Langmuir 2009, 25, 1311-1315
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Articles Redox-Switched Amphiphilic Ionic Liquid Behavior in Aqueous Solution Be´ne´dicte Chamiot, Ce´cile Rizzi, Laurent Gaillon, Juliette Sirieix-Ple´net,* and Joe¨l Lelie`vre UPMC UniVersite´ Paris 06 UMR 7575, Laboratoire d‘Electrochimie et de Chimie Analytique CNRS-ENSCP UMR 7575 Laboratoire d’Electrochimie et de Chimie Analytique, case 39, 4 place Jussieu 75252 Paris Cedex 05, France ReceiVed September 30, 2008. ReVised Manuscript ReceiVed NoVember 24, 2008 A new redox amphiphilic ionic liquid (AIL) containing ferrocene as a redox-active group was synthesized, 1-(11ferrocenylundecyl)-3-methylimidazolium bromide (Fc11MIm+). Adsorption and aggregation of both reduced and oxidized forms of this ferrocenated AIL in aqueous solution were studied by surface tension measurements. The micellization was favored for the reduced ferrocenated AIL (Fc11MIm+) as compared with the oxidized ferrocenated AIL (Fc+11MIm+). Minimum areas at the air/aqueous solution interface were identical whereas limiting surface tensions were slightly different. This corroborated the formation of an expanded monolayer of redox active AIL at the interface. The electrochemical behavior of redox active AIL was investigated. The electrochemical responses of Fc11MIm+ aqueous solution interestingly differed, depending on its concentration. Below the cmc, the electrochemical reaction was dominated by ferrocenated AIL adsorbed onto the electrode surface; then above the cmc, it was controlled by the Fc11MIm+ diffusing to the electrode. For the latter, the electrochemical mechanism was suggested to couple with the disruption reaction of the reduced form micelles.
Introduction Ionic liquids (ILs) have received much interest as an alternative to volatile organic solvents for applications in synthesis, electrochemical, and separation processes because of remarkable physical and chemical properties such as negligible vapor pressure, high ionic conductivity, and nonflammablility.1,2 Owing to the molecular structure of ILs associating an organic cation and an inorganic/organic anion, their physicochemical properties can be easily modulated by changing one of the ions. Davis et al. initiated the development of a new family of ionic liquids designed for specific tasks such as organic synthesis, catalysis, nanoparticles synthesis, conductive materials improvement, extraction, dissolution, etc. These ILs, in which a functional group is covalently tethered to either the cation or the anion (or both), were referred to as “task-specific ionic liquids” (TSILs) by these authors.3 It is noteworthy that most TSIL design was focused on the incorporation of a functionality into a branch appended to the cation, generally of the imidazolium type.4-6 For example, TSILs have already been synthesized with groups * Corresponding author. E-mail:
[email protected]. (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003. (3) Davis, J. H., Jr Chem. Lett. 2004, 33, 1072. (4) Lee, S.-g. Chem. Commun. 2006, 1049. (5) Gao, Y.; Twamley, B.; Shreeve, J. M. Inorg. Chem. 2004, 43, 3406. (6) Balasubramanian, R.; Wang, W.; Murray, R. J. Am. Chem. Soc. 2006, 128, 9994. (7) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., Jr J. Am. Chem. Soc. 2002, 124, 926. (8) Fraga-Dubreuil, J.; Famelart, M.-H.; Bazureau, J.-P. Org. Process Res. DeV. 2002, 6, 374. (9) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026. Kim, K. S.; Demberelnyamba, D.; Lee, H. Langmuir 2004, 20, 556.
such as amines for capture of CO2,7 ether or alcohol in liquidsupported synthesis,8 thiol for stabilizing nanomaterials,9 silicate for sol-gel processing,10 and urea or thiourea for depollution.11 Even with a short chain substituted ion, inherent amphiphilic nature of ILs has recently attracted much attention.12 A variation of the length of the alkyl chain on either cation or anion allows the lipophilicity to be adjusted. In recent years, the aggregation properties of various ILs from the imidazolium family with a long alkyl chain appended to ions in close resemblance to classical surfactants were also demonstrated at low concentrations in water.13-20 Such compounds are called amphiphilic ionic liquids (AILs). The functionalization of AILs appears as another way to modulate their hydrophobic/hydrophilic balance and consequently their aggregation. For classical surfactants, pH change and photochemical or redox reactions are known external stimuli to control aggregation process.21-23 The introduction of a redox active group should allow the design of an AIL that can reversibly (10) Leniewski, A.; Niedziolka, J.; Palys, B.; Rizzi, C.; Gaillon, L.; Opallo, M. Electrochem. Commun. 2007, 9, 2580. (11) Visser, A. E.; Swaltloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H., Jr.; Rogers, R. D. EnViron. Sci. Technol. 2002, 36, 2523. (12) Evans, K. O. Colloids Surf., A 2006, 274, 11. Canongia Lopes, J. N. A.; Padua, A. A. H. J. Phys. Chem. B. 2006, 110(7), 3330. (13) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133. (14) Miskolczy, Z.; Sebo˜k-Nagy, K.; Biczo´k, L.; Go¨ktu¨rk, S. Chem. Phys. Lett. 2004, 400, 296. (15) Sirieix-Ple´net, J.; Gaillon, L.; Letellier, P. Talanta 2004, 63, 979. (16) Gaillon, L.; Sirieix-Ple´net, J.; Letellier, P. J. Sol. Chem. 2004, 33, 1333. (17) Vanyur, R.; Biczo´k, L.; Miskolczy, Z. Colloids Surf., A 2007, 99, 256. (18) El Seoud, O. A.; Pires, P. A. R.; Abdel-Moghny, T.; Bastos, E. L. J. Colloid Interface Sci. 2007, 313, 296. (19) Dong, B.; Li, N.; Zheng, L.; Yu, L.; Inoue, T. Langmuir 2007, 23, 4178. (20) Dong, B.; Zhao, X.; Zheng, L.; Zhang, J.; Li, N.; Inoue, T. Colloids Surf., A 2008, 317, 666. (21) Lair, V.; Bouguerra, S.; Turmine, M.; Letellier, P. Langmuir 2004, 20, 8490.
10.1021/la803212q CCC: $40.75 2009 American Chemical Society Published on Web 01/07/2009
1312 Langmuir, Vol. 25, No. 3, 2009
Figure 1. Chemical structures of the reduced form, Fc11MIm+, and of the oxidized form, Fc+11MIm+.
switch between reduced and oxidized states exhibiting different properties. Synthesis of a few electroactive ILs incorporating redox-active constituents, including triiodide/iodide,24 ferrocene,6,25,26 and polyoxometalates,27 were already reported. In this paper, we describe the synthesis of a new ferrocenated redoxswitched AIL, 1-(11-ferrocenylundecyl)-3-methylimidazolium bromide (Figure 1). The amphiphilic and electrochemical properties of both reduced and oxidized forms were evaluated and discussed.
Experimental Section Materials. Reagents and solvents were purchased from Acros, Fluka, Aldrich, and Prolabo and used as received except ferrocene and 11-bromoundecanoic acid which were recrystallized from ethyl acetate and cyclohexane respectively before use. Dichloromethane was dried with CaCl2, distilled over P2O5, and stored over 4 Å molecular sieves. Ultrapure water was filtered with an ELGA UHQ II system. Column chromatography was carried out on silica gel of particles size 0.063-0.2 mm (Fluka). 1H and 13C spectra were recorded on a Bruker AC-200 spectrometer at 200 and 50 MHz, respectively. Electron ionization mass spectra (EI-MS) were obtained using a Varian model Saturn 2000 Ions trap mass spectrometer operating at 70 eV coupled to a Varian Model 3800 gas chromatograph. Low resolution positive electrospray mass spectra (ESI-MS (positive)) was obtained using a triple quadrupole mass spectrometer (Quattro I Micromass, Manchester, UK). Synthesis. The 1-(11-ferrocenylundecyl)-3-methylimidazolium bromide was prepared using a procedure adapted from the method described by Saji et al.28 (11-Bromoundecanoyl)ferrocene. A solution of 11-bromoundecanoic acid (12 g, 45.2 mmol) in 16.5 mL of thionyl chloride was stirred under reflux for 1 h. The excess of SOCl2 was evaporated (22) Sakai, H.; Matsumura, A.; Yokoyama, S.; Saji, T.; Abe, M. J. Phys. Chem. B 1999, 103, 10737. (23) Luk, Y.-Y.; Abbott, N. L. Curr. Opin. Colloid Interface Sci. 2002, 7, 267. (24) Kawano, R.; Watanabe, M. Chem Commun. 2003, 330. Kawano, R.; Watanabe, M. Chem. Commun. 2005, 2107. Jovanovski, V.; Orel, B.; Jese, R.; Vuk, A. S.; Mali, G.; Hocevar, S. B.; Grdadolnik, J.; Stathatos, E.; Lianos, P. J. Phys. Chem. B 2005, 109, 14387. (25) Ghilane, J.; Fontaine, O.; Martin, P.; Lacroix, J.-C.; Randriamahazaka, H. Electrochem. Commun. 2008, 10, 1205. (26) Bildstein, B.; Malaun, M.; Kopacka, H.; Ongania, K.-H.; Wurst, K. J. Organomet. Chem. 1998, 552, 45. Gao, Y.; Twamley, B.; Shreeve, J. M. Inorg. Chem. 2004, 43, 3406. (27) Antonio, R.; Firestone, M. A.; Kubatko, K.-A.; Szreder, T.; Wishart, J. F.; Dietz, M. L. Dalton Trans. 2007, 529. (28) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450.
Chamiot et al. to give the crude 11-bromoundecanoyl chloride. A solution of the crude compound in 20 mL of dry CH2Cl2 was added dropwise, under argon, to a solution of 7 g (37.6 mmol) of ferrocene and 6 g (45.2 mmol) of AlCl3 in 50 mL of anhydrous CH2Cl2. Reflux was maintained for 1h 30 min. After being stirred at room temperature overnight, the solution was poured into an ice-saturated aqueous solution of NaCl. The organic layer was separated, washed two times with NaCl brine, dried (MgSO4), and concentrated in vacuo. The crude product was extracted with 20 mL of hot methanol and evaporated to dryness in vacuo. The residue was purified by silica gel chromatography (cyclohexane/ethyl acetate, 9:1) to give 7.26 g (51%) of an orange solid. 1H NMR (CDCl3, 200 MHz): δ (ppm) ) 1.20-1.40 (m, 14H, (CH2)7, 1.70 (m, 2H, CH2), 1.85 (quint, J ) 7.1 Hz, 2H, CH2CH2Br), 2.69 (t, J ) 6.7 Hz, 2H, CH2COFc), 3.41 (t, J ) 6.9 Hz, 2H, CH2Br), 4.19 (s, 5H, Fc), 4.49 (s, 2H, Fc), 4.78 (s, 2H, Fc). 13C NMR (CDCl3, 50 MHz): δ (ppm) ) 25.0, 28.6, 29.2, 29.8, 29.9, 30.0, 33.2, 34.6, 40.2, 69.7, 70.1, 72.5, 79.5, 205.2. EIMS (70 eV): m/z (%): 434 (M(81Br)+, 98), 432 (M(79Br)+, 100), 352 ((M-Br)+, 35). (11-Bromoundecyl)ferrocene. The 11-bromoundecanoylferrocene was converted to 11-bromoundecylferrocene by a Clemmensen reduction. The preparation of amalgamated zinc was as follows: 0.9 g (3.34 mmol) of HgCl2 and 0.9 mL of 37 wt. % HCl were added to 8.3 g of zinc dust in 10 mL of water. After stirring for 10 min, at room temperature, the solution was decanted and the supernatant was eliminated. A 2.52 g (6.68 mmol) amount of 11-bromoundecanoylferrocene was added to the amalgamated zinc in 10 mL of ethanol. The reaction mixture was heated to reflux for 3 h and cooled down to room temperature before 20 mL of water was added. The liquid phase was extracted with Et2O (3 × 30 mL). The combined organic layers were washed with a saturated aqueous NaHCO3 solution and then with water. They were dried over MgSO4, filtered, and evaporated under vacuum. The residue was purified by silica gel chromatography (cyclohexane) to give (11-bromoundecyl)ferrocene as an orange solid (1.67 g, 60%). 1H NMR (CDCl3, 200 MHz): δ (ppm) ) 1.28-1.52 (m, 16H, (CH2)8), 1.86 (quint, J ) 7.1 Hz, 2H, CH2CH2Br), 2.31 (t, J ) 7.6 Hz, 2H, CH2Fc), 3.41 (t, J ) 6.8 Hz, 2H, CH2Br), 4.03-4.09 (m, 9H, Fc). 13C NMR (CDCl3, 50 MHz): δ (ppm) ) 28.1, 28.7, 29.3, 29.4, 29.6, 31.0, 32.7, 34.0, 66.8, 67.9, 68.3, 89.4. EI-MS (70 eV): m/z (%): 420 (M(81Br)+, 98), 418 (M(79Br)+, 100), 339 ((M - Br)+, 60). 1-(11-Ferrocenylundecyl)-3-methylimidazolium Bromide (Fc11MIm+) To 1 g (2.39 mmol) of (11-bromoundecyl)ferrocene in 10 mL of ethyl acetate was added 0.19 mL (2.39 mmol) of 1-methylimidazole. The reaction was stirred at reflux for 27 h. Two layers (organic and ionic liquid) were formed. The top organic layer was eliminated, and the ionic liquid layer was washed with ethyl acetate (3 × 1 mL). After traces of organic solvent were removed under reduced pressure, the compound Fc11MIm+ was obtained as a yellow solid (0.26 g, 27%). mp ) 60 °C. 1H NMR (CDCl3, 200 MHz): δ (ppm) ) 1.25 (m, 16H, (CH2)8), 1.91 (m, 2H, CH2CH2N+), 2.29 (t, J ) 7.0 Hz, 2H, CH2Fc), 4.07 (m, 12H, Fc + CH3N+), 4.30 (t, J ) 7.4 Hz, 2H, CH2N+), 7.22 (s, 1H, Him), 7.29 (s, 1H, Him), 10.58 (s, 1H, H2im). 13C NMR (CDCl3, 50 MHz): δ (ppm) ) 25.9, 28.7, 29.2, 29.3, 30.0, 30.8, 36.5, 49.9, 66.8, 67.8, 68.3, 89.4, 121.5, 123.3, 137.1. ESI-MS (positive): m/z: 421 [M - Br]+. The surfactant Fc11MIm+ was oxidized to Fc+11MIm+ using standard electrochemical procedure. The electrochemical oxidation of Fc11MIm+ was performed at a fixed potential of 0.3 V against a saturated-calomel electrode (SCE) used as reference. The working and counter electrodes were platinum meshes. The process of oxidation was stopped when a constant current was observed for at least 30 min. Completion of the Fc11MIm+ reduced form oxidation was controlled by linear voltammetry using a platinum disk electrode as the rotating working electrode. We used freshly prepared solution for all experiments. No change of color was observed in typically pH ) 5.6 solution we used. Electrochemistry. The cyclic voltammetry was carried out at 25 °C, with a conventional three-electrodes cell using a potentiostat (PGZ301 Voltalab Radiometer analytical corp.). The working
Redox-Switched Amphiphilic Ionic Liquid BehaVior
Langmuir, Vol. 25, No. 3, 2009 1313 Table 1. Surface Properties of the Ferrocenated AIL and Surfactant in Aqueous Solutions: Critical Micellar Concentration (cmc), Limiting Surface Tension (γlim) and Minimum Area per Molecule of Surfactant (Amin) compound +a
Fc11MIm
Fc+11MIm+ a reduced FTMAb oxidized FTMAb
cmc (mol L-1) γlim (mN/m) Amin (Å2/molecule) 5.5 × 10-4 c 3.5 × 10-5 1 × 10-3 1 × 10-4 3 × 10-2
49c 48 44 50
