Liquid Crystalline Phases Self-Organized from a ... - ACS Publications

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J. Phys. Chem. B 2009, 113, 2024–2030

Liquid Crystalline Phases Self-Organized from a Surfactant-like Ionic Liquid C16mimCl in Ethylammonium Nitrate Yurong Zhao, Xiao Chen,* and Xudong Wang Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, People’s Republic of China ReceiVed: December 2, 2008; ReVised Manuscript ReceiVed: December 23, 2008

The aggregation behavior of a surfactant-like ionic liquid, 1-hexadecyl-3-methylimidazolium chloride (C16mimCl), in a room temperature ionic liquid ethylammonium nitrate (EAN) has been investigated. With increasing C16mimCl concentration, a series of ordered aggregates including micelles, normal hexagonal (H1), lamellar (LR), and reverse bicontinuous cubic (V2) liquid crystalline (LC) phases can be detected over a large temperature range by using polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) techniques. When comparing such a phase behavior with that of the C16mimCl/H2O binary system, an additional V2 phase is identified and could be attributed to the different affinities of C16mimCl to EAN and water. Higher temperatures induce smaller lattice spacing for the LC phases, which may be due to the softening of the solvophobic chains of C16mimCl molecules. Both the imidazolium ring structure and alkyl chain of C16mimCl molecule are proved to play important roles for the LC phase formation. Dissipative particle dynamic simulations are also carried out at room temperature, and the obtained intuitive three-dimensional (3D) models can help us better understand the self-assembled structures, which are considered to be supplements for the experimental results. Introduction 1,2

Because of their unique properties, the room temperature ionic liquids (RTILs) have currently caught scientists’ eyes for the applications as solvents in organic synthesis,3 catalysis,4-6 electrochemistry,7,8 liquid/liquid extraction,9-11 and preparation of novel materials.12,13 Some recent investigations have concentrated on the self-assembled structures formed in 1-alkyl3-methylimidazolium salts ([Cnmim]+, where n is the alkyl chain length). Micellization behaviors of traditional surfactants and amphiphilic block copolymers in [Cnmim]+ have been both included in recent studies.14-16 Lyotropic liquid crystalline (LLC) phases or vesicles formed in [Bmim+][PF6-] have been respectively reported by us17 and Hao’s group.18 In addition, microemulsions including [Cnmim+]X- have also been investigated by many groups.19-23 Except for these aggregates formed in imidazolium-based molten salts, the self-assembling behavior of surfactants in another ionic liquid EAN, a low-melting fused salt showing more analogous properties than other RTILs to water,24 has also attracted much attention. The aggregations of surfactants or lipids into micelles or lamellar liquid crystal phases in EAN were reported by Evans et al. over two decades ago.25,26 Lis et al. subsequently studied the obtained aggregate structures as well as the phase transition mechanism in such a nonaqueous media.27,28 Both the micelles and LC phases formed from a series of nonionic surfactants, CnEm, in EAN were reported by Warr et al.29,30 They also found that CnEm, alkanes, and EAN could construct microemulsions and lamellar phases.31 Other surfactants, like the cationic hexadecyltrimethylammonium bromide (CTAB), the nonionic myverol 18-99 K, and phytantriol, were investigated in many protic ILs including EAN by Drummond et al.32,33 * To whom correspondence should be addressed: e-mail [email protected], Fax +86-531-88564464, Tel +86-531-88365420.

Recently, we have explored the amphiphilic block copolymer P123/EAN binary system and observed several aggregate structures including normal micellar cubic, normal hexagonal, lamellar, and reverse bicontinuous cubic phases.34 It is noted that the self-assembling behavior of P123 in EAN is different from that in water, even though both solvents have great similarities. Motivated by this study, we choose the long-chained imidazolium-based ILs, which is often called surfactant-like ILs and can form various aggregate structures in water,35 to investigate its phase behavior in EAN to extend the knowledge on the similarities and differences between EAN and water. Such a research, however, to the best of our knowledge, is rarely reported. Only the aggregation behavior of two surfactant-like ILs, C16mimCl and C16mimBF4 in EAN, was investigated by Kunz et al. with their focus on the micellization behavior at relative lower concentration regions.36 Here, the aggregation behavior of C16mimCl in EAN is investigated over a whole concentration range. Different LC phases were identified by POM and SAXS techniques and then compared with those formed in water. Meanwhile, the dissipative particle dynamics (DPD) approach is also applied to simulate the present binary system, which may provide us microphase separation information and is considered as a supplement for investigations of both the imidazolium-based surfactive ILs and EAN. Experimental Section Materials. C16mimCl was prepared according to the procedures reported previously.37,38 The compound 1-methylimidazole and an excess amount of 1-hexadecyl chloride were mixed in a flask, refluxed at 90 °C for 24 h, and then cooled to room temperature. The product (a white waxy solid) was purified by recrystallizing the mixture in fresh tetrahydrofuran (THF) at least three times and then dried under vacuum conditions for 48 h.

10.1021/jp810613c CCC: $40.75  2009 American Chemical Society Published on Web 01/23/2009

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Figure 1. Schematic representation of the simulation model. The C16mimCl molecule is fragmented into two DPD particles, C and H, which are connected together with a harmonic spring. EAN is represented by a single DPD particle E.

The product purity was ascertained by surface tension measurement and 1H NMR (nuclear magnetic resonance) spectrum in D2O. EAN was synthesized as described by Evans et al.;25 a portion of ∼3 M nitric acid was slowly added to the ethylamine solution while stirring and cooling in an ice bath. Water in the resulted solution was first removed with a rotary evaporator and then with a lyophilizer (MartinchristerALPHA1-2). The residual water content of the final product was determined by Karl Fischer titration to be 0.7 wt %, and its melting point was about 11 °C. Methods. Sample Preparation and Phase Diagram Mapping. The process for mapping the phase diagram has been described elsewhere.34,39 Samples were prepared by mixing C16mimCl and EAN with designed compositions (in weight percent, wt %, thereinafter). These mixtures were homogenized by repeat mixing and centrifugation. Then they were equilibrated for at least 3 months before further investigations. The phase change was detected by ocular observation and visual inspection through crossed polarizers. Liquid crystal types were determined by polarized optical microscopy and small-angle X-ray scattering techniques. The composition interval was first selected as 5% for a rough mapping and then 2% for the determination of the phase boundaries. Characterization. The obtained LC phases were characterized by an HMBG-SAX X-ray small-angle scattering system (Austria) with a Ni-filtered Cu KR radiation (0.154 nm) operating at 50 kV and 40 mA. The distance between the sample and detector was 27.8 cm. Photographs of sample birefringence were taken by a Motic B2 polarizing optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WV-CP460). Differential scanning calorimetry (DSC) measurements were carried out on a SDT Q600 Simultaneous DSC-TGA (TA Instruments). The linear heating rate of 10 °C /min was employed on the sample over the temperature range from 25 to 400 °C under nitrogen at a flow rate of 50 mL/min. Dissipative particle dynamic simulations were carried out by using a CeriusII software on SGI workstation,40 and the detailed information for such a simulation method can be seen in our previous paper.41 In the DPD simulations, the molecules are substituted by particles. Each particle represents a small volume of fluid containing many atoms. In order that the underlying chemistry of the materials is not lost, it is important to represent chemically distinct units by particles of different types. Besides, all particles should contain similar masses of material. According to these two reasons, in the simulations here, the C16mimCl molecule is represented by a dimeric model as shown in Figure 1, where the amphiphilic molecule is divided into two parts, the solvophilic part H and the solvophobic part C, which are connected by a harmonic spring. The EAN molecules are represented by the monomer particle E. The interaction parameters aij between different particles were as follows: aC-C ) aH-H ) aE-E ) 15, aC-H ) 80, aC-E ) 82, and aH-E ) 0. The

Figure 2. Phase diagram for C16mimCl/EAN binary system, in which H1, LR, and V2 denote respectively the normal hexagonal, lamellar, and reverse bicontinuous phases.

dynamics of 5000 DPD particles, starting from a random distribution, is simulated in a 10 × 10 × 10 cubic box under periodical boundary conditions. The step size for the integration of the Newton equation is set to ∆t ) 0.05. The temperature is set to 298 K. For each system, 20 000 time steps per simulation are carried out. The DPD method used here is somewhat different from the MesoDyn approach referred in our previous studies,42,43 which is more frequently applied for Pluronic systems. Results and Discussion The obtained phase diagram for the C16mimCl/EAN binary system is shown in Figure 2. At relative low C16mimCl concentrations (less than 46%), the solutions are homogeneous and isotropic, which are then considered to be normal micelle phase (L1) as that reported previously.36 With the increase of C16mimCl concentration, a series of LC phases including normal hexagonal, lamellar, and reverse bicontinuous cubic can be formed, all of which go through a large temperature range. Our discussions on the phase behavior of this binary system below are therefore divided into two parts: at room temperature and at relatively higher temperatures. Phase Behavior at Room Temperature. The isotropic phase at lower C16mimCl concentrations can be ascertained by the POM paragraph with no birefringence, as shown at the top of Figure 3a for a representative system with 45% C16mimCl. Besides, only one scattering peak in its corresponding SAXS curve (Figure 3a, bottom) makes it to be considered as a normal micelle phase. With the increase of surfactant concentration, this binary system goes through an H1 phase in the range of 47-57% C16mimCl, where the sample appearance is similar to that of the micelle phase, that is, transparent and clear. However, the system now becomes more viscous and birefringent as can be seen from the POM photographs shown at the top of Figures 3b and 3c for the samples respectively at 50% and 55% C16mimCl concentrations, where both textures are bright and characteristic. Their SAXS patterns shown at the bottom of Figures 3b and 3c exhibit four Bragg peaks with the relative positions of 1:3:2:7 relationship, indicating a hexagonal phase. The third Bragg peaks for both curves are quite weak and even hardly to be identified. Such a phenomenon has also been found in other systems and may be attributed to the cancelation between the form factor and the structure factor of the SAXS intensity.34,44-47 With further increasing C16mimCl concentration, the sample appearance changes greatly and becomes less transparent and somewhat cloudy. Detailed

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Figure 3. POM photographs (top) and SAXS curves (bottom) at room temperature for different C16mimCl concentrations (wt %): (a) 45, (b) 50, and (c) 55.

Figure 4. Simulated isodensity profiles for the micelle and hexagonal phases formed at room temperature and different C16mimCl concentrations (wt %): (a) 15, (b) 25, (c) 35, (d) 45, (e) 50, and (f) 55.

investigation shows that they are mixtures of solid and lyotropic liquid crystals. In order to get more intuitive information on the aggregates formed at room temperature, computer simulations have also been carried out. The simulated aggregate structures for the C16mimCl/EAN binary system at lower concentrations are shown in Figure 4a-c. It can be clearly seen that, when C16mimCl concentration is above the critical aggregation concentration (cac) value of 1.62 × 10-2 mol/L as reported by Kunz et al.,36 the micellar aggregates can be formed. Spherical micelles with certain defects (Figure 4a) are produced first and then reach a more ordered status at a higher C16mimCl concentration (Figure 4b). Keep on increasing the surfactant concentration, the rodlike micelles can be clearly seen as shown in Figure 4c. Such simulated results on the micelle structure evolution are in good accordance with that deduced experimentally. With C16mimCl concentration increasing to 45%, the rodlike micelles begin to arrange themselves to a certain pattern. But, no obvious hexagonal phase can be identified from the isodensity profiles shown in Figure 4d, which agrees well with the results obtained from POM and SAXS measurements. When C16mimCl concentration arrives at 50%, the rodlike micelles accumulate themselves to present nearly a well-hexagonal pattern, as can be clearly seen from the 3D models in Figure 4e. Further increasing C16mimCl concentration to 55%, the H1 phase is still

Figure 5. Calculated hexagonal structures for 50% C16mimCl system at different simulation time steps at room temperature: (a) 1000, (b) 2000, (c) 5000, and (d) 20 000.

maintained, though the simulated structure becomes less ordered once again (Figure 4f). Such an ordering change of the H1 phase may imply an approaching to the phase boundary at 55% C16mimCl concentration, which is almost the same as the experimentally determined phase boundary 57% from the POM and SAXS results. Simulations at further higher C16mimCl concentration are not performed then due to the complexities of the formed mixtures, which may lead to quite different interaction parameters between varied particles. Except for the intuitive 3D models, the knowledge on aggregation process is also of great importance. For this aim, the gradual phase morphology evolutions for the 50% C16mimCl system with increasing time steps are presented in Figure 5. A gyroid phase formed at initial stage (Figure 5a) is unstable and gradually changes to a more ordered status with time steps increasing to exhibit an embryo of the hexagonal phase (Figure 5b). With time extended, the aggregate structure becomes much more ordered (Figure 5c) and reaches a regular hexagonal phase at the time steps of 20 000 (Figure 5d). Then the structure behaves much more steady and will change little with further increasing time steps. Such an evolution process commonly happens in tens of microseconds and is very difficult to observe directly by experimental techniques. These simulated results are

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Figure 7. SAXS curves accompanying the corresponding POM images for the system with 75% C16mimCl concentration at different temperatures.

Figure 6. POM images (a-d) and SAXS curves (e) for the sample with 60% C16mimCl concentration at different temperatures (°C): (a) 35, (b) 75, (c) 105, and (d) 122.

therefore considered to be a beneficial supplement for the experimental observations and can provide us more microphase separation information. Liquid Crystals Formed at Relative Higher Temperatures. As referred to above, the C16mimCl/EAN binary system can form LLC phases at room temperature. Meanwhile, it has been found that thermotropic LC phases could be formed by such long chained imidazolium ILs at higher temperature.48-50 Then, what might be the structures of this binary system at higher temperatures? To answer this question, the following content will concentrate on the various liquid crystal phases formed at relative higher temperatures. Normal Hexagonal Phase. After the appearance of micelles, the normal hexagonal phase goes through a large area in the phase diagram as having been seen in Figure 2. With increasing temperature for a typical sample at 60% C16mimCl concentration, the changed POM textures can be seen in Figure 6. At a relative low temperature (e35 °C), the fan-shaped or pleatedribbon texture seems not quite uniform and exhibits small or narrow pleated ribbons (Figure 6a). With the increase of temperature, these ribbons become larger and clear (Figure 6b,c) and reach a critical point at 122 °C (Figure 6d). Above this value, the sample would turn into an isotropic phase. The SAXS patterns for the sample with above composition are measured at different temperatures (35, 45, and 55 °C) and shown in Figure 6e. Four Bragg peaks can be observed with the relative positions corresponding to a ratio of 1:3:2:7, all indicating a hexagonal structure. Besides, the appearance of this sample changes from cloudy at room temperature to more transparent with increasing temperature, which may be also an indication for the H1 phase formation. Further investigations show that higher temperatures will lead the first SAXS peak position to a bigger q (scattering factor) value, corresponding to a smaller lattice spacing. As we know, the H1 phase is formed by the rodlike micelles arranged into a hexagonal pattern, where the solvophobic tails are in the interior part of the rods while

the solvophilic heads outside are solvated by EAN molecules. With the increase of temperature, the solvophobic chains of C16mimCl molecules become soften and extruded to each other. Meanwhile, the solvophilic heads and EAN outside also compel them to arrange more tightly, all of which will lead to the decrease of the rod radius and then result in lower lattice spacing. Lamellar Phase. When mapping the phase diagram, we find two regions of lamellar structure following the emergence of the H1 phase, as can be seen from Figure 2. The small one is located at relative low temperatures, but another region holds a larger area and extends to higher temperatures. The typical structural information in the first region is expatiated by using a system with 75% C16mimCl. From the SAXS patterns shown in Figure 7, two Bragg peaks with their relative positions of 1:2 can be clearly observed, denoting well a lamellar structure. The corresponding POM images of this system are shown accompanying the SAXS curves, which change little with increasing temperature. As for the lamellar region at higher temperatures, different representative POM photographs have been chosen for illustration. Figure 8a-d shows the POM images evolved with increasing temperature for the sample with 80% C16mimCl. At relatively low temperatures (e72 °C), the sample is isotropic and no optical textures can be observed (Figure 8a). Such a phase is classified to be a reverse bicontinuous cubic phase, and more details will be discussed in the following part. With increasing temperature, a bright field with some marbling textures can be viewed, which is considered to be an indication for the lamellar phase formation (Figure 8b). At a temperature of 150 °C, the image is not so bright, but the characteristic marbling textures for lamellae become easier to identify (Figure 8c). When the temperature arrives at 155 °C, most of the textures disappear and the sample becomes more fluid (Figure 8d). Therefore, the lamellar phase at this composition can exist in an approximate temperature range of 75-155 °C. For samples with higher C16mimCl concentrations, the POM textures are somewhat different. As shown in Figure 9a,b for the sample with 85% C16mimCl, the characteristic Maltese cross textures for lamellar phase can be clearly observed at two different temperatures. For more concentrated samples like the one with 90% C16mimCl, the marbling textures become more obvious (Figure 9c,d). When comparing the POM textures between these two lamellar phase regions, we can easily find that the more crystallike textures in the first small region are typical for the condensed lamellar phase. However, those in the second large

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Figure 10. DSC curves for pure EAN, the H1 phase with 60% C16mimCl, and pure C16mimCl.

Figure 8. POM images (a-d) and SAXS curves (e) for the sample with 80% C16mimCl concentration at different temperatures (°C): (a) 50, (b) 100, (c) 150, and (d) 155.

Figure 9. POM photographs for the samples with 85% (a, b) or 90% (c, d) C16mimCl at different temperatures (°C): (a) 90, (b) 110, (c) 65, and (d) 75.

region are more typical for the common lamellar phase. Besides, the textures for the second region are much more similar to that of the thermotropic LC phases formed by single C16mimCl powder at relative higher temperature, which may be due to the higher C16mimCl content for the samples in this region. ReWerse Bicontinuous Cubic Phase. From Figure 2, a reverse bicontinuous cubic phase can also be seen to locate between two lamellar regions at C16mimCl concentrations from 75 to 85%, which may goes through a temperature range from 36 to 70 °C. Instead of any texture, only a dark background can be observed with POM measurements for samples here, indicating an isotropic phase. Figure 8e presents the SAXS patterns for a representative sample with 80% C16mimCl measured at different temperatures (45, 55, and 65 °C). Two strong Bragg peaks can be detected for all experimental temperatures. These two peak positions satisfy the relationship of 3:4 and are consistent with the first two reflections (211 and 220) of the Ia3d structure,

which is the most conventional reverse bicontinuous cubic phases.34 Such a phase was ever reported to locate at a little higher polymer concentrations than those for the lamellar phase in ternary systems containing block copolymers.51-53 Meanwhile, in a similar binary system with amphiphilic block copolymer P123 and EAN, we have also seen such an Ia3d reverse bicontinuous structure at relative higher P123 concentrations.34 Just on the basis of these reasons, we infer the formed aggregate in this region as V2 phase with Ia3d structure. Although these two peaks could also correspond to the second (111) and third (200) reflections of the Pn3m structure or the third (211) and fourth (220) reflections of the Im3m structure, such two matching styles were thought to be impossible. This is for the reason that the most intense peak always corresponding to the first reflection of the Pn3m or Im3m bicontinuous phase was not detected.34,54,55 Further investigation on the SAXS curves shows that the first peak position for the V2 phase also tends to move toward a higher q value with increasing temperature, which is in accordance with the changing trend for H1 phase and can still be ascribed to the softening of the solvophobic chains of C16mimCl molecules. However, such q shifts in the V2 phase are not as obvious as that in the H1 phase. As we know, the space group Ia3d is usually described by the interconnected rod network (ICR) model, in which the Ia3d structure is filled by two networks of rods having the same dimensions. The solvent and the solvophilic heads fill the rods whereas the solvophobic chains cover the outside.56 Under this circumstance, the outside chains are less affected by the inside EAN and solvophilic heads. Thus, the decreased lattice spacing is supposed to mostly result from the softened solvophobic chains themselves and will not be as obvious as that of the H1 phase. When comparing the phase behavior of this binary system with that of the C16mimCl/H2O system,35,57 the V2 phase obtained here is fresh. A possible explanation is from the affinity difference to solvent for the alkyl chains of C16mimCl. The higher affinity to EAN than to water might reduce the effective area of the solvophilic imidazolium ring structure and increase the volume of the solvophobic part,34,58 which then leads to the formation of such a V2 phase because of a higher critical packing parameter (CPP). Mechanism on the LC Phase Formation. In previous reports, the hydrogen bonding and solvophobic interactions are thought to be the significant driving forces for the LC phase formation in EAN.34,55 What might be that for the present system? To answer this question, two parallel systems have been

Liquid Crystalline Phases in C16mimCl/EAN investigated for comparison. First, as a conventional surfactant, the cetyltrimethylammonium chloride (CTAC) with a different headgroup but the same solvophobic chain is chosen to replace C16mimCl to construct the binary system with EAN. The obtained results show that no LC phase could be formed in all concentration range even the temperature is higher than 100 °C. This reminds us that the C16mimCl headgroup is of great importance for the LC phase formation. Second, it is noted that the alkyl chain of C16mimCl also has a great influence on the production of the LC phases. If [C8mim]Cl was used to substitute C16mimCl in the present binary system, no LC phase could be identified, either. Such an observation could demonstrate clearly the important role of the solvophobic interaction played in the assembly of the LC phase.59 As a protic solvent, EAN possesses a number of similarities to water and has been extensively included in the studies on aggregations of traditional surfactants or block copolymers as referred to above.25-34 Its ability to form a 3D hydrogen-bonding network, similar to that in water, is supposed to be a basis for self-assembly.36 In our investigated system, the EAN molecule is more preferable to interact with the headgroup of C16mimCl through the potential hydrogen-bonding network formed between the oxygen atoms in NO3- and the hydrogen atoms in the imidazolium ring structure or between the hydrogen atoms in CH3CH2NH3+ and the nitrogen atoms in imidazolium headgroup, which will promote the separation of the head and tail groups of C16mimCl molecules. Then, the binary system may be microphase separated into two parts: EAN molecules with C16mimCl headgroups and the solvophobic chains. At different concentrations of C16mimCl, their relative volume changes are different, which will influence the interfacial curvatures of the aggregates and then lead to the formation of various organized phases as discussed above. Besides, such relative volume changes of these two separated parts may also be caused with increasing temperature and therefore will lead to phase transition at higher temperatures. DSC measurements were also used to judge the interactions between the C16mimCl and EAN molecules. Figure 10 shows the obtained results for the two pure components and a sample with 60% C16mimCl. It can be clearly seen that there is an intensive exothermic peak at about 245 °C in the DSC curve of pure EAN, which resulted from the explosion of EAN at higher temperature and similar to the value reported previously.34 For the C16mimCl component, three endothermic peaks can be observed. The first peak corresponds to the melting point of C16mimCl and somewhat different from the value of 68 °C in our previous report,57 which may be attributed to the higher heating rate now, that is, 10 °C/min. Then it undergoes a phase transition at about 185 °C and decomposes at about 255 °C. Some interesting changes, however, are brought about when mixing these two components. Except for the new emerged peak corresponding to the phase transition temperature at about 120 °C, certain changes for the exothermic peak of EAN and the decomposition peak of C16mimCl have also taken place, that is, the exothermic peak of EAN shifts to a higher temperature but the decomposition peak of C16mimCl to a lower temperature. These changes should also be considered as an indication for the interactions existing between C16mimCl and EAN molecules.34 Conclusions The phase behavior for the binary system of C16mimCl/EAN has been investigated. As a part of continuing research on our previous works, that is, the aggregation behavior of the triblock

J. Phys. Chem. B, Vol. 113, No. 7, 2009 2029 copolymer P123 in both [Bmim+][PF6-] and EAN, this work is a supplement to the self-assembly behavior of polymers and conventional surfactants in room temperature ionic liquid. With the increase of C16mimCl concentration, aggregates of different morphologies have been identified and show similar properties to those formed in some traditional systems with water, which may be attributed to the great analogous property of EAN to water. The fresh V2 phase obtained in the present system has reflected the different affinities for the alkyl chain of the C16mimCl molecule to EAN and H2O, which may be of potential applications in many fields. The potential hydrogenbonding network formed between C16mimCl headgroup and EAN is considered to be the major driving force for the LC phase formation, which may be expanded to a large number of other systems with imidazolium ring structure or a certain part easily to form hydrogen-bonding network with the existing solvents. The simulated results obtained at room temperature have provided us the microphase separation information and are in good accordance with the experiments. Acknowledgment. We are thankful for the financial support from the National Natural Science Foundation of China (20573066, 20773080) and Natural Science Fund of Shandong Province (Y2005B18). References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (3) Holbrey, J. D.; Seddon, K. R. Clean Prod. Proc. 1999, 1, 223. (4) Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Z. Catal. Today 2002, 74, 157. (5) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667. (6) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (7) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106. (8) Lagrost, C.; Carrie, D.; Vaultier, M.; Hapiot, P. J. Phys. Chem. A 2003, 107, 745. (9) Swatloski, R. P.; Visser, A. E.; Reichert, W. M.; Broker, G. A.; Farina, L. M.; Holbrey, J. D.; Rogers, R. D. Chem. Commun. 2001, 2070. (10) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. Chem. Commun. 2003, 572. (11) Domanska, U.; Bogel-Lukasik, E.; Bogel-Lukasik, R. J. Phys. Chem. B 2003, 107, 1858. (12) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477. (13) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316. (14) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (15) Tran, C. D.; Yu, S. F. J. Colloid Interface Sci. 2005, 283, 613. (16) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745. (17) Wang, L. Y.; Chen, X.; Chai, Y. C.; Hao, J. C.; Sui, Z. M.; Zhuang, W. C.; Sun, Z. W. Chem. Commun. 2004, 2840. (18) Hao, J. C.; Song, A. X.; Wang, J. Z.; Chen, X.; Zhuang, W. C.; Shi, F.; Zhou, F.; Liu, W. M. Chem.sEur. J. 2005, 11, 3936. (19) 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. (20) Gao, Y. A.; Han, S. B.; Han, B. X.; Li, G. Z.; Shen, D.; Li, Z. H.; Du, J. M.; Hou, W. G.; Zhang, G. Y. Langmuir 2005, 21, 5681. (21) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302. (22) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X.; Hou, W. G.; Li, G. Z. Green Chem. 2006, 8, 43. (23) Gao, Y. A.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W.; Yu, L. ChemPhysChem 2006, 7, 1554. (24) Evans, D. F.; Chen, S.-H.; Schriver, G. W.; Arnett, E. M. J. Am. Chem. Soc. 1981, 103, 481. (25) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89. (26) Evans, D. F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J. Phys. Chem. 1983, 87, 3537. (27) Tamura-Lis, W.; Lis, L. J.; Quinn, P. J. J. Phys. Chem. 1987, 91, 4625. (28) Tamura-Lis, W.; Lis, L. J.; Quinn, P. J. Biophys. J. 1988, 53, 489.

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