Ionic Liquids Systems

Aug 13, 2009 - Argentina. Received April 27, 2009. Revised Manuscript Received July 20, 2009. The microenvironment of the polar core generated in diff...
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On the Formation of New Reverse Micelles: A Comparative Study of Benzene/Surfactants/Ionic Liquids Systems Using UV-Visible Absorption Spectroscopy and Dynamic Light Scattering R. Darı´ o Falcone,* N. Mariano Correa, and Juana J. Silber* Departamento de Quı´mica, Universidad Nacional de Rı´o Cuarto, Agencia Postal # 3. X5804ZAB Rı´o Cuarto, Argentina Received April 27, 2009. Revised Manuscript Received July 20, 2009 The microenvironment of the polar core generated in different ionic liquid reverse micelle (IL RM) systems were investigated using the solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB) as an absorption probe and dynamic light scattering (DLS) technique. The novel RM systems consist of two different ILs;1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N);sequestrated by two different surfactants;Triton X-100 (TX-100) and benzyl-n-hexadecyldimethylammonium chloride (BHDC);in order to make IL/surfactant/benzene RMs. The effect of the variation of Ws (Ws = [IL]/[surfactant]) on the QB spectroscopy was used to characterize these nonaqueous RMs. DLS results confirm the formation of these IL RM systems because increasing Ws increases the droplet sizes. Moreover it is demonstrated that the structure of the sequestrated ILs depends strongly on the type of surfactant use to create the RMs.

Introduction Reverse micelles (RMs) and water-in-oil (W/O) microemulsions have attracted considerable attention as a result of their ability to host hydrophilic components in organic solvents. These systems are suitable media for processes that involve hydrophobic and hydrophilic reactants providing “nanoreactors” for a variety of chemical *Corresponding authors. E-mail: [email protected] (R.D.F.); [email protected] (J.J.S.). (1) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (2) Silber, J. J.; Biasutti, M. A.; Abuin, E.; Lissi, E. Adv. Colloid Interface Sci. 1999, 82, 189. (3) Shinoda, K. J. Phys. Chem. 1985, 89, 2429. (4) Georges, J. Spectrochim. Acta. Rev. 1990, 13, 27. (5) Wong, M.; Thomas, J. K.; Gr€atzel, M. J. Am. Chem. Soc. 1976, 98, 2391. (6) Politi, M. J.; Chaimovich, H. J. Phys. Chem. 1986, 90, 282. (7) (a) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. (b) Fendler J. H. Membrane Mimetic Chemistry; Wiley Interscience, New York, 1982; Chapter 3. (8) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78. (9) (a) Ravey J. C., Buzier M. In Macro and Microemulsions Theory and Applications; Shah, D. O., Ed.; American Chemical Society: Washington, DC, 1985; p 253. (b) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sci. 1984, 97, 9. (10) Friberg S. E. In Interfacial Phenomena in Apolar Media; Eicke, H. F., Parfitt, G. D., Eds.; Marcel Dekker: New York, 1987; p 93. (11) Zhu, D.; Schelly, Z. A. Langmuir 1992, 8, 48. (12) (a) Gu, J.; Schelly, Z. A. Langmuir 1997, 13, 4256. (b) Mandal, D.; Datta, A.; Kumar Pal, S.; Bhattacharyya, K. J. Phys. Chem. B 1998, 102, 9070. (c) Zhu, D. M.; Wu, X.; Schelly, Z. A. Langmuir 1992, 8, 1538. (d) Zhu, D. M.; Feng, K.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 2382. (13) Zhu, D.-M.; Wu, X.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 7121. (14) Vasilescu, M.; Caragheorgheopol, A.; Almgren, M.; Brown, W.; Alsins, J.; Johannsson, R. Langmuir 1995, 11, 2893. (15) Sawada, K.; Ueda, M. J. Chem. Technol. Biotechnol. 2004, 79, 369. (16) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 1995, 172, 71. (17) Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 1996, 184, 570. (18) McNeil, R.; Thomas, J. K. J. Colloid Inteface Sci. 1981, 83, 57. (19) Novaira, M.; Biasutti, M. A.; Silber, J. J.; Correa, N. M. J. Phys. Chem. B 2007, 111, 748. (20) (a) Costa, S. M. B.; Brookfield, R. L. J. Chem. Soc. Faraday Trans. 2 1986, 82, 991. (b) Costa, S. M. B.; Aires de Barros, M. R.; Conde, J. P. J. Photochem. 1985, 28, 153. (c) Kikuchi, K.; Thomas, J. K. Chem. Phys. Lett. 1988, 148, 245. (d) Borsarelli, C. D.; Cosa, J. J.; Previtali, C. M. Langmuir 1993, 9, 2895. (21) Dhami, S.; Cosa, J. J.; Bishop, S. M.; Phillips, D. Langmuir 1996, 12, 293. (22) Borsarelli, C. D.; Previtali, C. M.; Cosa, J. J. J. Colloid Interface Sci. 1996, 179, 34.

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and biological reactions.1,2 There are a wide range of surfactants; anionic, cationic, and nonionic;that form RMs in nonpolar solvents.1-23 Among the cationic surfactant, benzyl-n-hexadecyl dimethylammonium chloride (BHDC) (Scheme 1), forms RMs in aromatic solvents without the addition of a cosurfactant.17-22 Triton X-100 (TX-100) (Scheme 1) is probably one of the most nonionic surfactants used in RM preparation in different hydrocarbons and without the presence of cosurfactant.11-13,23 The majority of the studies in RMs utilize water as the polar component. In recent years, attempts have been made to prepare and study waterless RMs. In this effort, water has been replaced by polar solvents, which have relatively high dielectric constants and are immiscible in hydrocarbon solvents.24 These nonaqueous RMs are essentially oil continuous25 and have attracted much interest from both theoretical (thermodynamics, particle interactions) and practical (potential use as novel reaction media) viewpoints.26-42 (23) Pant, D.; Levinger, N. E. Langmuir 2000, 16, 10123. (24) Martino, A.; Kaler, E. W. J. Phys. Chem. 1990, 94, 1627. (25) Ray, S.; Moulik, S. P. Langmuir 1994, 10, 2511. (26) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. Soc., Faraday Trans 1 1984, 80, 3307. (27) Lopez-Cornejo, P.; Costa, S. M. B. Langmuir 1998, 14, 2042. (28) Das, K. P.; Ceglie, A.; Lindman, B. J. Phys. Chem. 1987, 91, 2938. (29) Arcoleo, V.; Aliotta, F.; Goffredi, M.; La Manna, G.; Turco Liveri, V. Mater. Sci. Eng., C 1997, 5, 47. (30) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (31) Mathew, C.; Saidi, Z.; Peyrelasse, J.; Boned, C. Phys. Rev. 1991, A 43, 873. (32) Laia, C. A. T.; Lopez-Cornejo, P.; Costa, S. M. B.; d’Oliveira, J.; Martinho, J. M. G. Langmuir 1998, 14, 3531. (33) Lattes, A.; Rico, I.; de Savignac, A.; Ahamd-Zadeh Samii, A. Tetrahedron 1983, 43, 1725. (34) Ahamd-Zadeh Samii, A.; de Savignac, A.; Rico, I.; Lattes, A. Tetrahedron 1985, 41, 3683. (35) Gautier, M.; Rico, I.; Ahamd-Zadeh Samii, A; de Savignac, A.; Lattes, A. J. Colloid Interface Sci. 1986, 112, 484. (36) Rico, I.; Lattes, A.; Das, K. P.; Lindman, B. J. Am. Chem. Soc. 1989, 111, 7266. (37) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Kirk, A. J.; Swansbury, P. Langmuir 1993, 9, 523. (38) Martino, A.; Kaler, E. W. Langmuir 1995, 11, 779.

Published on Web 08/13/2009

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Falcone et al. Scheme 1. Molecular Structures of QB, bmimþ, BF4-, Tf2N-, TX-100, and BHDC

Ionic liquids (ILs) are attractive as a powerful alternative to conventional molecular organic solvents and they have received much attention as a class of neoteric solvents.43-46 Recently studies on RMs with ILs as a component has become an attractive topic.47-56 Nonaqueous ionic liquid reverse micelles (IL RMs) where the ILs are sequestrated by nonionic surfactants are an interesting area since they provide hydrophobic or hydrophilic nanodomains expanding the potential uses of the ILs in microheterogeneous systems as reaction, separation, and/or extraction media.53 Moreover, to the best of our knowledge, there are not reports about the use of cationic surfactant to prepare IL RMs

Letter

and the investigation of the ILs structure inside the RMs. It is important to learn about the properties of ILs encapsulated inside RMs, and how these properties can be modified to improve their potential application as a nanosized microheterogeneous solvent in the nanomaterials field. We have been interested for several years in the study of the interaction of small molecules inside RMs trying to elucidate the properties of these organized systems. Thus, we have studied, for example, the micropolarity of aqueous and nonaqueous sodium bis-2-ethylhexyl-sulfosuccinate (AOT) RMs16,17,19,42,57 by following the solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB) because its absorption spectrum is highly sensitive to its local environment.58,59 The aim of this work is to improve the basic understanding about the microenvironment created by the IL encapsulated in the RM interior, such as polarity and hydrogen bond interactions, using an optical probe. Thus, in the present contribution, we studied IL RMs of benzene/TX-100 and benzene/BHDC using 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N) as the polar domain (Scheme 1). These RM systems were investigated by UV-visible absorption spectroscopy following the solvatochromic behavior of the molecular probe QB. The micellar systems were investigated at a fixed surfactant concentration with different Ws (Ws = [IL]/[surfactant]) values. To the best of our knowledge, this is the first time that bmimTf2N is used as a polar component in TX-100 and BHDC RMs.

Experimental Section The details of the experiments are provided in the Experimental Section of the Supporting Information.

Results and Discussion (39) Tessy, E. I.; Rakshit, A. K. Bull. Chem. Soc. Jpn. 1995, 68, 2137. (40) Riter, R. E.; Undiks, E. P; Kimmel, J. R.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 7931. (41) Elles, C.; Levinger, N. Chem. Phys. Lett. 2000, 317, 624. (42) (a) Correa, N. M.; Falcone, R. D.; Biasutti, M. A.; Silber, J. J. Langmuir 2000, 16, 3070. (b) Silber, J. J.; Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Abuin, E.; Lissi, E.; Campodonico, P. Langmuir 2003, 19, 2067. (c) Falcone, R. D.; Biasutti, M. A.; Correa, N. M.; Silber, J. J.; Lissi, E.; Abuin, E. Langmuir 2004, 20, 5732. (d) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J. J. Colloid Interface Sci. 2006, 296, 356. (43) (a) Zhao, H; Malhotra, S. V. Aldrichimica Acta 2002, 35, 75. (b) OlivierBourbigou, H.; Magna, L. J. Mol. Catal. A 2002, 182, 419. (c) Zhao, D.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157. (d) Sheldon, R. Chem. Commun. 2001, 2399. (e) Gordon, C. M. Appl. Catal., A 2001, 222, 101. (f) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (g) Welton, T. Chem. Rev. 1999, 99, 2071. (44) Welton, T.; Wasserscheid, P. Ionic Liquids in Synthesis; VCH-Wiley: Weinheim, Germany, 2002. (45) Welton, T. Coord. Chem. Rev. 2004, 248, 2459. (46) (a) Freemantle, M. Chem. Eng. News 1998, 76, 32. (b) Crowhurst, L.; Falcone, R.; Lancaster, N. L.; Llopis-Mestre, V.; Welton, T. J. Org. Chem. 2006, 71, 8847. (47) (a) Liu, J.; Cheng, S.; Zhang, J.; Feng, X.; Fu, X.; Han, B. Angew. Chem., Int. Ed. 2007, 46, 3313. (b) Cheng, S.; Zhang, J.; Zhang, Z.; Han, B. Chem. Commun. 2007, 2497. (48) (a) Moniruzzaman, M.; Kamiya, N.; Nakashima, K.; Goto, M. ChemPhysChem 2008, 9, 689. (b) Moniruzzaman, M.; Kamiya, N.; Nakashima, K.; Goto, M. Green Chem. 2008, 10, 497. (c) Moniruzzaman, M.; Kamiya, N.; Goto, M. Langmuir 2009, 25, 977. (49) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (50) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302. (51) Gao, Y.; Han, S.; Han, B.; Li, G.; Shen, D.; Li, Z.; Du, J.; Hou, W.; Zhang, G. Langmuir 2005, 21, 5681. (52) (a) Chakrabarty, D.; Seth, D.; Chakratorty, A.; Sarkar, N. J. Phys. Chem. B 2005, 109, 5753. (b) Gao, Y. A.; Zhang, J.; H. Xu, Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W.; Yu, L. Chem. Phys. Chem. 2006, 7, 1554. (53) Yan, F.; Texter, J. Chem. Commun. 2006, 2696. (54) Gao, Y.; Hilfert, L.; Voigt, A.; Sundmacher, K. J. Phys. Chem. B 2008, 112 (12), 3711. (55) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, J.; Cao, Q.; Zhao, M. W.; Li, Z.; Zhang, G. Y. Chem.;Eur. J. 2007, 13(9), 2661. (56) Li, N.; Gao, Y.; Zheng, L.; Zhang, J.; Yu, L.; Li, X. Langmuir 2007, 23, 1091.

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The results obtained for the IL RMs media studied in this work are discussed in two sections. In the first section, we show the data using dynamic light scattering (DLS) technique in order to confirm the presence of such organized systems. In the second part, we report results obtained using QB as the absorption molecular probe. I. DLS Studies. In this work we present results on the novel systems: benzene/TX-100/bmimTf2N, benzene/BHDC/bmimTf2N, and benzene/BHDC/bmimBF4. Also, the previously reported benzene/TX-100/bmimBF4 RM system55 was investigated for comparison. DLS is used to assess whether the ILs are encapsulated by the surfactant to create RM media, because it is a powerful technique to evaluate the formation of these new organized systems.1,2,8,26,30,50,60 Thus, if the IL is really encapsulated to form RMs, the droplets size must increase as the Ws value increases with a linear tendency (swelling law of RMs) as it is well established for water or polar solvents/surfactant RM systems.2,26,40,61 This feature can also demonstrate that the IL RM media consist of discrete spherical and noninteracting droplets of ILs stabilized by the surfactant. Deviation from the linearity could be due to several factors, with the most relevant being the droplet-droplet interaction and/or other RM shape.2,61

(57) Correa, N. M.; Levinger, N. E. J. Phys. Chem. B. 2006, 110, 13050. (58) Ueda, M.; Schelly, Z. A. Langmuir 1989, 5, 1005. (59) Saxena, J. P.; Stafford, W. H.; Stafford, W. L. J. Chem. Soc. 1959, 1579. (60) Gao, Y.; Li, N.; Zheng, L.; Bai, X.; Yu, L.; Zhao, X.; Zhang, J.; Zhao, M.; Li, Z. J. Phys. Chem. B 2007, 111, 2506. (61) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochem. Biophys. Acta 1988, 947, 209.

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Figure 2. Variation of the B1 band as a function of Ws in benzene/ TX-100/bmimBF4 (9), benzene/TX-100/bmimTf2N (O), benzene/ BHDC/bmimBF4 (2), and benzene/BHDC/bmimTf2N (r). [BHDC] = 0.3 M; [TX-100] = 0.7 M; [QB] = 1  10-4 M. The λmax B1 value for neat bmimBF4 (---) and neat bmimTf2N ( 3 3 3 3 ) are included for comparison.

Figure 1. Diameter values (nm) of the IL RMs obtained at 25 °C varying Ws: (A) bmimBF4 RMs; (B) bmimTf2N RMs. (9) BHDC; (O) TX-100. [TX-100] = 0.7 M, [BHDC] = 0.3 M.

On the other hand, if the IL is not sequestrated by the surfactant, the droplets sizes should be insensitive or decrease with the polar solvent addition.30 In Figure 1A,B, we report the droplet size values obtained in benzene/TX-100/bmimTf2N, benzene/BHDC/bmimTf2N, benzene/BHDC/bmimBF4 and, benzene/TX-100/bmimBF4 RMs. In all the systems studied, it can be observed that there is an increase in the droplets size when the IL content increases, showing that both ILs are sequestrated by the surfactants yielding IL RMs. The linear tendency observed at Ws values lower than 1.5 for the bmimBF4 RM systems (Figure 1 A) and in the whole Ws range for the bmimTf2N RMs (Figure 1 B) indicates that, under these conditions, the IL RMs are discrete and noninteracting spherical droplets.2,61 Nevertheless, the fact that the bmimBF4 RM system shows a deviation from the linearity at Ws values higher than 1.5 could indicate that now droplet-droplet interaction is favored, changing the shape of the RMs. However, at this point we do not have enough evidence to confirm this assumption. On the other hand, it is clear that the RMs containing bmimBF4 as the polar domain (Figure 1 A) are much larger than the one containing bmimTf2N (Figure 1 B). This unexpected result is difficult to explain; however, we attempt to give the following explanation: The difference in the ILs RMs droplets sizes values can be explained considering that bmimBF4 interacts 10428 DOI: 10.1021/la901498e

strongly with the surfactants (see QB in the IL RMs section) and consequently penetrates the RM interfaces. Thus, it is possible that this penetration leads to an increase in the effective surfactant headgroup area (a) as it is well established for AOT in isooctane/ AOT/water RMs.62 Maitra demonstrated that the AOT’s a value increases from 36 to 51 A˚2 as the W = [H2O]/[AOT] value increases from 4 to 20 because the water molecules bind to the AOT polar headgroup at the RM inteface.62 It is known that the RM droplet sizes depend, among many other variables, on the effective packing parameter of the surfactants, v/alc, in which v and lc are the volume and the length of the hydrocarbon chain, respectively, and a is the surfactant headgroup area. The RM sizes are larger when the surfactant packing parameter values are smaller.63,64 Thus, as the Ws values increase, bmimBF4 penetrates the RM interfaces, increasing the surfactants’ a values with the consequent decrease in the surfactant packing parameter and the increase in both the RM droplet size and the interfacial fluidity. It must be noted that our results in the benzene/TX-100/ bmimBF4 system shows differences in size with the data reported previously.55 We strongly believe that the higher TX-100 concentration used in that work makes the droplet-droplet interaction easier, changing the shape of the RMs with the consequent differences in the droplet size values. Moreover, Gao et al.55 claim an ellipsoidal shape for their RMs systems. II. Studies Using QB As an Absorption Molecular Probe. It is important to note that, despite the fact that QB is soluble in benzene and can experience a partition process between two different pseudophases (the RMs and the organic solvent), we have previously demonstrated16,17 that, at the surfactant concentrations used in this work ([BHDC] = 0.3 M and [TX-100] = 0.7 M), the molecular probe exists mainly at the RMs interface. Thus, QB will monitor the changes at the interface properties. QB in Benzene/TX-100 RMs. Figure S2 in the Supporting Information, shows typical QB absorption spectra in benzene/ TX-100/IL, varying Ws at constant [TX-100]. Figures 2 and S3 show the λmax B1 and the Abs B2/Abs B1 values for QB in TX-100 (62) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (63) Li, Q.; Li, T.; Wu, J. J. Colloid Interface Sci. 2001, 239, 522. (64) Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 226.

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Figure 3. Variation of Abs B2/Abs B1 as a function of Ws in benzene/BHDC/bmimBF4 (9) and benzene/BHDC/bmimTf2N (O). [BHDC] = 0.3 M; [QB] = 1  10-4 M.

RMs varying Ws at [TX-100] = 0.7 M, respectively. At Ws = 0, the λmax B1 value is 521 nm, which is quite different from the value in pure benzene and TX-100 (Table S1 in the Supporting Information). Moreover, the data show that the absorbance ratio value is around 3.2, which is lower than the value obtained in benzene (Table S1) but higher than the one obtained in neat TX-100 (around 2.6). 6 0), a hypsochromic shift is observed in When IL is added (Ws ¼ the B1 band, showing a polarity increase sensed by QB.16,17,42a As it can be seen in Figure 2, the λmax B1 values shift from 521 nm at Ws = 0 up to around 497 nm at Ws = 2 using bmimBF4 and up to 500 nm for bmimTf2N at the same Ws. If the λmax B1 values obtained in the IL RM media at Ws = 2 are compared with the value obtained in the neat ILs (Table S1), it is possible to observe a difference of 12 nm for bmimBF4 and 8 nm for bmimTf2N. Those magnitudes suggest that the IL structures are not significantly disrupted upon encapsulation by the nonionic surfactant and also suggest what was deduced in the DLS section, in that bmimBF4 penetrates the interface. Similar results were found by other authors for bmimBF4/TX-100 RMs but using other molecular probes.52b,55 In those cases, the polarities sensed by the optical probes are close to the neat IL value, and a weak dipolar interaction between the electronegative oxygen atoms of oxyethylene units of TX-100 and the electropositive imidazolium ring55 have been suggested as the driving force for the solubilization of bmimBF4 into the core of the TX-100 RM aggregates.52b,55 Additionally, when the absorbance ratio changes are analyzed (Figure S3), although the magnitude changes are not large, it is possible to observe a clear tendency to increase the values when Ws increases. This increase of the absorbance ratio observed in both nonionic RMs could indicate that QB starts to sense the presence of the ILs encapsulated in TX-100 RMs and, in consequence, a less hydrogen donor microenvironment than TX-100.17 QB in Benzene/BHDC RM. Typical QB absorption spectra in benzene/BHDC/IL, varying Ws at constant [BHDC] is showed in Figure S4 in the Supporting Information. Figures 2 and 3 summarize the λmax B1 and the Abs B2/Abs B1 values, respectively, obtained in BHDC RMs varying Ws at constant [BHDC]. At Ws = 0, the λmax B1 value obtained is 534 nm, and, as it was found in the TX-100 RMs solutions, QB also exists at the BHDC RM interface. Furthermore, from previous results, we know that QB resides close to the head polar group of BHDC.17 The λmax B1 values obtained in BHDC RMs, using both ILs as the polar component, shift hypsochromically when Ws is increased Langmuir 2009, 25(18), 10426–10429

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(Figure 2). The hypsochromic shifting of the B1 band confirms an increment of the polarity interface in both RMs when the IL concentration is increased. As it can be seen in Figure 2, the λmax B1 value shifts from 534 nm at Ws = 0 up to around 518 nm at Ws = 2 for bmimBF4 and up to 524 nm at Ws = 2 for bmimTf2N. As it can be observed in Figure 2, it is possible to observe a difference from the λmax B1 value in neat ILs (Table S1) of 33 nm for bmimBF4 and 32 nm for bmimTf2N in BHDC at the maximum Ws reached. Moreover, the magnitude of the differences in the λmax B1 values between homogeneous and the cationic micelar media observed show an important perturbation of their microenvironments. This evidence is different in comparison with the behavior in nonionic RMs. Evidently, there are new effects (or interactions) that are not present in the ILs neat structure or in the nonionic ILs RMs. Figure 3 shows an interesting result because, although the magnitudes of the absorbance ratio changes are not large, it is possible to observe a different tendency for both ILs encapsulated in BHDC RMs. In the cationic system containing bmimBF4, as Ws increases, the absorbance ratio decreases until Ws ∼ 0.5, and, after that, the value is practically constant around 5.0. On the other hand, when bmimTf2N is used as the polar solvent, the Abs B2/Abs B1 values increase continuously with Ws in the whole range studied. Because of the ionic nature of the ILs and being that BHDC is a cationic surfactant (Scheme 1), we expect that the electrostatic interaction between the positive charge of the BHDC headgroup and the IL0 s anions are the main explaination of the results obtained. We think that this interaction can dramatically affect the IL properties when it is encapsulated in the cationic RMs. The positive charge is more localized in the BHDC headgroup than in the bmimþ ion, so we predict a stronger interaction between the IL’s anions and the BHDC headgroup than with its own bmimþ cations. Thus, we assume that the ion pair degree in both ILs must be altered when they are encapsulated inside the BHDC RMs. In addition, the strength of the BHDC-IL anions interaction should be different if the nature of the ions in the ILs is considered. If the basicity of the IL0 s anion is taking into account, we expect a stronger BF4-BHDC interaction (with a greater penetration to the interface) than for Tf2N-BHDC because BF4- is more basic than Tf2N- (Figure 1 A). Thus, the results obtained in benzene/ BHDC/bmimTf2N RMs show that bmimTf2N is less perturbed than bmimBF4 in the benzene/BHDC/bmimBF4 system. In summary, we have demonstrated the existence of different IL/ surfactant/benzene RMs and that the IL structures sequestrated depend strongly on the type of surfactant use to create the RMs. Acknowledgment. We gratefully acknowledge the financial support for this work by the Consejo Nacional de Investigaciones Cientı´ ficas (CONICET), Agencia Nacional de Promocion Cientı´ fica y Tecnica, Agencia Cordoba Ciencia, and Secretarı´ a de Ciencia y Tecnica de la Universidad Nacional de Rı´ o Cuarto. All the authors hold a research position at CONICET. We also want to thank reviewer 5 for his/her careful and thorough review of this manuscript. Supporting Information Available: Full experimental section, full discussion about QB results in neat ILs, a Table with QB absorption parameters in neat ILs, and the following Figures: Figure S1: QB absorption spectra in the neat ILs; Figure S2: QB absorption spectra in benzene/TX-100/ bmimTf2N as a function of Ws; Figure S3: QB Abs B2/Abs B1 ratio versus Ws for benzene/TX-100/ILs; and Figure S4: QB absorption spectra in benzene/BHDC/bmimBF4 as a function of Ws. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la901498e

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