Synthesis and thermophysical characterization of novel azide functionalized imidazolium based ionic liquids

Article abstract of DOI:10.1016/j.tca.2015.08.032

Several new monocationic and unsymmetrical dicationic azide functionalized ionic liquids (ILs) were synthesized. FT-IR, ¹H NMR, ¹³C NMR and elemental analyses were applied to identify the molecular structure of these compounds. The density of these ionic liquids was measured too. Unsymmetrical dicationic ionic liquids had higher density than monocationic analogues. The shear viscosity of a dicationic ionic liquid ((1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-vinyl-3H-imidazole-1-ium di-dicyanamide) and its corresponding monocationic analogue were also measured. Further thermal behaviour and heat capacity of this dicationic ionic liquid were investigated by TGA and DSC techniques. TGA analysis suggested that the decomposition temperature increases with increasing the heating rate and unsymmetrical dicationic ionic liquid has good thermal stability (up to 502.4-533.6 K) and heat capacity (298-343 K).

Full text of DOI:10.1016/j.tca.2015.08.032

Accepted Manuscript
Title: Synthesis and thermophysical characterization of novel
azide functionalized imidazolium based ionic liquids
Author: Reza Fareghi - Alamdari Razieh Hatefipour
PII:
S0040-6031(15)00352-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.tca.2015.08.032
TCA 77331
To appear in:
Thermochimica Acta
Received date:
Revised date:
Accepted date:
23-6-2015
24-8-2015
26-8-2015
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Synthesis and thermophysical characterization of novel azide functionalized
imidazolium based ionic liquids
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Reza Fareghi - Alamdari , Razieh Hatefipour
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Faculty of Chemistry and Chemical Engineering, Malek - Ashtar University of Technology,
Tehran, Iran.
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Abstract
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Several new monocationic and unsymmetrical dicationic azide functionalized ionic liquids (ILs)
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were synthesized. FT-IR, HNMR, CNMR and elemental analyses were applied to identify the
molecular structure of these compounds. The density of these ionic liquids was measured too.
Unsymmetrical dicationic ionic liquids had higher density than monocationic analogues. The
shear viscosity of a dicationic ionic liquid (1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-
vinyl-3H-imidazole-1-ium di-dicyanamide) and its corresponding monocationic analogue were
also measured. Further thermal behavior and heat capacity of this dicationic ionic liquid were
investigated by TGA and DSC techniques. TGA analysis suggested that the decomposition
temperature increases with increasing the heating rate and unsymmetrical dicationic ionic liquid
has good thermal stability (up to 502.4 – 533.6 K) and heat capacity (298 to 343 K).
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Keywords: Thermal properties; Unsymmetrical dicationic ionic liquids; Dicyanamide; Heat
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capacity.
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1. Introduction
Ionic liquids have attracted substantial interest in recent years. They are liquid at or near the
room temperature. They can also be considered organic salts. ILs are composed solely of cations
and anions. They exhibit several unique properties, such as extremely low vapor pressure, non-
flammability, and high thermal stability [1, 2]. Moreover, the physical and chemical properties of
ILs can be adjusted or tuned by controlling the nature and functionality of the cations and anions
[3]. Therefore, ILs have been considered for many applications, including the replacement of
organic solvents [4, 5], electrochemistry [6], enzyme catalyzed reactions [7], separation [8],
electrolyte [9], dye-sensitized solar cells (DSSC) [10], absorption of CO [11] and stationary
phases in gas chromatography [12].
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Recently, dicationic ILs, in which two monocations are combined into a dication have been
reported [13, 14]. These ILs are solids or melts near room temperature. So they could afford new
supramolecular IL architectures with potential applications ranging from lubrication [15] to
organic synthesis [16]. Dicationic ILs have been classified to geminal (the two monocations that
form the dication are the same) and unsymmetrical (the monocations are different). Thermal
stability measurements of 39 mono and dicationic imidazolium and pyrrolidium based ILs
∗
Corresponding authors: Fax: +98-21-44658251; Tel: +98-937-3381632.
E-mail: reza_fareghi@yahoo.com
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showed that dicationic ILs have higher thermal stabilities than monocationic ones [14]. Some
researchers reported that thermal stabilities of unsymmetrical dicationic ILs are higher than
monocationic and geminal dicationic ones [17-19]. Another research on some germinal
dicationic ILs showed that these ILs have higher densities, viscosities and longer liquid ranges
than monocationic ones [14, 16, 20, 21].
ILs based on dicyanamide (DCA) anion have been synthesized and characterized. DCA has
afforded highly fluid and conductive ILs made by combining heterocyclic cations such as 1,3-
dimethylimidazolium [22], and 1-ethyl-3-methylimidazolium [22, 23] as well as
diethylmethylsulfonium [24], and thereby have been utilized as components of electrolyte for
high performance dye-sensitized solar cell [25]. The DCA-based ILs exhibit lower viscosity than
their bis (trifluoromethanesulfonyl)imide (TFSI) counterparts [26].
Many works on the preparation of interesting energy-rich compounds containing organic azide
groups [27] have been reported recently. Several energetic azide based ionic liquids such as
methylated hydrazinium azides [28] and 1, 5-diamino-4-methyl-tetrazolium azide [29], have
been synthesized and characterized [30]. Schneider et al reported synthesis and characterization
of imidazolium cation azide based energetic ILS [31]. The importance of azide containing ILs, is
in their potential applications as liquid fuels in bipropellant systems. These ILs are thermally
stable and have significantly reduced sensitivity and toxicity characteristics because of their
negligible vapor pressure. Among different liquid propellants, hydrazine and its derivatives, such
as monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH), have been
widely used. Although they have high performance characteristics, they are also carcinogens. For
these reasons, it is desirable to replace hydrazine derivatives with ionic liquids which have
become species of green chemistry [32]. It seems that, azide containing ILs would be good
candidates to replace hydrazine and its derivatives in certain liquid fuel applications.
Although imidazolium based ILs and azide based ILs have desirable properties such as high
thermal stabilities, there have been no reports on the synthesis of dicationic azide functionalized
imidazolium based ILs. We report here, the synthesis, characterization and thermal properties of
the dicationic ionic liquid with dicyanamide, in which the unsymmetrical dications contain azide
functional group. In order to synthesize this dicationic IL, some monocationic ILs with or
without azide functional group were synthesized too. The reaction steps for the synthesis of azide
functionalized imidazolium based ILs are shown in scheme 1. In this research six novel azide
functionalized ILs were synthesized; two unsymmetrical dicationic ILs (ILs 4, 7) and four
monocationic ILs (ILs 2, 3, 5, 6). Characterization of synthesized ILs was carried out by use of
FT-IR and NMR spectroscopies as well as CHN analysis. The viscosity and the density of
dicationic ILs were also measured. Furthermore, the thermal stability and heat capacity of IL-7
was investigated by thermogravimetric analysis (TGA) and differential scanning calorimeter
(DSC).
<Scheme 1.>
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2. Experimental
2.1. Reagents and instruments
All of the solvents and chemicals were purchased from Sigma-Aldrich chemical company and
were used without further purification. H and CNMR spectra were recorded on a Brucker
Avance DRX – 500 MHz spectrometer, in CDCl and D O. TMS was used as an internal
standard. FT-IR spectra were recorded on a Nicolet 800 instrument. Elemental analyses were
conducted using Heraeous CHN analyzer (Germany) instrument. Thermo gravimetric analysis
(TGA) was conducted using a thermal analysis instrument Perkin Elmer (Pyris Diamond model)
in a nitrogen atmosphere. In this method, the temperature range was between 298 and 1173 K at
a heating rate of 5, 10 and 20 K.min . Runs were carried out in 40 µL alumina pans and under an
inert atmosphere of nitrogen. Nitrogen flow was 20 mL.min . Initial mass introduced in to the
pan was set to (10 1) mg. The melting point of synthesized ILs and the heat capacity of the
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unsymmetrical dicationic IL were measured using a differential scanning calorimeter (DSC) in a
Perkin Elmer instrument (Pyris 6 DSC model) in a nitrogen atmosphere with a temperature rate
of 20 K.min . The measurements were carried out using (10 1) mg of sample. The melting
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transition (T=429.31 K, 3.296 kg.mol ) of standard indium sample was used in order to calibrate
temperature and energy. The zero-heat flow producer described by TA instruments was flowed
for heat capacity measurements using a synthetic sapphire sample as the reference compound.
All these experiments allowed estimating an overall uncertainty of 0.1 K in temperature and
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3% in the heat capacities. Water content was eliminated in a treatment before each experiment
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that consisted of isothermal heating at 378 K for 0.5 h. This was found to be the optimal
condition for removing all water presented in the ILs matrix. The experiments were performed at
298 to 373 K, with a 20 K.min heating rate. Density was measured at room temperature using a
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00 Micromeritics Accupyc 1330 gas pycnometer, whereas viscosities, η, were analyzed using Anton
01 Paar MCR 300 rheometer with a cp 25-2 fixture (25 mm diameter, 2° cone angle). Density and
02 viscosity measurements were carried out at atmospheric pressure and temperature range of 298
03 to 373 K. The estimated uncertainty of measured densities is 5×10 g.cm . The relative
04 uncertainty in the dynamic viscosity was 1.1%, whereas the uncertainty in temperature was
05 within 0.1 K.
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07 2.2. Synthesis of ILs (1-7)
08 2.2.1. Synthesis of 1-allyl-3-methylimidazolium bromide [IL-1]
09 A mixture of 1-methylimidazole (10 mmol, 0.82 g) and allyl bromide (10 mmol, 0.86 mL), was
10 stirred at 353K for 24 h. The viscous product was washed three times using n-hexane to remove
11 any unreacted reactants. After drying the product under vacuum at 333K for 3h, brown, slightly
12 viscous liquid was obtained (Scheme 1, reaction 1).
13 H NMR (500 MHz, CDCl ): δ = 4.08 (s, 3H; CH ), 4.97 (d, 2H; CH ), 5.42 (d, 1H; CH ), 5.45
14 (d, 1H; CH ), 5.91-6.05 (m, 1H; CH), 7.39 (s, 1H; CH), 7.55 (s, 1H; CH), 10.33 (s, 1H; CH)
15 ppm; C NMR (125 MHz, D O): δ = 36.7, 51.6, 122.0, 122.1, 123.8, 130.2, 136.4 ppm. IR
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16 (KBr, ν/cm ): 3084.1, 2857.3, 1625.2, 1448.0, 1312.8, 948.0, 761.2, 1167.0, 1573.2, 1423.6,
17 843.2, 760.3. Anal. Calcd. for C H N Br: C, 41.40; H, 5.46; N, 13.79%; found: C, 41.38; H,
18 5.41; N, 13.68%.
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20 2.2.2. Synthesis of 1-(2-Azido-3-bromo-propyl)-3-methyl-3H-imidazol-1-ium bromide [IL-
21 2]
22 Bromine (10 mmol, 0.80 g) was added to an ice-cooled mixture of sodium azide (50 mmol, 3.25
23 g), dichloromethane (10 mL) and 30% HCl (2.5 mL). The mixture was stirred for 1 h. The
24 organic layer containing the bromine azide was decanted. Then sodium sulfate (1 spatula) was
25 added. The mixture was stirred for additional 30 minutes and then filtered. 1-allyl-3-
26 methylimidazolium bromide [IL-1] (10 mmol, 1.89 g) in nitromethane (10 mL) was added to
27 round-bottomed flask containing bromine azide solution at 293K and the mixture was stirred for
28 24h. The solvent was removed by a rotary evaporator and the product washed three times using
29 n-hexane and ethyl acetate. After drying the product under vacuum at 333K for 3h, light brown
30 viscous liquid was obtained (Scheme 1, reaction 2).
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31 H NMR (500 MHz, CDCl ): δ = 4.09 (s, 3H; CH ), 4.98 (d, 2H; CH ), 5.44 (d, 1H; CH ), 5.46
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32 (d, 1H; CH ), 5.98-6.04 (m, 1H; CH), 7.32 (s, 1H; CH), 7.42 (s, 1H; CH), 10.27 (s, 1H; CH)
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33 ppm; C NMR (125 MHz, D O): δ = 37.6, 54.5, 120.1, 124.0, 124.9, 131.4, 138.1 ppm. IR
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34 (KBr, ν/cm ): 757.2, 840.2, 949.8, 1164.3, 1312.6, 1339.2, 1424.5, 1460.8, 1573.3, 1648.0,
35 2107.1, 2798.9, 3073.1. Anal. Calcd. for C H N Br : C, 25.87; H, 3.41; N, 21.55%; found: C,
36 25.60; H, 3.38; N, 21.58%.
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38 2.2.3. Synthesis of 1-(3-Azido-2-bromo-propyl)-3-methyl-3H-imidazol-1-ium bromide [IL-
39 3]
40 Bromine (10 mmol, 0.80 g) was added to an ice-cooled mixture of sodium azide (50 mmol, 3.25
41 g), dichloromethane (10 mL) and 30% HCl (2.5 mL). The mixture was stirred for 1 h. The
42 organic layer containing the bromine azide was decanted. Then sodium sulfate (1 spatula) was
43 added. The mixture was stirred for additional 30 minutes and then filtered. 1-allyl-3-
44 methylimidazolium bromide [IL-1] (10 mmol, 1.89 g) in pentane (10 mL) was added to round-
45 bottomed flask containing bromine azide solution, under nitrogen atmosphere at 293K and the
46 mixture was stirred for 24h. The solvent was removed by a rotary evaporator and the product
47 washed three times using n-hexane and ethyl acetate. After drying the product under vacuum at
48 333K for 3h, light brown viscous liquid was obtained (Scheme 1, reaction 3).
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49 H NMR (500 MHz, CDCl ): δ = 4.03 (s, 3H; CH ), 4.94 (d, 2H; CH ), 5.35 (d, 1H; CH ), 5.39
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50 (d, 1H; CH ), 5.92-5.97 (m, 1H; CH), 7.44 (s, 1H; CH), 7.61 (s, 1H; CH), 10.06 (s, 1H; CH)
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51 ppm; C NMR (125 MHz, D O): δ = 37.3, 52.5, 122.5, 123.3, 124.8, 131.7, 137.2 ppm. IR
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52 (KBr, ν/cm ): 758.8, 841.6, 948.0, 1166.9, 1312.7, 1340.7, 1423.2, 1455.8, 1575.0, 1646.1,
53 2110.1, 2800.2, 3069.9. Anal. Calcd. for C H N Br : C, 25.87; H, 3.41; N, 21.55%; found: C,
54 25.77; H, 3.40; N, 21.51%.
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56 2.2.4. Synthesis of 1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-vinyl-3H-imidazole-
57 1-ium dibromide [IL-4]
58 1-vinylimidazole (12 mmol, 1.13 g) was slowly added into the 1-(2-Azido-3-bromo-propyl)-3-
59 methyl-3H-imidazol-1-ium bromide [IL-2] (10 mmol, 3.25 g) under solvent free condition. The
60 reaction mixture was stirred for 48h at 353K to form IL-4. After that, the reaction mixture was
61 cooled to room temperature, and then washed three times using ethyl acetate and toluene to
62 remove any unreacted reactants. After drying the product under vacuum at 333K for 3h, dark
63 brown and very viscous liquid was obtained (Scheme 1, reaction 4).
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64 H NMR (500 MHz, D O): δ = 4.12 (s, 3H; CH ), 5.01 (d, 2H; CH ), 5.08 (d, 2H; CH ), 5.09 (d,
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65 2H; CH ), 5.44-5.47 (dd, 2H; CH ), 5.99-6.02 (m, 1H, CH), 7.20 (t, 1H; CH), 7.30 (s, 1H; CH),
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66 7.38 (s, 1H; CH), 7.48 (s, 1H; CH), 7.64 (s, 1H; CH), 8.58 (s, 1H; CH), 10.20 (s, 1H; CH), ppm;
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67 C NMR (125 MHz, D O): δ = 37.2, 52.4, 110.7, 120.2, 120.7, 122.2, 123.1, 123.9, 124.6,
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68 128.7, 129.1, 136.9 ppm. IR (KBr, ν/cm ): 750.3, 825.4, 998.3, 1089.4, 1224.4, 1342.3, 1423.9,
69 1495.9, 1575.6, 1648.7, 2292.6, 3099.0, 3148.6, 3406.6. Anal. Calcd. for C H N Br : C, 34.39;
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70 H, 4.09; N, 23.39%; found: C, 34.26; H, 3.97; N, 23.31%.
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72 2.2.5. General procedure for anion exchange reaction
73 To a solution of IL (10 mmol) in distilled water (20 mL), was added silver dicyanamide (10
74 mmol, 1.74g for IL-1, IL-2, IL3, and 20 mmol, 3.48 g for IL-4). The reaction mixture was stirred
75 for 24 h at 313K. A green precipitate of silver bromide was formed. After that, the reaction
76 mixture was cooled to room temperature and filtered to remove any precipitate of silver bromide.
77 Water was removed by a rotary evaporator and the produced dicyanamide based IL finally dried,
78 under vacuum at 333K for 3h (Scheme 1, reactions 5-7).
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80 1-(2-Azido-3-bromo-propyl)-3-methyl-3H-imidazol-1-ium dicyanamide [IL-5]: H NMR
81 (500 MHz, CDCl ): δ = 4.05 (s, 3H; CH ), 4.91 (d, 2H; CH ), 4.95 (d, 1H; CH ), 5.41 (d, 1H;
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82 CH ), 5.82-6.02 (m, 1H; CH), 7.30 (s, 1H; CH), 7.38 (s, 1H; CH), 9.85 (s, 1H; CH) ppm; C
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83 NMR (125 MHz, D O): δ = 37.8, 54.1, 119.3, 123.6, 125.4, 130.1, 138.9 ppm. IR (KBr, ν/cm ):
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84 761.5, 841.2, 943.1, 1167.3, 1312.0, 1332.8, 1429.5, 1456.2, 1572.8, 1637.6, 2108.3, 2135.0,
85 2194.9, 2233.6, 2658.2, 3084, 3431.4. Anal. Calcd. for C H N Br: C, 34.74; H, 3.56; N,
86 36.01%; found: C, 34.59; H, 3.47; N, 36.09%.
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88 1-(3-Azido-2-bromo-propyl)-3-methyl-3H-imidazol-1-ium dicyanamide [IL-6]:
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89 H NMR (500 MHz, CDCl ): δ = 3.80 (s, 3H; CH ), 4.90 (d, 2H; CH ), 5.28 (d, 1H; CH ), 5.31
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90 (d, 1H; CH ), 5.46-5.54 (m, 1H; CH), 7.30 (s, 1H; CH), 7.47 (s, 1H; CH), 9.89 (s, 1H; CH) ppm;
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91 C NMR (125 MHz, D O): δ = 37.7, 51.8, 121.6, 123.5, 122.1, 130.9, 137.3 ppm. IR (KBr,
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92 ν/cm ): 760.5, 840.4, 946.1, 1166.2, 1312.4, 1333.9, 1425.7, 1454.7, 1574.6, 1645.4, 2118.3,
93 2133.5, 2190.7, 2236.2, 2649.2, 3081, 3427.4. Anal. Calcd. for C H N Br: C, 34.74; H, 3.56; N,
94 36.01%; found: C, 34.68; H, 3.51; N, 36.05%.
95
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96 1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-vinyl-3H-imidazole-1-ium di-
97 dicyanamide [IL-7]
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98 H NMR (500 MHz, D O): δ = 3.87 (s, 3H; CH ), 4.61 (d, 2H; CH ), 4.74 (d, 2H; CH ), 5.05 (d,
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99 2H; CH ), 5.29-5.39 (dd, 2H; CH ), 5.77-5.83 (m, 1H, CH), 7.02 (t, 1H; CH), 7.25 (s, 1H; CH),
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00 7.39 (s, 1H; CH), 7.44 (s, 1H; CH), 7.59 (s, 1H; CH), 8.64 (s, 1H; CH), 9.99 (s, 1H; CH), ppm;
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01 C NMR (125 MHz, D O): δ = 36.6, 52.2, 108.3, 120.2, 121.3, 121.9, 122.9, 123.5, 124.3,
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02 128.8, 131.0, 136.6 ppm. IR (KBr, ν/cm ): 750.9, 825.1, 993.4, 1086.3, 1228.0, 1344.3, 1419.8,
03 1511.2, 1573.0, 1647.9, 2134.9, 2194.1, 2239.4, 2292.6, 3095.7, 3144.3, 3414.4. Anal. Calcd. for
04 C H N : C, 51.66; H, 5.27; N, 43.07%; found: C, 51.60; H, 5.21; N, 43.09%.
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3. Results and discussion
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07 3.1. The density and melting point of synthesized ILs
08 Density is governed by molecular structures. Some properties such as high density make ILs
09 likely candidates for energetic applications. In Table 1, density values for synthesized ILs in this
10 study are compared with density of similar ILs reported in the literatures.
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<Table 1. >
14 It is clear from Table 1 that the density for the dicationic ILs is consistently higher than that for
15 the monocationic ILs with the same anions. VVDW/V (VVDW = van der Waals volume, V = molar
16 volume) in the dicationic ILs is larger than in monocationic ILs with the same anions, which
17 implies that the interionic interactions of the dicationic ILs are stronger than that in the
18 monocationic ILs [35].
19 As Table 1 shows, imidazolium based ILs which were synthesized in this study, showed higher
20 densities than ammonium based ILs (Table 1). Usually in ILs, the density decreases as the size of
21 the cation increases but there are exceptions, the density of the functionalized ILs is higher than
22 the density of the respective non-functionalized ILs (Table 1, entries 1, 5). The densities of ILs
23 are also affected by the identity of the organic anion; so that for ILs containing the same cation
24 species, increasing the anion mass corresponds to increasing the IL density. For this reason, the
25 density of ILs with bromide anion (Table 1, entries 3, 4, 7) is higher than corresponding ILs with
26 dicyanamide anion (Table 1 entries 5, 6, 8).
27 Table 1 also shows the melting points (T ) of the synthesized ILs. All of mono and dicationic
m
28 ILs are liquid at room temperature. We also found that T of the dicationic ILs are substantially
m
29 higher than that of the monocationic ILs.
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31 3.2. The viscosity of 1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-vinyl-3H-
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32 imidazole-1-ium di-dicyanamide [IL-7] and its corresponding mono cationic analogue
33 Viscosity is an important property of ILs because it strongly influences diffusion of species that
34 are dissolved or dispersed within ILs. IL viscosity is ordinarily influenced by the cation, anion
35 interactions, hydrogen bonding and the symmetry of the ions [36]. An increase in temperature
36 will substantially decrease the hydrogen bonding interactions, and therefore the viscosity will
37 decrease. The viscosity (η) of [IL-7] and its corresponding monocationic analogue, [IL-5], were
38 measured at several temperatures in the range 298–358 K. Viscosity – temperature dependence
39 for these ILs are shown in Figure 1. As expected, the viscosity of both [IL-5] and [IL-7]
Page 6 of 22

2
2
2
40 decreased with increase in temperature. In addition, the viscosity of the dicationic IL is larger
41 than that of the corresponding monocationic one.
42
2
2
2
43
<Figure 1. >
44
45 3.3. Thermal property of [IL-7]
2
2
2
2
46 The thermal stability of [IL-7] was investigated using TGA. Dynamic runs of [IL-7] were carried
47 out at three different heating rates (5, 10, 20 K.min ). The thermogravimetric (TG) and the
48 differential thermogravimetric (DTG) curves of this IL are shown in Figure 2.
49
-1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
50
<Figure 2.>
51
52 As shown in Figure 2, there are two steps where the mass gently decreases as temperature
53 increases. According to the weight percent of each ion in the IL, it is possible to determine which
54 ion decomposes first. For this reason, the weight percent of dicyanamide anion was calculated.
55 This was 20% for [IL-7]. So the first decomposition step (502 to 683 K), involves approximately
56 20% weight loss, can be attributed to the decomposition of dicyanamide anion. Decomposition
57 of the cation occurs from 623 to 843 K. It is important to highlight that the anion could modify
58 the beginning of the cation decomposition. At range 843 to 903 K little mass lost is observed.
59 This mass lost is related to inorganic compounds formed in the decomposition processes. This
60 constant pattern behavior occurs for all heating rates attributed to the presence of a similar
61 reaction mechanism. These data are in agreement with the literature [37, 38].
62 The decomposition temperatures (Tonset, Tpeak, T50%) at each heating rate are summarized in Table
63 2. As can be seen in Table 2, the lowest decomposition temperature was occurred at 5 K/min
64 heating rate. Thermal stability is closely dependent on time. Thus, thermal stability parameters
65 decrease when experimental time increase, the decomposition temperatures increase with the
66 heating rate. Tpeak could be increased by 65K when the heating rate goes up from 5 to 20 K/min.
67 However, the results confirm that the decomposition temperatures depend on the heating rate,
68 and they are in agreement with the literature [39, 40].
69
2
2
2
2
2
2
70
<Table 2.>
71
72 In order to compare the thermal stability of [IL-7] with other ILs, T50% for [IL-7] and several ILs
73 reported in the literatures are listed in Table 3.
74
75
<Table 3.>
2
2
2
2
2
2
2
2
2
76
77 As it can be observed in Table 3, thermal stability of IL-7 is higher than [emim]DCA and
78 [bmim]DCA. Generally, in ILs with the same anion, thermal stabilities of dicationic ILs are
79 significantly higher than their monocationic analogues, due to their higher harge, higher
80 molecular weight and greater intermolecular interactions [14].
81
82
83
84
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2
2
85 3.4. Heat capacity of 1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-vinyl-3H
86 -imidazole-1-ium di-dicyanamide [IL-7]
87 The heat capacity of the IL-7 was tested in the range 298–343 K. Figure 3 shows the variation of
88 heat capacity with temperature.
89
2
2
90
91
<Figure 3. >
2
2
2
2
2
2
2
2
3
3
3
3
3
3
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3
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3
3
3
3
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3
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3
3
3
92 As Figure 3 shows the variation in the heat capacity of [IL-7] with temperature is weak. For this
93 ionic liquid, a second order polynomial model was used to correlate the heat capacity (Cp)
94 against temperature (T) [42].
95
96 The heat capacity of several imidazolium based ILs with the dicyanamide anion have been
97 reported in literature, and is provided in Table 4.
98
99
<Table 4.>
00
01 Generally, energy can be stored in translational, vibrational, and rotational modes of freedom.
02 Therefore, one would expect that a molecule containing more atoms would have more energy
03 storage modes and thus a higher heat capacity. As shown in Table 4, for ILs with the same anion,
04 the higher the atom number, the higher the heat capacity.
05
06
4. Conclusion
07 Novel azide functionalized imidazolium based ionic liquids with the dicyanamide anion were
08 synthesized and characterized by NMR, IR spectroscopies as well as elemental analysis. The
09 densities and melting points of all synthesized ILs were measured. Among dicyanamide based
10 ILs 1-(2-azido-1-(2-azido-3-methyl-3H-imidazole-1-ium) propyl)-3-vinyl-3H-imidazole-1-ium
11 di-dicyanamide [IL-7] had the highest density. The viscosity of this IL at several temperatures
12 was compared with its corresponding monocationic analogue. Most common dicationic ILs have
13 high melting point and viscosity, but we found that [IL-7] has low viscosity and melting point, so
14 it is liquid at room temperature. For this reason, [IL-7] could be a suitable candidate for energetic
15 applications. Other thermophysical properties, heat capacity and thermal behavior were
16 investigated for this ionic liquid. Heat capacities of [IL-7] were measured from 298 to 373 K.
17 The obtained data show that this IL has a good heat storage capability; hence, it would also be
18 able to replace harmful heat transfer fluids. The thermal stability of [IL-7] was evaluated using
-1
19 TGA at three different heating rates (5, 10 and 20 K.min ). [IL-7] with one unsaturated
20 substituent and azide functionality had good thermal stability.
21 Based on the above results, it seems that the new synthesized dicationic ionic liquid with good
22 thermophysical properties could be used as an energetic material in propellant systems.
23
24 Acknowledgements
25 The authors are grateful to Malek - Ashtar University of Technology for supporting this work.
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3
26 References
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[(C F ) P(O)] N -Synthesis
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5 2
2
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70
71
72 Figure caption:
4
73 Scheme 1. Synthesis of new azide functionalized imidazolium based ILs.
4
4
4
4
4
4
4
4
4
4
4
74 Figure 1. Viscosity measurement of [IL-5] and [IL-7] as a function of temperature.
75 Shear Rate: d(gamma)/dt 500 1/s, Motor Correction Factor 1.
76 Uncertainty in the dynamic viscosity and temperature was 1.1% and 0.1 K, respectively.
77 Figure 2. The effect of the heating rate on the course of TG and DTG scans for [IL-7].
78 Figure 3. Heat capacity of [IL-7] at the rate of 20 K/min.
79
80
81
82
83
84
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19 Scheme 1.
5
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5
5
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5
20
21
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23
24
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34 Figure 1.
35
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40
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42
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48
49
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52 Figure 2.
53
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62
5
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63 Figure 3.
64
65
66
67
68
69
5
70
5
71 Table caption:
5
5
5
5
72 Table 1. Comparison the densities and melting points of the synthesized mono and dicationic ILs
73 and some reported ones.
74 Table 2. Dynamic TGA characteristic parameters for [IL-7].
75 Table 3. Comparison of T50% values between [IL-7] and dicyanamide based ILs reported in
5
5
5
5
5
5
5
5
5
5
5
5
5
76 literature.
77 Table 4. Comparison of heat capacity of imidazolium based ILs with the dicyanamide anion.
78
79
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96
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00
01
02
03
04
05
06
07
08
09
10
a
-3
Entry
1
Structure
T
m
(K)
(g.cm )
reference
[22],[33]
N(CN)2
263
282
1.06
1.15
N
N
CH3
N3
N
2
3
[34]
N(CN)2
N3
Br
N
N
283
290
271
1.36
1.34
1.31
This study
CH3
Br
Br
N3
Br
N
N
4
5
This study
This study
CH3
CH3
Br
N3
N
N
N(CN)2
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Br
N
N
6
279
1.28
This study
CH3
N(CN)2
N3
N3
N
N
N
N
N
N
N
N
7
8
296
292
1.56
1.49
This study
This study
CH3
CH3
2
Br
N3
2
N(CN)2
a
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11 Densities measured at 298K.
12 Table 1.
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17
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23
-1
Heating rate (K.min )
Tonset (K)a
5
10
20
502.4
548.6
793.8
513.3
569.2
796.8
533.6
613.5
824.0
Tpeak (K)b
T50% (K)
a
6
6
24 The maximum weight loss temperature in the first range that the mass decreases
b
25 The temperature at which the mass loss equals to 50% of the initial mass introduced
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6
6
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6
6
6
6
26 Table 2.
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29
30
31
32
33
34
35
36
37
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6
6
6
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6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
38
39
40
41
42
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52
53
54
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62
Ionic liquid
[bmim][DCA]
-1
a
b
Heating rate (K.min )
5
[emim][DCA]
614.6
[IL-7]
793.8
796.8
824.0
591.0
617.8
625.3
1
2
0
0
638.0
647.9
a
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6
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6
6
6
63 Ethylmethylimidazolium dicyanamide [41].
b
64 Butylmethylimidazolium dicyanamide [41].
65 Table 3.
66
67
68
69
6
6
6
6
6
70
71
72
73
74
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6
75
76
77
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79
80
81
82
83
84
85
86
87
Entry
Compound
T (K)
303
298
323
298
303
323
Method
DSC
DSC
DSC
DSC
DSC
DSC
C (J/mol.K)
328
389.8
375.7
400
409.7
452
Reference
[43]
[44]
p
a
1
2
3
4
5
6
[C mim]N(CN)2
2
b
[C CNmmim]N(CN)
3
2
c
[C mim]N(CN)2
[42]
4
[IL-7]
[IL-7]
[IL-7]
This study
This study
This study
a
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88 1-Ethyl-3-methyl-1H-imidazolium
b
89 3-(3-Cyanopropyl)-1,2-dimethyl-1H-imidazolium
C
90 1-Butyl-3-methyl-1H-imidazolium.
91 Table 4.
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6
6
6
6
6
92
93
94
95
96
97
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00
01 Highlights
02
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•
•
•
•
•
We synthesized new mono and asymmetric dicationic azide functionalized ionic liquids.
We measured the density of synthesized ILs at room temperature.
Asymmetric dicationic ILs have higher densities.
Thermal behavior of asymmetric dicationic IL with dicyanamide was investigated.
Dicationic IL with dicyanamide has good thermal stability and heat capacity.
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08
09
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