Ab Initio Molecular Orbital Theory, John Wiley: New York, 1986. 49. Firestone, Μ. Α.; Shank, M . L.; Sugar, S. G. Bohn, P. W. J. Am. Chem. Soc. 1996, 118, 9033-9041. 50. Elaiwi, Α.; Hitchcock, P. B.; Seddon, K. R.; Srinivasan, N . ; Tan, Y . ; Welton, T., Zora, J. A. J. Chem. Soc., Dalton Trans. 1995, 3467-3472. 51. Cammarata, L.; Kazarian, S. G.; Salter, P. Α.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192-5200.


Conventional Aspects of Unconventional Solvents: Room

Recommend Documents