J. Phys. Chem. B 2009, 113, 1007–1011
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SO2 Saturation of the Room Temperature Ionic Liquid [C2mim][NTf2] Much Reduces the Activation Energy for Diffusion Laura E. Barrosse-Antle,† Christopher Hardacre,‡ and Richard G. Compton*,† Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom, and School of Chemistry and Chemical Engineering/QUILL, Queen’s UniVersity Belfast, Belfast, Northern Ireland BT9 5AG, United Kingdom ReceiVed: October 3, 2008; ReVised Manuscript ReceiVed: NoVember 8, 2008
The physical effect of high concentrations of reversibly dissolved SO2 on [C2mim][NTf2] was examined using cyclic voltammetry, chronoamperometry, and ESR spectroscopy. Cyclic voltammetry of the oxidation of solutions of ferrocene, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), and chloride in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethanesufonyl)imide ([C2mim][NTf2]) reveals an increase in limiting current of each species corresponding to the addition of increasing concentrations of sulfur dioxide. Quantitative chronoamperometry reveals an increase in each species’ diffusion coefficient with SO2 concentration. When chronoamperometric data were obtained for ferrocene in [C2mim][NTf2] at a range of temperatures, the translational diffusion activation energy (29.0 ( 0.5 kJ mol- 1) was found to be in good agreement with previous studies. Adding SO2 results in apparent near-activationless translational diffusion. A significant decrease in the activation energy of rotational diffusion with the SO2 saturation of a 2,2,6,6tetramethyl-1-piperidinyloxyl (TEMPO) solution in [C2mim][NTf2] (29.9 ( 2.0 to 7.7 ( 5.3 kJ mol- 1) was observed using electron spin resonance (ESR) spectroscopy. The reversible physical absorption of SO2 by [C2mim][NTf2] should have no adverse effect on the ability of that ionic liquid to be employed as a solvent in an electrochemical gas sensor, and it is possible that the SO2-mediated reduction of RTIL viscosity could have intrinsic utility. 1. Introduction Room temperature ionic liquids (RTILs), a class of compounds that are composed entirely of ions and are liquid at and around 25 °C, are frequently touted as being “green” solvents.1,2 Their properties of near-zero volatility and high thermal stability enable RTILs to be recycled and reused, and they have been employed as solvents for various organic,3,4 inorganic,5 and enzymatic reactions6 and as well as solvents for analytical processes.7 In particular, electrochemical processes have been studied in RTILs, as the composition of these liquids eliminates the need for supporting electrolyte and extends the available electrochemical window beyond the limits imposed by using water or conventional organic solvents.1,8 High viscosity, another characteristic property of RTILs, results in slower response times than those obtained in more conventional solvents and is an important consideration in the investigation of RTILs for electrochemical applications.9 Ionic liquids range in viscosity from about 34 to 7453 cP,10,11 as compared to more conventional solvents such as acetonitrile and dicholoromethane, which have viscosities of 0.34 and 0.44 cP, respectively12 (all values are measured at 20 °C). Electrochemical gas sensing is one area in which ionic liquids have been explored as solvents.9 Many gases, such nitrogen dioxide,13 ammonia,14 hydrogen sulfide,15 and sulfur dioxide,16 have been found to have high solubilities in certain ionic liquids. The high solubility of these gases in RTILs, combined with the RTILs’ favorable properties of often negligible vapor pressure * Corresponding author. E-mail:
[email protected]. Tel: +44(0) 1865 275 413. Fax: +44(0) 1865 275 410. † Oxford University. ‡ Queen’s University Belfast.
and high thermal stability, suggests that these liquids may be successfully employed as gas sensors in conditions where sensors incorporating conventional solvents are prone to drying out or degrading over time. For this reason, it is important to understand the ways in which absorbing large amounts of gas affect the ionic liquids being employed, especially with regard to electrochemical sensing. Sulfur dioxide is a pollutant that has been linked to respiratory problems, particularly in asthmatics, as well as being thought to contribute to increased mortality.17 As a major contributor to acid rain, in addition to its other health risks, SO2 is an emission that is slated to be reduced by many countries.18–21 A previous publication investigated the use of ionic liquids as solvents for the electroreduction of SO2. High solubilities (as large as 3500 mM at 1 atm SO2 in one case) were observed, and the reduction mechanism was elucidated and found to differ from that followed in conventional solvents. However, the physical effects of gas absorption on the ionic liquids in question were not studied.16 The present work examines the effect of high concentrations of SO2 on the RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethanesufonyl)imide ([C2mim][NTf2]), a common and easy to work with ionic liquid (see Figure 1 for the structure, 34 cP at 20 °C).10 Cyclic voltammetry, chronoamperometry, and electron spin resonance (ESR) spectroscopy were employed to probe the physical impact of SO2 absorption on this ionic liquid. Near-activationless diffusion of species of various size and composition (ferrocene, N,N,N′,N′-tetramethyl-p-phenylenediamine, chloride, and 2,2,6,6-tetramethyl-1-piperidinyloxyl radical) is achieved when the ionic liquid is saturated with SO2.
10.1021/jp808755f CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009
1008 J. Phys. Chem. B, Vol. 113, No. 4, 2009
Figure 1. Structures and naming conventions of the cation and anion used as the RTIL in this study.
Barrosse-Antle et al. for 10 s. The nonlinear curve fitting function in Origin 7.0 (MicroCal Software Inc.) following the Shoup and Szabo25 approximation as employed by Evans et al.26 was used to fit the experimental data. The equations used in this approximation describe the current response within an accuracy of 0.6% and are given below:
I ) -4nFDcrd f (τ) 2. Experimental Section 2.1. Reagents and Instrumentation. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesufonyl)imide ([C2mim][NTf2]) was prepared using a standard literature procedure.10 Sulfur dioxide (CK Gas Products, 99.9%), ferrocene (Aldrich, 98%), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD, Fluka, g98%), tetra-n-butylammonium perchlorate (TBAP, Fluka, Puriss electrochemical grade, >99%), potassium chloride (Reidel-de Hae¨n, >99.5%), 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO, Aldrich, 98%), and acetonitrile (Fischer Scientific, dried and distilled, >99.99%) were used as received without further purification. Cyclic voltammetry (CV) was performed using a type II µAutolab (Eco Chemie, Utrecht, Netherlands), which was interfaced with a PC using GPES (version 4.9) software for Windows. Measurements were performed using a two-electrode cell consisting of a platinum working ultramicroelectrode (10 µm diameter) and a silver wire quasi-reference electrode (0.5 mm diameter). The electrodes were housed in a glass “T-cell” specially designed to control the environment of the RTIL.22,23 The microelectrode was modified using a portion of a disposable plastic micropipet tip to create a reservoir in which 20 µL of ionic liquid was placed. The T-cell containing the ionic liquid was kept under vacuum for at least 90 min prior to taking any measurements. Sulfur dioxide from a gas cylinder was introduced into the cell via PTFE tubing through one arm of the T-cell and removed through the other. The gas was then was directed into the fume cupboard, again via the nonreactive PTFE tubing. The system generally took 30-40 min after first introducing the gas to give a consistent voltammetric response. ESR spectra were obtained using a JEOL JES-FA100 X-band spectrometer equipped with a cylindrical (TE011) cavity resonator and an ES-DVT3 variable temperature controller. Ionic liquid solutions were kept under vacuum for at least 90 min prior to any measurements being taken or sulfur dioxide being added. In SO2-saturated samples, sulfur dioxide was bubbled into 400 µL of 1.2 mM TEMPO in [C2mim][NTf2] for 2 h prior to the solution being syringed into a glass tube of inner diameter 2 mm. The tube containing the ionic liquid solution was inserted into a standard NMR tube (4.2 mm inner diameter), and nitrogen was flowed over each sample for 30 min prior to analysis in order to minimize the samples’ absorption of atmospheric oxygen and water. 2.2. Electrode Preparation. Before use, the microelectrode was polished on soft lapping pads (Kemet Ltd., UK) using 1.0 and 0.3 µm aqueous alumina slurries (Buehler, IL). The radius of the microdisk electrode was electrochemically calibrated by analyzing the steady state voltammetry of a 2 mM ferrocene solution in acetonitrile, which contained 0.1 M TBAP as a supporting electrolyte. The diffusion coefficient value used was 2.3 × 10-9 m2 s-1 at 25 °C.24 2.3. Chronoamperometric Experiments. Chronoamperometric transients were achieved using a sample time of 0.01 s. The pretreatment step consisted of holding the potential at 0 V for 20 s, followed by a 2 s equilibration period. The potential was stepped to the required value, and the current was measured
(1)
f(τ) ) 0.7854 + 0.8863τ-1/2 + 0.2146 exp(-0.7823τ-1/2) (2) τ)
4Dt rd2
(3)
where n is the number of electrons transferred, F is the Faraday constant, D is the diffusion coefficient, c is the bulk concentration of parent species, rd is the radius of the microdisk electrode, and t is the time. The value of the electrode radius was fixed, having been previously calibrated. The software performed up to 100 iterations on the data, stopping when the experimental data had been optimized. A value for the diffusion coefficient, D, and the product of the number of electrons and the concentration of the parent species, nc, was thus obtained. 3. Results and Discussion 3.1. Voltammetry of Three Electroactive Species in [C2mim][NTf2] in the Presence of SO2. The oxidation of three electroactive speciessferrocene, N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), and chlorideswas observed in [C2mim][NTf2] using cyclic voltammetry at a 10 µm diameter platinum microelectrode. Clearly defined oxidative peaks were observed in all three cases, and limiting currents were achieved at a scan rate of 10 mV s-1. When 100% SO2 was added, the limiting currents increased drastically. Both chloride and ferrocene saw currents over 400% of those achieved in pure [C2mim][NTf2]. TMPD, which oxidizes first to the radical cation and then to the double cation form, showed 30% and 50% increases in limiting current, respectively. In all cases, an apparent shift of oxidation potential to more reducing potentials was seen. However, this is likely due to a shift in the potential of the silver quasi-reference electrode. Nitrogen gas was used to dilute the SO2 in order to better characterize the effect of varying the ionic liquid’s exposure to SO2. Increasing concentrations of SO2 (20, 40, 60, and 80% by volume) were added to the ionic liquid solutions and the cyclic voltammetry obtained is shown in Figure 2. In all cases, the limiting current increased with concentration of SO2. Potential step experiments were performed on the anodic waves in order to calculate the diffusion coefficients of each species in the presence of varying concentrations of SO2. The potential was stepped from 0 V to a potential past that at which the signal reached steady state (e.g., 0.5 V versus Ag for ferrocene), and the current was measured for 10 s. The experimental data were fitted to the Shoup and Szabo25 expression, giving values for the concentrations and diffusion coefficients of the species in their respective solutions. As with the limiting currents, the diffusion coefficient values of all of the analyzed species increased with increasing SO2 concentration (Figure 3). The fact that an increase in current and diffusion coefficient is observed for three very different species suggests
SO2 Saturation of [C2mim][NTf2]
Figure 2. Cyclic voltammograms of (a) ferrocene, (b) TMPD, and (c) chloride in [C2mim][NTf2] at a 10 µm platinum electrode (scan rate of 10 mV s-1). In each case, the limiting currents of the voltammograms are shown to increase as the solutions are treated with 0, 20, 40, 60, 80, and 100% SO2 in turn.
that the addition of SO2 results in a physical change to the ionic liquid rather than a chemical change to the diffusing species. This effect is seen to be reversibleswhen a T-cell containing a sample of chloride in [C2mim][NTf2] is saturated with SO2 and then evacuated for 30 to 45 min, the limiting current returns to its original (presaturation) level. The reversible absorption of SO2 in ionic liquids was previously observed by Huang et al. In that case, NMR was used to confirm that no chemical bonds were formed between the RTIL [C4mim][NTf2] and SO2, leading to the conclusion that SO2 is purely physcially absorbed by ionic liquids of this type.27 To support or disprove the assumption that a physical change to the ionic liquid occurs rather than some sort of interaction between the electroactive species and the SO2, 100% SO2 was added to solutions of TMPD in propylene carbonate and acetonitrile (0.1 M TBAP was used as a supporting electrolyte). After approximately 1 h of the solutions being bubbled with SO2, no increase in current was observed even though the characteristic shift in the quasi-reference electrode potential had occurred. A specially designed heated Faraday cage28 was used to obtain chronoamperometric data for solutions of ferrocene in [C2mim][NTf2] at 25, 28, 31, 34, and 37 °C. An Arrhenius plot was constructed, and the translational diffusion activation energy of ferrocene in pure [C2mim][NTf2] was calculated to be 29.0 ((0.5) kJ mol-1, a value which is in good agreement with the literature.29 The same temperatures were used to evaluate the diffusional activation energy of ferrocene in [C2mim][NTf2]
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Figure 3. Increase in diffusion coefficient of (a) ferrocene, (b) TMPD, and (c) chloride in [C2mim][NTf2] with the addition of 0, 20, 40, 60, 80, and 100% SO2.
Figure 4. Arrhenius plots obtained by measuring the diffusion coefficient of ferrocene at 25, 28, 31, 34, and 37 °C reveal a diffusional activation energy of 29.0 ((0.5) kJ mol-1 when no SO2 is added to the solution (filled dots) and near-activationless diffusion when the solution is saturated with SO2 (open dots).
saturated with SO2. As can be seen from Figure 4, the Arrhenius plot formed an approximately horizontal line, indicating apparently near-activationless translational diffusion, although the plot reflects the changing levels of SO2 solubility with temperature as well as the altering viscosity. 3.2. ESR Spectroscopy of TEMPO in SO2-Saturated [C2mim][NTf2]. Electron spin resonance spectroscopy was used to evaluate the viscosity of the ionic liquid with and without SO2 both qualitatively and quantitatively through the calculation of the rotational diffusion coefficient. In low-viscosity solutions,
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Figure 5. ESR spectrum for 1.2 mM TEMPO in [C2mim][NTf2] at 0 °C.
the anisotropic interactions of the species under study average to zero. However, in solutions of higher viscosity, incompletely averaged molecular tumbling is observed as asymmetry in the spectrum. Thus, the degree of asymmetry in an ESR spectrum of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), a muchstudied,30 stable free-radical species, in [C2mim][NTf2] can be used to approximate the viscosity of the ionic liquid. Figure 5 is an ESR spectrum of TEMPO in pure [C2mim][NTf2] at 0 °C. The intensities of the first two peaks (I+1 and I0) are roughly equal, but the third peak intensity (I-1) is markedly less, indicating the slower molecular tumbling caused by high viscosity. At ambient temperature, a solution of 1.2 mM TEMPO in the pure ionic liqiud had a I-1:I0 ratio of 0.85. When SO2 was bubbled into the solution, the asymmetry was reduced so that the ratio was found to be 0.93. In the fast motion range (10-11 e τ e 3 × 10-9 s), the rotational correlation coefficient (τR) can be calculated using the following equations:31
[ ]
(4)
[ ]
(5)
τR ) -6.1 × 10-10∆H(0)
τR ) +6.0 × 10-10∆H(0)
I0 I+1
I0 + I+1
I0 I-1
I0 -2 I-1
where In is the intensity of the ESR line corresponding to m1 ) n and ∆H(0) (measured in gauss) is the width of the central line. These two expressions should be equal in the absence of inhomogenous broadening. No correction was made for this effect, placing some small uncertainty in the calculated values of τR. However, the trends with regards to temperature variation should be preserved. Rotational activation energy values are thus attainable for the system.29 The values of τR were calculated for TEMPO in [C2mim][NTf2] at 0, 5, 10, 15, 20, and 25 °C with and without SO2 added to the solution. Arrhenius plots were constructed, and the value of the rotational diffusion energy of activation obtained for pure [C2mim][NTf2] correlated well with the literature value (29.9 ( 2.0 kJ mol-1).29 Saturating the solution with SO2 yielded an Ea of 7.7 ( 5.3 kJ mol-1, essentially reducing the energy required for rotational diffusion by a third. Greater divergence was observed between the rotational correlation coefficients calculated by the two expressions when SO2 was added, a fact reflected by the increased error associated with the SO2-saturated activation energy.
Recent research into the effect of sulfur dioxide saturation on 1-butyl-3-methylimidazolium bromide ([C4mim][Br]) has been carried out at the University of Sao Paulo.32,33 Siqueira et al. observed that flowing SO2 over the solid salt [C4mim][Br] resulted in liquidation, very tangibly lowering the viscosity. Raman spectroscopy and molecular dynamics simulations were used to determine that the bromide anion of the ionic liquid was shielded from long-range Coulombic interactions by the sulfur dioxide. No significant interaction was observed between the methylimidazolium cation and the sulfur dioxide.32 The previously cited work by Huang et al. seems to contradict these observations, suggesting that the anion does not influence the interaction of SO2 with the ionic liquid. This assertion is supported by comparing SO2 absorption in two ionic liquids which share the 1,1,3,3-tetramethylguanidine cation. Absorption was determined using FT-IR and 1H NMR spectra of the [BF4]and [NTf2]- ionic liquids into which the SO2 was bubbled and was seen to be almost equal for the two liquids. The nature of the cation was said to influence the RTILs’ absorption capabilities “slightly”.27 As we are exploring not the ability of [C2mim][NTf2] to absorb SO2 but the effect of saturating the RTIL with SO2, however much SO2 is needed to achieve saturation and whatever factors are at play in doing so, it seems that the study performed by Siqueira et al. is much more able to inform the present work. Thus, we posit a similar mechanism to that elucidated by Siqueira et al. for [C4mim][Br] in the case of [C2mim][NTf2]. As the cations are very similar in structure (a butyl side chain replaced by an ethyl, see Figure 1), it is reasonable to assume that the interaction between SO2 and [C2mim]+ is minimal. The anion [NTf2]-, though very different from the monatomic bromide anion, is likely shielded by the SO2, causing the disruption of ionic interactions in the ionic liquid. This disruption, in turn, causes the dramatic decrease in the activation energies of rotational and translational diffusion observed by electrochemical methods and ESR spectroscopy. 4. Conclusions The physical effect of high concentrations of SO2 on [C2mim][NTf2] was examined using cyclic voltammetry, chronoamperometry, and ESR spectroscopy. Absorption of SO2 causes a decrease in the RTIL’s viscosity, as evidenced by the increase in both limiting currents and diffusion coefficiencts of three electrochemically active species when plotted against SO2 concentration. Near-activationless translational diffusion of ferrocene through SO2-saturated [C2mim][NTf2] is found to occur in marked contrast to literature and measured activation energy values of around 30 kJ mol-1 for the pure ionic liquid.29 ESR spectroscopy was used to probe the system further. Trends in the rotational diffusion coefficients of TEMPO in pure and SO2-saturated [C2mim][NTf2] reveal a dramatic decrease in rotational activation energy. It is posited that SO2 shields the [NTf2]- anion from long-range ionic interactions, decreasing the viscosity of the ionic liquid and enhancing both rotational and translational diffusion of probe molecules through the solution. The absorption of SO2 by [C2mim][NTf2] is reversible and thus should have no adverse effect on the ability of that ionic liquid to be employed as a solvent in an electrochemical gas sensor. Additionally, in light of the disadvantages posed by the generally high viscosity of ionic liquids as a class, it is possible that the SO2-mediated reduction of RTIL viscosity could have intrinsic utility and certainly bears further investigation. Acknowledgment. L.E.B.A. thanks the Physical and Theoretical Chemistry Laboratory for financial support, and Leigh
SO2 Saturation of [C2mim][NTf2] Aldous for synthesising the ionic liquid employed in the completion of this work. References and Notes (1) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem 2004, 5, 1106–1120. (2) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391– 1398. (3) Vidisˇ, A.; Laurenczy, G.; Ku¨sters, E.; Sedelmeier, G.; Dyson, P. J. J. Phys. Org. Chem. 2007, 20, 109. (4) D’Anna, F.; Frenna, V.; Noto, R.; Pace, V.; Spinelli, D. J. Org. Chem. 2006, 71, 9637. (5) Xue, X.; Liu, C.; Lu, T.; Xing, W. Fuel Cells 2006, 6, 347. (6) Raab, T.; Bel-Rhlid, R.; Williamson, G.; Hansen, C.-E.; Chaillot, D. J. Mol. Catal. B: Enzym. 2007, 44, 60. (7) Yokozeki, A.; Shiflett, M. B. Appl. Energy 2007, 84, 351. (8) Silvester, D. S.; Compton, R. G. Z. Phys. Chem. 2006, 220, 1247. (9) Buzzeo, M. C.; Hardacre, C.; Compton, R. G. Anal. Chem. 2004, 76, 4583. (10) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (11) Ignat’ev, N. L.; Welz-Biermann, U.; Kucheryna, A.; Bissky, G.; Wilner, H. J. Fluorine Chem. 2005, 126, 1150–1159. (12) Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996. (13) Broder, T. L.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. B 2007, 111, 7778. (14) Ji, X.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2007, 111, 9562–9572. (15) O’Mahony, A. M.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 7725–7730. (16) Barrosse-Antle, L. E.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 3398–3404. (17) WHO. “WHO Air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide”; World Health Organization, 2006. (18) Hall, J.; Tipping, E.; Sutton, M.; Dragosits, U.; Evans, C.; Foot, J.; Harriman, R.; Monteith, D.; Broadmeadow, M.; Langan, S.; Helliwell,
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JP808755F
