Nonaqueous Lyotropic Liquid-Crystalline Phases Formed by Gemini

Dec 21, 2011 - The aggregation behaviors of three Gemini surfactants [(CsH2s-α,ω-(Me2N+CmH2m+1Br–)2, s = 2, m = 10, 12, 14] in a protic ionic liqu...
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Nonaqueous Lyotropic Liquid-Crystalline Phases Formed by Gemini Surfactants in a Protic Ionic Liquid Xudong Wang,† Xiao Chen,*,† Yurong Zhao,† Xiu Yue,† Qiuhong Li,† and Zhihong Li‡ †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

‡

S Supporting Information *

ABSTRACT: The aggregation behaviors of three Gemini surfactants [(CsH2s-α,ω-(Me2N+CmH2m+1Br−)2, s = 2, m = 10, 12, 14] in a protic ionic liquid, ethylammonium nitrate (EAN), have been investigated. The polarized optical microscopy and small-angle X-ray scattering (SAXS) measurements are used to explore the lyotropic liquid crystal (LLC) formation. Compared to the LLCs formed in aqueous environment, the normal hexagonal and lamellar phases disappear. However, with increasing the surfactant concentration, a new reverse hexagonal phase (HII) can be mapped over a large temperature range except for other ordered aggregates including the isotropic solution phase and a two-phase coexistence region. The structural parameters of the HII are calculated from the corresponding SAXS patterns, showing the influence of surfactant amount, alkyl chain length, and temperature. Meanwhile, the rheological profiles indicate a typical Maxwell behavior of the LLC phases formed in EAN. biomembranes.17,18 A nematic phase at the boundary between the micelle (L1) and normal hexagonal (HI) phases in the 12-212/water binary system was observed, whereas a bicontinuous cubic phase at high temperature in the 12-3-12/water system was formed.15 The concentration range for the lyotropic mesophases in 12-s-12/water binary system contracts as s is increased and completely disappears for s = 10 or 12.11 By increasing the alkyl chains, a richer lyotropic behavior for 15-s15 than 12-s-12 in water was mapped,16 due to the stronger hydrophobic effect. It was also reported that the formation of lyotropic phases for the m-6-m (m = 8, 10, 12)/water system was determined by the concentration, temperature, and length of alkyl chains.19 As we know, the solvent is another important factor to govern the phase behavior except for the amphiphile molecular structure.20 Exploration on the aggregation behaviors of the Gemini surfactant in nonaqueous solvents may prvode a better understanding of amphiphile self-assembly. Furthermore, such studies are also useful to improve the low thermodynamic stability of LLCs constructed in aqueous media. This is because the water evaporation at high temperature can destroy the structure of LLCs easily and limit their applications. Therefore, searching the suitable nonaqueous solvent may be an effective way to fabricate LLCs stable at high temperature, which is significant to extend the applications of both LLCs and Gemini surfactants. As a novel nonaqueous media, ionic liquids (ILs) have currently attracted much attention in the areas of organic

1. INTRODUCTION The Gemini (dimeric) surfactants are made up of two surfactant-like moieties connected by a spacer group.1,2 Much interest has been generated for their lower critical micelle concentration, higher surface activity, lower Krafft temperature, better wetting ability, and richer aggregate structures, compared to those properties of the corresponding monomeric counterparts. The structural character also makes it possible to greatly modify their aggregation behaviors in aqueous solution by modulating the length and the nature of both the spacer group and the alkyl chains.3−5 Because of their excellent properties, the Gemini surfactants have been attracting great attention in both academic and industrial fields and widely used in various areas, such as food industry,3 gene and drug delivery,6 synthesis of nanostructured materials,7 phase transfer catalysts,8 and oil recovery.9 Among so many Gemini surfactants, the cationic quaternary ammonium salt type molecules deserve to be mentioned for their interesting biological properties.6 Zana and co-workers have carried out many pioneering works on the synthesis of such kinds of Gemini surfactants with a general formula CsH2sα,ω-(Me2N+CmH2m+1Br−)2, referred to as m-s-m, and on the aggregate formation in water.10,11 Both the thermotropic and the lyotropic mesophase behaviors of them have been reviewed.12 Becasue they are amphiphilic molecules with special structures, the Gemini surfactants have the ability to create organized structures including micelles,13 microemulsions,14 vesicles,15 and lyotropic liquid crystals (LLCs)16 by self-assembling themselves either at surfaces or in bulk solutions. Thus, the LLC phases have been well documented and widely used as templates or media for chemical reactions and materials synthesis or as models to mimic the © 2011 American Chemical Society

Received: November 14, 2011 Revised: December 20, 2011 Published: December 21, 2011 2476

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synthesis and catalysis, biochemical engineering, materials science, electrochemistry, carbohydrate chemistry, inorganic nanomaterials, and separation techniques21−26 for their unique characteristics of low melting temperature, negligible vapor pressure, wide electrochemical window, nonflammability, good catalytic properties, high thermal stability, and ionic conductivity.21,27 Ionic liquids can be generally divided into two broad categories: the protic (PILs) and the aprotic (AILs). Because of the presence of the proton donor and acceptor sites, the PILs can form a hydrogen-bonded network which makes them good candidates as nonaqueous self-assembling media28 and makes it possible to design LLCs with high thermostabililty. The research on self-assemblies in PILs primarily started with ethylammonium nitrate (EAN),29 the first truly protic ionic liquid showing more analogous properties than other PILs to water.30,31 Evans and co-workers reported the lamellar liquid crystals formation by lipids in EAN.32,33 The nonionic polyoxyethylene alkyl ether surfactants were found by Warr et al. to form LLCs easily in EAN due to the hydrogenbonding.34 Drummond et al. have compared the solvophobic effect dominated LLCs formation by hexadecyltrimethylammonium bromide (CTAB) in 20 PILs.35 They also observed the LLCs formed by a monoolein and a phytantriol in a series of PILs, including EAN.36 The nonionic block copolymer EO20PO70EO20 (P123) can also exhibit liquid crystal behaviors in EAN as studied from our group.37 The normal micellar cubic, hexagonal, lamellar, and reverse bicontinuous cubic phases have been identified. We have also investigated the aggregation behaviors of a nonionic oleyl polyoxyethylene (10) ether (Brij 97) and a cationic 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) in EAN.38,39 Only a hexagonal phase in the Brij 97/EAN system was mapped, but a series of ordered aggregates including micelles, normal hexagonal, lamellar, and reverse bicontinuous cubic liquid crystalline phases could be observed in C16mimCl/EAN mixture. Motivated by these studies, it is natural to consider the phase behaviors of Gemini surfactants in PILs, expecting for the special aggregate structures and eminent properties. Unfortunately, such research is rarely reported. To our best knowledge, only Gin’s group reported that the LLCs could be formed by the imidazolium-based Gemini surfactants in ionic liquids with the [BF4−] anion. The reason is that BF4− can form hydrogen bonds in all directions with the protons present on imidazolium cations.40 To explore more about the aggregation properties of Gemini surfactant in PILs, we here choose the Gemini molecules of the well-known type of m-2-m (m = 10, 12, 14) to investigate their phase behaviors in EAN over a whole concentration range. The effects of Gemini molecule amount, temperature, and alkyl chain length on the formed LLC structures and thermodynamic stability are clarified. The obtained results could be expected to not only provide us further knowledge on the self-assembled structures and properties of Gemini surfactants in nonaqueous solvents but also extend the application fields of LLCs.

Figure 1. Structures of the studied m-2-m type Gemini surfactants (m = 10, 12, 14). vacuum conditions for 48 h. The product purity was ascertained by 1H NMR (nuclear magnetic resonance) spectrum in CDCl3 and the elementary analysis. The ionic liquid EAN was prepared according to the procedures reported previously.42 In a typical synthesis, a portion of ∼3 M nitric acid was slowly added to the ethylamine solution while stirring and cooling in an ice bath. The water in product 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 0.5 wt %, and its melting point was about 12 °C. The purity of EAN was ascertained by the 1H NMR spectrum in D2O. 2.2. Sample Preparation and Phase Diagram Mapping. The process for mapping the phase diagram has been described elsewhere.39,43 All samples were prepared by mixing the m-2-m surfactants 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 the crossed polarizers. The liquid crystal types were determined by the 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. 2.3. Characterization. 2.3.1. Small-Angle X-ray Scattering. The obtained LC phases were characterized by an HMBG-SAX X-ray small-angle scattering system (Austria) with a Ni-filtered Cu Kα radiation (0.154 nm) operating at 50 kV and 40 mA. The distance between the sample and detector was 27.8 cm. For comparison, one sample exhibiting the two-phase coexistence was determined with the SAXS beamline (1W2A) of the Beijing Synchrotron Radiation Facility (BSRF) in China under the experimental conditions of 0.154 nm incident X-ray wavelength, 1850 mm distance between the sample chamber and the detector, and a 400 s data accumulation time. Except for the enhanced intensity, no major difference in the scattering profiles could be found between these two X-ray sources. 2.3.2. Polarized Optical Microscopy. Photographs of samples with birefringence were taken by a Motic B2 polarizing optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WVCP460). 2.3.3. Rheological Measurement. The rheological measurements were carried out with a Haake Rheostress 6000 rheometer using a Rotor C35/1 system at 40.0 ± 0.1 °C. Frequency sweep measurements were performed in the linear viscoelastic region which was determined from the strain sweep measurement with the stress varying at a constant frequency of 1.0 Hz.

3. RESULTS AND DISCUSSION EAN molecules can form a three-dimensional hydrogen-bond network, which seemed to be an essential feature in its selfassembly.31,44−46 The aggregation of amphiphiles in EAN has been interpreted in terms of a solvophobic driving force analogous to the hydrophobic effect in water, which is dominated by the entropic contribution.35,47 There is a strong solvent−solvent interaction between EAN molecules and a weak van der Waals bonding between the EAN molecules and the amphiphile hydrocarbon chains. Upon the amphiphile molecules aggregation, the EAN-hydrocarbon interaction is reduced and the solvent is released from the ordered structure, resulting in an increase in entropy. The EAN mediated Gemini molecules self-assembly here should progress in a similar manner.

2. EXPERIMENTAL SECTION 2.1. Materials. The structures of studied m-2-m type Gemini surfactants are shown in Figure 1. They were prepared according to the procedures reported previously.10,41 N,N,N′,N′-Tetramethylethylenediamine (20 mmol) and the alkyl bromide (5 equiv) were refluxed in dry EtOAc or CH3CN (250 mL) for 2 days. After evaporation, the residue was recrystallized for at least three times and then dried under 2477

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sample at 25 °C, which are correlated with the H phase characteristic relative positions of 1:√3:2, (i.e., d100, d110, d200, ...).49 When the 12-2-12 concentration locates between 64 and 70%, a two-phase coexisting region appears as indicated by the POM image and SAXS pattern as shown in Figure 3b. A noncontinuous bright striated texture (Figure 3b, top), like a diluted fanlike one, can be seen, and their SAXS curves behave mostly like that for the micelle solution except with a small sharp peak overlapped (Figure 3b, bottom). The q value of this peak is 2.18 nm−1 and very close to that of the first peak for the H phase. The reliability of such a small peak has also been shown by the synchrotron SAXS measurement in BSRF as seen in the Supporting Information. Therefore, in this concentration range, it is reasonable to consider that the coexisting phases belong to L1 and H. As for the formed H phase, it is necessary to clarify its structural model: whether the surfactant alkyl chains in the cylinder will stretch inside (normal hexagonal phase: HI) or outside (reverse hexagonal phase: HII).11 Usually, the HI and HII phases appear respectively on the solvent-rich and solventpoor sides of the lamellar (Lα) phase.50 However, in our studied system now, no Lα phase is identified. To solve this problem, the structural parameters of the formed H phase are analyzed based on the SAXS pattern shown in Figure 3c. The lattice parameter, D (i.e., the distance between two centers of the neighboring cylinders), can be calculated from the scattering factor corresponding to the first Bragg peak with the formula of 4π/√3q1, and the calculated D value is 34.7 Å for 71.0% 12-2-12 concentration at 25 °C. From the D value, the area per molecule of amphiphile at the hydrophilic/hydrophobic interface (S) for an assumed HI phase (SI) can be obtained according to eqs S1−S2 (Supporting Information), which is 111.6 Å2 for the sample of 71.0% 12-212 concentration at 25 °C. In addition, the effective headgroup area (a0) of the 12-2-12 molecule at the critical micelle concentration in EAN (25 °C) is about 81.8 Å2, which is obtained from its adsorption at the air/EAN interface (Supporting Information). Usually, the S value decreases with increasing the surfactant concentration.12,20,51 However, the value of SI is bigger than that of a0, which exclude the possibility for a normal hexagonal phase. Then, such S data for an assumed

The phase behaviors of m-2-m type Gemini molecules in binary systems with the ionic liquid EAN have been investigated. On the basis of the POM textures and the SAXS patterns, their phase diagrams, especially the lyotropic liquid crystal phase regions, were identified. 3.1. Phase Diagrams of the 12-2-12/EAN Binary System. The mapped temperature−composition (T−X) phase diagram of 12-2-12/EAN binary system is shown in Figure 2. The identified phases include the isotropic solution

Figure 2. Phase diagram of 12-2-12/EAN binary system. L1 and HII denote respectively the normal micelle solution, and the reverse hexagonal phase.

(L1), the two-phase coexisting region and the reverse hexagonal (HII). The system behaves as a homogeneous, transparent, and isotropic solution at lower 12-2-12 concentrations (less than 64%). This phase is considered to be a normal micelle solution (L1) because only one weak and broad scattering peak was observed in its SAXS curve (Figure 3a, bottom).48 When the 12-2-12 concentration is between 70 and 80%, a large area of the hexagonal (H) phase is found, as confirmed by the typical fanlike texture under POM observation and the SAXS pattern (Figure 3c, top and bottom). The scattering factor (q) values corresponding to the first, second, and third Bragg peaks are respectively 2.09, 3.60, and 4.16 nm−1 for the 71.0% 12-2-12

Figure 3. POM images (top) and SAXS curves (bottom) at 25 °C for different 12-2-12 concentrations (wt %): (a) 58.8, (b) 65.4, and (c) 71.0. 2478

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CPP > 1. For a straight alkyl chain, the length lc and volume v of the solvophobic part can be obtained from the following Tanford equations:53,54

HII lyotropic phase (SII) and other structural parameters according to eqs S3−S4 (Supporting Information) are calculated and listed in Table 1. It is an intuitive judgment Table 1. Structural Parameters of HII Phases in the 12-2-12/ EAN Systema temperature (°C)

12-2-12 concentration (ω, wt %)

60 60 60 50 40 25

78.5 73.5 71.0 71.0 71.0 71.0

D (Å)

RII (Å)

dII (Å)

SII (Å2)

33.9 33.3 32.9 33.3 34.1 34.7

8.5 9.3 9.6 9.7 9.9 10.1

16.9 14.7 13.7 13.9 14.3 14.5

58.9 71.8 78.9 77.7 75.4 74.5

v = (27.4 + 26.9N ) Å3

(1)

lc = (1.5 + 1.265N ) Å

(2)

where N represents the number of carbon atoms of the alkyl chain which is incorporated in the hydrophobic core. The value of v obtained from the formula above is usually for amphiphiles with a single hydrocarbon chain. For the Gemini molecules studied here, it is just enlarged by a factor of 2.53 With the molecules of 12-2-12, the values of v and lc are calculated to be 700.4 Å3 and 16.68 Å, respectively. The areas a0 obtained from the adsorption of 12-2-12 at the air/solvent interface are respectively 102 (at air/H2O interface)55 and 81.8 Å2 (at air/EAN interface). Such a reduction of a0 in EAN is due to its ionic nature, which causes a large degree of charge screening and therefore leads to a more densely packing of the Gemini head groups.36 Thus, the CPP values of 12-2-12 are calculated as 0.41 in H2O and 0.51 in EAN, which therefore explains why the normal hexagonal and lamellar phases could be obtained in the 12-2-12/H2O system but the normal hexagonal phase disappears in 12-2-12/EAN mixtures. For the same reason, the EAN molecules can easily partition into the interface regions with the 12-2-12 molecules due to the charge screening.36 The ethyl group of EAN molecules here may squeeze into the 12-2-12 aggregates, which causes a net increase in the average volume of the 12-2-12 and thus an increase in the CPP. The affinity for solvent of the alkyl chains of Gemini molecules is also different. The higher affinity for EAN than for water might increase the volume of the solvophobic part,36,37,39 which also leads to a higher CPP and then results in the formation of a reverse phase. Moreover, the formation of a reverse phase has been confirmed by the calculated molecular packing parameter (MPP= V/al > 1) based on the structure parameters of a hexagonal phase (71.0%, 25 °C). Such a value of MPP is 1.30 calculated with V = 700.4, the real headgroup area (a = SII), and the length of the solvophobic part (l = dII/2) adopted from Table 1. All of these local molecular packing changes lead to a different phase behavior between EAN and water. 3.2. Effect of 12-2-12 Concentration on Reverse Hexagonal Liquid Crystals. The influences of the 12-2-12 concentration on the reverse hexagonal liquid crystal have been also explored. Because the phase region is narrow at 25 °C, such an effect is investigated at a higher temperature of 60 °C. Figure 5 shows the SAXS patterns of the HII phase constructed at three concentrations of 12-2-12 in EAN at 60 °C. As described above, the first scattering peak in the SAXS profile originates from the Bragg reflection of the (100) plane, which can help us to obtain the lattice parameter D. The HII phase consists of infinitely long cylinder-like aggregates packed in a hexagonal array and separated by a continuous hydrocarbon domains,51 where the solvophobic tails are located outside the cylinders, and the solvophilic heads are solvated by EAN molecules. According to the model of the HII phase, D can be divided into two parts: the twice radius of the EAN core in cylinder-like aggregate (RII) and the thickness of the outside hydrocarbon domains (dII) in between, They are in accord with the following equation:

a

RII is the radius of the reversed cylinder-like aggregates; dII is the thickness of hydrocarbon domains.

that the formed hexagonal phase should be a reverse one because the data of SII (74.5 Å2, 71.0%, 25 °C) is smaller than that of a0 (81.8 Å2). To understand more about the EAN solvent effects, it is better to compare them with the phase behavior of 12-2-12/ H2O binary system. As shown in Figure 4, compared to the

Figure 4. Phase diagrams for 12-2-12 binary systems with H2O and EAN at 40 °C. Symbol notification: L1 (normal micelle solution), HI (normal hexagonal phase), Lα (lamellar phase), and HII (reverse hexagonal phase).

phase diagram of the 12-2-12/H2O system,12 the HI and Lα phases are found to disappear with a HII phase instead in the 12-2-12/EAN system. It is known that the solvophobic effect is the main driving force to decide whether or not a surfactant could form certain self-assembly. Meanwhile, for a specific amphiphile molecule, its possibly formed liquid crystalline mesophases are dependent upon both the local molecular and the global aggregate packing constraints.35,36 Usually, the critical packing parameter (CPP) is adopted as a powerful and simple tool to interpret the possible self-assembled structures. CPP is 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.52 The general relationships between the organized aggregates and CPP are as follows: the spherical micelles for CPP < 1/3, the rod shaped micelles for 1/3 < CPP < 1/2, the bilayers and vesicles for 1/2 < CPP < 1, and the reverse structures when

D = 2RII + dII 2479

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(Figure 6a2). When the concentration is increased to 78.5%, the LLC phase structure could survive to a temperature of 110 °C, as shown in Figure 6c1. All of these results denote a more densely and ordered molecular packing in the HII phase at higher 12-2-12 content. A detailed discussion about the temperature effect on the reverse hexagonal phase is shown in the following text. 3.3. Temperature Effect on Reverse Hexagonal Liquid Crystals. To explore the relationship between the molecule arrangement and the thermal stability of the formed reverse hexagonal mesophase (HII), the effect of temperature is investigated. From the phase diagram shown in Figure 2, the region of HII can be seen to cover the 12-2-12 concentration range from 70 to 80% and may go through a temperature range from 25 to 115 °C. By increasing the temperature for a typical sample at 71.0% 12-2-12 concentration, the gradual changes of fanlike textures can be seen in Figure 7. The typical fan-like textures are clearly present at 25−60 °C (Figure 7a−d). At higher temperatures, such textures become gradually dimmed or broken and finally lose any birefringence when it is turned into an isotropic phase (Figure 7e−g). The SAXS patterns for the samples with the typical hexagonal phase texture at four different temperatures (25, 40, 50, and 60 °C) are shown in Figure 8. Three Bragg peaks with the relative positions corresponding to a ratio of 1:√3:2 indicate definitely a reverse hexagonal structure. Besides, increasing temperature leads the first SAXS peak position to a higher q value, corresponding to a smaller repeat lattice spacing. The corresponding phase structural parameters are also calculated and listed in Table 1. The reduction of the repeat lattice spacing can be considered from the temperature induced conformational fluctuation of the hydrophobic chains. The increased fluctuations at higher temperatures may shorten the average length of the alkyl chain and lead to the decrease of repeat lattice spacing. Meanwhile, large conformational fluctuations of the hydrophobic chains may also expand the average chain volume. Both factors may result in a thinner hydrocarbon domain with a higher inverse curvature in the HII phase.56 In addition, the breakage of hydrogen bonds in solvent with temperature increasing will also make EAN molecules more contracted. It has been suggested that the curvature of the phase structure is

Figure 5. SAXS curves for the 12-2-12/EAN system as a function of concentration at 60 °C. Insets are the corresponding POM images.

The measured D values are 32.9, 33.3, and 33.9 Å respectively for 71.0, 73.5, and 78.5% 12-2-12 concentrations, indicating a gradual increase with the concentration. From these D values, more structural parameters can be obtained according to eqs S3−S4 (Supporting Information), the values which are listed in Table 1. With the concentration of 12-2-12 increasing from 71.0 to 78.5%, the RII value decreases from 9.6 to 8.5 Å, whereas the dII increases from 13.7 to 16.9 Å at 60 °C. Both results suggest that the radius of the EAN solvent core inside the cylinder becomes contracted in the case of more 12-2-12, whereas the hydrocarbon layer of the cylinder expands. Furthermore, the area per molecule of 12-2-12 at the hydrophilic/hydrophobic interface in the reverse hexagonal phase, SII, decreases from 74.9 to 58.7 Å2 with the 12-2-12 amount increased reflecting a more dense packing. This can be supported by the fanlike texture in the images under the POM, which become more and more perfect upon adding more 12-212 as shown in the insets of Figure 5. Such a concentration effect is obviously due to the enhanced solvophobic effect. Meanwhile, the thermal stability of this LLC phase is improved with more 12-2-12 as observed in Figure 6. The collapse temperature of the HII phase at 71.0% 12-2-12 is about 71 °C based on the POM observation with the heating stage

Figure 6. POM images of the 12-2-12/EAN system for 12-2-12 concentrations of 71.0% (a), 73.5% (b), and 78.5% (c) at different temperatures (°C). a1: 60; a2: 71; b1: 100; b2: 103; c1: 107; c2: 109. 2480

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Figure 7. Gradual deteriorated fanlike textures for a typical 12-2-12/EAN sample with 12-2-12 concentration of 71.0% at different temperatures (°C). a, 25; b, 40; c, 50; d, 60; e, 65; f, 71; g, 73.

It should be mentioned that the lyotropic phases constructed in EAN exhibit better thermal stability because of the ionic liquid nature of EAN. The LLC from 12-2-12/EAN can exist stably at 60 °C for at least 6 months with the structural parameters little changed. However, the LLCs from the 12-212/water system can not be kept stably for long time at 60 °C. Only after several days, the water may escape from the assembly, which will destroy the LLC structure. 3.4. Effect of the Alkyl Chain Length on Reverse Hexagonal Liquid Crystals. With the same systematic phase identification procedures as described in section 3.1, the other two m-2-m/EAN systems (m = 10, 14) are found to exhibit a relatively similar phase behavior as seen in Figure 9. Only the reverse hexagonal LLC phase was identified. For the 10-2-10 system (Figure 9a) the HII phase region extends from 71 to 91% amphiphile concentration at temperatures up to 85 °C. Such a phase region for the 14-2-14 system (Figure 9b) starts at a relatively low concentration (61%) and extends the temperature maximum threshold to 135 °C. Their typical SAXS patterns and POM textures are shown in Figure 10. It is noted by comparison between Figures 2 and 9 that the three phase diagrams exhibit some differences with the change of the alkyl chain length. The thermal stabilities of HII phases are also enhanced with longer hydrocarbon chains because of the strengthened solvophobic effect. As shown in Figure 11, the highest temperatures when the typical POM birefringent textures of HII phase can be observed are proportional to the alkyl chain length. It decreases from 135 °C for 14-2-14, to 110

Figure 8. SAXS patterns for a 71.0% 12-2-12/EAN system at different temperatures (°C). a, 25; b, 40; c, 50; d, 60.

defined by the solvent channel radius. The smaller of such radii are, the highly curved phase structures could form.57 From Table 1, it is clearly seen that the EAN channel radius (71.0%), RII, decreases from 10.0 to 9.30 Å with the temperature increasing from 25 to 60 °C, which means a larger curvature should be obtained. Therefore, a correlation between the preferred cylinder curvature and the temperature effect seems reasonable: increasing temperature causes a higher curvature.

Figure 9. Phase diagrams of two m-2-m/EAN binary systems, m = 10 (a) and 14(b). L1 and HII denote respectively the normal micelle solution and the reverse hexagonal phase. 2481

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Figure 10. SAXS curves (a, b) and POM images (a1, b1) for the reverse hexagonal phases of the m-2-m/EAN systems at 60 °C (a: 10-2-10, 86.0% and b: 14-2-14, 67.5%).

Figure 11. POM images for the reverse hexagonal phases obtained in m-2-m/EAN systems at high temperatures (a: 14−2−14, 78.7%, 130 °C; b: 122-12, 78.5%, 105 °C; c: 10−2−10, 85.9%, 82 °C).

°C for 12-2-12, and 85 °C for 10-2-10. Such an effect is also reflected from the decreased micelle phase region with longer hydrocarbon chains. The highest m-2-m concentration boundary of the micelle solution decreases from 66% for 102-10 to 63% for 12-2-12 and 57% for 14-2-14. Similar phenomena have been also found in m-3-m/water systems.19 In addition, from the SAXS patterns shown in Figure 12, it is clear that the positions of the first peak shift left with longer alkyl chain for similar concentrations at 60 °C, which represents the larger lattice parameters (D) of the HII phases obtained from m-2-m/EAN system. This should be due to the extension of the thickness of hydrocarbon domains, which provides the possibility to control the dimensions of the HII phases. 3.5. Rheological Measurements. The viscoelasticity properties of obtained HII phase were characterized by the rheology measurements. Because all of the HII samples were found to show similar rheological behaviors, the results from the 12-2-12/EAN system are chosen here as an example. The LLC phase constructed by the 12-2-12/water system at the same concentration has also been investigated for comparison.

Figure 12. SAXS curves for the reverse hexagonal phases of m-2-m/ EAN systems at 60 °C (a: 10−2−10, 80.0%; b: 12-2-12, 78.5%; c: 14− 2−14, 76.5%).

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water shows bigger storage and loss moduli than those in EAN. Such a difference may be due to the distinct solvent differences between H2O and EAN, i.e., a tetrahedral H-bond network structure in H2O and only a three-dimensional network structure in EAN.31,44−46 The Gemini molecules exhibit therefore the stronger solvophobic effect in water than in EAN, which leads to bigger moduli.

The typical steady-shear rheograms (apparent viscosity as a function of shear rate) are shown in Figure 13. At low shear

4. CONCLUSIONS In summary, the aggregation behaviors of a series of Gemini surfactants (m-2-m) in EAN have been investigated. Compared to water, Gemini molecules are expected to pack more densely in EAN due to charge screening, which causes a higher CPP in EAN, and thus, a new reverse hexagonal phase (HII) can be formed in these systems. In addition, the thermal stability of the obtained reverse hexagonal phase is enhanced with increasing surfactant amount or longer alkyl chain. The obtained results provide further knowledge on Gemini molecules self-assembly in room-temperature ionic liquids. Considering its wide use in various fields, such as drug delivery,18 food applications,61 and material synthesis,62 the reverse hexagonal phase constructed by m-2-m dissolved in EAN with high thermal stability could expand the application area to nonaqueous and high temperature conditions, which will be an important supplement to the self-assembly behavior of a Gemini surfactant in nonaqueous solutions.

Figure 13. Steady-shear rheological data collected at 40 ± 0.1 °C for the 12-2-12/EAN and 12-2-12/water systems. The inset is a visual inspection photo on three LLC samples at 12-2-12 concentrations of 71.0% and 73.5% in EAN and 73.5% in water, from left to right.

rates, the samples whether in EAN or in water exhibit high apparent viscosity and look like gels, as seen by the inset image of unflowable phase on the top of inverse tubes. However, from the frequency sweep measurement data shown in Figure 14, the

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ASSOCIATED CONTENT

S Supporting Information *

The theory for calculation of structural parameters of the liquid crystalline phase and the calculations of a0. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-531-88365420. Fax: +86531-88564464.

■ Figure 14. Changes of storage modulus G′ (solid) and loss modulus G″ (empty) versus frequency for the LLC phase of the 12-2-12/EAN and 12-2-12/water systems at 40 °C.

ACKNOWLEDGMENTS We thank Prof. Zhonghua Wu and Dr. Zhihong Li for the kind help with the synchrotron SAXS measurements. The financial supports from the National Natural Science Foundation of China (20773080, 20973104, and 21033005), Shandong Provincial Science Fund (2009ZRB01147), and the SAXS station with the Beijing Synchrotron Radiation Facility (BSRF) in China are acknowledged.

dynamic moduli G′ and G″ are found to elevate with increasing frequency, which indicates that the samples are practically not gels but highly viscosity liquid crystals.58 As shown in Figure 14, the values of the storage modulus (G′) increase almost linearly versus the frequency, while the loss modulus (G″) increases with frequency at the beginning and then remains relatively constant. At low frequencies, G″ is larger than G′, showing a viscous behavior. At higher frequencies, however, G′ is larger than G″, exhibiting an elastic behavior.59 Such a rheological change belongs to the typical reverse hexagonal phase which has also been found in other systems.60 It is noticeable that both the storage and loss moduli for samples in EAN increase with more 12-2-12 content, accompanying with the higher viscosity, which reflects the more ordered LLC structure because of the stronger solvophobic effect. It is also clear that the LLC structure in

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