Liquid Crystalline Phases of the Amphiphilic Ionic Liquid N-Hexadecyl

Aug 17, 2010 - Experimental and DFT Studies on the Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Aromatic Counterions in...
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J. Phys. Chem. B 2010, 114, 11382–11389

Liquid Crystalline Phases of the Amphiphilic Ionic Liquid N-Hexadecyl-N-methylpyrrolidinium Bromide Formed in the Ionic Liquid Ethylammonium Nitrate and in Water Mingwei Zhao, Yanan Gao, and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, Shandong, 250100, People’s Republic of China ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: July 1, 2010

The phase behavior of a surfactant-like ionic liquid, N-hexadecyl-N-methylpyrrolidinium bromide (C16MPB), was studied in both water and a room temperature ionic liquid, ethylammonium nitrate (EAN). Polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) measurements were employed to investigate the phase behavior of the two systems and to determine which lyotropic liquid crystalline (LC) phases were formed. With increasing C16MPB concentration, an isotropic solution phase, a hexagonal (H1) phase, and a cubic phase (V2) are all present in either EAN or H2O. The structural parameters of the H1 phase were calculated from SAXS patterns, which show the structural changes as a function of the amount of C16MPB. The rheological results reveal that the H1 phase constructed by C16MPB in EAN displays a typical Maxwell behavior, whereas the H1 phase formed by C16MPB in water shows a gel-like behavior, unlike traditional cationic surfactants. POM and differential scanning calorimetry (DSC) results demonstrate that the lyotropic LC phase in EAN has a higher thermal stability than that formed in H2O, which may be important to extend the applications of the LC phase. 1. Introduction Finding environmentally benign solvents for industrial use is a great challenge to the scientific community. Ionic liquids (ILs) have emerged in the last two decades and show great potential to solve this puzzle.1-3 ILs are compounds which are liquid below 100 °C and are composed of ions. In recent years, ILs have received considerable attention due to their unique properties such as low melting temperature, nonflammability, high ionic conductivity, negligible vapor pressure, wide electrochemical window, and good catalytic properties. Thus, ILs have been employed widely in the areas of catalysis, preparation of novel nanomaterials, organic synthesis, electrochemistry, and liquid/liquid extraction.4-8 It is well-known that amphiphilic molecules play an important role in technological applications because of their unique ability to self-assemble both at surfaces and in bulk solutions. In general, surfactant molecules can form thermodynamically stable self-organized structures such as micelles,9 microemulsions,10 vesicles,11 and liquid crystals (LCs).12 Among these, LCs have become the most important subject, due to their wide applications in chemical reactions, material science, and pharmaceutical vehicles.13-16 Through characterization by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC), hexagonal (H1), lamellar (LR), and cubic (V2) phases of LCs can be determined. Due to their rich phase behavior, LCs have been widely investigated in the scientific field.17-20 Nevertheless, the low thermodynamic stability of LCs greatly limits their applications. This is because the traditional lyotropic LCs generally contain low-boiling-point solvents, like water and organic solvents, which can be rapidly evaporated at high temperature, and the structure of LCs is easily * To whom correspondence should be addressed. E-mail: lqzheng@ sdu.edu.cn. Fax: +86-531-88564750. Phone: +86-531-88366062.

destroyed. Exploring thermodynamically stable lyotropic LCs remains a great challenge. Due to their negligible vapor pressure, ILs provide a chance to design such thermodynamically stable lyotropic LCs. ILs are usually employed as solvents, and some recent investigations have concentrated on the lyotropic LCs. Friberg et al. first investigated the phase behavior of lyotropic LCs composed of water, nonionic surfactant C12EO4, and 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6).21 Small-angle X-ray diffraction results showed that a lamellar liquid crystal phase appeared and the location of IL in the LC phase was also clarified. After that, Warr et al. studied the phase behavior of LC phases formed by a number of polyoxyethylene nonionic surfactants in ethylammonium nitrate (EAN).22 In addition, some ILs with long alkyl chains have also been designed, which demonstrated good amphiphilic behavior.23-30 Firestone and Dzielawa described the formation of an LC phase composed of 1-decyl-3-methylimidazolium bromide and water.28 Our group also investigated the phase behavior of ternary mixtures of C16mimBr/p-xylene/water by SAXS, POM, and rheology measurements.29 H1 and LR phases are constructed, and the rheological properties were found to be related to the structures of the LCs. Although there are some investigations into LC phases constructed with ILs, the organic components or water in the LC systems still limit their applications. Up to now, there have been few reports about the construction of LCs with high thermal stability.30 Thus, in this work, our aim is to develop a new LC system with higher thermal stability. In the present study, we investigate two LC systems: N-hexadecyl-N-methylpyrrolidium bromide (C16MPB)/EAN and C16MPB/H2O. Through the investigation of the phase behavior and comparison of these two systems, the solvent effect on the phase diagrams, structural parameters, and rheological properties of LC is clarified. We expect this work to not only enrich our

10.1021/jp103728x  2010 American Chemical Society Published on Web 08/17/2010

Liquid Crystalline Phases of C16MPB knowledge of the phase behavior of surface active ILs but also provide a new way to prepare thermodynamically stable LCs. 2. Experimental Section 2.1. Materials. N-Methylpyrrolidine (97%) and 1-bromohexadecane (97%) were products of Sigma and used as received. Ethylamine (70%) was purchased from Alfa Aesar. Toluene (99%), diethyl ether (99%), and nitric acid (65%) were purchased from Beijing Chemical Reagent Company. Deionized water was used through the experiment. Synthesis of C16MPB. C16MPB was synthesized according to the procedures reported previously.31 N-Methylpyrrolidine (0.1 mol, 26.0 g) and an excess amount of 1-bromohexadecane (0.11 mol, 32.2 g) were mixed in toluene (200 mL) in a 500 mL round-bottom flask and then refluxed under a nitrogen atmosphere at about 80 °C for 48 h. Then, the product was cooled to room temperature and purified by recrystallization in fresh diethyl ether at least four times. Finally, the product was dried under a vacuum for 48 h. The purity of C16MPB was ascertained by the 1H NMR spectrum in CDCl3. Synthesis of EAN. EAN was synthesized according to the procedures presented by Evans et al.32 In a typical synthesis, a portion of nitric acid (50 mL) was slowly added to ethylamine solution (83 mL) dropwise while stirring in an ice bath. Then, water in the resulting mixture was removed with a rotary evaporator. The purity of EAN was ascertained by the 1H NMR spectrum in D2O. 2.2. Phase Diagram. The process of mapping the phase diagram has been described elsewhere.27,28,33 The samples were prepared by weighing the selected amount of components in stoppered glass vials to avoid evaporation. The mixtures were homogenized and equilibrated by repeated vortex mixing and centrifuging. Then, they were kept in a thermostat at least 1 month for equilibration. The phase diagram was established between 25 and 70 °C over the whole water-surfactant composition range. The optical nature of the samples (nonbirefringent or birefringent) was checked by crossed polarizers. In the case of LCs, polarized microscopy and SAXS were usually used to identify the type of LCs. The composition interval was first selected as 5 wt % for a rough mapping, while smaller intervals (2 wt %) were selected in the vicinity of phase boundaries. 2.3. Characterization. Polarized Optical Microscopy. A polarized optical microscope equipped with a charge-coupled device camera (Panasonic Super Dynamic II WV-CP460) was used to observe the LC phase. Birefringent textures from the optical microscope allow the determination of particular LCs. In general, the H1 phase displays a fanlike and angular texture,33 while the LR phase shows a distinct woven structure or a Maltese cross texture.34,35 The isotropic cubic LC is nonbirefringent and shows only background. Small-Angle X-ray Scattering. The SAXS technique was used to characterize the LC phase in an HMBG-SAX X-ray small-angle scattering system (Austria) with Cu KR radiation operating at 2 kW (50 kV and 40 mA). To remove the Kβ radiation, a 10 µm thick nickel filter was employed and a 2 mm tungsten filter was used to protect the detector from the primary beam. The distance between the sample and detector was about 278 mm. All samples were held in a vacuum steel holder to provide good thermal contact to the computercontrolled Peltier heating system (Hecus MBraun Austria). The structural parameters of the LC phase could be deduced from the Bragg peaks of the SAXS patterns.36,37 Rheological Measurement. Rheology measurements were performed with a HAAKE RS 75 rheometer. A cone-plate

J. Phys. Chem. B, Vol. 114, No. 35, 2010 11383 sensor was used with 20 mm diameter and 1° cone angle. The cone-plate distance was adjusted to 52 µm for all measurements. The temperature was kept at 25 ( 0.1 °C. Each sample was inserted onto the top of the cone template, and then, the plate was slowly elevated to its measuring position at a constant velocity. Any excess sample squeezed out from the sensor system was gently removed. Dynamic oscillation-shear measurements were employed, in which the stress (σ) was varied while the frequency was kept at 1.0 Hz. Once the linear viscoelastic region was determined, frequency sweep measurements were performed as a function of frequency at a constant stress. Differential Scanning Calorimetry. DSC was carried out on an SDT Q600 Simultaneous DSC-TGA (TA Instruments). The linear heating rate was always 10 °C/min, and the samples were heated under a nitrogen atmosphere at a flow rate of 50 mL/ min. 3. Results and Discussion 3.1. Phase Diagrams of the C16MPB/EAN and C16MPB/ H2O Systems. The phase diagram of C16MPB/EAN in the temperature range 25-70 °C is shown in Figure 1, together with the corresponding POM images and SAXS patterns. As shown in Figure 1a, the single-phase region consists of the isotropic solution phase, anisotropic H1 phase, and isotropic V2 phase. At low C16MPB content (less than 50 wt %), the homogeneous and isotropic solution phase appears. With the increase of C16MPB content, the H1 and V2 phases appear. When the C16MPB content is between 50 and 83 wt %, a large area of H1 phase is found, which is confirmed by the POM image and SAXS pattern shown in Figure 1b. The fanlike texture (Figure 1b (top)) is the typical character of the H1 phase, and the SAXS pattern (Figure 1b (bottom)) also confirms the existence of the H1 phase. Three scattering peaks are observed in the SAXS pattern, and the q values corresponding to the scattering peaks are in the ratio q1:q2:q3 ) 1:3:2, which are characteristic for the H1 phase.20 An isotropic V2 phase is found when the C16MPB content is between 83 and 87 wt % in the temperature range 35-70 °C. The POM image shown in Figure 1c (top) displays only background. Its SAXS pattern (Figure 1c (bottom)) gives two distinct Bragg peaks with relative ratios of 3:4, which are assigned to the (211) and (220) reflections of the Ia3d structure of the cubic phase.17 The phase diagram of the C16MPB/H2O mixture in the temperature range 25-70 °C is shown in Figure 2a. At low C16MPB content (less than 25 wt %), the mixture is an isotropic, clear, and transparent solution. With increasing C16MPB content, the system remains transparent but becomes more viscous. H1 and V2 phases were observed in the SAXS patterns. For instance, a H1 phase forms when the C16MPB content is in the range 25-70 wt % (Figure 2b). Three sharp scattering peaks are also detected which satisfy the ratios q1:q2:q3 ) 1:3:2, showing the existence of the H1 phase. A further increase in C16MPB content (70-85 wt %) leads to the appearance of the V2 phase. The three Bragg peaks in the SAXS spectrum (Figure 2c) can be indexed as the (211), (220), and (321) reflections of the Ia3d structure, with the ratio q1:q2:q3 ) 3:4:7. Through comparison of the phase diagrams of the two LC systems, some differences become clear, which may be due to the solvent effect between EAN and H2O. As previously reported, nonaqueous polar solvents reduce the LC region in comparison to aqueous mixtures.38,39 In our study, the C16MPB/ EAN system occupies a narrower LC region originating from 55 wt % C16MPB, while it begins from 25 wt % C16MPB in

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Figure 1. Phase diagram (a) for the C16MPB/EAN binary mixture, in which H1 and V2 denote the normal hexagonal and reverse bicontinuous phase. POM images (top, upon cooling from the isotropic phase) and SAXS curves (bottom) for the typical liquid crystal phases: (b) H1 and (c) V2. The scale bar is 200 µm.

Figure 2. Phase diagram for (a) the C16MPB/H2O binary mixture and typical SAXS curves for (b) H1 and (c) V2.

the C16MPB/H2O system. The V2 phase is constructed above 83 wt % C16MPB dissolved in EAN, while it appears from 70 wt % C16MPB in the C16MPB/H2O mixture. This means that it

is easier to construct the LC phase in H2O than in EAN. In general, the solvent effect on the formation of LC phases includes two relationships. The first relationship was provided

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by Wa¨rnheim and Jo¨nsson,40,41 who showed that the size of the LC regions was correlated to the solvent/hydrocarbon interfacial tension. A high interfacial tension suggests a considerable solvophobic interaction which favors extensive surfactant aggregation, and such a higher interfacial tension is always caused by the stronger polarity.42 It is well-known that the polarity of H2O is stronger than other solvents, so H2O forms LC phases more easily than EAN. The second relationship is that the size of the LC regions depends on the ability of the solvent to participate in hydrogen bonding.43,44 Comparing the structures of the two solvents used in our study, H2O exhibits a tetrahedral H-bond network structure, while EAN only forms a threedimensional network structure. The values of negative enthalpy and entropy changes for EAN are only half as large as those of H2O.32,45,46 According to these two relationships, we can conclude that LC phases are formed more easily in H2O than in EAN and the size of the LC regions is larger in H2O. Furthermore, large H1 phase regions exist in both systems, which may be due to the molecular structure of C16MPB. The critical packing parameter (CPP) is a very useful tool to interpret the self-assembled structure for a particular amphiphile.47 The CPP is determined as P ) V/(a · l), where P is the abbreviation of CPP, V is the volume of the solvophobic chains of surfactant molecules, a is the effective headgroup area of the surfactant molecules, and l is the effective chain length of surfactant molecules. In general, the values of CPP are as follows: for spherical micelles, P e 1/3; for cylindrical aggregates, 1/3 < P e 1/2; for bilayers, vesicles, and bicontinuous cubic LC phase, 1/2 < P e 1; and for inverted structures, P > 1. For C16MPB molecules, the hydrocarbon chain length l and volume of the solvophobic chains V can be obtained from the following Tanford equations:48

V ) 27.4 + 26.9N

(1)

l ) 1.5 + 1.265N

(2)

where N is the number of carbon atoms in the hydrocarbon chain. N ) 16 for C16MPB. From the equations above, V and l are determined to be 457.8 Å3 and 21.74 Å. The area a is obtained from the adsorption of C16MPB at the air/water interface, and the value is about 45.2 Å2 (Supporting Information). Thus, P of C16MPB is calculated to be 0.46. The result shows a strong preference for C16MPB molecules to pack in a hexagonal array. Thus, a large region of H1 phase appears in both the C16MPB/EAN and C16MPB/H2O systems. In the following section, we will focus on the effect of the amount of C16MPB on the structure of the H1 phase in the two systems and their different thermal stabilities. 3.2. The Effect of C16MPB Amount on the Phase Behavior of the Two LC Systems. 3.2.1. SAXS Measurements. The SAXS characterization of the LC phase is based on the longrange order in the LC state, which leads to the Bragg reflections. The positions of these Bragg reflections are characteristic of the different types of LC phases, and the relevant structural parameters can be deduced from the SAXS patterns. Figure 3 shows the SAXS patterns of the H1 phase constructed by different amounts of C16MPB in EAN at 25 °C. The first peak shown in the SAXS patterns originates from the Bragg reflection of the (100) plane, which can help us to obtain the lattice parameter (R0) according to eqs S1 and S2 (shown in the Supporting Information).20 The lattice parameter R0 represents the distance from the center of one cylinder to another, including the total diameter of the cylinder and the thickness

Figure 3. SAXS curves of different amounts of C16MPB in the H1 phase of the C16MPB/EAN system at 25 °C.

TABLE 1: Structural Parameters of H1 Phases in the C16MPB/EAN and C16MPB/H2O Systems at 25 °C sample (wt %) R0 (Å) φL dW (Å) dH (Å) as (Å2) C16MPB/EAN H1 (298 K)

C16MPB/H2O H1 (298 K)

55 60 65 70 35 40 45 50 55 60 65 70

47.28 46.56 45.87 45.57 61.14 63.24 58.41 55.92 53.32 52.41 51.24 49.58

0.55 0.60 0.65 0.70 0.31 0.36 0.41 0.45 0.50 0.56 0.61 0.66

10.48 8.680 6.650 5.600 25.38 23.38 19.11 16.50 13.70 11.21 9.200 7.260

18.40 18.94 19.61 20.25 17.88 19.93 19.65 19.71 19.81 20.60 21.02 21.16

49.76 48.34 46.69 45.21 51.21 45.94 46.60 46.45 46.22 44.45 43.56 43.27

of the solvent layer. From the SAXS patterns shown in Figure 3, it is clear that the positions of the first peak shift right with increasing concentration of C16MPB. Through calculation, the R0 values are assigned to be 47.28, 46.56, and 46.10 Å for 55, 60, and 70 wt % C16MPB, respectively, which means that R0 decreases with the increase in C16MPB concentration. The increased amount of C16MPB causes a decrease of the distance between the adjacent cylinders. To explain this phenomenon, we should first make clear the structure of the H1 phase. The H1 phase consists of infinitely long cylinder-like aggregates packed in a hexagonal array and separated by a continuous solvent region,38 where the solvophobic tails are located in the interior part of the cylinder-like aggregates and the solvophilic heads are solvated by EAN molecules. According to the model of the H1 phase, R0 can be divided into two parts: the radius of the cylinder-like aggregates (dH) and the thickness of the solvent layer (dW). They accord well with the following equation:

R0 ) 2dH + dW

(3)

Then, the following question arises: Which factor plays the major role in the structural change of the H1 phase? To answer this question, we should obtain the values of dH and dW first, from eq S3 in the Supporting Information and the above eq 3. The values of dH and dW are listed in Table 1. When the amount of C16MPB is from 55 to 70 wt %, dW decreases from 10.48 to 5.600 Å, while dH increases from 18.40 to 20.25 Å. The results suggest that the radius of the cylinder-like aggregates becomes larger with the increasing amount of C16MPB, while the solvent layer becomes thinner. In other words, the larger the concentration of C16MPB in the H1 phase, the more densely the C16MPB molecules pack in a hexagonal array. These two factors both lead to the decrease of R0 as the amount of C16MPB increases. Furthermore, we can deduce that the more dense packing certainly causes a decrease of the area per C16MPB molecule (as) in the H1 phase. To verify this statement, the values of as

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Figure 4. SAXS curves of different amounts of C16MPB in the H1 phase of the C16MPB/H2O system at 25 °C.

were calculated according to eq S4 in the Supporting Information and are listed in Table 1. It is clear that as decreases from 49.76 to 45.21 Å2 with the C16MPB amount from 55 to 70 wt %, which agrees well with the above statement. Thus, in the H1 phase of the C16MPB/EAN mixture, with the increase of C16MPB, the cylinder-like aggregates pack more densely, while the EAN solvent layer becomes thinner. Investigations into the BmimPF6/H2O/Brij-30 (tetraethylene glycol lauryl ether, C12E4) system showed similar changes in the structural parameters.49 For the C16MPB/H2O system, Figure 4 shows the SAXS patterns of the H1 phase with different amounts of C16MPB. The first peak for all SAXS patterns also shifts to the right with the increase of C16MPB concentration. The data show that R0 becomes smaller when more C16MPB molecules construct the H1 phase. Through calculation of the structural parameters (dH, dW, and as), the structural changes of the H1 phase are similar to those of the H1 phase in the C16MPB/EAN system. With the increase of the amount of C16MPB, the radius of the cylinderlike aggregates increases and the area per C16MPB molecule decreases due to the extension of the hydrophobic chains of the surfactant. The water layer becomes thinner with the decrease of the water content. Comparing the two H1 phases constructed by C16MPB in EAN and in water, the effect of C16MPB concentration on the structural parameters (R0, dH, dW, and as) is almost the same. However, the structural parameters (R0, dH and dW) except as in the C16MPB/H2O mixture are always larger than those of the C16MPB/EAN system with the same amount of C16MPB. This result may be due to the density difference of H2O (1.0 g/mL) and EAN (1.2 g/mL). The higher density makes the volume of EAN in the H1 phase smaller than that of H2O, which results in smaller structural parameters in the H1 phase of C16MPB/EAN. Furthermore, the value of as in the C16MPB/ EAN mixture is larger than that in the C16MPB/H2O system. The larger parameter dW in the C16MPB/H2O mixture results in the smaller as, which is consistent with previous results.20,29,50 In order to better show the difference between the H1 phases formed in these two systems, Figure 5 shows the ideal schematic diagram of the packing of these two lyotropic LC systems. The structures of these two H1 phases are different due to the solvent effect. As shown in Figure 5, the lattice parameter (a0) of the C16MPB/H2O system is larger, while the area per C16MPB molecule is smaller than in the C16MPB/EAN system. Furthermore, the ideal schematic diagram directly demonstrates that the C16MPB molecules pack more densely in H2O than in EAN. 3.2.2. Rheological Measurements. To find more information about the structure of the H1 phase, the rheological properties were also investigated. For the C16MPB/EAN mixture, the dynamic viscoelastic behavior is shown in Figure 6. The values of the storage modulus (G′) increase versus the frequency, with different slopes

Figure 5. Schematic diagram of the packing mode of the C16MPB/ H2O and C16MPB/EAN systems.

Figure 6. Storage moduli G′ (solid) and loss moduli G′′ (empty) versus frequency for the H1 phase of the C16MPB/EAN system at 25 °C.

for different wt % of C16MPB, while the loss modulus (G′′) increases with increasing frequency at the beginning and then remains constant. At low frequencies, the loss modulus G′′ is larger than the storage modulus G′, showing viscous behavior. At higher frequencies, G′ is larger than G′′, exhibiting elastic behavior.50 The rheological results mean that the H1 phase shows viscoelastic behavior, which is consistent with the typical rheological behavior of H1 phases. The frequency dependence of G′ and G′′ when EAN is substituted by H2O is shown in Figure 7. The moduli G′ and

Figure 7. Storage moduli G′ (solid) and loss moduli G′′ (empty) versus frequency for the H1 phase of the C16MPB/H2O system at 25 °C.

Figure 8. Typical curves of storage moduli (G′) and loss moduli (G′′) versus shear rate for the H1 phases in the C16MPB/EAN (a) and C16MPB/H2O (b) mixtures at a frequency of 1.0 Hz.

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TABLE 2: Rheological Parameters of H1 Phases in the C16MPB/EAN and C16MPB/H2O Systems at 25 °C sample (wt %) σc/Pa G′/Pa σc/Pa C16MPB/EAN (298 K) C16MPB/H2O (298 K)

55 60 65 35 40 45 50 55 60 65 70 75

1048 392.0 790.0 245.8 180.5 312.3 588.0 765.9 1242 2015 2449 2724

18265 37484 39919 5649 3768 7647 10990 14362 25098 43124 14894 57565

980.0 370.0 1141 368.7 265.3 470.2 879.8 1172 2133 3424 2880 2848

G′′/Pa tan δ 18457 31609 39919 753.4 625.5 1096 1635 2184 4263 5765 3706 15334

1.01 0.843 1.00 0.130 0.166 0.143 0.149 0.152 0.170 0.134 0.249 0.266

G′′ remain stable in the frequency region and are nearly frequency-independent. The modulus G′ is always higher than

G′′. The H1 phase shows more elastic than viscous properties throughout the whole frequency region. Therefore, the H1 phase exhibits elastic gel-like rheograms. In general, it is usual for the lamellar phase to display such rheograms,51 while for the H1 phase the moduli G′ and G′′ increase versus the frequencies with different slopes, as shown in Figure 6. However, in the C16MPB/H2O system, the H1 phase shows clear elastic gel-like behavior, not following the classical model of the H1 phase.52-54 This may be due to the strong interactions among the cylinderlike aggregates, which help to form the network structure of the H1 phase. In addition, through stress sweep measurements, we can further understand the structures of the H1 phase constructed by C16MPB dissolved in EAN and water. Figure 8a shows the typical curves of G′ and G′′ as a function of the stress at a constant frequency (1.0 Hz) for the C16MPB/EAN mixture. The moduli G′ and G′′ are nearly the same in the linear viscoelastic

Figure 9. POM images for the sample with 60 wt % C16MPB in the C16MPB/H2O system at different temperatures, 25 (a) and 70 °C (b), and the corresponding DSC curves (c). The scale bar is 200 µm.

Figure 10. POM images for the sample with 60 wt % C16MPB in the C16MPB/EAN system at 65 °C (a) (after cooling from the liquid phase) and 105 °C (b) and their corresponding DSC curve (c). POM image of the sample with 90 wt % C16MPB at 158 °C (d). The scale bar is 200 µm.

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region, which means that the elastic properties are nearly equal to the viscous properties. This is also in accordance with the results listed in Table 2, in which the values of tan δ (Supporting Information) are nearly equal to 1. For the C16MPB/H2O system, the typical curves are shown in Figure 8b. The modulus G′ is always larger than G′′ in the linear viscoelastic region. The H1 phase exhibits more elastic than viscous behavior and is also in agreement with the values of tan δ listed in Table 2. The difference of tan δ between these two H1 phases may be due to the distinct differences between EAN and H2O. Fumino and Ludwig studied the hydrogen bonding in EAN and H2O.45,46 They found that the tetrahedral H-bond network only existed in H2O, while three-dimensional network structures appeared in EAN. Thus, the structural differences between EAN and H2O may result in the rheological difference in the two systems. On the basis of the above results, we can see that the rheological properties are different due to the different solvents. In addition, through simple comparison, the modulus G′ in the C16MPB/EAN mixture is always smaller than that in the C16MPB/H2O system, suggesting that the network strength of the H1 phase in H2O is larger than that in EAN. In accordance with the results obtained from SAXS, the C16MPB molecules pack more densely in water than in EAN. 3.3. Thermal Stability of the C16MPB/H2O and C16MPB/ EAN Systems. POM and DSC measurements were employed to investigate the thermal stabilities of the LC phases. Figure 9a and b shows the typical POM images of the H1 phase in the C16MPB/H2O system. The birefringent textures are displayed and show the existence of the H1 phase (Figure 9a). The birefringent textures can also be observed in Figure 9b, indicating the H1 phase at 70 °C. To support the results obtained from the POM images, DSC was carried out to provide further evidence. As shown in Figure 9c, there is a strong endothermic peak in the DSC curve at about 90 °C, which indicates the phase transition of the LC phase. The H2O evaporates quickly in the LC mixture at high temperature, which results in damage to the H1 phase. In comparison with H2O, the boiling point of EAN is about 245 °C, which may inspire us to use EAN to construct LC phases with high thermal stability. Figure 10a shows the typical fanlike textures of LCs, indicating the presence of the H1 phase. The fanlike textures of the H1 phase remain unchanged until the temperature increases to about 105 °C. The textures disappear slowly, as shown in Figure 10b, which means that the samples melt at about 105 °C. To further confirm this phase transition, DSC was also employed and the typical curve is shown in Figure 10c. A distinct endothermic peak is shown at about 108 °C, which indicates damage to the H1 phase in the C16MPB/EAN system. The DSC curves are in good accordance with the POM results. Furthermore, the thermal stability of the C16MPB/EAN LC phase increases with the amount of C16MPB. When the C16MPB amount is increased to 90 wt %, the structure of the LC phase is not broken until the temperature is increased to 158 °C, as shown in Figure 10d. Therefore, the C16MPB/ EAN LC system displays higher thermal stability than the C16MPB/H2O mixture. 4. Conclusion In summary, we have investigated the phase behavior of the C16MPB/EAN and C16MPB/H2O systems using POM and SAXS measurements. Through comparison of these two LC systems, we can see that the C16MPB molecules pack more densely in H2O than in EAN, which may be due to the structural differences between EAN and H2O. In addition, through constructing the LC phase with EAN, we have obtained an LC phase with high

Zhao et al. thermal stability. This may inspire us to design LC samples with higher thermal stability and other functionality in ionic liquids, thus enriching the phase behavior of lyotropic LC phases and opening wide potential applications in catalysis and nanomaterial synthesis. Acknowledgment. The work was supported by the National Natural Science Foundation of China (Nos. 20773081, 50972080), National Basic Research Program (2007CB808004, 2009CB930101), and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, TIPC, CAS. The authors thank Dr. Pamela Holt for proofreading the manuscript. Supporting Information Available: The theory for calculation of structural parameters of the liquid crystalline phase and the calculations of as and tan δ. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Poliakoff, K.; Fitzpatrick, J. M.; Farren, T. R.; Anastas, P. T. Science 2002, 297, 807–810. (2) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. 2000, 2047–2048. (3) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792–793. (4) Ranu, B. C.; Banerjee, S. Org. Lett. 2005, 7, 3049–3052. (5) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.; Morris, R. E. Nature 2004, 430, 1012–1016. (6) Kuang, D. B.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534–10535. (7) Endres, F.; Bukowski, M.; Hempelmann, R.; Natter, H. Angew. Chem., Int. Ed. 2003, 42, 3428–3430. (8) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765–1766. (9) Abe, M.; Uchiyama, H.; Yamaguchi, T.; Suzuki, T.; Ogino, K. Langmuir 1992, 8, 2147–2151. (10) Schulman, J. H.; Stoeckenius, W.; Prince, L. M. J. Phys. Chem. 1959, 63, 1677–1680. (11) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371–1374. (12) Park, J. W.; Bak, C. S.; Labes, M. M. J. Am. Chem. Soc. 1975, 97, 4398–4400. (13) Taubert, A. Angew. Chem., Int. Ed. 2004, 43, 5380–5382. (14) Gin, D. L.; Gu, W. Q.; Pindzola, B. A.; Zhou, W. J. Acc. Chem. Res. 2001, 34, 973–980. (15) Torres, W.; Fox, M. A. Chem. Mater. 1992, 4, 583–588. (16) Nesseem, D. I. J. Pharm. Biomed. Anal. 2001, 26, 387–399. (17) Imura, T.; Hikosaka, Y.; Worakitkanchanakul, W.; Sakai, H.; Abe, M.; Konishi, M.; Minamikawa, H.; Kitamoto, D. Langmuir 2007, 23, 1659– 1663. (18) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831–838. (19) Ropers, M. H.; Ste´be´, M. J.; Schmitt, V. J. Phys. Chem. B 1999, 103, 3468–3475. (20) Wang, Z. N.; Diao, Z. Y.; Liu, F.; Li, G. Z.; Zhang, G. Y. J. Colloid Interface Sci. 2006, 297, 813–818. (21) Friberg, S. E.; Yin, Q.; Pavel, F.; Mackay, R. A.; Holbrey, J. D.; Seddon, K. R.; Aikens, P. A. J. Dispersion Sci. Technol. 2000, 21, 185– 197. (22) Araos, M. U.; Warr, G. G. J. Phys. Chem. B 2005, 109, 14275– 14277. (23) Wang, J. J.; Wang, H. Y.; Zhang, S. L.; Zhang, H. C.; Zhao, Y. J. Phys. Chem. B 2007, 111, 6181–6188. (24) Dong, B.; Li, N.; Zheng, L. Q.; Yu, L.; Inoue, T. Langmuir 2007, 23, 4178–4182. (25) Vanyu´r, R.; Biczo´k, L.; Miskolczy, Z. Colloids Surf., A 2007, 299, 256–261. (26) Inoue, T.; Dong, B.; Zheng, L. Q. J. Colloid Interface Sci. 2007, 307, 578–581. (27) Li, X. W.; Zhang, J.; Dong, B.; Zheng, L. Q.; Tung, C. H. Colloids Surf., A 2009, 335, 80–87. (28) Firestone, M. A.; Dzielawa, J. A.; Zapol, P.; Curtiss, L. A.; Seifert, S.; Dietz, M. L. Langmuir 2002, 18, 7258–7260. (29) Zhang, J.; Dong, B.; Zheng, L. Q.; Li, N.; Li, X. W. J. Colloid Interface Sci. 2008, 321, 159–165. (30) Zhao, Y. R.; Chen, X.; Wang, X. D. J. Phys. Chem. B 2009, 113, 2024–2030.

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