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Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements
Fluid Phase Equilibria 299 (2010) 229–237
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Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements Mériem Anouti a,∗ , Patrice Porion b , Catherine Brigouleix a , Hervé Galiano c , Daniel Lemordant a a b c
Université Franc¸ois Rabelais, Laboratoire PCMB (EA 4244), équipe de Chimie-physique des Interfaces et des Milieux Electrolytiques (CIME), Parc de Grandmont, 37200 Tours, France Centre de Recherche sur la Matière Divisée, CNRS-Université d’Orléans, UMR6619, 1b rue de la Férollerie, 45071 Orléans Cedex 02, France CEA, Le Ripault, Laboratoire Synthèse et Transformation des Polymères, F-37260 Monts, France
a r t i c l e
i n f o
Article history: Received 20 June 2010 Received in revised form 26 September 2010 Accepted 28 September 2010 Available online 5 November 2010 Keywords: Protic ionic liquids Pyrrolidinium Hydrogen sulfate Trifluoroacetate Pulsed-field gradient spin-echo NMR Self-diffusion coefficients Stokes–Einstein
a b s t r a c t Conductivity and viscosity measurements of pyrrolidinium hydrogen sulfate, [Pyrr][HSO4 ], and pyrrolidinium trifluoroacetate [Pyrr][CF3 COO] were performed at various temperatures over a wide temperature range (i.e., from T = 273 K to 353 K). The results were utilized in the Stokes–Einstein equation to investigate the proton conductivity in both PILs. The self-diffusion coefficients (D) of the cation and anion species in both studied PILs were independently determined in the same temperature range by observing 1 H and 19 F nuclei with the pulsed-field gradient spin-echo NMR technique. With regard to the mechanism of self-diffusion, based on the values of the coefficients, a relatively large difference was observed between the two ionic liquids (ILs). Independently of the temperature, the D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five times higher than those of the pyrrolidinium cation or hydrogenosulfate anion in [Pyrr][HSO4 ], and twofold those in the case of [Pyrr][CF3 CO2 ]. In order to comment d.c. conductivities results, the self-diffusion coefficients determined by PGSE NMR were converted into charge diffusivity D by means of the Nernst–Einstein equation. In a similar way, a viscosity-related diffusivity D was calculated with the aid of the Stokes–Einstein equation. The temperature-independent cation transference number and the effective hydrodynamic radius were also deduced from these equations. Such parameters play an important role in charge and mass transports in ILs. Moreover, proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms, which confirms that Brønsted acid–base ionic liquid systems are good candidates as proton conductors in fuel cells or supercapacitor electrolyte devices operating under anhydrous conditions at elevated temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The term ionic liquids (ILs) is broadly used in order to describe a large class of low-melting organic salts that are liquids below 373 K. Protic ionic liquids (PILs) are a subset of ILs formed by the combination of a Brønsted acid with a Brønsted base [1–4]. When a PIL is synthesized by mixing a strong acid with a strong base, the proton is fixed very tightly to the base; the PIL is most likely composed entirely of ions with possible ion complexation and aggregate formation [5]. Because of their advantageous properties, these materials may be used as alternatives for conventional solvents in the context of “green chemistry” as well as to reduce emissions to the environment. ILs have received a great deal of attention as a class of solvents with a wide range of potential applications including organic and inorganic synthesis [6], energy
∗ Corresponding author. Tel.: +33 247366951; fax: +33 247367360. E-mail address: [email protected] (M. Anouti). 0378-3812/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2010.09.035
storage devices [7], separations [8,9], and catalysis [10]. The most notable characteristics of many ILs include their low vapor pressure, non-flammability, thermal stability, wide liquid range, and solvating properties for diverse substances [11,12]. Since the main feature of PILs are their ability to form hydrogen bonds, they have been investigated as amphiphile self-assembly media [13], biological solvents [14], and in polymer membrane fuel cells [15]. Aggregate formation in ILs is an important issue still subject to debate [16,17]. The formation of ionic pairs and larger agglomerates (or supramolecules) [16] is intimately connected with the structural organization of RTILs at nano-scales [18–21]. Furthermore, the occurrence of such agglomerates can explain the discrepancies found between mass and charge transport in these liquids [22]. A high ionic concentration, resulting in the significant Coulombic interactions, implies that the differences in the ionic state and ion dynamics significantly contribute to the variations in physicochemical properties of ILs. However, microscopic information regarding the ionic state and ion dynamics has long been insufficient. It was only recently recognized that a pulsed-field gradient
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spin-echo (PGSE) nuclear magnetic resonance (NMR) method can be applied in order to determine the self-diffusion coefficients of the individual ionic species [23–25]. In this context, it was considered of considerable importance to investigate transport phenomena in ILs over a wide temperature range; not only from an engineering viewpoint but also to obtain more fundamental knowledge. This work presents two PILs synthesized and characterized with regard to viscosities, diffusivity, and conductivities, at various temperatures. The outstanding feature of this paper is the fact that it provides a complete set of transport data as a function of temperature for each ionic liquid. Specifically, this includes the self-diffusion coefficient for cations and anions from pulsedgradient spin-echo nuclear magnetic resonance (PGSE-NMR) measurements, the ionic conductivity, and the shear viscosity from cone-plate viscometer experiments. The temperature dependence of each transport quantity is described by separate fits of the Vogel–Tammann–Fulcher equation. Comparisons of the selfdiffusion with either the conductivity or the viscosity were based on the Nernst–Einstein and Stokes–Einstein equations, respectively. This procedure yielded information about the degree of cation–anion association as well as the hydrodynamic radius of both ionic species. 2. Experimental 2.1. Materials Pyrrolidine is commercially available from Fluka (>99.0%) and was used without further purification. A sulfuric acid (68% in water) solution and trifluoric acid (>99.0%) were obtained from Sigma Aldrich. 1,2-dichloroethane (DCE) (>99,0%) was purchased from Sigma Aldrich, and water was purified with a Milli-Q 18.3 M water system. 2.2. Preparation of PILs Pyrrolidinium trifluoroacetate and pyrrolidinium hydrogen sulfate were synthesized according to the procedure described in a previous publication [26]. 2.2.1. Preparation of pyrrolidinium hydrogen sulfate [Pyrr][HSO4 ]. Pyrrolidine (26.78 g; 0.37 mol) was introduced in a two-necked round-bottomed flask immersed in an ice bath and equipped with a plastic dropping funnel, for the addition of sulfuric acid (96% in water), and a thermometer, to control the temperature. Under vigorous stirring, sulfuric acid (37.80 g; 0.37 mol) was dropwise added to the flask in approximately 60 min (mixture temperature < 308 K). Stirring was maintained for 4 h at ambient temperature before adding 120 mL of DCE. Then, with the aim of removing residual water, this mixture was distilled under normal pressure until the water-DCE hetero-azeotropic boiling point was reached (346 K). DCE was finally evaporated from the mixture under reduced pressure so that a pale yellow and viscous liquid could be collected (recovery: 96%). 2.2.2. Preparation of pyrrolidinium trifluoroacetate [Pyrr][CF3 COO] Pyrrolidine (38.55 g; 0.85 mol) was charged to a three-necked round-bottomed flask immerged in an ice bath and equipped with a reflux condenser, a dropping funnel to add the acid and a thermometer to monitor the temperature. Under vigorous stirring, trifluoric acid (96.921 g; 0.85 mol) was added dropwise to the pyrrolidine (60 min). As this acid–base reaction was strongly exothermic, the temperature of the mixture was, by means of an ice
bath, maintained below 298 K during the addition of the acid. Stirring was carried out for 4 h at ambient temperature. A pale yellow viscous liquid was obtained. The residual pyrrolidine or acid was evaporated under reduced pressure and the remaining liquid was further dried at 353 K, also under reduced pressure (1–5 mmHg), to obtain the solid PIL (133.70 g; recovery: 98.7%) (Tm = 328 K). Since both PILs were very hygroscopic compounds, they were dried overnight at 343 K under high vacuum (1 Pa) prior to the characterizations. PILs were analyzed for water content using coulometric Karl–Fischer titration prior to the measurements. For [Pyrr][CF3 COO], the water content was 6000 ppm (0.6% ww ) just after distillation, and this value increased to 22 000 ppm (2.2% ww ) after a few weeks. The PIL, forming a eutectic mixture with water, was then liquid. In the case of [Pyrr][HSO4 − ], the water content varied between 200 and 30,000 ppm (3.0% ww ) after storing of the PILs for several weeks. The DSC thermogram and thermal properties for both studied PILs are presented in supplementary section. 2.3. Measurements 1 H NMR spectra were obtained using a Bruker 200 MHz spectrometer, with CDCl3 as the solvent and TMS as the internal standard. Viscosities were measured using a TA Instrument rheometer (AR 1000) with a conical geometry at various temperatures (from 298 to 378 K). Ionic conductivities were obtained with a Crison (GLP 31) digital multi-frequency conductimeter. The temperature control (from 293 to 393 K) was ensured by a JULABO thermostated bath (T ± 0.1 K). Differential Scanning Calorimetry (DSC) was carried out on a PerkinElmer DSC6 under a N2 atmosphere. Samples for DSC measurements were sealed in aluminum pans. The temperature and heat capacity were calibrated using cyclohexane, indium, and tin as standards. Pulse-Gradient Spin-Echo NMR (PGSE-NMR) All the PGSE-NMR experiments were performed on a Bruker DSX100 spectrometer with a 2.35 T superconducting magnet (Larmor frequencies: 0 = 100.13 MHz for 1 H and 0 = 94.22 MHz for 19 F) equipped with a 10-mm microimaging probe (Micro5 Bruker). For each ionic species, the runs were carried out in a temperature range from 273 K to 373 K with an accuracy of ±1 K and a step T = 5 K. The samples were thermally equilibrated at each temperature for 30 min before any measurements were performed.
3. Results and discussion For more clarity this manuscript is divided in two sections. Firstly transport properties of studied PILs, secondly the PGSE-NMR measurements. 3.1. Transport properties 3.1.1. Viscosity The viscosity is a very important parameter in electrochemical studies due to its strong effect on the rate of mass transport within a solution. Generally, ILs are more viscous than common molecular solvents. Indeed, while the viscosity of water, for instance, is only 0.8903 cP at 298 K, the viscosities of ILs typically range from 30 to 100 cP at room temperature, and can reach values as high as 1000 cP in certain cases [27–29]. PILs are very sensitive to nature of substituent [29], for example EAF, DEAF and TEAF have respectively 32 cP, 5.4 cP and 5.8 cP [27]. Moreover, IL viscosities appear to be governed also by van der Waals interactions [30] and cationic size [31].
M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
231
0,028
0,24
a 500
0,16
300 200 100 0
0,12
0,024
400
η (Pa.s)
Shear stress (Pa)
η (Pa.s)
0,20
0
500
1000 1500 2000 2500 3000
0,020
0,016
heated 1 heated 2
Shear rate (s )
0,08
0,012
300
0,04 300
310
320
330
320
330
340
b 0,28
0,21
η (Pa.s)
The identities of the anion and the cation composing an ionic liquid have a huge effect on its corresponding viscosity. With respect to the anionic species, a higher basicity, larger size, and more pronounced relative capacity to form hydrogen bonds result in more viscous ILs. Thus, due to the acid/base character of PILs, their viscosity depends significantly on hydrogen bonds [27]. At 298 K, the studied PILs displayed viscosity values of 187.6 and 25.7 cP for [Pyrr][HSO4 ] and [Pyrr][CF3 COO], respectively. The larger viscosity of [Pyrr][HSO4 ] as compared to that of [Pyrr][CF3 COO] may be the result of strong hydrogen bonds due to charge density on the hydrogen sulfate anion. Indeed, the basic character of the hydrogen sulfate anion increased its ability to accept hydrogen bonds, as well as the lability of the hydrogen on the nitrogen atom. Fig. 1 presents the temperature dependence of the viscosity data for [Pyrr][HSO4 ]. The viscosity can be seen to decrease from 240 cP to 50 cP as the temperature is raised from 293 to 343 K. This was the result of the higher mobility of the ions. In the studied temperature interval, the material exhibited a Newtonian behavior. Indeed, the shear stress increased linearly with the shear rate, as illustrated in the inset of Fig. 1. For the pyrrolidinium trifluoro acetate, sample viscosities were recorded during heating from 293 to 353 K, followed by cooling back down to 293 K. Two series of measurement were performed successively and the results are given in Fig. 2a and b. During heating, it was observed that water impurities evaporated at 333 K, and that the initial viscosity of pure [Pyrr][CF3 COO] was shifted from = 26 cP (water content = 22 000 ppm) before heating to = 24 cP (water content = 6000 ppm) after heating (the uncertainly of viscosity: is 0.1 cP). During cooling, subcooling phenomena were
0,14
cooled 1 cooled 2
0,07
0,00
300
310
320
330
noticeable by a shift of the maximum of viscosities caused by the crystallization of the dry PILs. 3.1.2. Conductivity Since ILs are composed entirely of ions, they are supposed to be among the most concentrated electrolytic fluids that exist, with a large number of charge carriers per unit volume. When these charge carriers are mobile, it becomes possible to obtain very high conductivities. ILs have reasonably good ionic conductivities (up to 10 mS cm−1 ) as compared to organic solvents/electrolyte systems [32]. PILs, on the other hand, possess generally higher conductivities than their aprotic counterparts [27]. At 298 K, the conductivity values of [Pyrr][HSO4 ] (water content = 300 ppm) and [Pyrr][CF3 COO] (water content = 8000 ppm)
b
T = 328 K
-1
σ (mS.cm )
-1
σ (mS.cm )
60
10
40
20
300
325
T/K
350
375
350
Fig. 2. The variation in viscosity with temperature for [Pyrr][CF3 COO] (a) during heating, and (b) during cooling. All measurements were performed at a scan rate v = 2 K mn−1 .
80
20
340
T/K
a
275
350
0,35
Fig. 1. The variation in viscosity of [Pyrr][HSO4 ] with temperature. Inset: the shear stress vs. shear rate at 298 K.
0 250
340
T/K
T/K
30
310
0 250
275
300
325
350
375
400
T/K
Fig. 3. The temperature dependence of the ionic conductivity for (a) [Pyrr][HSO4 ] and (b) [Pyrr][CF3 COO].
425
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-3,6
a
1,0 -1
Ln(σ/mS.cm )
-3,8
Ln(η/Cp)
b
1,5
-4,0 -4,2
0,5 0,0 -0,5
-4,4 -1,0
-4,6
3,0
3,1
3,2
3,3
3,4
2,6
2,8
3,0
3,2
3,4
3,6
3,8
4,0
-1
-1
1000/T(K )
1000/T(K )
Fig. 4. Arrhenius plots illustrating the temperature dependence of (a) the viscosity of [Pyrr][CF3 COO] and (b) the conductivity of [Pyrr][HSO4 ].
2,0
a
1,5
1,5
Ln(σ) / mS.cm
-1
-1
1,0
Ln(σ) / mS.cm
b
0,5
0,0
T = 55 °C 1,0
0,5
-0,5 0,0
-1,0
6
8
10
12
14
16
3,0
3,5
4,0
4,5
5,0
-1
-1
1000/(T-T0) (K )
1000/(T-T0) (K )
Fig. 5. VTF plots of ionic conductivities for (a) [Pyrr][HSO4 ] and (b) [Pyrr][CF3 COO]. The solid lines represent VTF fittings based on the parameters indicated in Table 2.
were 4.6 and 4.0 mS cm−1 , respectively, and at 363 K, these values were raised to respectively 31.2 and 36.3 mS cm−1 (Fig. 3). The obtained conductivity values were similar to those reported in previous studies for PILs based either on the [HSO4 ] or [CF3 COO] anion: (ethylammonium hydrogen sulfate, = 4.4 mS cm−1 , [2]); (1-methylimidazolium hydrogen sulfate, = 6.5 mS cm−1 , [33]); (N-methylpyrrolidinium trifluoroacetate, = 1.0 mS cm−1 , [34]); (2-methylpyrrolidinium trifluoroacetate, = 3.0 mS cm−1 , [4]). The temperature dependence of the ionic conductivity for [Pyrr][HSO4 ] and [Pyrr][CF3 COO] is shown in Fig. 3a and b. As expected, the conductivities increased with the temperature, up to 68 mS cm−1 at 148 ◦ C in the case of [Pyrr][CF3 COO]. After the heating, the water content increases up to 30,000 ppm and 27,000 ppm for [Pyrr][HSO4 ] and [Pyrr][CF3 COO] respectively. The variation of conductivities is clearly affected by water content as we shown in previously work [35]. Moreover, residual conductivities were
a
noticed: close to 0.17 mS cm−1 at −16 ◦ C for the [Pyrr][HSO4 ] and 0.48 mS cm−1 at −7 ◦ C for the [Pyrr][CF3 COO]. In the case of pyrrolidinium trifluoroacetate, a modification of its behavior with regard to the variation in conductivity was observed at T = 55 ◦ C. 3.2. Fitting procedure The viscosities or conductivities of PILs typically follow a nonArrhenius behavior, i.e., a distinct downward curvature at lower temperatures in an Arrhenius plot. Examples are presented in Fig. 4a and b, displaying the conductivity vs. temperature in the case of [Pyrr][HSO4 ] and the viscosity vs. temperature in the case of [Pyrr][CF3 COO], respectively. The viscosities , conductivities , and molar conductivities cond calculated from the ionic conductivity and the molar volume, can be fitted to the Vogel–Tamman–Fulcher (VTF) equation
b
Fig. 6. VTF plot of (a) the viscosity, , and (b) the molar conductivity, cond , for the studied PILs. The solid lines represent VTF fittings based on the parameters indicated in Table 1.
M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
233
Table 1 VTF equation parameters for the viscosity and the ionic conductivity (0 0 , Bi , T0, R2 ). PILs
Viscosity
[Pyrr][HSO4 ] [Pyrr][CF3 COO]
T0 (K) 230 230
0 (Cp) 7.322 1.792
B1 (K) 226.09 176.37
R2 a 0.9993 0.9991
T0 (K)
0 (mS cm−1 )
B2 (K)
R2 a
193.0 75.5 75.5
16.77 27.11 1130.03
227.9 501.1 1435.6
0.9995 0.9945 0.9998
0 (S.m2 mol−1 )
B3 (K)
R2 a
Conductivity
[Pyrr][HSO4 ] [Pyrr][CF3 COO] T > 55 ◦ C [Pyrr][CF3 COO] T < 55 ◦ C
Molar Conductivity cond T0 (K) [Pyrr][HSO4 ] [Pyrr][CF3 COO] a
2.334 × 10−3 5.192 × 10−3
227 229
267.8 298.3
0.9943 0.9977
Correlation coefficient.
according to
B1 = 0 exp (T − T0 ) = 0 exp
(1)
−B 2
(2)
(T − T0 )
cond = 0 exp
−B 3
(3)
(T − T0 )
where Bi , 0 , 0 and 0 are constants usually referred to as the pseudo activation energy and the zero-mobility temperature, for viscosity, conductivity and molar conductivity respectively. The experimental data can then be fitted with Eqs. (1)–(3), taking T0 as the ideal glass transition temperature for each compound, cf. Figs. 5 and 6. Here, 0 (mS cm2 mol−1 ), Bi (K), and T0 (K) are constants. Table 1 lists the best-fit parameters for this VTF equation. 3.3. Pulse-gradient spin-echo NMR (PGSE-NMR) 3.3.1. Method of measurement The PGSE-NMR method [36] is used to measure the selfdiffusion of a variety of ionic species. For each of the PILs in the present study, cationic and anionic self-diffusion coefficients were determined by proton (1 H) and/or fluorine (19 F), NMR respectively. Because of the difference between the longitudinal relaxation time (T1 ) and its transverse counterpart (T2 ) (T2 values are usually shorter than T1 ), a modified stimulated spin-echo sequence was utilized [37]. It consisted in a 13-interval PGSTE pulse sequence [38] with a bipolar-gradient pair to improve the self-diffusion coefficient measurements (cf. Fig. 7). Indeed, at high temperatures (T > 358 K) in such media, the T1 /T2 ratio in 1 H NMR can exceed 25 for certain chemical groups of the PILs. For example, at high temperature (T = 373 K) for the methyl group of the pyrrolidinium cation in [Pyrr][CF3 COO], the relaxation times T1 and T2 (1 H NMR) are 1160 ms and 41 ms respectively and the ratio T1 /T2 reaches 28.3. Self-diffusion coefficients were calculated by measuring the decrease of the NMR echo signal intensity through increasing magnetic field gradients (g). Self-diffusion coefficients, D, were then obtained by a simple linear least-square fitting of the echo attenuation E(q, ) according to Eq. (4):
E(g, ) =
I(g, ) = exp − 2 g 2 ı2 D I(0, )
−
ı 3
(4)
Here, I(g, ) and I(0, ) are the echo intensities, respectively, measured with and without the field gradient g (varying between 0 and
gmax ), ı is its duration, is the gyromagnetic ratio of the nuclei, is the diffusion time (fixed to 20 ms) and D is the self-diffusion coefficient. For all 1 H and 19 F PGSTE-NMR runs, the magnetic field gradient duration (ı) was set to 3 ms and the maximum value of the magnetic field gradient (gmax ) was 1.5 T/m. Depending on the longitudinal relaxation time T1 value, the recovery delay was varied between 2 s and 10 s. In addition, the spectrometer dead time was 100 s and 350 s, in 1 H and 19 F NMR measurements respectively. 3.3.2. Self-diffusion coefficient measurements Fig. 8 shows the 1 H NMR spectra, performed at 100 MHz, of the PILs together with the assignments of the various nuclei in both compounds. The PGSE-NMR method allows the determination of the ions’ self-diffusion coefficient without the use of any additional probe molecules affecting the diffusion processes. Since the PILs used in this study included NMR-sensitive 1 H and 19 F nuclei in the cations and/or anions, each of the cationic and anionic self-diffusion coefficients could be independently determined. Fig. 9a and b shows the temperature dependencies of the 1 H and 19 F nuclei self-diffusion coefficients in [Pyrr][CF COO] and 3 [Pyrr][HSO4 ]. Sample studied here content initially 12,000 and 25,000 ppm for [Pyrr][CF3 COO] and [Pyrr][HSO4 ] respectively. Preliminary experiments have shown that no dependence of the self-diffusion coefficients occurs when the diffusion time is varied from 20 ms to 100 ms. Such a behavior suggests that the diffusion process of the ions in the PILs can be considered as a simple Fickian phenomenon in a homogeneous fluid. In the studied tem-
Fig. 7. (a) A schematic view of the 13-interval PGSTE-NMR pulse sequence used for the measurement of the self-diffusion coefficients. This sequence produced a stimulated spin-echo at t = 4 1 + 2 . Between the third pulse and the fourth pulse, during the time 2 , the magnetization was stored along the static magnetic field B0 and therefore subjected only to the longitudinal relaxation time T1 . The diffusion time was = 2 1 + 2 . (b) The coherence transfer pathway (p = 0 → − 1 → + 1 → 0 → + 1 → − 1) resulting from an adequate phase cycling to take into account only the stimulated echo.
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1.2
[Pyrr][HSO [Pyrr][HSO4] ] 4
1.0
1
1.2
NH
+
2
0.4
OH
0.2 0.0
0.6
5
NH
+
2
0.4 0.2 0.0
a
(a)
-0.2 10
(b)
0
b
-0.2 10
-5
5
Chemical shift (ppm) 1
Fig. 8.
-10
10
T≈55°C
-
[Pyrr] [CF3COO ]
Diffusion coefficient D (m 2/s)
Diffusion coefficient D (m 2/s)
-5
H NMR spectra of (a) [Pyrr][HSO4 ] and (b) [Pyrr][CF3 COO]. The chemical shift reference was arbitrary.
-9
-10
+
Self-diffusion NH2 Self-diffusion CH2 10
0
Chemical shift (ppm)
10
10
2
2
0.8
Intensity (a. u.)
Intensity (a. u.)
2
H NMR
CH
CH
CH
0.6
1
33
1.0
CH
2
0.8
-
[Pyrr][CF COO [Pyrr][CF COO ] ]
H NMR
a
Self-diffusion CF3
-11
2.6
2.8
3.0
-
[Pyrr] [HSO4 ]
-11
10
+ 2
Self-diffusion NH
Self-diffusion OH
3.2
3.4
3.6
3.8
10
b
Self-diffusion CH2
-12
2.6
2.8
3.0
-1
3.2
3.4
3.6
1000/T (K-1)
1000/T (K )
Fig. 9. Temperature dependencies of 1 H and 19 F nuclei self-diffusion coefficients in [Pyrr][CF3 COO] (a) and [Pyrr][HSO4 ] (b). The solid lines represent the best fits using the Arrhenius law.
perature range, it was first of interest to compare the self-diffusion coefficients of ions in [Pyrr][HSO4 ] and [Pyrr][CF3 COO]: one can observe that diffusion was always faster for ions in [Pyrr][CF3 COO], which was in accord with its lower viscosity (cf. Section 3.1). As shown by the graphs in Fig. 9a and b, the diffusion coefficient values of the –CH2 groups belonging to the pyrrolidinium ring (1 H NMR) were very similar to those of the –CF3 groups in the CF3 COO− anion (19 F NMR) or the –OH group in the HSO4 − anion (1 H NMR). This clearly indicated that cations and anions diffused at the same speed and that they were, most probably, attached together as ion pairs or larger aggregates. Nevertheless, the D values for the pyrrolidinium ring were higher in [Pyrr][CF3 COO] than in [Pyrr][HSO4 ]. At 298 K, for [Pyrr][CF3 COO]: DPyrr = 6.07 × 10−11 m2 s−1 and whereas for [Pyrr][HSO4 ]: DCF3 COO = 5.84 × 10 − 11 m2 s−1 DPyrr = 7.93 × 10−12 m2 s−1
DHSO4 = 8.37 × 10−12 m2 s−1 .
and
10
The diffusivity ratios of cations or anions in the two PILs were almost equal to the fluidity ratio, according to D(Pyrr)[Pyrr][CF3 COO] D(Pyrr)[Pyrr][HSO4 ] = 6.98 ∼ =
D(CF3 COO) = 7.65 ∼ = D(HSO4 ]
˚[Pyrr][CF3 COO] ˚[Pyrr][HSO4 ]
= 7.3
(5)
where the fluidity is defined as the inverse of the viscosity (˚ = −1 ). Moreover, the diffusion coefficient of the proton bound to the nitrogen atom was faster at all temperatures in both ILs, however the difference was more pronounced in [Pyrr][HSO4 ] as opposed to in [Pyrr][CF3 COO], as demonstrated in Fig. 10a and b. This means that, especially in [Pyrr][HSO4 ], the proton attached to the nitrogen atom diffused faster than the pyrrolidinium cation. It is possible
b
a 10
10 Λ
10
10
2,6
2,8
3,0
3,2
1000/T(K )
3,4
3,6
10
2,6
2,8
3,0
3,2
3,4
3,6
1000/T(K )
Fig. 10. The temperature dependency of the molar conductivity for [Pyrr][HSO4 ] (a), and [Pyrr][CF3 COO] (b) calculated from Eq. (8).
M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237 Table 2 Arrhenius equation parameters for the self-diffusion coefficients D0 and Ea for [Pyrr][HSO4 ]. R2 is the correlation coefficient. D0 (m2 s−1 )
Group
−9
7.22 × 10 3.65 × 10−9 3.09 × 10−9
+
NH2 from [Pyrr] CH2 from [Pyrr] OH from [HSO4 ]
Ea (kJ mol−1 )
R2
13.7 15.2 14.6
0.98780 0.99802 0.99592
that this rapid diffusion was enabled by a Grotthuss mechanism, where free protons are able to jump from an acidic site (–NH2 + or HSO4 − ) to a basic site (–NH2 or SO4 2− ). In [Pyrr][CF3 COO], this effect was only observed at temperatures above 328 K, i.e., when the IL had adsorbed a sufficient amount of water (30,000 ppm for [Pyrr][HSO4 ]). The influence of the temperature on the self diffusion coefficients was quantified by Arrhenius plots of [Pyrr][HSO4 ] and the experimental data was fitted according to: D(T ) = D0 exp
E a
(6)
RT
where D0 (m2 s−1 ) is a pre-exponential factor and Ea is the activation energy for the diffusion process. Best-fit values are reported in Table 2. The activated energy Ea was approximately the same for all ionic species with a mean value of 14.5 ± 0.8 kJ/mol. This was expected since the diffusion was mainly controlled by the viscosity of the media, as seen before. For the [Pyrr][CF3 COO] sample, the evolution of the selfdiffusion coefficient followed two temperature-dependent behaviors as demonstrated by the break in the curves observed near T = 328 K a temperature corresponding to the fusion temperature (Tm ) of the anhydrous IL. Moreover, an equivalent break was observed in the curves from the ionic conductivity measurements. On either side of this break, the self-diffusion coefficients followed an Arrhenius law, but with different activation energies. This was due to Ea being larger at high temperatures than at low ones. The best-fit parameters are listed in Table 3. At low temperatures, the activation energy Ea of all ionic species presented very similar values (13.3 ± 1.5 kJ mol−1 ), whereas above Tm , Ea was four times higher with a mean value of 41.0 ± 1.8 kJ mol−1 . The diffusion coefficients measured by PGSE-NMR were illustrative of the differing behaviors of the two ILs. Obviously, changing the anion significantly influenced the intermolecular interactions as illustrated by the following equilibrium: Pyrr . . . .H+ . . .− OSO3 H PyrrH+ + HSO4 − PyrrH+ , HSO4 − Pyrr . . . .H+ . . .− OOCF3 PyrrH+ + − OOCF3 PyrrH+ ,− OOCF3 Several authors have used self-diffusion coefficients determined by PGSE-NMR to calculate the molar conductivity (NMR ) according to the Nernst–Einstein equation: [39,40] NMR =
F 2 (Dcation + Danion ) RT
(7)
Here, F is the Faraday constant, and R is the universal gas constant. The self-diffusion coefficients obtained by PGSE-NMR pointed at a
235
translational motion (self-diffusion) of the NMR-sensitive nuclei. Thus, the NMR values were derived from the assumption that all of the diffusing species detected during the PGSE-NMR measurement contributed to the molar conductivity. On the other hand, the cond values (i.e., the direct current, d.c., conductivity) were based on the migration of the charged species under an electric field. Hence, the Nernst–Einstein equation holds only if species involved in diffusion were also responsible for conduction. In the presence of neutral ion pairs, the NMR values were expected to differ from those determined from conductivity (cond ). Moreover, another important point was that the strong ion-ion interactions occurring in ILs should affect the D and values by separate mechanisms and hence to varying extents. In order to calculate the molar conductivity, it was necessary to take into account all kinds of mobile species: the mobile proton (H+ ), the pyrrolidinium ring (Pyrr+ ) and the anion (HSO4 − or CF3 COO− ). As the respective contributions of H+ and Pyrr+ to the conductivity were unknown, a parameter ˛, ranging from 0 to 1, was introduced in Eq. (8): NMR =
F2 ((1 − ˛)DPyrr+ + ˛(DH+ ) + DA− ) RT
(8)
Fig. 10a reports on the temperature dependency of the molar conductivity calculated from Eq. (8) for [Pyrr][HSO4 ], using ˛ = 1 and ˛ = 0 as limits. For the sake of comparison, the experimental conductivities exp for the two PILs have been plotted in the same graph. cond was found to be intermediate between calculated values for respectively ˛ = 1 and ˛ = 0. At high temperatures, the conductivity was mainly due to proton jumps and, conversely, at low temperature, Pyrr+ cations provided the major contribution to the conductivity. Nevertheless, at a temperature above 370 K or below 285 K, the curvature of the cond vs. T−1 trace indicated certain discrepancies between calculated and experimental values. This signified that the Nernst equation had become invalid. At low temperatures, the underlying reason for this might be the formation of ion pairs. Fig. 10b presents the temperature dependency of the molar conductivity for [Pyrr][CF3 COO], as calculated from Eq. (8). Since the contribution of the Grotthuss mechanism was low in this case, the curves corresponding to ˛ = 1 and ˛ = 0 were almost superposed. At all temperatures, low as well as high, NMR was greater than cond . Fig. 11 displays the ratio cond /NMR plotted against the temperature, and which was found to be sensitive to changes thereof. Consequently, the cond /NMR ratio indicated the percentage of ions (charged species) contributing to the ionic conduction in the diffusing species. It should be noted here that it was not possible to distinguish the difference between free ions and associated ionic species in the NMR chemical shifts within the NMR time scale (10−9 –10−10 s). This implies that the association/dissociation rate of the ionic species, if any, was much faster than the time scale of the d.c. conductivity measurements. In the time scale of the conductivity measurements, an individual ion under an electrical field migrates for a certain period when it exists as a charged species, but it may associate to form non-charged pairs and aggregates during another
Table 3 Arrhenius equation parameters (D0 and Ea ) for the self-diffusion coefficients of the ions in [Pyrr][CF3 COO]. R2 is the correlation coefficient. Temperature
Group ◦
Low temperature T < 55 C
High temperature T > 55 ◦ C
+
NH2 from [Pyrr] CH2 from [Pyrr] CF3 from [CF3 COO] NH2 + from [Pyrr] CH2 from [Pyrr] CF3 from [CF3 COO]
D0 (m2 s−1 ) −9
11.20 × 10 11.23 × 10−9 25.68 × 10−9 3.09 × 10−4 8.39 × 10−4 3.20 × 10−4
Ea (kJ mol−1 )
R2
12.1 12.8 15.0 39.8 43.0 40.1
0.98254 0.98839 0.97360 0.99756 0.99658 0.99870
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M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
1,0
observing 1 H and 19 F nuclei with the pulsed-field gradient spinecho NMR technique. It was shown that a fast diffusion of protons occurred at all temperature in [Pyrr][HSO4 ] but that this was not the case for [Pyrr][CF3 COO]. Instead, diffusion of mobile protons increased with the temperature. Independently of the temperature, D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five time higher than the pyrrolidinium cation or hydrogen sulfate anion in [Pyrr][HSO4 ] and twofold that of [Pyrr][CF3 COO]. The proton conduction in the PILs followed a combination of Grotthuss- and vehicle-type mechanisms. These results were confirmed by conductivity and viscosity measurements which were used to investigate the ion transport motilities in both PILs by means of the Stokes–Einstein equation.
[Pyrr][CF3COO] [Pyrr][HSO4]
Λcond/ΛRMN
0,8
0,6 T = 55°C
0,4
0,2
0,0
280
300
320
340
360
380
T(K) Fig. 11. Molar conductivity ratios (cond /NMR ) [Pyrr][HSO4 ] PILs plotted against temperature.
1,2
11
-2
1/D (10 m .s)
1,0
for
[Pyrr][CF3 COO]
and
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fluid.2010.09.035. References
[Pyrr][CF3COO] [Pyrr][HSO4]
0,8 0,6 0,4 0,2 0,0 0,0
-4
1,0x10
-4
2,0x10
-4
3,0x10
-4
4,0x10
-4
5,0x10
-1
η/T (Pa.s.K ) Fig. 12. The relationship between 1/D and /T for species in [Pyrr][HSO4 ] and [Pyrr][CF3 COO]. The solid lines were obtained by linear regression of the experimental data.
period. When the ion exists as a non-charged species, it does not contribute to the d.c. conduction. Thus, the cond /NMR ratios can be a measure of the ionic association of the PIL. The diffusion coefficient, D, can also be related to the viscosity of solution, , by the well-known Stokes–Einstein equation on the basis of the hydrodynamic model that a solute sphere moves through a continuum fluid: 1 6 r = D kB T
Appendix A. Supplementary data
(9)
Here, kB is the Boltzmann constant and r is the hydrodynamic radius. In Fig. 12, 1/D is plotted against /T for [Pyrr][HSO4 ] and [Pyrr][CF3 COO]. Here, is the viscosity. It was found that 1/D was not proportional to /T through the origin in the case of [Pyrr][HSO4 ] for the studied temperature range. This fact indicates that the macroscopic viscosity of the solution was not the predominant factor in the translational motions of the cation and anion species at low temperature. 4. Conclusions This work has demonstrated that self-diffusion coefficients (D) of the ion species in two studied PILs, i.e., pyrrolidinium hydrogen sulfate and pyrrolidinium trifluoroacetate, could be determined by
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Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements Mériem Anouti a,∗ , Patrice Porion b , Catherine Brigouleix a , Hervé Galiano c , Daniel Lemordant a a b c
Université Franc¸ois Rabelais, Laboratoire PCMB (EA 4244), équipe de Chimie-physique des Interfaces et des Milieux Electrolytiques (CIME), Parc de Grandmont, 37200 Tours, France Centre de Recherche sur la Matière Divisée, CNRS-Université d’Orléans, UMR6619, 1b rue de la Férollerie, 45071 Orléans Cedex 02, France CEA, Le Ripault, Laboratoire Synthèse et Transformation des Polymères, F-37260 Monts, France
a r t i c l e
i n f o
Article history: Received 20 June 2010 Received in revised form 26 September 2010 Accepted 28 September 2010 Available online 5 November 2010 Keywords: Protic ionic liquids Pyrrolidinium Hydrogen sulfate Trifluoroacetate Pulsed-field gradient spin-echo NMR Self-diffusion coefficients Stokes–Einstein
a b s t r a c t Conductivity and viscosity measurements of pyrrolidinium hydrogen sulfate, [Pyrr][HSO4 ], and pyrrolidinium trifluoroacetate [Pyrr][CF3 COO] were performed at various temperatures over a wide temperature range (i.e., from T = 273 K to 353 K). The results were utilized in the Stokes–Einstein equation to investigate the proton conductivity in both PILs. The self-diffusion coefficients (D) of the cation and anion species in both studied PILs were independently determined in the same temperature range by observing 1 H and 19 F nuclei with the pulsed-field gradient spin-echo NMR technique. With regard to the mechanism of self-diffusion, based on the values of the coefficients, a relatively large difference was observed between the two ionic liquids (ILs). Independently of the temperature, the D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five times higher than those of the pyrrolidinium cation or hydrogenosulfate anion in [Pyrr][HSO4 ], and twofold those in the case of [Pyrr][CF3 CO2 ]. In order to comment d.c. conductivities results, the self-diffusion coefficients determined by PGSE NMR were converted into charge diffusivity D by means of the Nernst–Einstein equation. In a similar way, a viscosity-related diffusivity D was calculated with the aid of the Stokes–Einstein equation. The temperature-independent cation transference number and the effective hydrodynamic radius were also deduced from these equations. Such parameters play an important role in charge and mass transports in ILs. Moreover, proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms, which confirms that Brønsted acid–base ionic liquid systems are good candidates as proton conductors in fuel cells or supercapacitor electrolyte devices operating under anhydrous conditions at elevated temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The term ionic liquids (ILs) is broadly used in order to describe a large class of low-melting organic salts that are liquids below 373 K. Protic ionic liquids (PILs) are a subset of ILs formed by the combination of a Brønsted acid with a Brønsted base [1–4]. When a PIL is synthesized by mixing a strong acid with a strong base, the proton is fixed very tightly to the base; the PIL is most likely composed entirely of ions with possible ion complexation and aggregate formation [5]. Because of their advantageous properties, these materials may be used as alternatives for conventional solvents in the context of “green chemistry” as well as to reduce emissions to the environment. ILs have received a great deal of attention as a class of solvents with a wide range of potential applications including organic and inorganic synthesis [6], energy
∗ Corresponding author. Tel.: +33 247366951; fax: +33 247367360. E-mail address: [email protected] (M. Anouti). 0378-3812/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2010.09.035
storage devices [7], separations [8,9], and catalysis [10]. The most notable characteristics of many ILs include their low vapor pressure, non-flammability, thermal stability, wide liquid range, and solvating properties for diverse substances [11,12]. Since the main feature of PILs are their ability to form hydrogen bonds, they have been investigated as amphiphile self-assembly media [13], biological solvents [14], and in polymer membrane fuel cells [15]. Aggregate formation in ILs is an important issue still subject to debate [16,17]. The formation of ionic pairs and larger agglomerates (or supramolecules) [16] is intimately connected with the structural organization of RTILs at nano-scales [18–21]. Furthermore, the occurrence of such agglomerates can explain the discrepancies found between mass and charge transport in these liquids [22]. A high ionic concentration, resulting in the significant Coulombic interactions, implies that the differences in the ionic state and ion dynamics significantly contribute to the variations in physicochemical properties of ILs. However, microscopic information regarding the ionic state and ion dynamics has long been insufficient. It was only recently recognized that a pulsed-field gradient
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spin-echo (PGSE) nuclear magnetic resonance (NMR) method can be applied in order to determine the self-diffusion coefficients of the individual ionic species [23–25]. In this context, it was considered of considerable importance to investigate transport phenomena in ILs over a wide temperature range; not only from an engineering viewpoint but also to obtain more fundamental knowledge. This work presents two PILs synthesized and characterized with regard to viscosities, diffusivity, and conductivities, at various temperatures. The outstanding feature of this paper is the fact that it provides a complete set of transport data as a function of temperature for each ionic liquid. Specifically, this includes the self-diffusion coefficient for cations and anions from pulsedgradient spin-echo nuclear magnetic resonance (PGSE-NMR) measurements, the ionic conductivity, and the shear viscosity from cone-plate viscometer experiments. The temperature dependence of each transport quantity is described by separate fits of the Vogel–Tammann–Fulcher equation. Comparisons of the selfdiffusion with either the conductivity or the viscosity were based on the Nernst–Einstein and Stokes–Einstein equations, respectively. This procedure yielded information about the degree of cation–anion association as well as the hydrodynamic radius of both ionic species. 2. Experimental 2.1. Materials Pyrrolidine is commercially available from Fluka (>99.0%) and was used without further purification. A sulfuric acid (68% in water) solution and trifluoric acid (>99.0%) were obtained from Sigma Aldrich. 1,2-dichloroethane (DCE) (>99,0%) was purchased from Sigma Aldrich, and water was purified with a Milli-Q 18.3 M water system. 2.2. Preparation of PILs Pyrrolidinium trifluoroacetate and pyrrolidinium hydrogen sulfate were synthesized according to the procedure described in a previous publication [26]. 2.2.1. Preparation of pyrrolidinium hydrogen sulfate [Pyrr][HSO4 ]. Pyrrolidine (26.78 g; 0.37 mol) was introduced in a two-necked round-bottomed flask immersed in an ice bath and equipped with a plastic dropping funnel, for the addition of sulfuric acid (96% in water), and a thermometer, to control the temperature. Under vigorous stirring, sulfuric acid (37.80 g; 0.37 mol) was dropwise added to the flask in approximately 60 min (mixture temperature < 308 K). Stirring was maintained for 4 h at ambient temperature before adding 120 mL of DCE. Then, with the aim of removing residual water, this mixture was distilled under normal pressure until the water-DCE hetero-azeotropic boiling point was reached (346 K). DCE was finally evaporated from the mixture under reduced pressure so that a pale yellow and viscous liquid could be collected (recovery: 96%). 2.2.2. Preparation of pyrrolidinium trifluoroacetate [Pyrr][CF3 COO] Pyrrolidine (38.55 g; 0.85 mol) was charged to a three-necked round-bottomed flask immerged in an ice bath and equipped with a reflux condenser, a dropping funnel to add the acid and a thermometer to monitor the temperature. Under vigorous stirring, trifluoric acid (96.921 g; 0.85 mol) was added dropwise to the pyrrolidine (60 min). As this acid–base reaction was strongly exothermic, the temperature of the mixture was, by means of an ice
bath, maintained below 298 K during the addition of the acid. Stirring was carried out for 4 h at ambient temperature. A pale yellow viscous liquid was obtained. The residual pyrrolidine or acid was evaporated under reduced pressure and the remaining liquid was further dried at 353 K, also under reduced pressure (1–5 mmHg), to obtain the solid PIL (133.70 g; recovery: 98.7%) (Tm = 328 K). Since both PILs were very hygroscopic compounds, they were dried overnight at 343 K under high vacuum (1 Pa) prior to the characterizations. PILs were analyzed for water content using coulometric Karl–Fischer titration prior to the measurements. For [Pyrr][CF3 COO], the water content was 6000 ppm (0.6% ww ) just after distillation, and this value increased to 22 000 ppm (2.2% ww ) after a few weeks. The PIL, forming a eutectic mixture with water, was then liquid. In the case of [Pyrr][HSO4 − ], the water content varied between 200 and 30,000 ppm (3.0% ww ) after storing of the PILs for several weeks. The DSC thermogram and thermal properties for both studied PILs are presented in supplementary section. 2.3. Measurements 1 H NMR spectra were obtained using a Bruker 200 MHz spectrometer, with CDCl3 as the solvent and TMS as the internal standard. Viscosities were measured using a TA Instrument rheometer (AR 1000) with a conical geometry at various temperatures (from 298 to 378 K). Ionic conductivities were obtained with a Crison (GLP 31) digital multi-frequency conductimeter. The temperature control (from 293 to 393 K) was ensured by a JULABO thermostated bath (T ± 0.1 K). Differential Scanning Calorimetry (DSC) was carried out on a PerkinElmer DSC6 under a N2 atmosphere. Samples for DSC measurements were sealed in aluminum pans. The temperature and heat capacity were calibrated using cyclohexane, indium, and tin as standards. Pulse-Gradient Spin-Echo NMR (PGSE-NMR) All the PGSE-NMR experiments were performed on a Bruker DSX100 spectrometer with a 2.35 T superconducting magnet (Larmor frequencies: 0 = 100.13 MHz for 1 H and 0 = 94.22 MHz for 19 F) equipped with a 10-mm microimaging probe (Micro5 Bruker). For each ionic species, the runs were carried out in a temperature range from 273 K to 373 K with an accuracy of ±1 K and a step T = 5 K. The samples were thermally equilibrated at each temperature for 30 min before any measurements were performed.
3. Results and discussion For more clarity this manuscript is divided in two sections. Firstly transport properties of studied PILs, secondly the PGSE-NMR measurements. 3.1. Transport properties 3.1.1. Viscosity The viscosity is a very important parameter in electrochemical studies due to its strong effect on the rate of mass transport within a solution. Generally, ILs are more viscous than common molecular solvents. Indeed, while the viscosity of water, for instance, is only 0.8903 cP at 298 K, the viscosities of ILs typically range from 30 to 100 cP at room temperature, and can reach values as high as 1000 cP in certain cases [27–29]. PILs are very sensitive to nature of substituent [29], for example EAF, DEAF and TEAF have respectively 32 cP, 5.4 cP and 5.8 cP [27]. Moreover, IL viscosities appear to be governed also by van der Waals interactions [30] and cationic size [31].
M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
231
0,028
0,24
a 500
0,16
300 200 100 0
0,12
0,024
400
η (Pa.s)
Shear stress (Pa)
η (Pa.s)
0,20
0
500
1000 1500 2000 2500 3000
0,020
0,016
heated 1 heated 2
Shear rate (s )
0,08
0,012
300
0,04 300
310
320
330
320
330
340
b 0,28
0,21
η (Pa.s)
The identities of the anion and the cation composing an ionic liquid have a huge effect on its corresponding viscosity. With respect to the anionic species, a higher basicity, larger size, and more pronounced relative capacity to form hydrogen bonds result in more viscous ILs. Thus, due to the acid/base character of PILs, their viscosity depends significantly on hydrogen bonds [27]. At 298 K, the studied PILs displayed viscosity values of 187.6 and 25.7 cP for [Pyrr][HSO4 ] and [Pyrr][CF3 COO], respectively. The larger viscosity of [Pyrr][HSO4 ] as compared to that of [Pyrr][CF3 COO] may be the result of strong hydrogen bonds due to charge density on the hydrogen sulfate anion. Indeed, the basic character of the hydrogen sulfate anion increased its ability to accept hydrogen bonds, as well as the lability of the hydrogen on the nitrogen atom. Fig. 1 presents the temperature dependence of the viscosity data for [Pyrr][HSO4 ]. The viscosity can be seen to decrease from 240 cP to 50 cP as the temperature is raised from 293 to 343 K. This was the result of the higher mobility of the ions. In the studied temperature interval, the material exhibited a Newtonian behavior. Indeed, the shear stress increased linearly with the shear rate, as illustrated in the inset of Fig. 1. For the pyrrolidinium trifluoro acetate, sample viscosities were recorded during heating from 293 to 353 K, followed by cooling back down to 293 K. Two series of measurement were performed successively and the results are given in Fig. 2a and b. During heating, it was observed that water impurities evaporated at 333 K, and that the initial viscosity of pure [Pyrr][CF3 COO] was shifted from = 26 cP (water content = 22 000 ppm) before heating to = 24 cP (water content = 6000 ppm) after heating (the uncertainly of viscosity: is 0.1 cP). During cooling, subcooling phenomena were
0,14
cooled 1 cooled 2
0,07
0,00
300
310
320
330
noticeable by a shift of the maximum of viscosities caused by the crystallization of the dry PILs. 3.1.2. Conductivity Since ILs are composed entirely of ions, they are supposed to be among the most concentrated electrolytic fluids that exist, with a large number of charge carriers per unit volume. When these charge carriers are mobile, it becomes possible to obtain very high conductivities. ILs have reasonably good ionic conductivities (up to 10 mS cm−1 ) as compared to organic solvents/electrolyte systems [32]. PILs, on the other hand, possess generally higher conductivities than their aprotic counterparts [27]. At 298 K, the conductivity values of [Pyrr][HSO4 ] (water content = 300 ppm) and [Pyrr][CF3 COO] (water content = 8000 ppm)
b
T = 328 K
-1
σ (mS.cm )
-1
σ (mS.cm )
60
10
40
20
300
325
T/K
350
375
350
Fig. 2. The variation in viscosity with temperature for [Pyrr][CF3 COO] (a) during heating, and (b) during cooling. All measurements were performed at a scan rate v = 2 K mn−1 .
80
20
340
T/K
a
275
350
0,35
Fig. 1. The variation in viscosity of [Pyrr][HSO4 ] with temperature. Inset: the shear stress vs. shear rate at 298 K.
0 250
340
T/K
T/K
30
310
0 250
275
300
325
350
375
400
T/K
Fig. 3. The temperature dependence of the ionic conductivity for (a) [Pyrr][HSO4 ] and (b) [Pyrr][CF3 COO].
425
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-3,6
a
1,0 -1
Ln(σ/mS.cm )
-3,8
Ln(η/Cp)
b
1,5
-4,0 -4,2
0,5 0,0 -0,5
-4,4 -1,0
-4,6
3,0
3,1
3,2
3,3
3,4
2,6
2,8
3,0
3,2
3,4
3,6
3,8
4,0
-1
-1
1000/T(K )
1000/T(K )
Fig. 4. Arrhenius plots illustrating the temperature dependence of (a) the viscosity of [Pyrr][CF3 COO] and (b) the conductivity of [Pyrr][HSO4 ].
2,0
a
1,5
1,5
Ln(σ) / mS.cm
-1
-1
1,0
Ln(σ) / mS.cm
b
0,5
0,0
T = 55 °C 1,0
0,5
-0,5 0,0
-1,0
6
8
10
12
14
16
3,0
3,5
4,0
4,5
5,0
-1
-1
1000/(T-T0) (K )
1000/(T-T0) (K )
Fig. 5. VTF plots of ionic conductivities for (a) [Pyrr][HSO4 ] and (b) [Pyrr][CF3 COO]. The solid lines represent VTF fittings based on the parameters indicated in Table 2.
were 4.6 and 4.0 mS cm−1 , respectively, and at 363 K, these values were raised to respectively 31.2 and 36.3 mS cm−1 (Fig. 3). The obtained conductivity values were similar to those reported in previous studies for PILs based either on the [HSO4 ] or [CF3 COO] anion: (ethylammonium hydrogen sulfate, = 4.4 mS cm−1 , [2]); (1-methylimidazolium hydrogen sulfate, = 6.5 mS cm−1 , [33]); (N-methylpyrrolidinium trifluoroacetate, = 1.0 mS cm−1 , [34]); (2-methylpyrrolidinium trifluoroacetate, = 3.0 mS cm−1 , [4]). The temperature dependence of the ionic conductivity for [Pyrr][HSO4 ] and [Pyrr][CF3 COO] is shown in Fig. 3a and b. As expected, the conductivities increased with the temperature, up to 68 mS cm−1 at 148 ◦ C in the case of [Pyrr][CF3 COO]. After the heating, the water content increases up to 30,000 ppm and 27,000 ppm for [Pyrr][HSO4 ] and [Pyrr][CF3 COO] respectively. The variation of conductivities is clearly affected by water content as we shown in previously work [35]. Moreover, residual conductivities were
a
noticed: close to 0.17 mS cm−1 at −16 ◦ C for the [Pyrr][HSO4 ] and 0.48 mS cm−1 at −7 ◦ C for the [Pyrr][CF3 COO]. In the case of pyrrolidinium trifluoroacetate, a modification of its behavior with regard to the variation in conductivity was observed at T = 55 ◦ C. 3.2. Fitting procedure The viscosities or conductivities of PILs typically follow a nonArrhenius behavior, i.e., a distinct downward curvature at lower temperatures in an Arrhenius plot. Examples are presented in Fig. 4a and b, displaying the conductivity vs. temperature in the case of [Pyrr][HSO4 ] and the viscosity vs. temperature in the case of [Pyrr][CF3 COO], respectively. The viscosities , conductivities , and molar conductivities cond calculated from the ionic conductivity and the molar volume, can be fitted to the Vogel–Tamman–Fulcher (VTF) equation
b
Fig. 6. VTF plot of (a) the viscosity, , and (b) the molar conductivity, cond , for the studied PILs. The solid lines represent VTF fittings based on the parameters indicated in Table 1.
M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
233
Table 1 VTF equation parameters for the viscosity and the ionic conductivity (0 0 , Bi , T0, R2 ). PILs
Viscosity
[Pyrr][HSO4 ] [Pyrr][CF3 COO]
T0 (K) 230 230
0 (Cp) 7.322 1.792
B1 (K) 226.09 176.37
R2 a 0.9993 0.9991
T0 (K)
0 (mS cm−1 )
B2 (K)
R2 a
193.0 75.5 75.5
16.77 27.11 1130.03
227.9 501.1 1435.6
0.9995 0.9945 0.9998
0 (S.m2 mol−1 )
B3 (K)
R2 a
Conductivity
[Pyrr][HSO4 ] [Pyrr][CF3 COO] T > 55 ◦ C [Pyrr][CF3 COO] T < 55 ◦ C
Molar Conductivity cond T0 (K) [Pyrr][HSO4 ] [Pyrr][CF3 COO] a
2.334 × 10−3 5.192 × 10−3
227 229
267.8 298.3
0.9943 0.9977
Correlation coefficient.
according to
B1 = 0 exp (T − T0 ) = 0 exp
(1)
−B 2
(2)
(T − T0 )
cond = 0 exp
−B 3
(3)
(T − T0 )
where Bi , 0 , 0 and 0 are constants usually referred to as the pseudo activation energy and the zero-mobility temperature, for viscosity, conductivity and molar conductivity respectively. The experimental data can then be fitted with Eqs. (1)–(3), taking T0 as the ideal glass transition temperature for each compound, cf. Figs. 5 and 6. Here, 0 (mS cm2 mol−1 ), Bi (K), and T0 (K) are constants. Table 1 lists the best-fit parameters for this VTF equation. 3.3. Pulse-gradient spin-echo NMR (PGSE-NMR) 3.3.1. Method of measurement The PGSE-NMR method [36] is used to measure the selfdiffusion of a variety of ionic species. For each of the PILs in the present study, cationic and anionic self-diffusion coefficients were determined by proton (1 H) and/or fluorine (19 F), NMR respectively. Because of the difference between the longitudinal relaxation time (T1 ) and its transverse counterpart (T2 ) (T2 values are usually shorter than T1 ), a modified stimulated spin-echo sequence was utilized [37]. It consisted in a 13-interval PGSTE pulse sequence [38] with a bipolar-gradient pair to improve the self-diffusion coefficient measurements (cf. Fig. 7). Indeed, at high temperatures (T > 358 K) in such media, the T1 /T2 ratio in 1 H NMR can exceed 25 for certain chemical groups of the PILs. For example, at high temperature (T = 373 K) for the methyl group of the pyrrolidinium cation in [Pyrr][CF3 COO], the relaxation times T1 and T2 (1 H NMR) are 1160 ms and 41 ms respectively and the ratio T1 /T2 reaches 28.3. Self-diffusion coefficients were calculated by measuring the decrease of the NMR echo signal intensity through increasing magnetic field gradients (g). Self-diffusion coefficients, D, were then obtained by a simple linear least-square fitting of the echo attenuation E(q, ) according to Eq. (4):
E(g, ) =
I(g, ) = exp − 2 g 2 ı2 D I(0, )
−
ı 3
(4)
Here, I(g, ) and I(0, ) are the echo intensities, respectively, measured with and without the field gradient g (varying between 0 and
gmax ), ı is its duration, is the gyromagnetic ratio of the nuclei, is the diffusion time (fixed to 20 ms) and D is the self-diffusion coefficient. For all 1 H and 19 F PGSTE-NMR runs, the magnetic field gradient duration (ı) was set to 3 ms and the maximum value of the magnetic field gradient (gmax ) was 1.5 T/m. Depending on the longitudinal relaxation time T1 value, the recovery delay was varied between 2 s and 10 s. In addition, the spectrometer dead time was 100 s and 350 s, in 1 H and 19 F NMR measurements respectively. 3.3.2. Self-diffusion coefficient measurements Fig. 8 shows the 1 H NMR spectra, performed at 100 MHz, of the PILs together with the assignments of the various nuclei in both compounds. The PGSE-NMR method allows the determination of the ions’ self-diffusion coefficient without the use of any additional probe molecules affecting the diffusion processes. Since the PILs used in this study included NMR-sensitive 1 H and 19 F nuclei in the cations and/or anions, each of the cationic and anionic self-diffusion coefficients could be independently determined. Fig. 9a and b shows the temperature dependencies of the 1 H and 19 F nuclei self-diffusion coefficients in [Pyrr][CF COO] and 3 [Pyrr][HSO4 ]. Sample studied here content initially 12,000 and 25,000 ppm for [Pyrr][CF3 COO] and [Pyrr][HSO4 ] respectively. Preliminary experiments have shown that no dependence of the self-diffusion coefficients occurs when the diffusion time is varied from 20 ms to 100 ms. Such a behavior suggests that the diffusion process of the ions in the PILs can be considered as a simple Fickian phenomenon in a homogeneous fluid. In the studied tem-
Fig. 7. (a) A schematic view of the 13-interval PGSTE-NMR pulse sequence used for the measurement of the self-diffusion coefficients. This sequence produced a stimulated spin-echo at t = 4 1 + 2 . Between the third pulse and the fourth pulse, during the time 2 , the magnetization was stored along the static magnetic field B0 and therefore subjected only to the longitudinal relaxation time T1 . The diffusion time was = 2 1 + 2 . (b) The coherence transfer pathway (p = 0 → − 1 → + 1 → 0 → + 1 → − 1) resulting from an adequate phase cycling to take into account only the stimulated echo.
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M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
1.2
[Pyrr][HSO [Pyrr][HSO4] ] 4
1.0
1
1.2
NH
+
2
0.4
OH
0.2 0.0
0.6
5
NH
+
2
0.4 0.2 0.0
a
(a)
-0.2 10
(b)
0
b
-0.2 10
-5
5
Chemical shift (ppm) 1
Fig. 8.
-10
10
T≈55°C
-
[Pyrr] [CF3COO ]
Diffusion coefficient D (m 2/s)
Diffusion coefficient D (m 2/s)
-5
H NMR spectra of (a) [Pyrr][HSO4 ] and (b) [Pyrr][CF3 COO]. The chemical shift reference was arbitrary.
-9
-10
+
Self-diffusion NH2 Self-diffusion CH2 10
0
Chemical shift (ppm)
10
10
2
2
0.8
Intensity (a. u.)
Intensity (a. u.)
2
H NMR
CH
CH
CH
0.6
1
33
1.0
CH
2
0.8
-
[Pyrr][CF COO [Pyrr][CF COO ] ]
H NMR
a
Self-diffusion CF3
-11
2.6
2.8
3.0
-
[Pyrr] [HSO4 ]
-11
10
+ 2
Self-diffusion NH
Self-diffusion OH
3.2
3.4
3.6
3.8
10
b
Self-diffusion CH2
-12
2.6
2.8
3.0
-1
3.2
3.4
3.6
1000/T (K-1)
1000/T (K )
Fig. 9. Temperature dependencies of 1 H and 19 F nuclei self-diffusion coefficients in [Pyrr][CF3 COO] (a) and [Pyrr][HSO4 ] (b). The solid lines represent the best fits using the Arrhenius law.
perature range, it was first of interest to compare the self-diffusion coefficients of ions in [Pyrr][HSO4 ] and [Pyrr][CF3 COO]: one can observe that diffusion was always faster for ions in [Pyrr][CF3 COO], which was in accord with its lower viscosity (cf. Section 3.1). As shown by the graphs in Fig. 9a and b, the diffusion coefficient values of the –CH2 groups belonging to the pyrrolidinium ring (1 H NMR) were very similar to those of the –CF3 groups in the CF3 COO− anion (19 F NMR) or the –OH group in the HSO4 − anion (1 H NMR). This clearly indicated that cations and anions diffused at the same speed and that they were, most probably, attached together as ion pairs or larger aggregates. Nevertheless, the D values for the pyrrolidinium ring were higher in [Pyrr][CF3 COO] than in [Pyrr][HSO4 ]. At 298 K, for [Pyrr][CF3 COO]: DPyrr = 6.07 × 10−11 m2 s−1 and whereas for [Pyrr][HSO4 ]: DCF3 COO = 5.84 × 10 − 11 m2 s−1 DPyrr = 7.93 × 10−12 m2 s−1
DHSO4 = 8.37 × 10−12 m2 s−1 .
and
10
The diffusivity ratios of cations or anions in the two PILs were almost equal to the fluidity ratio, according to D(Pyrr)[Pyrr][CF3 COO] D(Pyrr)[Pyrr][HSO4 ] = 6.98 ∼ =
D(CF3 COO) = 7.65 ∼ = D(HSO4 ]
˚[Pyrr][CF3 COO] ˚[Pyrr][HSO4 ]
= 7.3
(5)
where the fluidity is defined as the inverse of the viscosity (˚ = −1 ). Moreover, the diffusion coefficient of the proton bound to the nitrogen atom was faster at all temperatures in both ILs, however the difference was more pronounced in [Pyrr][HSO4 ] as opposed to in [Pyrr][CF3 COO], as demonstrated in Fig. 10a and b. This means that, especially in [Pyrr][HSO4 ], the proton attached to the nitrogen atom diffused faster than the pyrrolidinium cation. It is possible
b
a 10
10 Λ
10
10
2,6
2,8
3,0
3,2
1000/T(K )
3,4
3,6
10
2,6
2,8
3,0
3,2
3,4
3,6
1000/T(K )
Fig. 10. The temperature dependency of the molar conductivity for [Pyrr][HSO4 ] (a), and [Pyrr][CF3 COO] (b) calculated from Eq. (8).
M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237 Table 2 Arrhenius equation parameters for the self-diffusion coefficients D0 and Ea for [Pyrr][HSO4 ]. R2 is the correlation coefficient. D0 (m2 s−1 )
Group
−9
7.22 × 10 3.65 × 10−9 3.09 × 10−9
+
NH2 from [Pyrr] CH2 from [Pyrr] OH from [HSO4 ]
Ea (kJ mol−1 )
R2
13.7 15.2 14.6
0.98780 0.99802 0.99592
that this rapid diffusion was enabled by a Grotthuss mechanism, where free protons are able to jump from an acidic site (–NH2 + or HSO4 − ) to a basic site (–NH2 or SO4 2− ). In [Pyrr][CF3 COO], this effect was only observed at temperatures above 328 K, i.e., when the IL had adsorbed a sufficient amount of water (30,000 ppm for [Pyrr][HSO4 ]). The influence of the temperature on the self diffusion coefficients was quantified by Arrhenius plots of [Pyrr][HSO4 ] and the experimental data was fitted according to: D(T ) = D0 exp
E a
(6)
RT
where D0 (m2 s−1 ) is a pre-exponential factor and Ea is the activation energy for the diffusion process. Best-fit values are reported in Table 2. The activated energy Ea was approximately the same for all ionic species with a mean value of 14.5 ± 0.8 kJ/mol. This was expected since the diffusion was mainly controlled by the viscosity of the media, as seen before. For the [Pyrr][CF3 COO] sample, the evolution of the selfdiffusion coefficient followed two temperature-dependent behaviors as demonstrated by the break in the curves observed near T = 328 K a temperature corresponding to the fusion temperature (Tm ) of the anhydrous IL. Moreover, an equivalent break was observed in the curves from the ionic conductivity measurements. On either side of this break, the self-diffusion coefficients followed an Arrhenius law, but with different activation energies. This was due to Ea being larger at high temperatures than at low ones. The best-fit parameters are listed in Table 3. At low temperatures, the activation energy Ea of all ionic species presented very similar values (13.3 ± 1.5 kJ mol−1 ), whereas above Tm , Ea was four times higher with a mean value of 41.0 ± 1.8 kJ mol−1 . The diffusion coefficients measured by PGSE-NMR were illustrative of the differing behaviors of the two ILs. Obviously, changing the anion significantly influenced the intermolecular interactions as illustrated by the following equilibrium: Pyrr . . . .H+ . . .− OSO3 H PyrrH+ + HSO4 − PyrrH+ , HSO4 − Pyrr . . . .H+ . . .− OOCF3 PyrrH+ + − OOCF3 PyrrH+ ,− OOCF3 Several authors have used self-diffusion coefficients determined by PGSE-NMR to calculate the molar conductivity (NMR ) according to the Nernst–Einstein equation: [39,40] NMR =
F 2 (Dcation + Danion ) RT
(7)
Here, F is the Faraday constant, and R is the universal gas constant. The self-diffusion coefficients obtained by PGSE-NMR pointed at a
235
translational motion (self-diffusion) of the NMR-sensitive nuclei. Thus, the NMR values were derived from the assumption that all of the diffusing species detected during the PGSE-NMR measurement contributed to the molar conductivity. On the other hand, the cond values (i.e., the direct current, d.c., conductivity) were based on the migration of the charged species under an electric field. Hence, the Nernst–Einstein equation holds only if species involved in diffusion were also responsible for conduction. In the presence of neutral ion pairs, the NMR values were expected to differ from those determined from conductivity (cond ). Moreover, another important point was that the strong ion-ion interactions occurring in ILs should affect the D and values by separate mechanisms and hence to varying extents. In order to calculate the molar conductivity, it was necessary to take into account all kinds of mobile species: the mobile proton (H+ ), the pyrrolidinium ring (Pyrr+ ) and the anion (HSO4 − or CF3 COO− ). As the respective contributions of H+ and Pyrr+ to the conductivity were unknown, a parameter ˛, ranging from 0 to 1, was introduced in Eq. (8): NMR =
F2 ((1 − ˛)DPyrr+ + ˛(DH+ ) + DA− ) RT
(8)
Fig. 10a reports on the temperature dependency of the molar conductivity calculated from Eq. (8) for [Pyrr][HSO4 ], using ˛ = 1 and ˛ = 0 as limits. For the sake of comparison, the experimental conductivities exp for the two PILs have been plotted in the same graph. cond was found to be intermediate between calculated values for respectively ˛ = 1 and ˛ = 0. At high temperatures, the conductivity was mainly due to proton jumps and, conversely, at low temperature, Pyrr+ cations provided the major contribution to the conductivity. Nevertheless, at a temperature above 370 K or below 285 K, the curvature of the cond vs. T−1 trace indicated certain discrepancies between calculated and experimental values. This signified that the Nernst equation had become invalid. At low temperatures, the underlying reason for this might be the formation of ion pairs. Fig. 10b presents the temperature dependency of the molar conductivity for [Pyrr][CF3 COO], as calculated from Eq. (8). Since the contribution of the Grotthuss mechanism was low in this case, the curves corresponding to ˛ = 1 and ˛ = 0 were almost superposed. At all temperatures, low as well as high, NMR was greater than cond . Fig. 11 displays the ratio cond /NMR plotted against the temperature, and which was found to be sensitive to changes thereof. Consequently, the cond /NMR ratio indicated the percentage of ions (charged species) contributing to the ionic conduction in the diffusing species. It should be noted here that it was not possible to distinguish the difference between free ions and associated ionic species in the NMR chemical shifts within the NMR time scale (10−9 –10−10 s). This implies that the association/dissociation rate of the ionic species, if any, was much faster than the time scale of the d.c. conductivity measurements. In the time scale of the conductivity measurements, an individual ion under an electrical field migrates for a certain period when it exists as a charged species, but it may associate to form non-charged pairs and aggregates during another
Table 3 Arrhenius equation parameters (D0 and Ea ) for the self-diffusion coefficients of the ions in [Pyrr][CF3 COO]. R2 is the correlation coefficient. Temperature
Group ◦
Low temperature T < 55 C
High temperature T > 55 ◦ C
+
NH2 from [Pyrr] CH2 from [Pyrr] CF3 from [CF3 COO] NH2 + from [Pyrr] CH2 from [Pyrr] CF3 from [CF3 COO]
D0 (m2 s−1 ) −9
11.20 × 10 11.23 × 10−9 25.68 × 10−9 3.09 × 10−4 8.39 × 10−4 3.20 × 10−4
Ea (kJ mol−1 )
R2
12.1 12.8 15.0 39.8 43.0 40.1
0.98254 0.98839 0.97360 0.99756 0.99658 0.99870
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M. Anouti et al. / Fluid Phase Equilibria 299 (2010) 229–237
1,0
observing 1 H and 19 F nuclei with the pulsed-field gradient spinecho NMR technique. It was shown that a fast diffusion of protons occurred at all temperature in [Pyrr][HSO4 ] but that this was not the case for [Pyrr][CF3 COO]. Instead, diffusion of mobile protons increased with the temperature. Independently of the temperature, D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five time higher than the pyrrolidinium cation or hydrogen sulfate anion in [Pyrr][HSO4 ] and twofold that of [Pyrr][CF3 COO]. The proton conduction in the PILs followed a combination of Grotthuss- and vehicle-type mechanisms. These results were confirmed by conductivity and viscosity measurements which were used to investigate the ion transport motilities in both PILs by means of the Stokes–Einstein equation.
[Pyrr][CF3COO] [Pyrr][HSO4]
Λcond/ΛRMN
0,8
0,6 T = 55°C
0,4
0,2
0,0
280
300
320
340
360
380
T(K) Fig. 11. Molar conductivity ratios (cond /NMR ) [Pyrr][HSO4 ] PILs plotted against temperature.
1,2
11
-2
1/D (10 m .s)
1,0
for
[Pyrr][CF3 COO]
and
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fluid.2010.09.035. References
[Pyrr][CF3COO] [Pyrr][HSO4]
0,8 0,6 0,4 0,2 0,0 0,0
-4
1,0x10
-4
2,0x10
-4
3,0x10
-4
4,0x10
-4
5,0x10
-1
η/T (Pa.s.K ) Fig. 12. The relationship between 1/D and /T for species in [Pyrr][HSO4 ] and [Pyrr][CF3 COO]. The solid lines were obtained by linear regression of the experimental data.
period. When the ion exists as a non-charged species, it does not contribute to the d.c. conduction. Thus, the cond /NMR ratios can be a measure of the ionic association of the PIL. The diffusion coefficient, D, can also be related to the viscosity of solution, , by the well-known Stokes–Einstein equation on the basis of the hydrodynamic model that a solute sphere moves through a continuum fluid: 1 6 r = D kB T
Appendix A. Supplementary data
(9)
Here, kB is the Boltzmann constant and r is the hydrodynamic radius. In Fig. 12, 1/D is plotted against /T for [Pyrr][HSO4 ] and [Pyrr][CF3 COO]. Here, is the viscosity. It was found that 1/D was not proportional to /T through the origin in the case of [Pyrr][HSO4 ] for the studied temperature range. This fact indicates that the macroscopic viscosity of the solution was not the predominant factor in the translational motions of the cation and anion species at low temperature. 4. Conclusions This work has demonstrated that self-diffusion coefficients (D) of the ion species in two studied PILs, i.e., pyrrolidinium hydrogen sulfate and pyrrolidinium trifluoroacetate, could be determined by
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