Ionogels of Sugar Surfactant in Ethylammonium Nitrate: Phase

Article pubs.acs.org/JPCB

Ionogels of Sugar Surfactant in Ethylammonium Nitrate: Phase Transition from Closely Packed Bilayers to Right-Handed Twisted Ribbons Xiaolin Wang and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: In the simplest ionic liquid, ethylammonium nitrate (EAN), ionogels with high mechanical strength were prepared from a surfactant with a disaccharide polar head. Phase structures from closely packed bilayers to right-handed twisted ribbons were determined via freeze-fracture transmission electron microscopy (FF-TEM) observations. The phase transition mechanism was investigated deeply and systematically. The temperature contributes to suitable tail chain conformations of surfactant molecules for adapting to different self-assembled structures including right-handed twisted ribbons and bilayers. Two different arrays were revealed for different bilayers by the small-angle X-ray scattering (SAXS) measurements. The rheological and tribological properties of the ionogels were investigated. The better lubricating property and antiwear capability of the ionogels compared to the EAN may be attributed to the structure characteristics and the good thixotropic properties.

1. INTRODUCTION For developing green chemistry, much attention has been devoted to the exploration and design of green materials, and has broadly converged toward synthesis and application of “fully green” and biodegradable ionic liquids with sugar moieties on them.1 As everyone knows, ionic liquids are identified as green solvents because of their negligible volatility. In addition, they perform many other attractive properties, such as high ionic conductivity, thermal stabilities, chemical inertness, and so on.2,3 Ethylammonium nitrate (EAN) has been extensively used as an aggregate-formation medium because of its water-like properties especially the formation of three-dimensional hydrogen-bonded networks.4,5 Aggregates formed in EAN include micelles,6,7 lyotropic liquid crystals,8,9 vesicles,10 sponge phase,11 and microemulsions,12 but few study about chiral aggregates in EAN has been reported. Green and environmental friendly sugar surfactants possess various excellent properties and are gradually developed as attractive amphiphilic molecules for constructing aggregates. Sugar surfactants are really inspiring to meet the demand of green chemistry.13 The favorable applications are mainly based on their specific structure characteristics including the multiple and directional hydroxyls and the extensive stereochemistry. Because of their structure characteristics, sugars are very essential building blocks of many components in living cells and organisms which are involved in interactions among cells such as the recognition.14 The multiple hydroxyls make sugar surfactants easily form hydrogen-bonding networks for realizing the molecular self-assembly.15 Attributed to their easy © XXXX American Chemical Society

formation of hydrogen-bonding networks, solvophobic interactions of tail chains, sugar surfactants with different polar heads are all good gelators for various solvents. They can form hydrogels, organogels, and ionogels, and the microstructures are almost all fibers and ribbons.16−18 Combining with the properties of room-temperature ionic liquids and gelators, the functional ionogels motivate increasing interest. The first self-assembling ionogels were reported by Kimizuka et al. in 2001.19 The structures of the ionogels formed by the glycolipid in ionic liquids are bilayer membranes. When the samples were heated, the structures transformed from fibrous assemblies to vesicles. The application fields of the ionogels involve electrochemical devices, solid membrane materials for gas separation, drug delivery and dye-adsorbing agents and so on.20−23 Among the applications, electrochemical applications such as solid electrolytes demanded in lithium batteries, dye sensitized solar cells and electrochemiluminescent devices draw the most attention for their good electroconductibility, nonvolatility and noninflammability.24−28 Compared to hydrogels or organogels, ionogels show excellent mechanical properties such as higher mechanical strength, tunable elastic response and rapid recovery ability after the destruction.29−31 In addition, ionic liquids are good lubricants with high-performance,32 therefore, ionogels are expected to be good lubricants combining the good lubricating quality of ionic Received: August 8, 2015 Revised: September 22, 2015

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DOI: 10.1021/acs.jpcb.5b07712 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

°C, a small amount of it was dropped on the specimen carrier and was inserted rapidly into the liquid ethane at −175 °C which was cooled with liquid nitrogen. To obtain the microstructure of the 10 wt % sample at high temperature, the sample was first heated to 80 °C and then was handled according to above steps. After that, under a condition of −175 °C and 10−7 Pa, samples were fractured and replicated by using the Leica EM BAF 060 equipment. Pt/C at 45° and C at 90° were sprayed onto the fracture surface forming a 2.5 nm thick film and a 18 nm thick film, respectively. The replicas were observed with a JEOL JEM-1400 electron microscope operating at an accelerating voltage of 120 kV. 2.6. Rheological Measurements. The rheological properties of samples with high viscosity were measured on a Haake RheoStress 6000 rheometer with a cone−plate sensor system (C35/1° Ti L07116, diameter: 35 mm, core angle: 1°). The constant temperatures were kept by using circulator HAAKE DC10 cyclic water bath (Karlsruhe, Germany). First, stress sweep measurements were performed at a fixed oscillatory frequency of 1 Hz to obtain a linear viscoelastic region. An appropriate stress was selected from the linear viscoelastic region to perform the oscillatory shear measurements in the frequency range of 0.1−100 Hz. The viscosity was determined by the steady shear under the CR mode. To obtain the thixotropy of samples, the shear rate was increased from 0.01 S−1 to 1000 S−1 and then decreased from 1000 S−1 to 0.01 S−1 in the steady shear measurements. 2.7. Tribological Tests. The tribological properties of EAN and ionogels were evaluated by using an Optimol SRV-IV oscillating reciprocating friction and wear tester. The frictional pairs were composed of the upper running ball (10 mm in diameter, AISI 52100 steel) and the lower stationary disk (ø 24 mm × 7.9 mm, AISI 52100 steel). All tests were achieved under the load of 50 N, the oscillation frequency of 25 Hz, the amplitude of 1 mm and the relative humidity of 30−50%. The test temperature was 25 °C and the friction curves were recorded by the SRV test rig. After the friction and wear tests, wear volume losses of lower discs were measured by a MicroXAM-3D surface mapping microscope profilometer.

liquids and the semisolid state property without problems of the liquid oil leak and evaporation loss. Reports about ionogel lubricants are rare.33 The main problem may be the nearly same lubricity of ionogels and pure ionic liquids. The urgent issue needing to be solved is the exploration of lubricity-enhanced gelators for pure ionic liquids. The exploration on the dependence of lubrication performance on the structure and rheological properties is also particularly important. In present work, we investigated the microcosmic structures and phase transition mechanism of sugar ionogels formed by NOctadecyllactobionamide (C18G2, Supporting Information (SI), Figure S1) in EAN using the differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), freeze-fracture transmission electron microscopy (FF-TEM), and rheological measurements. The tribological results indicate that the sugar surfactant is a promising lubricity-enhanced additive for ionic liquids. Our work could provide a deep understanding for various microcosmic structures of aggregates formed in ionic liquid medium. Meanwhile, the results help us to better understand the correlation between the structures and the tribological properties.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. N-Octadecyllactobionamide (C18G2, high purity) was a gift from Professor Dr. Heinz Hoffmann, Bayreuth University, Germany. The high purity was demonstrated by 1H NMR [Bruker Avance 400 MHz, [D6]DMSO, 25 °C, tetramethylsilane (TMS)], FTIR (VERTEX 70/70v) and ESI-MS (Q-TOF6510). (SI, Figures S2, S3, and S4). The chemicals were directly used without further purification. EAN was obtained according to the method described by Evans et al.34 The melting point was measured to be 14 °C, in good agreement with the former results.6,7,34 Its density is 1.2 g/cm3 at 25 °C, and the water content is less than 0.4 wt %, measured by Karl Fischer titration. The purity of EAN was proved by 1H NMR and 13C NMR spectra and FT-IR spectrum (SI, Figures S5 and S6). 2.2. Sample Preparation. Samples were obtained by weighting calculated amounts of C18G2 and EAN in glass vials. Then the mixtures were homogenized by heating to 80 °C, stirring until solids completely dissolved. Clear and transparent solutions were obtained. The solutions solidified to white and nontransparent gels when cooled to the room temperature within several minutes. Samples were kept at 25 °C for 4 weeks. 2.3. Differential Scanning Calorimetry (DSC) Measurements. DSC (DSC8500, PerkinElmer, USA) measurements were performed to determine transition temperatures of samples. The samples were measured in aluminum pans under the nitrogen flow and an empty aluminum pan was used as a reference. The samples were heated from 25 to 100 °C at 5 °C/min and the enthalpy changes associated with phase transitions were determined with the Pyris Software 5.0. 2.4. Small Angle X-ray Scattering (SAXS) Measurements. SAXS experiments were performed on the SAXSess mc2 X-ray scattering system (Anton Paar) operated at 50 kV and 40 mA. The distance of the sample to the detector was 264.5 mm and the X-ray wavelength used in this study was 0.1542 nm (Cu Kα). The exposure time for samples was 900 s. 2.5. Freeze-Fracture Transmission Electron Microscopy (FF-TEM) Observations. FF-TEM measurements were carried out to obtain microstructures of samples at different phase states. For the sample with 10 wt % concentration at 25

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. As seen from Figure 1, different phases formed by C18G2 in EAN were mapped. Below phase transition temperature, three phases were observed within studied concentrations. When the concentration of C18G2 is

Figure 1. Phase diagram for C18G2 in EAN, the phase transition temperature against C18G2 concentration. The pictorial insets represent different aggregate structures in different conditions. B


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Figure 2. (a) DSC curves for samples with different concentrations and (b) the enthalpy changes for the phase transition against the concentration of C18G2.

Figure 3. Typical SAXS patterns of lamellar phases at 25.0 ± 0.1 °C. The mass fraction of C18G2 is 10 wt % (a) and 30 wt % (b).

(a and b) and Figure S8 (SI), the ratio of scattering peaks is in keeping with 1:2:3 which corresponds well to the (001), (002) and (003) planes, typically indicating the lamellar phases at room temperature. The interlayer spacing (d) of bilayers was calculated from the first peak of SAXS pattern (d = 2π/q1) which can be used to speculate the arrangement of surfactant molecules in lamellar structures. Figure S9 (SI) shows the variation of the interlayer spacing with the C18G2 concentration. It is clear to see that the interlayer spacing is approximately 5.5 nm when the concentration ranges from 5 wt % to 25 wt % and it decreases to 3.5−4.5 nm when the concentration exceeds 30 wt %. The calculated length of one C18G2 molecule is 2.74 nm. Thus, two arrays could be suspected to interpret the interlayer spacing of lamellar phases as is shown in Scheme 1. When the concentration is less than 30 wt %, the interlayer spacing is about twice the length of one C18G2 molecule with fully extended hydrocarbon chain. The schematic representation of Scheme 1b presents the detail of arrays of C18G2 molecules. One can see that the chains are rigid and parallel and the Lβ phase is formed. The traditional Lβ phase is always observed in the presence of small amounts of water, because the untilted surfactant molecules with low hydration have smaller area occupied by the polar heads in the case.37,38 Obviously, one can see that the Lβ phase formed by C18G2 molecules in EAN is different from the common ones. Seddon et al. has reported that the tilted Lβ′ phase and rippled Pβ′ phase of the pure phosphatidylcholine are substituted for the Lβ phase in the system containing 1:2 (mol/mol) phosphatidylcholine/fatty acid mixtures. They believed that the formation of Lβ phase

lower than 4 wt %, a precipitate phase was obtained. With an increase of the C18G2 concentration, nontransparent ionogels form. The characterizations reveal that they are composed of two different lamellar structures, i.e., vertical and inclining closely packing bilayers. 3.2. DSC Data and Enthalpy Changes for Phase Transition. DSC results (Figure 2) show well-defined sharp endothermic peaks during heating scan which are associated with the chain melting temperature.35,36 Combined with the SAXS data (SI, Figure S7) which indicate that the phase transition temperature of the 10 wt % sample is between 55 and 60 °C, we can draw the conclusion that the chain melting temperature is in agreement with the phase transition temperature. It is noted that the enthalpy change increases initially with the increasing concentration, and then decreases slightly when the concentration exceeds 50 wt % (Figure 2). Two parts are included in enthalpy changes for the melting of lamellar ionogels, one is the transition for C18G2 molecules from the rigid solid chain state to the flexible melted chain state, and the other is the redistribution of EAN molecules. It is obvious that the enthalpy change increases with the increase of C18G2 concentration because more molecules need to change from the solid state to the melted state. Tiny changes in the arrangement of 60 and 70 wt % samples give rise to the slight decrease. Detailed explanation will be shown in the following part. 3.3. SAXS Measurements. To obtain the structure information on bilayer phases, we performed SAXS measurements and investigated effects of the C18G2 concentration and the temperature on lamellar structures. As seen from Figure 3 C


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deep understanding for various lamellar structures in ionic liquid medium. We noted in particular that samples are very sensitive to the temperature because they transformed soon from the fluid isotropic liquid to nontransparent gels when the samples were taken from the high temperature to the room temperature. SAXS measurements at the same temperature achieved by different ways show that the phase transition is completely reversible (SI, Figure S10). Similar results have been reported for liquid crystalline phases formed by the glycolipid in water.41 They measured the ratio of the vibronic peak intensities of the probe pyrene (I1/I3) by fluorescence method and found that values measured after cooling from the high temperature correspond well to those measured before heating. In another study, authors investigated the reversible phase transition for monolinolein and water binary system by SAXS measurements.42 They found that phase structures are in thermodynamic equilibrium with the continuous water bulk and can not be damaged by the heating or cooling process. Further experiments confirmed that the phase transition is indeed very sensitive to the temperature and can reach to the balance as soon as possible (SI, Figure S11). However, no pronounced difference can be seen from SAXS patterns of lamellar phases at four temperatures (SI, Figure S12), implying that the interlayer spacing is independent of the temperature. 3.4. FF-TEM Observations. Definite microstructures of both samples at the room temperature (Figure 4) and the high

Scheme 1. Visualization of Two Kinds of Lamellar Phases with Different Arraysa

(a) A model for one C18G2 molecule; (b) the Lβ phase; (c) the Lβ′ phase. a

instead of Lβ′ and Pβ′ phases in the range of all studied water content is attributed to the change in hydration degree of phosphatidylcholine molecules caused by the fatty acids, so they prefer the untilted packing.39 For the system, the paucity of more available data makes it difficult to confirm the detailed formation mechanism of Lβ phase with large amounts of solvent EAN. It is most likely because of the modification of the attractive and repulsive forces between head groups of surfactants caused by EAN.40 The free energy of transfer in EAN medium is different from the water medium,40 and it is also an important factor for the preference of untilted array. With an increase of C18G2 concentration, another arrangement is adopted and Lβ′ phase is formed (Scheme 1c). Similar to those in Lβ phase, molecules in Lβ′ phase are also stiff and fully extended. The most difference is that chains tilt at appropriate angles in regard to the normal to the lamellar plane. The terminal −CH3 groups of the opposite monolayers are aligned.37 If the thickness of the EAN layer is ignored, the hydrocarbon chain tilt angle (θ, see Scheme 1c) with respect to the bilayer normal can be roughly calculated. With the concentration ranging from 30 wt % to 50 wt %, the C18G2 molecules are almost at a fixed tilt angle of 48°. The tilt angle decreases to 45° for the 60 wt % sample. The smallest observed θ is 40° when the concentration increases to 70 wt %. That is, above the concentration of 50 wt %, the tilt angle decreases with the decreasing EAN content and consequently the interlayer spacing increases. It is similar to the previous report in aqueous solution that the tilt angle increases with the increase of the water content.37 The solvation degree decreases with the decreasing EAN content and it gives rise to the smaller tilt angle of C18G2 molecules. It can be speculated that the ideal extreme case at a high enough concentration is the formation of Lβ phase which is the traditional β phase with small amount of solvents. The adjustment of the phase structure leads to the decrease of the energy needed for the phase transition and interprets the decline tendency in Figure 2b well. In Figure 2a, the decrease in the phase transition temperature is observed when the concentration is higher than 50 wt %. We believe that the decline is also attributed to the decreasing tilt angle. The arrangement of the molecules makes it easier for them to move. Similarly, it is easier for the structures to transform when the samples are heated. Although different lamellar structures have been comprehensively studied in aqueous solution, their formation in ionic liquids is rare, our work could provide a

Figure 4. FF-TEM images of the lamellar phase of 10 wt % C18G2 in EAN at 25 °C. Image (b) shows a magnified area in image (a).

temperature (Figure 5) are visually demonstrated via FF-TEM observations. Terraced planar bilayers can be clearly observed for the sample at the room temperature. As for the 10 wt % sample, the interlayer spacing calculated from FF-TEM images by the method of statistic analysis is 8.38 ± 0.86 nm which is D


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easier to form the twisted ribbon instead of helical ribbon. It is because the ribbon possesses a lower free energy in this condition.47 Our results are consistent with above conclusions. C18G2 molecules can be tuned in their chain conformations by absorbing or releasing the heat, and then the change in the molecular conformation gives rise to different aggregate structures. Results shown in Figure 5 are obtained by inserting the sample into the liquid ethane before it became opaque and white gel. If the preparation for the sample at the high temperature is not as soon as possible, twisted ribbons with low twist degree (SI, Figure S13) were observed. We believe that the twist degree is closely related to the temperature. The system absorbs more energy at higher temperatures, consequently, twisted ribbons have high twist degree. The twist degree has a dependence on the temperature which has been studied by the previous report.48,49 In the case, the ribbon undergoes the transformation from the flat condition to the twisted ribbon with different twisted pitches then to the helical ribbon, and finally turns into a tube with the increase of temperature.48 In another report, authors found that the handedness of the aggregates is profoundly affected by the cooling rate which can be seen distinctly from the CD spectra.49 A proposed mechanism was shown in Scheme 2. At the adequately high temperature, the twisted ribbon is composed of

Figure 5. FF-TEM images of twisted ribbons formed by 10 wt % C18G2 in EAN at 80 °C. Image (b) shows a magnified area in image (a).

Scheme 2. Proposal Mechanism for the Formation of Different Aggregates by C18G2 in EAN

larger than the value (5.51 nm) obtained from the SAXS measurements. It is clearly seen from Figure 5 that twisted ribbons with right-handedness are formed by C18G2 in EAN at the high temperature. That is, D-form C18G2 molecules self-assembled into right-handed twisted ribbons in EAN via noncovalent interactions including hydrogen bonding and solvophobic interactions.43 Previous study has demonstrated that the multiple and directional hydroxyl groups in the sugar moiety greatly contribute to the hydrogen bonding networks. The amide group is also involved in the hydrogen bond formation and it benefits well to the molecular self-assembly.15 The statistical width of twisted ribbons is about 40−50 nm and a single bilayer is speculated to form the ribbon which is supported by the appearance of dark twisted edges with thickness of about 6 nm. Combining with interlayer spacing results from FF-TEM and SAXS measurements, we could conclude that values of thicknesses of edges are some what lager than the real thicknesses. As is mentioned above, a stretched and rigid conformation is adopted by the molecules at the room temperature. When temperature is higher than the chain melting temperature, a different conformation is expected for tail chains to accommodate to another aggregate structure. At the sufficiently high temperature, chains of C18G2 molecules are in liquid-like state, that is, a flexible and melted state. In fact, the one bilayer of the ribbon belongs to the Lα phase which is more analogous to the membranes. Different from the β type, the α type of lamellae is more disordered, consequently, the movement of molecules are more complex.37 It is well-known that the twisted ribbons have Gaussian or saddle-like curvatures and are closely related to the “fluid membrane” composed of melted hydrocarbon chains.44−46 The bilayer in the fluid phase is

a single fluid bilayer and the molecule chains are in melted states. The fluid bilayer can obtain more energy when the temperature is raised and the twist degree increases accordingly. With the decrease of the temperature, the bilayer sheets with dozens of nanometers can form lamellar structures by means of accumulation and grow in the size until the results observed in Figure 4, in which molecule chains adopt a rigid and fully stretched state. 3.5. Rheological Properties. As for ionogels with lamellar structures, the mechanical properties and viscoelasticity are important for guidelines of their practical and commercial applications.50 To estimate the mechanical strength of iongels, the measurements of stress sweep in oscillatory mode were carried out. We use the magnitudes of both the elastic modules (G′) and the yield stress (τ) to measure the mechanical strength. G′ refers to the plateau value of the stress sweep curve and τ refers to the stress when G′ drops sharply. One can see clearly from Figure 6 that the ionogels have very high mechanical strength and the order of magnitude of G′ values is 105 when the concentration exceeds 20 wt %. For the 50 wt % sample, the G′ value is about 4 × 105 Pa and the τ is about 4 × 103 Pa (SI, Figure S14). Compared to the hydrogel, the above results display the excellent mechanical strength of the ionogel. In previous work, the authors replaced the water of the E


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samples show good thixotropy which is really needed by the lubricant. As seen from Figure 7 and Figure S16 (SI), hysteresis loops are observed for samples. It leads to the conclusion that structures were destroyed during the shear process and could not recover back soon during the experimental time because of the lower viscosity values of down curves than those of up curves. It is known that, the area between the up and down curves is the measure of thixotropy, so we can draw the conclusion that the thixotropy of the same sample decreases with the increasing temperature. It may because of the lower viscosity caused by the higher temperature. 3.6. Tribological Properties. The specific lamellar structures and the good thixotropic property of ionogels inspired us to explore the lubricating properties. One has realized that EAN is not a good lubricating material in previous work,51 especially compared to those excellent imidazoliumbased ionic liquid lubricants,32 the satisfactory tribological results for C18G2/EAN ionogels were really unexpected and encouraging. The evolution of the friction coefficient with the time is shown in Figure 8, and the high friction coefficient of

Figure 6. Relationship between the elastic module (G′) and the concentration. T = 25.0 ± 0.1 °C.

hydrogel to ionic liquid and organic fluid to achieve the transition of the hydrogel to the ionogel or organogel. The frequency sweep results revealed that the G′ and the viscous modulus (G″) values for the ionogel were significantly greater than those of the hydrogel on multiples of 15 and 37, respectively, and the G′ and G″ values for the organogel were in the middle state.29 As shown in Figure 6, the mechanical strength becomes stronger with the increasing concentration which may contribute to the more dense structure packing. From 25 wt % to 50 wt %, both the G′ and the enthalpy change increase with the increasing concentration. However, the phase transition temperature and interlayer spacing d almost remain constant in the concentration range. We believe that the molecules adopt more dense arrangement with the increasing amount of the molecules. Meanwhile, the interaction forces including the hydrogen bonding and solvophobic interactions between both the molecules and the bilayers become stronger. Therefore, when the structures are destroyed, more heat is needed and it contributes to the increasing enthalpy change. The viscoelasticity and microcosmic structure information are revealed in oscillatory rheologies, as shown in Figure S15 (SI). The rheological properties are usually found in lamellar structure systems.35 The difference for two samples is ascribed to the different lamellar structures. Steady shear measurements were conducted to obtain the viscosity and thixotropic behavior of different samples. The shear rate was set to increase first and then decrease, that is, a cycle variation mode, to examine the thixotropic behavior of the samples. Surprisingly, all studied

Figure 8. Evolution of the friction coefficients with time for EAN and ionogels (applied load: 50 N; frequency: 25 Hz; stroke: 1 mm; duration: 10 min; temperature: 25 °C).

EAN (about 0.17) reveals its poor lubricating property. However, when the EAN was gelated by C18G2, its friction coefficient can be lowered pronouncedly especially for the sample with concentration of 8 wt %. The friction coefficient of the 8 wt % sample is about 0.1 which has already reached the normal level for ionic liquid lubricants. In the first report about ionogels as lubricants, the unfortunate reality is that the

Figure 7. Hysteresis phenomenon for samples at different temperatures. The mass fraction of C18G2 is 10 wt % (a) and 45 wt % (b). F


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surfactant molecules in a flexible liquid-like chain state. While fully stretched solid chain state is adopted when temperature is lowered to form lamellar structures. The concentration gives rise to two different lamellar structures including the Lβ phase and the Lβ′ phase, and the distinction of them consists in the tilt angle of surfactant molecules. We believe that this part of the study could give people a deep understanding for both the α type and the β type lamellaes in ionic liquid medium. Although the samples are very sensitive to the temperature, the interlayer spacing of lamellar structures is independent of the temperature. The C18G2/EAN ionogels display high mechanical strength and good thixotropy, and it is exactly needed by the lubricant. The tribological results reveal the better lubricating property and antiwear capability of the sugar ionogel compared with EAN, which presenting the fact that the sugar surfactant may be a promising lubricity-enhanced additive for ionic liquid lubricants. We believe that the ionogels formed by the sugar surfactant could be developed to effective lubricants which are green and environmental friendly. Encouraged by the successful outcomes, the subsequent work about sugar ionogels is methodically in progress.

tribological properties of ionogels and the ionic liquid are almost the same.33 The advantage for this system is that the ionogels formed by C18G2 in EAN can promote effectively the lubricating property of poor ionic liquid lubricant EAN. The conjecture for the enhanced lubricity is that the easy slide of sheets in lamellar structures during the sliding process contributes to the lubricity of ionogels. Another favorable factor may depend strongly on the hydroxyl-rich structure characteristic for the sugar surfactant. The coordination interaction between sugar surfactants and metal ions produced during the sliding process makes ionogels easy to produce a protective film on the substrate surface to further enhance their lubricating property.52 The results could indicate that the sugar surfactant may be a good lubricity-enhanced additive for ionic liquid lubricants. On the one hand, sugar surfactant molecules have special structure character, that is, abundant hydroxyls; on the other hand, it is easy to form the lamellar structure for the sugar surfactant in ionic liquids which is also found in our subsequent work (SI, Figure S17), and the formation mechanism is still being studied. To further investigate the antiwear capability of ionogels, wear volume losses of lower discs were measured and results are shown in Figure 9. Almost the same law for the wear

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b07712. Structural formula of C18G2, characterizations of C18G2 and EAN, additional data on characterizations of ionogels including SAXS, FF-TEM and rheological measurement results, and the 3D optical microscopic morphology of worn substrate surfaces (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel.: +86-531-88366074. Fax: +86531-88564750. Notes

The authors declare no competing financial interest.

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Figure 9. Wear volumes of steel discs lubricated by EAN and ionogels with different C18G2 concentrations for the duration of 30 min. Inserts are the 3D optical microscopic morphology of worn substrate surfaces lubricated by EAN and ionogels with 8 wt % C18G2, respectively.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21420102006 and 21273134).

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volume loss and the friction coefficient with the increasing concentration was found. More visual results can be obtained from the 3D optical microscopic morphology of worn surfaces in Figure S18 (SI). When the concentration is 8 wt %, the wear volume loss is the lowest, that is, the ionogel at this concentration has the most effective antiwear capability. The wear volume loss of the pure EAN is about 15 times of the 8 wt % ionogel. The moderate viscosity and thixotropy of the 8 wt % sample may interpret for its most effective lubricating and antiwear properties.

REFERENCES

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4. CONCLUSIONS In conclusion, this work shows the structure transition induced by the temperature and the concentration for a sugar surfactant with a disaccharide polar head in EAN. The surfactant molecules can tune the hydrocarbon chain conformations by absorbing and releasing heat to adapt to different aggregate structures. When temperature is higher than the chain melting temperature, right-handed twisted ribbons are formed by G


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Ionogels of Sugar Surfactant in Ethylammonium Nitrate: Phase

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