Physicochemical properties of piperidinium, ammonium, pyrrolidinium and morpholinium cations based ionic liquids paired with bis(trifluoromethylsulfonyl)imide anion

Accepted Manuscript Physicochemical properties of piperidinium, ammonium, pyrrolidinium and morpholinium cations based ionic liquids paired with bis(trifluoromethylsulfonyl)imide anion Muna Hassan Ibrahim, Maan Hayyan, Mohd Ali Hashim, Adeeb Hayyan, Mohamed K. Hadj-Kali PII:

S0378-3812(16)30304-1

DOI:

10.1016/j.fluid.2016.06.028

Reference:

FLUID 11146

To appear in:

Fluid Phase Equilibria

Received Date: 5 March 2016 Revised Date:

11 June 2016

Accepted Date: 15 June 2016

Please cite this article as: M.H. Ibrahim, M. Hayyan, M.A. Hashim, A. Hayyan, M.K. Hadj-Kali, Physicochemical properties of piperidinium, ammonium, pyrrolidinium and morpholinium cations based ionic liquids paired with bis(trifluoromethylsulfonyl)imide anion, Fluid Phase Equilibria (2016), doi: 10.1016/j.fluid.2016.06.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

Ethyl-dimethyl-propylammonium [EDMPAmm]+

ACCEPTED MANUSCRIPT Physicochemical Properties of Piperidinium, Ammonium, Pyrrolidinium and Morpholinium Cations Based Ionic Liquids Paired with Bis(trifluoromethylsulfonyl)imide Anion

Mohamed K. Hadj-Kalie a

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Muna Hassan Ibrahima,b, Maan Hayyanb,c*, Mohd Ali Hashima,b, Adeeb Hayyanb,d*,

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Department of Chemical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia b University of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysia c Department of Civil Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia d Institute of Halal Research University of Malaya (IHRUM), Academy of Islamic Studies, University of Malaya, 50603, Kuala Lumpur, Malaysia e Chemical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia *Email: [email protected]; [email protected], Phone: +6-012-3002949,

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Fax: +6 -03-79675311.

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ACCEPTED MANUSCRIPT Abstract This study aims to investigate the temperature dependence of the physicochemical properties of five cations paired with bis(trifluoromethylsulfonyl)imide anion, namely 1-(2methoxyethyl)-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide, 1-(2-methoxyethyl)bis(trifluoromethylsulfonyl)imide,

methylmorpholinium

bis(trifluoromethylsulfonyl)imide,

methoxyethylammonium

bis(trifluoromethylsulfonyl)imide

N-methoxyethyl-N-

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1-methylpyrrolidinium

N-ethyl-N,N-dimethyl-2and

ethyl-dimethyl-

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propylammonium bis(trifluoromethylsulfonyl)imide. The density, viscosity, conductivity and surface tension of the resulting ionic liquids (ILs) have been determined within the

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temperature range (298.15 - 353.15) K. It has been found that all ILs exhibited Arrhenius behavior for conductivity and viscosity while the surface tension and density followed a linear trend. A satisfactory agreement was obtained between our experimental densities and those predicted by the group contribution model. This study is essential in the application of

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these ILs for process design and will reinforce the development of new correlations and other predictive methods in particular when the physicochemical properties are scarce, like in the

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case of morpholinium-based ILs.

Keywords: ionic liquid; physical properties; chemical process; industrial scale; green

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technology; surface tension.

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ACCEPTED MANUSCRIPT 1. Introduction Ionic liquids have been the subject of great interest in recent years due to their many attractive properties. ILs are salts which melt at 100 °C or below [1, 2], usually composed of an organic cation combined with an organic or inorganic anion [3-6]. ILs have many

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desirable characteristics such as low volatility and thermal stability [3]. Furthermore ILs are highly tunable and can be designed with a specific task in mind, hence the term ‘designer solvents’ that is sometimes used to describe them [7]. For this reason ILs have been explored

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for a wide range of applications including catalysis [8, 9], electrochemistry [10, 11], fuel cells

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[12] and separation processes [13].

The high electrostatic interactions between the ions of ILs give their characteristic physicochemical properties which distinguish them from conventional organic solvents [14]. Several of their physical properties, such as density and viscosity can be easily tuned or tailored by variation of cation and anion [15-17]. The determination and understanding of

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these fundamental physical and transport properties is indispensable for process design [18]. Furthermore, the collection of a large data bank for physicochemical properties of various ILs

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is essential, not only for product process design but also for the development of predictive methods and the design of ILs [19].

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Bis(trifluoromethylsulfonyl)imide [TFSI] is one of the most commonly used anions in ILs, as a result of the favorable properties of the resulting ILs. [TFSI] based ILs commonly are a liquid above 0 °C and exhibit low viscosity, which makes them favorable as solvents [20]. In addition, ILs with hexafluorophosphate and tetrafluoroborate anions have been found to be disadvantageous for industrial applications due to their possible decomposition to toxic hydrofluoric acid [15].

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ACCEPTED MANUSCRIPT Most studies on physical properties at various temperatures have focused on imidazolium cations [18, 21, 22], other families of cations have been less extensively studied. In particular reports on the physicochemical properties of morpholinium based ILs are scarce [23, 24]. This is despite the low cost of morpholinium cation sources [23]. Furthermore, morpholinium

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based ILs have been the subject of much interest due to their structural properties particularly for ionic liquid crystals [25]. There have also received attention for electrochemical applications and as heat stabilizers, catalysts and antioxidants [24]. In addition, morpholinium

makes them desirable as media for O2•− generation [26].

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based cations have been reported to have high stability for superoxide ion (O2•−) which

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In this work, we have investigated the physicochemical properties of five ILs, namely 1-(2methoxyethyl)-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide [MOEMPip][TFSI], 1-(2-methoxyethyl)-1-methylpyrrolidinium [MOEMPyrr][TFSI],

bis(trifluoromethylsulfonyl)imide

N-methoxyethyl-N-methylmorpholinium

[MOEMMor][TFSI],

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bis(trifluoromethylsulfonyl)imide

N-ethyl-N,N-dimethyl-2-

methoxyethylammonium bis(trifluoromethylsulfonyl)imide [N112,1O2][TFSI] and ethyldimethyl-propylammonium bis(trifluoromethylsulfonyl) imide [EDMPAmm] [TFSI]

in

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the temperature range of 25 to 80 °C (298.15 to 353.15 K) at atmospheric pressure. The

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essential physicochemical properties of density, conductivity, viscosity and surface tension have been obtained as a function of temperature.

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ACCEPTED MANUSCRIPT 2. Methods and materials 2.1. Chemicals In this study, synthesis grade ILs provided by Merck (Germany) were used. These ILs are summarized in Table 1 with the abbreviation adopted in this work. The ILs were dried

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overnight in a vacuum oven at 50 °C and the purity of these ILs was checked by 1H NMR analysis. The spectra results are provided as supplementary material in Fig S1 which confirms that no impurities were detected. Moreover, FT-IR characterization was used to

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identify the functional groups of three ILs among those used in this work. Fig S2 shows the FT-IR spectra and Table S1 lists the functional groups identified. The water content in each

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IL was determined by Karl Fischer titration and is also listed in Table 1. Scheme 1 and Scheme 2 show the structure of cations and anion used respectively.

Table 1: ILs studied in this work Abbreviation

1-(2-Methoxyethyl-1methylpiperidinium bis(trifluoromethylsulfonyl)imide

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IL

Molecular weight

Formula

Purity* (%)

Halides* (%)

≤ 0.1

Water (ppm) (KF)

[MOEMPip][TFSI]

452.44

C12H22F6N2O5S2

≥ 98.0

[MOEMMor][TFSI]

440.38

C10H18F6N2O6S2

≥ 98.0

≤ 0.1

232

[MOEMPyrr][TFSI]

424.38

C10H18F6N2O5S2

≥ 98.0

≤ 0.1

190

N-Ethyl-N,N-dimethyl-2methoxyethylammonium bis(trifluoromethylsulfonyl)imide

[N112,1O2][TFSI]

412.37

C9H18F6N2O5S2

≥ 98.0

≤ 0.1

318

Ethyl-dimethyl-propylammonium bis(trifluoromethylsulfonyl)imide

[EDMPAmm][TFSI]

396.37

C9H18F6N2O4S2

≥ 98.0

≤ 0.1

243

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N-Methoxyethyl-Nmethylmorpholinium bis(trifluoromethylsulfonyl)imide

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1-(2-Methoxyethyl)-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

*Merck MSDS

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Scheme 1: Structure of cations of studied ILs

Scheme 2: Structure of anion of studied ILs

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ACCEPTED MANUSCRIPT 2.2. Technical methodology In this study, all physical properties were measured in temperature range of 25–80 ° C. Fresh samples were analyzed to avoid air moisture and contaminants which may have an impact on the physical properties of ILs. Density was determined using Mettler Toledo DM40 density

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meter. The density meter was calibrated using four Mettler Toledo liquid standard reference substances of water, dodecane, 2,4-dichlorotoluene and 1- bromonapthalene.

Conductivity was measured using Trans instruments BC3020 conductivity meter, calibrated

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using standard conductivity solutions. The standard conductivity solutions used were Eutech Instruments standard potassium chloride solution of 84 µS at 25 °C , 1413 µS at 25 °C and

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500 µS at 25 °C. Temperature variation was achieved using a water circulator (Protech 631D).

The viscosities of the ILs were measured with Brookfield R/S plus rheometer. The rheometer

water circulator.

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was calibrated to zero point before each set of experiments. Temperature was controlled by a

The surface tension was obtained using KSV Sigma 702 tensiometer using platinum Du

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Noüy ring. A water-bath with temperature control was used for temperature variation. Table

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2 shows the experimental uncertainties in the measurement of each physical property.

Table 2: Uncertainties for measured physiochemical properties Property

Uncertainty

Density

± 0.0001 g cm−3

Viscosity

± 5 % of measured value

Conductivity

± 0.2 mS cm−1

Surface tension

± 0.2 mN m−1

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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Density Fig. 1 and Table S2 show the temperature dependence of density (ρ) for the five [TFSI]anion-based

ILs.

The

densities

decreased

in

the

order

[MOEMMor][TFSI]>

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[MOEMPyrr][TFSI]>[MOEMPip][TFSI] > [N112,1O2][TFSI] >[EDMPAmm] [TFSI]. At 298.15 K, the values of density were 1.50 g cm−3 for [MOEMMor][TFSI], 1.45 g cm−3 for [MOEMPyrr][TFSI], 1.43 g cm−3 for [MOEMPip][TFSI], 1.42 g cm−3 for [N112,1O2][TFSI]

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and 1.40 g cm−3 for [EDMPAmm][TFSI]. At room temperature, the density of the ILs was higher than that of H2O which 0.997 g cm−3 [27]. The densities obtained did not show a

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relationship with molecular weight of the cation but depended on the structure of the cation. Seki et al. also found density did not show a relationship with molecular weight of the cation [28].

The rate of decrease in density with temperature was similar for all samples and the slope of

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dρ/dT varied from 9.09 to 10.26×10−4 g cm−3 K−1. This was in accordance with literature values (8.8 to 10.6×10−4 g cm−3 K−1) for 1-alkyl-3-methylimidazolium and 1,3dialkylimidazolium ionic liquids containing the same [TFSI] anion [14, 21, 29].

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At 298.15 the denisty obtained for [EDMPAmm][TFSI], 1.4030 g cm−3 was in agreement with the value obtained by Řehák et al. [30], 1.4035 g cm−3. The density of [N112,1O2] at

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298 K (1.41 g cm−3) was 97.2% of the value reported by Zhou et al. (1.45 g cm−3) [31, 32]. The value of density at 298.15 K for [MOEMPyrr][TFSI] (1.45 g cm−3) was in agreement to that reported by Zhou et al. and Regueira et al. [15, 32, 33]. For [MOEMPyrr][TFSI], the values obtained at 313.15 K and 333.15 K were identical to those reported by Regueira et al. ( 313.15 K, 1.439 g cm−3 and 333.15 K, 1.421 g cm−3) [15].

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1.52 [MOEMMor][TFSI] [MOEMPyrr][TFSI]

1.50

[MOEMPip][TFSI]

[N112,1O2][TFSI] [EDMPAmm][TFSI]

1.48

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ρ/g cm-3

1.46 1.44 1.42 1.40

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1.38

1.34 290

300

310

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1.36

320

330

340

350

360

T/K

Fig. 1. Densities of ILs as a function of temperature

ρ = b − aT

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Generally, the relationship of density ρ (g cm−3) with temperature, follows Eq. (1) [28]: (1)

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where a and b are constants which represent coefficient of volume expansion (g cm−3 K−1)

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and extrapolated density at 0 K (g cm−3), respectively and T is temperature (K)[28]. Calculated values of a and b are listed in Table 3.

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ACCEPTED MANUSCRIPT Table 3: Calculated parameters for a and b of density of [TFSI]-based ILs using Eq. (1) R2

b

g cm−3 K−1 × 104

g cm−3

[EDMPAmm][TFSI]

9.12

1.675

0.99952

[MOEMPip][TFSI]

9.09

1.704

0.99996

[MOEMPyrr][TFSI]

9.32

1.731

0.99996

[N112,1O2][TFSI]

10.3

1.732

0.99866

[MOEMMor][TFSI]

9.18

1.774

0.99999

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a

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IL

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Density values within the studied temperature range are compared with available literature values in Fig. 2. For [MOEMPip] (a), the values are in agreement with the measurements obtained by Marciniak et al.[34]. When [MOEMMor] (b) is compared to 4-(2-methoxyethyl)4-methylmorpholinium by Marciniak et al. (2012b) [35], the data fall on the same line. For

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[MOEMPyrr][TFSI] (c) there is a close agreement with both Gacino et al. [36], Marciniak et al. [34] and Regueira et al. [15]. However for [N112,1O2] (d), the values obtained are higher than results obtained by Zoubeik, et al. [37], though it should be noted that the density

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reported by Zhou et al. (1.45 g cm−3) [31, 32] at 298.15 K was higher than the measured value. The slight variation in the results is expected as the synthesis method and purity of the

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stdeis ILs were different. However, in order to test and validate our density measurements against an efficient predictive model, we have applied the group contribution model developed by Jacquemin et al. [38] to predict the molar volume of all ILs investigated in this work, except for [EDMPAmm][TFSI] IL because the cation [EDMPAmm] is not available in the database provided in the original work of Jacquemin et al.. The comparison between the experimental and predicted densities is shown in Table 4. As can be seen, the relative error increases with temperature but in overall a very good agreement is obtained since the error

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ACCEPTED MANUSCRIPT doesn't exceed 2% for all ILs for the whole temperature range. These results confirm the high prediction capability of this model. a) [MOEMPip]

(b) [MOEMMor] compared to 4-(2methoxyethyl)-4-methylmorpholinium

1.44

1.51

4-(2-methoxyethyl)-4-methyl morpholinium Marciniak et al. [MOEMMor] present work

1.50

1.42

1.49

1.41

1.40

1.48

1.47

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ρ/g cm-3

ρ/g cm-3

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[MOEMPip] Marciniak et al [MOEMPip] present work

1.43

1.39 1.46

1.38

1.45

300

310

320

330

340

350

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290

T/K

1.44

290

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1.42

1.41

1.40

1.39

300

320

330

340

350

360

370

T/K

[N112,1O2] Gacino et al. [N112,1O2] present work [N112,1O2]] Zhou et al

1.44

1.42

ρ/g cm-3

1.43

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ρ/g cm-3

1.44

320

1.46

[MOEMPyrr] Gacino et al. [MOEMPyrr] present work [MOEMPyrr] Marciniak et al [MOEMPyrr] Regueira et al

1.45

310

(d) [N112,1O2]

(c) [MOEMPyrr][TFSI] 1.46

300

1.40

1.38

1.36

1.34 300

340

360

320

340

T/K

T/K

Fig. 2. Density comparison with literature data within studied temperature range (a)[MOEMPip][TFSI](b) [MOEMMor] compared to 4-(2-methoxyethyl)-4methylmorpholinium (c) [MOEMPyrr][TFSI] (d) [N112,1O2][TFSI].

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ACCEPTED MANUSCRIPT Table 4: Comparison between experimental measured densities and density calculated using the group contribution method proposed by Jacquemin et al. [38]

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T/K 298.15 303.15 313.15 323.15 333.15 343.15 353.15

[MOEMMor][TFSI] Cal exp Error (%) 1.5026 1.5002 0.16% 1.4999 1.4957 0.28% 1.4945 1.4864 0.55% 1.4892 1.4772 0.81% 1.4838 1.4680 1.07% 1.4784 1.4589 1.33% 1.4730 1.4498 1.60% [N112,1O2][TFSI] Cal exp Error (%) 1.4204 1.4249 0.32% 1.4156 1.4205 0.34% 1.4109 1.4110 0.01% 1.4061 1.4015 0.33% 1.4014 1.3903 0.80% 1.3967 1.3792 1.27% 1.3920 1.3689 1.69%

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T/K 298.15 303.15 313.15 323.15 333.15 343.15 353.15

[MOEMPip][TFSI] Cal exp Error (%) 1.4358 1.4332 0.18% 1.4334 1.4290 0.31% 1.4285 1.4198 0.61% 1.4235 1.4107 0.91% 1.4186 1.4017 1.20% 1.4136 1.3925 1.52% 1.4087 1.3833 1.83% [MOEMPyrr][TFSI] Cal exp Error (%) 1.4548 1.4530 0.13% 1.4522 1.4487 0.24% 1.4470 1.4393 0.54% 1.4418 1.4299 0.83% 1.4365 1.4207 1.12% 1.4313 1.4112 1.42% 1.4260 1.4019 1.72%

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Moreover, for the omitted IL [EDMPAmm][TFSI], our new experimental densities can easily be used to regress the missing parameters of the cation ([EDMPAmm]) in the existing

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database.

On the other hand, based on the structure of ILs used in this work, it is possible to investigate

when

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the effect of adding one oxygen atom on the density of ILs. Indeed, this effect is palpable comparing

(i)

[MOEMMor][TFSI]

with

[MOEMPyrr][TFSI]

and

(ii)

[N112,1O2][TFSI] with [EDMPAmm][TFSI]. However, in the first case the oxygen atom is added to a cyclic structure while in the second case it is added inside the aliphatic chain. In both cases, the addition of one oxygen atom will result in an increase of IL density. But this increase is more important in the first case (by about 3% for the whole temperature range) than it is in the second case, where it doesn't exceed 2%.

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ACCEPTED MANUSCRIPT 3.2 Viscosity The viscosity η is a physiochemical property which results from species interactions such as hydrogen bonding, van der Waals and columbic forces [39]. Viscosity is thought to be affected by the size and shape of the IL ions. Mass transport phenomena and conductivity are

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affected by viscosity. Thus, viscosity may restrict the suitability of particular ILs for some applications [28, 39]. The viscosity of some ILs is frequently significantly higher than that of conventional organic fluids commonly used in industry. High viscosity could be unfavorable

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for some industrial applications due to its negative impact on processes such as stirring, pumping and mass transfer operations [40]. In numerous applications such as fuel cell,

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synthetic solvents and field-effect transistors, high viscosity is a serious problem [28]. However, large viscosities can be desirable for some applications such as lubrication [40]. Fig. 3 depicts the viscosities of the ILs vs temperature. At room temperature, the IL with the morpholinium based cation [MOEMMor]+ shows a profoundly greater viscosity compared to

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the other ILs, Table S3. At room temperature, the [MOEMMor]+ based IL showed a dramatically higher viscosity (257.6 mPa s) than the other cations based ILs which had similar viscosities in the range of 52.19 mPa s [MOEMPyrr] to 69.89 mPa s [MOEMPip]. The values

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obtained for the ammonium based ILs were similar at 59.39 mPa s [N112,1O2] and 58.79

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mPa s [EDMPAmm]. However at higher temperatures, the difference in viscosities between [MOEMMor] and the other ILs was less significant. The observed decrease in viscosity with temperature is expected. This is due to the increased movement of the ions at higher temperature, this weakens the interionic forces between the ions and the liquid becomes less resistant to flow [41] At room temperature, the IL viscosities were much higher than that of H2O which is 0.8901 mPa s [27]. Like density, the viscosities obtained did not depend on molecular weight of the cation but instead depended on the cation structure as was found by Seki et al. [28]. On the contrary, in the same study Seki et al. found that the viscosity

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ACCEPTED MANUSCRIPT increases with the increase in molecular weight of the anion. This study confirms that cation molecular weight does not have this relationship. At 298 K, the viscosity obtained for [MOEMPyrr] (52.19 mPa s) was in agreement and 98.47% of the value reported by Zhou et al. (53 mPa s) [32, 33]. However the value for

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[EDMPAmm] (58.79) was 70.83 % of the value reported by Macfarlane et al. (83 mPa s )

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[32, 42].

6.0 [MOEMMor][TFSI] [MOEMPip][TFSI] [EDMPAmm][TFSI] [MOEMPyrr][TFSI] [N112,1O2][TFSI]

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5.5

4.5 4.0 3.5

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ln (η/mPa s)

5.0

3.0 2.5

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2.0 0.0028

0.0030

0.0031

0.0032

0.0033

(1/T)/K-1

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0.0029

Fig. 3. Ln (viscosity, η) of ILs as a function of (1/T)

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ACCEPTED MANUSCRIPT The viscosities of the studied ILs were fitted using the Arrhenius model as shown below, Eq. (2) [43]:

(2)

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η = η0e

 Eη  −   RT 

Where T is temperature (K), R is gas constant (J mol−1K−1), Eη is activation energy (J mol−1), η0 is pre-exponential constant (mPa s) and η is viscosity (mPa s). A good regression of the

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experimental data with the Arrhenius model is shown by the linear trend of ln(viscosity) as a function of ln(1/T), the values of R2 ranged from 0.975-0.998. Calculated values of −(Eη/R)

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and η0 are listed in Table 5.

Table 5: Calculated parameters for −(Eη/R) and η0 of viscosity of [TFSI]-based ILs using Eq. (2) −(Eη/R)

η0

K

mPa s

[MOEMMor][TFSI]

4379.80

1.07×10−4

0.9974

[EDMPAmm] [TFSI]

2835.30

4.45×10−3

0.9975

[MOEMPip][TFSI]

3164.90

1.61×10−3

0.9837

[N112,1O2][TFSI]

2951.40

2.76×10−3

0.9749

[MOEMPyrr][TFSI]

2627.70

7.80×10−3

0.9981

R²

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IL

The values of viscosity within the studied temperature range are compared with literature values in Fig. 4 in which (a) [MOEMPyrr] compared with Gacino et al.[36] and Marciniak et al. [34]. (b) [MOEMPip] compared with Marciniak et al. [34] (c) [MOEMMor] compared to 4-(2-methoxyethyl)-4-methylmorpholinium by Marciniak et al. [34]. It can be noticed that there is a similar trend and agreement by indicating that the viscosity decreases with temperature increase. 15

ACCEPTED MANUSCRIPT a) [MOEMPyrr]

(b) [MOEMPip] 120

60

[MOEMPyrr] Present work [MOEMPyrr] Gacinio et al. [MOEMPyrr] Marciniak et al

50

80

30

60

40

20

20

10

0

0

290

290

300

310

320

330

340

350

360

300

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η/mPa s

η/mPa s

40

310

320

330

340

350

360

T/K

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T/K

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(c) [MOEMMor] compared to 4-(2methoxyethyl)-4-methylmorpholinium 350

[MOEMMor] Present work 4-(2-methoxyethyl)-4-methyl morpholinium Marciniak et al.

300

250

200

150

100

50

0 290

300

310

320

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η/mPa s

[MOEMPip] Present work [MOEMPip] Marciniak et al.

100

330

340

350

360

EP

T/K

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Fig. 4. Viscosity values compared with literature data within studied temperature range (a) [MOEMPyrr][TFSI] (b) [MOPMPip][TFSI] (c) [MOEMMor] compared to 4-(2methoxyethyl)-4-methylmorpholinium. 3.3 Conductivity

Though ILs are totally comprised of ions, the conduction processes does not involve all of the ions. The resulting conductivity (γ) is dependent on the nature and structure of the IL [44]. Fig. 5 and Table S4 illustrate the obtained conductivities of the samples as a vs temperature. The morpholinium cation displays the poorest conductivity at all temperatures. This is expected as it also shows the highest viscosity which affects conductivity [39]. This low conductivity and high viscosity of morpholinium based ILs was also obtained by Yeon et al. 16

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N-(2-hydroxyethyl)-N-methyl

morpholinium

based

ILs

[HEMMor][BF4]

and

[HEMMor][TFSI] [23]. Generally, the conductivity increased at a faster rate at higher temperatures except for [MOEMPyrr]+ in which it increases at a slower rate at higher temperature. Conductivity

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decreases in the order [MOEMPyrr](3.45 mS cm−1)>[N112,1O2] (2.88 mS cm−1) >[EDMPAmm](2.48 mS cm−1)>[MOEMPip](2.09 mS cm−1)>[MOEMMor] (0.653 mS cm−1). This order of conductivity was maintained at all temperatures with the exception of 80 °C at

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which the conductivities of the ammonium based ILs were almost identical.

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The conductivity of [MOEMPyrr]+ at 298.15 K (3.45 mS cm−1) was similar but slightly lower (93.24%) of the value reported by Zhou et al. (3.7 mS cm−1) [33]. The value for [N112,1O2] (2.88 mS cm−1) was also slightly lower (92.90%) of the value reported by the same research group (3.1 mS cm−1) [31]. The value obtained for [EDMPAmm] (2.48 mS cm−1) was double

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the result reported by Macfarlane et al. (1.2 mS cm−1) [32, 42].

17

ACCEPTED MANUSCRIPT

2.5

2.0

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ln (γ/mS cm-1)

1.5

1.0

0.0

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0.5

-0.5

-1.0 0.0028

0.0029

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[MOEMPyrr][TFSI] [N112,1O2][TFSI] [EDMPAmm][TFSI] [MOEMPip][TFSI] [MOEMMor][TFSI]

0.0030

0.0031

0.0032

0.0033

0.0034

(1/T)/K-1

Fig. 5. Ln (Conductivity, γ) of ILs as a function of (1/T).

(3)

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γ = γ 0e

 Eγ  −   RT 

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The Arrhenius equation was used to fit the behavior as shown below in Eq (3) [43]

where γ the conductivity in (mS cm−1), γo is a constant (mS cm−1), E γ is the activation energy

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of conductivity (J mol−1 ) , R is the gas constant (J mol−1 K−1) and T is temperature (K). The data showed a good fit with the Arrhenius model with R2 range of 0.973-0.991. Calculated values of −(Eγ/R) and γo are listed in Table 6.

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ACCEPTED MANUSCRIPT Table 6: Calculated parameters for −(Eγ/R) and γo of conductivity of [TFSI]-based ILs using Eq. (3) γo

R²

K

mS cm−1

[MOEMPip][TFSI]

1930.60

1353.70

0.9901

[MOEMPyrr][TFSI]

1292.10

281.63

0.9733

[EDMPAmm][TFSI]

1687.80

705.85

0.9770

[N112,1O2][TFSI]

1478.80

404.12

0.9893

[MOEMMor][TFSI]

3042.10

17483.28

0.9910

SC

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−(Eγ/R)

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IL

3.4 Surface tension

Surface tension (σ) is an important physical property which is the force required to close a cut on the liquid surface [45]. Thermodynamically, surface tension is defined as the change of

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surface free enthalpy G per area A (σ = δG/δA) [46]. The determination of surface tension of ILs is a way to indirectly determine important information on the interactions between the IL ions [45]. Surface tension occurs as a result of the molecular orientation at the surface as well

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as intermolecular interactions in the bulk cohesive energy [46]. Other studies have shown that

[45].

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ILs with the [TFSI]− anion generally exhibit lower surface tension values than other anions

Fig. 6 depicts the obtained surface tension of the ILs. At room temperature, surface tension of the ILs is less than that of H2O (71.98 mN m−1) [27] and decreases in the order at 50.14 [MOEMMor]> 49.22 [MOEMPyrr]> 47.56 [N112,1O2]> 46.56 [EDMPAmm]> 44.65 mN m−1 [MOEMPip], Table S5. The values are in agreement with the reported surface tension of ILs which typically fall between the surface tension of octane and water [45] (21.14 to 71.99 m−1) [47]. The surface tension decreased with temperature following a linear trend, at higher 19

ACCEPTED MANUSCRIPT temperatures of 60 to 80 °C the surface tension of the ammonium based ILs ([N112,1O2]+ and [EDMPAmm]+ was very similar.

52

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[MOEMMor][TFSI] [N112,1O2][TFSI] [N112,1O2][TFSI] [EDMPAmm][TFSI] [MOEMPip][TFSI]

48

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46

44

42

40

38 300

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Surface Tension(σ)/mNm

-1

50

320

340

360

T/K

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Fig. 6. Surface tensions of ILs as a function of temperature

Eq (4):

(4)

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σ = b − aT

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The surface tension as a function of temperature was modeled using the following equation,

Where, σ is surface tension (mN m−1) and T is temperature (K) . While a (mN m−1 K−1) and b (mN m−1) are parameters which vary with the ILs. Table 7 shows the calculated values of a and b for Eq. (4).

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ACCEPTED MANUSCRIPT Table 7: Calculated parameters for a and b of surface tension [TFSI]-based ILs using Eq. (4) R2

b

mN m−1 K−1× 102

mN m-1

[EDMPAmm][TFSI]

9.88

75.918

0.993

[MOEMPip][TFSI]

10.35

75.495

0.997

[MOEMPyrr][TFSI]

12.03

85.101

0.991

[N112,1O2][TFSI]

12.03

85.101

0.992

[MOEMMor][TFSI]

9.85

79.305

0.992

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a

SC

IL

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The obtained linear trend is expected as surface tension is known to decreases linearly with temperature between freezing to the boiling temperature though it vanishes non-linearly close to the critical point [48]. The gradient of the variation of surface tension with temperature , i.e. surface entropy, can be used to characterize fluids in terms of the molecular energetics

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and surface microstructure [45, 48].

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ACCEPTED MANUSCRIPT 3.5 Walden Plot The Walden plot is a useful method to provide a qualitative measure of the ionicity of ILs [49, 50]. The Walden rule is given by the following equation [43]

Λη = k

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(5)

Where Λ is molar conductivity, η is viscosity and k is a parameter. The Walden plot of log(Λ) vs log(1/η) is used to compare the ionicity of compounds [51].

log ( Λ ) = log C + klog (1 / η )

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(6)

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Where log C is the intercept and k is the slope. Calculated values of log C and k are listed in Table 8.

Table 8: Calculated parameters for k and log C of Walden plot of [TFSI]-based ILs using Eq. (6) R2

Slope

Intercept

k

log C

0.6144

− 0.2936

0.9637

[MOEMPip][TFSI]

0.6144

− 0.2944

0.9558

[MOEMPyrr][TFSI]

0.5175

− 0.1124

0.9775

[N112,1O2][TFSI]

0.5066

− 0.2118

0.9377

[MOEMMor][TFSI]

0.6893

− 0.4541

0.978

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[EDMPAmm][TFSI]

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IL

The “ideal” Walden line runs through a square diagram from corner to corner for the units chosen and represents data for dilute KCl (aq) where the ions have equal mobility and are fully dissociated

[50, 52]. The ionicity of the IL can then be determined from this

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ACCEPTED MANUSCRIPT comparison, and the IL can be classified as ‘superionic’(above the line), ‘good ionic’ (close to the line) or ‘subionic’ (far below the line) [51, 53]. Deviations from the ideal line can be quantified by the vertical distance to the ideal line (∆W)

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[44, 54]. ∆W value of 1 indicates an IL exhibits of 10% of the ionic conductivity for the KCl(aq) line [44]. ILs with ∆W greater than 1 are considered “poor” ILs and have low ionicty [44, 54] while ILs with ∆W less than one can be considered high ionicity ILs [54]

For the studied ILs, ∆W is less than 0.6, meaning the ioncity of all studied ILs is greater than

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25% of the ionicity expected for KCl(aq), on the basis of viscosity. The ILs can be considered

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‘good ionic’.

Upon inspection of the Walden plot (Fig. 7), it can be seen that all the studied ILs are located below the ideal line. This is expected as the vast majority of ILs fall below the line, the extent of which depends on the IL structure [44, 55]. ILs typically fall below the ideal due to the strong interaction between ions, which affects the movement of the ions [54]. The walden

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plots of the ILs are relatively close together. The ILs appear to have similar ionicity, with [MOEMPyrr][TFSI] having the highest ionicity and [MOEMMor][TFSI] having the lowest

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same ionicity.

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ionicity. [N112,1O2][TFSI], [EDMPAmm][TFSI] and[MOEMPip][TFSI] appear to have the

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KCl (aq) [MOEMPyrr][TFSI] [EDMPAmm][TFSI] [N112,1O2][TFSI] [MOEMPip][TFSI] [MOEMMor][TFSI]

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0.5

0.0

-0.5

SC

2 -1 Log [molar conductivity (S cm mol )]

1.0

-1.0

-0.5

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-1.0 0.0

0.5

1.0

-1

Log [1/viscocity(Poise )]

Fig. 7.Walden plot for KCl(aq) and ILs

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It has been found that ionicity is dependent on the structure of both the cation and anion. Fraser et al. found that for tetradecyltrihexyl phosphonium cation, the deviation from the KCl line follows the order chloride ~ cyclamate ~ dodecylbenzenesulfonate > saccharinate >

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dicyanamide ~ [TFSI]− [54].

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Ueno et al. [50] also found the anion had a noticeable effect on deviation for for 1-butyl-3methylimidazolium ILs which follows trend trifluoroacetate, > trifluoromethanesulfonate, > [TFSI] > tetrafluoroborate > hexafluorophosphate. In addition, they also found the cation had an effect on ionicity. The alkyl chain length had an effect with deviation from KCl(aq) line for 1-alkyl-3-methylimidazolium [Cnmim][TFSI] ILs with deviation decreasing in the order [C8mim] > [C6mim] >[C4mim] > [C2mim] ~ [C1mim]. For [TFSI]− ILs with the same butyl group substituent on different cationic backbone structures (imidazolium, pyridinium, pyrrolidinium and ammonium), the deviations were not clear. In contrast, in the present work 24

ACCEPTED MANUSCRIPT when comparing [MOEMPyrr][TFSI] and [MOEMMor][TFSI], it can be seen than pyrrolidinium has higher ionicity than piperidinium from its closer proximity to the ideal KCl(aq) line. 4. Conclusion

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The physicochemical properties of five bis(trifluoromethylsulfonyl)imide anion-based ILs have been obtained experimentally in the temperature range 298.15 to 353.15 K. The density, viscosity, conductivity and surface tension were measured as function of temperature at

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atmospheric pressure. All ILs exhibited Arrhenius behavior for both conductivity and viscosity while the surface tension and density followed a linear trend. [MOEMMor][TFSI]

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IL showed the poorest conductivity and highest viscosity, surface tension and density. However the viscosity of the morpholinium based IL was close to that of the other ILs at higher temperatures. Hence, the high viscosity of morpholinium based ILs which is unfavorable for some applications is less pronounced at higher temperatures.

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Acknowledgments

This research was funded by the financial support of the University of Malaya Grant No. HIR- MOHE (D000003-16001) in collaboration with the Deanship of Scientific Research at

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King Saud University through the group project number RGP-VPP-108. The authors are also

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grateful to Miss Shahidah Nusailah Rashid and Dr. Lahssen El blidi for their help in conducting and analyzing some experiments. 5. References

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ACCEPTED MANUSCRIPT List of symbols Activation energy of viscosity (J mol−1K−1)

Eγ

Activation energy of conductivity (J mol−1K−1)

R

Gas constant (mol−1K−1)

T

Temperature (K)

Λ

Molar conductivity (S cm2 mol-1)

∆W

Vertical distance to the KCl(aq) line on Walden plot

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Eη

Greek Symbols

Electrical conductivity (mS cm−1)

γo

Constant (mS cm−1)

η

Viscosity (mPa s)

η0

Pre-exponential constant (mPa s)

ρ

Density (g cm−3)

σ

Surface tension (mN m−1)

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γ

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Research Highlights

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• Physicochemical properties of five ionic liquids (ILs) at 298.15 to 353.15 K were reported. • Bis(trifluoromethylsulfonyl)imide was fixed as anion in all tested ILs. • [MOEMPip], [MOEMMor], [MOEMPyrr], [N112,1O2] and [EDMPAmm] based cations were examined. • ILs possess superior performance comparing to conventional solvents. • ILs are promising fluids can be implemented in a wide range of applications.