Article pubs.acs.org/jced
Properties of Sodium Tetrafluoroborate Solutions in 1‑Butyl-3methylimidazolium Tetrafluoroborate Ionic Liquid Viktoriya A. Nikitina,† Andreas Nazet,‡ Thomas Sonnleitner,‡ and Richard Buchner*,‡ †
Department of Electrochemistry, Moscow State University, Leninskie Gory 1-str. 3, 119991 Moscow, Russian Federation Institut für Physikalische und Theoretische Chemie, Universität Regensburg, D-93040 Regensburg, Germany
‡
S Supporting Information *
ABSTRACT: Electrical conductivities, κ, of sodium tetrafluoroborate solutions in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) ionic liquid were measured in the wide temperature range of (238.15 to 458.15) K. Additionally, their densities, ρ, viscosities, η, and molar conductivities, Λ, are reported for the temperature range (278.15 to 358.15) K, supplemented by dielectric data at 298.15 K. The values for η and κ are well-described by the Vogel−Fulcher−Tammann equation. Walden plots, log(Λ) vs log(η−1), for the NaBF4 solutions coincide with the straight line found for neat [bmim][BF4], indicating that the solute has only limited impact on the structure of the ionic liquid. This is corroborated by the similarity of the standard molar volume of NaBF4 and its intrinsic volume, which suggests that solute-induced electrostriction is weak. Within the experimental uncertainty, the dielectric properties of the most concentrated NaBF4 solution (0.1739 mol·kg−1) were found to be identical with those of pure [bmim][BF4].
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INTRODUCTION
Meanwhile, also sodium ion conducting electrolytes are gaining increased attention as alternatives to lithium ion based electrolytes.15 The unique RTIL properties, especially their nonflammability, electrochemical stability, and very low vapor pressure, render such systems attractive as candidates for replacing organic electrolytes in sodium ion based energy storage devices.16,17 Therefore, the investigation of the details of ion transport for Na+ in RTILs is not only of considerable importance for our understanding of solvation and association mechanisms in such media but also for potential applications. In this study we report data for ρ, κ, and η of sodium tetrafluoroborate (NaBF4) solutions in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) over a wide temperature range. As the salt is not very soluble in [bmim][BF4] (< 0.2 mol·kg−1 at room temperature) and properties were found to be only moderately dependent on salt concentration, κ and η were determined at m = (0.0580, 0.1159, and 0.1739) mol·kg−1 to avoid precipitation at low temperatures; for ρ additional samples were studied. For the solution with m = 0.1739 mol·kg−1 the dielectric properties at 298.15 K are also reported. To the best of our knowledge no such data are available for NaBF4 solutions in [bmim][BF4] RTIL so far.
Metal−salt solutions in room-temperature ionic liquids (RTILs) have attracted considerable attention in view of their potential application to reactive metal electrodeposition and as electrolytes for electrochemical devices. Several studies of alkali metal electrodeposition from chloroaluminate1−3 RTILs, as well as from “second generation” air and water stable RTILs4,5 have been reported. Such systems are also intensively investigated as electrolytes or electrolyte components for lithium batteries.6−8 Many of these studies focused on the concentration dependence of viscosity, η, electric (dc) conductivity, κ, density, ρ, and diffusion coefficients, D, of lithium salt solutions in RTILs to gain information on Li+ solvation.9−11 Results obtained indicate that RTIL solutions of Li+, and thus probably also of the other alkali metal ions, differ strongly from solutions in traditional solvents. Whereas solvation in aqueous and aprotic media leads to the dissociation of the salt and the formation of solvated cations, the introduction of metal salts into RTILs enhances association due to the increased number of anions coordinating the metal cation, leading to the formation of [MXn+1]n− aggregates. This view is supported by all studies of transport properties as they found decreasing ionic conductivities and ion self-diffusion coefficients with increasing alkali metal salt concentration. In all reported cases diffusion coefficients follow the sequence DLi+ < Dan < Dcat (Dan and Dcat are the diffusion coefficients of the RTIL’s anion and cation); that is, the nominally smallest ion is moving slowest. Aggregate formation is also supported by spectroscopic studies.12−14 © 2012 American Chemical Society
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EXPERIMENTAL SECTION Chemicals. Neat [bmim][BF4] (> 99 % purity) was purchased from Iolitec and dried prior to use on a high Received: June 5, 2012 Accepted: September 26, 2012 Published: October 4, 2012 3019
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Table 1. Densities, ρ, of Neat [bmim][BF4] and the NaBF4 Solutions of Molality, m, Investigated in the Temperature (T) Range of (278.15 to 358.15) K at Pressure p = 0.1 MPaa ρ/kg·m−3 at T/K m/mol·kg 0 0b 0.0273 0.0540 0.0580 0.0676 0.0903 0.1069 0.1159 0.1377 0.1739
−1
278.15
288.15
298.15
308.15
318.15
328.15
338.15
348.15
358.15
1216.3 1215.43 1218.0 1219.4 1219.2 1220.7 1221.3 1222.4 1222.8 1224.3 1226.2
1209.0 1209.33 1210.7 1212.2 1212.1 1213.4 1214.0 1215.1 1215.5 1217.0 1218.9
1201.8 1202.08 1203.5 1204.9 1205.0 1206.1 1206.8 1207.9 1208.3 1209.8 1211.6
1194.6 1194.78 1196.3 1197.7 1198.0 1199.0 1199.6 1200.7 1201.1 1202.6 1204.4
1187.6 1187.79 1189.2 1190.7 1190.9 1191.9 1192.5 1193.6 1194.0 1195.5 1197.3
1180.6 1180.62 1182.2 1183.6 1183.9 1184.9 1185.5 1186.6 1187.0 1188.4 1190.3
1173.6 1173.51 1175.3 1177.0 1176.8 1177.9 1178.6 1179.6 1180.0 1181.5 1183.3
1166.7
1159.9
1168.4 1169.8 1169.8 1171.0 1171.7 1172.7 1173.1 1174.6 1176.4
1161.6 1163.0 1162.7 1164.2 1164.8 1165.9 1166.3 1167.7 1169.6
a Standard uncertainties u are u(m) = 0.002·m, u(T) = 0.01 K, u(p) = 10 kPa, and the combined expanded uncertainty Uc is Uc(ρ) = 0.2 kg·m−3 with a 0.95 level of confidence (k ≈ 2). bref 19.
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vacuum line (p < 10−8 bar) for 7 days at ∼ 310 K, yielding a water mass fraction < 15·10−6 (coulometric Karl Fischer titration, Mitsubishi Moisturemeter MCI CA-02). The dried RTIL was stored in a nitrogen-filled glovebox. Sodium tetrafluoroborate (NaBF4, VWR Prolabo, 0.986 mass fraction purity) was dried for 2 days at a high vacuum line at ∼ 340 K. Solutions were prepared in a glovebox using an analytical balance without buoyancy corrections. Salt molalities, m, were thus accurate to about ± 0.2 %. Nitrogen protection was maintained during all subsequent steps of sample handling, including measurements. Samples recovered after finishing the measurement program still had water mass fractions < 15·10−6. Density. The density, ρ, of neat RTIL and its NaBF4 solutions was determined at 278.15 ≤ T/K ≤ 358.15 (stability 330 K. Surprisingly, values of Vm0(NaBF4) in other solvents are rare. Only for acetonitrile Vm0(NaBF4) = 14 cm3·mol−1 can be calculated from the ionic standard molar volumes published by Marcus and Hefter.21 This value is considerably smaller than the intrinsic volume21 of NaBF4, Vint = 57.8 cm3·mol−1, obtained from the 3020
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Figure 1. Standard molar volume, Vm0, of NaBF4 in [bmim][BF4] as a function of temperature, T.
ion radii and the packing efficiency, which indicates strong electrostriction exerted by BF4− and especially Na+ on the surrounding acetonitrile. In contrast to that, electrostriction, and thus the impact of the solute on the RTIL structure, is marginal for NaBF4 in [bmim][BF4] as judged from the small difference of Vint and the standard molar volumes of Figure 1. Additionally, this difference decreases with increasing temperature. The electric conductivities, κ, determined in this study for neat [bmim][BF4] and the investigated NaBF4 solutions are summarized in Table 3 and Figure 2a; Table 4 and Figure 3a Table 3. Electrical Conductivities, κ, of Neat [bmim][BF4] and the NaBF4 Solutions of Molality, m, Investigated in the Temperature (T) Range of (238.15 to 458.15) K at Pressure p = 0.1 MPaa
Figure 2. (a) Electrical conductivities, κ, of neat [bmim][BF4] (■) and its NaBF4 solutions as a function of temperature, T. Solid lines represent VFT fits, eq 3, with parameters of Table 5. For clarity data for (●, 0.0580; ▲, 0.1159 and ◆, 0.1739) mol·kg−1 NaBF4 are divided by factors 2, 5, and 10, respectively. (b) Relative deviation, δκ = 100·(κ − κVFT)/κVFT, of experimental conductivities of neat [bmim][BF4] from present VFT fit: ■, this work; +, Stoppa et al.;19 △, Tokuda et al.;23 ○, Schreiner et al.;24 □, Harris et al.;25 ▽, Vila et al.26
κ/S·m−1 for m/mol·kg−1 T/K
0
0.0580
0.1159
0.1739
238.15 258.15 278.15 298.15 318.15 338.15 358.15 378.15 398.15 418.15 438.15 458.15
0.00095 0.0227 0.113 0.353 0.826 1.548 2.53 3.74 5.13 6.68 8.36 10.13
0.00083 0.0211 0.1075 0.341 0.794 1.500 2.47 3.66 5.04 6.58 8.24 10.00
0.00077 0.0201 0.103 0.328 0.769 1.464 2.40 3.57 4.93 6.45 8.11 9.85
0.00071 0.0188 0.0986 0.316 0.745 1.425 2.35 3.50 4.84 6.35 7.98 9.70
Table 4. Dynamic Viscosities, η, of Neat [bmim][BF4] and the NaBF4 Solutions of Molality, m, Investigated in the Temperature (T) Range of (278.15 to 358.15) K at Pressure p = 0.1 MPaa η/mPa·s for m/mol·kg−1
a Standard uncertainties u are u(m) = 0.002·m, u(T) = 0.005 K, u(p) = 10 kPa, and the combined expanded uncertainty Uc is Uc(κ) = 0.005·κ with a 0.95 level of confidence (k ≈ 2).
summarize the dynamic viscosities of the samples under study. At a given temperature κ is always highest for the neat RTIL and decreases with increasing salt concentration. This effect becomes more pronounced with decreasing temperature, in line with the behavior of lithium salts in [NTf2]-based10,11 and [BF4]-based9 RTILs. Also in line with investigations for Li+ salts,9,10 and corroborating the findings for κ, a pronounced
T/K
0
0.0580
0.1159
0.1739
278.15 288.15 298.15 308.15 318.15 328.15 338.15 348.15 358.15
384.5 197.7 112.3 68.89 45.36 31.49 21.99 16.54 12.84
400.2 204.4 115.5 70.66 46.10 31.69 22.75 16.93 13.00
421.4 215.5 121.4 74.52 48.94 33.98 23.85 17.63 13.53
454.8 232.6 130.4 79.37 51.72 35.69 23.94 17.97 14.01
a Standard uncertainties u are u(m) = 0.002·m, u(T) = 0.01 K, u(p) = 10 kPa, and the combined expanded uncertainty Uc is Uc(η) = 0.005·η with a 0.95 level of confidence (k ≈ 2).
increase of η with increasing NaBF4 concentration can be noticed. 3021
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Table 6. Parameters Aη, Bη, and T0,η and Corresponding Variance, χ2, for Fits of the Viscosity of NaBF4 Solutions with eq 3 m/mol·kg−1 0 0.0580 0.1159 0.1739
Transport properties (Y = κ or η) of glass-forming liquids are commonly described by the empirical Vogel−Fulcher− Tammann (VFT) equation,22
(3)
where AY, BY, and T0,Y are fit parameters; T0,Y is generally called the VFT temperature. Table 5 lists the values of Aκ, Bκ, and T0,κ for electrical conductivity, together with the corresponding variance of the fit, χ2. The VFT parameters for viscosity are summarized in Table 6. The lines shown in Figures 2a and 3a Table 5. Parameters Aκ, Bκ, and T0,κ and Corresponding Variance, χ2, for Fits of the Electrical Conductivity of NaBF4 Solutions with eq 3 m/mol·kg−1 0 0.0580 0.1159 0.1739
Aκ/S·m−1 120 120 123 121
± ± ± ±
1 1 1 1
Bκ/K −688 −689 −702 −700
± ± ± ±
T0,κ/K 4 4 3 3
179.9 180.8 179.3 180.4
± ± ± ±
0.6 0.6 0.5 0.4
0.100 0.100 0.109 0.089
± ± ± ±
0.007 0.001 0.009 0.013
Bη/K 941 937 935 996
± ± ± ±
17 1 20 40
T0,η/K 164 165 165 160
± ± ± ±
1 1 2 3
χ2 0.06 0.0001 0.11 0.39
were calculated from these data. For both data sets AY, BY, and T0,Y are essentially independent of NaBF4 concentration (Tables 5 and 6). This is almost certainly a reflection of the rather low solubility of this salt in [bmim][BF4] as for LiBF4 in 1-ethyl-3-methylimidazolium tetrafluoroborate T0,κ increased with salt concentration, which rose from (0.25 to 1.5) mol·L−1.9 However, the present results also underline the limited impact of Na+ on RTIL structure, as already inferred from Vm0(NaBF4) (Figure 1). The smooth variation of κ and η with temperature and NaBF4 concentration (Figures 2a and 3a) as well as the obtained VFT-parameter values (Tables 5 and 6) indicate selfconsistent data sets for electrical conductivity and viscosity. Thus, relative changes within the investigated concentration series can be reasonably discussed. However, this information allows no inference on the accuracy of κ and η. The sensitivity of physical properties of [bmim][BF4] against impurities and RTIL hydrolysis is notorious. Therefore, the present data of electrical conductivity and viscosity were compared in Figures 2b and 3b with good quality literature data for neat [bmim][BF4] covering large temperature ranges. As generally experimentally investigated temperatures did not match, relative deviations, δx = 100·(x − xVFT)/xVFT (x = κ, η), are displayed, where xVFT is calculated from the VFT parameters of the RTIL (Tables 5 and 6). A striking feature of Figure 2b is the marked relative deviation of the present κ values (Table 3) from their VFT fit at T ≤ 278.15 K. To some extent this is due to the rapidly decreasing magnitude of κ which therefore approaches the uncertainty of the experiment. Thus, separate low-T experiments with matched conductivity cells should improve the situation. However, also the data sets of refs 19, 23, and 24 follow the same trend at low T with only small deviations (< 4 %) from the present data. Thus, it appears that eq 3 does not yield a fully satisfying fit at T ≤ 278.15 K. Accordingly, the absolute values of the VFT parameters for κ (Table 5) should be taken with a grain of salt. Although such a systematic discrepancy between our experimental values and their fit is absent for η (Figure 3b), also the VFT parameters of Table 6 have to be taken with care because of the smaller temperature range. With deviations < 1 %, the κ values obtained for the commercial RTIL sample of this study are in good agreement with data reported previously19 for a [bmim][BF4] sample synthesized in our lab (Figure 2b). Only the present value at 238.15 K appears to be systematically too low, which might hint at partial crystallization of the samples. Figure 2b also shows δκ for conductivities determined by Tokuda et al.,23 Harris et al.,25 Vila et al.,26 and Schreiner et al.24 While conductivities of refs 23 and 24 are in good agreement with the present data, systematic deviations from the two other sets are obvious. At low temperatures the κ values of Harris et al.25 are in quantitative agreement with the present results and those of
Figure 3. (a) Dynamic viscosities, η, of neat [bmim][BF4] (■) and its NaBF4 solutions as a function of temperature, T. Solid lines represent VTF fits, eq 3, with parameters of Table 6. For clarity data for (●, 0.0580; ▲, 0.1159 and ◆, 0.1739) mol·kg−1 NaBF4 are multiplied by factors 2, 5, and 10, respectively. (b) Relative deviation, δη = 100·(η − ηVFT)/ηVFT, of experimental viscosities of neat [bmim][BF4] from present VFT fit: ■, this work; △, Tokuda et al.;23 ○, Schreiner et al.24 and two data sets of Harris et al.,27 samples □, BB1 and ◇, BB2.
⎞ ⎛ B Y ⎟⎟ Y = AY exp⎜⎜ ⎝ T − T0, Y ⎠
Aη/mPa·s
105·χ2 2 2 1 0.9 3022
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refs 23 and 24, but with increasing temperature δκ increases considerably (Figure 2b). On the other hand, for the data of Vila et al.26 δκ = 24 at 298 K but reaches −13 at 433 K. Interestingly, the viscosity data23,24,27 selected from the literature for comparison with the present η are all smaller and run roughly parallel (Figure 3b). The best agreement is found for sample BB2 of Harris et al.,27 δη < 3 on average, and the data of Schreiner et al.,24 δη < 5. As observed for κ, also the corresponding molar conductivities, Λ = κ/c (with molar concentration, c, calculated from m and ρ), decrease with increasing NaBF4 concentration (Table 7), indicating that the ionic mobility and/or the number Table 7. Molar Conductivities, Λ, of Neat [bmim][BF4] and the NaBF4 Solutions Investigated in the Temperature Range of (278.15 to 358.15) K Λ/104 S·m2·mol−1 for m/mol·kg−1 T/K
0
0.0580
0.1159
0.1739
278.15 298.15 318.15 338.15 358.15
0.210 0.670 1.573 2.98 4.93
0.198 0.634 1.496 2.862 4.766
0.188 0.606 1.437 2.767 4.596
0.178 0.578 1.379 2.668 4.445
Figure 4. Walden plot (molar conductivity, Λ, vs fluidity, η−1) for neat [bmim][BF4] (□) and its NaBF4 solutions at m = (○, 0.0580; ◇, 0.1159 and △, 0.1739) mol·kg−1. The solid line represents the “ideal” KCl line, broken line as a guide to the eye.
of charge carriers decreases upon NaBF4 addition to [bmim][BF4]. Since the decrement ΔΛ = (Λ(0) − Λ(mmax))/Λ(0), where mmax = 0.174 mol·kg−1, decreases from 0.152 at 278.15 K to 0.098 at 358.15 K, it appears that the retarding effect of Na+ on the ionic mobility of NaBF4 + [bmim][BF4] mixtures is less important at high temperatures. This parallels the trend in Vm0(NaBF4) (Figure 1), so that one may speculate that even the limited electrostriction operative in the present samples is a relevant factor for Λ. The Walden plot, log(Λ) vs log(η−1), is a convenient tool to infer on the ionicity of RTILs.28,29 With increasing distance of the data from the “ideal” KCl line (of slope 1·10 −3 S·m2·Pa·s·mol−1 and representative for independently moving ions), the motions of anions and cations become more and more correlated and the system thus less able to hold an electric current. The line defined by the (η−1, Λ) data of neat [bmim][BF4] is rather close to the KCl line (Figure 4) indicating high ionicity of this “good ionic liquid”. However, the slightly smaller slope of (0.927 ± 0.005)·10−3 S·m2·Pa·s·mol−1 suggests that with increasing temperature the number of free ions slightly decreases. Interestingly, the data for the studied NaBF4 solutions are practically on top of the [bmim][BF4] line (Figure 4), which indicates that at a given temperature the fraction of ions participating in charge transport is independent of the solute concentration. This may hint at the formation of slowly moving [Na(BF4)n+1]n− aggregates in [bmim][BF4], similar to what was claimed for Li+ solutions in various RTILs.9−14 However, these aggregatesshould they exist have no dipole moment. Otherwise, they should show up in the dielectric spectrum.30 Thus, the aggregates cannot be conventional ion pairs with a lifetime exceeding their rotational correlation time, which is expected to be in the order of a few hundred picoseconds.30 Figure 5 compares the spectra of relative permittivity, ε′(ν), and dielectric loss, ε″(ν), of two independently prepared solutions of m = 0.1739 mol·kg−1 NaBF4, recorded at 298.15 K in the frequency range 0.2 ≤ ν/GHz ≤ 50 (see Supporting Information for data), with the corresponding spectra of pure
Figure 5. Spectra of relative permittivity, ε′(ν), and dielectric loss, ε″(ν), of neat [bmim][BF4] (solid lines31) and two independently prepared solutions of 0.1739 mol·kg−1 NaBF4 (open and closed symbols) at 298.15 K. Also indicated is the total loss, η″(ν), for neat [bmim][BF4] (broken line31) and one of the solutions (◊).
[bmim][BF4]. The latter was calculated from the parameters (static permittivity ε = limν→0 ε′(ν) = 14.6; relaxation strengths SCC = 10.0 and SD = 2.0; relaxation times τCC = 284 ps and τD = 0.6 ps; CC shape parameter αCC = 0.52) of a Cole−Cole +Debye (CC+D) model used by Hunger et al.31 to fit their RTIL spectra. Also indicated are the total loss spectra, η″(ν), for one of the solutions and the neat RTIL. Within experimental uncertainty ε′(ν) and ε″(ν) of solution and solvent are identical, and the spectra of η″(ν) only differ by their difference in κ. Thus, the fit parameters31 of the CC+D model describing pure [bmim][BF4] are also sufficient to model the dielectric properties of its NaBF4 solutions at least up to m = 0.1739 mol·kg−1. This includes the static permittivity of the solution, ε = 14.6. Such behavior again shows that within the limited solubility range of NaBF4 (< 0.2 mol·kg−1) this electrolyte has no major effect on the structure of liquid [bmim][BF4]. This is in contrast to conventional electrolyte solutions, where already salt concentrations of ∼0.15 mol·kg−1 3023
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lead to a significant reduction of ε (more accurately, the solvent relaxation strength) due to ion solvation.30 Since the CC relaxation can be assigned to cation reorientation, whereas the fast D mode subsumes the onset of intermolecular vibrations and librations dominating the far-infrared region,30,31 the lack of solute-specific features in the solution spectrum (Figure 5) also indicates that in the investigated solutions neutral NaBF4 ion pairs are either not present in a significant amount or/and their lifetime is too short to allow their detection by dielectric spectroscopy.
(3) Piersma, B. J.; Ryan, D. M.; Schumacher, E. R.; Riechel, T. L. Electrodeposition and Stripping of Lithium and Sodium on Inert Electrodes in Room Temperature Chloroaluminate Molten Salts. J. Electrochem. Soc. 1996, 143, 908−913. (4) Wibowo, R.; Jones, S. E. W.; Compton, R. G. Kinetic and Thermodynamic Parameters of the Li/Li+ Couple in the Room Temperature Ionic Liquid N-Butyl-N-methylpyrrolidinium Bis(trifluoromethylsulfonyl) Imide in the Temperature Range 298−318 K: A Theoretical and Experimental Study Using Pt and Ni Electrodes. J. Phys. Chem. B 2009, 113, 12293−12298. (5) Wibowo, R.; Aldous, L.; Rogers, E. I.; Jones, S. E. W.; Compton, R. G. A Study of the Na/Na+ Redox Couple in Some Room Temperature Ionic Liquids. J. Phys. Chem. C 2010, 114, 3618−3626. (6) Garcia, B.; Lavallée, S.; Perron, G.; Michot, C.; Armand, M. Room Temperature Molten Salts as Lithium Battery Electrolyte. Electrochim. Acta 2004, 49, 4583−4588. (7) Guerfi, A.; Duchesne, S.; Kobayashi, Y.; Vijh, A.; Zaghib, K. LiFePO4 and Graphite Electrodes with Ionic Liquids Based on Bis(fluorosulfonyl)imide (FSI)− for Li-ion Batteries. J. Power Sources 2008, 175, 866−873. (8) Lux, S. F.; Schmuck, M.; Jeong, S. S.; Passerini, S.; Winter, M.; Balducci, A. Li-Ion Anodes in Air-Stable and Hydrophobic Ionic Liquid-Based Electrolyte for Safer and Greener Batteries. Int. J. Energy Res. 2010, 34, 97−106. (9) Hayamizu, K.; Aihara, Y.; Nakagawa, H.; Nukuda, T.; Price, W. S. Ionic Conduction and Ion Diffusion in Binary Room-Temperature Ionic Liquids Composed of [emim][BF4] and LiBF4. J. Phys. Chem. B 2004, 108, 19527−19532. (10) Monteiro, M. J.; Bazito, F. F. C.; Siqueira, L. J. A.; Ribeiro, M. C. C.; Torresi, R. M. Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid. J. Phys. Chem. B 2008, 112, 2102−2109. (11) Umebayashi, Y.; Hamano, H.; Seki, S.; Minofar, B.; Fujii, K.; Hayamizu, K.; Tsuzuki, S.; Kameda, Y.; Kohara, S.; Watanabe, M. Liquid Structure of and Li+ Ion Solvation in Bis(trifluoromethanesulfonyl)amide Based Ionic Liquids Composed of 1-Ethyl-3-methylimidazolium and N-Methyl-N propylpyrrolidinium Cations. J. Phys. Chem. B 2011, 115, 12179−12191. (12) Castriota, M.; Caruso, T.; Agostino, R. G.; Cazzanelli, E.; Henderson, W. A.; Passerini, S. Raman Investigation of the Ionic Liquid N-Methyl-N-propylpyrrolidinium Bis(trifluoromethanesulfonyl)imide and Its Mixture with LiN(SO2CF3)2. J. Phys. Chem. A 2005, 109, 92−96. (13) Lassègues, J.-C.; Grondin, J.; Talaga, D. Lithium Solvation in Bis(trifluoromethane-sulfonyl)imide-Based Ionic Liquids. Phys. Chem. Chem. Phys. 2006, 8, 5629−5632. (14) Lassègues, J.-C.; Grondin, J.; Aupetit, C.; Johansson, P. Spectroscopic Identification of the Lithium Ion Transporting Species in LiTFSI-Doped Ionic Liquids. J. Phys. Chem. A 2009, 113, 305−314. (15) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652−657. (16) Kumar, D.; Hashmi, S. A. Ionic Liquid Based Sodium Ion Conducting Gel Polymer Electrolytes. Solid State Ionics 2010, 181, 416−423. (17) Plashnitsa, L. S.; Kobayashi, E.; Noguchi, Y.; Okada, S.; Yamaki, J. Performance of NASICON Symmetric Cell with Ionic Liquid Electrolyte. J. Electrochem. Soc. 2010, 157, A536−A543. (18) Stoppa, A.; Hunger, J.; Buchner, R. Conductivities of Binary Mixtures of Ionic Liquids with Polar Solvents. J. Chem. Eng. Data 2009, 54, 472−479. (19) Stoppa, A.; Zech, O.; Buchner, R.; Kunz, W. The Conductivity of Imidazolium-Based Ionic Liquids from (− 35 to 195) °C. A. Variation of Cation’s Alkyl Chain. J. Chem. Eng. Data 2010, 55, 1768− 1773. (20) Schrödle, S; Hefter, G.; Kunz, W.; Buchner, R. Effects of the non-ionic surfactant C12E5 on the cooperative dynamics of water. Langmuir 2006, 22, 924−932.
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CONCLUSIONS In this study electrical conductivities, densities, and viscosities were reported for three concentrations of sodium tetrafluoroborate in [bmim][BF4] over a wide temperature range. The data reveal a decrease in ionic conductivity (Table 3) and a corresponding increase in viscosity (Table 4) and density (Table 1) with increasing NaBF4 concentration, whereas the dielectric properties (Figure 5) are not measurably affected. Since the standard molar volume of the solute (Figure 1) is only slightly smaller than its intrinsic volume, NaBF4 induced electrostriction is small. The temperature dependence of κ and η is well-described by the Vogel−Fulcher−Tammann equation (Tables 5 and 6). The Walden product, Λη, is nearly constant for the neat RTIL and all three salt concentrations so that all data collapse on a single line in the Walden plot (Figure 4), which differs only slightly in slope from the ideal KCl line. All together, this indicates that while the overall dynamics is slowed down by the introduction of NaBF4 into [bmim][BF4], the number of charge carriers does not change significantly for the studied salt concentrations nor is the RTIL structure significantly affected. These findings are compatible with the assumption that [Na(BF4)n+1]n− aggregates are formed. However, these must be rather short-lived (
