Performance assessment of new perturbed hard-sphere equation of state for molten metals and ionic liquids: Application to pure and binary mixtures

Journal of Non-Crystalline Solids 358 (2012) 1753–1758

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Performance assessment of new perturbed hard-sphere equation of state for molten metals and ionic liquids: Application to pure and binary mixtures S.M. Hosseini a,⁎, M.M. Papari b, F. Fadaei Nobandegani a, J. Moghadasi a a b

Department of Chemistry, Shiraz University, Shiraz, 71454, Iran Department of Chemistry, Shiraz University of Technology, Shiraz, 71555-313, Iran

a r t i c l e

i n f o

Article history: Received 8 March 2012 Received in revised form 11 May 2012 Available online 8 June 2012 Keywords: Equation of sate; Molten metals; Ionic liquids

a b s t r a c t This work addresses modeling the pressure–volume–temperature (PVT) properties of molten metals and ionic liquids (ILs) using a new perturbed hard-sphere equation of state (PHS EOS). Two temperaturedependent parameters appeared in the EOS, are correlated with two scaling constants σ and ε. Knowing these parameters, the proposed EOS is applied to these classes of liquids. The reliability of the proposed model is checked by comparing with 3177 experimental density data points. The average absolute deviations (AAD) of predicted densities of molten metals and ILs from literature data are found to be 1.35% and 0.56%, respectively. The extension of PHS EOS to binary metal alloys and IL + IL is also discussed. Generally, 609 data points for binary mixtures have been examined. The AAD of the predicted results are found to be 1.03%. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The knowledge of thermophysical properties of molten metals is of considerable interest in industry. Liquid metals have a number of interesting characteristics, which make them applicable in hightemperature operations as heat transfer agents and reactor coolants. For example, liquid refractory metals are ideal for use in very high temperature operations such as liquid and solid rocket motors. Due to the problems associated with experimental measurements, a few experimental data are available on liquid metals, especially at very high temperatures and pressures. Furthermore, in the case of ILs, although some experimental data on densities exist, prediction of their PVT properties is still an important task considering that they have been in focus recently as new materials offering several highly promising applications [1–4], so, the prediction of PVT properties of molten metals and ILs is an important task. In these circumstances the development of EOS and corresponding states correlation methods to predict volumetric properties can be highly useful. In this respect, several researchers proposed different methods for the estimation of density of both molecular as well as ionic liquids. For instance, Hofmann [5] has developed a fast estimation method based on the average atom volume (AAV) for organic or metalorganic crystals. Generally, density of both neutral molecule as well as ionic specie can be computed based on the high-level ab-initio calculations which are very expensive and time-consuming [6,7]. Besides, Rebelo et al. [8,9] have proposed a simple model for

⁎ Corresponding author. E-mail address: [email protected] (SM. Hosseini). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.05.014

predicting the molar volume of ILs. In their work, the molar volume of an IL is considered as the sum of the effective molar volumes occupied by the cation and the anion. This work is the continuation of our previous studies on the modeling of the pressure–volume–temperature (PVT) properties of molten metals and ionic liquids from perturbed hard-sphere equations of state (PHS EOS). The main objective of the present work is to develop a new PEOS based on the first-order perturbation theory of liquids and the generalized corresponding states correlation for aforecited liquids. To construct a new equation of state, a primary hard-sphere (HS) model is chosen as reference term and a perturbation term was added to the reference part. In this respect, the hard sphere equation proposed by Malijevsky and Veverka [10] (MV) is chosen as reference term. This term has a form of the rescaled virial series and uses the first seven virial coefficients. The reason for selecting the MV hard-sphere expression as reference system is that, the packing fraction of hard spheres (η) has been located at nearly close to unity. Actually, only real and positive pole of the HS expression of MV, is located at η = 1 and hence it seems to be appropriate for representation of the volumetric properties of dense fluids. We evaluate two temperature-dependent parameters appeared in the perturbed Malijevsky–Veverka equation of state (PMV EOS) via the corresponding states correlation method. In this regard, we follow the procedure developed by Eslami et al. [11,12] for determining the above-mentioned parameters. In the present work, the scaling constants σ and ε are used to develop two new corresponding states correlations for a(T) and b(T) parameters in the PHS EOS. These constants are good alternative for critical properties because critical properties are scarce in literature. Only for 16 metals in the periodic table the critical constants have been reported in the literature.

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Moreover, the reported values for these metals are not consistent with each other [13]. Finally, the proposed model is extended to mixture to predict the volumetric properties of fluid mixtures including ILs and metals. Basically, the volumetric properties of mixtures are important because they depend on the composition and/or temperature, and are of great importance in understanding the nature of molecular aggregation that exists in the binary mixtures. Extension of the PMV EOS to binary metal alloys and mixtures of ILs for the estimation of their PVTx properties will also be discussed.

2.1. New perturbed hard-sphere equation of state for pure fluid The general frame of the PMV EOS can be expressed in terms of compression factor: 2

ð1Þ

where P is the pressure, ρ is the number (molar) density, and kT is the thermal energy per molecule. The first term represents a reference physical model and the second term is a perturbation part. The reference system is hard-sphere (HS) model expressed by Malijevsky and Veverka equation [10] and the attractive part of the van der Waals equation of state is introduced as the perturbation term. η is the packing fraction of hard-spheres defined as:

η¼

bðT Þρ : 4

P 1 þ 1:056ηm þ 1:6539η2m þ 0:3262η3m  ¼
ρkT 1 þ 0:056ηm þ 0:5979η2m þ 0:3076η3m ð1−ηmÞ3 −

ð7Þ

ρ ∑ ∑ x x aðT Þij kT i j i j

ηm ¼

ρ ∑ x bðT Þi : 4 i i

ð8Þ

In the case of binary mixtures, the hard-sphere co-volumes, b(T)ij are additive according to following expression:

3

P 1 þ 1:056η þ 1:6539η þ 0:3262η aðT Þρ  ¼
; − ρkT kT 1 þ 0:056η þ 0:5979η2 þ 0:3076η3 ð1−ηÞ3

The mixture version of the proposed PMV EOS can be expressed in compression factor as [17]:

where xi and xj are the mole fractions of i'th and j'th components, respectively. ηm is the packing fraction of mixtures of hard-spheres [18]. This parameter is defined by the following expression:

2. Theory

Z¼

2.2. Extension to binary mixtures

ð2Þ

h i 1=3 1=3 3 bðT Þij ¼ 1=8 bðT Þi þ bðT Þj :

The attractive forces between two hard-sphere species of a mixture including i and j components can be written as follows: aðT Þij ¼

2π 3 σ ε F ðkT=εÞij : 3 ij ij a

ð10Þ

The present method for calculating the two temperaturedependent parameters can be extended to mixtures by using a simple geometric mean and an arithmetic mean for the adjustable scaling constants; i.e., ðε=kÞij ¼

The two temperature-dependent parameters a(T) and b(T) must be evaluated. It has been already shown that these scaling constants can be obtained from macroscopic properties such as compressibility, density and vapor-pressure [14–16]. These parameters are correlated with the effective hard-sphere diameter, σ and the non-bonded interaction energy between two spheres, ε according to the following equations:

ð9Þ

σ ij ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðε=kÞi ðε=kÞj

 1 σi þ σj 2

FaðkT=εÞij ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi F a ðkT=εÞii F a ðkT=εÞjj :

ð11Þ ð12Þ ð13Þ

3. Results and discussion aðT Þ ¼

2π 3 σ εF a ðkT=εÞ 3

ð3Þ

bðT Þ ¼

2π 3 σ F b ðkT=εÞ: 3

ð4Þ

Fa and Fb are the universal functions of reduced temperature (kT/ ε), which can be written as the following empirical formula: 2

ð5Þ

2

ð6Þ

F a ¼ a1 þ a2 ðkT=εÞ þ a3 ðkT=εÞ

F b ¼ b1 þ b2 ðkT=εÞ þ b3 ðkT=εÞ :

The coefficients a1–a3 and b1–b3 in Eqs. (5) and (6) were allowed to vary using the experimental PVT data over a broad range of pressures and temperatures by non-linear least squares method. Their numerical values are a1 = 2.11202, b1 = 0.280228, a2 = 1.03184, b2 = 0.945775, a3 = − 0.813189, and b3 = − 1.15401.

A simple analytical equation of state (PMV EOS) has been developed to model the volumetric properties of molten metals and IL systems. The scaling constants ε/k and σ required by the EOS have been tabulated in Table 1. Fig. 1 illustrates the calculated values of P/ρRT using PMV EOS versus temperature for typical liquid potassium and [C4mim][OcSO4] as two representatives of molten metal and IL, respectively. As it is obvious from Fig. 1, the P/ρRT at constant T is nearly proportional to P. It means that ρ is independent of P. This behavior implies that the liquids are incompressible. A close inspection of Fig. 1 reveals that P/ρRT in terms of temperature is nearly constant at given pressure. Exception of this trend can be observed for molten metals at high pressures. This behavior can be attributed to this fact that packing fraction η is close to unity. It has been found that along the contour defined by Z = 1, where the compressibility factor is the same as for an ideal gas, the density of studied liquids is nearly linear function of temperature [19]. Along the entire Zeno contour, the attractive and repulsive contributions to intermolecular potential are in dynamic balance. This regularity can be a means for testing equations of state. Because the Zeno behavior appears to be a generic property of pure fluids, it is advantageous to incorporate it in EOS models. In the present study, we have

SM. Hosseini et al. / Journal of Non-Crystalline Solids 358 (2012) 1753–1758 Table 1 The scaling constants of studied systems.

Metal Lithium (Li) Sodium (Na) Potassium (K) Rubidium (Rb) Cesium (Cs) Rhenium (Rh) Molybdenum (Mo) Titanium (Ti) Zirconium(Zr) Hafnium (Hf) Niobium (Nb) Ionic liquid [C4mim][OcSO4] [C2mim][NTf2] [C3mim][NTf2] [C4mim][NTf2] [C5mim][NTf2] [C2mim][EtSO4] [C2mim][Triflate] [(C6H13)3P(C14H29)][Ac] [(C6H13)3P(C14H29)][Cl] [C2mim][BF4] [C4mim][PF6] [C6mim][BF4] [C4mim][BF4] [C4mim][Triflate] [N1114][NTf2] [C3mpyr][NTf2] [C4mpyr][NTf2] [C6mim][NTf2]

ε/k(K)

σ(nm)

19512.0 5083.5 9244.1 8994.5 8436.8 10668.0 11818.0 49709.0 5083.5 5576.8 69111.0

0.49685 0.59857 0.72817 0.77962 0.83911 0.35477 0.36282 0.46187 0.39153 0.39361 0.46819

1003.80 838.90 1839.90 860.08 1329.9 893.38 808.33 1202.10 1135.9 913.20 926.28 966.77 935.78 837.83 909.92 983.12 861.33 824.80

1.13090 1.03020 1.15880 1.07780 1.15880 0.93679 0.92634 1.43410 1.40210 0.87266 0.96690 0.98953 0.93397 0.97885 1.07300 1.07680 1.08790 1.11570

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examined the PMV EOS whether Zeno condition can be satisfied. Figs. 2 and 3, respectively, illustrate the Zeno contour for several molten metals and ionic liquids. As Figs. 2 and 3 demonstrate, this linearity can properly predicted by our model for two classes of fluids. The reliability of the PMV EOS was further checked through predicting the liquid density of the studied systems and comparing it with the literature data. For example, the densities of 11 molten metals have been determined by means of the proposed model and have been compared with available literature values [20–23]. The results have been summarized in Table 2. In general, for 335 data points examined for liquid metals, AAD was found to be 1.35%. It may be interesting to compare our model with other models. In this regard, we have compared the outcomes of our calculations with those obtained from the model proposed by Eslami [11,12] and Maftoon et al. [14]. As it is clear from Table 2, the harmony between the predicted densities from our model and the literature works is acceptable. The capability of the present model in predicting the volumetric properties of ILs is also checked. In this respect, the proposed EOS has been employed to predict the mass density of 18 various ILs over a wide range of temperatures from 293 K to 472 K and pressures from 0.1 to 200 MPa. The measured values of densities were taken from [24–40]. The outcome of the results has been tabulated in Table 3. For 2842 data points the AAD was found to be 0.56%. Besides, the reliability of the PMV model in predicting the volumetric properties of ILs has been evaluated by comparing with other methods. Table 4 contains the AAD of the calculated densities of several ILs by the present work, the model developed by Hosseini et al. [15,16] and the method of Gardas and Coutinho (GC) [41]. All calculations have been compared with the literature values [24,27–32,34]. As it is clear from Table 4, the predicted densities obtained from PMV EOS have almost the same accuracy as those obtained from Ref. [41]. The interesting point of the present study is that the average AAD of the present model is 0.61% which is lower than those obtained from our previous model [15,16]. Generally speaking, the overall AAD for 3177 data points for all studied liquids was found to be 0.60%. The PMV model was successfully extended to mixtures. First the EOS was employed to predict the PVTx properties of binary alloys including K + Na and K + Cs. In this regard, Fig. 4 depicts the relative deviation of the calculated densities from those reported in literature [43]. In order to show the ability of the mixture version of our model, a comparison has also been made with the one presented by Sabzi et al. [42] in Fig. 4. It is obvious that the accuracies of our calculations are of the order of ±3.36% while the accuracies of the model proposed by Sabzi et al. [42] are within ±6.77%. It should be mentioned that for 225 experimental data points examined for the aforementioned

1100

R² = 0.9927 1000 900

T/ K

800 700

R² = 0.9995

600 500

R² = 0.9981 400

R² = 0.9921

R² = 0.9929 300 300

800

1300

1800

2300

ρ / kg. m-3 Fig. 1. Plot of P/ρRT versus temperature for (a) Potassium at 5 MPa (◊), 10 MPa (*), 20 MPa (○), and 30 MPa (●). (b) [C4mim][OcSO4] at 10 MPa (♦), 30 MPa (◊), 50 MPa (●), 70 MPa (○), and 90 MPa (▲).

Fig. 2. The Zeno line for liquids potassium (Δ), lithium (▲), sodium (○), rubidium (●), and caesium (*) using PMV EOS. The solid lines are the best straight-lines passing through the points together with their R-squared (R2).

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410

Table 3 Predicted density of pure ILs using the PMV EOS.

R² = 0.9908 390

R² = 0.9883 370

Ionic Liquid

ΔP (MPa)

ΔT (K)

NPa

AADb(%)

Reference

[C2mim][NTf2]

0.1–30 0.1 0.1–60 0.1–60 0.1–35 0.1–35 0.1–35 0.2–65 0.2–65 0.1–35 0.1–65 0.1–60 0.1–10 1–200 0.1–60 0.1–20 1–200 0.1–30 1–200 0.1–60 0.1–35 1–200 0.1–35 0.1–35 1–200 0.7–40 0.09–202

293–393 293–353 298–333 298–328 283–333 293–393 293–393 298–333 298–333 283–318 293–338 293–333 293–393 313–452 298–332 293–353 313–472 293–393 313–472 298–333 298–353 313–472 293–393 293–393 312.8–432 298–373 298–343

096 013 165 168 063 080 091 144 134 072 155 163 077 168 068 020 180 096 180 099 036 178 091 091 174 045 014 2842

0.33 0.36 0.75 0.30 0.81 0.54 0.36 0.59 0.55 1.05 0.46 0.35 0.26 0.59 1.12 0.88 0.75 0.90 0.61 0.64 0.55 0.58 0.11 0.38 0.58 0.57 1.48 0.56

[24] [25] [26] [27] [29] [28] [28] [30] [30] [31] [32] [27] [33] [34] [35] [36] [37] [38] [37] [26] [39] [34] [28] [28] [34] [24] [40]

R² = 0.995 [C3mim][NTf2] [C4mim][NTf2] [C2mim][EtSO4]

T/ K

350

R² = 0.9983 330

[C2mim][Triflate] [(C6H13)3P(C14H29)][Ac] [(C6H13)3P(C14H29)][Cl]

310 290

R² = 0.9992 R² = 0.9942

R² = 0.99 [C6mim][NTf2]

270

[C4mim][Triflate]

250 800

1000

1200

1400

1600

ρ / kg. m-3

[C4mim][BF4] [C2mim][BF4]

Fig. 3. The Zeno line for liquids [(C6H13)3P(C14H29)][Cl] (○), [C6mim][NTf2] (▲), [C4mim][OcSO4] (●), [C4mpyr][NTf2] (◊), [N1114][NTf2] (*), [C4mim][NTf2] (♦), and [C2mim][EtSO4] (Δ) using PMV EOS.

binary alloys, the AAD of our results from the literature values [42] were found to be 1.30% and ±1.84%, respectively. Second, the mixture version of PMV EOS was further evaluated by applying to predict the PVTx properties of binary mixtures containing ionic liquids. In this respect, three deviation plots have been shown in Fig. 5. Fig. 5 plots the relative deviation percent of the calculated mixture density of the three binary mixtures of IL as a function of mole fractions and at several temperatures from the literature data [44]. From 381 experimental data points studied, the overall AAD of calculated densities from those reported in literature using proposed model were found to be 1.14%. From the calculations of volumetric properties regarding the mixtures it can be concluded that the accuracies associated with our model are within ±1.65%. Also, for 609 data points for binary metal alloys and IL + IL, overall AAD were found to be 1.03%. 4. Conclusion A hard-sphere equation of state was developed by taking Malijevsky and Veverka equation as reference part and the attractive part of the van der Waals equation of state as the perturbation term. The temperature-dependent parameters that appeared in the proposed

Table 2 Predicted density of liquid metals using the PMV EOS (this work), and models proposed by Eslami [11,12] and Maftoon et al. [14], compared with the experiment [20–23]. Metal

ΔT (K)

NPa

This work

AAD (%)b Eslami [11,12]

Maftoon et al. [14]

Lithium Sodium Potassium Rubidium Cesium Rhenium Molybdenum Titanium Zirconium Hafnium Niobium Overall

600–1600 600–1600 400–1500 400–1400 400–1300 3453–5725 2896–5033 1650–2050 1850–2750 2300–2650 2320–2950

18 18 84 77 52 26 11 09 19 08 13 335

0.84 0.95 1.37 2.18 1.40 1.38 1.37 0.36 0.72 0.16 0.23 1.35

1.63 1.18 1.29 1.30 1.10 1.48 0.25 0.65 1.28 0.38 0.43 1.18

1.57 0.85 0.87 0.99 0.94 – – – – – –

a b

NP represents the number of data points examined. NP P AAD ¼ 100=NP jρi;Cal: −ρi;Exp: j=ρi;Exp: . i¼1

[C6mim][BF4] C5mim][NTf2] [N1114][NTf2] [C4mim][OcSO4] [C3mpyr][NTf2] [C4mpyr][NTf2] [C4mim][PF6] Overall a b

NP represents the number of data points examined. NP P AAD ¼ 100=NP jρi;Cal: −ρi;Exp: j=ρi;Exp: . i¼1

EOS were correlated by molecular energy and size scaling constants. This EOS was checked against a large number of experimental data points over wide range of temperatures for molten metals as well as ionic liquids. Further, our model has been compared with other models such as those proposed by Gardas and Coutinho for ionic liquids and Eslami for molten metals. In each case, results are in favor of capability of our EOS to model the PVT properties of these classes of fluids. The Zeno line regularity can well be predicted by our model for both ionic liquids and molten metals. Finally, the performance of PMV EOS in predicting the volumetric properties of selected mixtures including binary alloys and IL + IL has been successfully evaluated on a wide space of temperatures and

Table 4 Average absolute deviation (AAD) of the calculated densities of studied ILs using the present, previous [15,16] and GC [41] models. ΔT (K)

Ionic liquid

[C2mim][NTf2] [C4mim][NTf2] [C4mim][OcSO4] [C2mim][EtSO4] [C2mim][Triflate] [(C6H13)3P (C14H29)][Cl] [(C6H13)3P (C14H29)][Ac] [C6mim][NTf2] [C3mpyr][NTf2] [C4mpyr][NTf2] Overall a b

NPa

AAD (%)b previous work [15,16]

this work

GC [41]

Ref.

293–393 298–333 313–472 283–333 293–393 298–333

096 168 174 063 091 134

2.25 1.15 1.62 2.48 0.82 1.00

0.33 0.30 0.56 0.81 0.36 0.55

0.09 0.22 1.41 0.46 0.18 2.43

[24] [27] [34] [29] [28] [31]

298–334

126

1.21

0.59

1.69

[30]

293–338 293–393 293–393

156 091 091 1190

1.34 0.75 1.28 1.37

0.46 0.11 0.38 0.61

0.34 0.09 0.09 0.78

[32] [28] [28]

NP represents the number of data points examined. NP P AAD ¼ 100=NP jρi;Cal: −ρi;Exp: j=ρi;Exp: . i¼1

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Fig. 4. Deviation plots for the calculated density of mixtures (a-plot): x potassium + (1 − x) cesium at mole fractions: 0.1 (♦), 0.505 (▲), 0.7 (■), and 0.9 (●) and (bplot): x Na + (1 − x) K at mole fractions: 0.3 (■), 0.5 (▲), 0.68 (♦), and 0.9 (●) in terms of temperature. The filled and open markers represent our and Sabzi et al. results [42], respectively. The experimental data were taken from Ref. [43].

mole fractions. Moreover, our model has been compared with other models reported in literature. Results show that the densities obtained from the mixture version of the present equation of state agree well with those reported in literature and that this model outperforms Sabzi's model studied in the present work. 5. Nomenclature and units

Z a (T) b (T) x P R T k Fa and Fb a1–b3

Compression factor strengths of attractive forces, J/m 3 van der Waals co-volume, m 3 mole fraction pressure, Pa gas constant, J/mole K absolute temperature, K Boltzmann constant, J/K universal functions coefficients used in Eqs. (5) and (6)

Greek letters ρ molar density η packing fraction ε non-bonded interaction energy parameter/ J σ effective hard-sphere diameter/ nm

Fig. 5. Deviation plots for the calculated density of mixtures:(a-plot) x[C6mim][BF4] + (1 − x) [C2mim][BF4] at 298.15 K (◊), 303.15 K (□), 301.15 K (▲), 305.15 K (●), and 308.15 K (○), (b-plot): x[C4mim][BF4] + (1 − x) [C6mim][BF4] at 298.15 K (◊), 301.15 K (□), 303.15 K (Δ), 305.15 K (○), and (c-plot): x[C4mim][PF6] + (1 − x) [C4mim] [BF4] at 298.15 K (◊), 301.15 K (□), 303.15 K (Δ), 305.15 K (○), and 308.15 K (●) in terms of mole fraction, compared with experiment [44].

Subscripts m mixture

Abreviations [C4mim][PF6] 1-butyl-3-methylimidazolium hexafluorophosphate [C4mim][OcSO4] 1-butyl-3-methylimidazolium octylsulfate [C4mim][NTf2] 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide

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[C5mim][NTf2] 1-pentyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C2mim][NTf2] 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C4mim][Triflate] 1-butyl-3-methylimidazolium trifluoromethanesulfonate [C2mim][BF4] 1-ethyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4] 1-hexyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] 1-butyl-3-methylimidazolium tetrafluoroborate [(C6H13)3P(C14H29)][Cl] trihexyltetradecylphosphonium chloride [(C6H13)3P(C14H29)][Ac] tetradecyl (trihexyl)phosphonium acetate [C6mim][NTf2] 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C4mpyr][NTf2] 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [C3mpyr][NTf2] 1-propyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [C3mim][NTf2] 1-propyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [N1114][NTf2] butyltrimethylammonium bis(trifluoromethylsulfonyl)imide [C2mim][Triflate] 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2mim][EtSO4] 1-ethyl-3-methylimidazolium ethylsulfate

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