A novel series of cyclophosphazene derivatives containing imidazolium ionic liquids with variable alkyl groups and their physicochemical properties

Journal of Molecular Liquids 295 (2019) 111722

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

A novel series of cyclophosphazene derivatives containing imidazolium ionic liquids with variable alkyl groups and their physicochemical properties Aathira M. Sadanandhan, Praveen K. Khatri ⁎, Suman L. Jain ⁎ Chemical and Material Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India

a r t i c l e

i n f o

Article history: Received 26 May 2019 Received in revised form 9 August 2019 Accepted 8 September 2019 Available online 11 September 2019 Keywords: Cyclophosphazene Imidazolium Ionic liquids Physicochemical studies Viscosity

a b s t r a c t A series of substituted cyclophosphazene derivatives containing imidazolium ionic liquids having variable alkyl groups were successfully synthesized and characterized by different spectroscopic techniques. The synthesized ionic liquids with butyl, pentyl, hexyl, heptyl, octyl and decyl groups were designated as IL1, IL2, IL3, IL4, IL5, and IL6 respectively. Physicochemical studies in terms of viscosity, density, thermal stability and conductivity were carried out as a function of temperature and molecular weight to see the lengthening effect of the alkyl group. The viscosity and density of the ionic liquids tend to decrease linearly with temperature. Further, experimental values of viscosity and density calculated the viscosity index, thermal expansion coefficient and molar volume. Similarly, all other physicochemical properties were evaluated to study structure-property correlation. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Owing to the distinctive physicochemical properties, such as negligible vapor pressure, intrinsic polarity, high thermal and chemical stability, recyclability, non-flammability and outstanding solvation capacity, ionic liquids have gained considerable scientific interest over the past few decades [1,2]. They have found a broad spectrum of applications in various fields including electrochemistry [3], gas separation [4,5], crude oil dissolution [6,7], catalysis [8–11], reaction media [12], corrosion inhibitors [13–15], and lubrication [16–18]. Determination of the fundamental physicochemical properties is the crucial aspect to explore the potential of the ionic liquids for different kinds of applications [16,19–23]. More specifically, hetero-atoms such as nitrogen-rich ionic liquids have found extensive applications in the realm of lubrication and energy materials. Among the numerous nitrogen-containing ionic liquids such as tetrazine, triazolium, tetrazolium, imidazolium, picrate, urotropinium reported [20,21,24], imidazolium ILs have been widely investigated mainly due to their low melting points and excellent chemical and electrochemical stabilities [25–31]. Ghanem and co-workers synthesized a series of amino acid-derived ionic liquids containing 1-octyl-3-methylimidazolium cation to conduct the physicochemical studies [26]. Kim et al. reported the effect of alkyl group in a series of imidazolium ionic liquids on the interfacial properties of the organic electrolyte [30]. Recently, Pillai and co-workers reported the imidazolium ionic liquids containing ⁎ Corresponding authors. E-mail addresses: [email protected] (P.K. Khatri), [email protected] (S.L. Jain).

https://doi.org/10.1016/j.molliq.2019.111722 0167-7322/© 2019 Elsevier B.V. All rights reserved.

variable alkyl chain length as an alternative to surfactants for enhancing the oil recovery (EOR). Crosthwaite et al. [32] performed a systematic study to compare the liquid phase behaviour of imidazolium-based ionic liquids containing variable alkyl groups with extraction solvents, such as alcohols. They illustrated the effect of alkyl chain lengthening in both ILs and alcohols through data generation of mutual solubility for both ionic liquid-water and ionic liquid-alcohol phase. Recently Zheng et al. [33] studied the working performance of imidazolium ionic liquids as an absorbent in conjunction with refrigerants like ammonia, water, alcohol and hydrofluorocarbons for absorption cycle. Abdouss and coworkers studied the effect of an alkyl group, nature of anion and temperature on the physicochemical properties of hydrophilic ionic liquids having imidazolium cation and dihydrogen phosphate, chloride and glycinate as anions [34]. In contrast to reported literature on imidazolium ionic liquids, the nitrogen-rich cyclophosphazene substituted imidazolium ionic liquids have been rarely explored [35,36]. In this context, Shreeve and co-workers demonstrated the synthesis, physicochemical properties and applicability of cyclophosphazene derivatives containing poly (trimethylammonium) and poly (N-methylpyridinium) ionic liquids as lubricant additives for the Si3N4/Si3N4 ceramic interface in aqueous lubrication [35]. Feng et al. reported the synthesis and application of alkoxy-cyclophosphazene derivatives containing imidazolium ionic liquids as lubricant additives [36]. However, the reported phosphazene derived imidazolium ionic liquids have been associated with certain drawbacks, such as multi-step synthetic procedure, unsymmetrical substitution and poorly defined compounds due to the mixture of various possible structural motifs.

2

A.M. Sadanandhan et al. / Journal of Molecular Liquids 295 (2019) 111722

In continuation to our on-going research on the use of ionic liquids for different applications [37–39], we herein describe the synthesis, characterization and physicochemical studies of a novel series of ionic liquids containing cyclophosphazene substituted imidazolium cation having variable alkyl groups. Importantly, the effect of alkyl group of imidazolium on the physicochemical properties like thermal expansion coefficient, density, viscosity and molar volume was determined as a function of the temperature. 2. Experimental section 2.1. Materials Phosphonitrilic chloride trimer (cyclophosphazene, 98%), 1bromobutane (98%), 1-bromopentane (99%), 1-bromohexane (99%), 1-bromoheptane (98%), 1-bromooctane (98%), 1-bromodecane (98%), imidazole (99%) were purchased from Alfa Aesar and used without any further treatment. Tetrahydrofuran (THF), diethyl ether and acetonitrile were of analytical grade and used as received. 2.2. General procedure of synthesis of IL 2.2.1. Synthesis of imidazole substituted cyclophosphazene [40] In a typical synthesis, a solution of phosphonitrilic chloride trimer (3 mmol) in 15 ml of dry tetrahydrofuran (THF) was added gradually to the flask containing imidazole (25 mmol) in dry THF, and the mixture was heated at 70 °C for 2 h under N2 atmosphere. Subsequently, the mixture was concentrated under reduced pressure followed by extraction with dry THF to remove the unreacted imidazole. The resulted residue was dried under vacuum at 60 °C for 24 h to give the desired imidazole substituted cyclophosphazene as a colorless liquid. 2.2.2. Synthesis of cyclophosphazene containing imidazolium ILs In the next step, a solution of alkyl halide (0.06 mol) in 20 ml of acetonitrile was added gradually to the solution of imidazole substituted cyclophosphazene (0.01 mol) in acetonitrile. The resulting reaction mixture was refluxed at 80 °C with constant stirring for 24 h under N2 atmosphere. After completion of the reaction, the mixture was concentrated under reduced pressure. The resulting viscous liquid was washed with diethyl ether, concentrated and dried at 80 °C for 24 h under vacuum to give the desired ILs (1–6) in more than 90% yield as a pale yellow to brown colored liquid. 2.3. Characterization The synthesized ionic liquids were characterized by NMR (1H, 13C and 31P NMR) spectroscopy and elemental analysis. All the NMR spectra were recorded on a Brucker NMR spectrometer of 500 MHz by using CDCl3 as a reference solvent and tetramethylsilane as an internal standard. Phosphoric acid 85 wt% in water used as an external reference for 31P NMR spectral analysis.

Table 1 Physical appearance and yield of the ionic liquids (IL1–6). Ionic liquid

IL1

IL2

IL3

IL4

IL5

IL6

Yield (%) Physical appearance

90 Semi-solid

91 Liquid

92 Liquid

91 Liquid

91 Liquid

90 Liquid

Table 2 Elemental analysis data of Ionic liquids.a Ionic liquid

Carbon (%)

Hydrogen (%)

Nitrogen (%)

IL1 IL2 IL3 IL4 IL5 IL6

35.21 (37.11) 38.61 (39.94) 41.93 (42.45) 42.70 (44.17) 47.14 (46.74) 49.46 (50.25)

4.12 (5.34) 5.02 (5.86) 5.99 (6.33) 6.17 (6.75) 6.18 (7.13) 6.44 (7.78)

13.44 (15.45) 12.78 (14.55) 12.63 (13.75) 11.56 (13.03) 11.13 (12.39) 10.09 (11.27)

a

Values in parenthesis are the calculated one.

2.4. Viscosity and density measurements Viscosity and density of ionic liquids were measured using an Anton Paar viscometer (SVM 3000 Stabinger) as per the ASTM D7042 method. The obtained results were found to be identical to ISO 3104/ASTM D445. 2.5. Conductivity measurements The conductivity of the ionic liquids was determined at ambient temperature using a Eutech PC700 Thermo Scientific instrument. The instrument was calibrated with a standard solution at regular intervals. 2.6. Thermal stability Thermal stability of the ionic liquids was determined over a Perkin Elmer TG-DTA (TGA 4000 system) instrument in the temperature range between of 30 to 700 °C with an increment of 10 °C/min under nitrogen atmosphere. 3. Results and discussion 3.1. Synthesis and characterization of the ILs A series of ionic liquids containing cyclophosphazene substituted imidazolium ILs having butyl, pentyl, hexyl, heptyl, octyl and decyl groups were synthesized via a simple two-step procedure as outlined in Scheme 1. The ionic liquids were designated as IL1, IL2, IL3, IL4, IL5 and IL6, respectively. All ionic liquids except IL1 were liquid at room temperature as displayed in Table 1. The structural identity of the ionic liquids was confirmed by elemental and NMR (1H NMR, 13C NMR and 31P NMR) spectroscopic analysis. The results of these analyses are summarized in Tables 2 and 3, respectively. Copies of the spectra are provided in the Supplementary

Scheme 1. Synthesis of ionic liquid.

A.M. Sadanandhan et al. / Journal of Molecular Liquids 295 (2019) 111722

3500

1

13

H NMR (ppm)

C NMR (ppm)

\CH_CH), IL1 10.1 (\ \CH_N+), 8.32 (N\ 7.28 (CH_CH\ \N+), 3.94 (N+\ \CH2 (α)\ \CH2), 1.9 (H2C\ \CH2(β)\ \CH2), 1.25 (H2C\ \CH2(γ)\ \CH2), 0.82 (H2C \ \CH3(δ)). IL2

IL3

IL4

IL5

IL6

133.12 (\ \CH_N+), 122.01 (N\ \CH_CH), 122 (CH_CH\ \N+), 49.04 (N+\ \CH2(α)\ \CH2), 32.34 (H2C\ \CH2(β)\ \CH2), 19.98 (CH2\ \CH2(γ)\ \CH2), 13.08 (CH2\ \CH3(δ)). 10.15 (\ \CH_N+), 8.7 (N\ \CH_N+), 121.23 \CH_CH), 135.10 (\ 7.5 (CH_CH\ (N\ \CH_CH), 121.1 (\ \N+), 4.31 (N+\ \CH2 \CH_N+), \CH2(α)\ (α)\ \CH2), 4.03 (H2C\ \CH2(β)\ \CH2), 49.73 (N+\ \CH2), 35.64 1.9 (H2C\ (H2C\ \CH2(γ)\ \CH2), 1.27 \CH2(β)\ \CH2), 28.68 (CH2\ \CH2(δ)\ \CH2), 0.84 (CH2\ \CH3 (CH2\ \CH2(γ)\ \CH2), 21.63 (CH2\ (ε)) \CH2(δ)\ \CH2), 13.21 (CH2\ \CH3(ε)) 10.16 (\ \CH_N+), 8.62 135.92 (\ \CH_N+), 122.32 (N\ \CH_CH), 7.7 (CH_CH\ (NCH_CH), 122 (CH_CH\ \N+), \N+), 4.32 (N+\ \CH2(α)\ 49.86 (N+\ \CH2(α)\ \CH2), 4.03 \CH2), 46.31 \CH2(β)\ \CH2), 3.26 (H2C\ \CH2 (H2C\ \CH2(β)\ \CH2), 30.04 (H2C\ (γ)\ \CH2), 1.92 (H2C\ \CH2(δ)\ \CH2), (CH2\ \CH2(γ)\ \CH2), 25.62 1.31 (CH2\ (CH2\ \CH2(ε)\ \CH2), 0.88 \CH2(δ)\ \CH2), 22.17 (CH2\ (CH2\ \CH3(ζ)). \CH2(ε)\ \CH2), 13.77 (CH2\ \CH3(ζ)). 10.19 (\ \CH_N+), 8.8 (N\ \CH_N+), 122.16 \CH_CH), 135.0 (\ 7.5 (CH_CH\ (N\ \CH_CH), 122.1 (CH_CH\ \N+), 4.32 (N+\ \CH2 \N+), 49.8 (N+\ \CH2(α)\ (α)\ \CH2), 4.1 (H2C\ \CH2(β)\ \CH2), \CH2), 46.13 3.15 (H2C\ (CH2\ \CH2(γ)\ \CH2), 1.99 (H2C \CH2(β)\ \CH2), 31.92 \ \CH2(δ)\ \CH2), 1.86 (CH2\ \CH2(ε)\ \ (CH2\ \CH2(γ)\ \CH2), 28.73 CH2), 1.2 (CH2\ (CH2\ \CH2(ζ)\ \CH2), 0.81 \CH2(δ)\ \CH2), 26.51 (CH2\ (CH2\ \CH3(η)). \CH2(ε)\ \CH2), 22.34 (CH2\ \CH2(ζ)\ \CH2), 13.10 (CH2\ \CH3(η)). 134.46 (\ \CH_N+), 122.46 10.28 (\ \CH_N+), 7.96 (N\ \CH_CH), 7.2 (CH_CH\ \CH_CH), 122.3 (CH_CH\ \N+), 3.9 (N\ \N+), (N+\ \CH2(α)\ \CH2(α)\ \CH2), 3.68 (CH2\ \CH2 49.15 (N+\ \CH2), 45.33 (β)\ \CH2), 3.43 (CH2\ \CH2(γ)\ \CH2), (CH2\ \CH2(β)\ \CH2), 30.51 2.77 (CH2\ (CH2\ \CH2(δ)\ \CH2), 2.61 \CH2(γ)\ \CH2), 29.28 (CH2\ (CH2\ \CH2(ε)\ \CH2), 1.6 (CH2\ \CH2 \CH2(δ)\ \CH2), 28.67 (ζ)\ \CH2), 1.05 (CH2\ \CH2(η)\ \CH2), (CH2\ \CH2(ε)\ \CH2), 25.16 0.63 (CH2\ (CH2\ \CH3(θ)). \CH2(ζ)\ \CH2), 21.47 (CH2\ \CH2(η)\ \CH2), 13.18 (CH2\ \CH3(θ)). 136.13 (\ \CH_N+), 123.0 9.99 (\ \CH_N+), 8.7 (N\ \CH_CH), 7.4 (CH_CH\ (N\ \CH_CH), 122.32 (CH_CH\ \N+), 4.2 (N+\ \CH2 \N \CH2(α)\ (α)\ \CH2), 3.89 (H2C\ \CH2(β)\ \CH2), +), 49.86 (N+\ \CH2), 49.26 3.6 (H2C\ (H2C\ \CH2(γ)\ \CH2), 3.01 \CH2(β)\ \CH2), 46.31 (CH2\ \CH2(δ)\ \CH2), 2.43 (CH2\ \CH2 (CH2\ \CH2(γ)\ \CH2), 36.61 (CH2\ (ε)\ \CH2), 1.93 (CH2\ \CH2(ζ)\ \CH2), \CH2(δ)\ \CH2), 31.61 1.7 (CH2\ (CH2\ \CH2(η)\ \CH2), 1.2 \CH2(ε)\ \CH2), 29.25 (CH2\ \CH2(θ)\ \CH2), 1.11 (CH2\ \CH2 (CH2\ \CH2(ζ)\ \CH2), 29.10 (CH2\ (ι)\ \CH2), 0.7 (CH2\ \CH3(κ)). \CH2(η)\ \CH2), 26.04 (CH2\ \CH2(θ)\ \CH2), 22.43 (CH2\ \CH2(ι)\ \CH2), 13.92 (CH2\ \CH3(κ)).

Kinematic Viscosity(mm2/s)

Table 3 NMR data (1H and 13C) of the ionic liquids. IL

3

3000 2500 2000 1500 1000 500 0 0

20

40

60

Viscosity of ILs (mm2/s)

10

20

30

40

100

Fig. 1. The kinematic viscosity of ILs with respect to temperature.

information (Figs. S1–S15). In the 31P NMR spectra, a peak in the region δ −10 to −12 ppm corresponded to phosphazene phosphorus clearly indicated the presence of phosphazene moiety in the ionic liquids. Furthermore, emergence of a single peak in all spectra suggested the substitution of all six chloride ions to give symmetrically substituted ionic liquid derivatives as depicted in Scheme 1.

3.2. Viscosity The viscosity of the ionic liquids was measured over a large temperature range from 10 to 100 °C with an interval of 10 °C. The values of viscosities are summarized in Table 4, and a curve between viscosity vs temperature is depicted in Fig. 1. It can be observed that the viscosity of ionic liquids tends to reduce linearly with temperature (Fig. 1). To understand the effect of variation in alkyl group in the ionic liquid on the viscosity, a plot between viscosity and molecular weight is shown in Fig. 2. The viscosity of ionic liquids increased linearly with the \\CH2 units that can be explained on the basis of the van der Waals forces of interaction that increases with the alkyl chain length [41,42]. As per the experimental data, the IL1 has the lowest viscosity while IL6 possesses the highest viscosity at all temperatures ranging from 10 to 100 °C. At high temperature, the observed deviation in the viscosity might be due to the variation of fluctuating polarization of cations and anions.

Table 4 The kinematic viscosity (mm2/s) of pure IL as a function of temperature (°C).

IL

80 0

Temperature, oC 50 60

70

80

90

100

IL 1

725.81

302.84 145.18 77.722 45.501 28.698 19.243

13.568 9.9640

7.5493

IL 2

1032.7

481.53 247.16 137.79 80.838 51.137 33.769

26.85

19.314

14.269

IL 3

1136.1

527.01 271.01 151.91 91.297 57.948 38.906

27.348 19.933

15.015

IL 4

1737.1

765.17 372.87 199.47 115.81 71.679 46.921

32.229 23.062

17.077

IL 5

2988.9

1267.7 599.05 311.73 176.04 106.4

45.546 31.9

23.221

IL 6

3056

1286.1 613.30 322.33 183.69 111.99 72.448

48.849 34.583

25.393

67.921

4

A.M. Sadanandhan et al. / Journal of Molecular Liquids 295 (2019) 111722

1.40

350

IL1 IL2 IL3 IL4 IL5 IL6

250

1.35

1.30

Density(g/cm3)

Viscosity (mm2/s)

300

200

150

1.25

1.20

1.15

100

1.10 50 660

680

700

720

740

1.05

760

0

20

Molecular weight (g/mol)

40

60

80

100

Temperature(0C)

Fig. 2. Viscosity of ILs as a function of molecular weight at 40 °C.

Fig. 3. The plot of ILs density vs temperature.

3.3. Density A slight fall in density with temperature was observed for each ionic liquid in a linear manner, as shown in Table 5 and Fig. 3. In addition, the density of ILs containing longer alkyl chain was found to be lower as compared to those having shorter alkyl chain. For example, for IL1, the value was 1.3049 g/cm3; whereas for IL6 having the longest alkyl chain, the value obtained 1.1198 g/cm3. As per the observation of Table 5, the order of density in ILs remained IL1 N IL2 N IL3 N IL4 N IL5 N IL6 at a given temperature, which is in accordance with earlier conducted studies for other ionic liquids having variable alkyl chain either in cation or anion [29,43,44]. The addition of CH2 unit in alkyl chain leads to the dispersion of charge centre with dwindling of electrostatic attraction between cation and anion. Furthermore, due to the lengthening of the alkyl chain, non-polar moiety increases that occupy more space and causes a drop in the density [44–46]. Similarly, in order to correlate the density with IL structure, a plot between density vs molecular weight is represented in Fig. 4. The density value of IL was found to be decreased with increasing the molecular weight due to the addition of \\CH2 units. Standard deviation with respect to temperature was calculated by using the following equation: ρ ¼ A1 þ A2 T

1.30

Density (g/cm3)

1.25

1.20

1.15

1.10

660

680

700

720

Fig. 4. The density of IL as a function of molecular weight at 40 °C.

ð2Þ

Table 5 The density (g/cm3) of ILs as a function of temperature (°C).

Temperature, oC

Density of ILs (gm/cm3)

Ionic Liquid

10

20

30

40

740

Molecular weight (g/mol)

ð1Þ

ln η ¼ A3 þ A4 ð1=TÞ

IL1 IL2 IL3 IL4 IL5 IL6

50

60

70

80

90

100

IL 1

1.3049

1.2973 1.2899 1.2828 1.2756 1.2684 1.2612

1.2540 1.2469

1.2387

IL 2

1.2776

1.2703 1.2629 1.2558 1.2489 1.2420 1.2346

1.2326 1.2204

1.1920

IL 3

1.2268

1.2196 1.2124 1.2055 1.1987 1.1917 1.1849

1.1780 1.1711

1.1642

IL 4

1.2155

1.2086 1.2018 1.1947 1.1878 1.1811 1.1744

1.1677 1.1609

1.1543

IL 5

1.1751

1.1683 1.1611 1.1541 1.1474 1.1406 1.1338

1.1270 1.1203

1.1138

IL 6

1.1198

1.1131 1.1062 1.0993 1.0926 1.0858 1.0791

1.0726 1.0656

1.0590

760

A.M. Sadanandhan et al. / Journal of Molecular Liquids 295 (2019) 111722

5

900

Table 6 Fitting parameter for Eq. (1) and SD values. A1

A2 · 104

SD

R2

IL1 IL2 IL3 IL4 IL5 IL6

1.7044 1.51042 1.4226 1.40788 1.36797 1.31098

−13.512 −8.1315 −6.9315 −6.8048 −6.8236 −6.75818

0.051494 0.025437 0.020987 0.020603 0.020660 0.020461

0.58748 0.92881 0.99994 0.99992 0.99988 0.99996

Molar volume(cm3/mol)

850

IL

Table 7 Fitting parameter for Eq. (2) and SD values. IL

A3 · 101

A4 · 104

SD · 103

R2

IL1 IL2 IL3 IL4 IL5 IL6

−1.2367 −1.0862 −1.0964 −1.1777 −1.2210 −1.1888

0.52845 0.49776 0.50389 0.53837 0.56589 0.55700

0.22604 0.32288 0.35491 0.54375 0.93633 0.95553

0.98723 0.98937 0.99291 0.99233 0.99297 0.99213

800 750 700 650 600 550 500 0

20

40

60

80

100

Temperature (0C) Fig. 5. Change in molar volume of ILs with temperature.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N u 1 X s¼t ðxi −xÞ2 ; N−1 i¼1

atmospheric pressure as per the following equation:

ð3Þ

αp ¼ −

where ρ, η and s represent density, viscosity and standard deviation (SD), respectively. Moreover the A1, A2, A3, A4 are the adjustable parameters and are calculated by linear regression analysis. In Eq. (3), xi is the sample value, x is the sample mean and N is the no. of the sample. The data values are summarized in Tables 6 and 7.

  1 δρ ρ δT

where, αp, T, and ρ represent the thermal expansion coefficient, absolute temperature and density, respectively. The values of αp are given in Table 9 and their plots as a function of temperature are displayed in Fig. 7. The data presented in Table 9 clearly indicates a marginal variation of thermal expansion coefficient with temperature, which is in well agreement with the existing literature [28,34,47]. However, an unexpected drop in the thermal expansion of IL2 in the temperature range 345 to 355 K was observed as shown in Fig. 7.

3.4. Molar volume Molar volumes (Vm) of ILs with respect to temperature were calculated from molecular weights and experimental values of density by using the following equation: Vm = MWIL / ρ, where MWIL is molecular weight of IL and ρ is the experimental density of IL. The obtained values are summarized in Table 8 and in Fig. 5. As expected a linear increase in the molar volume with alkyl chain length was observed [43]. It could be assumed due to the more occupancy of space with lengthening of non-polar alkyl chain [44]. Similarly, the molar volume was found to be increased with molecular weight as illustrated in Fig. 6.

3.6. Viscosity index Viscosity index (VI) of each ionic liquid was calculated from kinematic viscosity values at 40 °C and 100 °C by the online calculator of ASTM D2270 method. The values of VI are presented in Fig. 8. The viscosity index for IL1 was found to be lowest; whereas for IL2–6 the values obtained in the range of 91to102. The higher values of VI in comparison to the reported phosphazene containing imidazolium ILs suggested that the IL2–6 can be used as improved additives for lubrication properties [36].

3.5. Thermal expansion coefficient Thermal expansion coefficients (volume expansivity) of ionic liquids were determined from density values with respect to temperature at Table 8 Molar volumes (cm3/mol) of ILs as a function of temperature (°C).

Temperature, oC

Molar volume (cm3/mol)

IL

10

20

30

40

50

60

70

80

90

100

IL1

516.51

519.54

522.52

525.41

528.37

531.37

534.41

537.48

540.54

544.11

IL2

538.50

541.60

544.77

547.85

550.88

553.94

557.26

558.16

563.74

577.18

IL3

572.22

575.59

579.01

582.33

585.63

589.07

592.45

595.92

599.43

602.98

IL4

589.05

592.42

595.77

599.31

602.79

606.21

609.67

613.76

616.76

620.28

IL5

621.22

624.83

628.71

632.52

636.22

640.01

643.85

647.73

651.61

655.41

IL6

676.90

680.98

685.22

689.52

693.75

698.10

702.43

706.69

711.33

715.76

A.M. Sadanandhan et al. / Journal of Molecular Liquids 295 (2019) 111722

700

Molar Volume (cm3/mol)

680 660 640

IL1 IL2 IL3 IL4 IL5 IL6

620 600 580 560 540 520 4

5

6

7

8

9

10

No.of CH Groups Fig. 6. Molar volume of ILs as a function of the number of CH2 groups at 40 °C.

Thermal Expansion Coefficient

6

3.7. Conductivity measurements

9 Conductivities of the ILs were performed at 300 K using a multiparameter Eutech PC 700 instrument of Thermo Scientific. The data illustrated in Fig. 9 showed the values in the range 1.0 to 4.5 μS/cm and a good linear correlation with the addition of CH2 units in the alkyl chain of the ionic liquid. As conductivity is regulated by number and mobility of charge carriers [48], lengths of alkyl chain increase the van der Waals forces and reduce the mobility of charge carrier imidazolium nitrogen [49]. Therefore, the conductivity decreased with increasing the alkyl chain length in ionic liquids. 3.8. Thermal stability Thermogravimetric analysis over a temperature range 30–700 °C determined the thermal decomposition pattern of the ionic liquids as shown in Fig. 10. All the ionic liquids were found to be stable up to 250 to 350 °C followed by a steady and gradual weight loss till 450 °C. The thermal stability of the ionic liquids was increased with lengthening of alkyl chain that can be explained on the basis of enhanced Van der Waals forces of interaction with the expansion of alkyl chain length [25,50].

6 3 0 290

300

310

320

330

340

Temp. αp (10−4 K K−1) IL1

αp (10−4 K−1) IL2

αp (10−4 K−1) IL3

αp (10−4 K−1) IL4

αp (10−4 K−1) IL5

αp (10−4 K−1) IL6

293 K 303 K 313 K 323 K 333 K 343 K 353 K 363 K

5.74 5.85 5.65 5.52 5.55 5.99 1.62 9.99

5.90 5.93 5.72 5.67 5.87 5.73 5.85 5.89

5.70 5.65 5.94 5.80 5.67 5.70 5.73 5.85

5.82 6.20 6.06 5.83 5.96 5.99 6.03 5.98

6.01 6.23 6.27 6.13 6.26 6.20 6.06 6.56

5.85 5.73 5.53 5.64 5.67 5.70 5.74 5.69

370

Fig. 7. A data plot between thermal expansion coefficient and temperature.

viscosity, density, thermal stability, conductivity etc. at atmospheric pressure. The density and viscosity of ionic liquids were found to be decreased with temperature in a linear fashion. The conductivity of ionic liquids decreased with the addition of the CH2 group in the imidazolium ring. Furthermore, other physicochemical properties such as molar volume, thermal expansion coefficient, viscosity index, etc. were calculated from the experimental density and viscosity values using the empirical formulas. The synthesized ionic liquids exhibited moderate to higher thermal stability and VI as compared to the reported ones. Based on these studies, it is believed that the synthesized ionic

100

80

Viscosity Index

Table 9 The thermal expansion coefficient of ILs at different temperatures.

360

Temperature (K)

4. Conclusion We have demonstrated the synthesis and physicochemical characterization of a novel series of cyclophosphazene derivatives containing imidazolium ionic liquids having variable alkyl groups. The effect of alkyl chain length was studied on physicochemical properties, such as

350

60

40

20

0

Fig. 8. The viscosity index of ILs.

A.M. Sadanandhan et al. / Journal of Molecular Liquids 295 (2019) 111722

References

4.5 4.0

Conductivity( S/cm)

7

3.5 3.0 2.5 2.0 1.5 1.0

Fig. 9. Conductivity of IL as a function of temperature at 300 K.

Fig. 10. TGA curve of synthesized ILs.

liquids can be established as promising additives for lubrication formulations for tribological applications.

Acknowledgements Authors gratefully acknowledge Director IIP for granting permission to publish these findings. Analytical Sciences Division of the institute is kindly acknowledged for providing analytical support. Council of Scientific and Industrial Research (CSIR), New Delhi is acknowledged for providing financial assistance to carry out the work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111722.

[1] K. Ghandi, Green and Sustainable Chemistry, 4, 2014 44–53. [2] K. E. Gutowski, Physical Sciences Reviews. 3 (2018) doi.org/https://doi.org/10.1515/ psr-2017-0191. [3] M.P. Mousavi, B.E. Wilson, S. Kashefolgheta, E.L. Anderson, S. He, P. Bühlmann, A. Stein, ACS Appl. Mater. Interfaces 8 (2016) 3396–3406. [4] D. Shang, X. Liu, L. Bai, S. Zeng, Q. Xu, H. Gao, X. Zhang, Current Opinion in Green and Sustainable Chemistry 5 (2017) 74–81. [5] B. Sasikumar, G. Arthanareeswaran, A.F. Ismail, J Mol. Liquids 266 (2018) 330–341. [6] S. Sakthivel, R.L. Gardas, J.S. Sangwai, Energy Fuels 30 (2016) 2514–2523. [7] P. Pillai, A. Kumar, A. Mandal, J. Ind. Eng. Chem. 63 (2018) 262–274. [8] J.P. Hallet, T. Welton, Chem. Rev. 111 (2011) 3508–3576. [9] A.D. Swapnil, Res. J. Chem. Sci. 2 (2012) 80–85. [10] M.J. Earle, K.R. Seddon, Pure Appl. Chem. 72 (2000) 1391–1398. [11] M.V. Reddy, K.R. Byeon, S.H. Park, D.W. Kim, Tetrahedron 73 (2017) 5289–5296. [12] S.A. Dake, S. R. Sarda, R. P. Marathe, R.B. Nawale, U. A. Deokate, S. S. Khadabadi, and R.P. Pawar, in Green Chemistry: Synthesis of Bioactive Heterocycles 2014, 201-230 in K. L. Ameta, A. Dandia, (Eds.), Springer , New Delhi. [13] N.V. Likhanova, M.A. Domínguez-Aguilar, O. Olivares-Xometl, N. Nava-Entzana, E. Arce, H. Dorantes, Corrosion Sci 52 (2010) 2088–2097. [14] Y. Guo Q1, B. Xub , Y. Liua, W. Yanga, X. Yina , Y. Chena, J. Lea and Z. Chen. J. Ind. Eng. Chem. 56 (2017) 234–247. [15] P. Kannan, T.S. Rao, N. Rajendran, J. Mol. Liq. 222 (2016) 586–595. [16] P.K. Khatri, C. Joshi, G.D. Thakre, S.L. Jain, New J. Chem. 40 (2016) 5294–5299. [17] A. García, R. González, A. HernándezBattez, J.L. Viesca, R. Monge, A. FernándezGonzález, M. Hadfield, Tribol. Int. 72 (2014) 42–50. [18] Y. Zhou, J. Qu, ACS Appl. Mater. Interfaces 9 (2017) 3209–3222. [19] E.J. González, B. Gonzalez, E.A. Macedo, J. Chem. Eng. Data 58 (2013) 1440–1448. [20] G.H. Tao, Y. Guo, Y.H. Joo, B. Twamley, J.M. Shreeve, J. Mater. Chem. 18 (2008) 5524–5530. [21] R.P. Singh, R.D. Verma, D.T. Meshri, J.M. Shreeve, Angew. Chem. Int. Ed. 45 (2006) 3584–3601. [22] M. Kermanioryani, M. I. A. Mutalib, Y. Dong, K. C. Lethesh, O. B. O. B. Ghanem, K. A. K., N. F. Aminuddin and J.-M. Leveque, .J. Chem. Eng. Data 61(6) (2016) 2020-2026. [23] M. Yousefi, M. Abdouss, A.A.M. Beigi, A. Naseri, Korean J. Chem. Eng. 34 (9) (2017) 2527–2535. [24] D. Singh, R.L. Gardas, J. Phys. Chem. B 120 (2016) 4834–4842. [25] I.H.J. Arellano, J.G. Guarino, F.U. Paredes, S.D. Arco, J. Therm. Anal. Calorim. 103 (2011) 725–730. [26] O.B. Ghanem, M.A. Mutalib, J.-M. Lévêque, G. Gonfa, C.F. Kait, M. El-Harbawi, J. Chem. Eng. Data 60 (2015) 1756–1763. [27] S.V. Dzyuba, R.A. Bartsch, ChemPhysChem 3 (2002) 161–166. [28] Z. Gu, J.F. Brennecke, J. Chem. Eng. Data 47 (2002) 339–345. [29] N. Zec, M. Vraneš, M. Bešter-Rogač, T. Trtić-Petrović, A. Dimitrijević, I. Čobanov, S. Gadžurić, J. Chem. Thermodyn. 121 (2018) 72–78. [30] J.W. Kim, D. Kim, C.S. Ra, G.B. Han, N.-K. Park, T.J. Lee, M. Kang, J. Ind. Eng. Chem. 20 (2014) 372–378. [31] C.P. Fredlake, J.M. Crosthwaite, D.G. Hert, S.N. Aki, J.F. Brennecke, J. Chem. Eng. Data 49 (2004) 954–964. [32] J.M. Crosthwaite, S.N.V.K. Aki, E.J. Maginn, J.F. Brennecke, J. Phys. Chem. B 108 (2004) 5113–5119. [33] D. Zheng, L. Dong, W. Huang, X. Wu, N. Nie, Renew. Sust. Energ. Rev. 37 (2014) 47–68. [34] M. Yousefi, M. Abdouss, A.A. Miran Beigi, A. Naseri, Korean J. Chem. Eng. 34 (2017) 2527–2535. [35] B.A. Omotowa, B.S. Phillips, J.S. Zabinski, J.M. Shreeve, Inorg. Chem. 43 (2004) 5466–5471. [36] J. Li, D. Feng, Y. Liang, Y. Xia, W. Liu, Ind. Lubr. Tribol. 62 (2010) 161–167. [37] P.K. Khatri, G.D. Thakre, S.L. Jain, Ind. Eng. Chem. Res. 52 (2013), 15829. [38] P.K. Khatri, M.S. Aathira, S.L. Jain, J. Ind. Eng. Chem. 64 (2018) 420–429. [39] P. Nagendramma, P.K. Khatri, G.D. Thakre, S.L. Jain, J. Mol. Liq. 244 (2017) 219–225. [40] J.L. Zhu, P.S. Ren and S. Feng, US Patent Pub. No. 20130035456 A1, 2013. [41] M. Tariq, P.J. Carvalho, J.A.P. Coutinho, M.M. Marrucho, J.N.C. Lopes, L.P.N. Rebelo, Fluid Phase Equilib. 301 (2011) 22–32. [42] M.H. Ghatee, M. Bahrami, N. Khanjari, J. Chem. Thermodyn. 65 (2013) 42–52. [43] S. Yeganegi, V. Sokhanvaran, A. Soltanabadi, Mol. Simulat. 39 (2013) 1070–1078. [44] C. Kolbeck, J. Lehmann, K. Lovelock, T. Cremer, N. Paape, P. Wasserscheid, A. Froba, F. Maier, H.-P. Steinruck, J. Phys. Chem. B 114 (2010) 17025–17036. [45] Y. Wang, G.A. Voth, J. Am. Chem. Soc. 127 (2005) 12192–12193. [46] J.N.A.C. Lopes, A.A.H. Padua, J. Phys. Chem. B 110 (2006) 3330–3335. [47] M. Tariq, P.A.S. Forte, M.F.C. Gomes, J.N.C. Lopes, L.P.N. Rebelo, J. Chem. Thermody. 41 (2009) 790–798. [48] S. De Santis, G. Masci, F. Casciotta, R. Caminiti, E. Scarpellini, M. Campetella, L. Gontrani, Phys. Chem. Chem. Phys. 17 (2015) 20687–20698. [49] D.-J. Tao, Z. Cheng, F.-F. Chen, Z.-M. Li, N. Hu, X.-S. Chen, J. Chem. Eng. Data 58 (2013) 1542–1548. [50] I.H.J. Arellano, J.G. Guarino, F.U. Paredes, S.D. Arco, J. Therm. Anal. Calorim. 103 (2011) 725–730.