Strong Tendency of Homeotropic Alignment and Anisotropic Lithium

Aug 26, 2013 - Karel Goossens , Kathleen Lava , Christopher W. Bielawski , and Koen Binnemans. Chemical Reviews 2016 116 (8), 4643-4807. Abstract | Fu...
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Strong Tendency of Homeotropic Alignment and Anisotropic Lithium Ion Conductivity of Sulfonate Functionalized Zwitterionic Imidazolium Ionic Liquid Crystals Rohini Rondla,† Joseph C. Y. Lin,† C. T. Yang,† and Ivan J. B. Lin*,† †

Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan S Supporting Information *

ABSTRACT: Here, we report the first attempt to investigate the liquid crystal (LC) behavior of SO3− functionalized imidazolium zwitterionic (SO3−ImZI) salts, which display homeotropic alignment on a glass slide without the aid of any aligning approach. Doping lithium salt to ImZI salts lowers the melting temperatures and raises the clearing temperatures substantially to form room temperature ImZILCs. Excellent anisotropic lithium ion conductivity is achieved; which is strengthened by their tendency for homeotropic alignment.

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INTRODUCTION Zwitterionic-type ionic liquids (ZILs) constitute a unique subclass of ILs composed of both positive and negative charge in an intramolecular form.1 They can dissolve a variety of inorganic salts due to their high dipole moment and are thus proven to be excellent ion dissociators, useful for liquidizing solid salts or strong acids.2−8 They have been reported as good ion carriers for electrochemical applications due to their lack of component ion-pair migration.3−9 ZILs with suitable design could provide good target ion transporting abilities to overcome the disadvantages of conventional ILs, such as their low target ion transport number and lack of selectivity, as reported by Ohno and co-workers.3 Over the past decade, task specific sulfonate (SO3−) functionalized zwitterionic imidazolium ionic liquids (SO3−ImZILs) have attracted great interest for use as ion conductive matrixes or electrolytes for batteries,2,10−13 antifouling thin films,14 nanoparticle stabilizers,15 catalysts,16−18 N-heterocyclic carbene ligands,19−21 ion exchange membranes,22,23 and for separation processes.24,25 The oxygen atoms of the SO3− group are negatively charged and, in principle, can coordinate with positive ions. It has been reported that the SO3− groups are good exchange sites of lithium ions, because it is easy for Li+ to associate or dissociate with the SO3− group and thus play a role in transfering lithium ions from one site to another.2,26,27 The good proton exchange ability of SO3−ImZILs/brønsted acid mixtures is also noteworthy.28,29 Recently, significant proton conduction efficiency was achieved for a liquid crystal (LC) of 4-(octadecyloxy) phenylsulfonic acid with the help of an aligning agent.30 A recent survey of the literature reveals that preparing highly ordered materials and achieving uniform molecular alignment are challenging factors in fulfilling a range of applications, © 2013 American Chemical Society

including those in transportation, mechanical, biotechnological, molecular electronics and optoelectronic devices, and LC display manufacturing.31 Herein we report the first attempt to investigate the LC behavior of SO3−ImZI salts, which display a strong tendency to align perpendicularly on a glass surface, presumably due to the favored hydrogen bonding interaction between the glass substrate and hydrophilic SO3− group and the high local symmetry around the S atom. These highly aligned SO3− ImZILCs show excellent lithium ion transporting ability.

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EXPERIMENTAL SECTION

Chemicals and Instrumentations. Chemicals and solvents were purchased from Aldrich and were used as received, unless otherwise specified. The 1H NMR and13C NMR spectra were recorded on Bruker AVANCE DPX-300 spectrometer operated at 300 MHz and 75 MHz, respectively. Elemental analysis was carried out using PerkinElmer 2400 elemental analyzer. DSC analysis was performed using a Mettler Toledo DSC 822 differential scanning calorimeter. The decomposition temperatures were determined by thermogravimetric analysis using a Mettler Toledo TGA851. Optical studies were carried out using Zeiss Axioplan-2 imaging polarizing microscope equipped with a Mettler Toledo FP 82HT hot stage and a FP 90 processor. Powder X-ray diffraction data were collected on Bruker D8 diffractometer from National Dong Hwa University. The ionic conductivities were measured with three types of cells: comb-shaped gold electrodes reported by Kato et al,32 gold plates, and ITO cell. Synthesis. Two series of sulfonate (SO3−) functionalized zwitterionic imidazolium ionic liquids (SO3−ImZILs); denoted as [Cn−Im−C3SO3] (n = 12, 14, 16, and 18) and [Cn (2-OH)−Im− Received: June 20, 2013 Revised: August 3, 2013 Published: August 26, 2013 11779

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are shown in Table 1. In the [Cn−Im−C3SO3] series, the n = 12 salt shows melting and clearing temperatures at 85.2 and

C3SO3] (n = 12, 14, 16, and 18), and their lithium doped mixtures are synthesized. 1-Alkylimidazoles and 1-(2-hydroxyalkyl) imidazoles were initially prepared as reported.33−36 Then the commercially available starting material 1,3-propanesultone was selected to react with them through ring-opening substitution to give the zwitterionic salts. Scheme 1 illustrates the synthetic route for SO3−ImZILs. A typical

Table 1. Phase transition Temperatures [°C] of [Cn−Im− C3SO3] (n = 12, 14, 16, and 18) salts and [Cn−Im−C3SO3] + LiClO4 (6:1) Mixtures during Heating Process Determined by DSC at a Rate of 10 °C min−1a,b

Scheme 1. Illustration of the Synthetic Route for Imidazolium Based Zwitterionic Salts

experimental procedure for each of the series is given below. The overall synthetic procedure presented here is convenient and easy to purify, and also provides good product yields. The physical properties and characterization details of these compounds are summarized in the Supporting Information, SI. Preparation of [C18−Im−C3SO3]. 1,3-Propane sultone (0.24 g, 2.0 mmol) and 1-octadecylimidazole (0.64 g, 2.0 mmol) were mixed in a round-bottom flask and stirred at 100 °C. After 24 h, the reaction mixture was cooled down to room temperature. The crude product was recrystallized by CH2Cl2/ether for three times. The resulted product was dried by a vacuum system to give white solid product [C18−Im−C3SO3] of yield 96%. Results of 1H NMR characterization and elemental analysis are given as follows: white solid, 1H NMR (300 MHz, DMSO-d6, ppm): δ = 9.05 (s, 1H, CH), 7.76 (m, 2H, CH), 4.29 (t, 3J = 7.0 Hz, 2H, CH2), 4.13 (q, 3J = 7.0 Hz, 2H, CH2), 2.40 (t, 3J = 7.0 Hz, 2H, CH2), 2.08, (t,3J = 7.0 Hz, 2H, CH2), 1.79 (m, 2H, CH2), 1.22 (m, 30H, CH2), 0.83 (t, 3J = 6.4 Hz, 3H, CH3). Calcd. For C24H46N2O3S·H2O: C 62.57, H 10.50, N 6.08. Found: C 62.35, H 10.24, N 6.35. A similar procedure was followed for compounds of different alkyl chain lengths. Preparation of [C18 (2-OH)−Im−C3SO3]. Equal amounts of 1-(2hydroxyoctadecanyl) imidazole (MW = 336.31, 1.0 g, 1equiv) and 1, 3-propanesultone (MW = 122.14, 037 g, 1 equiv) were placed in a 50 mL round-bottom flask and mixed in 30 mL acetone at room temperature. The reaction mixture was stirred for 24 h. The solid was collected by filtration and rinsed several times by ether to obtain a dry white powder solid of yield 90%. 1H NMR (400 MHz, DMSO-d6): δ = 8.99 (s, 1H, CH), 7.67 (s, 1H, CH), 7.60 (s, 1H, CH), 4.98 (s, 1H, OH), 4.34 (t, 3J = 7.0 Hz, 2H, CH2), 4.22 (dd, H, CH2), 3.96 (dd, H, CH2), 3.76 (s, 1H, CH), 2.14 (m, 2H, CH2), 1.39−1.24 (m, 30H, CH2), 0.85 (t, 3J = 7.0 Hz, 3H, CH3). Anal. Calcd for C24H46N2O4S: C, 62.84; H, 10.11; N, 6.11 Found: C, 62.89; H, 10.31; N, 6.00. A similar procedure was followed for compounds of different alkyl chain lengths.

(ΔH in kJ mol−1 in parentheses) [2nd cycle]. bCr = crystal phase; S,S1, S2 = soft phase; SmA = smetic A mesophase; I = isotropic; d = decompse; a = observed by POM. a

139.2 °C respectively in the heating process. Salt of n = 14 has two soft crystal phase transitions (S1 and S2 of the SI) and salts of n = 16 and 18 have one soft crystal phase transition (S) before melting into SmA phase. The melting and clearing temperatures of these n = 14, 16, and 18 salts are 73.6, 82.3, 89.1 and 220, 250, 270 °C, respectively. All of these salts show wide mesophase range and that increases with increasing alkyl chain length. Addition of LiClO4 (6:1 molar ratio) induces them to form room temperature liquid crystals (RTLCs) or near RTLCs and thus enhances the mesophase ranges. In the [Cn(2-OH)−Im− C3SO3] series, salts of n = 12, 14, 16, and 18 show LC behavior in the heating process with the melting temperatures at 153.8, 155.2, 154.6, 155.7 °C and clearing temperatures at 206.4, 250, 260, 270 °C, respectively. The mesophase range of these salts increases with increasing alkyl chain length. Li doped mixtures of these salts show slightly reduced melting temperatures and slightly raised clearing or decomposition temperatures (Table 1). However, the melting temperatures of the [Cn(2-OH)− Im−C3SO3] series of salts are significantly higher than the [Cn−Im−C3SO3] series of salts, which can be attributed to the

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RESULTS AND DISCUSSION Thermotropic Liquid Crystal Properties. Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) studies were employed to study the thermotropic phase behavior of these SO3−ImZILs and their lithium perchlorate (LiClO4) adducts. Phase transition temperatures for two series of SO3−ImZI salts and their Li doped mixtures 11780

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Figure 1. Polarized optical microscopy image of (a) [C12−Im-C3SO3] at 195 °C on plane glass and (b) the XRD pattern of [C16−Im−C3SO3] in the mesophase at 120 °C.

Table 2. PXRD Data for [Cn−Im−C3SO3] and [Cn(2-OH)− Im−C3SO3] Series of Zwitterionic Saltsa

possibility of additional hydrogen bonding interactions in the presence of hydroxyl group. These compounds are thermally unstable at high temperature and are partially decomposed. Under POM, these salts show a high degree of homeotropic phenomenon upon cooling from isotropic, suggesting the strong tendency toward uniform alignment of the molecules on the glass. This alignment is retained even after repeated heating and cooling cycles. A typical example of the POM image for the Cn−Im−C3SO3 series of salts is shown in Figure 1(a). It is noteworthy that this perpendicular alignment occurs easily without any pretreatment of the glass. These results may be due to the good hydrogen bonding of the SO3− group with the glass surface hydroxyl groups. Similar homeotropic black domains also appeared for Li doped SO3−ImZILCs mixtures, as shown in Figure 1(a). This well-aligned homeotropic LC medium is very important because it provides a regular pathway for transportation of target ions. Diffractograms for these compounds in the mesophase show the characteristic SmA pattern with a strong (001) and weak (002) reflections at the small angle region and a halo at the middle angle region. A typical diffractogram is shown for [C16− Im−C3SO3] in Figure 1(b). The crystal phase d-spacings of all these compounds show a linear increment with chain length (n), indicating a similar structure for all these compounds (Table 2). Diffractograms at the crystal phases show that the dspacing of these salts are longer than the calculated molecular length. For example, the calculated molecular length for C12 salt is 21.3 Ao (based on Chem 3D software) and the diffractogram shows the d-spacing of 33.3 Ao, which suggests the bilayer fashion. A schematic illustration of smectic structures of SO3−ImZILC and the Li doped SO3−ImZILC mixtures are represented in Figure 2. A diffractogram of the C16−Im− C3SO3+ LiClO4 (6:1) mixture shows two reflections at the small angle region. The d-spacing for C16−Im−C3SO3 is 35.3 Ao, and that for the C16−Im−C3SO3+ LiClO4 (6:1) mixture is 40.0 Ao. A PXRD diffractogram for the C16−Im−C3SO3+ LiClO4 (6:1) mixture is included in the SI. Single crystal data reported for 1-methylimidazolium-3-propanesulfonate shows that the negatively charged sulfonate group is surrounded by four imidazolium head groups with six close contacts of C− H...O hydrogen bonding network,37 therefore repulsion between SO3− ions is relaxed. This suggests that the increased d-spacing in the lithium doped mixture is probably due to

Cn−Im−C3SO3]

Cn

temp (°C)

d-spacing (Ao)

phase

12

RT 120 RT 120 RT 120 RT 120 RT 180 RT 180 RT 180 RT 180

33.3 29.4 37.5 32.2 38.0 35.3 38.6 37.6 25.46 29.42 28.01 32.10 30.42 34.62 33.31 37.56

soft SmA soft SmA Cr SmA Cr SmA Cr SmA Cr SmA Cr SmA Cr SmA

14 16 18 [Cn(2-OH)−Im−C3SO3]

12 14 16 18

a

SmA: smectic A; Cr: crystalphase ; Soft: high order mesophase.

Figure 2. Schematic illustration of smectic structures and increment of d-spacing from (a) SO3−ImZILC to its (b) lithium doped mixture.

opening of the C−H...O hydrogen bonding network, allowing coordination with the lithium ions. Ion Conductivity. Temperature dependent ionic conductivities of [Cn−Im−C3SO3]/LiClO4 (6:1), and [Cn(2OH)−Im−C3SO3]/LiClO4 (6:1) are measured by three types of cells: comb shaped gold electrodes, gold plates, and indium tin oxide (ITO) electrodes. The samples in gold finger electrodes cell show homeotropic alignment under POM. In 11781

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Figure 3. Temperature dependence of the ionic conductivity for [Cn−Im−C3SO3]/LiClO4 (n = 12, 14, 16 and 18) (ratio of 6:1), and [C3SO3ImCn (2-OH)]/LiClO4 (n = 12,14,16, and18) (ratio of 6: 1) mixtures measured in gold finger electrode cell shown in (a) and (b); and in ITO cell shown in (c) and (d), respectively. Part (e) shows the comparison of conductivity results measured for the [C12−Im−C3SO3]/LiClO4 (6:1) mixture with gold finger electrodes, gold plates, and ITO cell. 11782

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103 times higher than those for the σi⊥ values between the gold plates and the ITO cell. These results suggest that the formation of long-range conducting pathways in the homeotropically aligned smectic mesophases are contributing high ion conductivities.38,39 SO3−ImZI salt could enhance the dissociation of lithium salt and allow the high lithium ion transport numbers needed for enhanced conductivities. Previously, Ohno and co-workers reported the use of SO3−ImZI salts as dissociation-enhancers for LITFSI and also reported good ionic conductivity results for polymer gel electrolytes by adding 1-ethylimidazolium-3propane (or butane) sulfonate/LiTFSI mixtures.3,4 The electrolytes containing mixtures of model electrolyte (LiPF6 in a mixture of EC, DMC, EMC solvents)/SO3−ImZI salt with an ether group at C-2 position exhibited good conductivity.40 Low melting oxyethylene functionalized SO3−ImZIL/LiN(Tf2) mixtures have also been reported with good conductivity results.41 However, it should be noted that the self-assembled SO3−ImZILC/LiClO4 smectic mixtures used in our study show greater ionic conductivity than those reported SO3− based imidazolium zwitterionic mixtures. Recently, good anisotropic proton conduction through lyotropic columnar LCs of phosphonium type SO3−zwitterions/HTf2N mixtures has been investigated; although it includes the role of water.42 We assume that a well-aligned ZILC medium or selforganization of ZILCs is preferable for target ion transportation. The oxygen atoms of the SO3− group can coordinate easily with the Li+ ions. This complex is labile, and thus provides ordered ionic channels for transportation of target ions.

this case, the ion conductivity along with the direction parallel (σi∥) to the smectic layer plane has been measured. For the series of [Cn−Im−C3SO3] /LiClO4 (6:1) mixtures, ionic conductivities increase gradually with increasing temperature in the mesophase (Figure 3(a)). The ionic conductivities for [Cn(2-OH)−Im−C3SO3]/LiClO4 (6:1) mixtures shown in (b) are from soft materials to liquid crystal phase. Their structural alteration and viscosity are probably not different enough upon phase transition, therefore the ionic conductivities increase gradually with increasing temperature with no apparent changes. The highest ionic conductivity is obtained for the mixture of [C12−Im−C3SO3] /LiClO4 (6:1); this can be attributed to its room temperature liquid crystal nature. Further, using ITO electrodes, the ion conductivity along with the direction perpendicular (σi⊥) to the smectic layer plane has been measured for all of these mixtures. The results are shown in Figure 3, parts (c) and (d). The mixtures in ITO cell show fan shaped texture with small homeotropic domains, indicating the formation of polydomain alignment. Ionic conductivities measured in ITO cell are lower than that obtained from the gold finger electrode cell. However, the ion conductivity along with the direction perpendicular (σi⊥) to the smectic layer plane has been measured for [C12−Im−C3SO3]/ LiClO4 (6:1) mixture by a cell with gold plates. This result is shown in Figure 3(e). It should be noted that the ionic conductivity measured from gold finger electrode cell is significantly higher than that between the gold plates. Thus, it is reasonable to expect that the high anisotropic conductivity has a contribution from the highly ordered monodomain orientation of the smectic layers between the two glass substrates in the gold finger electrode cell, in which ionic channels along the transverse direction provide effective migration of Li+ ions from one site to another (Figure 4). As shown in Figure 3(e), the ionic conductivity of the [C12−Im− C3SO3] /LiClO4 (6:1) mixture in three types of cells is on the order of (σi∥ gold finger electrodes) > (σi⊥ gold plates) > (σi⊥ ITO cell). The σi∥ values in the gold finger electrode cell are nearly 102−

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CONCLUSIONS Herein we report the first attempt to investigate the LC behavior and anisotropic Li+ ion transporting ability of SO3−ImZILCs. Highly ordered homeotropic molecular alignment is achieved on the glass surface without any pretreatment of the glass. Addition of LiClO4 promotes the formation of RTLC systems. It is worthwhile to investigate the target ion transportation in the well-aligned ZILC medium. Effective interactions between Li+ ions and SO3− group through wellordered pathways lead to the enhancement of the ionic conductivities. We thus expect that this type of SO3−ImZILCs could be an excellent choice for target ion carrier in the electrochemical applications and also may help in the design of similar ZILCs for further investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The characterization details of imidazolium zwitterionic salts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +886-3-863-3570; e-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

Figure 4. Schematic illustration of anisotropic Li+ ion conductivity via ordered channels formed by SO3−ImZILCs.

The authors declare no competing financial interest. 11783

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ACKNOWLEDGMENTS We thank the National Science Council (NSC) of Taiwan for financial support of this work (NSC 101-2113-M-259-005MY2), and National Dong Hwa University Nano-Science and Technology Research Center for providing research facilities. We also thank Jing C. W. Tseng for some of the ion conductivity measurement.

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dx.doi.org/10.1021/la402336n | Langmuir 2013, 29, 11779−11785