Cyclohexane

The validity of the second virial coefficient was confirmed by the percolation behavior of different ionic liquid microemulsion solutions of Triton X-...
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Langmuir 2008, 24, 2973-2976

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Second Virial Coefficient of bmimBF4/Triton X-100/ Cyclohexane Ionic Liquid Microemulsion as Investigated by Microcalorimetry Na Li, Shaohua Zhang, Liqiang Zheng,* Yan’an Gao, and Li Yu Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, China ReceiVed December 7, 2007. In Final Form: February 15, 2008 The second virial coefficient of the ionic liquid (IL) microemulsion was obtained for the first time using microcalorimetry. The heat of dilution of the microemulsion solutions was measured by isothermal titration microcalorimetry (ITC), and the second virial coefficient was derived from the heat of dilution and the number density of the IL microemulsion solutions on the basis of a hard-sphere interaction potential assumption and as a function of the second-order polynomial. The validity of the second virial coefficient was confirmed by the percolation behavior of different ionic liquid microemulsion solutions of Triton X-100 in cyclohexane with or without added salts. The information obtained from the second virial coefficient shows that the interactions between ionic liquid microemulsion droplets are much stronger than those for traditional microemulsions, which may be attributed to the relatively larger size of the microemulsion droplets.

Introduction Ionic liquids (ILs) are receiving significant attention as novel and environmentally benign solvent systems for chemical reactions and separations.1,2 Their unique properties such as thermal stability, nonflammability, high ionic conductivity, and low volatility make them highly desirable in many reactions of industrial importance.3-5 Recently, many attempts have been made to prepare and study IL-containing microemulsions.6-12 In these microemulsions, water has been replaced by ILs that are immiscible in hydrocarbon solvents. The IL microemulsions have a number of distinct advantages over aqueous ones and have been applied to produce polymer nanoparticles, gels, and open-cell porous materials, which expands the potential applications of ILs and microemulsions in many fields.13,14 With the discovery of novel IL microemulsions, their formation mechanism and microstructure have been explored by various techniques. Han and co-workers6 first discovered that 1-butyl-3-methylimidazolium tetrafluoroborate could act as polar nanosized droplets dispersed in cyclohexane. The microstructures, swelling behavior, and morphology of the microemulsions were * Corresponding author. E-mail: [email protected]. Fax: 86-53188564750. Tel: 86-531-88366062. (1) Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.; Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932. (2) Gao, Y. A.; Li, Z. H.; Du, J. M.; Han, B. X.; Li, G. Z.; Hou, W. G.; Shen, D.; Zheng, L. Q.; Zhang, G. Y. Chem.sEur. J. 2005, 11, 5875. (3) Welton, T. Chem. ReV. 1999, 99, 2071. (4) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. 2000, 2047. (5) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (6) 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. (7) Eastoe, S.; Gold, S. E.; Rogers, A.; Paul, T.; Welton, R. K.; Heenan, I. G. J. Am. Chem. Soc. 2005, 127, 7302. (8) Chakrabarty, D.; Seth, D.; Chakraborty, A.; Sarkar, N. J. Phys. Chem. B 2005, 109, 5753. (9) Gao, Y. A.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W.; Yu, L. ChemPhysChem 2006, 7, 1554. (10) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Langmuir 2007, 23, 1091. (11) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, J.; Cao, Q.; Zhao, M. W.; Li, Z.; Zhang, G. Y. Chem.sEur. J. 2007, 13, 2661. (12) Gao, Y. A.; Li, N.; Zheng, L. Q.; Bai, X. T.; Yu, L.; Zhao, X. Y.; Zhang, J.; Zhao, M. W.; Li, Z. J. Phys. Chem. B. 2007, 111, 2506. (13) Li, Z. H.; Zhang, J. L.; Du, J. M.; Han, B. X.; Wang, J. Q. Colloids Surf., A 2006, 286, 117. (14) Li, N.; Dong, B.; Yuan, W. L.; Gao, Y. A.; Zheng, L. Q.; Huang, Y. M.; Wang, S. L. J. Dispersion Sci. Technol. 2007, 28, 1030.

detected by conductivity, dynamic light scattering (DLS), and freeze-fracture electron microscopy (FFEM). Subsequently, Eastoe et al.7 investigated the same system by small-angle neutron scattering (SANS). Regular swelling behavior was obtained, which is also in accordance with water-in-oil (W/O)-type microemulsions. These results indicate that IL microemulsion systems behave similarly to traditional aqueous ones. In our recent studies, the formation mechanism of IL microemulsions has been proposed.9 Micropolarities and solubilization behaviors of IL/O microemulsions have been characterized by UV-vis spectroscopy.10 Moreover, it has been discovered that small amounts of water have a great effect on the microstructure and stability of IL/O microemulsions.11,12 Although IL microemulsions have been widely studied, to the best of our knowledge there have been no reports on the interactions between IL microemulsion droplets. Understanding microemulsion droplet interactions supplies an important theoretical basis for many applications, such as drug carriers and chemical reaction media.13-15 Therefore, it is necessary to obtain some information about novel IL microemulsions. Generally, it is very difficult to measure the interaction forces between droplets directly. However, a thermodynamic parameter, that is, the second virial coefficient, is feasible for describing different droplet interactions.16 The second virial coefficient has been experimentally determined by measuring the osmotic pressure17 and diffusion coefficient as a function of volume fraction.18 The magnitude of the second virial coefficient is correlated with the aggregation behavior of colloids. Using microcalorimetry technology to obtain the second virial coefficient is a widely accepted method of detecting the interactions of traditional aqueous microemulsions and vesicle systems.16,19 Furthermore, this method has successfully characterized the protein solutions.20 Considering the similar microstucture of IL microemulsions with respect to traditional ones, the second virial coefficient of IL microemulsions was obtained (15) Lv, F. F.; Zheng, L. Q.; Tung, C. Int. J. Pharm. 2005, 301, 237. (16) Chen, W. Y.; Kuo, C. S.; Liu, D. Z. Langmuir 2000, 16, 300. (17) Stauffer, D. Introduction to Percolation Theory; Taylor and Francis: London, 1985. (18) Cassin, G.; Illy, S.; Pileni, M. P. Chem. Phys. Lett. 1994, 221, 205. (19) Liu, D. Z.; Chen, W. Y.; Tasi, L. M.; Yang, S. P. Colloids Surf., A 2000, 172, 57. (20) Huang, S. L.; Lin, F. Y.; Yang, C. P. Eur. J. Pharm. Sci. 2005, 24, 545.

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2974 Langmuir, Vol. 24, No. 7, 2008

Figure 1. Phase diagram of the bmimBF4/Triton X-100/cyclohexane three-component system at 25 °C. For line a, the initial Triton X-100 weight fraction is 0.55.

from this method. In this study, the heat of dilution of IL microemulsion solutions was measured using isothermal titration microcalorimetry (ITC), and the second virial coefficient of the IL microemulsions was calculated on the basis of statistical thermodynamics derivations and the hard-sphere assumption. Furthermore, the percolation behavior of IL microemulsions with or without solubilized metal salts verifies that the second virial coefficient obtained from ITC can successfully characterize the interactions between IL microemulsion droplets. Experimental Section Materials. Triton X-100, ((CH3)3CCH2C(CH3)2C6H4(OCH2CH2)9.5OH, Mw ) 646.86, d ) 1.063 g/mL), a typical nonionic surfactant, was obtained from Alfa Aesar and evaporated under vacuum at 80 °C for 4 h to remove any excess water before use. The IL, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), was prepared as reported earlier.21 The purity of the product was checked by 1H NMR spectroscopy (400 MHz, acetone-d6) δ: 1.04 (t, 3 H), 1.46 (m, 2 H), 2.03 (m, 2 H), 4.22 (s, 3 H), 4.54 (t, 2H), 8.01 (s, 1 H), 8.08 (s, 1 H), 9.70 (s, 1 H). Cyclohexane was provided by the Beijing Chemical Reagent Company. Metal salts FeCl3 and CuCl2 were purchased from the Shanghai Chemical Reagent Company. Apparatus and Procedures. Microemulsion Preparation. The IL microemulsions were prepared by mixing an appropriate quality of pure IL or an IL salt-containing solution with a micelle solution of Triton X-100 in cyclohexane. Subsequently, the microemulsions were placed in Teflon-stoppered test tubes and were left to equilibrate for 1 day before the conductivity and microcalorimetric measurements. Isothermal Titration Microcalorimetry. The ITC experiments were carried out using the model µRCSYS-0011 titration calorimeter from Thermal Hazard Technology (THT). Briefly, 0.67 mL of a Triton X-100-in-cyclohexane microemulsion was placed in the calorimeter and titrated with pure IL or a salt-containing IL solution along line a in Figure 1; 20 × 4 µL injections of IL were added to the reaction vessel with a 1000 s interval between each injection. That means that every injection added 0.597 wt % of titrant until a total of 0.08 mL titrant had been added (11.94 wt %).

Results and Discussion Phase Behavior. The phase diagram of bmimBF4/Triton X-100/cyclohexane ternary system at 25 °C is shown in Figure 1. The phase diagram was constructed by titration with bmimBF4 as follows: First, the required masses of Triton X-100 and (21) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 1999, 79, 236.

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bmimBF4 were mixed. For each titration, the bmimBF4-to-Triton X-100 weight ratio (I) was fixed. The phase boundaries were determined by observing the transition from turbidity to transparency or from transparency to turbidity. In Figure 1, the region marked “µE” was transparent, and the region marked “two phases” was turbid. By repeating the experiment for other I values, the phase diagram was established. The microregions of the microemulsion were identified by conductivity measurements according to previous reports,9 and three microregions (i.e., IL/O, bicontinuous, and O/IL microemulsions) were also plotted in Figure 1. On the basis of the phase diagram, several samples in the IL/O microregion along dilute line a were chosen for the ITC and conductivity measurements. Measurements of the Second Virial Coefficient. The virial coefficient that appears in the equation of state of a gas and is a function of intermolecular potential involves the simultaneous interaction of three molecules.22 It is known that, for colloidal systems, the virial coefficient also carries information about the sign and importance of any weak interparticle interactions.23 In this work, the measured heat of dilution of the IL microemulsion solutions versus the concentration of the microemulsions was combined with the virial equation for nonideal behavior of microemulsion solutions. The corresponding second virial coefficient values can thus be obtained by virial equation analysis under a hard-sphere interaction potential assumption.16,19,20 In general, the heat of dilution of the microemulsion solutions with the microemulsion droplet number density can be fitted by a function of the second-order polynomial as

d (Qdilution/NkBT) d(E/NkBT) = ) b2 + b3F + b4F2 dF dF where E is the internal energy (J), F is the microemulsion droplet number density (mL/particle), which equals N/V (V is the volume of the solution (mL) and N is the number of particles, i.e., the number of microemulsion droplets in the solution), kB is the Boltzmann constant (J/K), T is the absolute temperature (K), and b2, b3, and b4 are the virial coefficients. Specifically, b2 is the second virial coefficient. For an open liquid system, there is no change in pressure, the dilution volume can be neglected compared with the total solution volume of the system, and the heat of dilution should be equal to the internal energy change of the system. According to earlier reports,16,19,20 a negative b2 value indicates attractive forces between microemulsion droplets, and a larger magnitude of b2 implies aggregation of the microemulsion droplets. Figure 2 shows the heat of dilution of the microemulsion solutions with various IL microemulsion droplet number densities (Triton X-100/bmimBF4/cyclohexane with 30 mM CuCl2) as an example. The heat of dilution generated in Figure 2 against the IL microemulsion droplet number density is plotted in Figure 3 with a second-order polynomial fit. The b2 values obtained from the fitting equation for various systems are listed in Table 1. The b2 values indicate that the added salts increase the magnitude of b2, showing that the interactions between the microemulsion droplets become stronger. Table 2 shows the various values of b2 at different temperatures. As the temperature increases, the magnitude of b2 decreases, showing that the interaction between the droplets is weakened. Compared with traditional microemulsions and micelle systems, both the heat of dilution and the magnitude of b2 of IL microemulsion systems are much greater,16,19,20 so the interaction between IL microemulsion (22) Sherwood, E.; Prausnitz, J. M. J. Chem. Phys. 1964, 41, 413. (23) Kurnaz, M. L.; Maher, J. V. Phys. ReV. E 1997, 55, 572.

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Langmuir, Vol. 24, No. 7, 2008 2975

Figure 2. Heat of dilution of the microemulsion solution with various bmimBF4 volume fractions. Each peak indicates a titration of 4 µL of bmimBF4 into the reverse micelle solution (bmimBF4/Triton X-100/cyclohexane with 30 mM CuCl2, initial Triton X-100 weight fraction of 0.55, and cyclohexane weight fraction of 0.45).

Figure 3. Second-order polynomial fitting of the heat of dilution of the microemulsion solution vs the IL microemulsion droplets number density. (The components of the samples are the same as those in Figure 2.) Table 1. b2 Values and the Conductivity Percolation Temperatures of Various IL Microemulsion Solution Systems microemulsion solution (25 °C)

tp (°C)

b2 (mL/particle)

Triton X-100/cyclohexane/bmimBF4 Triton X-100/cyclohexane/bmimBF4/ CuCl2 ) 30 mM Triton X-100/cyclohexane/bmimBF4/ FeCl3 ) 30 mM

40 37

-1.41 × 10-18 -1.57 × 10-18

35

-1.86 × 10-18

Table 2. b2 Values of the IL Microemulsion Solutions at Different Temperaturesa microemulsion solution

b2 (mL/particle)

Triton X-100/cyclohexane/bmimBF4 (25 °C) Triton X-100/cyclohexane/bmimBF4 (35 °C) Triton X-100/cyclohexane/bmimBF4 (45 °C)

-1.41 × 10-18 -1.23 × 10-18 -0.98 × 10-18

a The Triton X-100 weight fraction is 0.495, the cyclohexane weight fraction is 0.405, and the bmimBF4 weight fraction is 0.10.

droplets must be much stronger. Consequently, IL microemulsions can be treated as a new reaction medium that provides a unique environment for chemical reactions depending on the collision of the droplets. The specific properties of the IL microemulsions could offer advantages over traditional colloids, and this discovery may help the development of new applications for IL microemulsions. Validity of the Second Virial Coefficient Confirmed by Conductivity. The structure of the microemulsions remarkably depends upon the composition and temperature.24,25 The conductivity measurement is a powerful tool for assessing the structural changes occurring in microemulsion systems.9,10,15,24,25 (24) A ¨ lvarez, E.; Garcı´a-Rı´o, L.; Mejuto, J. C.; Navaza, J. M.; Perez-Juste, J. J. Chem. Eng. Data. 1999, 44, 850. (25) Mehta, S. K.; Sharma, S. J. Colloid Interface Sci. 2006, 296, 690.

Figure 4. Conductivity of the IL microemulsion solutions with or without salts vs the solution temperature (Triton X-100 weight fraction of 0.495, cyclohexane weight fraction of 0.405, and bmimBF4 weight fraction of 0.10).

Figure 5. Determination of the percolation temperature by the Kim method.28 (The components of the samples are the same as those in Figure 4.)

It has been reported that the percolation value or percolation temperature of microemulsions can reflect the interactions between the microemulsion droplets to some extent.26,27 The electrical percolation of the IL microemulsions with and without solubilized metal salts was thus studied (Figure 4). Values of the specific conductivity/temperature, κ/t, were measured, and the value of the percolation threshold was determined. The percolation threshold was calculated from κ/t using the treatment illustrated in Figure 5, where the plot of dκ/κ dt versus t shows a maximum that corresponds to the percolation temperature, tp.28 For comparison, the percolation temperatures obtained from Figure 5 are also listed in Table 1. It can be seen that the systems with solubilized metal salts show a lower percolation temperature, suggesting that the interactions between IL microemulsion droplets were increased after the metal salts were added. The result is in agreement with the second virial coefficients obtained by ITC measurements. As mentioned, the driving force for the formation of IL microemulsions was the electrostatic attraction between the electronegative oxygen atoms of the OE units of Triton X-100 and the positively charged imidazolium ring.9 After CuCl2 and FeCl3 were added to the IL microemulsions, the salts were sequestered in the IL cores of the microemulsions. It can be deduced that there is stronger electrostatic attraction between the added metal salts and IL or OE units, which accordingly decreases the interactions between IL and Triton X-100. As a result, the interfacial curvature of the Triton X-100 film will bend toward the continuous cyclohexane phase and hence cause an increased droplet size. The sizes of microemulsions after adding metal salts were also measured by dynamic light scattering (DLS). The diameter of the IL microemulsion droplets is 58.3 nm without salt and 66.5 and 75.8 nm in the presence of 30 mM CuCl2 and 30 mM FeCl3, respectively (Supporting Information, Figure S1.) Therefore, we can conclude that the increased size of the IL (26) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (27) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. (28) Kim, M. W.; Huang, J. S. Phys. ReV. A 1986, 34, 719.

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microemulsions leads to the low percolation temperature and large magnitude of the second virial coefficient. Furthermore, Fe3+ should be more effective at influencing the microemulsion size, which agrees with the result of DLS; that is, a larger size is obtained for the FeCl3 system than for the CuCl2 system. Interestingly, we found that the conductivity of the IL microemulsions was unusually decreased when metal salts were added, as shown in Figure 4. The phenomenon may be due to the decreased movement of conducting bmimBF4. Cu2+ and Fe3+ had stronger electric fields that can attract the surrounding ions of bmimBF4. The movement of liquid bmimBF4 will thus be remarkably restricted and finally leads to the decrease in solution conductivity. The restricted movement of bmimBF4 due to the addition of CuCl2 and FeCl3 was also observed by 1H NMR spectra (Supporting Information, Figure S2.).

interactions between IL microemulsion droplets. There are much stronger interactions between IL microemulsion droplets, which may be attributed to their relatively larger droplet size compared with that of traditional microemulsions. The validity of the method was further confirmed by the percolation study and DLS. This method may also be used to investigate other ILs-containing colloids and may be helpful in understanding the microstructure and interaction mechanism of IL microemulsions.

Conclusions

Supporting Information Available: Sizes, size distributions, and 1H NMR spectra of bmimBF4-in-cyclohexane microemulsions. This material is available free of charge via the Internet at http://pubs.acs.org.

We have used a microcalorimetry method to obtain the second virial coefficient of the IL microemulsions. The second virial coefficient was found to be feasible for characterizing the

Acknowledgment. We are grateful to the National Natural Science Foundation of China (grant nos. 50472069 and 20773081) and the National Basic Research Program (2007CB808004). We also thank Dr. Pamela Holt for editing the manuscript.

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