Gels and Lyotropic Liquid Crystals: Using an Imidazolium-Based

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Gels and Lyotropic Liquid Crystals: Using an Imidazolium-Based Catanionic Surfactant in Binary Solvents Ni Cheng,† Qiongzheng Hu,‡ Yanhui Bi,† Wenwen Xu,†,§ Yanjun Gong,† and Li Yu*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China Department of Chemistry, University of Houston, Houston, Texas 77204, United States § School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, PR China ‡

S Supporting Information *

ABSTRACT: The self-assembly behavior of an imidazolium-based catanionic surfactant, 1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]), was investigated in water−ethylammonium nitrate (EAN) mixed solvents with different volume ratios. It is particular interesting that this simple surfactant could not only form lyotropic liquid crystals (LLC) with multimesophases, i.e., normal hexagonal (H1), lamellar liquid crystal (Lα), and reverse bicontinuous cubic phase (V2), in the water-rich environment but also act as an efficient low-molecular-weight gelator (LMWG) which gelated EAN-abundant binary media in a broad concentration range. The peculiar nanodisk cluster morphology of gels composed of similar bilayer units was first observed. FT-IR spectra and density functional theory (DFT) calculations reveal that strong H bonding and electrostatic interactions between EAN and the headgroups of [C4mim][C12H25SO4] are primarily responsible for gelation. The selfassembled gels displayed excellent mechanical strength and a thermoreversible sol−gel transition. It is for the first time that a rich variety of controllable ordered aggregates could be observed only by simply modulating the concentration of a single imidazolium-based catanionic surfactant or the ratio of mixed solvents. This environmentally friendly system is expected to have broad applications in various fields, such as materials science, drug delivery systems, and supramolecular chemistry.

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behave as both hydrogelators and organogelators.19 Ramakanth reported fibrous gels of cetylpyridinium chloride in binary CHCl3/H2O solvent mixtures and found a lamellar organization with an interdigitated bilayer structure.20 Cai et al. synthesized a novel imidazolium-based surfactant that gelate normal ionic liquids through collective intermolecular interactions.21 The obtained gels displayed good characteristics of conductivity, anticorrosion, and antioxidation. The focus of most present studies is on this kind of cationic surfactant with a relatively small anion (e.g., halogen anion). The salt-free catanionic surfactants are more chemically tunable (both the cations and the anions) and environmentally friendly than the halogen-containing surfactants as mentioned above. Currently, there are only a few reports8,22−24 of the aggregation behavior of imidazolium-based catanionic surfactant. Eastoe et al.23 synthesized systematic catanionic surfactants containing 1butyl-3-methyl-imidazolium and studied their physicochemical, surface-active properties and pointed to possible applications in electrochemistry. Blesic et al.24 investigated alkyl-chain effects on the micellar formation of 1-alkyl-3-methylimidazolium alkylsulfonate salts. They found this series of compounds to

INTRODUCTION Aggregation behaviors of a new class of surfactants, formed by a large organic cation (e.g., imidazolium, pyridinium, pyrrolidinium, or a quaternary ammonium cation) under different conditions, have attracted increasing attention.1−6 Various aggregates include micelles, vesicles, microemulsions, liquid crystals, and gels can be produced.7−11 Recently, lyotropic liquid crystals (LLCs), an important type of self-assembled aggregates, have attracted considerable interest for their potential applications in biomaterials and electrooptics. For example, 1-alky-3-methylimidazolium bromides (CnmimBr) were demonstrated to form liquid crystals in concentrated water solutions.12−14 In addition, Chen’s group reported liquidcrystalline aggregates of 1-hexadecyl-3-methylimidazolium chloride (C16mimCl)15 and N-alkyl-N-methylpiperidinium bromides (CnPDB, n = 12, 14, 16) in water,16 respectively. Another ordered aggregate of this kind of surfactant is the supramolecular gel, which is formed through noncovalent interactions. Supramolecular gels have attracted a great amount of attention because they possess superior properties, e.g., high ionic conductivity, potential applications in electrochemical devices,17 good mechanical strength,10 and versatility in the design and synthesis of gelators.18 D’Anna and coworkers studied the gelation of geminal diimidazolium salts in solvents with different polarities and demonstrated that they could © 2014 American Chemical Society

Received: May 24, 2014 Revised: July 11, 2014 Published: July 15, 2014 9076

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(FT-IR) were measured from 400 to 4000 cm−1 with a resolution of 4 cm−1 using a Bruker VERTEX-70 infrared spectrophotometer. The gel samples were coated onto the KBr plate. Differential scanning calorimetry (DSC) measurements were performed using a Mettler Toledo DSC822e thermal analyzer. The gel samples were analyzed from −30 to 80 °C with a heating−cooling−heating cycle at a heating rate of 5 °C/min. The temperature was kept at (25 ± 0.1) °C for all of the measurements except those noted.

possess amphiphilic character in both cations and anions, with lower CMC values, and enhanced surface activity compared to that of the corresponding imidazolium halide salts. Although there still is a growing interest in the aggregation ability at low concentration in a unitary solvent,25 the influence of binary solvents with different proportions on the selforganized aggregates of imidazolium-based catanionic surfactant has rarely been studied systematically. Ethylammonium nitrate (EAN), which has the excellent ability to build up networks of H bonds in different systems,26−28 is a promising candidate for composing mixed solvent systems with water. Although EAN possesses specific physicochemical properties of ionic liquids, here it is selected and just acts as a polar cosolvent. Our investigation is aimed at examining the aggregation behavior of 1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]) in water−EAN media at varied compositions over a wide range of concentrations, together with the characterization of the controllable ordered aggregates. This is the first report on rich aggregates such as lyotropic liquid crystals (LLC) and gels formed by just a single imidazolium-based surfactant without any additives. [C4mim][C12H25SO4] is more environmentally friendly than a majority of imidazolium-based surfactants with halogen atoms, which avoid environmental hazards and toxicity concerns and lays a foundation for their potential biodegradability application.

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RESULTS AND DISCUSSION Aggregation Tests in the Binary H2O−EAN Mixtures. Figure 1 shows that the lyotropic liquid crystals (LLC) phase

EXPERIMENTAL SECTION

Figure 1. Phase diagram of [C4mim][C12H25SO4] in H2O−EAN mixed solvents with different volume ratios. L1, H1, Lα, and V2 indicate micellar, hexagonal, lamellar, and reverse bicontinuous cubic phases, respectively. W and G represent weak gel and gel, respectively.

Materials. 1-Methylimidazole (99%) was purchased from Acros Organics. Butyl chloride (98%) was obtained from Shanghai Aladdin Chemistry Co. Ltd. of China. Sodium dodecyl sulfate (SDS, 99.9%) was purchased from Alfa Aesar. Ethylamine aqueous solution (65−70 wt %) was obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Nitric acid (65−68 wt %) was obtained from Kangde Chemical Reagent Factory (Laiyang, China). [C4mim][C12H25SO4] was freshly prepared according to the ion-exchange procedures described in the previous work.8 The synthesis and purification of EAN were performed as described by Evans et al.29 More detailed information is shown in the Supporting Information. Methods. [C4mim][C12H25SO4] was weighed and transferred into test tubes with a screw cap, and then corresponding mixed solvents at different volume ratios (i.e., KH2O/EAN = 1:0, 3:1, 1:1, 1:3, 0:1) were added to obtain the prescribed compositions (in weight percent, wt %, hereinafter). As reported, the dielectric constants of EAN and H2O are 26 and 78,30 respectively. Thus, the corresponding dielectric constants of the binary mixtures (KH2O/EAN = 3:1, 1:1, 1:3) are calculated to be 77, 72, and 56, respectively, according to the formula reported elsewhere.31 The macroscopic manifestation of successful gelation is the absence of observable flow upon inversion of the test tube. More details are provided in the Supporting Information. Characterization. Photographs of sample birefringence were taken with a Motic B2 polarizing optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WV-CP460). Small-angle X-ray scattering (SAXS) measurements were performed using an Anton-Paar SAX Sess mc2 system with Ni-filtered Cu Kα radiation (1.54 Å) operating at 50 kV and 40 mA. The gel structures were characterized by transmission electron microscopy (TEM) (JEM-100CX II (JEOL)) and scanning electron microscopy (SEM) (JMS-6700 (JEOL)). Gels were cast onto carbon-coated Cu grids and then vacuum-dried before observation. The rheology measurements were carried out on a Haake RS75 rheometer equipped with a cone−plate sensor (Ti, diameter 35 mm, cone angle 1°, distance 52 μm). Frequency sweep measurements were performed at a constant stress which was determined from the strain sweep measurements. UV/vis transmittance spectra measurements were performed using a Hitachi U-4100 spectrophotometer with a 10 mm path length quartz cell. The scan rate for each measurement was 300 nm·min−1. Fourier transform infrared spectra

was obtained in the water-rich environments (K = 1:0, 3:1) while gels were constructed at higher EAN content in the mixed solvents (K = 0:1, 1:3, 1:1). We also found that the gradual addition of EAN to the LLC sample can lead to the phase transforming to gels. The transformation from gels to LLC could not be determined because of the concentration window limitation (Figure 1). It is very interesting to observe that the self-assembly of [C4mim][C12H25SO4] has such a strong dependence on the solvent composition. In pure water (K = 1:0), when the concentration of [C4mim][C12H25SO4] increased up to 35 wt %, a normal hexagonal phase (H1) was observed, which is optically clear and displays a birefringent appearance (Figure 2a). In the K = 3:1 mixed solvent system, the initial phase behaviors (isotropic micelle phase and H1 phase) are similar to that in pure water. However, the H1 assembly showed up at a higher concentration (65 wt %). We speculate that the driving forces that induce the

Figure 2. POM images of (a) the LLC phase for 60 wt % [C4mim][C12H25SO4] (K = 1:0) and (b) a typical gel phase for 40 wt % [C4mim][C12H25SO4] (K = 0:1) at 25 °C. 9077

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Figure 3. SAXS curves for the LLC phase samples: (a) K = 1:0, with different concentrations of [C4mim][C12H25SO4] (wt %) at 25 °C; (b) K = 1:0, 60 wt % [C4mim][C12H25SO4] at various temperatures.

higher temperatures. SAXS patterns of other LLC phases are included in the Supporting Information. On the basis of the model of the H1 phase, a series of structural parameters, including the repeat lattice parameter (a0), the total diameter of a cylindrical micelle (2dH), the thickness of the solvent layer (dW) between cylinders, and the area per molecule at the hydrophilic/hydrophobic interface (as), are calculated according to the reported procedures.33 As listed in Table 1, the distance between two centers of the

formation of LLC, i.e., solvophobic interactions, were weakened due to the addition of a modicum of EAN. As shown in Figure 1, when the EAN volume ratio increased (K = 1:1, 1:3, 0:1), [C4mim][C12H25SO4] could form opaque gels within a broad concentration range. Phase transitions from micelles to the weak gel (collapse after several days) and then the gel phase with varied [C4mim][C12H25SO4] contents were deduced from simple UV/vis transmittance experiments, as depicted in Figure S1. It is noteworthy that at concentration >5 wt %, gel phases were stable at room temperature for more than 1 year. A strong birefringence property was found through the POM observation (Figure 2b). During the gelation process, we noticed that small well-shaped gel spheres were formed first at the air−water interface and tended to adsorb more [C4mim][C12H25SO4] molecules due to the higher interface energy. Then the small gel spheres spread to the whole vessel, and gels were formed completely. In pure EAN, the gelation process occurred immediately when samples were cooled at room temperature. As for the other two mixed solvents (K = 1:1, 1:3), gels were not formed immediately and the solution remained clear and transparent for several hours. This might be accounted for by a thermomechanical history-dependent nucleation process.32 Thus, by adjusting the [C4mim][C12H25SO4] concentration and solvent formation, the assembly process can be readily manipulated to control the properties of ordered aggregate, such as aggregate states and transparency, the gelation concentration and time, and, as we will show, the thermal and mechanical properties. SAXS Patterns. The SAXS patterns of the H1 phase obtained in pure water (K = 1:0) with different concentrations of [C4mim][C12H25SO4] are shown in Figure 3a. Three periodic reflection peaks with their relative positions (q scattering vector) of 1/31/2/41/2 denote a hexagonal columnar liquid-crystalline structure (H1). The first Bragg peaks were observed to shift right at a higher [C4mim][C12H25SO4] concentration, corresponding to a lower distance between adjacent cylinders. The effect of temperature on the H1 phase was also investigated. As depicted in Figure 3b, an obviously larger q value shows up in the first peak position at higher temperature, suggesting a smaller lattice spacing caused by the softening of the solvophobic tails located in the interior part of cylinder-like micelles.16 Figure 3b also shows that H1 phase formed by [C4mim][C12H25SO4] in water was still stabilized even when the temperature reached 60 °C. Considering the volatility of water, we did not perform further investigations at

Table 1. Structural Parameters of the H1 Phase Formed by [C4mim][C12H25SO4] in Pure Water (K = 1:0) at 25 °C [C4mim][C12H25SO4] (wt %)

a0 (Å)

dW (Å)

dH (Å)

aS (Å2)

35 50 55 60 70

56.29 46.21 45.52 43.71 43.42

23.38 13.30 11.38 9.36 6.10

16.45 16.46 17.07 17.17 18.66

42.57 42.56 41.04 40.79 37.54

adjacent columns a0 (a0 = dW + 2dH) shows a declining tendency. Meanwhile, dW decreases and 2dH increases with increasing concentration of [C4mim][C12H25SO4]. This suggests that a higher surfactant concentration results in a larger radius of the cylinder-like aggregates, and thus the solvent layer is compressed. Both of these factors lead to the denser aggregation of cylinder units. Compared to the parameters of the hexagonal phase formed by the SDS/H2O system at the same concentration,34 a0 values in the [C4mim][C12H25SO4]/H2O system are all smaller. This can be attributed to the stronger electrostatic screening effect offered by [C4mim]+, which decreases the electrostatic repulsions between polar headgroups. Therefore, denser cylindrical aggregates with a more positively spontaneous curvature were formed and a smaller lattice parameter (a0) was obtained for the [C4mim][C12H25SO4]/H2O system. The SAXS patterns of the gels phase are presented in Figure 4. For all of the gel samples the scattering vectors q1/q2 exactly obeyed 1:2, indicative of a significantly highly ordered lamellar organization. The average lattice spacing (d) between two lamellar structures estimated using Bragg’s Law d = 2π/q1 is all around 24 Å, indicating that a basic aggregate unit existed for the assembled gels. This result is quite different from that of other lamellar systems which show a gradual reduction in lattice spacing with increasing gelator concentration.27,35,36 The network morphologies of the gels investigated by TEM (Figure 9078

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Figure 4. SAXS curves for the gels formed by different concentrations of [C4mim][C12H25SO4] in various H2O−EAN mixed solvents: (a) K = 0:1, (b) K = 1:3, and (c) K = 1:1.

5) and SEM (Figure 6) observation clearly display ordered particular nanodisk clusters with the dimension of hundreds of

Figure 6. SEM images of gels formed by 10 wt % [C4mim][C12H25SO4] in various binary H2O−EAN mixtures: (a) K = 0:1, (b) K = 1:3, and (c, d) K = 1:1. Figure 5. TEM images of gels formed by 10 wt % [C4mim][C12H25SO4] in various binary H2O−EAN mixtures: (a) K = 0:1, (b) K = 1:3, and (c, d) K = 1:1.

were determined for pure EAN as well as the gels formed by 40 wt % [C4mim][C12H25SO4] in EAN (K = 0:1). As observed in Figure 7, once gels were formed, both the asymmetric deforming vibration (1621 cm−1) and stretching vibration (3059 cm−1) of NH3+ in pure EAN generated blue shifts of 6 and 10 cm−1, respectively, revealing the existence of strong intermolecular hydrogen bonding between NH3+ and C12SO4−. At the same time, the change in the asymmetric stretching vibration of NO3− from 1400 to 1389 cm−1 demonstrates the H-bonding interaction between the NO3− ion and the

nanometers, which are quite rarely observed in self-assembled gels.25 More interestingly, we found that these nanodisks could further self-assemble to form linear aggregates (Figure 5d and insets in Figure 6), and then the chains were interconnected to produce an extended branched network (Figure 5c). Fourier Transformation Infrared (FT-IR) Spectra. To gain insight into the H-bonding interactions, FT-IR spectra 9079

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In our calculation mode, we did not put the EAN molecule near the alky chain of [C4mim][C12H25SO4] during the process of optimizing the structure of [C4mim][C12H25SO4]/EAN binary complexes because there were slight noncovalent interactions between them once aggregates were formed. As shown by the dashed line in Figure 8b, there existed very strong H-bonding interactions between EAN and the polar headgroup of [C4mim][C12H25SO4]. The shortest distance of (O···H) is 1.56 Å, and the others vary from 1.52 to 2.40 Å, which reveals how the gelators immobilized the solvents during the gelation.44 To understand quantitatively the difference in the interactions between [C 4 mim][C 12 H 25 SO 4 ]−H 2 O and [C4mim][C12H25SO4]−EAN, DFT calculations were also carried out on both of the them. Sixteen structures for binary complex [C 4 mim][C 12 H 25 SO 4 ]−H 2 O and [C 4 mim][C12H25SO4]−EAN with different mole ratios were selected randomly for the theoretical calculations. The most stable optimized structures are presented in Figure S4, and their interaction energies (Eint), calculated by the stabilization energy difference between complexes and monomers,45 are listed in Table S1. It is obvious that the calculated interaction energy of the [C4mim][C12H25SO4]−EAN complex is larger than that of [C4mim][C12H25SO4]−H2O. The primary reasons for this stronger interaction are strong H-bonding and electrostatic interactions in the EAN system. Mechanism of Ordered Aggregates. What causes different aggregation behavior of [C4mim][C12H25SO4] molecules (hexagonal liquid crystal and lamellar gels) in various H2O−EAN binary mixtures? On the basis of all of the investigations above, the possible mechanism of aggregates formed by [C4mim][C12H25SO4] is proposed. The critical packing parameter (CPP), defined as v/(a0lc), where v, a0, and lc are the average volume of the amphiphile, the effective headgroup area, and the effective length in the molten state, respectively,46 are used to interpret the self-assembled structures. The general relationships between the organized aggregates and CPP are as follows: spherical micelles for CPP < 1 /3, cylindrical 1/3 < CPP < 1/2, and planar 1/2 < CPP < 1. The v, a0, and lc values for [C4mim][C12H25SO4] are about 350 Å3, 60 Å2, and 14 Å, respectively. These parameters were obtained from Tanford equations,47 a surface tension measurement,8 and a DFT calculation, respectively. On the basis of these parameters, the CPP value for [C4mim][C12H25SO4] is 0.42,

Figure 7. FT-IR spectra of EAN and gels formed by 40 wt % [C4mim][C12H25SO4] in pure EAN (K = 0:1) at 25 °C.

hydrogen atom of the imidazolium ring proton in the C2 position that has acidic character.37,38 Compared to EAN, a weak shoulder peak on the higher-wavenumber side (2396 cm−1) disappeared for the gel system. As reported previously,39 this band was assigned to the contact ion pair of NH4NO3 for pure EAN. Thus, its vanishment in the gel sample indicates that the ion-pair binding was weakened due to the formation of intermolecular H bonding. DFT Calculations. In order to understand the interactions between [C4mim][C12H25SO4] and EAN better, we also conducted density functional theory (DFT) calculations by using the Gaussian 09 package40 at the level of M06-2X/631G(d, p).41,42 To consider the effects of the solvent, the polarizable-continuum model (PCM)43 was used in our calculations. Frequency calculations were performed to verify that all of the optimized geometries correspond to a local minimum that has no imaginary frequency. Figure 8 illustrates the optimized structural models for [C4mim][C12H25SO4], EAN, and [C4mim][C12H25SO4]/EAN. As depicted, the alky chain length of [C4mim][C12H25SO4] is about 14 Å (Figure 8a). Therefore, the interplanar spacing (d) of approximately 24 Å obtained by SAXS is larger than the alky chain length of [C4mim][C12H25SO4] but less than twice its value, indicating a highly interdigitated bilayer structure. The diameter of a cylindrical micelle (2dH) in a hexagonal columnar liquidcrystalline structure is larger than 30 Å (Table 1), which means that the hydrophobic chain stretches over the entire hexagonal phase.

Figure 8. Geometries of (a) [C4mim][C12H25SO4] (in vacuo), (b) 3EAN, and (c) [C4mim][C12H25SO4]-3EAN (in EAN with the PCM procedure) optimized at the M06-2X/6-31G(d,p) level. Color code for atoms: blue, nitrogen; red, oxygen; dark gray, carbon; light gray, hydrogen. 9080

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Figure 9. Strain sweep line and frequency sweep line for (a, c) the H1 phase, K = 1:0, 60 wt % and (b, d) gel, K = 0:1, 40 wt %.

For a fixed concentration of [C4mim][C12H25SO4], when the volume ratio of EAN in mixed solvent increases, almost invariant layer thickness was observed, which suggests that excess EAN molecules might insert into the original solvent layer via H-bonding and electrostatic interactions, thereby enhancing the connections between different layers.53 For a certain volume fraction of EAN, an increase in concentration might cause the hydrophobic tails of [C4mim][C12H25SO4] molecules to interdigitate more tightly, and their headgroups are organized into well-defined arrangement due to the formation of hydrogen bonds and π−π interactions. As a result, constant d values (24 Å) are obtained for all of the gels, which also indicates a stable molecular packing for [C4mim][C12H25SO4] gels (K = 1:3, 1:1, 0:1). It is clearly shown in Figure 1 that the gel-phase region in the K = 1:1 system is narrower than that of K = 1:3 and 0:1. This can be ascribed to the destruction of interactions between [C4mim][C12H25SO4] and EAN upon an increase in water composition. Compared to EAN, the stronger polarity of water induces a greater solvophobic interaction, which is hazadous to gel formation. Since new H bonds may formed between H2O and EAN molecules, original intermolecular H bonds between [C4mim][C12H25SO4] and EAN might be damaged. These two factors reduce the gel-phase region in 1:1 H2O−EAN mixtures.

suggesting a preferable hexagonal phase formation for [C4mim][C12H25SO4] in water. The formation of a lamellar gel phase (K = 0:1, 1:1, 1:3) can be ascribed to the different interactions of [C4mim][C12H25SO4] with EAN and water. The stronger interactions between the headgroup of [C4mim][C12H25SO4] and EAN (mainly H bonding and electrostatic interactions) may pull the alky chain further out of the cylindrical surface toward the solvent phase, which reduces spontaneous curvature from positive to approximately zero and favors the formation of a lamellar phase.48 The ionic nature of EAN may also cause a large degree of charge screening, thereby lessening the effective headgroup area of [C4mim][C12H25SO4]. The ethyl group of the EAN molecule can also be easily squeezed into the aggregates due to the charge screening, which causes an increase in the average volume of [C4mim][C12H25SO4].49 Compared to water, a higher affinity of EAN may also increase the volume of the solvophobic part of [C4mim][C12H25SO4].50 All of these factors can increase the CPP value and result in the formation of aggregates with more dense packing and relatively little spontaneous curvature, which is a characteristic of the planar lamellar phase.51 One can see that the strong interactions between EAN and the headgroups of [C4mim][C12H25SO4] play a crucial role in the formation of lamellar gel structures. During the formation of lamellar gel structures, [C4mim][C12H25SO4] molecules form bilayer units (d = 24 Å) that are stabilized by π−π interactions between the imidazolium rings and hydrophobic interactions between the alkyl chains at the beginning.52 Then H bonds and electrostatic interactions between EAN and headgroups of [C4mim][C12H25SO4] connect these units to form laminar nanodisk assemblies with one unit stacking above the other. During this process, the solvent molecules were confined and gelation was completed.

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PROPERTIES OF ORDERED AGGREGATES

Thermostability. A thermally reversible sol−gel transition, melting upon heating, and turning back into a gel upon cooling were observed for all gels. This also indicates that the [C4mim][C12H25SO4] molecules assemble into a network through noncovalent interactions. DSC thermograms of three gel systems (Figure S5) show a gel−sol phase-transition temperature (Tgel) shift to a temperature 10−25 °C higher than the sol−gel conversion temperature, indicating a characteristic 9081

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obtained for the first time by employing ecofriendly imidazolium-based surfactant [C4mim][C12H25SO4] in H2O− EAN mixed solvents. The molecular orientation of the controllable ordered aggregates in H2O−EAN mixtures shows a significant dependence on the solvent volume ratios. The formation of LLC is driven by the solvophobic interaction while H-bonding and electrostatic interactions are essential for the formation of gel aggregates. Beyond the formation of rich phases of liquid crystals, this work also provides a facile method to compose the nanodisk-based network of gels with superior properties, e.g., nearly constant interlayer spacing and high mechanical strength. This halogen-free surfactant in binary media might have a strong impact on broad applications in drug delivery systems and electrochemical fields.

of hysteresis behavior.50 Moreover, Tgel was relatively low (35.8, 37.0, and 28.2 °C for K = 0:1, 1:3, and 1:1, respectively), compared to that of other LMWG gels formed by macrocycleequipped54 and amino acid-functionalized gelators in imidazolium-based ILs55 and bis(benzimidazolylidene)-based gelators in type IL ammonium salts.44 The peculiar nanodisk clusters (Figures 5 and 6) contribute to our gelation of [C4mim][C12H25SO4], while long and entangled fibrous structures lead to the networks in their gel. The former texture might be less resistant to high temperature than the latter. Rheological Properties. Strain sweep measurements were performed at 1 Hz with the applied stress elevated. Obviously, the rheograms of LLC (Figure 9a) and gel (K = 0:1 in Figure 9b; K = 1:3; 1:1 in Figure S6) share some common features but also show significant disparities. Both the storage modulus (G′) and loss modulus (G″) are initially independent of the stress and then decrease dramatically beyond a critical yield stress. It should be pointed out that the large yield stress values (nearly 1000 Pa) of gel samples demonstrate the significantly high mechanical strength. For the LLC samples (Figure 9a), both moduli G′ and G″ are nearly the same, displaying virtually equal elastic and viscous properties. However, for gel samples, modulus G′ is always larger than G″, showing an elastic response in the linear viscoelastic region. Figure 9c,d presents rheograms of the oscillatory shear for the LLC (K = 1:0) and gel (K = 0:1). Rheograms of gels in K = 1:3, 1:1 are shown in Figure S7. For gels formed in K = 0:1, 1:3 systems, the complex viscosity (|η*|) decreases with a slope of −1 and the storage modulus (G′) and loss modulus (G″) are almost frequency-independent over the whole investigated frequency range (Figures 9d and S6a), indicating the behavior of a typical Bingham fluid. In addition, both moduli (G′ and G″) values are about 1 order of magnitude apart, displaying an elastic property. Furthermore, the G′ value of gels in pure EAN is nearly 1000 kPa, and the other two gels in binary media also possess relatively high G′ values, exhibiting a good resistance against mechanical stress. G′ values of gels obtained in the present work are much higher than those of gels formed by other LMWGs, such as sodium laurate,27 macrocyle-based gelators,54 and amino acid derivatives,56 as well as gels obtained from polymeric gelators,57 which suggests their exceptional rigidity. For the gel in the K = 1:1 system, G′ < G″ at lower frequency shows a viscous characteristic and G′ > G″ at higher frequency exhibits elastic behavior (Figure S7b), which is consistent with the rheological behavior of H1 phases (as shown in Figure 9c) but does not follow the classical model of the gel phase. This may be due to the higher water content that causes the rheological behavior of [C4mim][C12H25SO4] close to that in pure water, i.e., the H1 phase. Typical shear-thinning curves of gels (Figure S8a) in steadyshear measurements show a shear-thinning characteristic with high apparent viscosities (about 2000−12 000 Pa·s). Shear thinning is a characteristic of gels assembled by LMWGs through noncovalent forces, which causes the disruption of noncovalent interactions57 and then the collapse of networks at sufficiently high shear rates.54 Compared to the gels, LLC samples also behaved as a shear-thinning feature but with a lower apparent viscosity (Figure S8b).

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ASSOCIATED CONTENT

S Supporting Information *

More details about phase behaviors, DFT calculations, DSC curves, and rheograms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-531-88364807. Fax: +86-531-88564750. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (no. 21373128), the Natural Science Foundation of Shandong Province of China (no. ZR2011BM017), and the Sinopec Project (no. P13045).

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REFERENCES

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CONCLUSIONS In this work, multiple-state ordered aggregates, i.e., lyotropic liquid crystals (LLC) (normal hexagonal, lamellar liquid crystal, and reverse bicontinuous cubic phase) and laminar gels, were 9082

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