Organic Solvents Induce the Formation of Oil-in-Ionic Liquid

Jan 12, 2009 - The role of four organic solvents in the formation process of 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) based ionic liqui...
0 downloads 9 Views 2MB Size
Download PDF
J. Phys. Chem. B 2009, 113, 1389–1395

1389

Organic Solvents Induce the Formation of Oil-in-Ionic Liquid Microemulsion Aggregations Yanan Gao, Na Li, Shaohua Zhang, Liqiang Zheng,* Xinwei Li, Bin Dong, and Li Yu Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, China ReceiVed: September 25, 2008; ReVised Manuscript ReceiVed: NoVember 10, 2008

The role of four organic solvents in the formation process of 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) based ionic liquid (IL) microemulsions is investigated. The results showed that the addition of Triton X-100 remarkably decreased the conductivity of bmimBF4. The added organic solvents provided a strong apolar environment for the hydrophobic tails of Triton X-100 and caused the surfactant molecules to aggregate into the interfacial film of oil-in-bmimBF4 (O/IL) microemulsions. As a result, the conductivities of the solutions were initially increased because the insulative Triton X-100 molecules were assembled, which corresponded to increasing the concentration of continuous bmimBF4 solutions. The hydrophobic interaction between the dispersed organic solvents and the hydrophobic tails of Triton X-100 may be the driving force for the formation of O/IL microemulsions. The droplets of O/IL microemulsions were successively swollen by organic solvents, and a bicontinuous IL-containing microemulsion was observed by freeze-fracture transmission electron microscopy for the first time. The current study can help in further understanding the ILscontaining microemulsions and thereby improve microemulsion theory. Introduction Ionic liquids (ILs), as one of the unique types of liquids, are considered as promising alternatives to traditional organic solvents in the future for their desirable properties, such as nonvolatility, nonflammability, high thermal stability, and wide electrochemical window.1 Such favorable properties have led to intense interest in these materials as environmentally benign solvents in a range of synthetic, catalytic, and electrochemical applications.2 Particular interest has involved the self-aggregations of amphiphilic molecules in the ILs, such as micelles,3-10 microemulsions,11-17 liquid crystals,18-24 ion gels,25-28 and vesicles.29 The microstructure and unique properties of these aggregations have been significantly investigated. Microemulsion is an optically isotropic, transparent, and thermodynamically stable medium formed by two or more immiscible liquids that are stabilized by an adsorbed surfactant film at the liquid-liquid interface.30 Self-assembled structures of different types can be formed, ranging, for example, from spherical and cylindrical microemulsions to lamellar phases and bicontinuous microemulsions, which may coexist with predominantly oil or aqueous phases. Recent work has also concerned the formation mechanism of the surfactant aggregations and more attention is especially paid for the ILs-containing reverse microemulsions stabilized by nonionic surfactants.17,31-33 It has been revealed that several common ILs, including 1-butyl-3methylimidazolium tetrafluoroborate (bmimBF4), can be dispersed in some organic solvents to form the reverse IL-in-oil (IL/O) microemulsions.12,15-17,34 The regular swelling behavior was found to be in accordance with traditional water-in-oil (W/ O) microemulsions, indicating that these unusual systems behaved akin to common W/O microemulsions.11,12 The dropletshaped microstructure was observed for the bmimBF4-incyclohexane microemulsion, which is also similar to the typical spherical structure of W/O systems.12 However, an unusual * To whom correspondence should be addressed. Phone: 86-531 88366062; fax: 86-531 88564750; e-mail: [email protected].

feature is the requirment of high background levels of nonionic surfactants.16 Besides, the relatively larger aggregate size was generally obtained.12,31,32,35 These unusual features are probably ascribed to the unique structure and properties of both ILs and nonionic surfactants. Small-angle neutron scattering (SANS) data have been treated in accordance with an ellipsoid form factor, and the ellipsoid model gave the best statistical fits, even compared to polydisperse spherical particles.11 Recently, Cheng et al. also discovered that 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) was the dispersed phase in ethylene glycol (EG) in the presence of nonionic surfactant Triton X-100. The single-phase microemulsion region was also divided into bmimPF6-in-EG, bicontinuous, and EG-in-bmimPF6 microstructures.36 Subsequently, they found another nonvolatile IL microemulsion, that is, bmimPF6-in-propylammonium formate (ILin-IL) microemulsion stabilized by anionic surfactant, AOT.37 Goto and co-workers reported the formation of reverse microemulsions in a hydrophobic IL comprising sodium bis(2-ethyl1-hexyl) sulfosuccinate (AOT) as the surfactant, in which various enzymes can be solubilized without loss of their catalytic activity.38,39 This kind of nonvolatile microemulsions may have potential applications with some advantages due to the waterfree and nonvolatile natures. Furthermore, hydrophobic ILs have been used to replace organic solvents to create immiscible hydrophobic ILs/water microemulsions. It was demonstrated that the hydrodynamic diameter (Dh) of the bmimPF6-in-water microemulsions is nearly independent of the water content but increases with increasing bmimPF6 content owing to the swelling of the microemulsions by bmimPF6.14 Sarkar and co-workers utilized steady state and picosecond time-resolved emission spectroscopy to explore the solvation dynamics and rotational relaxation of Coumarin 153 in bmimBF4/Triton X-100/cyclohexane microemulsions.40 In addition, water was found to have a great effect on the microstructures of IL/O microemulsions. The added water was solubilized within the total palisade layers of bmimBF4-in-benzene microemulsion and contributed to the stability of the microemulsion,17 whereas water molecules were

10.1021/jp808522b CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

1390 J. Phys. Chem. B, Vol. 113, No. 5, 2009 only located on the periphery of the bmimBF4-in-cyclohexane microemulsions and led to the decrease of microemulsion droplet size.41 The IL microemulsions have been used in the preparation of nanostructured materials. For instance, Han and co-workers reported42 the synthesis of silica microrods with nanosized pores using water/Triton X-100/bmimPF6 microemulsions. The IL, bmimPF6, played an important role in the formation of the final morphology of silica synthesized, as the imidazolium species are expected to be strongly complexed to silica surfaces during TEOS hydrolysis. Microemulsions containing ILs as a solvent phase have successfully been used to prepare nanoparticles. The microemulsion of guanidinium ILs in supercritical CO2 was used to prepare spherical gold nanoparticles or gold networks,33 and bmimPF6 in water was used to prepare tetragonal ZrO2 nanoparticles.43 So far, most efforts were merely focused on the microstructure, formation mechanism, properties, and potential application of IL/O reverse microemulsions. The driving force of the IL/O microemulsions has been considered to be the electrostatic attraction between the positively charged imidazolium cation of ILs and the electronegative oxygen atoms of oxyethylene (OE) units of nonionic surfactants.31 However, bicontinuous and oil-in-IL (O/IL) type of microemulsions were scarcely investigated. In this contribution, we aim to explore the major role of organic solvents involving toluene, cyclohexane, benzene, anp p-xylene and various intermolecular interaction mechanisms in the O/IL microemulsions. The result showed that the addition of organic solvents induced the formation of O/IL microemulsions. The swelling behavior and microstructures of the microemulsions were further characterized by conductivity, two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (2D ROESY) and freeze-fracture transmission electron microscopy (FF-TEM) technologies. Experimental Section Materials. The nonionic surfactant Triton X-100 and toluene (99.8%) were obtained from Sigma-Aldrich. Triton X-100 was evaporated under vacuum at 80 °C for 4 h to remove any excess water before use. Cyclohexane (99.5%) and deuterated toluene (C6D5CH3) were purchased from Merck. Toluene, benzene, deuterium oxide (99.9%), and p-xylene were provided by Beijing Chemical Reagents Company. The IL bmimBF4 was synthesized by the quaternization of 1-methylimidazole with 1-chlorobutane.44 The imidazolium chloride salt was crystallized in ethyl acetate at -30 °C. The postmetathesis product was obtained by ion exchange of 1-butyl-3-methylimidazolium chloride (bmimCl) and potassium tetrafluoroborate in distilled water, washed with dichloromethane, and then dried under high vacuum. The purity of the product was checked using 1H spectroscopy. To avoid water, the containers with the materials were sealed tightly to avoid any further contact with air before use. Apparatus and Procedures. A low-frequency conductivity meter (Model DDS-307, Shanghai Cany Precision Instrument Co., Ltd.) with an accuracy of ( 1% was used to measure the solution conductivities. The surface tensions of the IL soluitons were measured by a surface tensiometer (Model JYW-200B, Chengde Dahua Instrument Co.) equipped with a platinum ring. The temperature of the solutions was controlled by a thermostat (F31C, Julabo) with an accuracy of (0.1°C. Freeze-fracture transmission electron microscopy (FF-TEM) observations on the replication of samples were performed using a JEOL TEM 200CX electron microscope. For the preparation of the replicas, a small amount of sample was placed in a gold cup. The

Gao et al.

Figure 1. Variation of conductivity k as a function of organic solvent weight fraction, F, at different initial Triton X-100 weight fraction, R, for toluene (a), cyclohexane (b), benzene (c), and p-xylene (d) systems at 25.0 °C.

temperature of the gold cup was first kept at 25 °C by high thermal capacity instruments before preparing the sample replicas. Then the gold cup was swiftly plunged into liquid Freon that was cooled with liquid nitrogen. The frozen samples were fractured and replicated in a freeze-fracture apparatus BAF 400 (Bal-Tec, Balzer, Liechtenstein) at -140 °C. Pt/C was deposited at an angle of 45°. 1H measurements were first carried out with a Bruker AVANCE 600 NMR spectrometer at 298 K. The instrument was operated at a frequency of 600.13 MHz using tetramethylsilane as an internal reference. The spectrometer was fitted with a 5 mm CPTXI-1H-13C/15N/2H probe head with z-gradients. The standard 2D ROESY pulse sequence was used with a low power spin-lock pulse. The relaxation delay was 2 s. Complex data (2k) were collected in 256 increments with 8 transients each. The spin-lock field strength was 3200 Hz with a mixing time of 200 ms. The phase-sensitive 2D time domain were recorded and processed using TPPI protocol. A pure squared cosine window function was used in both dimensions prior to filling and Fourier transformation. A capillary tube loaded with D2O was inserted into the NMR tube to lock field for the 2D ROESY measurements of Triton X-100/bmimBF4 binary system. Results and Discussion Conductivity Measurements. An important feature of microemulsions is that they are macroscopically homogeneous; however, the structure is heterogeneous on a microscopic scale. Ordered microstructures such as oil-in-water (O/W) or W/O microdroplets can form at high water or oil content. Furthermore, a bicontinuous microstructure, such as a network of water tubes in an oil matrix or a network of oil tubes in a water matrix, with hydrocarbon and water regions stretching over large distances, has been identified in the intermediate regions.45,46 Conductivity has proven to be powerful tool to estimate structure and structural changes in microemulsions on the base of percolation theory. ILs are essentially molten salts and therefore have a very high conductivity. For pure bmimBF4, its conductivity is about 3393 µs/cm at 25.0 °C. The variations of conductivity k as a function of added organic solvent weight fraction, F (F ) Woil/(Woil + WTriton X-100 + WIL)), at different initial Triton X-100 weight fraction, R (R ) WTriton X-100/(WTriton X-100 + WIL)), are shown in Figure 1. The addition of nonionic surfactant Triton X-100

Oil-in-Ionic Liquid Microemulsion Aggregations decreased the conductivity of pure bmimBF4 to the order of 20-600 µs/cm because Triton X-100 is a nonconductive compound. The decreased conductivity is mainly ascribed to the dilution of highly conductive bmimBF4 by nonionic Triton X-100. The interaction of Triton X-100 with the imidazolium cation, bmim+ and/or BF4- via ion-dipole (ion of IL and dipole of Triton X-100) and/or hydrogen bonding (hydroxyl H of Triton X-100 and BF4- and/or hydroxyl/ethoxy oxygens of Triton X-100 with acidic C-2 hydrogen of bmim+), was also found to decrease the overall ionic character of the solution and resulted in decreased solution conductivity.47 When organic solvents were initially added to the mixtures, it can be seen from Figure 1 that the conductivities of all four systems increased considerably, which is somewhat unexpected because organic solvents are highly nonconductive and the addition of such compounds will commonly decrease the conductivity of solutions. It is known that the exceptional case also happened to traditional water/oil types of microemulsions. In that case, when small amounts of organic solvents were added to O/W microemulsions, the solution conductivity would also increase, which is due to the increase of conducting microdroplet number and size. However, the case is much different from the ILs-containing microemulsions because ILs are essentially molten salts. The conducting microdroplets can only increase the solution conductivity of O/W microemulsions by tens of microseconds per centimeters, they are impossible to increase the solution conductivity as much as those shown in Figure 1 (hundreds of µs/cm). Our previous studies have found that the IL/O microemulsions showed a similar conductive behavior to W/O microemulsions, and electrical conduction of IL/O microemulsions was attributed to the conducting open IL channels, which may be regarded as a two-component system in which conducting spherical droplets of ILs covered by a surfactant film are embedded in a continuous, insulative oil medium.31,48 Besides, the conductivity of the surfactant/oil solution was found to always increase on progressively increasing IL content, until it approached the conductivity of pure ILs,48 which is different from the water/oil type of microemulsions where the solution conductivity will finally decrease when water is added to a certain extent. At that stage, O/W microemulsions are formed and the decreased conductivity is due to the fact that the continuous aqueous phase was progressively diluted with water. This indicated that the conductivity behavior of O/IL microemulsions is much different from the traditional O/W microemulsions. We believe that highly conductive ILs are responsible for the electrical conduction of both bicontinuous and O/IL types of microemulsions rather than conducting microemulsion droplets, especially considering that the nonionic surfactant Triton X-100 would not form conducting microemulsion droplets. Therefore, the increased conductivity of bmimBF4 solution by the addition of organic solvents is not ascribed to the increase of conducting microdroplet number and size. 2D ROESY Spectroscopy. This interesting phenomenon may be attributed to the special intermolecular interaction or appearance of certain newly self-assemblied structures. 2D ROESY spectroscopy has proven to be a powerful tool to measure dipolar interactions between protons.49,50 The microstructure of micelle or microemulsion can be outlined by 2D NMR to a certain extent from the cross-peak intensities of the ROESY spectra. Figure 2 describes the contour plot of the 2D ROESY spectrum for the Triton X-100/bmimBF4 binary system (R ) 0.1). For clarity, the entire spectra were split into two sections (shown in Figure 2, panels a and b). The numbering of atomic positions of the two key components of our microemulsion, bmimBF4 and Triton

J. Phys. Chem. B, Vol. 113, No. 5, 2009 1391

Figure 2. 2D ROESY data of Triton X-100/bmimBF4 solution with R ) 0.1 at T ) 25.0 °C.

CHART 1: Chemical structure and atom numbering for bmimBF4 and Triton X-100

TX-100, is shown in Chart 1. It is evident that many interactions happen among all the protons of bmimBF4, for instance, H-2/ H-4, H-2/H-5 (not shown here), H-2/H-6, H-2/H-7, H-2/H-8, H-4/H-6, H-4/H-7, H-4/H-8, H-5/H-6, H-5/H-7, H-5/H-8, H-6/ H-8, H-6/H-9, and H-7/H-10. It was reported that there is an ordered structure for the pure bmimBF4 solution.51 These wide interactions therefore suggest that the ordered structure was destroyed by the addition of Triton X-100. Moreover, the cross peaks of H-2/H-a, H-2/H-b, H-2/H-c, H-2/H-h, H-4/H-h, H-5/ H-h, H-4/H-a, H-5/H-a, H-4/H-b, H-5/H-b, H-4/H-c, H-5/H-c,

1392 J. Phys. Chem. B, Vol. 113, No. 5, 2009 H-a/H-9, H-a/H-10, H-6/H-h, H-7/H-h, H-8/H-h, H-9/H-h, and H-10/H-h were also observed, indicating that there is close interaction between the Triton X-100 and bmimBF4. This is in accordance with our previous report that the electronegative oxygen atoms of Triton X-100 electrostatically attracted the positively charged imidazolium cation of bmimBF4.31 From these results, we can see that Triton X-100 is dissolved in bmimBF4 solution and that Triton X-100 molecules may be in a free state. In addition, we also characterized the solution system by freeze-fracture transmission electron microscopy (FFTEM) and no micellar aggregation was observed in bmimBF4 solution when the concentration was less than 0.56 mol/L (not shown here). Our surface tension measurements revealed that Triton X-100 formed micellar structure only when the concentration of Triton X-100 in bmimBF4 was higher than 0.56 mol/L or 30 wt % (see Supporting Information). Such high critical micellar concentration (CMC) is related to the properties of both Triton X-100 and bmimBF4. Triton X-100 is an amphiphilic molecule with a long hydrophilic polyoxyethylene (POE) headgroup and a short hydrophobic alpha-[4-(1,1,3,3-tetramethylbutyl)phenyl] tail, which thus leads to weak hydrophobicity. Moreover, ILs have been reported to possess a similar polarity to short-chain alcohols.52,53 Thus, the solvophobicity between Triton X-100 surfactant molecules and bmimBF4 is so weak that it is difficult for Triton X-100 molecules to form micellar aggregates. So, it can be deduced that there may be a molecular solution of bonded Triton X-100 or premicelles solution when the concentration of Triton X-100 was less than CMC. However, when organic solvents were added to the Triton X-100 based bmimBF4 solution, a newly self-assembled structure can be formed. As mentioned above, the Triton X-100 molecules are difficult to aggregate into micelles due to their weak solvophobicity. After adding organic solvents, we can image that these organic solvents are possibly highly dispersed and induce the formation of Triton X-100 aggregates. In other words, the organic molecules provide a stronger apolar environment and cause the hydrophobic tails of Triton X-100 to aggregate together. In these aggregates, organic solvents were dispersed as apolar cores and Triton X-100 molecules separated the immiscible oil and bmimBF4 with hydrophilic POE chains toward the continuous bmimBF4 and with hydrophobic tails toward the organic solvent inner cores. The similar phenomenon was also observed when some ionic fluorine surfactants were used to solubilize water molecules into continuous supercritical CO2. Ionic fluorine surfactants are difficult to dissolve in supercritical CO2 owing to the high crystallization of ionic surfactants, whereas the added water molecules hydrate the ionic head groups and form the hydration interaction between the water molecules and the head groups of surfactants. The waterin-supercritical-CO2 reverse microemulsions were thus induced to form.54-56 The difference is that organic solvents induced the formation of O/IL microemulsion in our investigated system. The aggregation of nonconductive Triton X-100 molecules actually corresponds to increasing the concentration of continuous bmimBF4. As a consequence, the solution conductivity was greatly increased. A schematic diagram of bmimBF4 solution environments before and after adding organic solvents is demonstrated in Figure 3. Here we have to mention that the OE units of Triton X-100 actually acted as an IL-philic group in the O/IL microemulsions. It has been proposed that the electrostatic attraction between the positively charged imidazolium cation, bmim+, and the electronegative oxygen atoms of OE units of nonionic surfactants drove the formation of IL/O microemulsions.31 The

Gao et al.

Figure 3. A schematic diagram of solution environments before and after adding organic solvents to the mixture of Triton X-100 and bmimBF4. (left) A molecular solution or micellar solution. (right) An induced O/IL microemulsion.

Figure 4. 2D ROESY spectroscopy of Triton X-100/bmimBF4/toluene ternary system with R ) 0.1 and F ) 0.48.

electrostatic attraction occurred within the inner cores of IL/O microemulsion and behaved like hydrogen bonds or hydration in aqueous W/O microemulsions. For the here investigated O/IL microemulsions, the same electrostatic attraction also happened but was located at the outer shell of O/IL microemulsion droplets. The hydrophobic interaction between the dispersed organic solvents and the hydrophobic group of Triton X-100 acted as glue to stick the Triton X-100 molecules and to build up the interfacial film between the immiscible oil and bmimBF4 two phases. The formation of O/IL microemulsions was also proved by 2D ROESY spectroscopic analysis. As a typical example, Figure 4 shows the 2D ROESY spectroscopy of Triton X-100/ bmimBF4/toluene ternary system with R ) 0.1 and F ) 0.13 (the composition is in O/IL microemulsion region according to the phase diagram15). It is evident that the methyl group of toluene intensively interacted with H-a, H-b, and H-c (they were marked as CH3/a, CH3/b, and CH3/c) of the hydrophobic alpha-

Oil-in-Ionic Liquid Microemulsion Aggregations [4-(1,1,3,3-tetramethylbutyl)phenyl] group of Triton X-100. This indicates that there is strong hydrophobic interaction between the hydrophobic chain of Triton X-100 and toluene. Moreover, three new cross peaks of H-a/H-b, H-a/H-c, and H-b/H-c appeared after toluene was added, showing that the surfactant molecules of Triton X-100 are now confined in an ordered structure with their hydrophobic tails close to each other. Also, the cross peak intensity of H-a/H-6, H-a/H-9, H-a/H-10, and H-a/H-10 were greatly decreased, which suggests that bmimBF4 was separated with the hydrophobic group of Triton X-100. From these results, it can be concluded that a new interface film of the surfactant Triton X-100 was now formed, in other words, microemulsion aggregate was induced to appear by the addition of organic solvents. It is also worthy mentioning that Triton X-100 micelles appear when the concentration of Triton X-100 in bmimBF4 is higher than CMC. Even so, the dissociated Triton X-100 concentration is also very high, and the conductivity of bmimBF4 will be remarkably decreased by the added Triton X-100 as well. Besides, FF-TEM images have shown a very crowded micellar structure of nonionic surfactants in ILs,57 which is also against the higher conductivity of bmimBF4. In the same way, the addition of organic solvents will also induce the appearance of Triton X-100 aggregated microemulsion structure and thus lead to the increased solution conductivity. The schematic diagram of Triton X-100 aggregated micelles in bmimBF4 solution before and after adding organic solvents is also demonstrated in Figure 3. To study swelling behavior of the microemulsions, as an example the toluene-based microemulsion system with R ) 0.5 was chosen because the O/IL microemulsion with R ) 0.1 is too close to the phase boundary.48 With successively increasing content of toluene, more and more Triton X-100 molecules were induced to assemble together, and the conductivity was successively increased accordingly (Figure 1a). However, the solution conductivity decreased when toluene content is above ca 30 wt %. This is because with increasing toluene content the droplet size of oil-in-bmimBF4 (O/IL) microemulsion will be simultaneously increased because of the progressive swelling of oil core, which will, in turn, decrease the solution conductivity because Triton X-100-surrounded oil microdroplets are relatively nonconductive. The similar phenomena were also observed for other three systems. On the base of percolation theory, the total swelling process can be consequently explained as following: the initial increase in the electrical conductivity reveals the appearance of IL/O microemulsions; the following nonlinear decrease in the curve indicates that the medium undergoes further structural transitions, and a bicontinuous microstructure, ascribed to the progressive growth and interconnection of the oil microdomains, appears; the third linear decrease is due to the formation of IL microdomains resulting from the partial fusion of clustered inverse microdroplets. The phenomenon suggests that an IL/O microemulsion is formed in this low ILcontent gap; the final nonlinear decrease in the electrical conductivity reveals the existence of a percolation phenomenon that is attributed to inverse IL microdroplet aggregation. FF-TEM Images. Freeze-fracture transmission electron microscopy (FF-TEM) was used to detect the swelling process of oil-in-bmimBF4 microemulsions with successively increasing organic solvent content. Figure 5 shows the FF-TEM images of the toluene/Triton X-100/bmimBF4 system with R ) 0.5 and F ) 0.10, 0.15, 0.20, 0.25, and 0.30. A near-spherical droplet structure was observed when F is less than 0.25, and their average diameter increased with increasing toluene content,

J. Phys. Chem. B, Vol. 113, No. 5, 2009 1393

Figure 5. FF-TEM images of the toluene-in-bmimBF4 microemulsion aggregates with R ) 0.5 and F ) 0.10 (a), 0.15 (b), 0.20 (c), 0.25 (d), and 0.30 (e). Scale bar is 100 nm for all samples in the figure.

indicating that the microdroplets were gradually swollen, which is consistent with the volume of dispersed nanodomains being directly proportional to the amount of added organic solvents. This is common to many droplet microemulsions, for example, poly(ethylene oxide)-poly(propyleneoxide)-poly(ethylene oxide) (PEO-PPO-PEO) stabilized O/W microemulsions,58 which further indicates the formation of O/IL microemulsions. A tendency toward aggregation of the microemulsion droplets can be observed at increased toluene content (F ) 0.25, Figure 5d). The aggregates consist of two or three single microdroplets, which are marked by a square in Figure 5d. With further increasing toluene content (F ) 0.30), an irregular bicontinuous microemulsion in which bmimBF4 and oil solutions are both local continuous phase was formed (Figure 5e). This is the first time that a bicontinuous microstructure was observed in ILscontaining aggregations. We also investigated the cyclohexane/ Triton X-100/bmimBF4 system. The swelling behavior of cyclohexane-in-bmimBF4 (O/IL) microemulsion with successive addition of cyclohexane was shown in Figure 6. A near-spherical droplet structure of bmimBF4-in-cyclohexane (IL/O) microemulsion has been demonstrated by Han and co-workers.12 At a lower cyclohexane content (F ) 0.05), large amounts of small droplet aggregations can be seen, and these aggregations are crowded tightly. With increasing cyclohexane content (F ) 0.075, 0.10, and 0.125), these small microstructures are swollen, and the original crowded structure becomes loose. The loosened microstructure is especially obvious when F ) 0.125 (Figure 6d). The same phenomenon was also observed for the toluenebased system. The reason, we believe, is that the addition of organic solvents induced the aggregation of Triton X-100 molecules in bmimBF4 solution. With the successive swelling of microemulsion droplets, more and more Triton X-100 molecules are collected, and the small microdroplets may be simultaneously digested and turned into larger ones. Therefore, the concentration of continuous bmimBF4 phase is enhanced and the solution conductivity of O/IL microemulsion was thus increased. This result is in accord with our conclusion. However, we can not obtain the bicontinuous microstructure for the

1394 J. Phys. Chem. B, Vol. 113, No. 5, 2009

Gao et al. is proved by the fact that further addition of organic solvents leading to the appearance of a bicontinuous microemulsion and that some droplets are still observable in Figure 7d. Therefore, the image in Figure 7d can be regarded as the intermediate state of droplet microemulsion and bicontinuous microemulsion. When p-xylene content is F ) 0.325, the bicontinuous microstructure appears (Figure 7e). At this stage, the bicontinuous ILs-containing microemulsion was further confirmed. When more organic solvent is added to the system, the bicontinuous microstructure will change into IL/O microemulsions. The droplet structure has been observed, and their formation mechanism and properties of such microemulsions have been intensively investigated by recent work.12,17,31 So, it is not necessary to discuss this here any further. Conclusions

Figure 6. FF-TEM images of the cyclohexane-in-bmimBF4 microemulsion aggregates with R ) 0.7 and F ) 0.05 (a), 0.075 (b), 0.10 (c), and 0.125 (d). Scale bar is 100 nm for all samples in the figure.

The effect of successively adding organic solvents to the Triton X-100/bmimBF4 binary system was investigated. The addition of oils provided a strong hydrophobic environment for the hydrophobic groups of Triton X-100 molecules and induced them to aggregate into interface film. The oil and ionic liquid (IL) bmimBF4 two-phases was separated by the Triton X-100 interface film, and an oil-in-IL (O/IL) microemulsion was thus formed. The aggregation behavior of nonconductive Triton X-100 led to the increase of continuous bmimBF4 concentration and thus the solution conductivity. The hydrophobic interaction between the added organic solvents and hydrophobic groups of surfactant molecules was considered to play a crucial role in driving the formation of O/IL microemulsions. The formation mechanism is different from the IL/O microemulsions where the electrostatic attraction between the positively charged imidazolium cation of bmimBF4 and the electronegative oxygen atoms of Triton X-100 was the driving force for the solubilizing bmimBF4 into the cores of Triton X-100 aggregates. The formed O/IL droplet microemulsion was successively swollen by the addition of organic solvents, and a bicontinuous microstructure was finally observed for the first time. The current study can help in understanding the aggregation behaviors of ILscontaining self-assemblies and thus better build them as potential reaction media. Acknowledgment. This work was supported by Natural Scientific Foundation of China (Grant No. 20773081) and the National basic Research Program (2007CB808004). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. FF-TEM images of the p-xylene-in-bmimBF4 microemulsion aggregates with R ) 0.6 and F ) 0.05 (a), 0.10 (b), 0.20 (c), 0.30 (d) and 0.325 (e). Scale bar is 100 nm for all samples in the figure.

cyclohexane-based system because phase separation occurs with further addition of cyclohexane. To support our observation, the swelling behavior of p-xylenein-bmimBF4 microemulsion aggregates was also investigated by FF-TEM. The FF-TEM images of the microemulsion with increasing p-xylene content are shown in Figure 7. Similarly, the gradually increased aggregate size and loosened microstructure were also observed (F ) 0.10 and 0.20; Figure 7, panels a and b). Also, the assembly tendency of several single microdroplets combining into a larger one is more obvious in this case (F ) 0.30; Figure 7c). This assembly tendency is actually an earnest of the bicontinuous microemulsion to come, which

References and Notes (1) Wasserscheid, P. Nature 2006, 439, 797. (2) Welton, T. Chem. ReV, 1999, 99, 2071–2084. (3) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem Commun. 2003, 2444–2445. (4) Fletcher, K. A.; Pandey, S. Langmuir, 2004, 20, 33–36. (5) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89–96. (6) Evans, D. F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J. Phys. Chem., 1983, 87, 3537–3541. (7) Araos, M. U.; Warr, G. G. J. Phys. Chem. B 2005, 109, 14275– 14277. (8) Atkin, R.; Warr, G. G. J. Am. Chem. Soc., 2005, 127, 11940–11941. (9) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de Lauth-Viguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99–101. (10) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745–2750. (11) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K. J. Am. Chem. Soc. 2005, 127, 7302–7303.

Oil-in-Ionic Liquid Microemulsion Aggregations (12) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R. Phys. Chem. Chem. Phys. 2004, 2914–2916. (13) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X. Green Chem, 2006, 8, 43–49. (14) Gao, Y. A.; Han, S. B.; Han, B. X.; Li, G. Z. Langmuir, 2005, 21, 5681–5684. (15) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Langmuir, 2007, 23, 1091–1097. (16) Atkin, R; Warr, G. G. J. Phys. Chem. B. 2007, 111, 9309–9316. (17) Gao, Y. A.; Li, N.; Zheng, L. Q.; Bai, X. T.; Yu, L.; Zhao, X. Y. J. Phys. Chem. B. 2007, 111, 2506–2513. (18) Evans, D. F.; Kaler, E. W.; Benton, W. J. J. Phys. Chem. 1983, 87, 533–535. (19) Tamura-Lis, W.; Lis, L. J.; Quinn, P. J. J. Phys. Chem. 1987, 91, 4625–4627. (20) Tamura-Lis, W.; Lis, L. J.; Quinn, P. J. Biophys. J. 1988, 53, 489– 492. (21) Wang, L. Y.; Chen, X.; Chai, Y. C.; Hao, J. C.; Sui, Z. M.; Zhuang, W. C. Chem Commun 2004, 2840–2841. (22) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. J. Phys. Chem. B 2007, 111, 4082–4088. (23) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Langmuir 2007, 23, 402–404. (24) Wang, Z. N.; Liu, F.; Gao, Y.; Zhuang, W. C.; Xu, L. M.; Han, B. X. Langmuir 2005, 21, 4931–4937. (25) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759–6761. (26) He, Y. Y.; Lodge, T. P. Chem Commun 2007, 2732–2734. (27) He, Y. Y.; Boswell, P. G.; Buhlmann, P.; Lodge, T. P. J. Phys. Chem. B 2007, 111, 4645–4652. (28) Lee, J.; Panzer, M. J.; He, Y.; Lodge, T. P.; Frisbie, C. D. J. Am. Chem. Soc. 2007, 129, 4532–4523. (29) Hao, J. C.; Song, A. X.; Wang, J. Z.; Chen, X.; Zhuang, W. C.; Shi, F. Chem.-Eur. J. 2005, 11, 3936–3940. (30) Shinado, K. SolVent Properties of Surfactant Solutions; Marcel Dekker: New York, 1967. (31) Gao, Y. A.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W. ChemPhysChem 2006, 7, 1554–1561. (32) Gao, Y.; 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, 2661–2670. (33) Liu, J. H.; Cheng, S. Q.; Zhang, J. L.; Feng, X. Y.; Fu, X. G; Han, B. X. Angew. Chem., Int. Ed. 2007, 46, 3313–3315. (34) Li, J. C.; Zhang, J. L.; Gao, H. X.; Han, B. X.; Gao, L. Colloid Polym. Sci. 2005, 283, 1371–1375. (35) Gao, Y. A.; Vogit, A.; Hilfert, L.; Sundmacher, K. ChemPhycChem 2008, 9, 1603–1609.

J. Phys. Chem. B, Vol. 113, No. 5, 2009 1395 (36) Cheng, S. Q.; Fu, X. G.; Liu, J. H.; Zhang, J. L.; Zhang, Z. F.; Wei, Y. L.; Han, B. X. Colloid Surf. A 2007, 302, 211–215. (37) Cheng, S. Q.; Zhang, J. L.; Zhang, Z. F.; Han, B. X. Chem Commun 2007, 2497–2499. (38) Zaman, M. M.; Kamiya, N.; Nakashima, K.; Goto, M. ChemPhysChem 2008, 9, 689–692. (39) Zaman, M. M.; Kamiya, N.; Nakashima, K.; Goto, M. Green Chem. 2008, 10, 497–500. (40) Chakrabarty, D.; Seth, D.; Chakraborty, A.; Sarkar, N. J. Phys. Chem.B, 2005, 109, 5753–5758. (41) Gao, Y.; Hilfert, L.; Voigt, A.; Sundmacher, K. J. Phys. Chem.B, 2008, 112, 3711–3719. (42) Li, Z. H.; Zhang, J. L.; Du, J. M.; Han, B. X.; Wang, J. Q. Colloids Surf. A, 2006, 286, 117–120. (43) Li, N.; Dong, B.; Yuan, W. L.; Gao, Y. A.; Zheng, L. Q.; Huang, Y. M.; Wang, S. L. J. Disper. Sci. Technol. 2007, 28, 1030–1033. (44) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 1999, 79, 236–241. (45) Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982, 86, 2294–2304. (46) Mackay, R. A.; Myers, S. A.; Bodalbhai, L.; Brajter-Toth, A. Anal. Chem. 1990, 62, 1084–1090. (47) Behera, K.; Dahiya, P.; Pandey, S. J. Colloid Interface Sci. 2007, 307, 235–245. (48) Gao, Y.; Wang, S.; Zheng, L.; Han, S.; Zhang, X.; Lu, D.; Yu, L.; Ji, Y.; Zhang, G. J. Colloid Interface Sci. 2006, 301, 612–616. (49) Yuan, H. Z.; Zhao, S.; Cheng, G. Z.; Zhang, L.; Miao, X. J.; Mao, S. Z.; Yu, J. Y.; Shen, L. F.; Du, Y. R. J. Phys. Chem. B, 2001, 105, 4611– 4615. (50) Yuan, H. Z.; Cheng, G. Z.; Zhao, S.; Miao, X. J.; Yu, J. Y.; Shen, L. F.; Du, Y. R. Langmuir 2000, 16, 3030–3035. (51) Dupont, J. J. Braz. Chem. Soc. 2004, 15, 341–350. (52) Reichardt, C. Green Chem. 2005, 7, 339–351. (53) Wakai, C.; Oleinikova, A.; Ott, M.; Weinga¨rtner, H. J. Phys. Chem. B, 2005, 109, 17028–17030. (54) Harrison, K. L.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536–3541. (55) Sagisaka, M.; Yoda, S.; Takebayashi, Y.; Otake, K.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M. Langmuir 2003, 19, 8161–8167. (56) Gao, Y.; Wu, W.; Han, B.; Li, G.; Chen, J.; Hou, W. Fluid Phase Equilib. 2004, 226, 301–305. (57) Zhang, S.; Li, N.; Zheng, L.; Li, X.; Gao, Y.; Yu, L. J. Phys. Chem. B. 2008, 112, 10228–10233. (58) Lettow, J. S.; Lancaster, T. M.; Glinka, C. J.; Ying, J. Y. Langmuir 2005, 21, 5738–5746.

JP808522B