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Volatilisation of ferrocene from ionic liquids: kinetics and mechanismw Chaopeng Fu,a Leigh Aldous,a Edmund J. F. Dickinson,a Ninie S. A. Mananbc and Richard G. Compton*a Received 21st April 2011, Accepted 11th May 2011 DOI: 10.1039/c1cc12336j The evaporation of dissolved ferrocene from non-volatile ionic liquids under a flow of nitrogen gas has been monitored voltammetrically and modelled mathematically. The rate of volatilisation was found to depend on the surface tension of the ionic liquid, and a model is presented. Ionic liquids are fascinating, relatively new solvents, with many unusual or unique combinations of physical properties.1 Their low volatility2 has led to their widespread application in a variety of areas.3 Theoretically, ionic liquids are easily recycled with minimal volatile organic pollution4 and volatile solutes (such as reaction products) can be removed by suitable treatment in vacuo5 or with a flow of gas.6 Volatilisation of compounds from ionic liquids is therefore of widespread significance and importance. The low volatility of conventional ionic liquids also contributes to their apparent non-flammability in the bulk liquid state7 (within reason8). The volatilisation of ferrocene from both the solid state9,10 and ionic liquids11–13 has been noted. A methodology of investigating the volatilisation of ferrocene from the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C4mpyrr][NTf2], has recently been developed.14 Indirect voltammetric monitoring of the ferrocene concentration with mathematical modelling of the diffusion to the interface highlighted that the volatilisation of ferrocene under a flow of dry nitrogen gas was kinetically limited (1st order with k = 1.6 107 cm s1 at 298 K) with a relatively high activation energy of volatilisation (85 2 kJ mol1).14 In this communication the physical process is probed mechanistically in a range of ionic liquids, and the activation energy of volatilisation found to be related to the surface tension of the ionic liquid. A 10.2 mm diameter platinum microelectrode was positioned facing up and a cylindrical collar fashioned from a modified, disposable micropipette tip. 15 mL of ionic liquid (containing ca. 10 mM ferrocene) was then deposited on top of the
microelectrode, where it formed a well defined, approximately cylindrical body of liquid. This was housed in a specially designed glass ‘T-cell’ (for full experimental details please see ref. 14). Dry nitrogen gas was then flowed through the cell at a rate of 150 cm3 min1, and the entire experimental set-up was thermally controlled and equilibrated. Periodic measurement of the system was performed, using cyclic voltammetry and chronoamperometry. Simulation of the results using established methods15,16 provided simultaneous concentration and diffusion coefficient values for ferrocene in the ionic liquid. Fig. 1 displays cyclic voltammograms recorded for an initial concentration of 11.1 mM ferrocene in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide, [C2mim][NTf2], as a function of time. It can clearly be observed that the limiting current for ferrocene oxidation decreases as a function of time due to loss of ferrocene. Experiments using 10 mM solutions of cobaltocenium hexafluorophosphate, a non-volatile compound, demonstrated no significant changes in the voltammetry or limiting current during similar experiments, demonstrating that the decrease for ferrocene corresponds to its volatilisation at the ionic liquid–gas interface, as opposed to any other mechanisms such as electrode fouling. This is in good agreement with previous voltammetric11–13 and spectroscopic13 observations highlighting the volatilisation of dissolved ferrocene from ionic liquids.
a
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK. E-mail:
[email protected] b School of Chemistry and Chemical Engineering, The QUILL Centre, Queen’s University Belfast, Belfast BT9 5AG, UK c Department of Chemistry, Faculty of Science, University of Malaya, 50603, Kuala Lumpur, Malaysia w Electronic supplementary information (ESI) available: CVs in the different ionic liquids as a function of temperature. See DOI: 10.1039/ c1cc12336j
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Fig. 1 Cyclic voltammetry displaying the oxidation of ferrocene (initial concentration 11.1 mM) for 15 mL [C2mim][NTf2] on a 10.2 mm platinum microelectrode at 310.15 K, after (a) 300, (b) 540 and (c) 780 min flow of dry nitrogen.
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Mathematical modelling was then used to extract to rate of volatilisation from the electrochemical data. Full details of the model and fitting techniques can be found in ref. 14. Briefly, the decrease in ferrocene at the electrode|ionic liquid interface can be related to the rate of volatilisation at the ionic liquid|gas interface. Volatilisation of ferrocene was assumed to take place as a first-order reaction with the rate constant k (eqn (1)). The resulting change in concentration of ferrocene throughout the body of liquid with length L was then solved using Fick’s 2nd Law of Diffusion (eqn (2)). The equation set was solved numerically by discretising with a fully implicit centrally differenced finite difference method across a regular space and time grid, and solving the discretised equations using the Thomas (tri-diagonal matrix) algorithm. @½Fc D ¼ k½Fc ð1Þ @x x¼L @½Fc @ 2 ½Fc ¼D @t @x2
ð2Þ
As the length, L, the diffusion coefficient of ferrocene, D, initial ferrocene concentration, [Fc]0, and ferrocene concentration at the electrode as a function of time, [Fc], are all known from the initial experimental design and subsequent electrochemical measurements, the unknown parameter k could be derived by simulation. Fig. 2(a) displays a plot of the electrochemically measured ferrocene concentration (symbols, normalised with respect to the [Fc]0 used for the simulation) as a function of time and
Fig. 2 Experimental (symbols) and simulated (—) concentration profiles for ferrocene in 15 mL of (a) [C2mim][NTf2] and (b) [C4mim][BF4] over the temperature range of 298.15–310.15 K. Also displayed is the simulation for ferrocene in [C2mim][NTf2] at 298.15 K where k has been (---) increased and ( ) decreased by 0.2 107 cm s1.
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recorded at four temperatures in [C2mim][NTf2]. Concentration and diffusion coefficient values were determined simultaneously by fitting measured chronoamperograms with the Shoup and Szabo equation.15,16 Ionic liquids are known to rapidly accumulate moisture5 and dissolved gases11 under ambient conditions, which results in a reduction of viscosity and therefore an increase in solute diffusion coefficients.17 A flow of dry gas such as nitrogen results in rapid removal of these contaminants from the ionic liquid.6 As moisture-contamination was unavoidable during this work while dealing with mL-quantities of ionic liquid, the diffusion coefficient decreased for the first ca. 100–200 min of dry gas flow, corresponding to gradual moisture removal. However, after this period D stabilised, and Fig. 2 displays D values that are the average of the values recorded for the (dried) ionic liquid. Simulation of the trend in [Fc] vs. time was performed using the average measured D value, the known L value (80 mm) and by varying [Fc]0,sim and k in order to obtain the best fit.z Fig. 2(a) displays the best fits for the simulations over all temperatures in [C2mim][NTf2], as well as highlighting the effect of minor changes in k on the quality of the fit at 298.15 K. Similar data are also displayed in Fig. 2(b) for the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [C4mim][BF4]. The rate constants of volatilisation increased with temperature, and Fig. 3 displays an Arrhenius plot for the two ionic liquids. From these plots it was determined that the activation energy of volatilisation of ferrocene was 89 2 kJ mol1 in [C2mim][NTf2] and 97 1 kJ mol1 in [C4mim][BF4]. These are comparable to the value of 85 2 kJ mol1 for [C4mpyrr][NTf2], which was determined previously using a similar methodology.14 The activation energy of diffusion for the ferrocene in the ionic liquids, derived from Arrhenius plots using the obtained D values, gave 28.2,14 24.8 and 23.9 kJ mol1 for [C4mpyrr][NTf2], [C4mim][BF4] and [C2mim][NTf2], respectively. That the trend in activation energy of diffusion of ferrocene as a function of ionic liquid ([C4mpyrr][NTf2] c [C4mim][BF4] 4 [C2mim][NTf2]) does not correlate with the trend in activation energy of volatilisation ([C4mim][BF4] c [C2mim][NTf2] 4 [C4mpyrr][NTf2]) highlights that this process is kinetically controlled by the nature of the ionic liquid|gas interface and is not under diffusional control.
Fig. 3 Arrhenius plot of ln(k) vs. T1 over the temperature range 298.15–310.15 K for ferrocene volatilisation in [C4mim][BF4] (R2 4 0.99) and [C2mim][NTf2] (R2 4 0.97).
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concentration as a function of temperature and nitrogen gas flow time has yielded activation energies of volatilisation. The activation energies related to the surface tension of the ionic liquid, and a model has been proposed for this process.
Notes and references z It was necessary to make [Fc]0,sim slightly lower than the measured [Fc]0 as the simulation did not take into account the slightly elevated D values at short times. y Surface tension values at 293.15 K are taken from references [C4mim][BF4],21 [C4mpyrr][NTf2]22 and [C2mim][NTf2].23 Surface tension changes less than 2% within the 12 K range were investigated. The Ea value for [C4mpyrr][NTf2] has been taken from ref. 14. Fig. 4 (left) Diagram highlighting (a) ferrocene (Fc) moving to the interface from the bulk, (b) occupying a surface site, with the additional surface area to be generated upon volatilisation (–) highlighted, and (c) after volatilisation. (right) Plot of activation energy of volatilisation of ferrocene as a function of ionic liquid surface tension at 293.15 K.y
Several studies have highlighted that ionic liquid|gas and ionic liquid|vacuum interfaces possess one well-ordered layer of anions and cations at the interface, although techniques differ in their assignment of the precise molecular orientations.18 However, it is clear that in order to volatilise, ferrocene molecules have to approach the ionic liquid|gas interface, and the limiting step corresponds to displacement of ions at the structured interface, followed by an increase in the surface area of the interface upon volatilisation. This can be approximated by eqn (3), Ea = DU + 2pNArlg
(3)
where DU corresponds to the internal energy per mole required to exchange with ionic liquid at the interface with a ferrocene molecule from the bulk (and lead to partial or likely near total desolvation of the ferrocene), NA is the Avogadro constant, g is the surface tension of the ionic liquid, and 2pNArlg corresponds to the energy associated with the additional surface area generated by a cylindrical hole of radius r and length l (per mole) in the surface of the ionic liquid phase. This is shown schematically in Fig. 4 (left). Fig. 4 (right) displays a plot of activation energy vs. surface tension for the Ea values determined in this and other work.14 The equation of the line drawn is Ea (kJ mol1) = 51.5 (1.3 kJ mol1) + 1.01 106 (0.03 m2 mol1) g (kJ m2) showing that both terms in eqn (3) contribute significantly to Ea. While DU is likely to change from one ionic liquid to another, if it is assumed approximately constant for the data in Fig. 4 (right), a value for the additional surface area of 1.7 1018 m2 per ferrocene molecule volatilised is obtained. Simplifying the surface vacancy to a cylinder with 2r = l gives a radius, r, of ca. 3.6 A˚ in reasonable agreement with known solvodynamic radii in ionic liquids19 and in crystalline ferrocene.20 The volatilisation of ferrocene has been investigated in a range of ionic liquids. Modelling of the decrease in ferrocene
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1 D. S. Silvester and R. G. Compton, Z. Phys. Chem., 2006, 220, 1247–1274. 2 J. M. S. S. Esperanca, J. N. C. Lopes, M. Tariq, L. M. N. B. F. Santos, J. W. Magee and L. P. N. Rebelo, J. Chem. Eng. Data, 2010, 55, 3–12. 3 S. Kuwabata, T. Tsuda and T. Torimoto, J. Phys. Chem. Lett., 2010, 1, 3177–3188. 4 T. Welton and P. Wasserscheid, Ionic liquids in synthesis, WileyVCH, Weinheim [Cambridge], 2003. 5 A. M. O’Mahony, D. S. Silvester, L. Aldous, C. Hardacre and R. G. Compton, J. Chem. Eng. Data, 2008, 53, 2884–2891. 6 C. Zhao, A. M. Bond, R. G. Compton, A. M. O’Mahony and E. I. Rogers, Anal. Chem., 2010, 82, 3856–3861. 7 K. Ui, K. Yamamoto, K. Ishikawa, T. Minami, K. Takeuchi, M. Itagaki, K. Watanabe and N. Koura, J. Power Sources, 2008, 183, 347–350. 8 D. M. Fox, J. W. Gilman, A. B. Morgan, J. R. Shields, P. H. Maupin, R. E. Lyon, H. C. De Long and P. C. Trulove, Ind. Eng. Chem. Res., 2008, 47, 6327–6332. 9 G. Wilkinson, M. Rosenblum, M. C. Whiting and R. B. Woodward, J. Am. Chem. Soc., 1952, 74, 2125–2126. 10 V. N. Emel’yanenko, S. P. Verevkin, O. V. Krol, R. M. Varushchenko and N. V. Chelovskaya, J. Chem. Thermodyn., 2007, 39, 594–601. 11 L. E. Barrosse-Antle, L. Aldous, C. Hardacre, A. M. Bond and R. G. Compton, J. Phys. Chem. C, 2009, 113, 7750–7754. 12 X. J. Huang, L. Aldous, A. M. O’Mahony, F. J. del Campo and R. G. Compton, Anal. Chem., 2010, 82, 5238–5245. 13 M. A. Vorotyntsev, V. A. Zinovyeva, D. V. Konev, M. Picquet, L. Gaillon and C. Rizzi, J. Phys. Chem. B, 2009, 113, 1085–1099. 14 C. Fu, L. Aldous, E. J. F. Dickinson, N. S. A. Manan and R. G. Compton, ChemPhysChem, 2011, DOI: 10.1002/ cphc.201100204. 15 D. Shoup and A. Szabo, J. Electroanal. Chem., 1982, 140, 237–245. 16 C. A. Paddon, D. S. Silvester, F. L. Bhatti, T. J. Donohoe and R. G. Compton, Electroanalysis, 2007, 19, 11–22. 17 U. Schro¨der, J. D. Wadhawan, R. G. Compton, F. Marken, P. A. Z. Suarez, C. S. Consorti, R. F. de Souza and J. Dupont, New J. Chem., 2000, 24, 1009–1015. 18 C. Aliaga, C. S. Santos and S. Baldelli, Phys. Chem. Chem. Phys., 2007, 9, 3683–3700. 19 M. C. Lagunas, W. R. Pitner, J. A. van den Berg and K. R. Seddon, ACS Symp. Ser., 2003, 856, 421–438. 20 P. Seiler and J. D. Dunitz, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1979, 35, 2020–2032. 21 M. G. Freire, P. J. Carvalho, A. M. Fernandes, I. M. Marrucho, A. J. Queimada and J. A. P. Coutinho, J. Colloid Interface Sci., 2007, 314, 621–630. 22 C. Kolbeck, J. Lehmann, K. R. J. Lovelock, T. Cremer, N. Paape, P. Wasserscheid, A. P. Froba, F. Maier and H. P. Steinruck, J. Phys. Chem. B, 2010, 114, 17025–17036. 23 P. J. Carvalho, M. G. Freire, I. M. Marrucho, A. J. Queimada and J. A. P. Coutinho, J. Chem. Eng. Data, 2008, 53, 1346–1350.
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