ARTICLE pubs.acs.org/JPCA
Thermal, Rheological, and Ion-Transport Properties of Phosphonium-Based Ionic Liquids Matthew D. Green,† Christian Schreiner,‡ and Timothy E. Long‡,* †
Departments of Chemical Engineering and ‡Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, Virginia 24061, United States
bS Supporting Information ABSTRACT: Phosphonium-based ionic liquids with varying counteranions from commercially available ionic liquid precursors enabled tunable viscosity, ionic conductivity, and thermal stability. Thermogravimetric analysis revealed a relationship between thermal stability and anion composition where anions with lower basicity remained stable to higher temperatures. Determination of glass transition temperatures and melting temperatures using differential scanning calorimetry revealed supercooling, crystallization, and dependence on anion composition. Rheological and ionic conductivity measurements determined the temperature-dependence of the viscosity and ionic conductivity of the phosphonium-based ionic liquids. Arrhenius analyses of conductivity and viscosity provided activation energies, which showed a decrease toward larger, more delocalized anions. An assessment according to the Walden plot displayed their efficacy relative to other ionic liquids.
’ INTRODUCTION Ionic liquids (ILs) are typically defined as salts with a melting temperature (Tm) below 100 °C.1 A special subset of ILs with Tms below room temperature are defined as room temperature ionic liquids (RTILs). The unique and desirable characteristics of ILs include their high thermal stability, low or negligible volatility, high ionic conductivity, wide electrochemical window, and tunable chemical structure.2,3 While significant research currently focuses on imidazolium-based ILs and the ability to tailor the substituents on the imidazolium ring, the imidazolium cation remains as a benchmark.46 However, studies focusing on novel classes of ILs through changes in the chemical structure of the cation, specifically phosphonium-based ILs, are not receiving the same literature attention as their imidazolium analogues. Based on their desirable properties, phosphonium ILs are under investigation for a variety of applications similar to their imidazolium counterparts.7,8 MacFarlane et al. nicely summarized the history, commercial synthesis, and several of the enhanced properties of phosphonium ILs relative to ammonium and imidazolium ILs, including an extensive table of commercially available phosphonium ILs.7 In summary, MacFarlane described the superior stability in basic environments; high chemical, electrochemical, and thermal stability; and typical densities less than water. Specific structural effects discussed concluded that the length and symmetry of the alkyl substituents on the phosphonium cation significantly influenced the physical state, while anion selection favored smaller charge-diffuse ions with limited hydrogen bonding sites. Potential applications summarized in the review included battery electrolytes, electrolytes for r 2011 American Chemical Society
dye-sensitized solar cells, capacitor electrolytes, corrosion inhibitors, and use as reaction media. Several of the specific studies summarized in MacFarlane’s review include Clyburne et al. who studied the use of phosphonium ILs as reaction media for strong bases.9,10 Phosphonium ILs are also under consideration for their application as extraction agents,11,12 where Chen et al. studied the adsorption of Cr(III) and Cr(IV) onto silica-based systems embedded with phosphonium ILs.11 Their work displayed improved thermal stability, higher and faster metal extraction, and nearly quantitative desorption of chromium ions relative to ammonium IL counterparts. Frackowiak et al. investigated phosphonium ILs for their potential as supercapacitors.13 They analyzed tri(hexyl) tetradecylphosphonium bis(trifluoromethanesulfonyl)imide (TFSI) and tri(hexyl) tetradecylphosphonium dicyanamide. Their findings suggested that these phosphonium ILs provided an enhanced electrochemical window relative to currently available ammonium ILs, while providing applicationsuitable capacitance values at reasonable applied voltages and scan rates. The primary limiting property of the phosphonium IL was the high viscosity, which was remedied through dilution with acetonitrile. Tsunashima et al. studied the potential of phosphonium ILs as battery electrolytes; their studies revealed that through chemical tuning, the phosphonium IL viscosity reduced to values that provided high discharge capacities relative to ammonium IL counterparts.14,15 Finally, Downard et al. Received: June 29, 2011 Revised: October 24, 2011 Published: October 25, 2011 13829
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The Journal of Physical Chemistry A discussed the potential for commercialization of phosphonium ILs, citing synthetic procedures, structural tuning through phosphonium-substituent and anion choice, physical properties, and the potential for application-specific materials.16 A continually expanding focus of research is the potential application of ILs as green, recyclable reaction media. Phosphonium ILs received attention in this area, as they have displayed improved reaction conditions for a variety of synthetic organic reactions. DielsAlder reactions, Grignard reactions, Michael reactions, and aryl halide oxidations display improved efficiency and reactivity when performed in IL media.1720 Janus and Stefaniak utilized tri(hexyl) tetradecylphosphonium TFSI as a solvent for the DielsAlder reaction.19 They showed quantitative yields at room temperatures with only 2 h reaction times and observed no loss in catalyst activity with time. Furthermore, the product is easily distilled from the IL reaction media due to the thermal stability and negligible volatility of the phosphonium IL. Pawar et al. also analyzed the use of tri(hexyl) tetradecylphosphonium chloride as a phosphonium IL reaction media for Michael additions.17 Their work focused on 1,4-addition of thiols to olefins at room temperature in high yield (82% represented the lowest obtained for a variety of reactions) with short reaction times, less than 3 h. Their unique reaction workup allowed for isolation of the products through extraction. They added water and hexanes to the reaction mixture to form a three-phase system, allowing for extraction of product based on solubility. Pawar et al. also noted the lower potential for chemical side reactions and intermolecular interactions among phosphonium ILs. Most phosphonium ILs lack acidic protons that undergo abstraction or aromatic rings that physically interact with other compounds. MacFarlane et al. discussed the phase diagrams for three-phase systems comprised of phosphonium ILs, water, and organic solvents, and they demonstrated potential for extraction processes.21 Dreisinger et al.22 discussed the physical properties of a variety of commercially available phosphonium ILs including density, viscosity, and conductivity. Their study revealed that the conductivity of the IL directly related to the anion volume and inversely correlated with the cation volume. They also investigated how cation symmetry influenced physical properties, and they cited a decrease in viscosity and slight increase in conductivity with the introduction of molecular asymmetry. In general, they also measured higher viscosities for phosphonium ILs relative to ammonium ILs. They used the VogelFulcher Tammann (VFT) equation to fit conductivity and temperature data, which allowed them to calculate T0. T0 describes the temperature at which molecular transport ceases, and values for T0 ranged from 160 to 20 °C, the highest T0 value occurring at 33 °C below the observed Tms. Diamond et al.23 discussed the local structure present in phosphonium ILs, and they related the structure to classical theory for solutions. They investigated the unique ability of ILs to solvate compounds and related the unique polar and nonpolar regions within homogeneous IL solutions to solvating power. Xanthos et al. discussed the hydrolytic and thermal degradation of poly(lactic acid) within two phosphonium ILs, tri(hexyl) tetradecylphosphonium tetrafluoroborate (BF4) and tri(hexyl) tetradecylphosphonium decanoate.24 Their findings suggested that the decanoate anion resulted in more pronounced degradation, that is, the decanoate anion catalyzed the hydrolytic degradation. On the other hand, the degradation in the BF4-containing phosphonium IL related to the generation of HF in the presence atmospheric moisture
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and the presence of the phosphonium IL actually increased the thermal stability. They discussed the need for more extensive studies related to the investigation of toxicity and interactions with solutes or neighboring chemical species prior to commercialized applications. There are numerous advantages associated with phosphonium ILs; however, their applications will remain limited without further characterization of thermal and physical properties.
’ EXPERIMENTAL METHODS Tri(isopropyl) methylphosphonium tosylate (Cyphos 106, [iBu3MeP][Tos]) and ethyl tri(butyl)phosphonium diethylphosphate (Cyphos 169, [Bu3EtP][Et2PO4]) were dried for several days at 65 °C under vacuum. Acetonitrile (Fisher, HPLC grade) was dried by distillation from calcium hydride. Sodium BF4 (Aldrich, 98%), lithium TFSI (LiTFSI, Alfa Aesar, 98%), oxalic acid (Aldrich, anhydrous, p.a.), dichloromethane (Fisher, reagent grade), hexanes (Fisher, HPLC grade), and trimethylsilyl chloride (Aldrich, 98%) were used as received. Anion metathesis of both Cyphos ILs to the corresponding tetraalkylphosphonium BF4 and TFSI salts were performed using well-established methods in aqueous solutions. The hydrophobic products either precipitated or formed a second liquid layer. Dichloromethane was used to extract the products from the aqueous phases. The organic solutions were washed multiple times with small amounts of water, dried with sodium sulfate overnight, and filtered. After solvent removal under vacuum at 25 °C, the products were dried in a vacuum oven at 60 °C for a minimum of 3 days. The phosphonium difluorooxalatoborate ILs were synthesized from their phosphonium BF4 precursors and bis(trimethylsilyl)oxalate, as described in literature.25 Temperature-dependent viscosities were measured with a TA Instruments AR-G2 rheometer, using a concentric cylinder geometry with a required liquid volume of 8 mL. The cup and spindle were enclosed with a custom-prepared glass cover to minimize water uptake from the air. The cover was fitted with a hose barb allowing a gentle stream of argon to pass over the liquid. The temperature was equilibrated for 10 min prior to any measurements. Temperature-dependent conductivities were measured with an Oakton Acorn 6 conductometer in a temperature-controlled bath capable of holding the temperature within (0.05 K. The ILs and the conductometer’s probe were placed in a special glass vessel purged with argon first and then sealed airtight to minimize water uptake. The temperature was equilibrated for 10 min prior to any measurements. Differential scanning calorimetry (DSC) was performed on a TA Q800 instrument under a nitrogen atmosphere at a 10 °C/ min heating and cooling ramp. Glass transition temperatures, crystallization temperatures, and melting temperatures were measured on the second heat. Thermogravimetric analysis (TGA) was performed using a TA Q500 under a nitrogen atmosphere with a temperature ramp of 10 °C/min. Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FT-IR) was performed using a Nicolet Impact 400 Spectrometer following viscosity and conductivity measurements to analyze qualitative moisture uptake (Supporting Information). Karl Fischer titration was performed using a Metrohm 831 KF Coulometer, and determined that water levels in the ILs were less than 10 ppm following a drying protocol of reduced pressure (0.5 mmHg) at 60 °C for at least 30 h. 13830
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The Journal of Physical Chemistry A Synthesis of Bis(trimethylsilyl)oxalate. Trimethylsilylchloride (175 g, 1.61 mol) was added to a suspension of oxalic acid (50.0 g, 555 mmol) in 110 mL of 1,2-dichloroethane. The mixture was first refluxed at 5070 °C until the HCl gas evolution slowed down and then for three more days at 90 °C until all solid starting material was dissolved and a clear solution was obtained. Then the solvent was distilled off, and the residue was purified in two steps to obtain the final product. First, the product was distilled at 95 °C at reduced pressure (12 mbar), dissolved in hexanes, and filtered. The product was stored as a solution until needed when hexanes was removed at reduced pressure (0.5 mmHg). Synthesis of [iBu3MeP][TFSI]. An aqueous solution of i [ Bu3MeP][Tos] (1.4603 g, 3.758 mmol in 2.0 mL) was mixed with an aqueous solution of LiTFSI (1.0800 g, 3.762 mmol in 2.0 mL); upon mixing, a white waxy solid precipitated. A total of 1.0 mL of water and 3.0 mL of dichloromethane were added to the solution. Two clear phases were observed, and the organic phase was washed with 4.0 mL of an aqueous solution of 4.0 mg/ mL LiTFSI followed with 3.0 mL of water three times. The organic phase was evaporated, and the product was dried at reduced pressure (0.5 mmHg) at 60 °C for 3 d; 1.4571 g of product were isolated (78% yield). 1 H NMR (400 MHz, DMSO) δ = 1.01 (d, 7 Hz, 18 H), 1.89 (d, 14 Hz, 3 H), 2.01 (m, 3 H), 2.16 (dd, 14 Hz, 6 Hz, 6 H). 19F NMR (376 MHz, DMSO) δ = 78.77 (s). 31P NMR (162 MHz, DMSO) δ = 29.17 (s). Synthesis of [iBu3MeP][BF4]. An aqueous solution of i [ Bu3MeP][Tos] (20.61 g, 53.03 mmol in 30 mL) was mixed with an aqueous solution of NaBF4 (6.06 g, 55.19 mmol in 20 mL) and a white precipitant formed immediately. The solution was diluted with 20 mL of water and extracted with 80 mL of dichloromethane. The organic phase was washed with a dilute aqueous solution of NaBF4 followed with 20 mL of water four times. The dichloromethane was evaporated and the product was dried at reduced pressure (0.5 mmHg) at 60 °C for 3 d; 12.10 g of product were isolated (75% yield). 1 H NMR (400 MHz, DMSO) δ = 1.00 (d, 7 Hz, 18 H), 1.89 (d, 14 Hz, 3 H), 2.01 (m, 3 H), 2.16 (dd, 14 Hz, 6 Hz, 6 H). 13C NMR (100 MHz, DMSO) δ = 6.55 (d, 49 Hz), 23.16 (d, 4.4 Hz), 24.65 (d, 9 Hz), 29.59 (d, 46 Hz). 19F NMR (376 MHz, DMSO) δ = 148.01 (s, 10BF4̅ ), 148.07 (s, 11BF4̅ ). 31P NMR (162 MHz, DMSO) δ = 29.16 (s). Synthesis of [iBu3MeP][DFOB]. Bis(trimethylsilyl)oxalate (5.070 g, 21.63 mmol) was used to prepare a 30 mL solution with acetonitrile, which was added dropwise to [iBu3MeP][BF4] (6.5788 g, 21.629 mmol) in 20 mL of acetonitrile. After stirring the solution at 45 °C for 3 d, the solvent was evaporated at 60 °C and reduced pressure (0.5 mmHg) to give the product in quantitative yield. 1 H NMR (400 MHz, DMSO) δ = 1.01 (d, 7 Hz, 18 H), 1.90 (d, 14 Hz, 3 H), 2.02 (m, 3 H), 2.17 (dd, 13 Hz, 6 Hz, 6 H). 11B NMR (160 MHz, DMSO) δ = 2.00 (t, 3 Hz), 2.21 (q, 1 Hz, 3% BF4). 13C NMR (100 MHz, DMSO) δ = 6.60 (d, 50 Hz), 23.18 (d, 4.3 Hz), 24.64 (d, 9 Hz), 29.66 (d, 47 Hz), 159.8 (s). 19F NMR (376 MHz, DMSO) δ = 150.73 (s + q, 3 Hz), 148.07 (s + s, 3% BF4). 31P NMR (162 MHz, DMSO) δ = 29.16 (s). Synthesis of [Bu3EtP][TFSI]. An aqueous solution of [Bu3 EtP][Et2PO4] (11.40 g, 29.65 mmol in 20 mL) was mixed with an aqueous solution of LiTFSI (8.94 g, 31.14 mmol in 10 mL) and two phases separated immediately upon mixing. The solution was diluted with 30 mL of water and the product extracted
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Figure 1. Commercially available phosphonium ILs Cyphos #106 ([iBu3MeP][Tos]) (a) and Cyphos #169 ([Bu3EtP][Et2PO4]) (b).
with 60 mL of dichloromethane. The organic phase was washed with a dilute aqueous solution of LiTFSI followed with 20 mL of water four times. The dichloromethane was evaporated and the product was dried at reduced pressure (0.5 mmHg) at 60 °C for 3 d, and 11.68 g of product were isolated (77% yield). 1 H NMR (400 MHz, DMSO) δ = 0.89 (t, 7 Hz, 9 H), 1.11 (dt, 18 Hz, 8 Hz, 3 H), 1.33 1.51 (m, 12 H), 2.09 2.25 (m, 8 H). 13 C NMR (100 MHz, DMSO) δ = 5.56 (d, 4.9 Hz), 11.70 (d, 48 Hz), 13.50 (s), 17.36 (d, 48 Hz), 23.00, (d, 4.4 Hz), 23.74 (d, 16 Hz), 119.9 (q, 323 Hz). 19F NMR (376 MHz, DMSO) δ = 78.91 (s). 31P NMR (162 MHz, DMSO) δ = 35.44 (s), 39.17 (s, 2% secBu isomer). Synthesis of [Bu3EtP][BF4]. An aqueous solution of [Bu3EtP][Et2PO4] (26.09 g, 67.86 mmol in 40 mL) was mixed with an aqueous solution of NaBF4 (7.888 g, 71.84 mmol in 20 mL) and a white precipitant formed immediately. The solution was diluted with 30 mL of water and the product was extracted with 100 mL of dichloromethane. The organic phase was washed with a dilute aqueous solution of NaBF4 followed with 20 mL of water four times. The dichloromethane was evaporated and the product was dried at reduced pressure (0.5 mmHg) at 60 °C for 3 d; 16.41 g of product were isolated (76% yield). 1 H NMR (400 MHz, DMSO) δ = 0.90 (t, 7 Hz, 9 H), 1.11 (dt, 18 Hz, 8 Hz, 3 H), 1.33 1.52 (m, 12 H), 2.09 2.26 (m, 8 H). 13 C NMR (100 MHz, DMSO) δ = 5.65 (d, 5.6 Hz), 11.66 (d, 49 Hz), 13.65 (s), 17.30 (d, 48 Hz), 23.00, (d, 4.5 Hz), 23.79 (d, 15 Hz). 19F NMR (376 MHz, DMSO) δ = 148.32 (s, 10BF4̅ ), 148.38 (s, 11BF4̅ ). 31P NMR (162 MHz, DMSO) δ = 35.47 (s), 39.19 (s, 2% secBu isomer). Synthesis of [Bu3EtP][DFOB]. Bis(trimethylsilyl)oxalate (10.506 g, 44.821 mmol) was dissolved in 70 mL of acetonitrile and added dropwise to [Bu3EtP][BF4] (14.050 g, 44.157 mmol) in 50 mL of acetonitrile. After stirring the solution at 45 °C for 3 d, the solvent was evaporated at 60 °C and reduced pressure (0.5 mmHg) to give the product in quantitative yield. 1 H NMR (400 MHz, DMSO) δ = 0.89 (t, 7 Hz, 9 H), 1.11 (dt, 18 Hz, 8 Hz, 3 H), 1.33 1.51 (m, 12 H), 2.09 2.25 (m, 8 H). 11 B NMR (160 MHz, DMSO) δ = 2.00 (s), 2.28 (s, 2% BF4). 13 C NMR (100 MHz, DMSO) δ = 5.60 (d, 6 Hz), 11.69 (d, 49 Hz), 13.57 (s), 17.35 (d, 49 Hz), 22.99, (d, 4.6 Hz), 23.77 (d, 16 Hz), 159.8 (s). 19F NMR (376 MHz, DMSO) δ = 151.94 (s + q, 3 Hz), 148.32 (s + s, 2% BF4). 31P NMR (162 MHz, DMSO) δ = 35.48 (s), 39.19 (s, 2% secBu isomer).
’ RESULTS AND DISCUSSION Commercially available phosphonium-based IL precursors, Cyphos 106, hereafter termed [iBu3MeP][Tos], and Cyphos 169, hereafter termed [Bu3EtP][Et2PO4], Figure 1, facilitated the synthesis of a series of phosphonium-containing ILs. The two cations are nearly regioisomers of each other, with the [iBu3MeP] cation possessing iso-butyl alkyl chains and the [Bu3EtP] cation 13831
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Scheme 1. Anion Exchange to Produce Bu3-Based Phosphonium ILs, [Bu3EtP][BF4], [Bu3EtP][TFSI], and [Bu3EtP][DFOB] from Top to Bottom
Figure 2. Thermogravimetric analysis of phosphonium ionic liquids. Phosphate and DFOB ILs display a two-step degradation at lower temperatures, indicating more reactive anions.
Scheme 2. Anion Exchange to Produce iBu3-Based Phosphonium ILs, [iBu3MeP][BF4], [iBu3MeP][TFSI], and [iBu3MeP][DFOB] from Top to Bottom
possessing n-butyl alkyl substituents. This structural variation provided valuable information relating the cationic structure to thermal, physical, and electrical properties of the ILs. Anion exchanges presented a facile synthetic technique to tune the properties of the ionic species, and anion selection provided control over viscosity, ionic conductivity, and solubility. These exchanges resulted in phase changes for the products of the metathesis reactions, simplifying purification. The product either precipitated as a solid, or in the case of the lower Tm products formed a second liquid layer. As stated, anion exchanges significantly impact the solubility, Tg, thermal stability, conductivity, and viscosity of the IL, while also opening a seemingly endless synthetic toolbox to tune the IL chemical structure. Exchange of the diethyl phosphate counteranion present on the commercially available phosphonium-based IL, [Bu3EtP][Et2PO4] (Scheme 1) to BF4, DFOB, and TFSI provided a series of ILs for characterization. Measurements of viscosity, ionic conductivity, and the thermal properties with varying counteranion revealed the effect of counteranion on the physical properties of the phosphonium-based ILs.
Exchange of the tosylate counteranion available on [iBu3 MeP][Tos] to BF4, DFOB, and TFSI provided a structural variation in IL structure to compare to the [Bu3EtP]-based series (Scheme 2). These six salts provided a series of phosphoniumbased ILs to analyze and compare the thermal stability, viscosity, and ionic conductivity relative to a common imidazolium IL, 1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO4]. Thermal analysis of the phosphonium-based ILs provided information on the thermal stabilities of the phosphonium-based ILs. Trends in thermal stability for the phosphonium-based ILs with varying counteranions showed that the thermal stability for the [Bu3EtP] cation increased in the order of Et2PO4 < DFOB < TFSI = BF4 anion, and for the [iBu3MeP] cation, the thermal stability of the IL increased in the order of DFOB < TFSI = BF4 < tosylate anion (Figure 2). Although the mechanism of degradation was not studied, the composition of the anion controlled the degradation pathway. All ILs containing a BF4 or TFSI counteranion experienced single-step degradations. However, [Bu3EtP][EtPO4] and both ILs containing the DFOB counteranion displayed two-step degradations. Further thermal analysis using DSC revealed the values of Tg, Tc, and Tm for the phosphonium-based ILs. All the phosphonium-based ILs studied met the current criteria for IL status with a Tm below 100 °C with the exception of those with BF4 counteranions. The BF4 counteranion was the smallest counteranion under investigation, thus leading to the highest values for Tm of 227 and 161 °C for [iBu3MeP][BF4] and [Bu3EtP] [BF4], respectively. Interestingly, the phosphonium salt with the branched substituent, [iBu3MeP][BF4], possessed the higher Tm of 227 °C, most likely due to the compact and spherical shape of the cation. Only four of the phosphonium-based ILs displayed a stepwise glass transition temperature in the DSC, correlated to a Tg for ILs.26 Table 1 presents a summary of the thermal properties of the phosphonium ILs with varying counteranion. The onset thermal degradation temperature of imidazolium-based ILs 1-butyl-3-methylimidazolium BF4 ([BMIm][BF4]) and 1-ethyl-3-methylimidazolium BF4 ([EMIm][BF4]) were 403 and 412 °C, respectively, and a series of 1-alkyl-3-methylimidazolium ILs with the TFSI counteranion and an increasing alkyl substituent (methyl, ethyl, butyl, hexyl, and octyl) displayed decreasing onset thermal degradation temperatures as the alkyl substituent increased, 444, 439, 427, 428, and 425 °C, all of which 13832
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Table 1. Thermal Analysis Results for Phosphonium Ionic Liquids Tdeg (°C)
Tg (°C)
[ Bu3MeP][Tos]
439
44
[Bu3EtP][Et2PO4] [iBu3MeP][TFSI]
313 429
69
[Bu3EtP][TFSI]
391
28a
[iBu3MeP][BF4] [Bu3EtP][BF4]
ionic liquid i
a
Tm (°C)
Tc (°C)
48
21
26
3
434
227
169
433
161
112
[iBu3MeP][DFOB]
327
58
43
[Bu3EtP][DFOB]
327
40
22
17
Solidsolid transition.
Figure 4. Arrhenius plot of phosphonium ILs’ specific conductivities.
Table 2. Activation Energy for Conductivity and Viscosity for Phosphonium ILs EA,K (J/mol)
EA,η (J/mol)
[EMIm][EtSO4]
28.5
33.4
[Bu3EtP][DFOB]
33.4
34.8
[Bu3EtP][TFSI] [Bu3EtP][Et2PO4]
35.0 45.4
37.4 50.0
[iBu3MeP][Tos]
61.3
63.8
ionic liquid
Figure 3. Arrhenius plot of phosphonium ILs’ viscosity.
were below the phosphonium ILs investigated with the corresponding anion.27,28 Similar ammonium ILs tetra(butyl) ammonium BF4 and tri(octyl)ammonium TFSI displayed onset thermal degradation temperatures of 325 and 357 °C, which were also below values observed for the phosphonium ILs investigated.29,30 Viscosity measurements for the phosphonium ILs with varying temperature revealed the impact of cation and anion selection on physical properties (Figure 3). Fitting the natural log of viscosity with Arrhenius-type relationships displayed linear fitting against 1000/T, in good agreement with the Arrhenius equation. However, slight curvatures in Figure 3 suggested that over a more extended temperature range the empirical VogelFulcherTammann (VFT) fitting would provide more suitable fittings.3133 Viscosity increased in the order of [Bu3EtP][TFSI] < [Bu3EtP][DFOB] < [Bu3EtP][Et2PO4] < [iBu3MeP][Tos]. [EMIm][EtSO4] was selected as a commercially available reference to imidazolium ILs with a significantly lower viscosity than those observed for the phosphonium ILs. The phosphonium ILs with lower viscosities possessed highly fluorinated counteranions, which resulted in weaker ionic interactions. [iBu3MeP][Tos] phosphonium IL with the iso-butyl substituents displayed the highest viscosity among phosphonium ILs. Intuitively, the more spherical cation structure should lead to smaller viscosities compared with the n-butyl substituents indicating the importance of anion selection. As a reference, at 60 °C imidazolium-based ILs 1-butyl-3-methylimidazolium BF4 ([BMIm][BF4]) and [BMIm][TFSI] had viscosities of 23 and 15 mPa 3 s, which fell well below the values for the phosphonium
ILs.34 A similar ammonium IL tri(octyl)ammonium TFSI displayed a viscosity of 43.0 mPa 3 s at 60 °C, which was higher than the observed value for the phosphonium ILs with corresponding anions.30 The conductivity of the phosphonium ILs nearly followed the opposite trend observed for viscosity. Bulk conductivity measurements revealed the temperature dependence of ionic conductivity for the phosphonium ILs. Analysis of only those compounds with melting temperatures near room temperature was possible due to experimental limitations. Analysis of the natural log of ionic conductivity using the Arrhenius relationship provided linear fitting against 1000/T. The ionic conductivity increased in the order of [iBu3MeP][Tos] < [Bu3EtP][Et2PO4] < [Bu3EtP][TFSI] < [Bu3EtP][DFOB] relating to the IL that contained a highly fluorinated, and large or bulky counteranion (Figure pt?>4). The ionic conductivity of [EMIm][EtSO4] provided a suitable reference, representing a commercially available imidazolium IL. The inverse relationship between viscosity and ionic conductivity indicated that the mobile or conducting species in the IL is restricted as the viscosity of the liquid increased. The overall superior performance of the imidazolium IL indicated the need to further tailor the chemical structure of the phosphonium cation or anion to rival the thermal and electrical properties of existing imidazolium ILs. As a reference, at 60 °C imidazolium-based ILs 1-butyl-3-methylimidazolium BF4 ([BMIm][BF4]) and [BMIm][TFSI] had ionic conductivity values of 13.1 and 11.3 mS/cm, which fell well above the values for the phosphonium ILs.34 A similar ammonium IL trimethylbutylammonium TFSI displayed an ionic conductivity of 8.6 mS/ cm at 60 °C, which is slightly less than an order of magnitude above the reported values for the phosphonium IL with the TFSI anion.35 13833
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Table 3. Fitting Parameters for the Fractional Walden Plot According to eq 1 α
ionic liquid
Figure 5. Walden plot of phosphonium ILs with [EMIM][EtSO4] for comparison.
Arrhenius analysis of the temperature-dependent conductivity and viscosity determined the activation energies for ion conduction and flow for the ILs. These activation energies, summarized in Table 2, showed an increase in the order of [EMIm][EtSO4] < [Bu3EtP][DFOB] < [Bu3EtP][TFSI] < [Bu3EtP][Et2PO4] < [iBu3 MeP][Tos]. The activation energies of the ILs with DFOB and TFSI counteranions were very similar due to a slightly higher viscosity for the DFOB counteranion and slightly lower conductivity. The imidazolium IL, [EMIm][EtSO4], possessed the lowest activation energy for both viscosity and conductivity, as expected from the overall lower viscosity and higher conductivity profiles. The phosphonium IL with the tosylate counteranion possessed the highest activation energy. The Fractional Walden’s rule36,37 (eq 1) describes the relationship between viscosity and ionic conductivity, where Λm is the molar conductivity, η is the IL viscosity, C’’ is a constant that accounts for cation and anion radius and shifts plots vertically in the Walden plot, and α is an exponent allowing for deviations from slopes of unity in the Walden plot. log Λm ¼ log C0 þ α log η1 Λm ¼ kVm ¼ k
M F
ð1Þ ð2Þ
Equation 2 allowed for calculation of the molar conductivities for the phosphonium ILs. In this equation, k is the specific conductivity, Vm is the molar volume, M is the molar mass, and F is the density. Plotting molar conductivity against viscosity in the Walden plot (Figure 5) revealed their relative performance to one another. An “ideal” line for an aqueous 0.1 M KCl solution extrapolated in both directions with a slope of unity serves as a reference for completely dissociated ions, as well as the imidazolium IL [EMIm][EtSO4]. The ionic conductivity and viscosity for the phosphonium ILs containing DFOB and TFSI exhibited optimal properties among the phosphonium ILs analyzed. Likewise, they displayed the closest proximity to the imidazolium reference in the Walden plot. The [iBu3MeP][Tos] phosphonium IL displayed the next best properties in the Walden plot followed with the [Bu3EtP][Et2PO4] phosphonium IL. This result is not clear based on the superior viscosity and conductivity of [Bu3EtP][Et2PO4] relative to [iBu3MeP][Tos] in addition to the higher activation energy of [iBu3MeP][Tos]. This may suggest a more enhanced
log(C0 )(S 3 cm2 3 mol1)
EA,K/EA,η
[EMIm][EtSO4]
0.898
0.101
0.853
[Bu3EtP][DFOB] [Bu3EtP][TFSI]
0.960 0.934
0.255 0.271
0.960 0.936
[iBu3MeP][Tos]
0.956
0.402
0.908
[Bu3EtP][Et2PO4]
0.934
0.567
0.961
performance based on the structure of the cation and anion. Despite encouraging viscosity, conductivity, and enhanced thermal stability of the phosphonium ILs, they proved inferior relative to the imidazolium reference on the plot of Walden’s rule. Linear fitting of the data on the Walden plot determined the exponent α and constant C0 in the Fractional Walden Rule for each phosphonium IL, Table 3. The exponent α, which is unity for an ideal system, is higher for all phosphonium IL systems compared to the imidazolium IL reference. The inferior performance may relate to the cation and anion size, which significantly reduces the C0 value compared to the imidazolium IL. Additionally, the ratio of the temperature-dependent activation energies for conductivity and viscosity reflected the exponents extracted from linear fitting of the Fractional Walden Rule.38 Deviations occurred for the ILs that displayed nonlinear or non-Arrhenius type behavior, indicating the error of averaging the activation energy calculation over the full temperature range in this case. However, the improved exponent on the Fractional Walden Plot indicated potential for phosphonium IL upon further derivatization to improve viscosity and conductivity for emerging applications.
’ CONCLUSIONS Imidazolium ILs currently dominate the literature for applications in a variety of fields ranging from green solvents to actuator diluents. However, phosphonium ILs are continuing to gain attention as optimization of performance and properties expand their applicability. Anion exchange to BF4, DFOB, and TFSI provided new variants of commercially available phosphonium IL precursors. The enhanced thermal and electrochemical stability of phosphonium ILs are critical attributes as the demand for ILs with larger temperature and voltage application windows than imidazolium counterparts provide continue to grow. The phosphonium ILs possessed thermal stabilities above 300 °C with several well above 400 °C. DSC revealed crystalline salts with Tm values for most phosphonium ILs below 100 °C and several displayed a Tg, which is characteristic of IL small molecules. The IL viscosity-temperature relationship exhibited Arrhenius-type behavior with activation energies favoring bulky, fluorinated anions. Similarly, activation energies derived from ionic conductivities favored the phosphonium ILs with bulky fluorinated anions. Analysis of molar conductivities using the Walden plot indicated near-unity α values, however, low C0 values hampered placement compared with imidazolium ILs on the Walden plot. This indicated that the trend for molar conductivities of phosphonium-containing ILs were smaller than for imidazolium-based ILs presumably due to stronger intermolecular interactions. Phosphonium-containing ILs require further optimization prior to outperforming imidazolium IL counterparts, however, applications in a wide variety of settings remain feasible based on their desirable properties. 13834
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’ ASSOCIATED CONTENT
bS
Supporting Information. ATR-FTIR after viscosity measurements confirming the absence of moisture uptake. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This material is based on work supported by the U.S. Army Research Office under Grant No. W911NF-07-1-0452 Ionic Liquids in Electro-Active Devices (ILEAD) MURI. This material is based upon work supported by the Army Research Office (ARO) under Award No. W911NF-10-1-0307. ’ REFERENCES (1) Fannin, A. A.; Floreani, D. A.; King, L. A.; Landers, J. S.; Piersma, B. J.; Stech, D. J.; Vaughn, R. L.; Wilkes, J. S.; Williams John, L. J. Phys. Chem. 1984, 88, 2614. (2) Chen, H.; Choi, J.-H.; Salas-de la Cruz, D.; Winey, K. I.; Elabd, Y. A. Macromolecules 2009, 42, 4809. (3) Wasserscheid, P.; Hal, R. v.; Bosmann, A. Green Chem. 2002, 4, 400. (4) Green, M. D.; Long, T. E. Polym. Rev. 2009, 49, 291. (5) Kottsieper, K. W.; Stelzer, O.; Wasserscheid, P. J. Mol. Catal A: Chem. 2001, 175, 285. (6) Wasserscheid, P.; Keim, W. Angew. Chem. 2000, 39, 3772. (7) Fraser, K. J.; MacFarlane, D. R. Aust. J. Chem. 2009, 62, 309. (8) Del Sesto, R. E.; Corley, C.; Robertson, A.; Wilkes, J. S. J. Organomet. Chem. 2005, 690, 2536. (9) Ramnial, T.; Ino, D. D.; Clyburne, J. A. C. Chem. Commun. 2005, 325. (10) Ramnial, T.; Taylor, S. A.; Bender, M. L.; Gorodetsky, B.; Lee, P. T. K.; Dickie, D. A.; McCollum, B. M.; Pye, C. C.; Walsby, C. J.; Clyburne, J. A. C. J. Org. Chem. 2008, 73, 801. (11) Liu, Y.; Guo, L.; Zhu, L.; Sun, X.; Chen, J. Chem. Eng. J. 2010, 158, 108. (12) Campos, K.; Vincent, T.; Bunio, P.; Trochimczuk, A.; Guibal, E. In Solvent Extraction & Ion Exchange; Taylor and Francis Ltd.: U.K., 2008; Vol. 26, p 570. (13) Frackowiak, E.; Lota, G.; Pernak, J. Appl. Phys. Lett. 2005, 86, 164104. (14) Tsunashima, K.; Yonekawa, F.; Sugiya, M. Chem. Lett. 2008, 37, 314. (15) Tsunashima, K.; Yonekawa, F.; Sugiya, M. Electrochem. SolidState Lett. 2009, 12, A54. (16) Bradaric, C. J.; Downard, A.; Kennedy, C.; Robertson, A. J.; Zhou, Y. Green Chem. 2003, 5, 143. (17) Sarda, S. R.; Jadhav, W. N.; Shete, A. S.; Dhopte, K. B.; Sadawarte, S. M.; Gadge, P. J.; Pawar, R. P. Synth. Commun. 2010, 40, 2178. (18) Ramnial, T.; Taylor, S. A.; Clyburne, J. A. C.; Walsby, C. J. Chem. Commun. 2007, 2066. (19) Janus, E.; Stefaniak, W. Catal. Lett. 2008, 124, 105. (20) Dake, S. A.; Kulkarni, R. S.; Kadam, V. N.; Modani, S. S.; Bhale, J. J.; Tathe, S. B.; Pawar, R. P. Synth. Commun. 2009, 39, 3898. (21) Chowdhury, S. A.; Scott, J. L.; MacFarlane, D. R. Pure Appl. Chem. 2008, 80, 1325. (22) Vaughan, J. W.; Dreisinger, D.; Haggins, J. ECS Trans. 2006, 2, 381.
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