Transport Properties of Tributylphosphonium Tetrafluoroborate Protic

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Transport Properties of Tributylphosphonium Tetrafluoroborate Protic Ionic Liquid Laure Timperman and Mériem Anouti* Laboratoire PCMB (EA 4244), Equipe de Chimie-Physique des Interfaces et des Milieux Electrolytiques (CIME), Université François Rabelais, Parc de Grandmont, 37200 Tours, France ABSTRACT: The conductivity and viscosity of pure tributylphosphonium tetrafluoroborate protic ionic liquid (PIL) as well as the mixture with acetonitrile were measured from 20 to 80 °C. Those electrolytes exhibit a Newtonian behavior, and the temperature dependence of their viscosity, η, can be described by the Vogel−Tamman−Fulcher (VFT) equation. These transport properties show that the Walden product, ησ, is independent of the temperature signifying that disassociation of ions was the dominating factor in combined conductivity and viscosity parameters. The viscosity deviations, Δη, for PIL + CH3CN solution, calculated at several temperatures from T = 20 °C to T = 80 °C, present negative deviation from ideality with minima in the rich-PIL region composition. Finally, thanks to the Debye−Onsager plot, the infinite dilution molar conductivity of the PIL could be determined and thus, the infinite dilution conductance of the cation phosphonium was evaluated. Because of the favorable transport properties (σ, η), tributylphosphonium tetrafluoroborate PIL pure and in mixture with acetonitrile appear as promising electrolytes for electrochemical devices.

1. INTRODUCTION Ionic liquids (ILs) are organic molten salts with melting points below 100 °C. Because of their unique physicochemical properties, for example, the favorable solubility of organic and inorganic compounds, relatively high ionic conductivity, low vapor pressure, high thermal stability, and low flammability, they have been intensively investigated for various applications such as recyclable solvents for organic reactions and separation processes,1−4 lubricating fluids,5−7 active pharmaceutical ingredients,8,9 electrolytic media in various electrochemical systems,10 and so forth. By modifying the cations and anions, the IL physical properties (such as the melting point, viscosity, density, hydrophobicity, or hydrophilicity) can be customized.11 Thus, ILs can be designated “designer solvents”. The ability to carefully and predictably control physical properties has led to a vast number of papers concerning the use of ILs as solvents.4,12−16 ILs can be classified into two groups: protic ILs (PILs) and aprotic ILs (AILs).17,18 Generally, PILs are synthesized with equimolar amounts of a Brønsted acid and a Brønsted base.19−21 The proton transfer from the acid to the base creates proton donor and acceptor sites and can lead to the formation of hydrogen bonds.19 Because of the recent interest in proton-conducting electrolytes, PILs appear as promising materials for aqueous batteries, fuel cells, double-layer capacitors, solar cells, or actuators. Phosphonium-based AILs, particularly those involving quaternary phosphonium, have received great interest as potential substitutes of the corresponding ammonium counterparts thanks to practical advantages of phosphonium ILs include high chemical and electrochemical stabilities.22,23 It has been found that phosphonium ILs with bis(trifluoromethylsulfonyl) amide (TFSA) as the anion exhibited a relatively high thermal stability compared to the corresponding ammonium.24,25 Moreover, AILs based on phosphonium © 2012 American Chemical Society

cations are of considerable interest with regard to electrochemical aspects. Several electrochemical applications have previously been reported, such as voltammetric measurements for various redox couples,26 anodic polymerization of pyrrole,27 supercapacitors,28 and dye-sized solar cells.27,29 However, PILs based on phosphonium cations have not received much attention in the literature, except for catalysis. Pd-catalyzed reactions like the dimerization of methyl acrylate,30,31 annulation of anilines,32 formate reduction of allylic carbonates,33 arylation of N-boc-pyrrolidine,34 alkylative cyclization of alkynals and alkynones,35 or C-H functionalization36 were reported in phosphonium-based PILs. Phosphonium-based PILs have also been used for the regio- and strereoselective catalytic hydroamidation of alkynes with RuCl3.37 We presented in a recent work the activity of tributylphosphonium tetrafluoroborate PIL for electrochemical applications (supercapacitors).38 The present work reports on the preparation and characterization of a tributylphosphonium tetrafluoroborate PIL. The physicochemical properties of this PIL are presented as well as the temperature effect on the transport properties of the pure PIL and in a mixture with acetonitrile.

2. EXPERIMENTAL SECTION 2.1. Materials. Tributylphosphine was commercially available from Sigma Aldrich (mixture of isomers, 97%) and was used without further purification. The tetrafluoroboric acid solution (48% in water), anhydrous acetonitrile (99.8%), and 1,2-dichloroethane (DCE) (>99%) were purchased from Sigma Received: Revised: Accepted: Published: 3170

October 20, 2011 December 23, 2011 January 9, 2012 January 9, 2012 dx.doi.org/10.1021/ie202412u | Ind. Eng.Chem. Res. 2012, 51, 3170−3178

Industrial & Engineering Chemistry Research

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Aldrich. The water was purified using a Milli-Q 18.3 MΩ water system. 2.2. Preparation of PIL. Tributylphosphonium tetrafluoroborate PIL was synthesized according to the following procedure. First, tributylphosphine (30 g; 0.148 mol) was introduced into a three-necked round-bottom flask immersed in an ice bath and topped by a reflux condenser. A dropping funnel was utilized to add the acid, and a thermometer to control the temperature. A tetrafluoroboric acid solution (22.13 g; 0.148 mol) was added dropwise to the phosphine under vigorous stirring (30 min). As the acid−base reaction was lightly exothermic, the ice bath was used to maintain the temperature below 25 °C. After addition of all the acid to the phosphine, the stirring was maintained for 2 h at ambient temperature before charging 233 g of DCE. Subsequently, to remove the water, this mixture was distilled under normal pressure until the water-DCE heteroazeotropic boiling point was reached (346 K). Residual DCE was finally evaporated under reduced pressure enabling a transparent highly viscous liquid to be collected. The PIL was analyzed for water content using coulometric Karl Fischer titration, and was found to contain approximately 30,000 ppm (3.0% ww) just after distillation. Finally, the PIL was dried under high vacuum using a trap with liquid nitrogen, during two days. The water content remained constant around 2.0−3.0%39. Miscibility tests were done for water in the PIL, and the experiment shows that beyond the residual amount of 3.0%, the water and the PIL are not miscible. However, with CH3CN there is a total miscibility in all proportions, whatever the temperature. According to the water content of 3.0% ww, the molar fractions of CH3CN were calculated taking into account PIL and water amount. 2.3. Measurements. 1H NMR spectra were obtained with a Bruker 400 MHz spectrometer. CDCl3 was used as the solvent and TMS as the internal standard. Densities were determined using a density meter Anton Paar (Δρ ± 0.001 g cm−3). Viscosities were determined using a TA Instrument Rheometer (AR 100) with a conical geometry in a temperature range from 20 to 80 °C (exactness: 2%, uncertainly: 0.5%). A Crison (GLP 31) digital multifrequency conductimeter was utilized to measure ionic conductivities. The temperature control, from 25 to 80 °C, was ensured by a thermostatted bath JULABO F25, with an accuracy of ± 0.2 °C. The conductimeter was calibrated using standard solutions of known conductivity (0.1 and 0.02 mol L−1 KCl), the uncertainty for conductivities did not exceed ± 2%. Each conductivity was recorded when the stability was better than 1% within 2 min.

Figure 1. NMR spectra of tributylphosphonium tetrafluoroborate in CDCl3.

cm−1).40 However, at room temperature, their conductivities are lower than those of concentrated acqueous electrolytes. By comparison with AILs, PILs have generally higher conductivities.38,39 At 25 °C, the conductivity of tributylphosphonium tetrafluoroborate was found to be 0.83 mS cm−1 and was in the same range as several values reported earlier for ILs based on phosphonium cations at 25 °C: tributylmethylphosphonium bis(trifluoromethylsulfonyl) amide, σ = 0.42 mS cm−1;41 benzyltriethylphosphonium bis(trifluoromethylsulfonyl)amide (P222(Bz)-TFSA), σ = 0.74 mS cm−1; P2225-TFSA, σ = 1.73 mS cm−1; P2228-TFSA, σ = 0.98 mS cm−1; P4448-TFSA, σ = 0.27 mS cm−1;42 butyltrihexylphosphonium iodides, σ = 0.213 mS cm−1; isobutyltrihexylphosphonium iodides, σ = 0.380 mS cm−1;27 triethyl(4-pentenyl)phosphonium bis(trifluoromethylsulfonyl)amide (P222(Pe)-TFSA), σ = 2.1 mS cm−1; P2225-TFSA, σ = 1.7 m S c m− 1 ; a n d a l l y l t r i b u t y l p h o s p h o n i u m b i s (trifluoromethylsulfonyl) amide (P444(Al)-TFSA), σ = 0.64 mS cm−1 (at 30 °C); (P4441-TFSA), σ = 0.42 mS cm−1.43 The nature and the size of the ions significantly influence the IL conductivity, some ILs based on phosphonium cations like phosphonium iodides exhibit lower values of conductivity: octyltrihexylphosphonium iodide and isobutyltrioctylphosphonium iodide, σ = 0.0933 and 0.055 mS cm−1 respectively.27 Tsunashima et al.25 reported on the tri-n-butyl-n-octylphosphonium with BF4− as the anion, and its conductivity was found to be lower than that of tributylphosphonium BF4−, and equal to 0.069 mS cm−1. Moreover, some researchers have observed higher conductivities for phosphonium-based ILs. Tsunashima et al.44 also studied triethylbutylphosphonium, triethylpentylphosphonium, triethyl(metoxy-methyl)phosphonium and triethyl(2-ethoxymethyl)phosphonium cations with bis(trifluoromethylsulfonyl)amide or bis(fluoromethylsulfonyl)amide anions, which demonstrated conductivities from 1.7 to 8.9 mS cm−1. Room temperature ILs (RTILs) like tri-nbutylmethylphosphonium fluorohydrogenate, tetra-n-butylphosphonium fluorohydrogenate, and tri-n-butyl-n-octylphosphonium fluorohydrogenate exhibited conductivities of 6.0, 3.7, and 1.5 mS cm−1, respectively.45 Additionally, dicyanamide (DCA)46 and bis(trifluoromethylsulfonyl)imide (TFSI)24 -based phosphonium were investigated and showed a wide range of conductivities from 0.45 to 12.8 mS cm−1 and from 0.47 to 4.40 mS cm−1, respectively. Figure 2 presents the influence of the temperature on the conductivity of the PIL tributylphosphonium tetrafluoroborate. As expected, the conductivity increases with the temperature

3. RESULTS AND DISCUSSION 3.1. 1H NMR. The characteristics of the 1H NMR spectra of the PIL are presented in Figure 1 and listed in Table 1. The first peaks represent the hydrogen atoms (27) in the three butyl groups. One can note that for the labile proton carried by the phosphorus, the signal shows two septuplets, with δ equal to 4.9 and 6.5 ppm, corresponding to a coupling constant of 500 Hz. 3.2. Physical Properties of the Pure PIL. Conductivity. ILs are composed entirely of ions, and thus have their place among the most concentrated electrolytic fluids, with numerous charge carriers per unit volume. In fact, very high conductivities can be obtained when these charge carriers are mobile. Compared with organic solvents or electrolyte systems, ILs have reasonably good ionic conductivities (up to 10 mS 3171

dx.doi.org/10.1021/ie202412u | Ind. Eng.Chem. Res. 2012, 51, 3170−3178


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Table 1. Structure and 1H NMR Spectrum Characteristics of the PIL Tributylphosphonium Tetrafluoroborate.

Figure 2. Influence of the temperature on the conductivity of the tributylphosphonium tetrafluoroborate. The solid line serves as a guide to the eye.

Figure 4. Walden behavior, interdependence of conductivity and viscosity as a function of the temperature, for the tributylphosphonium tetrafluoroborate.

Figure 3. Influence of the temperature on the viscosity of the tributylphosphonium tetrafluoroborate. Inset: shear stress versus shear rate at 20 °C.

Figure 5. Arrhenius plot of the viscosities for the PIL tributylphosphonium tetrafluoroborate. The solid line represents the Arrhenius fitting.

from 0.83 to 5.42 mS cm−1 in a temperature range from 25 to 80 °C. Viscosity. Mostly, ILs are more viscous than common molecular solvents, for example, the viscosity of water is 0.8903 mPa s at 25 °C. The viscosity of ILs typically ranges from 30 to 100 mPa s at room temperature, although higher values like 500−600 mPa s18 or 1000 mPa s can sometimes be observed.39,47,48 The viscosity can be influenced by several parameters. First, for the anionic species, a higher alkalinity, size, and relative capacity to form hydrogen bonds increase the viscosity of ILs. Moreover, because of the acid/base character of PILs, the viscosity is highly dependent on hydrogen bonds. Additionally, van der Waals interactions49 and the size of the

cation50 significantly influence the IL viscosity. In the present case, the viscosity of the pure tributylphosphonium tetrafluoroborate was 143 mPa s at 25 °C, which is higher than for common PILs.39 Several studies have reported on phosphonium-based AILs and provided viscosity values that can be compared to that of the synthesized PIL. Indeed, octyltrimethylphosphonium bis(trifluoromethylsulfonyl)amide (P2228-TFSA) exhibits a higher viscosity with 248 mPa s,42 as opposed to the methyltributylphosphonium TFSA (P4441-TFSA) with 207 mPa s.43 The same researchers reported on a phosphonium3172



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mobility of ions. The inset in the figure exhibits a linear relation between the shear stress and the shear rate. According to this observation, one can conclude that this PIL has a Newtonian behavior in this temperature range. Walden Rule. Figure 4 shows the interdependence of the conductivity and the viscosity as a function of the temperature as the Walden product ση versus T. When the conductivity is completely offset by the viscosity, the Walden product is constant as a function of the temperature. As can be seen from Figure 4 in the whole range of temperature, constant ση values were obtained for PIL, indicating increases in the ionic charge with a disassociation of ions at same time decreases of van der Waals interactions and then viscosity. Fitting Procedure. Figure 5 represents an Arrhenius plot of viscosities as a function of temperature for the tributylphosphonium tetrafluoroborate, according to equation 1:

Figure 6. VFT plot of the ionic conductivities and viscosities for the tributylphosphonium tetrafluoroborate. The solid lines represent the VFT fitting.

⎡B ⎤ η = η0 exp⎢ 2 ⎥ ⎣T ⎦

based IL with a TFSI anion,24 showing a lower viscosity: P2225TFSI, η = 88 mPa s; P2228-TFSI, η = 129 mPa s; P222(101)-TFSI, η = 35 mPa s; P222(201)-TFSI, η = 44 mPa s; and another with a somewhat higher value: P222(12)-TFSI, η = 180 mPa s. Generally, phosphonium-based ILs demonstrate viscosities between 40 and 200 mPa s at 25 °C43−46 depending to length chain on alkyl group in phosphonium cation. However, some investigations have reported on more viscous materials. Matsumiya et al.51 presented results for triethylalkylphosphonium and tri-n-butylalkylphosphonium with TFSI anion with viscosities from 88 to 303 mPa s. (P4441-DCA) also showed a high viscosity of 245 mPa s.46 Figure 3 illustrates the influence of the temperature on the viscosity for the synthesized PIL. The viscosity decreases as the temperature rose from 20 to 80 °C, because of the higher

(1)

where η0 (mPa s), and B2 = Ea/R (K) are fitting parameters, R is the molar gas constant, Ea, the activation energy (kJ mol−1), and T the temperature (K). In Figure 5, it is clear that tributylphosphonium tetrafluoroborate exhibits a non-Arrhenius behavior for the viscosity. Consequently, the Vogel−Tamman−Fulcher (VFT) equation is used to represent the temperature dependence of the ionic conductivity and viscosity:

⎡ B′ ⎤ 1 σ = σ0 exp⎢ ⎥ ⎣ T − T0 ⎦

(2)

Table 2. Specific (σ), Molar Ionic Conductivity (λ), Density (ρ) for Mixture of [Bu3PH][BF4] + Acetonitrile as Function of Acetonitrile Molar Fraction and Concentration at 25°C. ρ

CIL xCH3CN 0 0.035 0.071 0.108 0.147 0.162 0.178 0.211 0.228 0.245 0.262 0.279 0.297 0.314 0.332 0.350 0.369 0.388 0.408 0.426 0.456 0.467 0.487 0.508 0.529

−3

mol dm 3.63 3.60 3.56 3.51 3.49 3.46 3.41 3.38 3.35 3.32 3.28 3.25 3.21 3.17 3.13 3.09 3.05 3.01 2.96 2.89 2.87 2.82 2.76 2.71

σ −3

g cm

1.094 1.096 1.095 1.091 1.089 1.087 1.081 1.078 1.074 1.071 1.067 1.063 1.059 1.055 1.051 1.047 1.043 1.039 1.035 1.029 1.027 1.023 1.019 1.015

λ −1

mS cm

1.436 1.725 1.929 2.18 2.5 2.67 3.02 3.06 3.57 3.94 4.2 4.48 4.72 5.05 5.4 5.85 6.42 7.03 7.6 8.49 9.36 10.24 10.92 12.23 13.45

2

ρ

CIL −1

xCH3CN

S cm mol

0.550 0.572 0.594 0.616 0.639 0.662 0.685 0.709 0.734 0.758 0.783 0.809 0.834 0.861 0.888 0.915 0.943 0.971 0.975 0.981 0.985 0.991 0.995 1.0

0.005 0.011 0.020 0.033 0.040 0.050 0.063 0.081 0.098 0.114 0.133 0.153 0.178 0.207 0.242 0.288 0.343 0.402 0.485 0.602 0.689 0.797 0.970 1.160 3173

−3

mol dm 2.65 2.59 2.52 2.45 2.38 2.29 2.20 2.11 2.00 1.88 1.75 1.61 1.45 1.28 1.08 0.85 0.61 0.32 0.28 0.22 0.17 0.11 0.05 0

σ −3

g cm

1.011 1.007 1.003 0.998 0.994 0.989 0.983 0.977 0.970 0.962 0.953 0.942 0.930 0.915 0.897 0.876 0.852 0.824 0.819 0.813 0.808 0.801 0.796 0.79

λ −1

mS cm 14.66 15.94 17.59 18.7 20.5 22.2 23.8 25.1 26.7 27.8 28.9 29.3 29.1 28.2 28.3 27.9 26 18.92 21.5 17.91 15.21 10.77 6.44 3.64

S cm mol−1 2

1.377 1.635 1.975 2.306 2.786 3.339 3.983 4.704 5.644 6.692 8.018 9.513 11.277 13.413 17.195 23.102 32.971 48.955 65.150 72.303 77.277 89.878 104.551



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Figure 7. (a) Conductivity for the mixture of [Bu3PH][BF4] with acetonitrile at 25 °C as a function of weight fraction of CH3CN (wCH3CN) in mixture. Inset: as a function of the molar fraction of CH3CN (xCH3CN). (b) Debye−Onsager plot of molar conductivity λ, against (CIL)1/2.

Figure 8. Influence of the temperature on the conductivity of the mixture of the PIL tributylphosphonium tetrafluoroborate with CH3CN (molar fraction: 0.809). The solid line serves as a guide to the eye.

⎡ B′ ⎤ 2 η = η0 exp⎢ ⎥ ⎣ T − T0 ⎦

Figure 10. Evolution of the viscosity as a function of the temperature for the mixture of ([Bu3PH][BF4] + acetonitrile) at different molar fractions of CH3CN (x2 = 0.147, x2 = 0.3144, x2 = 0.508, x2 = 1) Inset: shear stress versus shear rate at 20 °C for the two selected mixtures (x = 0.147 and 0.314).

mol−1). Figure 6 represents the VFT plots. According to (3)

equations 2 and 3, fitting parameters could be deduced for the

−1

Here, σ0 (mS cm ), η0 (mPa s), Bi′ (K), and T0 (K) are fitting parameters. The product Bi′R (where R is the molar gas constant) has the dimension of the activation energy (kJ

tributylphosphonium tetrafluoroborate, giving σ0 =104.21 mS cm−1, B′1 = 427.21 K, η0 = 61.7 mPa s, B′2 = 476.1 K, and T0 =

Figure 9. Arrhenius (a) and VFT (b) plot of ionic conductivities for the PIL tributylphosphonium tetrafluoroborate in a mixture with CH3CN (molar fraction: 0.809). The solid lines represent the Arrhenius or VFT fitting. 3174



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Table 3. Viscosity (η) as a Function of the Temperature, for Pure [Bu3PH][BF4], Acetonitrile, and for Mixture of ([Bu3PH][BF4] + Acetonitrile) for Different Molar Fraction of Acetonitrile (x2) η (mPa s) t (°C)

x2 = 0

x2 = 0.147

20 22 24 26 28 30 32 34 36 38 40 45 50 55 60 65 70 75 80

241.30 188.10 153 134.70 119.10 105.70 95.14 86.11 78.83 71.99 68.48 65.83 53.30 43.78 36.51 30.71 26.29 22.77 19.82

73.24 68.41 63.05 58.28 54.22 50.20 46.59 43.37 40.70 38.08 35.64 30.76 26.71 23.62 20.93 18.93 16.91 15.45 14.08

x2 = 0.314 x2 = 0.508 x2 = 0.809 x2 = 1 35.02 33.22 31.13 29.30 27.60 25.99 24.55 23.20 22.21 21.17 20.17 18.01 16.44 14.97 13.85 12.91 11.86 11.23 10.25

14.09 13.69 13.61 13.24 12.83 12.44 12.014 11.83 11.58 11.32 11.07 10.85 10.47

2.11 2.11 2.11 2.13 2.16 2.14 2.12 2.11 2.11 2.14 2.14

0.37 0.37 0.36 0.36 0.35 0.34 0.32 0.31 0.30 0.28 0.27 0.22 0.21 0.17 0.14 0.13 0.07 0.03 0.02

Figure 11. Evolution of Δη (ηid. − ηexp.) as a function of the molar fraction of CH3CN mixed with the PIL tributylphosphonium tetrafluoroborate for selected temperatures.

electrolyte solutions and in IL mixtures has been attributed to two opposing effects: (i) the increase in number of charge carriers, and (ii) the reduction in the mobility as the concentration increases.53 The molar conductivities, λ, of the PIL in solution were calculated from the ionic conductivity and the molar concentration for selected molar fraction of solvent (eq. 4).

⎛σ⎞ λ = 1000⎜ ⎟ ⎝C ⎠

211 K with a square correlation coefficient R2 equal to 0.9845 and 0.9985 respectively for ionic conductivity and viscosity. 3.3. Physical Properties of the PIL in a Mixture with Acetonitrile. Acetonitrile is the most common organic solvent for an electrochemical device like a supercapacitor; thus, it is interesting to understand transport properties for tributylphosphonium tetrafluoroborate in a mixture with acetonitrile for this application as presented in previous work.38 Conductivity. Table 2 and Figure 7a show the evolution of the conductivity with the addition of acetonitrile, to the PIL, at 25 °C. Molar fractions of acetonitrile were calculated taking into account the PIL and water (wH2O = 3%) amount. One can easily see that, at first, the conductivity increased quickly to reach a maximum at molar fraction of about xCH3CN = 0.809, σ = 29.3 mS cm−1. Subsequently, the conductivity stays almost constant until xCH3CN = 0.915; finally, it dropped very fast to the conductivity value of pure acetonitrile, 3.64.10−3 mS cm−1. Those three different zones can be clearly defined: the first, below xCH3CN = 0.809, that is, the acetonitrile in the PIL zone, a continuous increase in conductivity occurs as the viscosity decreases faster than the decrease in ionic species concentration occurring by dilution; the second, between xCH3CN = 0.809 and 0.915, a transition zone is observed for which the decreasing viscosity is compensated by the dilution effect; the third, beyond xCH3CN = 0.915, the dilution effect is predominant and the ionic conductivity decreases sharply.52 The existence of conductivity maxima, found here at xCH3CN = 0.809, in

(4)

The dependence of the molar conductivity, λ on the square root of the mole concentration in IL, (CIL)1/2 (CIL in mol dm−3) is presented in Figure 7b. From this curve, we observed that for a PIL concentration higher than 1.0 mol dm−3, λ decreases exponentially when CIL increases. This behavior is characteristic of a weak electrolyte, partially associated in acetonitrile. The infinite dilution conductance of the PIL was determined from the extrapolation of the curve λ as a function of (CIL)1/2 at low PIL concentrations (less than 0.5 mol1/2 dm−3/2). At these concentrations, the Debye−Onsager relation54 can describe evolution of λ as a function of (CIL)1/2 as follows:

λ = λ 0 + (a λ 0 + b) CIL

(5)

where λ is the molar conductivity, λ0 the infinite dilution molar conductivity, CIL the molar concentration, and a, b the empirical constants. The infinite dilution molar conductivity of the PIL λ0 was determined by fitting the experimental data using the eq 5, at low PIL concentrations range (less than 0.5 mol1/2 dm−3/2), and the result is 153.48 S cm2 mol−1. To evaluate the infinite dilution conductance of the cation or anions of the investigated PIL, the limiting molar conductivity can be decomposed into contributions from the different ions (law of independent migration of ions), and the addition law at infinite dilution was applied (eq. 6):

Table 4. Viscosity (η), Δη (ηid. − ηexp.) for the Mixture of [Bu3PH][BF4] + Acetonitrile as a Function of Acetonitrile Molar Fraction, at 25 °C. xCH3CN η (mPa s) Δη (mPa s)

0

0.071

0.147

0.228

0.314

0.508

0.809

1.0

142.90 0

82.13

60.65 −53.74

52.57

30.12 −55.77

13 −44.38

3.52 −16.81

0.37 0

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Figure 12. Arrhenius (a) and VFT (b) plot of the viscosities for the PIL tributylphosphonium tetrafluoroborate in a mixture with acetonitrile for selected molar fractions of CH3CN: x2 = 0.147, 0.314. The solid lines represent the Arrhenius or VFT fitting (parameters indicated in Table 5).

Table 5. VFT Equation Parameters for the Viscosity (η0, B2′, T0) of Selected Mixtures of Tributylphosphonium Tetrafluoroborate with Acetonitrile

a

xCH3CN

T0/K

η0/mPa s

B2′/K

R2

0 0.071 0.147 0.228 0.314 0.508

211 182 199 220 216 174

61.7 0.72 1.02 2.46 6.59 141.8

476.1 548.74 404.7 238.3 28.5 197.5

0.99855 0.9955 0.9999 0.9995 0.9993 0.9968

acetonitrile (x2) (x2 = 0, 0.147, 0.314, 0.508, 0.809, and 1) from 20 to 80 °C. Figure 10 represents the viscosity as a function of the temperature for the selected mixtures. Table 3 reports viscosity values as a function of the temperature, for selected mixtures, and for pure tributylphosphonium tetrafluoroborate and pure acetonitrile. As observed for the pure tributylphosphonium tetrafluoroborate, the viscosity decreases with the temperature when acetonitrile was added. For x2 = 0.508, we observe evaporation of acetonitrile after 50 °C, (these points are not shown in the graph). The inset of Figure 10 shows the shear rate as a function of the shear stress. The linear relation between those two parameters confirmed a Newtonian behavior for the studied solution in this temperature range. The viscosity deviations from ideality, Δη, for themixture (PIL + CH3CN) were calculated from the equation 8:56

Correlation coefficient. − λ 0 = z+λ + 0 + z−λ 0

(6)

According to the infinite dilution conductance of the anion BF4− reported in the literature,55 λ0− = 107.48 S cm2 mol−1, at 25 °C, the infinite dilution conductance of the cation phosphonium could be determined; λ0+= 46 S cm2 mol−1. Knowing that the optimum mixture obtained at 25 °C could evolve with the temperature, we chose to evaluate the mixture of PIL with acetonitrile, with a molar fraction of xCH3CN = 0.809, for the temperature dependence of conductivity. This corresponds to a PIL concentration of 1.61 mol dm−3. Figure 8 exhibits the evolution of the conductivity when increasing the temperature from 25 to 80 °C. As expected, the conductivity increased with temperature, from 29 mS cm−1 to 46.8 mS cm−1, in the studied range. The mixture of PIL and acetonitrile (xCH3CN = 0.809) presented a non-Arrhenius behavior as shown in Figure 9a, according to the equation 7:

⎡ −B ⎤ σ = σ0 exp⎢ 1 ⎥ ⎣ T ⎦

Δη = η − (x1η1 + x2 η2 )

(8)

where x1, x2 are the mole fractions of PIL and CH3CN, η, η1, and η2 are the viscosity of the solution, the pure PIL, and the pure acetonitrile, respectively (Table 4). Figure 11 shows the dependence of the viscosity deviations as a function of CH3CN molar fraction composition, x2, for different temperatures. It can be seen that, Δη is negative over the entire composition range and decreases with increasing temperature for all temperatures studied. The viscosity deviations, Δη, decrease when the temperature increases; this signifies that the experimental viscosity came closer to the ideal viscosity when the temperature increases. For 80 °C, Δη was close to zero, and the experimental viscosity thus approximately equaled the ideal viscosity. In fact, the temperature strongly influences the viscosity deviations, but the observed compositions at the maximum in viscosity deviations were found to be almost constant and independent of temperature. Furthermore, the viscosity deviations show minima near the PIL-rich region composition (x2 = 0.2). For selected mixtures of PIL with acetonitrile, Arrhenius and VFT plots are presented, respectively, in Figure 12a and 12b. It is clear that tributylphosphonium tetrafluoroborate in acetonitrile exhibited a non-Arrhenius behavior with regard to the viscosities. The same conclusion was made for other mixtures with acetonitrile not presented here, and fitting parameters are presented in Table 5. The pure PIL exhibits also a VFT behavior, like the mixture solution with acetonitrile. This behavior can be related to the fragility of electrolytes and

(7)

−1

where σ0 (mS cm ) and B1 = Ea/R (K) are fitting parameters, R is the molar gas constant, Ea, the activation energy (kJ mol−1), and T the temperature (K). Therefore, the Vogel−Tamman−Fulcher (VFT) expression (eq. 3) was used to determine the temperature dependence of the conductivity. Figure 9b represents the VFT plot, according to equation 3. The best-fit values were: σ0 = 141.8 mS cm−1, B1′ = 197.5 K, and T0 = 174 K with a square correlation coefficient R2 = 0.9968. Viscosity. The influence of the acetonitrile added on the viscosity was then investigated for different molar fraction of 3176



Article

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discussed with temperature dependence of the effective activation energy according to Leys et al.57 and our previously work.52

4. CONCLUSION Tributylphosphonium tetrafluoroborate PILs were prepared by neutralization of tributylphosphine with tetrafluoroboric acid. The obtained compound was very viscous at room temperature. The pure PIL as well as its mixture with acetonitrile exhibited a Newtonian behavior and could be described by the VFT equation when temperature increases from 20 to 80 °C. The molar conductivity value at infinite dilution of the PIL was determined from the Debye−Onsager equation (λ0 = 153.48 S cm2 mol−1), and allowed to evaluate the infinite dilution conductance of the cation phosphonium (λ0+ = 46 S cm2 mol−1). The conductivity of (PIL + acetonitrile) mixture at xCH3CN = 0.809 was evaluated for the temperature dependence. It was found that σ increases from 29 to 46.8 mS cm−1 in the temperature range from 25 to 80 °C. These mixtures exhibit a VFT behavior.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (33)247367360. Phone: (33)247366951.

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ACKNOWLEDGMENTS This research was supported by Conseil Régional de la region Centre through the Sup’Caplipe project.

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REFERENCES

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Transport Properties of Tributylphosphonium Tetrafluoroborate Protic

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