New fluorous ionic liquids function as surfactants in conventional room-temperature ionic liquids Travis L. Merrigan, Eleanor D. Bates, Scott C. Dorman and James H. Davis, Jr.* Department of Chemistry, University of South Alabama, Mobile, Alabama 36688 USA. E-mail: jdavis@jaguar1.usouthal.edu Received (in Columbia, MO, USA) 23rd June 2000, Accepted 13th September 2000 First published as an Advance Article on the web 2nd October 2000 Ionic liquids formulated from imidazole cations with ap- pended fluorous tails function as surfactants when added to conventional ionic liquids, facilitating the emulsification of fluoroalkanes with IL phases. The development of less-polluting, ‘neoteric’ solvents is a key element in an emerging agenda for sustainable industrial development. 1 The alternatives receiving the most attention as neoteric solvents are ionic liquids (ILs), supercritical CO 2 (sc- CO 2 ), supercritical/near-critical H 2 O, and perfluorocarbons. 2 Despite the intense interest in them, much of the basic chemistry of ionic liquids remains to be explored. Indeed, an under- standing of this basic chemistry is of vital importance to the development of ionic liquids for ‘green’ applications. 1,3 The thrust of our work with ionic liquids is the design and synthesis of salts composed of ions that incorporate structural or functional features that imbue them with particular proper- ties. 1d,4 In a recent paper, we reported the synthesis of the first imidazolium ion with a long fluorous ‘ponytail’, with which we formulated a low-melting salt, cf., a fluorous ionic liquid. 5,6 Our observation of the formation of a liquid crystalline phase by this structurally complex, fluorinated IL led us to speculate that the fluorocarbon tails of such ionic-liquid component cations associate. We subsequently reasoned that this property might be exploited to address a question of fundamental importance with regard to ionic liquid chemistry—the capacity of a designed, IL- based surfactant to alter the properties of a more conventional IL phase. Further, we deemed it feasible to capitalize upon such compounds to achieve a result of potential utility in biphasic systems—the use of the surfactant to promote and stabilize dispersions of a fluorous phase into an ionic liquid, two ordinarily immiscible neoteric solvent types. Note that Bren- necke, Johnson and Rogers have all recently demonstrated the potential importance of biphasic systems in which both solvent components were neoteric materials. 7 Here, we report the synthesis of four new fluorinated ionic liquids. The new compounds, 1–4, differ substantially from the handful of known fluorine-containing ionic liquids in that the cation head-group structure is conventional and the fluorous appendage long (Fig. 1). 8,9 All of the new compounds are surfactants that promote the formation and stabilization of dispersions of perfluorocarbons in a conventional ionic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate, [6-mim]PF 6 . Ours is the first report of the action of a fluorous surfactant on an ionic liquid, and only the second report of any surfactant action on or in an ionic liquid solvent. 10,11 The new fluorous ionic liquids are readily prepared from commercially available starting materials. Using an approach similar to that used to prepare our earlier fluorous IL, the imidazole cores 1-methyl- and 1-butylimidazole are dissolved in toluene, treated with a slight molar deficiency of 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluoro-8-iodooctane or 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluoro-10-iododec- ane and heated under reflux overnight. Cooling to rt followed by removal of the solvent in vacuo leaves soft, off-white waxes. These iodide salts are readily metathesized in acetone using AgPF 6 , giving ILs 1–4 in good yields. 12 Compounds 1–4 manifest several behaviors common to surfactants. Simple capillary-rise measurements reveal that the surface tensions of saturated solutions (0.3 mass percent, ~ 1–4 mmolar) of 1–4 in [6-mim]PF 6 range from 10 to 15% less than the surface tensions of the conventional ionic liquid alone. Further, Ostwald viscosimetry establishes that increases in the viscosity of [6-mim]PF 6 occur upon addition of compounds 1–4, a behavior common to known surfactants. 13 Still, the most dramatic surfactant behavior manifested by the new ILs is their capacity to act as agents to promote and stabilize dispersions of perfluorohexane in the ‘conventional’ IL [6-mim]PF 6 . In the absence of the new fluorous ILs, the IL and perfluorohexane phases remain well defined. Controlled, timed mechanical agitation of the two-phase system results in the dispersal of droplets of the fluorous phase into the IL, but the droplets are quite large and individually distinguishable. They visibly begin to coalesce back into a bulk phase upon cessation of agitation. When this experiment is repeated using saturated solutions of 1–4 dissolved in [6-mim]PF 6 , the dispersions that are formed persist for weeks without visible change. Transmittance spectroscopy of the dispersions of per- fluorohexane in the fluorous/conventional ionic liquid solution versus time graphically demonstrates the enhanced stability of those systems (Fig. 2). 14 Without fluorous IL, the initial turbidity of the system as determined by transmittance is slightly more than 40%. When any of the compounds 1–4 is dissolved in the bulk IL phase, the initial transmittance values are all near 10%. The rapid coalescence of the dispersed perfluorohexane droplets is apparent in the system without any Fig. 1 General structure of the new fluorous ionic liquids. Fig. 2 Transmittance of perfluorohexane dispersions in [6-mim]PF 6 as a function of time with (top to bottom) no added fluorous IL; added 2; added 1; added 4; added 3. The enhanced stability of the dispersions in the presence of the fluorous ILs is apparent. This journal is © The Royal Society of Chemistry 2000 DOI: 10.1039/b005418f Chem. Commun., 2000, 2051–2052 2051