Design, Synthesis, and Analysis of Thermophysical Properties for

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Design, Synthesis, and Analysis of Thermophysical Properties for Imidazolium-Based Geminal Dicationic Ionic Liquids Hang Zhang, Mingtao Li, and Bolun Yang* Department of Chemical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China S Supporting Information *

ABSTRACT: To enhance the thermal stability of ionic liquids (ILs) and increase the latent heat, the effect of amount of hydrogen bonds for geminal dicationic ionic liquids (DILs) was investigated and compared to that of monocationic analogues. A series of geminal dicationic ionic liquids with alkyl chain or electronegativity functional groups in the imidazolium were synthesized. Thermal stability was determined by TGA; melting point, heat of fusion, and heat capacity were investigated by DSC for synthetic DILs. The effect of molecular structure on the heat of fusion was examined by changes alkyl side-chain, linkage chain, C2−H of imidazole ring, and functional groups. Hydrogen bonding in DILs was studied, in the case of C2(eim)2(Br)2, by single-crystal X-ray diffraction. The thermal analysis results indicate that functionalized geminal dicationic ionic liquids show excellent thermal stability. The decomposition temperatures of geminal dicationic ionic liquids can be up to 603.74 K, and the latent heat can reach 159.35 J g−1. It is increased on average by 64.5% and 212.5%, respectively, as compared to alkyl chain ionic liquid (C4mim)Br. It can be expected that these geminal dicationic ionic liquids are suitable for thermal storage applications. PCMs. The largest ΔHf = 153 J g−1 was for [C16mim]Br at Tm = 372 K. Vijayaraghavan et al.13 have described a range of protic salts, some of which exhibit high heats of fusion (such as guanidinium methanesulfonate [Gdm][Ms] ΔHf = 190 J g−1), making them suitable as PCMs for TES application. More recently, our research group14 found that increasing the hydrogen-bond energy may be the key point to improve the heat of fusion of ionic liquids. Therefore, a series of functionalized ILs were synthesized by introducing an ester group, hydroxyl group, and carboxyl group to the alkyl chain of the imidazolium cation. The experimental results indicate that the heat of fusion and heat capacity of these functionalized ionic liquids increased by 34% and 86.5%, as compared to those of alkyl chain ILs. This work has indicated that the heat of fusion of ionic liquids largely depends on the cation−anion interactions. It is well-known that ionic liquids have the cation−anion interactions and mainly include the hydrogen-bond energy and electrostatic energy, as well as van der Waals interactions.15 Generally, the hydrogen bond is formed between a hydrogen atom and an electronegativity atom. Therefore, on one hand, introducing more H atoms in the cation of ionic liquids and halide anions can form more hydrogen bonds to increase the hydrogen-bond energy. On the other hand, introducing functional groups with electronegativity atoms in alkyl side-

1. INTRODUCTION Recently, a green and efficient energy conservation technology, phase change storage, has been promising and attractive for thermal energy storage (TES). Thermal energy transfer occurs when a phase change material (PCM) undergoes a change from the solid to liquid state or vice versa.1−3 Therefore, PCM is the core of this technology, and traditional PCMs include organic and inorganic PCMs.4 However, their inherent disadvantages are visible. For example, organic PCMs are generally low melting, volatile, and flammable, which can cause considerable safety concerns.5 For inorganic PCMs, they often suffer from supercooling, phase separation, and strong corrosive action, which can affect their thermal-storage capacity.6 These shortcomings impede the development of phase change storage technology. Hence, searching for new PCMs to overcome these shortcomings is important. Ionic liquids are molten organic salts that are composed of organic cation and inorganic anion with a wide liquidus range, and are typically chemically stable, relatively nonvolatile, and nonflammable, thus making them attract much attention as promising PCMs.2,7−11 Besides, their properties can be adjusted by changing the anions, cations, or alkyl substituents in the cations. Yet surprisingly, little research has been devoted to ionic liquids as PCMs. One of the few studies mainly focused on the monocationic imidazolium ionic liquids as PCMs. Zhu et al.12 have reported a series of imidazolium ILs of different alkyl and found that the heat of fusion increases with increasing alkyl chain length, which illustrates long-chain alkylimidazolium is suited for as © XXXX American Chemical Society

Received: September 19, 2017 Revised: December 28, 2017 Published: January 22, 2018 A

DOI: 10.1021/acs.jpcc.7b09315 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C chain of ionic liquids can also form more hydrogen bonds. In addition, the electrostatic energy of the ionic liquid depends on the charge distribution of ions. Thus, increasing the charge density of ionic liquids by increasing the number of cation can be considered an efficient approach to improve electrostatic energy. On the basis of the above discussion, a series of geminal dicationic ionic liquids (DILs) were synthesized in this work. The geminal dicationic ionic liquids consist of two head groups linked by a rigid or flexible spacer and two monoanions. They possess a wider liquid range and higher electrostatic energy than in monocationic ILs.16−19 Moreover, the geminal dicationic ionic liquids contain more H atoms in cation and two halide anions, and then can form more hydrogen bonds. Therefore, an increase of the thermal storage density of the ionic liquid and a fundamental strategy to construct new functional imidazolium geminal dicationic ionic liquids can be achieved.20

Scheme 2. Schematic Representation of the Synthesis of Geminal Dicationic Ionic Liquids with Dimethyl SideChains

Scheme 3. Schematic Representation of the Synthesis of Geminal Dicationic Ionic Liquids with Hydroxy or Carboxylic Side-Chains

2. EXPERIMENTAL SECTION 2.1. Materials. 1-Methylimidazole, 1-ethylimidazole, 1propylimidazole, 1-buthylimidazole, and 1-vinylimidazole (≥99 wt %) were vacuum distilled prior to use. 1,2Bibromoethane and 1,4-dibromobutane (≥99 wt %) were distilled before use. Methanol, ethanol, ethyl acetate, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF) (≥99 wt %) were distilled and then stored over molecular sieves in tightly sealed glass bottles, respectively. Imidazole was recrystallized twice from water and was dried under reduced pressure. 2-Bromoethanol and bromoacetic acid (≥99 wt %) were purchased from the Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis of Geminal Dicationic Ionic Liquids. Geminal dicationic ionic liquids with alkyl side-chains were synthesized according to Scheme 1 by mixing 0.2 mol of 1-

and water was removed under reduced pressure. The crude products were purified by washing with diethyl ether and dried under a vacuum for 24 h at 323.15 K. 1,4-Bis(imidazole-1yl)butane was obtained as white needle-like crystals. Next, 1,4bis(imidazole-1-yl)butane (1.90 g, 10 mmol) and 2-bromoethanol (5.60 g, 44.8 mmol) or bromoacetic acid (8.32 g, 60 mmol) were dissolved in DMF (10 mL), and the mixture was stirred at 323.15 K for 3 days. The reaction mixture was precipitated in diethyl ether (100 mL, twice), to obtain imidazolium bromide as a yellowish solid (3.8 g, 86.4%). The product was then further dried in a vacuum oven for 24 h. 2.3. Characterization. The samples were characterized by NMR and FT-IR. 1H NMR spectra were taken on a Bruker ACANCE II NMR spectrometer, using DMSO as the solvent and tetramethylsilane (TMS) as the internal standard. Fourier transform infrared (FT-IR) spectra of the samples were obtained using a Thermo Nicolet AVATAR-360 FT-IR spectrophotometer at a scanning number of 30 with the KBr sampling method.21 2.4. Thermal Analysis. Thermogravimetric analysis (TGA) dates were obtained in an alumina (Al2O3) pan on a TA Instrument. The heating rate was 10 K/min and the N2 flow rate was 60 mL/min, ranging from 303 to 873 K. Differential scanning calorimetry (DSC) experiments performed in sealed Al pans were carried out on a Q1000, a TA Instruments apparatus. Samples were heated from 303 to 623 K and at a scan rate of 5 K/min under a nitrogen atmosphere.22,23 2.5. X-ray Analysis. A crystal of the geminal dicationic salt was grown, and its structure was determined. The diffraction data of C2(eim)2(Br)2 were measured at room temperature using Mo Kα radiation on a Bruker APEX II CCD diffractometer equipped with a kappa geometry goniometer. The data set was reduced by EvalCCD and then corrected for absorption. The solution and refinement were performed by SHELX. The crystal structure was refined using full-matrix least-squares based on F2 with all non-hydrogen atoms anisotropically defined. Hydrogen atoms were placed in calculated positions by means of the “riding” model.16,20

Scheme 1. Schematic Representation of the Synthesis of Geminal Dicationic Ionic Liquids with Alkyl Side-Chains

alkylimidazole and 0.11 mol of 1,2-dibromoethane or 1,4dibromobutane in ethanol (15 mL) at 343.15 K for 24 h in a N2 charged round-bottom flask. Next, ethanol was removed from the product under vacuum at 323.15 K. Any excess starting materials were extracted three times with 15 mL of ethyl acetate. The product was then further dried in a vacuum oven for 24 h at 323.15 K.16 Geminal dicationic ionic liquids with dimethyl side-chains were synthesized according to Scheme 2 in a manner analogous to that used for synthesizing 1,2-bis(3-methylimidazolium-1-yl) ethane bromide. First, 1,4-bis(imidazole-1-yl) butane (BIm) was prepared according to Scheme 3. Imidazole (34 g, 0.5 mol), NaOH (20 g, 0.5 mol), and DMSO (60 mL) were placed into a flask. The mixture was heated at 333.15 K. 1,4-Dibromobutane (54 g, 0.25 mol) was added, and the mixture was stirred at 353.15 K for 4 h. After the removal of sodium bromide and DMSO by filtration and distillation, respectively, the liquid mixture was poured into 100 mL of water. Finally, the mixture was filtered, B

DOI: 10.1021/acs.jpcc.7b09315 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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MHz, D2O, ppm): δ 144.32, 122.73, 120.70, 47.53, 34.73, 25.72, 8.88. IR (500−4000 cm−1, KBr pellet): 3070, 2110, 1630, 1540, 1260, 1150, 760, 625. 1,4-Bis(imidazole-1-yl) Butane. BIm was obtained as a white solid. 1H NMR (400 MHz, DMSO-d6, ppm): δ 7.61 (s, 2H), 7.14 (d, 2H), 6.88 (d, 2H), 3.96 (t, 4H), 1.66−1.57 (m, 4H). 13 C NMR (101 MHz, DMSO-d6, ppm): δ 137.67, 128.87, 119.69, 45.71, 28.14. 1,4-Bis(3-ethoxylimidazolium-1-yl) Butane Bromide. C4(C2H5Oim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.31 (s, 2H), 7.86 (d, 2H), 7.81 (d, 2H), 5.18 (m, 2H), 4.28 (t, 4H), 4.24 (s, 4H), 3.73 (m, 4H), 1.82 (s, 4H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 137.10, 124.09, 122.71, 59.65, 52.15, 48.35, 26.49. 1,4-Bis(3-carboxymethyl-imidazolium)-1-yl Butane Bromide. C4(C2H3O2im)2(Br)2 was obtained as a slightly yellow solid. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.34 (s, 2H), 7.88 (d, 2H), 7.77 (d, 2H), 4.32 (t, 4H), 3.76 (m, 4H), 1.82 (s, 4H), 14.34 (t, 2H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 136.64, 122.47, 120.26, 47.98, 26.86. The NMR technology can effectively detect the density of electron cloud around the nucleus. Ionic liquids were first analyzed by 1H NMR spectroscopy, and a summary of the chemical shift values for the H imidazolium ring proton is provided in Table 1. The results showed that chemical shift

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of ILs. 1,2-Bis(3methylimidazolium-1-yl) Ethane Bromide. C2(mim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 8.71 (s, 2H), 7.43 (d, 2H), 7.47 (d, 2H), 3.82 (m, 10H). 13C NMR (101 MHz, D2O, ppm): δ 136.88, 124.77, 122.51, 48.83, 36.15. IR (500−4000 cm−1, KBr pellet): 3080, 1760, 1650, 1560, 1170, 1130, 760, 617. 1,2-Bis(3-ethylimidazolium-1-yl) Ethane Bromide. C2(eim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 8.78 (s, 2H), 7.52 (d, 2H), 7.38 (d, 2H), 4.17 (m, 8H), 1.38 (t, 6H). 13C NMR (101 MHz, D2O, ppm): δ 135.76, 123.42, 122.52, 48.66, 45.27, 14.30. IR (500−4000 cm−1, KBr pellet): 3150, 1780, 1670, 1560, 1330, 1160, 795, 636. 1,2-Bis(3-propylimidazolium-1-yl) Ethane Bromide. C2(pim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 8.72 (s, 2H), 7.47 (d, 2H), 7.37 (d, 2H), 4.01 (m, 8H), 1.71 (t, 4H), 0.70 (t, 6H). 13C NMR (101 MHz, D2O, ppm): δ 135.72, 123.65, 122.50, 51.59, 48.86, 22.80, 9.90. IR (500−4000 cm−1, KBr pellet): 3070, 1760, 1650, 1560, 1350, 1170, 764, 636. 1,2-Bis(3-butylimidazolium-1-yl) Ethane Bromide. C2(bim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 8.76 (s, 2H), 7.52 (d, 2H), 7.42 (d, 2H), 4.13 (m, 8H), 1.73 (t, 4H), 1.15 (m, 4H), 0.80 (t, 6H). 13C NMR (101 MHz, D2O, ppm): δ 135.75, 123.64, 122.07, 49.79, 48.89, 30.92, 18.58, 12.50. IR (500−4000 cm−1, KBr pellet): 3070, 1740, 1650, 1560, 1440, 1170, 791, 636. 1,4-Bis(3-methylimidazolium-1-yl) Butane Bromide. C4(mim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.34 (s, 2H), 7.87 (d, 2H), 7.77 (d, 2H), 4.27 (t, 4H), 3.77 (m, 4H), 1.81 (s, 6H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 137.10, 124.09, 122.71, 48.35, 36.30, 26.49. IR (500−4000 cm−1, KBr pellet): 3070, 2050, 1630, 1560, 1330, 1170, 791, 625. 1,4-Bis(3-ethylimidazolium-1-yl) Butane Bromide. C4(eim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 8.72 (s, 2H), 7.43 (d, 2H), 7.39 (d, 2H), 4.15 (t, 8H), 1.83 (m, 4H), 1.41 (s, 6H). 13C NMR (101 MHz, D2O, ppm): δ 135.06, 122.51, 48.67, 44.85, 25.95, 14.52. IR (500−4000 cm−1, KBr pellet): 3080, 1780, 1630, 1560, 1320, 1150, 791, 629. 1,4-Bis(3-vinylimidazolium-1-yl) Butane Bromide. C4(C2H4im)2(Br)2 was obtained as an off-white solid. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.74 (s, 2H), 8.28 (d, 2H), 8.03 (d, 2H), 7.35 (t, 4H), 6.00−5.43 (t, 4H), 4.32 (t, 4H), 1.89−2.09 (m, 4H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 135.76, 129.24, 123.72, 119.74, 109.10, 48.85, 26.18. IR (500−4000 cm−1, KBr pellet): 3090, 2070, 1650, 1550, 1320, 1190, 760, 594. 1,2-Bis(2,3-dimethylimidazolium-yl) Ethane Bromide. C2(mmim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 7.32 (d, 2H), 7.19 (d, 2H), 4.56 (t, 8H), 3.72 (s, 6H), 2.45 (s, 6H). 13C NMR (101 MHz, D2O, ppm): δ 145.00, 123.37, 120.90, 47.09, 34.99, 8.88. IR (500− 4000 cm−1, KBr pellet): 3060, 1780, 1650, 1540, 1270, 1120, 791, 648. 1,4-Bis(2,3-dimethylimidazolium-yl) Butane Bromide. C4(mmim)2(Br)2 was obtained as a white solid. 1H NMR (400 MHz, D2O, ppm): δ 7.21 (d, 2H), 7.18 (d, 2H), 4.02 (s, 6H), 3.62 (t, 4H), 2.46 (s, 6H), 1.72 (t, 4H). 13C NMR (101

Table 1. Chemical Shifts of Geminal Dicationic Ionic Liquids chemical shifts ionic liquids

C −H

C4−H

C5−H

C2(mim)2(Br)2 C2(eim)2(Br)2 C2(pim)2(Br)2 C2(bim)2(Br)2 C4(mim)2(Br)2 C4(eim)2(Br)2 C2(mmim)2(Br)2 C4(mmim)2(Br)2 C4(C2H4im)2(Br)2 C4(C2H5Oim)2(Br)2 C4(C2H3O2im)2(Br)2

8.71 8.78 8.72 8.76 9.34 8.72

7.47 7.52 7.47 7.52 7.87 7.43 7.32 7.21 8.28 7.86 7.88

7.43 7.38 7.37 7.42 7.77 7.39 7.19 7.18 8.03 7.81 7.77

2

9.74 9.31 9.34

values of imidazole ring C4−H and C5−H are slightly different. A large difference can be seen for the chemical shift of the H atom at position 2 of the imidazolium ring (abbreviated as C2− H). In general, the largest chemical shift values were observed when functional groups contain electronegativity atom in the imidazolium ring. It is well established that the hydrogenbonding ability of the functional groups can have a dramatic effect upon the chemical shift value of the more acidic C2−H proton. However, the overall range in values was relatively small (Δppm = 1.03) when compared to that of analogous alkyl sidechain DILs. It is hypothesized that this smaller range in chemical shift values reflects a suppressed interaction between the anion and the imidazolium C2−H proton because of the more preferred hydrogen bonding that exists between the anion and the functional group protons.24 3.2. Thermal Stability. The thermal stability of the geminal dicationic ionic liquids was measured by thermal gravimetric analysis.25 The thermal decomposition curves of the geminal dicationic ionic liquids are presented in Figure 1, and the decomposition temperatures are presented in Table 2. One C

DOI: 10.1021/acs.jpcc.7b09315 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Characteristic decomposition curves of the ionic liquids: (a) C2(mim)2(Br)2, (b) C2(eim)2(Br)2, (c) C2(pim)2(Br)2, (d) C2(bim)2(Br)2, (f) C4(eim)2(Br)2, (i) C4(C2H4im)2(Br)2, (j) C4(C2H5Oim)2(Br)2, and (k) C4(C2H3O2im)2(Br)2.

accumulation was hindered. Therefore, the decomposition temperature decreases. The substituents ground on the alkyl side-chain also affected the decomposition temperature of these geminal dicationic ionic liquids. Considering samples i−k, when the simple alkyl chain was replaced by a complex functional group, the decomposition temperature increases, as shown in Figure 1B. Because the complex functional groups increase the strength of the intermolecular interaction, there is more energy in the chemical bond. When the chemical bonds are broken, they need to absorb more energy, which increases the decomposition temperature. 3.3. Phase Change Behavior. Phase change behavior determines the cyclicity of ionic liquids, which is an important parameter for economic efficiency. DSC measurements for germinal dicationic ionic liquids contain different carbons in alkyl side-chains are shown in Figure 2. The measured temperature range was from 297 K to the temperature exceeding the individual melting point, followed by cooling; all measurements started with sample cooling from the liquid state down to 297 K. An endothermic peak on a heating trace and an exothermic peak on a cooling trace were registered. In Figure 2a, C2(mim)2(Br)2 has a distinct freezing point on the cooling curve and a distinct melting point on the heating curve; one is the exothermic peak at about 437 K, and the other is the endothermic peak at about 461 K. The exothermic freezing peak appeared at about 24 K lower temperature than the melting peak. The broken lines indicate crystal−crystal transitions. This small exothermic peak was observed in the cooling process just after crystallization. This trace suggests that sluggish phase transitions occurred in the cooling process. The stable existence of a supercooled state has been reported for many ionic liquids. Although supercooling of ionic liquids is common, the particular ionic liquid C2(mim)2(Br)2 readily crystallizes and does not form glasses, which indicates that this material is a favorable candidate for PCMs. In Figure 2c, the DSC curve of C2(pim)2(Br)2, a peak ascribed to a crystal−plastic crystal phase transition is observed at 330 K. Imidazolium cations have a high symmetry around the nitrogen atom and often form plastic crystal phases. Investigating the calorimetric curves, it is noted that there were significant differences in the melting and crystallization behaviors between C2(mim)2(Br)2 and C2(bim)2(Br)2. The crystallization of C2(bim)2(Br)2 in Figure 2d) is not observed in the DSC cooling curve until the temperature decreases to 320 K due to the supercooling effect. This result indicates that

must be careful in interpreting the onset temperature for thermal decomposition.26,27 Table 2. Acronyms, Molecular Weight, and Decomposition Temperatures of Imidazolium-Based Geminal Dicationic Ionic Liquids

Table 2 indicates that these ionic liquids are stable up to temperatures of 520 K (basically no weight loss) and decomposed rapidly at a constant, with little change. From this study, two main factors were found to affect the decomposition temperature of the various geminal dicationic ionic liquids. These factors are (1) the length of the alkyl sidechain and (2) the substituents ground on the alkyl side-chain. Considering first the ethyl linkage chain dicationic ionic liquids, longer alkyl side-chains result in a lowering of the decomposition temperature. As shown Figure 1A, the decomposition temperature (Td) for sample C2(mim)2(Br)2 is 583.81 K, and the Td value for sample C2(bim)2(Br)2 is 553.54 K. It is clear that sample C2(mim)2(Br)2 has better stability than sample C2(bim)2(Br)2. With the increase of the number of carbon atoms in the alkyl side-chain, the symmetry of imidazole dication was reduced and the crystal effective D

DOI: 10.1021/acs.jpcc.7b09315 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Differential scanning calorimetric thermograms for (a) C2(mim)2(Br)2, (b) C2(eim)2(Br)2, (c) C2(pim)2(Br)2, and (d) C2(bim)2(Br)2. Dotted lines indicate crystal−crystal phase transition.

the range to form hydrogen bonds. Thus, the decreasing intermolecular forces and hydrogen-bond energy lead to a lowering of the heat of fusion values for these ionic liquids.28,29 3.4.2. Effect of the Linkage Chain. The melting points and the heats of fusion values for ethyl and butyl linkage chains geminal dicationic ionic liquids are shown in Table 4. The

the C2(bim)2(Br)2 is generally hard to crystallize as the longer alkyl side-chains. This ionic liquid could be a potential PCM if the supercooling problem was solved by special nucleates. 3.4. Heat of Fusion Analysis. 3.4.1. Effect of the Alkyl Side-Chain. The melting points and the heats of fusion for geminal dicationic ionic liquids that contain different carbons in alkyl side-chains are shown in Table 3. The DSC measurements

Table 4. Effect of the Linkage Chain on the Heat of Fusion Values

Table 3. Effect of the Alkyl Side-Chain on the Heat of Fusion Values

a b c d

ionic liquids

melting temperature (K)

heat of fusion value (J g−1)

C2(mim)2(Br)2 C2(eim)2(Br)2 C2(pim)2(Br)2 C2(bim)2(Br)2

461.47 457.36 376.03 373.80

116.26 125.17 60.15 54.42

a e b f

ionic liquids

melting temperature (K)

heat of fusion value (J g−1)

C2(mim)2(Br)2 C4(mim)2(Br)2 C2(eim)2(Br)2 C4(eim)2(Br)2

461.47 388.60 457.36 404.49

116.26 115.69 125.17 102.13

length of the alkyl linkage chain separating the dications is observed to have only small effects on the melting points and the heats of fusion value. When the dications were connected by a butane linkage chain with the Br− anion, the melting points and the heat of fusion are decreased. For example, the melting points and the heat of fusion for sample C4(mim)2(Br)2 (388.60 K and 115.69 J g−1) are lower than those for sample C2(mim)2(Br)2 (461.47 K and 116.26 J g−1). Considering the longer alkyl linkage chains result in a lowering of the melting points and the heats of fusion value for dicationic ionic liquids, this trend is essentially related to the crystallinity of both salts. Generally, low symmetry and weak ion interaction (such as suppressing hydrogen bonding) tend to reduce the crystal lattice energy of the salts, thus resulting in low-melting salts.30 So the samples C2(mim)2(Br)2 and C2(eim)2(Br)2 of more symmetrical structure have higher melting points and heats of fusion.

of four ionic liquids are given in Figure 2. The single-peaked shape of the curve of four samples excludes the possibility of a solid−solid transition near melting. In the case of sample C2(mim)2(Br)2, which consists of the methylimidazolium dications connected by a ethane linkage chain with the Br− anion, the melting point is decreased by nearly 88 K by replacing the methyl groups with butyl groups to from sample C2(bim)2(Br)2. Meanwhile, the heat of fusion reduces 53.2%. Moreover, from Table 3, the heat of fusion values for C2(pim)2(Br)2 and C2(bim)2(Br)2 ionic liquids are observed to be lower than those for C2(mim)2(Br)2 and C2(eim)2(Br)2 ionic liquids. This result is attributed to the steric hindrance, and because the chain length of propyl and butyl side-chain ionic liquids is longer than that of methyl and ethyl side-chain ionic liquids, it hinders the ions from being close; therefore, the distance between the ions may be beyond E

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The Journal of Physical Chemistry C 3.4.3. Effect of the C2−H. Table 5 indicates that the methylation of the two positions of the imidazolium dications Table 5. Effect of the C2−H on the Heat of Fusion Values

a g e h

ionic liquids

melting temperature (K)

heat of fusion value (J g−1)

C2(mim)2(Br)2 C2(mmim)2(Br)2 C4(mim)2(Br)2 C4(mmim)2(Br)2

461.47 383.27 388.60 305.60

116.26 17.86 115.69 86.70

significantly decreases the melting point and the heat of fusion of these geminal dicationic ionic liquids. In the case of C2(mim)2(Br)2, which contains the 1-methylimidazolium dication connected by a ethane linkage chain, the melting point is nearly 461.47 K higher than that of the corresponding 1,2-dimethylimidazolium dication also connected by a ethane linkage chain. Similarly, the melting point of C4(mim)2(Br)2 is nearly 388.60 K higher than that of C4(mmim)2(Br)2. The heat of fusion values for C2(mim)2Br2 (ΔHf = 116.26 J g−1), C2(mmim)2Br2 (ΔHf = 17.86 J g−1), C4(mim)2Br2 (ΔHf = 115.69 J g−1), and C4(mmim)2Br2 (ΔHf = 86.70 J g−1) are also obviously different due to C2−H of the imidazolium ring forming hydrogen bonds with the other ions in ionic liquids. In addition, C2−H is the main interactive site with the anion in imidazolium ionic liquids; it exhibits a notable effect on the interaction force between two ions. This interaction force of ionic liquids weakens when methyl replaces C2−H because the interaction of acidic C2−H with anions disappears. So, the melting point and the heat of fusion are indirectly decreased because C2−H methylation decreases the interaction between cations and anions or lattice energy.31 3.4.4. Effect of Functional Groups. The substituents on the imidazolium ring also affected the heat of fusion values of geminal dicationic ionic liquids, as observed in Table 6. These

Figure 3. Melting curves of functionalized dicationic ionic liquids: (f) C4(eim)2(Br)2, (i) C4(C2H4im)2(Br)2, (j) C4(C2H5Oim)2(Br)2, and (k) C4(C2H3O2im)2(Br)2.

to change the temperature of a compound by a given amount. Heat capacity is related to the number of translational, vibrational, and rotational energy storage modes in the molecule.8,32 So, a molecule containing more atoms would have more energy modes and thus a higher heat capacity. Data have been obtained at atmospheric pressure and within 333.15−453.15 K for geminal dicationic ionic liquids in steps of 1 K. For simplicity, only values at some temperatures are shown in Table 7. Table 7. Experimental Heat Capacities for Functionalized Dicationic Ionic Liquids heat capacity (J g−1 K−1)

Table 6. Effect of Functional Groups on the Heat of Fusion Values

f i j k

ionic liquids

melting temperature (K)

heat of fusion value (J g−1)

C4(eim)2(Br)2 C4(C2H4im)2(Br)2 C4(C2H5Oim)2(Br)2 C4(C2H3O2im)2(Br)2

404.49 398.01 376.07 481.77

102.13 159.35 110.20 117.85

T/K

C4(eim)2(Br)2

C4(C2H4im)2(Br)2

C4(C2H5Oim)2(Br)2

373.15 378.15 383.15 388.15 393.15 398.15 403.15 408.15 413.15

0.94 1.00 10.7 1.13 1.21 1.30 1.44 1.67 1.97

0.81 0.85 0.91 0.97 1.06 1.20 1.42 1.88 2.99

1.41 1.64 2.09 2.57 2.93 4.54 1.68 1.39 1.38

The heat capacities of these functionalized dicationic ionic liquids at solid state were higher than 1.21 J g−1 K−1 and increased with increasing temperature. It is higher as compared to monocationic ionic liquids and performed good sensible heat storage. The reason is that the dicationic ionic liquids have more hydrogen bonds. This notable improvement is duo to the mutative vibration mode of the additional hydrogen bonds, which will absorb a large amount of heat during the increasing temperature period.12 3.6. Crystal Structure of C2(eim)2(Br)2. A single crystal of C2(eim)2(Br)2 was grown through methyl alcohol evaporation at room temperature, which resulted in colorless crystals with sizes of 0.49 × 0.43 × 0.34 mm3. The structure of C2(eim)2(Br)2 was determined by single-crystal X-ray diffraction. Crystal data and refinement results are given in Table 8. The structure of C2(eim)2(Br)2 in the solid state is shown in Figure 4. As shown in Figure 4, in the molecular structure of C2(eim)2(Br)2, the ethyl side-chains are turned almost perpendicular to the respective imidazolium ring. The

data are higher than those of the alkyl side-chain ionic liquids. It is likely to form more hydrogen bonds with C2−H, C4−H, and C5−H when introducing functional groups that contain electronegativity atom in the imidazolium ring. In addition, electronegativity functional groups will centralize the positive charge in the imidazolium ring, which increases the interaction force among the ions. Moreover, sample C4(C2H4im)2(Br)2 has a higher heat of fusion than other geminal dicationic ionic liquids. One of the possible reasons for this is the additional intermolecular interactions in the crystal packing of C4(C2H4im)2(Br)2, showing strong π−π conjugate between imidazolium rings and the CC double bond. Thus, the larger interaction force increases the binding energy and the heat of fusion indirectly. Figure 3 gives the DSC measurements of four ionic liquids. 3.5. Heat Capacity. Heat capacity (Cp) is the measurable physical quantity that characterizes the amount of heat required F

DOI: 10.1021/acs.jpcc.7b09315 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

well with the expected structures, which indicates that the preparation and purification methods reported are reliable. All of the ionic liquids reported in this work achieve the thermal stability with decomposition temperatures higher than 520 K. The heat of fusion of the geminal dicationic ionic liquids increased on average by 20%, and the heat capacities are greater than those of analogues monocationic ionic liquids. The crystal structure of the C2(eim)2(Br)2 indicates that dicationic ionic liquids can form the complicated hydrogen-bonding network. The geminal dicationic ionic liquids exhibit preferable thermal performances with experimental observations, and the hydrogen-bond energies of the samples are great and thus cause high heat of fusion values.

Table 8. Summary of the Crystal Data for C2(eim)2(Br)2 compound empirical formula molecular weight temperature (K) crystal color crystal size (mm) crystal system space group (Z) radiation, λ (Å) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z/density (calcd) (mg/m3) D (calcd) (Mg/m3) absorption coefficient (mm−1) F (000) (e) θ range for data collection (deg) R indices (all data)a a

C2(eim)2(Br)2 C12H20Br2N4 380.13 293(2) colorless 0.49 × 0.43 × 0.34 triclinic P1 MoKα, 0.71073 5.2258(7) 13.7832(1) 10.5862(2) 89.973(2) 101.109(2) 90.060(6) 748.2(3) 2 1.673 5.020 346 1.50−26.41 R1 = 0.0580, wR2b= 0.1343

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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.jpcc.7b09315. NMR and FT-IR spectra of the ionic liquids (DOC)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 29 82663189. Fax: +86 29 82663189. E-mail: [email protected].

R1 = ∑∥Fo| − |Fc||/∑|Fo|. bwR2 = {∑[ω(F2o − F2c )2/∑[ω(F2o)2]}1/2.

ORCID

Bolun Yang: 0000-0003-0340-4871 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work from the State Key Laboratory of Chemical Safety and Control (Sinopec Research Institute of Safety Engineering) is gratefully acknowledged.

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Figure 4. Structure of C2(eim)2(Br)2 in the solid state.

REFERENCES

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[C2(eim)2]2+ dication has a highly twisted configuration with two aromatic rings almost perpendicular to the C2 plane (angles of C6C7−N1C3N2C4C5 and C6C7−C10C9N4C8N3 are 92.1(6)° and 106.8(7)°, respectively). The imidazolium cation ring has a typical planar conformation, and all of the ring C−C and C−N distances and angles are within the normal range. The C−C bond of ethyl moiety is almost perpendicular to the plane of the aromatic ring, which is also typical for (N)CC fragments of alkylimidazolium crystal structures. The geometrical parameters of the aromatic rings for both structures are in good accord with each other except that the angle in C2(eim)2(Br)2 between the two ethylimidazolium rings is much smaller.20,33 Several hydrogen bonds are found in this structure, and these bonds influence the unique packing within the solid state of DILs. The crystal structure of C2(eim)2(Br)2 consists of zigzag bands of molecules along the crystallographic c-axis. Those bands are formed by separated stacks of isolated dications and bromide anions joined with hydrogen bonds typical for DILs.

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