Physicochemical and thermodynamic properties of imidazolium ionic liquids with nitrile and ether dual functional groups

Journal of Molecular Liquids 225 (2017) 281–289

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Physicochemical and thermodynamic properties of imidazolium ionic liquids with nitrile and ether dual functional groups Jaganathan Joshua Raj a,⁎, Cecilia Devi Wilfred b, Syed Nasir Shah a, Matheswaran Pranesh a, M.I. Abdul Mutalib a, Kallidanthiyil Chellappan Lethesh c,⁎ a b c

Centre of Research in Ionic Liquids (CORIL), Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia Center for Biofuel and Biochemical Research, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia

a r t i c l e

i n f o

Article history: Received 27 September 2016 Received in revised form 3 November 2016 Accepted 3 November 2016 Available online 23 November 2016 Keywords: Ionic liquids Dual functionalization Thermal stability Thermophysical property COSMO-RS

a b s t r a c t Synthesis of new dual functionalized imidazolium ionic liquids (ILs) containing nitrile functionality and ether group were synthesized and characterized. Bis(trifluoromethylsulfonyl)imide, trifluoroacetate and dicyanamide were used as the anion. The combination of two functional groups on an imidazolium cation was mainly developed to assess their potential in desulfurization of fuel oil. Although ILs based on this individual functional group has been reported to improve the desulfurization efficiency, no effort has been made to combine nitrile and ether functional groups on the same cation to enhance the desulphurization potential. In this report, we tailored both nitrile and ether functionality on imidazolium cation and studied their physicochemical properties in details. NMR (1H and 13C) spectroscopy and elemental analysis were used to characterize the synthesized ILs. Physical properties and optical properties of the ILs were investigated in a wide temperature range. The synthesized ILs showed good thermal stability. COSMO-RS was effectively utilized to discuss the σ surface, σ potential and σ profile of the synthesized ILs. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The intervention of ionic liquids (ILs) in solvent chemistry had created many positive implications towards scientific and industrial applications. In general, ILs, best described as “designer solvents”, have been known to be easily tunable by a selection of ionic species so as to own a plethora of chemical and physical properties. The advantages of ILs include the ability to remain liquid in a wide temperature range, very low vapor pressure, high thermal stability, wide electrochemical window, low flammability, high conductivity and others [1–4]. Combinations of cations and anions in ILs can be tuned to make them suitable for various applications such as separation [5], extraction, [6,7] electrochemical applications, [8,9] and catalysis [10,11]. Due to unique architectural platform, ILs had created an unstoppable popularity among applications such as solvents in organic synthesis, [12,13] electrolytes in dye sensitized solar cells [14], electro deposition of metals [15] etc. In recent years, synthesis of ILs carrying functional groups such as halogen, nitro, nitrile, ester and amide groups on cations have demonstrated remarkable physicochemical properties and have been actively linked with

⁎ Corresponding authors. E-mail addresses: [email protected] (J.J. Raj), [email protected] (K.C. Lethesh).

http://dx.doi.org/10.1016/j.molliq.2016.11.049 0167-7322/© 2016 Elsevier B.V. All rights reserved.

specific applications such as desulfurization of diesel fuel [16,17] and biomass dissolution [18]. Nevertheless, only limited number of research activities have been focusing on their application for dual functional ILs i.e., ILs having more than one functional group on cation [1,19,20]. In order to explore the full potential of dual functionalized ILs for various applications, detailed investigation is required to generate and compile comprehensive data on thermophysical properties, particularly the novel ones. Hence, the scope of this study focuses on synthesis, characterization and evaluation of the thermophysical properties of dual functionalized imidazolium cation comprising of nitrile and ether groups coupled with (trifluroacetate, bis(trifluoromethanesulfonyl)imide and dicyanamide) as anions. These ILs were mainly developed to assess their potential in desulfurization of fuel oil. Previous studies revealed the individual nitrile [21] and ether [22] functional group containing ILs showed improved desulfurization efficiency. However, there is no report regarding the synthesis and application of ILs by combining both these functional groups in a single cation. The novelty of this work is gained by combining both functional groups (nitrile and ether) in single cation and to study the thermophysical properties. Furthermore, the application of these ILs in desulphurization of fuels was also assessed using COSMO-RS study and molar polarizability data derived from refractive index values through Lorentz-Lorentz equation. Experiments on the desulfurization of fuel oils using these ILs are currently in progress in our laboratory and will publish elsewhere.

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2. Experimental

25]. On the other hand, trifluroacetate anion was introduced by neutralization reaction with trifluroacetic acid [26]. All structures of the synthesized ILs were confirmed using 1H and 13C NMR spectroscopy.

2.1. Chemicals and materials The materials and reagents used for this experiment are as follows; Imidazole (Sigma Aldrich, N99%), Acrylonitrile (Sigma Aldrich, N99%), 2-bromoethyl ethyl ether (Sigma Aldrich, N99%), Trifluoroacetic acid (Sigma Aldrich, N 99%), Lithium bis(trifluoromethanesulfonyl)imide (Solvionic, N99%), Sodium dicyanamide (Sigma Aldrich, N 98%), Silver nitrate (Merck), Ethyl acetate (Merck), Dichloromethane (HPLC grade). All the starting materials were used without further purification. 2.2. Characterization An elemental analyzer (EA-1110) was used to measure carbon, hydrogen and nitrogen content of the ILs. Structures of the synthesized ILs were confirmed by 1H and 13C NMR spectroscopy (Bruker Avance 500 MHz spectrometer). Viscosity and density were measured using Anton Paar viscometer (model SVM3000) and Anton Paar density meter (DMA5000) respectively. Coulometric Karl Fischer titrator (Mettler Toledo, model DL39) was used to analyze the water content in ILs. The refractive index was measured using refractometer (Mettler Toledo, RM40) and the surface tension was measured using surface tensiometer (model OCA 20) using pendant drop method. Thermal gravimetric analyzer (Perkin-Elmer, Pyris V-3.81) with heating profile from 323.15 K to 1073.15 K at a heating rate of 4.72 K·s−1 in inert (nitrogen atmosphere) condition was used to measure the thermal stability of ILs. Differential scanning calorimetry (Mettler Toledo, DSC Star 1/500) was used to measure the glass transition temperature and the melting point. The samples were heated in an inert atmosphere from 123.15 K to 403.15 K at a heating rate of 4.72 K·s−1. The uncertainty of the measurement is ±0.01 K. Ion chromatogram (Metrohm model 761 Compact IC) was used to measure the bromide content in the ILs. 3. Results and discussion 3.1. Synthesis of ILs The structures of the synthesized ILs in this study are shown in Fig. 1. The ILs synthesis in this work was conducted into two stages, namely synthesis of the dual functional cation followed by the introduction of the desired anion. Michael addition reaction between imidazole and acrylonitrile was applied to introduce nitrile functionality to the imidazole ring [19]. Ether unit was then tethered by SN2 reaction of imidazol-1-yl-acetonitrile with 2-bromoethyl ether according to previously published work [23].Three different anions (trifluroacetate, bis(trifluoromethanesulfonyl)imide and dicyanamide) were used in this study. Dicyanamide and bis(trifluoromethanesulfonyl)imide anions were introduced through the anion exchange reaction of the halide precursor ILs with metal salts (Ag or Li) of the corresponding anion [24,

3.2. Thermal profile Thermal stability and glass transition temperature of the ILs were measured and the results are shown in Figs. 2 and 3.The water and halide content of the ILs are shown in Table 1 (Supplementary information). The water and the halide content in the ILs are below 500 ppm and 1000 ppm respectively. TGA thermogram in Fig. 1 and Table 2 (supplementary info) indicates that the thermal stability of the ILs with [NTf2] anion is higher compared with other anions. According to previous studies, thermal stability of ILs depends mainly on the coordinating nature of the anion [27,28]. For instance, strongly coordinating anions tend to decrease the decomposition temperature of ILs and poorly coordinating anions ([NTf2] and [N(CN)2]) increases the thermal stability. Thermal stability of the ILs in this study increases in the order of [C2CNImEtOEt][CF3COO] b [C2CNImEtOEt][N(CN)2] b [C2CNImEtOEt] [NTf2]. In addition, the [C2CNImEtOEt] cation with [NTf2] and N(CN)2 anions are thermally less stable compared to the imidazolium ILs with simple alkyl chains. For example, [C6MIM][NTf2] and [C6MIM][N(CN)2] showed a decomposition temperature, (Td) of 684.15 K and 503.15 K respectively [27,29] compared to [C2CNImEtOEt][NTf2] and [C2CNImEtOEt] [N(CN)2] having Td of 596.15 K and 476.15 K respectively. This clearly indicates that the incorporation of nitrile and ether functionality in the cation lowered the thermal stability of the dual functionalized ILs. On another account, [C2CNImEtOEt][NTf2] and [C2CNImEtOEt][N(CN)2] possess lower Td compared to the imidazolium ILs containing only nitrile functionality ([C3CNMIm][NTf2] (Td = 657.45 K) and [C3CNMIm] [N(CN)2] (Td = 551.25 K)]) [25], which might be due to the presence of the ether functionality on the ILs in this study. The Glass Transition Temperature, (Tg) of the dual functionalized ILs used in this study are shown in Fig. 3 and Table 2 (Supplementary information). The Tg value increases in the order of [C2CNImEtOEt] [CF3COO] ≈ [C2CNImEtOEt][NTf2] b [C2CNImEtOEt][N(CN)2]. A small difference in Tg was observed between [CF3COO] and [NTf2] anions, whereas the lowest Tg was recorded for [N(CN)2] anion. In general, Tg for dual functionalized ILs are higher than ILs containing only nitrile functionality. For instance, [C2CNImEtOEt][NTf2] has Tg of 212.95 K, while [C3CNMIm] [NTf2] showed a lower Tg ( 202.45 K). Increase in Tg may be attributed to the presence of ether group on the cation that promotes hydrogen bond interactions [30]. Similar results were observed in the

Cation

O

N

N

C N

Anions

O F3C

F3C O

CF3 N S S O O OO

Fig. 1. Overview of the of ILs synthesized in this study.

NC

N

CN Fig. 2. TGA profile for ILs: [C2CNImEtOEt] [N(CN)2].

[C2CNImEtOEt][CF3COO],

[C2CNImEtOEt] [NTf2],

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283

Table 2 Surface thermodynamic functions of pure ILs at temperature 303.15 K: Surface entropy (Ss) and surface enthalpy (Es). ILs

103·Ss(mJ·K−1·m−2)

Es(mJ·m−2)

[C2CNImEtOEt][CF3COO] [C2CNImEtOEt][NTf2] [C2CNImEtOEt][N(CN)2]

39.31 41.43 44.93

51.87 36.25 35.28

3.3.2. Estimation of volumetric properties. The experimental density values were correlated with temperature using the equation below.
 ρ ¼ A0 −A1 ðT−298:15Þ=K ln g:cm−3

Fig. 3. DSC Profile for ILs: [C2CNImEtOEt] [N(CN)2].

[C2CNImEtOEt] [CF3COO],

[C2CNImEtOEt] [NTf2],

case of [N(CN)2] anion based dual functionalized ILs synthesized in this study. 3.3. Physical properties of ionic liquids The physical properties of ILs such as density, viscosity, surface tension and refractive index were studied in a relatively wide temperature range. All the properties were found to vary linearly with temperature. The fitting parameters for the linear correlation and the standard deviation obtained from the regression are shown in Table 3 in the supplementary information. The extremely low standard deviation observed for all the properties measure confirms the linear relationship with temperature. The findings agree well with previous studies conducted on various other ILs.

where Ao and A1are constants and its value with the resulting standard deviation for the correlation developed can be found in Table 3 (Supplementary information). In turn, the A1 value was used to calculate the value of thermal expansion coefficient (α) according to the equation below. A1 ¼

  α ∂lnρ ¼− K ∂ðT−298:15Þ p

V¼

M NA ρ

ρ (kg·m−3) V(nm3) S°(J·K−1·mol−1) UPOT(kJ·mol−1)

[C2CNImEtOEt][CF3COO] 1370.00 1453.70 [C2CNImEtOEt][NTf2] [C2CNImEtOEt][N(CN)2] 1247.20

0.3724 0.5418 0.3465

493.67 704.88 461.42

ð3Þ

S0 ð303:15Þ=J  K  mol −1

−1

  ¼ 1246:5 Vm =nm3 þ 29:5

ð4Þ

¼ 1981:2 ðρ=MÞ1=3 þ 103:8

ð5Þ

where M is molecular weight and NA is Avogadro's number. From the values of molecular volume, V in Table 1, it can be deduced that the bulkiness of anion is responsible for the increase in the molar volume when the cation remains same. On the same note, the molecular volume and standard molar entropy increases with increasing size of cation or anion and both results are in close proximity with already reported trend [2,34]. The value of lattice energy decreases with the bulkiness of the anion as can be seen in Table 1. The lattice energy values for the ILs with the anions [CF3COO], [NTf2] and [N(CN)2] are 336.46, 298.14 and 344.39 kJ·mol−1 respectively and the trend agrees with previous reported ILs [35–38]. These ILs have significantly lower lattice energy compared to the halides salts such as NaCl (lattice energy of 756 KJ/ mol) and CaCl2 (lattice energy of 2120 KJ/mol) [39]. This could be due to two reasons. In the case of alkali salts, the charge on the ions is significantly higher and secondly the ions are much closer compared to the ILs.

Table 3 The critical temperature (Tc) Normal boiling temperature (Tb) and enthalpy of vaporization Δg1Hom of ILs at temperature 303.15 K.

Table 1 Calculated values of Volume properties of ILs at temperature 303.15 K.

ILs

ð2Þ

The density values can also be further utilized to calculate many other important parameters such as molecular volumes, V, standard molar entropy (S0) and lattice energy of the ILs according to the Eqs. (3)-(5) respectively. All these values can be predicted with good accuracy using the quantum chemical calculations [32,33]. The values of all these parameters are displayed in Table 1.

UPOT =kJ=mol 3.3.1. Densities of ILs The effect of temperature on density (ρ) of the synthesized ILs was studied at temperature range of 293.15 K to 363.15 K and the results are given in Fig. 4. Fig. 4 and Table 4 (Supplementary information) clearly depicts the linear reduction of the density values decrease with increase in temperature. A nearly constant slope value was observed for density of ILs. Using the same cation, the order of density changes with respect to different anions following the sequence of [N(CN)2] b [CF3COO] b [NTf2]. It is interesting to observe that the bulky and weakly coordinating [NTf2] anion produced highest density value compared to [CF3COO] and [N(CN)2] anions. On another perspective, the increase in number of functional group with the cation leads to higher density values. At 293.15 K, the conventional ILs, [C6MIM] [NTf2] and [C6MIM][N(CN)2] have density values of 1370 kg·m− 3 and 1040 kg·m− 3 respectively [29,31]. However, due to the presence of additional functional groups, significant increase in density was observed only for [N(CN)2] anion based dual functionalized ILs where the density of [C2CNImEtOEt][N(CN)2] is 1250 kg·m−3.

ð1Þ

336.46 298.14 344.39

ILs [C2CNImEtOEt][CF3COO] [C2CNImEtOEt][NTf2] [C2CNImEtOEt][N(CN)2]

Guggenheim

Eötvos

Tc/K

Tb/K

Tc/K

Tb/K

Δg1Hom kJ·mol−1

1576.78 1031.54 921.09

946.06 618.92 552.66

1014.55 845.99 672.25

608.73 507.59 403.35

143.83 110.32 75.78

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Fig. 4. Densities ρ of ILs as a function of temperature T: ■, [C2CNImEtOEt][CF3COO]; ●, [C2CNImEtOEt] [NTf2]; ▲, [C2CNImEtOEt] [N(CN)2].

Fig. 5. Viscosities η of ILs as a function of Temperature T: ■, [C2CNImEtOEt][CF3COO]; ●, [C2CNImEtOEt] [NTf2]; ▲, [C2CNImEtOEt] [N(CN)2].

3.3.3. Viscosities of ILs Anton Paar viscometer was used to perform the viscosity (η) measurement within a temperature range of 293.15 K to 373.15 K. An increase in temperature decreases the viscosity. The viscosity versus temperature data are depicted in Fig. 5 and Table 4 (Supplementary information). The pattern of viscosity changes in ILs can be evaluated from the Coulombic and Van der Waals interaction alongside with hydrogen bond formation [4]. The interaction between cation and anion in ILs are important to determine their viscosity behavior. Fig. 5 clearly depicts the viscosity of the ILs decrease significantly at higher temperatures. The viscosity of ILs synthesized in this experiment varies in the order of anion N(CN)2 b NTf2 b CF3COO. It should also be mentioned that the presence of water as an impurity could also reduce the viscosity of ILs [40]. However, the water content of all ILs used in this study are below 500 ppm and they should not have substantial impact on the viscosity. On the other hand, the presence of ether functionality on the alkyl chain was known to reduce the viscosity of ILs based on previously reported work [23,41]. Nevertheless, the presence of nitrile functionality in the same cation acted contrary by increasing the viscosity of the dual functionalized ILs used in this study. For an example, [Bmim][NTf2] and [C3CNMIm][NTf2] have viscosity of 44 mPa·s and 286 mPa·s respectively at 298.15 K [30]. This clearly shows that the presence of nitrile functionality has increased the viscosity remarkably. Nitrile functionality is an electron donor that is responsible in hydrogen bond formation which leads to increase in viscosity. In addition, the presence of triple bonds on nitrile group increases the π–π interaction between the alkyl groups resulting in higher viscosity. Similar behavior can be observed between the conventional ILs, [C6MIM][NTf2], which has viscosity of 90.1 mPa·s [42] against [C2CNImEtOEt][NTf2] which has viscosity of 401.3 mPa·s at 273.15 K. Similar trend was also observed for dual functionalized ILs with [N(CN)2] anion used in this study.

3.3.4. Effect of temperature on surface tension of ILs Broadly speaking, surface tension can be defined as the force to close a cut of unit length in the surface of a liquid which is related to surface energy and intermolecular forces. Hence, it is essential to study these important physical characteristics of ILs. The surface tension of the studied ILs as a function of temperature is shown in Fig. 6. From Fig. 6, it can be seen that the surface tension shows linear dependence with temperature. Also the surface tension values of ILs changes with different anions in the order [CF3COO] N [NTf2] N [N(CN)2]. This trend can be explained by the effect of fluorination on the ILs. Fluorine containing anions like [NTf2] and [CF3COO] showed higher surface tension compared to non-fluorinated ones, which are in good agreements with previously published reports [43]. It is also interesting to note that all the ILs studied show lower surface tension than water (72.7 m·Nm−1) [44] at 293.15 K. However, only [N(CN)2] and [NTf2] based ILs exhibited lower surface tension compared to other common organic solvents like acetone (23.7 mN·m−1) [45], and methanol (22.6 mN·m−1) [45]. In addition, [NTf2] based ILs in this experiment depicted lower surface tension values than those ILs with simple cationic core (such as 1-alkyl-3-methylimidazolium) [46–48]. This could be

Table 4 The values of parameters of ILs for the interstice model at 303.15 K.

ILs

10−24v cm3

∑v cm3

∑v/v

104α(calc) K−1

104α(exp) K−1

[C2CNImEtOEt][CF3COO] [C2CNImEtOEt][NTf2] [C2CNImEtOEt][N(CN)2]

22.60 49.32 56.26

27.22 59.41 67.76

12.14 18.21 32.47

6.005 9.008 16.067

6.000 6.000 6.000

Fig. 6. Surface tension γ as a function of temperature T: ■, [C2CNImEtOEt] [CF3COO]; ●, [C2CNImEtOEt] [NTf2]; ▲, [C2CNImEtOEt] [N(CN)2].

J.J. Raj et al. / Journal of Molecular Liquids 225 (2017) 281–289

related to the additional functional groups present in the cationic core. Nitrile functionalized ILs such as [C3CNMIm][NTf2] and [C3CNMIm][N(CN)2] [30] recorded surface tension values of 52.2 mN·m−1 and 51.4 mN·m−1 at 298.15 K, which are much higher than the dual functionalized ILs [C2CNImEtOEt][NTf2] (24.05 mN·m−1) and [C2CNImEtOEt][N(CN)2] (22.03 mN·m−1) at 293.15 K). The lowering of surface tension of the dual functionalized ILs could be due to the presence of long alkyl chains that increases the extent of segregation of bulky alkyl chains to the surface. Moreover, the presence of ether functional group on the backbone of the alkyl chain could also be the reason behind lower surface tension for [NTf2] and [N(CN)2] based ILs [41]. The surface tension was fitted linearly with temperature using the following correlation.  σ= mNm−1 ¼ A4 −A5 T

ð6Þ

The corresponding values of A4 and A5 along with the standard deviation are shown in Table 3 in supplementary information. These fitting parameters were further used to estimate the surface entropy and surface enthalpy as given in the following equations.

285

is a constant, NA is Avogadro number and T is the temperature at which the surface tension was measured. The boiling temperature of ILs can be predicted using the equation Tb ≈ 0.6Tc predicted by Rebelo et al. [58]. The values of boiling temperature for all these ILs are shown in Table 3. 3.3.6. The interstice model for ionic liquids The intersticial volume of all the three dual functionalized based ILswas estimated using the interstice model developed by Yang et al. [59,60]. The volume properties such as the average volume of interstice (v) of the ILs can be found using the equation given below; 3=2

v ¼ 0:6791ðkb T=σ Þ

ð11Þ

where v is the interstice volume, kb is a constant, σ is the surface tension of the ILs and T is the temperature of the surface tension measurement. The molar volume of ionic liquids (Vm) is the summation of the inherent volume, Vi, and the total volume of the all interstices, ∑v = 2NAv, i.e.

  ∂σ Sa ¼ A5 ¼ − ∂T P

ð7Þ

V m ¼ Vi þ 2NA v

  ∂σ Es ¼ A4 ¼ σ − ∂T P

ð8Þ

The interstice model proposes that expansion of ILs volume is only because of the expansion of the interstice. Thus, the coefficient of thermal expansion, α, from the equation given below,

The values of these parameters are shown in Table 2. The values of surface entropy are in closed vicinity as compared to the already reported imidazolium based ILs [49,50]. However, these ILs have very low surface entropy in comparison to the organic solvents. For example, the surface entropy of water (0.138 mN m−1), pyridine (0.1369 mN m−1), benzene (0.13 mN m−1) and ethanol (0.086 mN m−1) are significantly lower as compared to all these ILs [51]. This higher surface entropy of these ILs might be due to the high surface organization and higher structured liquid phase, which is further verified by simulation as well as by X-ray reflectometry [52,53]. The value of the surface enthalpy for [CF3COO], [NTf2] and [N(CN)2] is 51.87, 36.25 and 35.28 mJ·m−2 respectively. The values of surface energy for all these ILs is quite close to benzene and octane that had surface enthalpy values of 67 and 51.1 mJ·m−2 respectively [54]. 3.3.5. Critical temperatures One of the most important properties of fluids is critical temperature which is mostly used for correlating the equilibrium and transport properties of fluids [55]. To estimate reliable data for determining critical temperature of ILs (Tc) is quite difficult because of the intrinsic nature of the ILs. The critical temperature of ILs is predicted mostly using Guggenheim [56] and Eötvos [57] equations as given below. The results of critical temperature using Guggenheim and Eötvos are shown in Table 3. Furthermore the enthalpy of vaporization was also found using Eq. (10). σ ¼E

σ

σ

1−

T

 α¼

1 Vm

  ∂V m 3NA v ¼ V mT ∂T P

ð12Þ

ð13Þ

where Vm is the molar volume of ionic liquids, T is the temperature and NA is the Avogadro number. The values of the average volume of the interstices along with the volume of ionic liquids and coefficient of thermal expansion for all the three ILs are given in Table 4. Furthermore, the experimental and calculated values of α were compared and both of these values were found to be in close proximity except for [C2CNImEtOEt][N(CN)2]. Thus, validating the applicability of the interstice model for most of the ILs. 3.3.7. Effect of temperature on refractive index of ionic liquids Optical properties of the synthesized ILs were studied by measuring their refractive indices, (ηD) within the temperature range of 293.15 to

!11 =

9

T Gc

 2 =  3 M ¼ K T Ec −T ρ

Δg1 H om ¼ 0:01121ðσ V 2=3 NA1=3Þ þ 2:4

ð9Þ

ð10Þ ð11Þ

where Eσ, is the total surface energy of ILs, which equals to the surface enthalpy because of the tiny volume difference due to thermal expansion at the temperatures that are not similar to the critical temperature TGC , σ is surface tension, M is molecular weight, ρ is density, K

Fig. 7. Refractive index ηD as a function of temperature T: ■, [C2CNImEtOEt] [CF3COO]; ●,[C2CNImEtOEt] [NTf2]; ▲, [C2CNImEtOEt] [N(CN)2].

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dielectric constant can be replaced by η2D, where ηD is the refractive index and the equation above can be written as;

Table 5 Molar volume, (Vm) and molar polarizability, (Rm) of ILs at 293.15 K. ILs

Vm/cm3·mol−1

Rm/cm3·mol−1

[C2CNImEtOEt][CF3COO] [C2CNImEtOEt][NTf2] [C2CNImEtOEt][N(CN)2]

222.87 324.24 207.53

63.87 85.83 63.13

η2 −1 α ¼ 3υ 2D ε0 ηD þ 2

343.15 K. Also the ηD of the ILs as a function of temperature are presented in Fig. 7 and Table 5 (Supplementary information). It can be clearly seen that the ηD values decrease linearly with increasing temperature. ηD of ILs varies with different anions in the order of [NTf2] b [CF3COO] b [N(CN)2]. Generally, ηD of most conventional ILs ranges from 1.4–1.5 and highest value of ηD reported is 1.57 for colorless ILs [61]. Taking this into account, it was observed that ηD at 293.15 K for [CF3COO] and [NTf2] based dual functionalized ILs are 1.49 and 1.44 respectively, whereas [N(CN)2] recorded 1.52 which falls slightly higher than the conventional ILs range, making it the highest ηD ILs. Higher ηD for [N(CN)2] based ILs could be due to the presence of more CN groups which results in higher electron mobility around CN group compared to other anions. Similar pattern was also observed for nitrile functionalized ILs, [C3CNMIm][NTf2] and [C3CNMIm][N(CN)2], which have ηD of 1.44 and 1.53 respectively [30]. Perhaps, very minimal variation on ηD values was observed between dual functionalized ILs with [NTf2] and [N(CN)2] anions used in this experiment with only nitrile-functionalized imidazolium ILs having same anions [30]. Meanwhile, these ηD values of ILs can effectively use to calculate molar refraction or molar polarizability (Rm) which would be helpful to provide useful data such as cohesive molecular internal energies which measures the forces between molecules (Dispersive and Coloumbic) [62]. In general, total polarizability, (α) of spherical and non-interacting molecules can be calculated using Clausius–Mossotti equation [63] which is expressed as: α ¼ 4πε0



 ε−1 3ν ε þ 2 4π

ð14Þ

where ν is molecular volume and ε0 is the dielectric constant in vacuum. Since the focus is to calculate the electronic polarizability (αe),

!

Eq. (15) is also known as Lorentz-Lorentz equation. Considering (υ = M / ρNA and f = Vm / NA), Molar polarizability, Rm can be computed using the following expression;

Rm ¼

! η2 −1 α ε NA ; ¼ υm 2D 3ε0 ηD þ 1

ð16Þ

where M is the molar mass, ρ the density, NA is the Avogadro's constant, and υm the molar volume. The computation of Rm values of all three ILs used in this study was done using Eq. (3) and tabulated in Table 5. It shows a clear relationship between polarizability and refractive index. Rm value is crucial in determining the overall molecular polarizability, which can be regarded as a measure of the dispersive forces within the fluid [62]. A study conducted by Karina Shimizu et al. clearly indicates that dispersive force in a molecule is strongly dependent on molar polarizability compared to Coloumbic force [62]. Therefore, solvents should be able to have strong dispersion forces, an indication for good solvents for species possessing high polarizability. ILs with high Rm values indicates strong dispersion forces and the application could be linked to desulfurization of fuel oil. ILs with high Rm values could exhibit possible dispersive force which can be utilized in the extraction of sulfur compounds. Table 5 indicates the calculated molar volume, (υm) and molar polarizability, (Rm) at 293.15 K for all the ILs synthesized in this study. Highest Rm values are observed for [NTf2] based ILs, followed by [CF3COO] and [N(CN)2]. The variations of Rm of ILs used in this study comes from the different anions used. Generally, large ions have large polarizabilities [64] where the Rm values obtained show good evidence on this clause. Size of anion used in this experiment increases from [N(CN)2] b [CF3COO] b [NTf2]. In addition, the presence of highly electronegative F atoms in [NTf2] and [CF3COO] anions leads to large dipole moment which increases the overall polarizability compared to [N(CN)2] anion. Moreover, similar pattern was also observed for highly

[C2CNImEtOEt] Cation

[CF3COO] anion

ð15Þ

[NTf2] anion

(N(CN)2)anion

Fig. 8. Sigma surface of cation and anions of ILs.

J.J. Raj et al. / Journal of Molecular Liquids 225 (2017) 281–289

symmetrical F atoms containing anions such as [BF4] and [PF6] anions based ILs [65]. 4. Cosmo-RS COMSMO-RS is a widely used molecular simulation tool among others for predicting thermodynamic properties of fluids [66,67]. It is an alternative to the structure interpolating group-contribution methods and works based on unimolecular quantum calculations [68, 69]. Complete information on COSMO-RS can be found in the work published by Klamt and Eckert [70,71]. Analysis of σ surface profile and potential for the respective ILs was performed using COSMO-RS, involving two steps. Primary step is to generate COSMO-RS cation and anion files. BP functional B88-p6 with

H-bond donor region

287

a triple –ξ valence polarized basis set (TZVP) was used to generate the files. The resolution of identity standard (RI) approximation was done using the TURBOMOLE 6.1 program package [72]. Final step involves structural analysis of ILs using COSMOtherm and element specific parameters were adopted. Structures producing lowest energy conformations was known to give better results [73], hence lowest energy conformations species were chosen for analysis. 4.1. σ surface, σ profile and σ potential Among the various advantages offered by COSMO-RS [71], it helps to identify the behavior of a molecule in both pure and mixture states. Thermodynamic properties of ILs synthesized were calculated from 3D molecular surface polarity distributions (σ surface) by COSMO-RS and

Non-polar region

H-bond accepto r region

(a)

Interaction with H-bond donor

Interaction with non-polar groups

Interaction with H-bond acceptor

(b)

INCREASING INTERACTION N

Fig. 9. (a) Sigma profiles for cation and anions of ILs (b) Sigma potentials for cation and anions of ILs.

, [C2CNImEtOEt];

, [CF3COO−];

, [NTf2];

[N(CN)2].

288

J.J. Raj et al. / Journal of Molecular Liquids 225 (2017) 281–289

results are shown in Fig. 8. Based on the quantum chemical calculations, data can be visualized in histogram function representing the σ profile and σ potential. Fig. 9a and b represents σ profile and σ potential of the studied ILs. Based on the σ surface, the electronic structure of the dual functionalized cation, [C2CNImEtOEt] contains one imidazolium ring with delocalized 3-centered -4 electron configuration across the N1-C2-N3 moiety. It has a double bond between C4 and C5 with a weak delocalization in the center of the aromatic ring C4 and C5. C2-H is more acidic compared to other hydrogen atoms attached to the C. Acidic nature is due to location of hydrogen between two electronegative nitrogen atoms. It can be clearly seen from Fig. 8 that the dark blue surface displays the positive charge due to acidic proton. Red color region on the left end is due to the cyano group and the other red patch near the right side of imidazolium ring is due to the presence of oxygen on the ether group. Regarding the surface of anions, red color surfaces indicates the ability of the anion to accept protons or preferable interaction with proton donors. From Fig. 9a, the histogram is divided into three regions. First region where σ b −1 e/A2 is called hydrogen bond donor region; the second region where σ lies between −1 and 1 e/A2 and is linked to the nonpolar characteristics of a molecule; the third region where σ N 1 e/A2 which is linked to the potential to act as a hydrogen bond acceptor. In the σ profile of the cation, a major peak was observed both on hydrogen bond donor region and non-polar region, while only a minor peak was detected in hydrogen bond acceptor region. The major peak located within the hydrogen bond donor region is due to the presence of acidic proton and the peak within the non-polar region is due to effect of alkyl groups. Whilst the minor peak in hydrogen bond acceptor region is due to the presence of oxygen (ether group). Peaks in hydrogen bond acceptor region and non-polar region were displayed for the studied anion. From Fig. 9b, COSMO-RS histogram is divided into three regions where σ b −1 e/A2 indicate the ability to interact with hydrogen bond donors followed by the second region where σ lies between −1 and 1 e/A2 and it represents the ability to interact with the non-polar molecules. The third region, where σ N 1 e/A2 shows the capacity to interact with hydrogen bond acceptors. Over y axis, the positive values indicate decreasing interaction and negative values indicate increasing interaction. The σ potential of cation has negative values in all the three regions. However, there is a slight portion of σ potential curve found in positive regions (interaction with hydrogen bond donor) and nonpolar region. This could be due to the presence of aromatic ring. Hence, the cation can exhibit the most favorable interaction with hydrogen bond acceptors and non-polar groups and favorable interaction with hydrogen bond donor groups. Anions show favorable interaction with hydrogen bond donor and non-polar groups. These ILs have the capability to interact with the groups in order of hydrogen bond acceptors b non-polar groups b hydrogen bond donors. Since these ILs exhibit strong interaction towards non-polar groups, they display high prospect for desulphurization of fuel oils. 5. Conclusion New dual functionalized ILs with various anions such as [CF3COO], [NTf2] and [N(CN)2] were synthesized and characterized. Thermophysical properties such as density, viscosity and surface tensions were studied in a wide temperature range. Optical properties (refractive index) at various temperatures were also reported. The water content in the ILs was kept below 500 ppm. The thermal stability of dual functionalized ILs varies with different anions and showed thermal stability up to 300 °C. The density and viscosity of dual functionalized ILs exhibited a linear dependency with temperature. Isothermal expansion coefficient of all the ILs was determined from experimental density values. The surface tension of the dual functionalized ILs decreased with increasing temperature. The surface entropy and surface enthalpy of the ILs were also calculated using the experimental surface tension

values and the critical temperatures of the ILs were computed using the empirical equations. The validity of the interstice model for ILs was established. The σ surface, potential and profile were analyzed for studied ILs by COSMO-RS.

Acknowledgement The authors acknowledge all the research officers in CORIL for helping with the analysis of ILs. The authors declare no competing financial interest. The present work has been supported by the Centre of Research in Ionic Liquids (CORIL), Universiti Teknologi PETRONAS. (PRF; 0153AB-A30)

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2016.11.049.

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