Interaction of Ionic Liquid with Water in Ternary Microemulsions (Triton

Aug 4, 2006 - On the other hand, in the case of C-151, with an increase in R the fast component of the solvation time gradually increases and the slow...
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Langmuir 2006, 22, 7768-7775

Interaction of Ionic Liquid with Water in Ternary Microemulsions (Triton X-100/Water/1-Butyl-3-methylimidazolium Hexafluorophosphate) Probed by Solvent and Rotational Relaxation of Coumarin 153 and Coumarin 151 Debabrata Seth, Anjan Chakraborty, Palash Setua, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, WB, India ReceiVed May 12, 2006. In Final Form: June 30, 2006 The interaction of ionic liquid with water in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])/ Triton X-100 (TX-100)/H2O ternary microemulsions, i.e., “[bmim][PF6]-in-water” microregions of the microemulsions, has been studied by the dynamics of solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 151 (C-151). The variation of the time constants of solvent relaxation of C-153 is very small with an increase in the [bmim][PF6]/TX-100 ratio (R). The rotational relaxation time of C-153 also remains unchanged in all micremulsions of different R values. The invariance of solvation and rotational relaxation times of C-153 indicates that the position of C-153 remains unaltered with an increase in R and probably the probe is located at the interfacial region of [bmim][PF6] and TX-100 in the microemulsions. On the other hand, in the case of C-151, with an increase in R the fast component of the solvation time gradually increases and the slow component gradually decreases, although the change in solvation time is small in comparison to that of microemulsions containing common polar solvents such as water, methanol, acetonitrile, etc. The rotational relaxation time of C-151 increases with an increase in R. This indicates that with an increase in the [bmim][PF6] content the number of C-151 molecules in the core of the microemulsions gradually increases. In general, the solvent relaxation time is retarded in this room temperature ionic liquid/watercontaining microemulsion compared to that of a neat solvent, although retardation is very small compared to that of the solvent relaxation time of the conventional solvent in the core of the microemulsions.

1. Introduction Room temperature ionic liquids (RTILs), a class of neoteric solvents, have been extensively used as “green substitutes” for toxic, hazardous, flammable, and volatile organic solvents.1-3 Some common RTILs are composed of heterocyclic pyridinium or imidazolium cations containing alkyl substituents and anions such as [PF6-], [BF4-], or [NO3-]. A few properties such as the viscosity, hydrophobicity, density, and solubility of RTILs can be tuned by selecting different combinations of cations and anions, to customize RTILs for some specific demands. These RTILs have some unique properties such as high conductivity, thermal stability, nonflammable nature, negligible vapor pressure, wide electrochemical windows, easy recyclability, and wide liquidous temperature range (-96 to ∼+300 °C) that make them quite useful as alternative and environmentally benign solvents and in large-scale industrial applications.1-4 They can be extensively used for organic chemical reactions5 and have some electrochemical applications.6 RTILs are not always “green”. Some are toxic and corrosive in nature; [PF6]- ion containing RTILs are hydrolyzed in the presence of moisture to produce volatile and * To whom correspondence should be addressed. E-mail: nilmoni@ chem.iitkgp.ernet.in. Fax: 91-3222-255303. (1) (a) Seddon, K. R. Nat. Mater. 2003, 2, 363. (b) Ionic Liquids: Industrial Applications for Green Chemistry; Rodgers, R., Seddon, K., Eds.; ACS Symposium Series 818; American Chemical Society: Washington, DC, 2002. (c) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (d) Sheldon, R. Chem. Commun. 2001, 2399. (2) Welton, T. Chem. ReV. 1999, 99, 2071. (3) (a) Baker, G. A.; Baker, S. N.; Pandey, S.; Bright, F. V. Analyst 2005, 130, 800. (b)Freemantle, M. Chem. Eng. News 1998, 76, 32. (c) Freemantle, M. Chem. Eng. News 2003, 81, 9. (4) Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2000, 105, 221. (5) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (6) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. J. Am. Chem. Soc. 2002, 124, 10276.

harmful HF, POF3, etc.7a,b However, RTILs can be tuned to be environmentally benign for large-scale applications in industry.3b RTILs are also used to increase the thermal stability of proteins compared to that in water.7c Several photophysical and ultrafast spectroscopic studies were undertaken in these RTILs.8-20 Aki (7) (a) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Green Chem. 2003, 5, 361. (b) Baker, G. A.; Baker, S. N. Aust. J. Chem. 2005, 58, 174. (c) Baker, S. N.; McCleskey, T. M.; Pandey, S.; Baker, G. A. Chem. Commun. 2004, 940. (8) (a) Aki, S. N. V. K.; Brennecke, J. F.; Samanta, A. Chem. Commun. 2001, 413. (b) Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R. J. Chem. Soc., Perkin Trans. 2 2001, 433. (c) Carmichael, A. J.; Seddon, K. R. J. Phys. Org. Chem. 2000, 13, 591. (9) (a) Fletcher, K. A.; Pandey, S. J. Phys. Chem. B 2003, 107, 13532. (b) Fletcher, K. A.; Baker, S. N.; Baker, G. A.; Pandey, S. New J. Chem. 2003, 27, 1706. (c) Baker, S. N.; Baker, G. A.; Bright, F. V. Green Chem. 2002, 4, 165.(d) Pandey, S.; Fletcher, K. A.; Baker, S. N.; Baker, G. A. Analyst 2004, 129, 569. (10) Reichardt, C. Green Chem. 2005, 7, 339. (11) Hyun, B.-R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. A 2002, 106, 7579. (12) (a) Chakrabarty, D.; Chakraborty, A.; Hazra, P.; Seth, D.; Sarkar, N. Chem. Phys. Lett. 2004, 397, 216. (b) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2002, 106, 4447. (c) Karmakar, R.; Samanta, A. Chem. Phys. Lett. 2003, 376, 638. (d) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2002, 106, 6670. (e) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2003, 107, 7340. (f) Saha, S.; Mandal, P. K.; Samanta, A. Phys. Chem. Chem. Phys. 2004, 6, 3106. (13) (a) Ingram, J. A.; Moog, R. S.; Ito, N.; Biswas, R.; Maroncelli. M. J. Phys. Chem. B 2003, 107, 5926. (b) Ito, N.; Arzhantsev, S.; Heitz, M.; Maroncelli, M. J. Phys. Chem. B 2004, 108, 5771. (c) Arzhantsev, S.; Ito, N.; Heitz, M.; Maroncelli, M. Chem. Phys. Lett. 2003, 381, 278. (d) Ito, N.; Arzhantsev, S.; Maroncelli, M. Chem. Phys. Lett. 2004, 396, 83. (14) Chakrabarty, D.; Hazra, P.; Chakraborty, A.; Seth, D.; Sarkar, N. Chem. Phys. Lett. 2003, 381, 697 and references therein. (15) Chowdhury, P. K.; Halder, M.; Sanders, L.; Calhoun, T.; Anderson, J. L.; Armstrong, D. W.; Song, X.; Petrich, J. W. J. Phys. Chem. B 2004, 108, 10245. (16) (a) Chakrabarty, D.; Chakraborty, A.; Seth, D.; Hazra, P.; Sarkar, N. Chem. Phys. Lett. 2004, 397, 469. (b) Chakrabarty, D.; Chakraborty, A.; Seth, D.; Sarkar, N. J. Phys. Chem. A 2005, 109, 1764. (17) Baker, S. N.; Baker, G. A.; Munson, C. A.; Chen, F.; Bukowski, E. J.; Cartwright, A. N.; Bright, F. V. Ind. Eng. Chem. Res. 2003, 42, 6457. (18) (a) Shim, Y.; Duan, J.; Choi, M. Y.; Kim, H. J. J. Chem. Phys. 2003, 119, 6411. (b) Kobrak, M. N.; Znamenskiy, V. Chem. Phys. Lett. 2004, 395, 127.

10.1021/la061356c CCC: $33.50 © 2006 American Chemical Society Published on Web 08/04/2006

Ionic Liquid-Water Interaction in Microemulsions

et al.8a determined the polarity of the imidazolium- and pyridinium-based RTILs using UV-vis absorption and fluorescence spectroscopy. Muldoon et al.8b determined the polarity of the RTILs using solvatochromic probes. Seddon et al.8c determined the polarity of several neat 1-alkyl-3-methylimidazolium-based RTILs using the solvatochromic dye Nile red. Using Prodan, pyrene, 1-pyrenecarboxaldehyde, Rechardt’s betain dye, and Rhodamine 6G as the solvatochromic probes, various bulk properties and the polarity of various RTIL/cosolvent mixtures were determined.9 Recently, one excellent review paper was published by Reichardt to determine the polarity of RTILs by means of solvatochromic betaine dyes.10 Femtosecond optical Kerr effect measurement was used to study the low-frequency vibrational motions.11 The photoisomerization reactions12a and intramolecular excimer formation kinetics12c have been studied in RTILs. There are also a few reports available on time-dependent solvation in neat RTILs,12-15 and also in RTIL/polar solvent mixtures.16,17 Using molecular dynamics simulation, Shim et al.18a interpreted that the fast component arises due to diffusional motion of the anion and the slower component due to collective motion of the cations and anions. However, in a recent paper, Kobrak et al.18b showed that the fast component originates from collective motions of both the cation and anion. The excitationwavelength-dependent fluorescence behavior of some dipolar molecules in room temperature liquids was investigated by Samanta and co-workers, which contributed to the unusual red edge effect in RTILs.19a Recently, Margulis et al.19b predicted the heterogeneity in RTILs and also observed the red edge effect in RTILs using molecular dynamics simulation, Samanta and co-workers20 reported the various optical properties of different neat RTILs using absorption and fluorescence spectroscopy, and ultrafast dynamics and vibrational relaxation in RTILs and RTILs containing reverse micelles were investigated.21 Recent studies showed that some surfactants might form micelles or microemulsions in RTILs.22,23 Anderson et al.22a reported micelle formation in the RTILs 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], and 1-butyl-3-methylimidazolium chloride, [bmim][Cl], with different surfactants such as Brij-35, Brij-700, etc. Fletcher et al.22b also reported micelle formation of Brij-35, Brij-700, Tween-20, and Triton X-100 (TX-100) in the low-viscosity ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N). RTILs containing microemulsions were prepared and characterized recently.23 In a recent work Gao et al.23b prepared and characterized [bmim][PF6]/TX-100/water-containing ternary microemulsions. The solvent relaxation is a useful technique to unravel the dynamics of water molecules in biologically relevant organized media.25-28 Water in oil microemulsions are important elegant models for biological membranes.24 AOT (dioctylsulfosuccinate, sodium salt) is the most common surfactant to form microemulsions. The solvent relaxation in AOT/water microemulsions (19) (a) Mandal, P. K.; Sarkar M.; Samanta A. J. Phys. Chem. A 2004, 108, 9048. (b) Hu, Z.; Margulis, C. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 831. (20) (a) Paul, A.; Mandal, P. K.; Samanta A. J. Phys. Chem. B 2005, 109, 9148. (b) Paul, A.; Mandal, P. K.; Samanta A. Chem. Phys. Lett. 2005, 402, 375. (c) Mandal, P. K.; Sarkar M.; Samanta A. J. Phys. Chem. A 2004, 108, 9048. (21) (a) Shirota, H.; Funston, A. M.; Wishart, J. F.; Castner, E. W., Jr. J. Chem. Phys. 2005, 122, 184512. (b) Sando, G. M.; Dahl, K.; Owrutsky, J. C. Chem. Phys. Lett. 2006, 418, 402. (22) (a) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (b) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (23) (a) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (b) Gao. Y.; Han, S.; Han, B.; Li, G.; Shen, D.; Li, Z.; Du, J.; Hou, W.; Zhang, G. Langmuir 2005, 21, 5681. (c) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302. (24) Luisi, P. L., Straube, B. E., Eds. ReVerse Micelles; Plenum Press: New York, 1984.

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was investigated by Bhattarcharyya and co-workers25 and Lundgren et al.26 In microemulsions, the dynamics were strongly dependent on the w ([water]/[surfactant]) values and a long bimodal relaxation time was observed compared to that in pure water. Other surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and TX-100 form microemulsions in alkane solvents in the presence of medium chain length primary alcohols such as 1-pentanol, 1-hexanol, etc. Solvent relaxation was also investigated in these systems.27 It was found that the solvent relaxation times in these quaternary microemulsions were affected very little with a change in the w value. Solvent relaxation was employed to understand the relaxation of water molecules in lipids or other biological membranes.28 To explain the bimodal relaxation in microemulsions, Nandi and Bagchi29 proposed a dynamic exchange model between free and bound water molecules. Riter et al.30 characterized various types of microemulsions using AOT as a surfactant and other polar solvents such as acetonitrile, methanol, formamide, ethylene glycol, and N,N-dimethylformamide (DMF) as cosolvents. Recently, solvation dynamics using methanol,30,31 acetonitrile,31,32 formamide,32,33 and DMF33 as the polar core has been investigated. Schelly et al.34 characterized the water/TX-100/ cyclohexane ternary microemulsions. Recently, some ternary microemulsions were prepared and characterized by Tomsic et al.35 using nonionic surfactant Brij 35 in water and various simple alcohols from ethanol to 1-octanol at 25 °C. They showed that, for higher alcohols starting from butanol, with an increase in the alcohol concentration the micellar size changed. With an increase in the concentration of higher alcohols such as hexanol, it penetrated to the miceller core with an increase in miceller size. Besides the experimental studies regarding solvent relaxation in confined cavities using water and other polar cosolvents, there are also several simulation studies36-39 to unravel the dynamical feature in confined assemblies. All these simulation studies (25) (a) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. ReV. 2000, 100, 2013. (b) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (c) Sarkar, N.; Das, K.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 10523. (d) Das, S.; Datta, A.; Bhattacharyya, K. J. Phys. Chem. A 1997, 101, 3299. (e) Pal, S. K.; Mandal, D.; Sukul, D.; Bhattacharyya, K. Chem. Phys. Lett. 1999, 312, 178. (26) Lundgren, J. S.; Heitz, M. P.; Bright, F. V. Anal. Chem. 1995, 67, 3775. (27) (a) Hazra, P.; Chakrabarty, D.; Chakraborty, A.; Sarkar, N. Chem. Phys. Lett. 2003, 382, 71. (b) Corbeil, E. M.; Levinger, N. E. Langmuir 2003, 19, 7264. (28) (a) Hof, M. Solvent Relaxation in Biomembranes. In Applied Fluorescence in Chemistry, Biology, and Medicine; Rettig, W., Strehmel, B., Schrader, S., Seifert, H., Eds.; Springer Verlag: Berlin, 1999; p 439. (b) Sykora, J.; Mudogo, V.; Hutterer, R.; Nepras, M.; Vanerka, J.; Kapusta, P.; Fidler, V.; Hof, M. Langmuir 2002, 18, 9276. (c) Sykora, J.; Kapusta, P.; Fidler, V.; Hof, M. Langmuir 2002, 18, 571. (29) (a) Nandi, N.; Bagchi, B. J. Phys. Chem. B 1997, 101, 10954. (b) Nandi, N.; Bagchi, B. J. Phys. Chem. A 1998, 102, 8217. (30) (a) Riter, R. E.; Undiks, E. P.; Kimmel, J. R.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 7931. (b) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (31) Shirota, H.; Horie, K. J. Phys. Chem. B 1999, 103, 1437. (32) (a) Hazra, P.; Sarkar, N. Phys. Chem. Chem. Phys. 2002, 4, 1040. (b) Hazra, P.; Chakrabarty, D.; Sarkar, N. Langmuir 2002, 18, 7872. (c) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2003, 371, 553. (d) Hazra, P.; Sarkar, N. Chem. Phys. Lett. 2001, 342, 303. (33) Shirota, H.; Segawa, H. Langmuir 2004, 20, 329. (34) (a) Zhu, D.-M.; Feng, K.-I.; Schelley, Z. A. J. Phys. Chem. 1992, 96, 2382. (b) Zhu, D.-M.; Schelley, Z. A. Langmuir 1992, 8, 48. (35) (a) Tomsie`, M.; Bester-Rogae`, M.; Jamnik, A.; Kunz, W.; Touraud, D.; Bergmann, A.; Glatter, O. J. Phys. Chem. B 2004, 108, 7021. (b) Tomsˇie`, M.; Besˇter-Rogae`, M.; Jamnik, A.; Kunz, W.; Touraud, D.; Bergmann, A.; Glatter, O. J. Colloid Interface Sci. 2006, 294, 194. (36) (a) Pal, S.; Bagchi, B.; Balasubramanian, S. J. Phys. Chem. B 2005, 109, 12879. (b) Balasubramanian, S.; Pal, S.; Bagchi, B. Phys. ReV. Lett. 2002, 89, 115505. (c) Pal, S.; Balasubramanian, S.; Bagchi, B. J. Phys. Chem. B 2003, 107, 5194. (37) (a) W. H. Thompson, J. Chem. Phys. 2004, 120, 8125. (b) Thompson, W. H. J. Chem. Phys. 2002, 117, 6618. (38) (a) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2005, 109, 6732. (b) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2001, 105, 11148.

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Seth et al.

Table 1. Composition and Hydrodynamic Diameter of [bmim][PF6]/TX-100/Water Microemulsions system no.

water concn (wt %)

TX-100 concn (wt %)

[bmim][PF6] concn (wt %)

R

Dh (nm)

1 2 3

83.9 83.3 81.5

15.0 15.1 15.7

1.1 1.6 2.8

0.17 0.24 0.41

8.3 12.6 18.9

suggested the existence of biexponential solvent relaxation in the confined assemblies. Like other polar solvents, RTILs can form microemulsions with suitable surfactants and may have some potential applications due to the unique feature of RTILs. We have chosen this RTILcontaining ternary microemulsion to investigate because this system resembles biological and cellular systems. Moreover, these systems are important for catalysis and synthesis of nanoparticles.40 The diameter of RTIL-containing microemulsions23 is much larger than that of microemulsions containing water or other polar liquids.25-27,30,31 Solvation dynamics in neat RTILs has been extensively studied.12-15 Recently, we reported the solvation dynamics in RTIL-confined nanometer-size microemulsions41a and in RTIL-containing micelles.41b In this paper we report solvent and rotational relaxation studies in [bmim][PF6]/TX-100/H2O ternary microemulsions. In a recent paper Gao et al.23b characterized [bmim][PF6]/TX-100/H2O ternary microemulsions by the phase behavior and dynamic light scattering (DLS) measurement, using cyclic voltammetry and UV-vis spectroscopy. They recognized three types of microstructures, water in [bmim][PF6], [bmim][PF6] in water, and bicontinuous, in the microemulsions. They showed that with the addition of [bmim][PF6] in the TX-100/water mixture the size of the droplet increases similarly to that of other microemulsions. The dynamic light scattering experiment showed that the structure of the microemulsions is spherical and the hydrodynamic diameter increases from 8.3 to 18.9 nm as R increases from 0.17 to 0.41 (see Table 1). Gao et al.23b also showed that with a change in water concentration keeping the [bmim][PF6]/TX-100 ratio (R) fixed the hydrodynamic diameter (Dh) of the microemulsions does not change very much. This indicates that, with successive addition of [bmim][PF6] to the TX-100/water binary system, [bmim][PF6] penetrated to the hydrophobic core of the TX-100 micelle and the micelle was swollen by [bmim][PF6]. In this study we chose [bmim][PF6]-in-water microemulsions to report solvent relaxation using two different types of probes, one rigid hydrophobic probe, coumarin 153 (C-153), and one flexible hydrophilic probe, coumarin 151 (C-151). The choice of using one hydrophilic (C-151) and one hydrophobic (C-153) probe allowed us to monitor the different regions of the microemulsion. Recently, Castner et al. used three probes of different hydrophobicities to investigate different regions of aqueous aggregates.42 Dutt et al. have investigated orientational friction with a pair of chromophores, one of which is a strong H-bond (39) (a) Senapati, S.; Berkowitz, M. L. J. Phys. Chem. A 2004, 108, 9768. (b) Senapati, S.; Keiper, J. S.; DeSimone, J. M.; Wignall, G. D.; Melnichenko, Y. B.; Frielinghaus, H.; Berkowitz, M. L. Langmuir 2002, 18, 7371. (c) Senapati, S.; Chandra, A. J. Phys. Chem. B 2001, 105, 5106. (40) (a) Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988. (b) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (41) (a) Chakrabarty, D.; Seth, D.; Chakraborty, A.; Sarkar, N. J. Phys. Chem. B 2005, 109, 5753. (b) Chakraborty, A.; Seth, D.; Chakrabarty, D.; Setua, P.; Sarkar, N. J. Phys. Chem. A 2005, 109, 11110. (42) (a) Grant, C. D.; DeRitter, M. R.; Steege, K. E.; Fadeeva, T. A.; Castner, E. W., Jr. Langmuir 2005, 21, 1745. (b) Grant, C. D.; Steege, K. E.; Bunagan, M. R.; Castner, E. W., Jr. J. Phys. Chem. B 2005, 109, 22273.

Scheme 1. Structures of C-153, C-151, TX-100, and [bmim][PF6]

donor and the other of which does not donate a H-bond.43 The structures of C-151, C-153, TX-100, and [bmim][PF6] are shown in Scheme 1. We have investigated solvent relaxation in three different compositions of the microemulsions as mentioned in Table 1. 2. Experimental Section C-153 and C-151 (laser grade, Exciton) were used as received. [bmim][PF6] was obtained from Acros Chemicals (98% purity) and purified using the literature procedure.14 The RTIL was dried in a vacuum for ∼24 h at 70-80 °C before use. TX-100 was purchased from Aldrich and used as received. Triply distilled water was used to prepare all solutions. The weight fraction of the TX-100 in the microemulsions was ∼0.15, and the RTIL/TX-100 molar ratios (R) were 0.17, 0.24, and 0.41 in the solvent and rotational relaxation studies. The final concentration of C-153 and C-151 in all experiments was kept at ∼5 × 10-5 M. All experiments were carried out at 298 K. The absorption and fluorescence spectra were measured using a Shimadzu (model no. UV-1601) spectrophotometer and a Jobin Yovon Fluoromax-3 spectrofluorimeter. The fluorescence spectra were corrected for the spectral sensitivity of the instrument. For steady-state experiments, all samples were excited at 408 nm. The detailed time-resolved fluorescence setup is described in our earlier publication.27 Briefly, the samples were excited at 408 nm using a picosecond laser diode (IBH, Nanoled), and the signals were collected at magic angles (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (3809U). The instrument response function of our setup was ∼90 ps. The same setup was used for anisotropy measurements. For the anisotropy decays, we used a motorized polarizer in the emission side. The emission intensities at parallel (I|) and perpendicular (I⊥) polarizations were collected alternately until a certain peak difference between parallel (I|) and perpendicular (I⊥) decay was reached. The peak differences depended on the tail matching of the parallel (I|) and perpendicular (I⊥) decays. The analysis of the data was done using IBH DAS, version 6, decay analysis software. The same software was also used to analyze the anisotropy data. All the decays were fitted with a biexponential function because χ2 lies between 1 and 1.2, which indicates a good fit. All decay parameters with χ2 are shown in the Supporting Information (SI). For viscosity measurement we used an advanced rheometer (TA instrument, AR 1000) at 25 °C. (43) (a) Dutt, G. B. J. Phys. Chem. B 2004, 108, 805. (b) Mali, K. S.; Dutt, G. B.; Mukherjee, T. J. Chem. Phys. 2005, 123, 174504.

Ionic Liquid-Water Interaction in Microemulsions

Langmuir, Vol. 22, No. 18, 2006 7771 Table 2. Absorption and Emission Maxima of C-153 and C-151 in Microemulsions and a TX-100/Water Mixture system

λmaxabs (nm)

λmaxflu (nm)

〈τ〉a (ns)

C-153 system 1 C-153 system 2 C-153 system 3 C-151 system 1 C-151 system 2 C-151 system 3 C-153 in a 15 wt % TX-100/water mixture C-151 in a 15 wt % TX-100/water mixture

427 427 427 382 380 380 416 374

528 528 528 484 484 484 528 484

4.52 4.50 4.52 5.00 4.90 4.60 4.52 4.95

a

Figure 1. (a) Absorption spectra of C-151 in system 1 (solid line) and in water (dashed-dotted line) and C-153 in system 1 (dashed line) and in water (dotted line). (b) Emission spectra of C-151 in system 3 (solid line) and in water (dashed-dotted line) and C-153 in system 3 (dashed line) and in water (dotted line).

3. Results 3.1. Steady-State Absorption and Emission Spectra. C-153 in water shows an absorption peak at 434 nm. In a ∼15 wt % TX-100 solution in water the absorption peak is blue shifted to 416 nm. With addition of [bmim][PF6] to this solution the absorption peak of C-153 is red shifted to 427 nm. With further addition of [bmim][PF6], i.e., with increasing [bmim][PF6]/TX100 molar ratio (R), the absorption peak of C-153 remains unchanged at 427 nm. In the case of C-151, the absorption peak maximum in water is at 364 nm and in a ∼15 wt % TX-100 solution in water the absorption peak of C-151 is red shifted to 374 nm. With addition of [bmim][PF6] to this solution the absorption peak is further red shifted to 382 nm. With a further increase in R the absorption peak maximum of C-151 remains unchanged at 380 nm. Thus, from the absorption spectra it is clear that the microenvironment of C-153 and C-151 in [bmim][PF6]-in-water microemulsions is completely different from that of a bulk water solvent or a TX-100/water binary system. The representative absorption spectra and peak positions of C-153 and C-151 in microemulsions are shown in Figure 1a and Table 2, respectively. The representative emission spectra of C-153 and C-151 in [bmim][PF6]-in-water microemulsions are shown in Figure 1b. The emission peaks in microemulsions are given in Table 2. The blue shift in the emission spectra compared to that of bulk solvent water confirms that both probes reside in a region in the microemulsions which is less polar than water. With a gradual increase in R the emission maxima of both probes remain the same compared to those of the initial composition (see Table 2). With a gradual increase in R the micelles are swollen by [bmim][PF6] as a result of the hydrodynamic diameter (Dh) increase,23b but the emission maximum of both probes does not change with loading of [bmim][PF6] in the microemulsions (see Tables 1 and

Error in experimental data of (5%.

2). Therefore, from steady-state data we can conclude that both probes reside in the surface of the microemulsions. The observed emission peak of C-153 in [bmim][PF6] is at 532 nm, and the emission peak of C-153 in the microemulsions is at 528 nm; i.e., C-153 resides in a region which is less polar than [bmim][PF6], probably in the interface of TX-100 and [bmim][PF6] in the microemulsions. We have observed emission maxima of C-151 at 484 nm at three different compositions of microemulsions. 3.2. Time-Resolved Studies. 3.2.1. SolVation Dynamics. We have observed a dynamic Stokes shift in the emission spectra of C-153 and C-151 in the microemulsions at different R values. The fluorescence decay of both probes is markedly dependent on the emission wavelength. At short wavelength, a fast decay is observed. At the red edge of the emission spectra the decay profile consists of a clear rise followed by the usual decay (which is shown in Figure SI-1 of the Supporting Information). The time-resolved emission spectrum (TRES) has been constructed following the procedure of Fleming and Maroncelli44 and described in our earlier publication.32 Each time-resolved emission spectrum was fitted by a log-normal function to extract the peak frequencies. These peak frequencies were then used to construct the decay of solvation correlation function (C(t)), which is defined as

C(t) )

ν(t) - ν(∞) ν(0) - ν(∞)

(1)

where ν(0), ν(t), and ν(∞) are the peak frequencies at time zero, t, and infinity, respectively. The representative TRES of C-153 and C-151 at R ) 0.24 is shown in Figure 2. The decay of C(t) was fitted by biexponential, triexponential, and stretched exponential functions. The biexponential model is superior to the other models. The stretched exponential model gives the worst fitting to our C(t) data, which is shown in the Supporting Information. The triexponential model gives a result similar to that of the biexponential fitting. Therefore, we chose the biexponential model to fit all of our C(t) curves. The decay of C(t) with time (Figure 3) was fitted by the biexponential function

C(t) ) a1e-t/τ1 + a2e-t/τ2

(2)

where τ1 and τ2 are the two solvation times with amplitudes of a1 and a2, respectively. The decay parameters of C(t) are summarized in Table 3. In the case of C-153, with an increase in the R value from 0.17 to 0.41 the fast component of the solvation time remains almost the same and we have observed a small change in the slow component. That is, the solvation time is practically unaffected with an increase in the [bmim][PF6] content or size of the microemulsions. In the case of C-151, with an (44) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1987, 86, 6221.

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Seth et al.

Figure 2. Time-resolved emission spectra of (a) C-153 in [bmim][PF6]/TX-100/water microemulsions (R ) 0.24) at (i) 0 (9), (ii) 200 (O), (iii) 1000 (2), and (iv) 4000 (×) ps and (b) C-151 in [bmim][PF6]/TX-100/water microemulsions (R ) 0.24) at (i) 0 (9), (ii) 200 (O), (iii) 1000 (2), and (iv) 4000 (×) ps. Table 3. Decay Parameters of C(t) for C-151 and C-153 in Microemulsions and a TX-100/Water Mixturea system

∆νb (cm-1)

a1

a2

τ1 (ns)

τ2 (ns)

C-151 system 1 C-151 system 2 C-151 system 3 C-153 system 1 C-153 system 2 C-153 system 3 C-151 in a TX-100/water mixture C-153 in a TX-100/water mixture

860 885 950 1565 1595 1550 690 1220

0.33 0.33 0.39 0.51 0.50 0.51 0.21 0.24

0.12 0.11 0.07 0.39 0.41 0.38 0.09 0.33

0.70 0.74 0.84 1.10 1.06 1.10 0.72 0.93

4.56 3.13 2.38 8.51 7.92 7.82 0.72 2.87

a

Error in experimental data of (5%. b ∆ν ) ν0 - ν∞.

increase in R from 0.17 to 0.41, the fast component of the solvation time gradually increases and the slow component gradually decreases. 3.2.2. Time-ResolVed Anisotropy Studies. Time-resolved fluorescence anisotropy (r(t)) is calculated using the following equation:

r(t) )

I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)

(3)

where G is the correction factor for detector sensitivity to the polarization direction of the emission and I|(t) and I⊥(t) are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. The G factor for our setup is 0.6. Using the tail-matching procedure, there is a possibility that we are losing the long-time relaxation process that is longer than the probe fluorescence lifetime.48 The anisotropy decay parameters are listed in Table 4. The rotational (45) Fee, R. S.; Maroncelli, M. Chem. Phys. 1994, 183, 235.

Figure 3. Decay of the solvent correlation function (C(t)) of (a) C-153 in [bmim][PF6]/TX-100/water microemulsions at (i) R ) 0.17 (9) and (ii) R ) 0.41 (3) and (b) C-151 in [bmim][PF6]/TX100/water microemulsions at (i) R ) 0.17 (O) and (ii) R ) 0.41 (9). Insets are given for better clarification of the initial part of the decay.

relaxation time of C-153 in the microemulsions is fitted to a biexponential function. With increase in R the average rotational relaxation time remains unchanged, which is shown in Figure 4a and Table 4. In the case of C-151, the rotational relaxation time in microemulsions is fitted to a triexponential function. It is found that with an increase in R the average rotational relaxation time of C-151 gradually increases from 1.79 ns in system 1 to 2.08 ns in system 3. The variation of the rotational relaxation time of C-151 and C-153 in different ways suggested different locations of the probes in the microemulsion. This is shown schematically in Scheme 2. 3.2.3. Viscosity Measurement. We also measured the viscosity of different microemulsions. With gradual addition of [bmim][PF6] the bulk viscosity of the solution gradually increases (Table 5).

4. Discussion From the steady-state results we can conclude that C-153 is located at the interface of the microemulsions. The rotational relaxation time of C-153 also does not change with an increase in R. We have also measured the lifetime of C-153 in different microemulsions at the corresponding emission peak of the dye, and we have observed that the lifetime of C-153 at the emission maximum is single exponential and remains unchanged for all microemulsions. This also suggests that C-153 is located at the interface of the microemulsions. The most interesting characteristic in these microemulsions is that the solvent reorganization time is not very sensitive to an increase in R or an increase in the size of the droplet. This observation is different from what (46) (a) Chapman, C. F.; Maroncelli, M. J. Phys. Chem. 1991, 95, 9095. (b) Bart, E.; Meltsin, A.; Huppert, D. J. Phys. Chem. 1994, 98, 10819. (47) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275. (48) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Academic/ Plenum Publishers: New York, 1999.

Ionic Liquid-Water Interaction in Microemulsions

Langmuir, Vol. 22, No. 18, 2006 7773

Table 4. Rotational Relaxation Parameters of C-151 and C-153 in Microemulsions and a TX-100/Water Mixture system

r0

a1r

τ1r (ns)

a2r

τ2r (ns)

C-151 system 1 C-151 system 2 C-151 system 3 C-153 system 1 C-153 system 2 C-153 system 3 C-151 in a TX-100/water mixture C-153 in a TX-100/water mixture

0.40 0.40 0.40 0.35 0.36 0.34 0.37 0.35

0.19 0.16 0.19 0.47 0.44 0.36 0.47 0.41

0.06 0.05 0.05 0.87 0.89 0.74 0.63 0.75

0.43 0.47 0.47 0.53 0.56 0.64 0.53 0.59

0.92 0.95 1.03 3.80 3.64 3.52 3.27 3.20

a

a3r

τ3r (ns)

〈τr〉a,b (ns)

0.38 0.37 0.34

3.65 4.0 4.67

1.79 1.93 2.08 2.42 2.43 2.52 2.03 2.19

〈τr〉 ) a1rτ1r + a2rτ2r + a3rτ3r. b Error in experimental data of (5%.

Scheme 2

Table 5. Bulk Viscosity of [bmim][PF6]/TX-100/Water Microemulsions at Different R Values

Figure 4. Decays of the time-resolved fluorescence anisotropy (r(t)) of (a) C-153 in [bmim][PF6]/TX-100/water microemulsions at R ) 0.41 (O) and (b) C-151 in [bmim][PF6]/TX-100/water microemulsions at (i) R ) 0.17 (9) and (ii) R ) 0.41 (O). (c) Standard deviation for the best fitted result for C-153 in system 3.

we have seen in AOT microemulsions.25-26,32 In previous solvation dynamics studies on AOT microemulsions the dye molecules migrated to the water/polar solvent pool of the microemulsions as the hydrodynamic radii or water content of the microemulsions increased.25-26,32 For this reason the solvent relaxation time gradually becomes faster with an increase in the water content of AOT microemulsions. Recently, we studied solvent relaxation of C-153 in RTIL-confined nanometer-sized microemulsions,41a and we found a change in the solvent relaxation time of C-153 with an increase in the RTIL/TX-100 ratio. In this work we do not observe a systematic variation of

R

viscosity (cP)

0.17 0.24 0.41

5.09 5.27 5.87

the solvent relaxation time, like in AOT microemulsions. This is probably due to the interfacial location of C-153 in the microemulsions, and with gradual addition of RTILs the position of C-153 remains more or less the same. C-153 is a hydrophobic probe, so it is more likely that C-153 resides at the interface of RTILs and TX-100 in the microemulsions. Recently, Corbeil et al.27b and Hazra et al.27a also showed a similar trend in the solvation time in SDS, CTAB, and TX-100 quaternary microemulsions. In our experiment, the fast component of the solvation time remained almost the same and we have observed a small change in the slow component with an increase in R from 0.17 to 0.41 (Table 3). We have also observed the solvation time of C-153 in a 15 wt % TX-100/water binary system, and it is described by two components of 930 ps (24%) and 2.87 ns (33%). The slowing of both components of the solvation time in [bmim][PF6]-in-water microemulsions compared to TX-100/water binary systems is due to addition of [bmim][PF6]. Due to addition of [bmim][PF6] the structure of the TX-100/water binary system has been changed. Consequently, probe molecules feel different microenvironments. In all RTIL-containing microemulsions, we have observed a bimodal solvation time and time constant and their relative weights are more or less the same in all microemulsions. The time constant of the fast component, ∼1100 ps (51%), and relative weight of the slow component, ∼39%, are the same in all microemulsions. We have observed only a small

7774 Langmuir, Vol. 22, No. 18, 2006

change in the time constant of the long component. In our earlier work14 we observed the solvation time of C-153 in neat [bmim][PF6]; the average solvation time is 3.35 ns with time constants of 536 ps (83%) and 17.10 ns (17%). Hence, the observed dynamics in RTIL-containing microemulsions is not the same as the dynamics in TX-100/water mixtures or in neat [bmim][PF6]; rather it implies the dynamics inside the microemulsions. Recently, we reported the solvation time of C-153 in neat [bmim][PF6] as ∼3 ns.41b Maroncelli et al.13a,c reported the solvation time in [bmim][PF6] as ∼1.8 ns using 4-aminophthalimide and 1.0 ns using C-153 as the probe. Samanta et al.12e reported the solvation time of Nile red in [bmim][PF6] as ∼1.0 ns. Thus, the solvation times reported by various groups are different. Even we have also observed different solvation times of C-153 in [bmim][PF6]14,41b in different experiments and various drying conditions. This is due to the fact that the qualities of the RTILs used by different groups are different and the presence of a small amount of impurities such as water and chloride ion vastly changes the viscosity of the RTILs.47 Recently, Maroncelli et al.13d reported that different results reported by different groups are due to different methods/setups used by different groups to represent the data. In this experiment the solvation dynamics is hindered in the pool of the microemulsions compared to the neat [bmim][PF6], but retardation is very small compared to the severalfold retardation of the solvation dynamics of conventional solvents inside the core of the microemulsions.25-28,32,33 In pure water solvent relaxation occurs on the femtosecond time scale. Jimenez et al.49 reported that solvent relaxation of C-343 in water consists of an initial decay of 55 fs (50%), attributed to the librational motion. Solvent relaxation of C-102 in water is bimodal with time constants of