Ionic Liquids IIIA - American Chemical Society

Chapter 18

Amphiphilic Self Organization in Ionic Liquids 1

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Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch018

Gary A. Baker and Siddharth Pandey

1Bioscience Division, Los Alamos National Laboratory, MS J586, Los Alamos, N M 87545 2Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India

Despite clear interest, progress toward the controlled formation of amphiphilic assemblies in ionic liquids (or their purely inorganic ancestor, the low-melting molten salt) is still in its infancy. In this chapter, we provide brief historical perspective, highlight results from the recent literature and speculate on the future of this exciting area of research.

Background 9

"Second-generation ionic liquids (air and moisture stable organiccontaining molten salts) have garnered global attention not least because they hold the simultaneous potential for novel, improved and/or eco-friendly chemistry. The latter point is manifest by strict adherence to the guiding principles of Green Chemistry (/). The first two aspects dealing with unique 234

© 2005 American Chemical Society

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

235 and advanced chemistry are bolstered by both the introduction of task-specific ionic liquids (2) and the fact that ionic liquids are, in general, 'modular' solvents that allow for facile synthetic variation. That is, for any given parent structure [A ][B~] one can in principle generate a truly huge superset of possibilities simply by iterative variation in the cation system, its substitution pattern and/or anion choice. Indeed, Seddon has remarked that over 10 simple salts that qualify as ionic liquids might be possible (3). For comparison, about 500 molecular organic solvents find use today; of these, a few dozen-mostly volatile organic compounds (VOCs)-comprise the bulk of industrial usage. It is worth stressing that at this juncture the number and variety of well characterized and/or commercial ionic liquids is still quite small compared with molecular organic solvents available to the chemist. Several principal strategies have emerged toward the creation of cleaner and more sustainable chemical technologies. The ultimate goal of course is elimination of the solvent altogether (4). For many applications performing a reaction 'neat' is not possible, however. Green alternatives to a solventless approach include the use of aqueous media (5), sub- or supercritical fluids, most notably supercritical carbon dioxide, scC0 (f5), fluorous phases (7), ionic liquids (8) and mixtures thereof. Using SCCO2 in concert with ionic liquids in particular has attracted the interest of several research groups recently (9). Taking a lesson from the ubiquitous and highly successful application of surfactants in scC0 (10), one suspects that similar utility might be found for them within ionic liquids. Surprisingly, there is very little on record concerning the use of surfactants within ionic liquids. The story actually begins much earlier than one might think. In fact, the term 'ionic liquid' is a constantly evolving one with an eminent prehistory including the field of classical 'molten salts' (//). Molten salts are also completely ionized solvents, however, they differ implicitly in both the melting temperature and the ion motif. That is, molten salts wholly consist of inorganic ions and even their eutectic mixtures do not melt below 150 °C. Ionic liquids on the other hand contain at least one organic ion, frequently a bulky and asymmetric cation based on an imidazolium, pyrrolidinium or phosphonium backbone. It is now widely accepted that the working definition of the term "ionic liquid" is reserved for salts that liquefy near or below 100 °C, an important subset of which is the room-temperature ionic liquid (8). In the section that follows we provide an account of research relating to the use of surfactants in ionic liquids. While the information presently available is relatively limited and progress has been slow, it has been punctuated with some significant victories. To the best of the authors' knowledge, Figure 1 provides a fairly inclusive and updated list of the surfactants studied in ionic liquids so far. Although this is a modest inventory it can be safely assumed that this will not +


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Figure I. Structures ofsurfactants studied in ionic liquids. 1 CTAB; 2 HDPB; 3 SDS; 4 Triton X-100; S Tween 20; 6 AOT; 7 CS; 8 Brij 35; 9 Brij 700.


237 remain the case for long. It is our hope that this chapter serve as a reveille to the growing ranks of ionic liquid researchers out there.

Surfactants in Ionic Liquids: A Brief Tour To date, reports of surfactants in molten salts or ionic liquids have been few and far between. In advance of any such account, Steigman and Shane communicated in 1965 that micelle formation was possible for long-chain fatty acids in concentrated sulfuric acid (12). Using surface tension and light scattering measurements these authors learned, for example, that stearic acid formed micelles in 97.3% (w/w) H S0 . Although this demonstrated that micelle formation was feasible in a highly ionized solvent it should be noted that this still constitutes a considerable water level on a molar basis (-2.8 M). On the other hand, similar water levels are often associated with ionic liquids in common use. The earliest evidence for micellization of surfactants in a 'lowmelting' molten salt is the work of Bloom and Reinsborough beginning a few years later in the late 1960s on pyridinium chloride melts near 150 °C (13). Although ethylammonium nitrate, [EtNH ][N0 ], an organic ionic liquid which melts around 14 °C, had been known since 1914 (this ionic liquid has the distinction of being the first one on record) (14) it was not until the early 1980s that Evans et al. (15) reported that [EtNH ][N0 ] supported micelle formation by certain surfactants. Using classical and quasi-elastic light scattering (15a\ hydrodynamic radii of 14 and 22 Â were estimated for tetradecylpyridinium bromide and hexadecylpyridinium bromide 2, respectively, in [EtNH ][N0 ]. From the change in critical micellar concentration (CMC) with surfactant chain length, the free energy of transferring a methylene group from the fused salt into the interior of a micelle was found to be -400 cal/mol compared with -680 cal/mol in water (15c). The fact that [EtNH ][N0 ] is a slightly better solvent than water for hydrocarbons accounts at least in part for the 7-10 fold higher CMCs observed in [EtNH ][N0 ]. Further, measured second virial coefficients were reasonably well described by a hard-sphere potential reflecting the effective electrostatic screening in this completely ionized solvent. Later, Chang studied a sodium dodecyl sulfate 3 + decane + ethylenediamine/ammonium/potassium nitrate molten salt (an energetic eutectic of interest to the explosives community, m.p.~104 °C) mixture using electrical conductivity (16). Although the results were not definitive, they did suggest a structural change from a globular droplet to a bicontinuous lamellar form as 1-pentanol was added. Although CMC


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238 values were 4 times higher than in water, direct comparison is not meaningful as these studies were conducted at 120 °C. More recently, Friberg et al. (17) investigated l-butyl-3methylimidazolium hexafluorophosphate, [bmim][PF ], solubilization in a Brij 30-water system (Brij 30 is equivalent to 8 for n=4) using small angle x-ray diffraction. Notably, the system formed a lamellar liquid crystal which solubilized large amounts of [bmim][PF ] (15% w/w) without changing the dimensions of the amphiphile layer or the water level dependence of interlayer spacing. In addition to its role as solvent, an appropriately designed ionic liquid may also function as a surfactant or cosurfactant. This is perhaps not too strained an assertion given the structural likeness of many existing ionic liquids to surfactants, phase transfer catalysts, electrolytes, ion exchangers and the like. A very interesting study that expands upon this concept was recently reported by Davis and co-workers (18). In this work, four novel ionic liquids formulated from imidazolium cations appended with long fluorous 'ponytails' were found to emulsify perfluorohexane into the 'conventional' ionic liquid l-hexyl-3methylimidazolium hexafluorophosphate, [C mim][PF ]. While perfluorohexane and [C mim][PF ] formed a biphase in the absence of surfactant immediately upon cessation of mechanical agitation, [C mim][PF ] saturated in any of these fluorous ionic liquids formed dispersions with perfluorohexane that persisted for weeks. In a similar approach, by dispersing amide group enriched glycolipids in ether-bearing ionic liquids, authors Kimizuka and Nakashima were able to form stable, thermally labile bilayer membranes, so-called 'ionogels' (19). In the last several months, three additional papers on the topic of surfactant action on or in an ionic liquid have found print. The first, a paper from the Armstrong laboratory explores the possibility of normal micelle formation for surfactants 3, 6, 7, 8, and 9 in [bmim][PF ] and [bmimJCI using surface tensiometry and inverse gas chromatography (IGC) (20). In line with earlier, studies (15, 16), their results indicate that CMCs in the ionic liquid are about an order of magnitude larger than in water. Their IGC results in concert with linear free energy relationships also show that above the CMC the hydrogen bond basicity increases dramatically for addition of the two nonionic surfactants 8 and 9 to [bmim][PF ]. The preparation of simple or multiple emulsions of ionic liquids stabilized solely by the interfacial adsorption of solid silica nanoparticles is also an interesting possibility realized by Binks et al. (21). Although surfactants are not involved here we include this contribution because these systems offer significant advantages in the pursuit of clean chemistry. 6


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239 The final topic of our discussion is a recent paper from one of the authors' labs (22). Using methods detailed earlier (23), the solvatochromic responses of two fluorescent probes-pyrene and l,3-bis(l-pyrenyl)propane (BPP)-were examined for increasing levels of surfactants 1, 3, 4, 5, 8, and 9 in the ionic liquid 1 -ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, temim][Tf N]. Our interest was based on the prospect of developing a rapid and convenient optical method for estimating CMC values using miniscule ionic liquid samples. The bases for the two probe responses are a pertubation in the


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Ionic Liquids IIIA - American Chemical Society

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