11888
2007, 111, 11888-11891 Published on Web 09/21/2007
Anomalous Diffusion of Water in [BMIM][TFSI] Room-Temperature Ionic Liquid Anne-Laure Rollet,*,† Patrice Porion,‡ Michel Vaultier,| Isabelle Billard,§ Michael Deschamps,† Catherine Bessada,† and Laurence Jouvensal⊥ Centre de Recherche sur les Mate´ riaux a` Haute Tempe´ rature (CRMHT) - CNRS 1D aVenue de la Recherche Scientifique, 45071 Orle´ ans Cedex 2 France, Centre de Recherche sur la Matie` re DiVise´ e (CRMD) - CNRS 1bis rue de la Fe´ rollerie, 45071 Orle´ ans Cedex 2 France, Institut de Recherches Subatomiques de Strasbourg (IReS) - IN2P3, UMR 7500, CNRS/IN2P3 - ULP, Chimie Nucle´ aire, Bat. 35, B.P. 28, 67037 Strasbourg Cedex 2, France, UniVersite´ de Rennes 1, CNRS UMR 6510 (SESO), Campus de Beaulieu, 35042 Rennes Cedex France, and Centre de Biophysique Mole´ culaire (CBM), CNRS, rue Charles Sadron, 45071 Orle´ ans Cedex 2 France ReceiVed: July 10, 2007; In Final Form: August 29, 2007
We have studied the self-diffusion properties of butyl-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide ([BMIM][TFSI]) + water system. The self-diffusion coefficients of cations, anions, and water molecules were determined by pulsed field gradient NMR. These measures were performed with increased water quantity up to saturation (from 0.3 to 30 mol %). Unexpected variations have been observed. The self-diffusion coefficient of every species increases with the quantity of water but not in the same order of magnitude. Whereas very similar evolutions are observed for the anion and cation, the increase is 25 times greater for water molecules. We interpret our data by the existence of phase separation at microscopic scale.
Introduction The room-temperature ionic liquids (RTILs) field has widely expanded since the early 1990s.1,2 Though their discovery dates back to 1914, they have been overlooked by scientists until synthetic organist chemists identified them as a new class of solvents. Since that moment, synthesis of new RTILs as well as their use as solvents,3 reactants, and catalysts4 exponentially increased. They have now applications in materials synthesis,5 biocatalysis,6 electrochemistry,7,8 extraction,9,10 etc. However, there is a need to collect data on the physical and chemical properties of RTILs because of a lag between the understanding of these solvents and their uses. For example, their sensibility to the presence of additional compounds such as water11 is known but few studies attempt to explain it. As a consequence, important discrepancies occur in literature data because of the presence of neglected impurities in studied samples.12 In this paper, we have studied how water influences the properties of one of the most used systems, the butyl-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide, [BMIM][TFSI]. Its chemical formula writes as follows:
Moreover, water is an omnipresent compound on the one hand because of the intrinsic hygroscopy of these systems and on * To whom correspondence should be addressed. E-mail: rollet@ cnrs-orleans.fr. † Centre de Recherche sur les Mate ´ riaux a` Haute Tempe´rature (CRMHT). ‡ Centre de Recherche sur la Matie ` re Divise´e (CRMD). § Institut de Recherches Subatomiques de Strasbourg (IReS). | Universite ´ de Rennes 1. ⊥ Centre de Biophysique Mole ´ culaire (CBM).
10.1021/jp075378z CCC: $37.00
the other hand because many of the processes (synthesis, extraction, etc.) involve water. We have chosen to address this issue of water impact by focusing on the self-diffusion of every species of the [BMIM][TFSI] + water mixture. Self-diffusion represents the ability of a particle to move inside its environment. This fundamental quantity is therefore involved in every transport equation either individual or collective. Among all the experimental techniques allowing us to get at the self-diffusion coefficients, NMR pulsed field gradients is probably one of the most convenient and reliable. Indeed, it is noninvasive, the system is kept at equilibrium, and different elements of the system can be measured selectively. In our case, the selfdiffusion coefficient of BMIM cation and water molecules was determined via 1H and the one of TFSI anion via 19F. Experimental Methods The pulsed gradient spin-echo NMR (PGSE NMR) method was used to measure the 1H (cation and water) and 19F (anion) self-diffusion coefficients. Because of the difference between the transverse relaxation time (T2) and the longitudinal relaxation time (T1) (the T2 values are always shorter than T1), we have used a modified stimulated spin-echo sequence13 based on the Cotts et al. sequence14 to improve the measurements. Selfdiffusion coefficients were calculated by measuring the decrease in the NMR echo signal intensity through increasing magnetic field gradients. The self-diffusion coefficients were obtained by nonlinear least-square fitting of the echo attenuation E(q, ∆) as
E(q, ∆) ) I(q, ∆)/I(0, ∆) ) exp[-4π2q2D(∆ - δ/3 - τ/2)] where I(q, ∆) and I(0, ∆) are the echo intensities, respectively, measured with and without the field gradient; q ) γgδ/2π, where γ is the gyromagnetic ratio of the nuclei, g is the intensity © 2007 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 111, No. 41, 2007 11889
TABLE 1: Self-Diffusion Coefficients of BMIM, TFSI, and H2O water molar fraction
Dcation m2/s (1H)
Danion m2/s (19F)
Dwater m2/s (1H)
0.03 ()W0) 0.12 0.22 0.3
2.3 × 10-11 2.6 × 10-11 2.7 × 10-11 2.9 × 10-11
2.0 × 10-11 2.3 × 10-11 2.5 × 10-11 2.6 × 10-11
3.5 × 10-11 13.4 × 10-11 20.6 × 10-11 26.8 × 10-11
of the applied magnetic field gradient, δ is its duration, ∆ is the diffusing time, D is the self-diffusion coefficient, and τ is a gradient delay. All the 1H self-diffusion measurements were performed on a DSX100 Bruker spectrometer equipped with a microimaging probe (Micro5 Bruker), the maximum gradient value was 1.2 T/m, and all the data were recorded for ∆ ) 50 ms and δ ) 5 ms. The 19F measurements were done on a DSX400 Bruker spectrometer on a 10 mm liquid probe (1H19F/X), the maximum gradient value was 0.55 T/m, and all the measurements were performed for ∆ ) 50 ms and δ ) 4 ms. All the experiments were conducted at room temperature (298 K). After [BMIM][TFSI] synthesis, water has been carefully removed by pumping the sample under vacuum at 50 °C. Increased water amounts have been then added to the samples. Finally, NMR tubes have been sealed to prevent sample evolution over time. Experiments were performed several days after sample preparation to ensure the equilibrium state of the sample. Indeed, the demixion occurs slowly in this system as it has been also observed by numerical simulations.15
Figure 1. Relative self-diffusion coefficients D(W)/D(W0) have been plotted as a function of water molar fraction W. W0 is the lowest water amount used in this work. Danion is represented by triangles and Dcation by diamonds.
Results and Discussion All the data are gathered in Table 1. First, the self-diffusion of both anion and cation in the system of lowest water amount is very slow. The pertaining coefficients are 2 orders of magnitude lower than the one of bulk water (2 × 10-9 m2/s). This result is in agreement with previous works16,17 on this very system or similar systems.18,19 Moreover, these results are consistent with the [BMIM][TFSI] viscosity12 that is 0.0635 Pa compared to 0.001 Pa for bulk water. Second, the anion and cation have close self-diffusion coefficients despite their very different size and shape. This phenomenon arises from the fact that BMIM and TFSI are strongly associated and therefore partially move together. Through the assumption of a fast exchange between free and associated species at NMR times scale, the self-diffusion coefficient of one species can thus be written as
Dmeasured ) RDfree + [1 - R]Dassociated where Dfree represents the self-diffusion coefficient when the species moves alone, Dassociated is when it moves together with the counterion, and R is the proportion of free species. [BMIM][TFSI] is known as being slightly hygroscopic and weakly miscible with water. At 298 K and atmospheric pressure, the maximal quantity of water in [BMIM][TFSI] is about 1.3 mass %.20,21 Usually and also in this work, the water amount is determined using Karl Fischer titration. This method gives the overall amount of water but no insight of the mixture microstructure. The water addition in [BMIM][TFSI] decreases the viscosity.22,12 This effect has been attributed to the screening of electrostatic interactions by water, which induces a decrease of RTIL cohesion.11 In this work, we have measured the selfdiffusion coefficient of every species of the system [BMIM][TFSI] with increased amount of water W. As presented in Figure 1 where D(W)/D(W0) is plotted against W, the variation
Figure 2. Relative self-diffusion coefficients D(W)/D(W0) have been plotted as a function of water molar fraction W. W0 ) 0.03 is the lowest water amount used in this work. Dwater is represented by circles, Danion by triangles, and Dcation by diamonds. The dotted line represents the relative self-diffusion coefficients of water calculated using the StokesEinstein relation.
for both anion and cation are very similar, almost superimposed. This is somehow surprising because water is expected to rise the ion pair dissociation rate and as a consequence to cause a greater increase of Danion as compared to Dcation with respect to their size (Rg anion < Rg cation). At the largest water amount, Danion and Dcation are larger by a factor 1.3 than at the lowest water amount. The amplitude of this variation is very close to the decrease of viscosity12, that is, around 30%, observed in the same conditions. These results indicate that the addition of water in [BMIM][TFSI] system does not lead to a significant increase of anion-cation pair dissociation, that is, to R. Water appears to interact quite weakly with ions pairs. Recent works23 suggest that water content is dependent on the interaction between the anion and water. The hydrophobicity of TFSI anions prevents water from dissociating ions pairs and being incorporated in RTILs in large amount. We have also measured the self-diffusion coefficient of water Dwater using the 1H nucleus. Here appears the striking result of this study: the evolution of Dwater is completely out of comparison with that of the ions. As it can be seen in Figure 2, Dwater rises 25 times quicker. What hypothesis can be put forward to explain this phenomenon? The thesis of the fluidizing effect of water can be rejected because if it was valid, all the self-diffusion coefficients (water and ions) would have been affected in the same order of magnitude. A second hypothesis would be to think of water as being linked to ions at low water amount; then, when increasing
11890 J. Phys. Chem. B, Vol. 111, No. 41, 2007
Figure 3. From left to right, artistic view of the microstructure of [BMIM][TFSI] + water systems for increased amount of water. RTIL is represented in gray and water in white.
the water amount water would be released and could thus diffuse much more rapidly. Such a hypothesis implicates the following evolution of Dwater with 2 domains: in the small water amount, Dwater should increase rapidly along with W and then reach a pseudo-plateau where Dwater should increase in the same order of magnitude as Dcation and Danion. Indeed, one can write the measured Dwateras free Dwater ) b(W)Dlinked water + [1 - b(W)]Dwater
where b(W) represents the proportion of bounded water molecules. Dlinked water is approximately equal to Dcation and Danion. can be estimated thanks to the Stokes-Einstein relaDfree water tion24,25 that links self-diffusion coefficient to viscosity. The -11 m2/s, which is much slower than Dfree water values are about 10 our experimental values. In Figure 2, the variation of Dwater(W)/ Dwater(W0) versus W, calculated using the Stokes-Einstein relation and assuming that the whole water is free, is plotted for comparison. This figure clearly demonstrates that this second hypothesis can also be rejected. The hypothesis we suggest to explain the particular evolution of the three species in [BMIM][TFSI] + water systems as a function of water amount W is that water and ions do not occupy the same domains. In other words, water is not homogenously mixed with RTIL but forms small aggregates whose size and connections increase with W. Figure 3 offers an artistic view of this phenomenon. Recent numerical simulations on this have shown the existence of a local water “pool” in several RTILs26,23 and particularly in [BMIM][TFSI].15 Our results show that these pools are connected and form a porous network at the local scale. According to Sieffert and Wipff,15 water molecules are generally hydrogen-bonded to two TFSI anions so as to form pore walls richer in TFSI than BMIM. Moreover they also found that this H-bond is stronger and shorter than the one between BMIM and water despite that BMIM may have long lifetime hydration shell at high dilution27 (in the [BMIM][Br] + H2O system). When increasing the water amount, the tortuosity of aqueous network is decreased. At 30% of water, the tortuosity28 θ is estimated using Dwater ) θ-1 Dbulk water and is about 10. Such values are comparable to those obtained in porous systems like Nafion ionomer membranes where the tortuosity is about 10 for 30% water volume fraction;29 one can notice that 30 mol % of water in [BMIM][TFSI] corresponds approximately to 8% in volume fraction indicating a great connectivity of the aqueous network. Above 30 mol % of water, macroscopic demixion occurs between water phase and RTIL phase. The method of pulsed field gradients NMR allows us to investigate diffusion phenomena in the time range from tens of milliseconds to a few seconds (time range depends on the sample), and the corresponding distances are about a micrometer. By varying the observation time ∆, one can determine the characteristic size of the pores or the pseudopores (heterogeneity of pores organization as may occur in polymer systems for example). If the volume explored by the probe is bigger than this characteristic size, no variation of D versus ∆ is observed; in the reverse case, D decreases versus ∆. We have performed a series of D measurements with ∆
Letters ranging from 50 to 400 ms on the sample with the highest water content where the pores size is the biggest. No variation of D versus ∆ is observed indicating than the characteristic size is much smaller than λ ) (2D∆)1/2 ) 5.2 µm. The porous network formed by water in [BMIM][TFSI] can be compared to bicontinuous phases (oil + surfactant + water) and could be defined as one of a particular kind. Indeed, the segregation between oil and water in the three-component bicontinuous phases is high and consequently the self-diffusion coefficients of oil and water are proportional to the phase volume each element occupies.30 Thus, the self-diffusion coefficient of water increases along with the water amount, whereas the oil self-diffusion coefficient decreases. In the [BMIM][TFSI] + water system, there is an affinity between the two components, as shown by the hygroscopy of [BMIM][TFSI]. An increase of Dcation and Danion versus water amount is observed that indicates the presence of water molecules in the RTIL phase, that is, a partial phase segregation. Conclusion We have measured the self-diffusion coefficients of BMIM, TFSI, and water in [BMIM][TFSI] + water system to study the modification of translational dynamics properties of this RTIL when water is added. The variation of the self-diffusion coefficients versus water amount indicates that water does not induce a significant increase of the ion pair dissociation but disturbs the RTIL cohesion. Moreover, whereas very similar evolutions are observed for anion and cation (increase of 30%), in the same range of water molar fraction the increase of Dwater is 25 times greater. It indicates that miscibility of water is not complete at the microscale and that the [BMIM][TFSI] + water system shows a partial segregation between [BMIM][TFSI] + some water molecules phase and water + some [BMIM][TFSI] ions phase. Acknowledgment. The authors thank Herve´ Meudal, AnneMarie Fauge`re, Alfred Delville, and Joe¨l Puibasset for their helpful discussions. This work has received support from GDR PARIS and ANR JCJC. References and Notes (1) Welton, T Chem. ReV. 1999, 99, 2071-2083. (2) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 13911398. (3) Pandey, S. Anal. Chim. Acta 2006, 556, 38-45. (4) Dupont, J.; de Souza, R. F.; Suarez P. A. Z. Chem. ReV. 2002, 102, 3667-3692. (5) Carmichael, A. J.; Haddleton, D. M. Polymer Synthesis in Ionic Liquids. In Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, 2003; pp 319-335. (6) Yang, Z.; Pan, W. B. Enzyme Microb. Technol. 2005, 37, 19-28. (7) Lagrost, C.; Hapiot, P.; Vaultier, M. Green Chem. 2005, 7, 468474. (8) Asanuma, N.; Harada, M.; Yasuike, Y.; Nogami, M.; Suzuki, K.; Ikeda, Y. J. Nucl. Sci. Technol. 2007, 44, 368-372. (9) Zhao, H.; Xia, S. Q.; Ma, P. S. J. Chem. Technol. Biotechnol. 2005, 80, 1089-1096. (10) Sieffert, N.; Wipff, G. J. Phys. Chem. A 2006, 110, 1106-1117. (11) Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 2275-2287. (12) Widegren, J. A.; Laesecke, A.; Magee, J.; Magee, W. Chem. Comm. 2005, 1610-1612. (13) Wu, D. H.; Chen, A. D.; Johnson, C. S. J. Magn. Reson., Ser. A 1995, 115, 260. (14) Cotts, R. M.; Hoch, M. J. R.; Sun, T.; Markert, J. T. J. Magn. Reson. 1989, 83, 252-266. (15) Sieffert, N.; Wipff, G. J. Phys. Chem. B 2006, 110, 13076-13085. (16) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593-16600.
Letters (17) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103-6110. (18) Every, H. A.; Bishop, A. G.; MacFarlane, D. R.; Ora¨dd, G.; Forsyth, M. Phys. Chem. Chem. Phys. 2004, 6, 1758-1765. (19) Bagno, A.; D’Amico, F.; Saielli, G. J. Mol. Liq. 2007, 131, 1723. (20) Heintz, A.; Lehmann, J. K.; Wertz, C.; Jacquemin, J.; J. Chem. Eng. Data 2005, 50, 956-960. (21) Crosthwaite, J. M.; Aki, S. N. V. K.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2004, 108, 5113-5119. (22) Pandey, S.; Fletcher, K. A.; Baker, S. N.; Baker, G. A. Analyst 2004, 129, 569-573. (23) Jiang, W.; Wang, Y.; Voth, G. A. J. Phys. Chem. B 2007, 111, 4812-4818.
J. Phys. Chem. B, Vol. 111, No. 41, 2007 11891 (24) Einstein, A. InVestigations on the Theory of Brownian Motion; Dover: New York, 1956. (25) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed. (revised); Butterworth: London, 1965. (26) Hanke, G. C.; Lynden-Bell, R. M. J. Phys. Chem. B 2004, 108, 10873-10878. (27) Nakakoshi, M.; Ishihara, S.; Utsumi, H.; Seki, H.; Koga, Y.; Nishikawa, K. Chem. Phys. Lett. 2006, 87-90. (28) Latour, L. L.; Kleinberg, R. L.; Mitra, P. P.; Sotak., C. H. J. Magn. Reson., Ser. A 1995, 112, 83-91. (29) Rollet, A.-L.; Simonin, J.-P.; Turq, P. Phys. Chem. Chem. Phys. 2000, 2, 1029-1034. (30) Boonme, P.; Krauel, K.; Graf, A.; Rades, T.; Junyaprasert, V. B. AAPS PharmSciTech. 2006, 7, Article 45.
