The transport and conductivity properties of the ionic liquid EMIMTCM

Journal of Molecular Liquids 201 (2015) 96–101

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

The transport and conductivity properties of the ionic liquid EMIMTCM Batchimeg Ganbold, Gang Zheng, Scott A. Willis, Gary R. Dennis, William S. Price ⁎ Nanoscale Organisation and Dynamics Group, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia

a r t i c l e

i n f o

Article history: Received 25 July 2014 Received in revised form 16 October 2014 Accepted 3 November 2014 Available online 7 November 2014 Keywords: Conductivity Diffusion NMR EMIMTCM EMIMTFSA EMIMFSA Viscosity

a b s t r a c t The ionic liquid 1-ethyl-3-methylimidazolium tricyanomethanide (EMIMTCM) is a very interesting, yet so far poorly studied, ionic liquid which has a low viscosity, high conductivity, and acceptable electrochemical stability for use as an electrolyte. In this study the self-diffusion coefficients, spin–lattice relaxation times, and spin–spin relaxation times of EMIMTCM were measured over the temperature range 263 to 343 K. To gain insight into the origin of the physical properties of EMIMTCM, the temperature dependence of the measured cation diffusion coefficient was compared with two structurally related EMIM-based ionic liquids which differed only in the choice of anion: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (EMIMFSA) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)amine (EMIMTFSA). The experimentally determined diffusion coefficient, conductivity, and viscosity values were analysed using the Stokes–Einstein–Sutherland, Stokes–Einstein–Debye and Vogel–Fulcher–Tamman equations, respectively. The correlation time (τcation) of the cations were calculated from the relaxation data using the Bloembergen–Purcell–Pound equation. The overall isotropic molecular reorientational correlation time, τc, and translational correlation time, τD, of the cations were calculated from the viscosity and translational diffusion coefficients, respectively. The results of this study provide information on reorientational and translational motions of EMIMTCM and the correlation times were found to be in the following order τD N τc N τcation and EMIMTCM diffused faster than EMIMTFSA and EMIMFSA, due to the smaller size of the TCM anion and its lower viscosity. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids (ILs) are composed only of cations and anions and have a melting point below 100 °C. They possess a unique combination of properties including high thermal stability, a wide range of solubility for different chemicals, high conductivity, low moisture sensitivity, low melting point, high heat capacity, non-flammable and negligible vapour pressure at room temperature — highly desirable properties for use as electrolytes in modern high performance batteries [1–4]. ILs have also found wide applications in many other fields such as solar panels, organic synthesis, and separation processes [1,3,5–8]. IL cations can be combined with various organic and inorganic anions to produce a wide range of ILs with modified physical and chemical properties. 1-Ethyl-3-methylimidazolium (EMIM) cation based ionic liquids are interesting candidates for battery electrolyte technology due to their high thermal stability, low viscosity and good ionic conductivity [9–14]. ILs containing TCM anions have been used as electrolytes in dyesensitized solar cells and as a solvent for the separation of sulphur compounds from aliphatic hydrocarbon due to their desired properties [15,16]. To date, however, the IL EMIMTCM has been rarely studied [7,11].

⁎ Corresponding author. E-mail address: [email protected] (W.S. Price).

http://dx.doi.org/10.1016/j.molliq.2014.11.010 0167-7322/© 2014 Elsevier B.V. All rights reserved.

The physical properties of three EMIM-based ILs are summarised in Table 1. EMIMTCM has a conductivity of 17.3 mS cm− 1 at 303 K which is larger than the conductivities of the related ILs, 8.8 mS cm− 1 for 1-butyl-3-methylimidazolium tricyanomethanide (BMIMTCM), 7.7 mS cm− 1 for 1-butyl-1-methylpyrrolidinium tricyanomethanide (BMPyrrTCM), 7.0 mS cm − 1 for 1-butyl-4methylpyridinium tricyanomethanide (Bu4PicTCM), 9 mS cm− 1 for EMIMTFSA and 15.4 mS cm−1 for EMIMFSA. The viscosity of EMIMTCM is 1.6 and 2.7 times smaller than the viscosity of EMIMFSA and EMIMTFSA, respectively (Table 1) [17]. The cations and anions make strong intermolecular bonds which can remove the charge symmetry and increase dynamics of the ions (e.g., decrease the viscosity and lower the melting point). The TCM anion has stronger intermolecular bonding with the EMIM cation in the EMIMTCM than the TFSA and FSA anions with the EMIM cation [18]. Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for probing the molecular motions and interactions of ILs [19–23]. Translational diffusion (i.e., self-diffusion or intradiffusion) is intimately related to the fundamental dynamic processes involved in ionic conductivity [24–26]. Pulsed gradient spin-echo NMR allows measurement of the self-diffusion coefficients, D (m2 s−1), of the individual electrolyte species [25]. The aim of this paper is to characterise the molecular dynamics properties of EMIMTCM by measuring the self-diffusion coefficients and relaxation time constants of the cation and anion over a range of

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Table 1 The chemical structures, molecular mass of anions and viscosity values of the ILs studied in this work. Ionic liquid

Cation structure

Anion structures

Manion (g mol−1)

Viscosity at 298 K (cP)

1-Ethyl-3-methylimidazolium tricyanomethanide (EMIMTCM)

90.06

12.1

1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (EMIMFSA)

180.13

19.5a

1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amine (EMIMTFSA)

280.14

32.9a

a

[19].

temperatures. To elucidate the physical and molecular dynamics properties of EMIMTCM, we compared the results to two related EMIMbased ILs that differed only by the choice of anion, EMIMFSA and EMIMTFSA (see Table 1). EMIMTCM was found to diffuse faster due to the smaller size of the TCM anion and its lower viscosity.

and
 1 C 5τcation 2τ cation 3τcation þ ¼ þ T2 2 1 þ ω2 τ cation 2 1 þ 4ω2 τ cation 2

ð4Þ

2. Theory

where ω is the 1H Larmor frequency (rad s−1) and τcation is the correlation time of the cation (s). C is defined by

2.1. Self-diffusion coefficient and its correlation with viscosity

C¼

It is possible to simplistically characterise the temperature and timedependence of the diffusive behaviour as follows. For free diffusion it is often found that the temperature dependence is described by an Arrhenius relation, D ¼ D0 expð−EA =RT Þ

ð1Þ

where R is the gas constant, EA is the activation energy of diffusion (J mol−1) and the pre-exponential constant D0 is a fitting constant (m2 s−1). EA is required energy to make a space for the next jump in diffusion. When the mean squared displacement of the diffusive motion scale linearly with the diffusion measuring time (i.e., Δ) the diffusive process is referred to as Fickian. The diffusion coefficient can be related to the solvent viscosity and effective hydrodynamic (or Stokes) radius (rs (m)) of the diffusing species using the Stokes–Einstein–Sutherland equation [27–29], kT D¼ cπηr s

ð2Þ

3 4 2X 1 γ ħ 10 r 6j j

ð5Þ

where γ is the 1H gyromagnetic ratio, ħ is Planck's constant/2π, and rj is the inter-proton distance (m). It should be noted that on account of the much smaller gyromagnetic ratios of 13C and 15N compared to 1H, together with the low natural abundance, the 13C (1.07%) and 15N (0.368%) contributions to 1H dipolar relaxation have been ignored. 2.3. The temperature dependence of the viscosity and conductivity It is commonly found that the product of the viscosity and conductivity of a solution are a constant (the Walden product [24,32]), ηκ ¼ constant:

ð6Þ

The temperature dependence of viscosity and conductivity data is often empirically described by Vogel–Fulcher–Tamman (VFT)-type relations [33],  η ¼ η0 exp

B Y−T 0



where k is the Boltzmann constant and the constant c takes the value 4 or 6 for the slip or stick boundary conditions, respectively.

  −B0 κ ¼ κ 0 exp 0 T−T 0

2.2. Relaxation times for ILs

where η0, B (K), T0 (K), κ0, B′ (K) and T0′ (K) are fitting constants.

ð7Þ

ð8Þ

1

Assuming that the H relaxation follows the dipolar relaxation mechanism, the 1H-T1 and T2 values can be described by the Bloembergen Purcell Pound (BPP) equations [30,31]:


1 τcation 4τcation ¼C þ T1 1 þ ω2 τcation 2 1 þ 4ω2 τcation 2

 ð3Þ

3. Experimental 3.1. Sample preparation EMIMTCM and EMIMFSA were purchased and used as received from Iolitec GmbH (N 99.8% purity). EMIMTFSA was purchased from Tokyo

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Kasei Co., Ltd. For NMR measurements, the samples were placed into 5mm NMR microtubes (BMS-005J, Shigemi, Tokyo) to a height of 5 mm under a helium atmosphere to exclude moisture and the tubes were then flame sealed. 3.2. NMR measurements — general All NMR measurements were performed on a Bruker Avance 500 MHz spectrometer. The sample temperature was calibrated by using ethylene glycol for high temperatures (N 300 K) and methanol for low temperatures (b300 K) [34,35]. The gradient strength was calibrated by measuring the diffusion of the residual HDO in D2O (500 μL) at 298 K [36] with a PGSTE pulse sequence. A recycle delay greater than five times the longest spin–lattice relaxation time (T1) value was typically used for all measurements, however, for the diffusion measurements of the TCM at four temperatures via 13C NMR, the recycle delay was 1 × T1. To confirm whether this gave accurate results (i.e., given the use of 1 × T1 and the inherent extra noise in natural abundance 13 C NMR) the lowest and highest temperature measurements for TCM via 13C NMR were repeated with a recycle delay N5 × T1 and the results were similar and the results for measurements with 1 × T1 were used. See also reference [37] for more regarding the use of shorter recycle delays in diffusion measurements. Integrals of appropriate resonances were used in the analysis of the NMR diffusion and relaxation data. 3.3. NMR diffusion measurements Diffusion measurements were performed using a Bruker Avance 500 MHz NMR with Micro5 probe and 5 mm insert. While the Micro5 is a triple axis gradient set the diffusion was measured along the z-axis (i.e., direction of the static magnetic field) and the maximum magnetic field gradient strength (gmax) along this axis (i.e., 100% gradient strength) was 2.94 T m−1. The pulsed gradient stimulated echo (PGSTE) NMR diffusion sequence [38,39] or double stimulated echo (DSTE) (for temperatures above 303 K to minimise convection effects) [40] were used to measure the diffusion coefficients of over the temperature range 263–343 K. Diffusion was measured for various diffusion times, Δ, from 20 to 100 ms. The magnetic field gradient pulse duration (δ) was set to 3 ms and the diffusion measurements at each temperature or Δ were made via typically varying the gradient (g) from 0.03 to 0.7 T m−1. Measurements were typically made with 11 gradient values. Each gradient point was typically averaged over 8 scans (1H) and 32 scans (13C). The 1H NMR spectrum of the cation EMIM at 298 K is shown in Fig. 1. The analysis of NMR diffusion measurements has been described in detail elsewhere [25]. The errors given in the figures are those estimated from the non-linear least squares analysis, however, the true uncertainty of each measurement is likely to be of the order of

1% [41]. Many sources of error are in theory possible (e.g., background/ internal magnetic field gradients, eddy currents, non-homogeneous applied/pulsed magnetic field gradients, temperature fluctuations and radiation damping [25]) — however, individually, the contribution from each of these should only be small. The cation diffusion coefficient was measured using the 1H resonance of the t-CH3 (i.e., the terminal CH3 group on the alkyl chain) group and the 13C resonance of TCM was used to determine the diffusion of the anion. 3.4. NMR relaxation measurements Spin–lattice and spin–spin relaxation measurements were performed using the inversion recovery and Carr–Purcell–Meiboom–Gill (CPMG) pulse sequences, respectively [43,44]. Typically 12 τ values or recovery values were used in the inversion recovery measurements or CPMG measurements, respectively. 3.5. Conductivity measurements All conductivity measurements were conducted using a Lazar Model-3101 programmable multi-range conductivity meter (LAZAR Research Lab. Inc., USA) which was calibrated using a standard solution of KCl (1413 μS cm−1) at 298 K. 3.6. Viscosity measurements All viscosity measurements were performed using a Brookfield DV-II + Pro viscometer (Brookfield Engineering Lab., Inc., USA) between 298 and 343 K. Temperature was controlled by a Brookfield TC-202 circulation type thermo regulated water bath. The viscometer was calibrated by using water as a standard. 3.7. Data analysis All data fitting was performed using OriginPro 9.0 (Originlab, MA, USA). 4. Results and discussion The temperature dependence of the self-diffusion coefficients, relaxation times, conductivity, and viscosity of EMIMTCM was measured. However, only the self-diffusion coefficients of EMIMTFSA and EMIMFSA were measured since their relaxation times, conductivity and viscosity experimental data have previously been reported [19, 45]. The viscosity (η) and conductivity (κ) of EMIMTCM were measured in the temperature range of 298–333 K and are given in Table 2. The viscosity and conductivity data were in good agreement with the studies of Larriba et al. [7]. The directly measured viscosity gave a better correlation with the Walden product (i.e., a constant) than the viscosity calculated from the measured diffusion coefficients (although they are still reasonably constant). The VFT fit parameters for the viscosity and conductivity data of EMIMTCM were determined by non-linear least squares regression Table 2 Experimentally measured viscosity, conductivity, and calculated viscosity (ηD) for EMIMTCM. ηD was calculated from the measured diffusion coefficients (see below) and the Stokes–Einstein–Sutherland equation (Eq. (2)), using rs = 0.303 nm and c = 4. The Walden products (Eq. (6)) are also given. T (K)

Fig. 1. The 1H NMR spectrum of EMIM in EMIMTCM at 298 K. While it is known that the presence of water in ILs affects their viscosity [42], no water peak was seen in the spectrum and so the sample did not absorb water during its preparation.

κ (mS cm−1)

298 7.6 ± 0.1 303 9.3 ± 0.1 313 12.5 ± 0.2 323 17.3 ± 0.1 333 21.2 ± 0.3

η (mPa s) 12.1 9.9 7.4 5.4 4.4

± ± ± ± ±

0.1 0.1 0.1 0.2 0.1

κη (mS mPa s cm−1) 92.0 92.1 92.5 93.4 93.3

± ± ± ± ±

0.2 0.2 0.3 0.3 0.4

ηD (mPa s) 11.5 9.8 7.0 5.3 4.3

± ± ± ± ±

0.2 0.1 0.2 0.1 0.1

κηD (mS mPa s cm−1) 87.4 91.1 87.5 91.7 91.1

± ± ± ± ±

0.3 0.2 0.4 0.2 0.4

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Table 3 VFT fit parameters for the viscosity and conductivity data of EMIMTCM (see Eqs. (7) and (8)), and EMIMTFSA and EMIMFSA (from refs [19,45]). The R-squared values ranged from 0.997 to 0.999 for the fitted functions. IL

η0 (10−2 cP s)

EMIMTCM EMIMTFSA EMIMFSA

27 ± 0.1 23 ± 0.3 24 ± 0.1

κ0 (mS cm−1)

B (K)

T0 (K)

367 ± 115 684 ± 3 668 ± 13

202 ± 17 160 ± 0.3 146 ± 1.5

25 ± 4 657 ± 10 236 ± 15

B′ (K)

T0′ (K)

16 ± 12 572 ± 5 315 ± 17

267 ± 16 164 ± 0.6 181 ± 3.3

and are given in Table 3 along with the VFT fit parameters for the viscosity and conductivity data of EMIMTFSA and EMIMFSA from the literature [19,45]. The measured self-diffusion coefficients at different temperatures for EMIM in EMIMTCM are plotted versus kT/πη in Fig. 2 (see Eq. (2)) and it can be clearly seen that D is strictly proportional to 1/η. The value of crs was calculated from this data as 1.1 ± 0.2 nm. The van der Waals radius of EMIM (i.e., 0.303 nm) [24] was taken to be the effective Stokes radius from which a c value of 3.6 was obtained indicating that the diffusion of EMIM follows the slip condition. Similarly, the c values for the EMIMTFSA, EMIMFSA and BMIM based ILs with [(C2F5SO2)2], [(CF3SO2)2N], [PF6], [CF3CO2] and [BF4] were determined to be smaller at 2.7, 2.4, 3.3, 3.4, 3.0, 3.8 and 3.3, respectively [19,46]. The measured D of EMIM appeared essentially Fickian although a slight Δ-dependence was observed at lower temperatures for Δ = 20–30 ms in Fig. 3. Similar experimental results were observed for the IL N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)amide (DEMETFSA) [47]. The temperature dependence of the diffusion of the EMIM cation (1H) and the TCM anion (13C) in EMIMTCM were measured and the results are summarised in Fig. 4. The diffusion of the EMIM cation was larger than the TCM anion at all of the measured temperatures. NMR self-diffusion coefficients and relaxation time constant measurements have previously been reported for EMIMFSA and EMIMTFSA ILs and the results agreed with our results [19]. The temperature-dependence of the measured D of EMIM in EMIMTCM, EMIMTFSA, and EMIMFSA are compared in Fig. 4 and the diffusion coefficients follow the order EMIMTCM N EMIMFSA N EMIMTFSA. Faster self-diffusion was observed for EMIMTCM compared to the imidazolium based ILs due to the smaller size of the TCM anion and its lower viscosity. The activation energies for the diffusion of EMIMTCM, EMIMFSA and EMIMTFSA were 20.3 ± 0.5, 22.1 ± 0.3, and 28.7 ± 0.8 kJ mol−1, respectively (the fitting

of Eq. (1) is shown in Fig. 4). This difference is attributed to the reduced steric effects with TCM being the smallest anion. These activation energy results indicate that the intermolecular interactions are stronger in EMIMFSA and EMIMTFSA and required higher activation energies to diffuse. Arrhenius plots of the 1H T1 values (i.e., equation analogous to Eq. (1), but written in terms of T1 instead of D) for various EMIM protons are shown in Fig. 5. As a general trend, the protons located near the centre of the molecules have T1 minima, such as NCH2CH2O and OCH3 in N, N-diethyl-N-methyl-N-(2-methoxylethyl)ammonium (DEME), t-CH3 in the propyl group in N,2-dimethyl-N-propylimidazolium (DMPIM) [48] and NCH3 and CH2 groups in EMIMTFSA and EMIMFSA [19]. In the present work the EMIM H4 and H5 protons gave the longest T1 values but didn't exhibit minima. However the N–CH3 and CH2 group exhibited a T1 minimum at ~ 273 K. The activation energies of H2, H5 and H4, CH2, NCH3 and t-CH3 protons of the EMIM cation were obtained (via fitting to the data in Fig. 5) as 10.6 ± 0.2, 11.5 ± 0.2, 14.9 ± 0.3, 12.7 ± 0.6 and 14.0 ± 0.3 kJ mol−1, respectively. Note, if the last points in Fig. 5 (i.e., the point after the T1 minima) for CH2 and NCH3 were masked prior to fitting an equation analogous to Eq. (1), the activation energies were 13.4 ± 0.2 and 15.3 ± 0.3 kJ mol−1, respectively. Arrhenius plots of the 1H T1 and T2 (i.e., equation analogous to Eq. (1), but written in terms of T1 or T2 instead of D) of t-CH3 in the EMIM of EMIMTCM are shown in Fig. 6. The T2 was much shorter than T1 with both continuously increasing with temperature. The relaxation rate 1/T1 is directly proportional to τcation in the region of ωτcation ≪ 1 (extreme narrowing condition). The minimum T1 value occurs at ωτcation = 2πυ0τc = 0.62. In this study, the 1H observation frequency, υ0 (1H) is 21.14 × 108 rad s−1 and τcation at the T1 minimum is calculated to be 2.45 × 10−10 s. C is calculated to be 1.39 × 109 from the T1 value obtained at the minimum in Fig. 5. The reorientational correlation time is affected by the viscosity of the IL and temperature. A

Fig. 2. Diffusion coefficients (●) are plotted versus kT/πη for the EMIM cation in EMIMTCM. Linear regression, shown by the solid line, and Eq. (2) gave crs = 1.1 ± 0.2 nm. The error in the diffusion coefficients is contained within the data points.

values at 263 (★), 273 ( ), 284 ( ), 298 (■), 303 (●), 313 (▲), 323 (▼), 333 (◄), and

Fig. 3. D of the t-CH3 moiety in the EMIM cation in EMIMTCM obtained with different Δ

◆

343 K (►). The diffusion was essentially Fickian at all but the lower temperatures and shortest Δ values. The error in the diffusion coefficients is contained within the data points.

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Fig. 4. A comparison of the temperature dependence of the EMIM cation in EMIMTCM (▲), EMIMFSA (■) and EMIMTFSA (●), and the TCM anion (▽) in EMIMTCM in the temperature range 263–343 K. The solid lines are the result of fitting Eq. (1) onto the data and clearly shows that the cations and anion exhibit Arrhenius behaviour. The error in the diffusion coefficients is contained within the data points.

plot of the τcation values calculated from Eq. (3) and the experimental T1 values is shown in Fig. 6. The measured τcation decreased with temperature. τcation is defined as the time for the librational flip of the whole molecule [47]. τc is the overall isotropic molecular reorientational correlation time, and can be related to the viscosity η using the Stokes–Einstein–Debye relationship [29,49,50], τc ¼

Vη kT

ð9Þ

where V is the effective molecular volume which is assumed to be equal to the van der Waals volume V ¼ 43 πr 3s . From Eqs. (2) and (9), the overall 4r2s isotropic molecular reorientational correlation time is τc ¼ 3cD where c, as determined above, has the value of 3.6 for EMIM in EMIMTCM. The calculated τc values are also2 shown in Fig. 7. The translational correlation time defined by τ D ¼ 2rDs [51] is also shown in Fig. 7.

Fig. 5. Arrhenius plots of 1H T1 values of H5 and H4 (●), H2 (■), t-CH3 (◆), CH2 (▲) and NCH3 (▼) groups in EMIM of EMIMTCM. The T1 minimum value of NCH3 and CH2 were 0.58 s and 0.53 s at 273 K, respectively. The largest T1 value was 2.84 s for the H5 and H4 groups at 343 K. The solid lines are the result of fitting an equation analogous to Eq. (1), but written in terms of T1 instead of D, onto the data.

Fig. 6. Arrhenius plots of 1H T1 (●) and T2 (■) values of the t-CH3 moiety in EMIM of EMIMTCM. The T1 was 1.16 s and T2 was 0.27 s at 298 K. The solid lines are the result of fitting equations analogous to Eq. (1), but written in terms of T1 or T2 instead of D, onto the data. The activation energies were determined to be 14.0 ± 0.4 and 24.6 ± 0.2 kJ mol−1, respectively.

The τcation obtained for the molecular motion of the EMIM cation in EMIMTCM changes from 0.38 ns at 263 K to 17 ps at 343 K. The τcation of DMPIM action in DMPIMTFSA was reported and the values were changed from 1.3 ns to 72 ps at 253 K–353 K [14]. The τcation of EMIM is much shorter than that of DMPIM, this is most likely a reflection of the side chains of EMIM being smaller than those of DMPIM. The correlation times of EMIM in EMIMTCM followed the order of τD (from diffusion coefficient) N τc (from viscosity) N τcation (from T1). The reason for this shorter τcation is that the intramolecular local motion is quicker than the whole molecular motion (e.g., the translational and the overall isotropic molecular reorientational correlation time). The same order was obtained for the cations in EMIMTFSA, EMIMTFSA-Li, EMIMFSA, EMIMFSA-Li, 1methyl-3-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13TFSI) and 1-methyl-3-propyl-pyrrolidinium bis(fluorosulfonyl) imide (P13FSI) [19,48].

Fig. 7. Arrhenius plots of EMIM correlation time τD (●) for translational motion, τc (▲) for overall isotropic molecular reorientational motion and τcation (▼) for librational motion. τD was 0.2 ns, τc was 0.11 ns and τcation was 0.08 ns at 298 K. The solid lines are the result of fitting equation analogous to Eq. (1), but written in terms of the respective correlation times (τD, τc and τcation) instead of D [52], onto the data and gave the activation energies to be 29.3 ± 0.1, 28.1 ± 0.1 and 26.6 ± 0.1 kJ mol−1, respectively.

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

Fig. 8. A plot of τcation calculated from 1H T1 versus viscosity ( ) for the EMIM in EMIMTCM. The solid line represents linear regression onto the data and indicates that τcation is strongly correlated with viscosity.

A plot of τcation versus η for the EMIM cation is shown in Fig. 8. τcation increases linearly with increasing viscosity. 5. Conclusions The self-diffusion coefficients, ionic conductivity and viscosity of EMIMTCM were measured and subsequently analysed using the Stokes–Einstein–Sutherland and Stokes–Einstein–Debye equations and fitted with the Vogel–Fulcher–Tamman equation. Good correlation was found between the viscosity and cation correlation time τcation. The temperature-dependence of the measured Dcation of EMIMTCM was compared with different ILs, EMIMFSA and EMIMTFSA. Faster self-diffusion was observed for EMIMTCM due to the smaller size of the TCM anion and its lower viscosity. τcation was observed to increase linearly with increasing viscosity. The overall isotropic molecular reorientational correlation time τc, translational correlation time τD and τcation were found to be in the order τD N τc N τcation at all of the temperatures studied. Acknowledgements

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]

This research was supported by an Endeavour International Postgraduate Research Scholarship from the University of Western Sydney. Prof. Kikuko Hayamizu at the National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Centre 2, Ibaraki 305-8568, Japan, is thanked for her valuable discussion regarding this work. The authors acknowledge the facilities and the scientific and technical assistance of the National Imaging Facility, University of Western Sydney.

[48] [49] [50] [51] [52]

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