Dicationic imidazolium based ionic liquids: Synthesis and properties

Dicationic imidazolium based ionic liquids: Synthesis and properties

    Dicationic Imidazolium Based Ionic Liquids: Synthesis and Properties Amir Sada Khan, Zakaria Man, Annie Arvina, Mohammad Azmi Bustam,...

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    Dicationic Imidazolium Based Ionic Liquids: Synthesis and Properties Amir Sada Khan, Zakaria Man, Annie Arvina, Mohammad Azmi Bustam, Asma Nasrullah, Zahoor Ullah, Ariyanti Sarwono, Nawshad Muhammad PII: DOI: Reference:

S0167-7322(16)32336-4 doi:10.1016/j.molliq.2016.11.131 MOLLIQ 6678

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

19 August 2016 24 November 2016 28 November 2016

Please cite this article as: Amir Sada Khan, Zakaria Man, Annie Arvina, Mohammad Azmi Bustam, Asma Nasrullah, Zahoor Ullah, Ariyanti Sarwono, Nawshad Muhammad, Dicationic Imidazolium Based Ionic Liquids: Synthesis and Properties, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.11.131

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ACCEPTED MANUSCRIPT Dicationic Imidazolium Based Ionic Liquids: Synthesis and Properties Amir Sada Khana,b*, Zakaria Mana, Annie Arvinaa, Mohammad Azmi Bustama, Asma

Centre of Research in Ionic Liquid, Department of Chemical Engineering Universiti

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a

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Nasrullahc, Zahoor Ullaha, Ariyanti Sarwonoa, Nawshad Muhammadd*

Teknologi PETRONAS, Tronoh 31750, Malaysia. b

Department of Chemistry, University of Science and Technology, Bannu 28100, Khyber

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Fundamental and Applied Science Department, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia.

Interdisciplinary Research Centre in Biomedical materials, COMSATS Institute of

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d

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Information Technology, Lahore, Pakistan.

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Corresponding authors; [email protected], [email protected]

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c

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Pakhtunkhwa, Pakistan.

ACCEPTED MANUSCRIPT Abstract:

In

the

present

work,

four

dicationic

ionic

liquids

(DILs)

having

1,1-Bis(3-

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methylimidazolium-1-yl) butylene ([C4(Mim)2]) cation with counter anions namely

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hydrogensulfate, methanesulfonate, trifluoromethanesulfonate, and paratoluene sulfonate

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were synthesized and characterized. The structures of ILs were confirmed by H1NMR, C13 and elemental analysis (CHNS). The effect of temperature and anion type on thermophysical properties such as refractive index, density and viscosity were studied. Density and viscosity

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values were measured within the temperature range of 293.15-273.15 K. Moreover, thermal expansion coefficient, molecular volume, molar volume, free volume, standard entropy, and lattice energy were derived from the density and viscosity values. Viscosity values were used

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for calculation of activation energy using the Arrhenius equation. Refractive index values were measured in the temperature range of 293.15-323.15 K and the values of refractive index were used further to estimate the electronic polarizability using Lorenz-Lorentz Thermal

properties

of

the

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equation.

synthesized

DILs

were

determined

using

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thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC).

Keywords: Dicationic ionic liquids, Density, Viscosity, Refractive index. Derived properties

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Thermal behavior

ACCEPTED MANUSCRIPT Introduction

Ionic liquids have been revealed as green reaction media due to their superior physical

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properties, having very low vapor pressure, high thermal and chemical stabilities, non-

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flammability, low melting point, large liquid temperature range and diversity of their structure accessibility make them a distinct candidate in various reactions [1]. This new class

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of chemical can reduce the use of common hazardous organic solvents which cause environmental pollution due to their low vapour pressure. In addition, ILs are designed solvent constituted by versatility of anions and cations and therefore provides opportunity to

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design desired ILs with functionality [2]. Due to their unique properties and tune ability, ILs are used in a variety of applications viz. separation process, biodiesel synthesis, biomass

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pretreatment, biotechnology and material engineering etc [3, 4]. DILs, in which cationic moieties are connected via linkers, are presently demonstrated to be promising materials because of their high potential for specific/targeted applications. Dicationic liquids are more

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tunable in comparison to monocationic ILs because of large number of possible combinations

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of various anions, cations and linker. Moreover, the physicochemical properties such as viscosity, density, thermal stability, melting point, and solubility behaviors can be varied

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(tuned) to a greater extent in multicationic ILs than the conventional monocationic ILs by changing the cation, anion and linkage chain length [5, 6]. Kim and Jadhav have synthesized dicationic ILs using ethylene glycol chains as a linker between two imidazolium cations with methanesulfonate and paratoluene sulfonate anions. These dicationic ILs containing

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methanesulfonate and paratoluene sulfonate anions were found effective in azidation of alkyl halides [7]. The same ILs were found effective in catalytic conversion of fructose to 5hdroxymethyl furfural and achieved 92.3% yield in 40 min at 70 oC [8]. In addition, different tailor made ILs especially, dicationic ILs are becoming popular as green solvent for various chemical reactions such as organic reactions, enzymatic treatment, as surfactant as well as in other various chemical analysis [6, 9-11]. It is also reported that dicationic ILs are capable of dissolving the cellulose and carbohydrates to a great extent [10, 12]. Furthermore trisimidazolium salts bearing hexadecyl chains and a bridging mesitylene were investigated as stabilizers for palladium nanoparticles [13]. In addition, two classes of trigonal tricationic ionic liquids containing triethylamine (Am) and 1,3,5-trimethylbenzene (Bn) cores were evaluated as surfactants in aqueous solution [14].

ACCEPTED MANUSCRIPT Industrial process design and the yield of ILs-based products depend on thermophysical properties, namely density, viscosity, refractive index, thermal behavior and surface tension of ILs. Therefore, an accurate and systematic thermophysical properties measurement of ILs

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is very important in both fundamental and applied research as it can help us in screening to

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find best ILs for required applications.

To scrutinize the effect of anions on the various thermophysical properties of ILs, a series of dicationic ILs composed of 1,1-Bis(3-methylimadazoilum-1-yl) butane cation and four anions derived from various Bronsted acids such as sulfuric acid (H2SO4), methanesulfonic acid

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(CH3SO3H), trifluoromethanesulfonic acid (CF3SO3H), and paratoluene sulphonic acid (ρTSA) were synthesized using the early reported approaches [6, 15]. To understand the nature

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of these synthesized dicationic ILs, their physicochemical properties such as density, viscosity, and refractive index were calculated and studied as function of anion’s type and temperature. The experimentally measured values of density were used further to estimate

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other properties like thermal expansion coefficients, molecular volume, standard entropy and

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lattice energy of dicationic ILs. The viscosity values were used in the calculation of activation energies. Refractive index values were used further to estimate the electronic

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polarizability of prepared DILs. TGA and DSC analysis was performed to study the thermal

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behavior of these DILs

ACCEPTED MANUSCRIPT Experimental section

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Materials

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The entire chemical used in this research work were of analytical grade and used without any

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further purification. 1-Methylimidazole (99%), Acetonitrile (99.5%), Ethyl Acetate (99.5%), Toluene (99.9%), Sulfuric acid (100%), Patratoluene-4-sulfonic acid monohydrate (99%) were supplied by Merck and 1,4-Dibromobutane (99%) was supplied by Sigma Aldrich.

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Synthesis of ionic liquids

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Four different ionic liquids composed of 1,1-Bis(3-methylimadazoilum-1-yl) butane [C4(Mim)2] cation and four different anions (HSO4-, CH3SO4-, CF3SO4- and ρ-TSA-) were synthesized using the already reported approaches [6, 15]. Initially, 1-methylimidazole (0.1

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mol) and 1,4-dibromobutane (0.05 mol) were charged into round bottom flask contain

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acetonitrile (10 mL) and was stirred for 6 h at 50°C under a nitrogen atmosphere. The synthesized white crystalline intermediate, 1,1-Bis(3-methylimadazoilum-1-yl) butylene

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bromide ([C4(Mim)2]2Br), was washed with ethyl acetate and toluene to remove unreactive residues. The solvents were removed by vacuum rotary evaporator at 55°C and the product was dried at 80°C in a vacuum oven for overnight. In the second step, [C4(Mim)2][2Br]

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(0.025 mol) was dissolved in water (20 mL) and then respective acid (0.05 mol) were added dropwise under the cooling condition for 30 min. After dropwise addition, the temperature of the mixture was increased to 50 oC and stir for 12 h under nitrogen reflux. The desired ILs were washed with water and toluene respectively to remove the unreacted materials and dried in a vacuum oven at 70 °C for 24 hours. The synthesis scheme for DILs is shown in supplementary material (Figure S1). Names, abbreviations and structures of anions and cation of synthesized dicationic ionic liquids are given in Table 1.

ACCEPTED MANUSCRIPT Characterizations

H NMR and

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C NMR (Bruker 500MHz spectrometer) spectra were used to confirm the

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1

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NMR and CHNS analysis

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structure of synthesized dicationic ILs using methanol (CH3OH) as internal solvent (Spectra are provided in supplementary Figures (S.Fig.1-8). CHNS-932 (LECO instruments) was used

Physiochemical properties measurements

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for elemental analysis.

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The thermophysical properties such as refractive index, density, viscosity and thermal properties of the synthesized dicationic ILs were measured and discussed subsequently below. Indices of refraction of synthesized samples were measured using an ATAGO digital

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refractometer (RX-5000α) with an accuracy of ± 4.5×10-5 in the temperature range of 293.15

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K to 333.15 K with 5 K intervals. Anton-Parr viscometer (model SVM3000) was used for viscosity and density measurement of synthesized dicationic ILs. The viscosity and density of

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samples were measured in the temperature ranging from 293.15 K to 373.15 K with 5 K interval at atmospheric pressure; the viscosity of [C4(Mim)2][2ρ-TSA] was measured in temperature range from 333.15 K to 373.15 K. The uncertainty of measurements was u(T) =

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± 0.01 K, u(η) = ± 0.32 % and u(ρ) = ± 5×10-6 g·cm−3 for temperature, viscosity and density, respectively. Before measurement, all the instruments were calibrated with Millipore quality water followed by ILs. The density, viscosity and refractive index measurement were performed in triplicate and the average values are reported.

Thermogravimetric analysis.

To

measure

thermal

decomposition

temperature

of

synthesized

dicationic

ILs,

thermogravimetric analyzer (Perkin-Elmer TGA, Pyris V-3.81) was used in the nitrogen atmosphere. Thermal properties were measured in the temperature range of 323 -773 K with a heating rate of 10 K/min.

ACCEPTED MANUSCRIPT Melting point and glass transition determination.

The melting point (Mp) and glass transition temperature (Tg) of samples were measured using

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differential scanning calorimetry (Perkin-Elmer, model pyris 1). The samples were weighted

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in aluminium pans and subjected to thermo-cycles in which initially the sample is heated in a nitrogen atmosphere with heating rate of 10 °C.min-1 from 0 °C to 110 °C and then cooled

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from 110 °C to -150 °C and then heated again to 110 °C at heating rate of 10 °C.min-1.

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Results and discussion

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H1& C13 NMR and CHNS analysis of dicationic ILs

The NMR analysis has been provided below (Spectra are given in supplementary Figures

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S. 2-9)

1,1-Bis(3-methylimidazolium-1-yl) butylene hydrogensulfate [C4(Mim)2][2HSO4]

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Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 8.988 (s, 1H, NCHN), 7.691 (s, 1H,

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NCHCHN), 7.598 (s, 1H, NCHCHN), 4.335 (b, 4H, NCH2CH2CH2CH2N), 3.974 (s, 3H, NCH3), 1.975-2.002 (b, 6H, NCH2CH2CH2CH2N).

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C NMR: 26.405, 35.331, 48.636,

122.333, 123.690, 163.703

CHNS analysis: C12H22N4O8S2, Mol. Wt.: 414.45

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Calculated; C, 34.78; H, 5.35; N, 13.52; O, 30.88; S, 15.47 Found: C, 34.71; H, 5.32; N, 13.48; O, 30.91; S, 15.51 1,1-Bis(3-methylimidazolium-1-yl) butylene metanesulfonate [C4(Mim)2][2CH3SO3] Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 8.457 (s, 1H, NCHN), 7.212 (s, 1H, NCHCHN), 7.165 (s, 1H, NCHCHN), 4.303 (b, 4H, NCH2CH2CH2CH2N), 3.974 (s, 3H, NCH3), 2.502 (s, CH3SO3), 1.620-1.647 (b, 6H, NCH2CH2CH2CH2N). 35.720, 48.623, 122.0, 123.617, 135.610. CHNS analysis: C14H26N4O6S2 Mol. Wt.: 410.51 Calculated: C, 40.96; H, 6.38; N, 13.65; O, 23.38; S, 15.62 Found: C, 41.15; H, 6.42; N, 13.73; O, 23.32; S, 15.56 1,1-Bis(3-methylimidazolium-1-yl) butylene trifluoromethanesulfonate [C4(Mim)2][2CF3SO3]

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C NMR: 26.090,

ACCEPTED MANUSCRIPT Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 8.875 (s, 1H, NCHN), 7.628 (s, 1H, NCHCHN), 7.564 (s, 1H, NCHCHN), 4.303 (b, 4H, NCH2CH2CH2CH2N), 3.939 (s, 3H, NCH3), 1.951-1.979 (b, 6H, NCH2CH2CH2CH2N).

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C NMR: 26.090, 35.762, 38.477,

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48.623, 122.008, 123.617, 135.610.

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CHNS analysis: C14H20F6N4O6S2, Mol. Wt.: 518.45

Calculated: C, 32.43; H, 3.89; F, 21.99; N, 10.81; O, 18.52; S, 12.37

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Found: C, 32.48; H, 3.95; F, 22.05; N, 10.87; O, 18.45; S, 12.39

1,1-Bis(3-methylimidazolium-1-yl) butylene paratoluenesulfonate [C4(Mim)2][2ρ-TSA] Spectroscopic data: 1H NMR (500 MHz, CH3OH): δ = 8.313 (s, 1H, NCHN), 7.357 (s, 1H,

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NCHCHN), 7.341 (s, 1H, NCHCHN), 7.037-7.080 (b, 2H, CHCCH), 6.964-6.980 (s, CHCCH), 3.815 (b, 4H, NCH2CH2CH2CH2N), 3.510 (s, 6H, NCH3), 2.014 (CCH3), 1.483-

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1.510 (b, 6H, NCH2CH2CH2CH2N). 1C NMR: 20.443, 26.018, 35.667, 48.517, 121.027, 123.545, 125.170, 129.258, 135.534, 139.752, 141.999. CHNS analysis: C19H26N4O3S, Mol. Wt.: 390.50

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Calculated: C, 58.44; H, 6.71; N, 14.35; O, 12.29; S, 8.21

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Found; C, 58.46; H, 6.75; N, 14.32; O, 12.31; S, 8.28

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Correlation of the physical properties of ILs

The refractive index, density and viscosity values of synthesized DILs were fitted by least

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squares method using the following linear equations:

(1)

(2)

Where T is the absolute temperature in Kelvin, and A0, A1, A2 and A3 are adjustable parameters. The standard deviations (SD) for physical properties were calculated using the following expression:

ACCEPTED MANUSCRIPT SD 



N

i

( Z exp  Z calc ) 2

(3)

N

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T

Refractive index analysis

The refractive index represents the ratio of the speed of light in a vacuum to that in the particular media. The refractive index is an important physical property and helpful to determine the purity of sample, concentration of solute in a solution and electronic

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polarizability (Rm). Figure 1 shows the effects of anion and temperature on the refractive indices values. Among these synthesized DILs, [C4(Mim)2]][2ρ-TSA] has the highest

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refractive index value which is 1.55766 at 292.15 K. On another side, [C2(Mim)2][2CF3SO3] has the lowest value which is 1.44344 at 293.15 K. The decreasing order of refractive indices of

synthesized

DILs

is

[C4(Mim)2][2CF3SO3]

˃

[C4(Mim)2][2HSO4]

˃

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[C4(Mim)2][2CH3SO4] ˃ [C4(Mim)2][2 ρ-TSA]. The lower values are noted for the

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[C4(Mim)2][2CF3SO3] while the highest values are observed for the ionic liquid [C4(Mim)2]][ρ-TSA] owing to more aromaticity (additional electron mobility) and bulky

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structure of the anion, therefore the chance for light to strike the molecule is more hence causes an increase in the refractive index [16, 17]. The values of refractive indices for these dicationic ionic liquids are higher than the monocationic ionic liquids as reported in the

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literature [18]. The values of refractive index for [Bmim][CH3SO3] and [Bmim][CF3SO3] are 1.4792 and 1.4368 respectively which are lower than the values of refractive indices of [C4(Mim)2][2CH3SO3] and [C4(Mim)2][2CF3SO3][18]. Due to the presence of limited literature on thermophysical properties of DILs, the comparison was made with monocationic ILs. Fig. 3 shows that the refractive indices values decrease linearly with the increase of temperature as reported elsewhere [19]. The estimated values of fitting parameters A0 and A1 and standard deviation of refractive indices for DILs obtained using Eq. 1 are summarized in the supplementary material (Table S1).

ACCEPTED MANUSCRIPT Density analysis

The effects of temperature and anion on the densities of synthesised ILs are shown in Figure

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2. The order of the density for synthesized DILs is [C4(Mim)2][2ρ-TSA] ˃

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[C4(Mim)2][2CH3SO3] ˃ [C4(Mim)2][2CF3SO3] ˃ [C4(Mim)2][2HSO4]. Among the

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synthesized DILs, [C4(Mim)2][2HSO4] has higher density value while [C4(Mim)2][2ρ-TSA] has lower value at 293.15 K. The higher value of density for [C4(Mim)2][HSO4] containing sulphate anion is due to smaller molecular size of anion compared to the other anions [20]. In general, the density of ILs increasing with the increase of anion molecular weight, however

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the density is not only linked to molecular weight of anion and sometimes density decreases with increasing the anion molecular weight. In the present study, the density is not only

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depend on molecular weight but also depend on anion and cation interaction [19, 21, 22]. The values of density for these DILs are higher than the monocationic ionic liquids reported in the literature [18]. The values of density for [Bmim][CH3SO3] and [Bmim][CF3SO3] at 303.2 K

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are 1.2025 and 1.300 g/cm3, respectively, which are lower than the values of density of

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[C4(Mim)2][2CH3SO3] (1.4371 g/cm3) and [C4(Mim)2][2CF3SO3] (1.4696 g/cm3). It is clear, that the liquid density for the dicationic ILs is higher than that for the monocationic ILs when

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the dicationic ILs are compared to the monocationic ILs having the identical anions and cations [18]. Moreover, the fact regarding the higher density values for the dicationic ILs than the monocationic ILs implies that the inter-ionic interactions of the dicationic ILs could

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be stronger than that of the monocationic ILs [23]. With the increase of temperature the linear decrease in density values of all DILs were observed, this decrease in density values with temperature might be due to the decrease in the van der Waal forces of interactions which consequently cause an increase in the mobility of the ions [24]. At high temperature the molecules parted away from each other, therefore cause a decrease in density. The decrease in density values of ILs with an increase of temperature is usual and also observed for other ILs [25]. The fitting parameter (A2 and A2) values with R2 and standard deviation (SD) for empirical correlation of density of the measured ionic liquids calculated by using Eq. 2 are provided in supplementary Table S2.

ACCEPTED MANUSCRIPT Thermal Expansion Coefficient

Density data were used further to calculate the value of thermal expansion coefficient (α) or

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1    A1 .     T  p A0  A1T

(4)

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p  

T

cubical expansion through the following expression:

Where ρ is the density of ILs, T is the temperature and A0 and A1 are density fitting

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parameters calculated using Eq.2. The values of thermal expansion coefficients of DILs as a function of temperature and type of anions are presented in Table 2. The variations of the thermal expansion coefficients with temperature and anions are significant for the present

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DILs. In the present study, the values of thermal expansion coefficient vary in the range of

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4.31×10-4 to 5.55×10-4 K-1 which are significantly lower than the values of common organic solvents [17, 22]. Among these synthesized ILs, [C4(Mim)2][2HSO4] has the highest α value

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(1.55510-4 K-1) while [C4(Mim)2][2ρ-TSA] has the lowest value (5.02104 K-1). The values are in the range measured for others dicationic ILs [26]. The α of ILs are considerably lower than that of common molecular organic liquids (10-3 K-1) while greater than classical molten

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salt (1-2×10-4 K-1). In this study, the small change was noted in thermal expansion coefficient values with change in temperature. The slightly increase in thermal expansion coefficient values with the increase in temperature is due to decrease in the ordering of ILs [22].

Standard molar volume

Standard molar volume (Vm) is the volume occupied by one mole of a substance at standard temperature and pressure. The Vm of ILs at various temperatures and at atmospheric pressure can be calculated from their molar mass (M) using the following equation:

(5)

ACCEPTED MANUSCRIPT Where, Vm is molar volume in cm3.mol-1, M is the molecular weight in g.mol-1 and ρ is the density in g.cm-3. The values of Vm calculated for the DILs at a temperature range of 293.15273.15 K are shown in Table 3. The molar volume values of [C4(Mim)2][2HSO4],

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[C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], and [C4(Mim)2][2ρ-TSA] are 2.993×103,

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3.060×103, 3.722×103, and 4.278×103 cm3.mol-1, respectively. The standard molar volumes of the studied ILs are higher than their corresponding monocationic ILs as reported in

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literature. The higher molar volume of the [C4(Mim)2][2ρ-TSA] is due to its large size of anion [27]. The lower molar volume of [C4(Mim)2][2HSO4] is due to the small size of the anion which leads to strong interactions between cation and anion. The same conclusion can

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be reached from the molecular volume of the DILs. The molecular volumes (V) of the ILs were calculated from molar volume and Avogadro’s constant (NA) using the following

(6)

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equation:

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Where, NA (Avogadro’s constant) = 6.02245×1023 molecule per mol. The molecular volumes calculated for DILs at various temperatures are tabulated in Table 3. The molecular volume

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of the calculated for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], and [C4(Mim)2][2ρ-TSA] are 0.497, 0.508, 0.168 and 0.710 nm, respectively. From the values of V presented in Table 3, it can be concluded that an increase in the temperature cause an

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increase of the molar volumes of ILs [28]. The molar volumes and free volumes calculated for DILs at nine selected temperatures between 293.15 K and 373.15 K are listed in Table 3.

Free volume (Vf)

Free volume is an important parameter which is helpful to understand the transport phenomena in ILs and calculated using the flowing expression:

(7)

Where Rm is the molar refraction and Vm is the molar volume. The values of free volume at several temperatures were also calculated for the DILs (Table 3). The above formula can be used to estimate the free volume of spherical molecules but it also applicable for ILs

ACCEPTED MANUSCRIPT molecules having non-spherical ions. The increasing order of the free volume at 273.15 K for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], and [C4(Mim)2][2ρTSA] are 2.110×102, 2.147×102, 2.734×102 and 2.942×102 cm3.mol-1, respectively. Among

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the synthesized DILs, [C4(Mim)2][2ρ-TSA] has the highest value of free volume while

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[C4(Mim)2][2HSO4] has the lowest value. The higher value of free volume for [C4(Mim)2][2ρ-TSA] is assigned to the large size of paratoluene sulfonate. The lower value

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of free volume for [C4(Mim)2][2HSO4] could be correlated with higher van der Waals forces between the opposite ions [29]. The free volume for ILs is increasing with the increase of the anion size, as expected. The free volume of ILs can be correlated to the solubility of various

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types of species, particularly the low molecular weight gases such as CO2, CH4, and C2H6 gases [24, 30]. The solubility of gases increasing with the increase of the free volume of ILs

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[30].

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Standard entropy

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The values of standard entropy (So) for the prepared DILs were calculated from their

(8)

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molecular volume using the relationship established by Glasser [31]:

Where, V is the molecular volume of the ILs. The values of the standard entropy calculated for the ILs are listed in Table 3. The standard entropy reported for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], and [C4(Mim)2][2ρ-TSA] are 6.490× 102, 6.630× 102, 7.999 × 102 and 9.151 × 102 J.K-1.mol-1, respectively. The lower value of standard entropy noted for [C4(Mim)2][2HSO4] (6.490× 102 J.K-1.mol-1 at 298.15 K) while higher value was noted for [C4(Mim)2][2ρ-TSA] (9.151 × 102 J.K-1.mol-1 at 298.15 K), is due to the presence of large size anion which leads to its less interaction with cations due to steric hindrance and ultimately causes higher standard entropy [32]. The values of standard entropy of these DILs are higher than the moncationic ILs contain same anion and cation. The lower value of standard entropy noted for [C4(Mim)2][2HSO4] (6.490× 102 J.K-1.mol-1 at 298.15 K) is higher than the value noted for [C4Mim][2HSO4] (4.09 × 102 J.K-1.mol-1 at 298.15 K) [33].

ACCEPTED MANUSCRIPT Lattice potential energy

The lattice potential or crystal energy (UPOT) of DILs can be calculated with the help of Eq.9

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developed by Glasser [31].

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(9)

where, ρ and M are the density (g.cm-3) and molecular mass (g.mol-1), respectively. The

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estimated lattice potential energies are summarized in Table 3. As expected the crystal energies of the studied DILs are much lower than those of inorganic fused salts. The crystal energy for cesium iodide, which has the lowest crystal energy among alkali-chlorides, is 613

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kJ.mol-1 [34]. The crystal energies estimated at 293.15 K for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], and [C4(Mim)2][2ρ-TSA] are 3.999×102, 3.977×102, 3.792×102 and 3.667×102 KJ.mol-1, respectively. The [C4(Mim)2][2HSO4] has the

D

highest crystal lattice energy value while [C4(Mim)2][2ρ-TSA] has the lowest value. The

TE

results suggested that the crystal lattice energy is inversely correlated to the volume of ions. The lower crystal energy value of [C4(Mim)2][2ρ-TSA] is due to the larger size of

CE P

paratoluenesulfonate anion which leads to less packing of ions, the higher crystal lattice energy value of [C4(Mim)2][2HSO4] is due to smaller anion size which causes close compactness of opposite charge ions. Moreover, the crystal energy values of synthesised

AC

DILs are almost lower than monocationic ILs containing same anion and cation [33].

Electronic polarizability

The electronic polarizability (Rm) represents the total polarizability for one mole of a substance and can be derived from its refractive index value. Rm

provide information

concerning the force between molecules or their behaviours in solution [35]. The electronic polarization can be derived from refractive index values using Lorenz-Lorentz equation [36]:

(10)

ACCEPTED MANUSCRIPT Where nD is the refractive index, M is the molar mass, ρ is the density, and Vm is the molar volume. The values of derived molar refraction for DILs containing various anions at different temperatures are tabulated in Table 3. The Rm value for [C4(Mim)2][2CH3SO3] is

T

0.913×102 cm3.mol-1 at 298.15 K which is greater than the Rm value noted for

IP

[C4Mim][CH3SO3] (0.5841×102 cm3.mol-1 at 298.15 K). It is clear, that the Rm value for the dicationic ILs is higher than that for the monocationic ILs when the dicationic ILs are

SC R

compared to the monocationic ILs contain same anion and cation. The results confirmed that values of Rm increase slightly with increasing of molecular weight of anion. The increase in the Rm values with the increase of molecular weight of anion was reported by Ziyada and

NU

Cecilia [36]. As seen from the experimental results, Rm values of DILs increase very slightly with the increase of temperature as reported elsewhere; yet it depends on upon the nature of

MA

anion and temperature [24, 36].

D

Viscosity analysis

TE

Viscosity is an important property of ionic liquids from an engineering point of view and therefore the knowledge of viscosity is important for many calculations such as mass transfer,

CE P

modelling, fluid flow, and equipment design. Figure 3 shows the effects of temperature and anions on the dynamic viscosities of DILs. The raise in temperature causes decrease in the dynamic viscosity of all DILs. This rapid decrease in viscosities of many ILs with temperature is usual and already been reported by many researchers [19, 37, 38]. The

AC

decrease in viscosities of ILs with respect to temperature is due to fast and freely movement of molecules and thereby decreases in viscosity. The dynamic viscosities for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], [C4(Mim)2][2ρ-TSA] are 3309.23, 278.25, 390.36, and 9050.11 mPa.s, respectively at 313.15 K. The higher viscosity for [C4(Mim)2][2ρ-TSA] at 313.15 K is due to larger anion size which have higher steric hindrance and cannot move freely. To realize the relationship of viscosities of ILs with the structure of anion is difficult, probably due to a large number of parameters such as ion size, shape, charge density, polarizability and flexibility of ions and planarity of molecular geometry which strongly affect the viscosities of ILs [37]. When the viscosities of dicationic ILs [C4(Mim)2][2HSO4] (11740.12 mPa.s) are compared to the monocationic ILs [C4C1Im][HSO4] (4935.01 mPa.s) having the same anions and

cation, the viscosity of

dicationic ILs is observed greater than the monocationic ILs [33]. Similar trend was observed

ACCEPTED MANUSCRIPT by Shirota et al. for comparison of the viscosity of monocationic and dicationic ionic liquids [23].

T

The variation in viscosity of ILs with temperature can be expressed using logarithmic form of

SC R

IP

Arrhenius equation as shown below:

temperature,

is the viscosity at infinite

is the energy of activation (J/mol) and T is temperature. The activation

) value for DILs can be calculated by plotting of

(Figure 3). The

MA

energy (

NU

Where, R is the universal gas constant (8.314J/K. mol),

(11)

values of

estimated are listed in supplementary Table S3. Energy of activation is

the least amount of energy required for the ions to move past others and therefore can be

D

linked with structural information of ILs. Lower value of Ea for ILs has been attributes to the

TE

lower electrostatic force acting between anion and cation, thereby more easily ions are capable to cross over each other’s. The higher value of Ea for [C4(Mim)2][2ρ-TSA] is due to

CE P

its higher molecular weight and complex structure of paratoluenesulfonate to move past each other. The values of Ea estimated for synthesised DILs are in the range of 25.50 to 77.613 KJ/mol. The change in Ea value with anion is assigned to change in its geometry and

AC

interaction with cation [39, 40].

Thermal Stability

Figure 4 shows the change in weight of DILs with an increase of temperature at a heating rate of 10 oC min-1. The results indicated that ILs containing [C4(Mim)2][2CF3SO3]shows higher stability than others DILs. The ILs contain derived from a strong acid such as H2SO4, CF3SO3H, CH3SO3H, and ρ-TSA shows high decomposition temperature compared to ILs derived from weak acids such as acetic acid or formic acid. These data are in good agreement with the reported literature [2, 41-43]. Miran et al, [44] reported that the DBU based protic ILs with different anions such as acetate, trifluoroacetate and methanesulfonate has the decomposition temperature in the range of 171-451 oC. The protic ionic liquids due to N—H

ACCEPTED MANUSCRIPT bonding in its cations structure is thermally unstable as compare to aprotic ionic liquids (AILs), which have alky side chain instead of proton in cations moiety such as 1-methyl-3ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]) [45]. Moreover, the

T

acetate base ionic liquids were observed more unstable as compared to other anion based

IP

protic ionic liquids which might be due to easy degradation of acetate anion. The thermal stability measured for these DILs were in reasonable range to be used in various applications.

SC R

It has been assumed that the decomposition temperature of the ionic liquids depends on upon the structure and molecular weight of cations and anions.

NU

Glass Transition

MA

Tg is an important thermal parameter which give considerable information about the cohesive energy of ILs, low Tg value representing low cohesive energies for ILs. The cohesive energy increased for ILs with the increase of attractive forces such as hydrogen bonding, van der

D

Waals and coulombic interaction, while it is decreased by the repulsive Pauli interaction due

TE

to overlapping of closed electron shells. It is worth to mention that low Tg values indicate that the ionic liquid probably has desirable physicochemical properties such as low viscosity and

CE P

high ionic conductivity. The glass transition temperatures for all the synthesised DILs are given in Table 4. The trend of DSC curves for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], [C4(Mim)2][2ρ-TSA] are similar to each other, but the positions of peak are different from each other. The glass-transition temperatures determined from DSC for

AC

measurements

[C4(Mim)2][2Br],

[C4(Mim)2][2HSO4],

[C4(Mim)2][2CH3SO3],

[C4(Mim)2][2CF3SO3], [C4(Mim)2][2ρ-TSA] are -4.88, -46.08, -47.95, -57.71 and –20.15 K, respectively. The change in Tg was observed with a change of anion, this effect of change of anion on Tg has been also previously reported by other research groups [44].

4.

Conclusions

In the present work, four ILs having same cation and different anions were successfully synthesized and structurally characterized using 1H NMR, 13C NMR and CHNS analysis. The effects of anion and temperature on physicochemical properties such as density, viscosity and refractive index have been studied. Density and viscosity were found to decrease with increasing of temperature for the range covered in the present work. Among the synthesized DILs, [C4(Mim)2][2HSO4] has higher density value while [C4(Mim)2][2ρ-

ACCEPTED MANUSCRIPT TSA] has lower value at 293.15 K. The dynamic viscosities for [C4(Mim)2][2HSO4], [C4(Mim)2][2CH3SO3], [C4(Mim)2][2CF3SO3], [C4(Mim)2][2ρ-TSA] are 3309.23, 278.25, 390.36, and 9050.11 mPa.s, respectively at 313.15 K. The values of molar volume Vm and

T

thermal expansion coefficient increased linearly with the increased of temperature. The Rm

IP

values for the dicationic ILs are higher than that for the monocationic ILs. The values of activation energy estimated for synthesized DILs are in the range of 25.50 to 77.613 KJ/mol.

SC R

The present DILs were found to be thermally stable in a wide range of temperature as reported for the most of the ILs. Tg values were observed greatly depend on the structure of anion.

NU

Acknowledgement

MA

The authors gratefully acknowledge the Ministry of Higher Education (MOHE) for funding the research work under the Fundamental Research Grant Scheme (FRGS 0153AB-i75) and Centre of Research in Ionic Liquid (CORIL), all the research officers and postgraduate

AC

CE P

TE

D

students for helping in all aspects.

ACCEPTED MANUSCRIPT References

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Table 1. The Name, abbreviation and structure, of anion and cation of dicationic ionic liquids Name

Cation

Abbreviation

Anion O

CE P AC

IP

O

OH

SC R

NU

[C4(Mim)2][2CH3SO3]

[C4(Mim)2][2CF3SO3]

OO

S

O

CH3

O

F3C

N

O-

S O O

N

[C4(Mim)2][2ρ-TSA]

TE

1,1-Bis(3-methylimidazolium-1-yl) butylene paratoluenesulfonate

S

N

MA

1,1-Bis(3-methylimidazolium-1-yl) butylene trifluoromethanesulfonate

O

N

(CH2) 4

1,1-Bis(3-methylimidazolium-1-yl) butylene metanesulfonate

[C4(Mim)2][2HSO4]

D

1,1-Bis(3-methylimidazolium-1-yl) butylene hydrogensulfate

T

CH3

H3C

CH3

S O

O-

ACCEPTED MANUSCRIPT

Table 2. Thermal expansion coefficient estimated at different temperature using Eq. (4).

104/(K-1) [C4(Mim)2][2HSO4]

[C4(Mim)2][2CH3SO3] [C4(Mim)2][2CF3SO3]

T

T/K

IP

[C4(Mim)2][2ρ-TSA] 5.55

5.15

5.40

5.02

303.15

5.58

5.18

5.42

5.05

313.15

5.61

5.21

5.45

5.07

323.15

5.64

5.23

5.48

5.10

333.15

5.67

5.26

5.51

5.12

343.15

5.70

5.29

5.55

5.15

353.15

5.74

5.32

5.58

5.18

363.15

5.77

5.35

5.61

5.20

273.15

5.80

5.38

5.64

5.23

NU

MA D TE

CE P AC

SC R

293.15

ACCEPTED MANUSCRIPT Table 3. Estimated values of molar volume (Vm), molecular volume (V), free volume (Vf), standard entropy (So) and crystal energy (UPOT) at different temperature.

T

V S × 102 Uop × 103 Vm × 102 Rm × 102 Vf × 102 (nm3) (J.K-1.mol-1) (KJ.mol-1) (cm3.mol-1) (cm3.mol-1) [C4(Mim)2][2HSO4] 6.490

3.999

2.993

0.882

2.110

303.15 0.499

6.525

3.994

3.009

0.885

2.124

313.15 0.502

6.564

3.988

323.15 0.505

6.594

3.983

333.15 0.508

6.630

3.977

343.15 0.511

6.670

3.971

353.15 0.514

6.705

3.966

3.097

363.15 0.517

6.741

3.960

3.114

373.15 0.520

6.777

3.955

3.131

IP

293.15 0.497

SC R

T/K

3.028

0.888

2.140

3.043

0.890

2.152

3.060

0.892

2.168

MA

NU

3.080

[C4(Mim)2][2CH3SO3] 3.977

3.060

0.913

2.147

303.15 0.510

6.664

3.972

3.077

0.915

2.162

313.15 0.513

6.699

3.967

3.093

0.916

2.177

323.15 0.516

6.731

3.962

3.109

0.916

2.192

333.15 0.519

6.764

3.957

3.125

0.917

2.208

343.15 0.521

6.797

3.952

3.141

353.15 0.524

6.831

3.947

3.157

363.15 0.527

6.870

3.941

3.176

273.15 0.530

6.904

3.936

3.193

CE P

AC

D

6.630

TE

293.15 0.508

[C4(Mim)2][2CF3SO3] 293.15 0.618

7.999

3.792

3.722

0.987

2.734

303.15 0.621

8.041

3.787

3.742

0.988

2.754

313.15 0.625

8.086

3.781

3.764

0.988

2.775

323.15 0.628

8.129

3.776

3.784

0.989

2.795

333.15 0.631

8.170

3.772

3.804

0.989

2.815

343.15 0.635

8.213

3.767

3.825

353.15 0.638

8.255

3.762

3.845

363.15 0.642

8.298

3.757

3.866

273.15 0.646

8.351

3.751

3.892

ACCEPTED MANUSCRIPT

293.15 0.710

9.151

3.667

4.278

1.378

---

303.15 0.714

9.198

3.662

4.301

1.382

---

313.15 0.718

9.254

3.657

4.328

1.386

2.942

323.15 0.722

9.298

3.652

4.349

1.388

T

[C4(Mim)2][2ρ-TSA]

333.15 0.725

9.332

3.649

4.366

1.388

343.15 0.728

9.379

3.645

4.388

353.15 0.732

9.426

3.640

4.411

363.15 0.736

9.478

3.635

4.436

373.15 0.740

9.531

3.630

4.462

IP

SC R

NU MA D TE CE P AC

2.961 2.977

ACCEPTED MANUSCRIPT

Table 4. Glass transition and melting point of synthesized dicationic ionic liquids. Tg (onset)

[C4(Mim)2][2CH3SO3]

-47.95

[C4(Mim)2][2CF3SO3]

-57.71

[C4(Mim)2][2ρ-TSA]

-20.15

NU MA D TE CE P AC

-44.22

T

-46.08

SC R

[C4(Mim)2][2HSO4]

Tmid

-46.79

IP

ILs

-57.20 -18.46

ACCEPTED MANUSCRIPT

1.568

T IP

1.504

SC R

[C4(Mim)2][2HSO4] [C4(Mim)2][2CH3SO3]

1.472

[C4(Mim)2][2p-TSA] [C4(Mim)2][HSO4]

[BMim][CH3SO3] [BMim][CF3SO3]

1.440

1.408 300

305

310

MA

295

NU

Refractive index (n)

1.536

315

320

325

330

T (K)

AC

CE P

TE

D

Figure 1. Refractive indices (nD) as a function of temperature for the dicationic ionic liquids

ACCEPTED MANUSCRIPT 1.53 [C4(Mim)2][2CF3SO4] [C4(Mim)2][2CH3SO4]

1.50

[C4(Mim)2][2P-TSA] [C4(Mim)2][2HSO4]

T IP

1.44 1.41

SC R

(g/cm3)

1.47

1.38

1.32 290

300

310

320

NU

1.35

330

340

350

360

370

MA

T (K)

AC

CE P

TE

D

Figure 2. Density (ρ) as a function of temperature for the dicationic ionic liquids

ACCEPTED MANUSCRIPT

10.4 [C4(MIm)2][2HSO4] [C4(MIm)2][2CH3SO4]

9.1

[C4(MIm)2][2CF3SO4]

T

[C4(MIm)2][2-TSA]

IP

6.5

SC R

ln(/mPa.s)

7.8

5.2

2.6 0.00330

0.00315

0.00300

MA

0.00345

NU

3.9

0.00285

0.00270

1/T (1/K)

AC

CE P

TE

D

Figure 3. Viscosities (ƞ ) as a function of temperature and anions for dicationic ionic liquid

ACCEPTED MANUSCRIPT

100

T

[C4(Mim)2][2HSO4] [C4(Mim)2][2CH3SO3] [C4(Mim)2][2CF3SO3]

IP

[C4(Mim)2][2-TSA]

60

SC R

Weight loss (%)

80

40

0 100

150

200

250

300

MA

50

NU

20

350

400

450

500

550

T (K)

AC

CE P

TE

D

Figure 4. Thermogravimetric analysis (TG) of dicationic ionic liquids

ACCEPTED MANUSCRIPT Highlights  Dicationic ionic liquids containing various anions were synthesized and characterized.  The effects of temperature and anions on thermophysical properties were investigated.

T

 Density and refractive index values were used to calculate other properties.

AC

CE P

TE

D

MA

NU

SC R

IP

 Thermal behavior was studied for the prepared ionic liquids.