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J. Phys. Chem. B 2007, 111, 6452-6456
Effect of Water and Organic Solvents on the Ionic Dissociation of Ionic Liquids Wenjing Li, Zhaofu Zhang, Buxing Han,* Suqin Hu, Ye Xie, and Guanying Yang Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
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ReceiVed: February 6, 2007; In Final Form: April 13, 2007
The effect of water and several organic solvents on the density, viscosity, and conductivity of ionic liquids (ILs) 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1-n-butyl-3-methylimidazolium trifluoroacetate ([bmim][CF3CO2]) was studied at 298.15 K in wide composition ranges. The density, viscosity, and conductivity of the three neat ILs were also determined at various temperatures. Upon the basis of the molar conductivity of the mixtures and that of the neat ILs of the same viscosity, the degree of dissociation of ILs in the solutions was investigated. It can be deduced that the organic solvents enhance the ionic association of the ILs, the effect depending on the solvent dielectric constant, while water promotes dissociation significantly due to its high dielectric constant and its ability to form strong hydrogen bonds with the anions of the ILs.
Introduction Ionic liquids (ILs) are organic salts that are liquids at temperatures below 100 °C. They have received considerable attention as alternatives to the traditional organic solvents. Because of their interesting physical and chemical properties, such as negligible vapor pressure, unique permittivity, high thermal stability, good solvents for both organic and inorganic substances, high electrical conductivity, and wide electrochemical window, ILs have been widely used as reaction media, separation solvents, and novel electrolytes.1-4 Due to the increasing interest in application in electrochemistry, many studies on the fundamental electrochemical properties of ILs have been carried out. The electrical conductivity of some pure ILs has been studied over a wide temperature range.5-8 It is therefore of importance to study the conductivity of mixtures of ILs with other solvents to expand the utility of ILs. Recently, studies on mixtures containing ILs have become more and more attractive and many useful data of the physical properties of the mixtures have been reported. For example, to improve the safety of lithium batteries, binary mixtures of lithium salts with ILs have been applied as electrolytes and the ionic conduction and ion diffusion in the mixtures have been investigated.9,10 It has been indicated that cosolvents can strongly affect the physical properties of ILs, such as viscosity,11-13 polarity,14 and solvation properties.15,16 The electrical conductivities of mixtures of water and organic solvents with ILs have also been studied,17-22 and the molar conductivity of ILs in solutions has been found to vary exponentially with the mole fraction of added solvents.22 However, the effect of common solvents on the ionic association behavior of ILs is not clear, though association of ILs has been shown by experimental and simulation studies to occur both in neat ILs and in solutions.23-25 To the best of our knowledge, only two reports have appeared on the ionic association behavior of ILs in mixtures. Watanabe et al.26 determined the conductivities and self-diffusion coefficients of the N,N-diethyl-N-2-methoxyethyl-N-methyl am* To whom correspondence should be addressed. E-mail: hanbx@ iccas.ac.cn.
monium ion with different anions and the mixtures formed from the ILs and propylene carbonate or 1,2-dichloroethane. They found that the two solvents could increase the ionic diffusion to a significant extent. From the conductivity and self-diffusion coefficients, they deduced that both of the solvents would promote the ionic association of the ILs. Another study has revealed that the ionic association of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) remains almost unchanged in the presence of water up to a mole fraction of 0.25.27 In this work, the density, viscosity, and conductivity of [bmim][PF6], 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1-n-butyl-3-methylimidazolium trifluoroacetate ([bmim][CF3CO2]) and their solutions formed with some solvents have been studied under different conditions. The effect of these solvents on the association behavior of the ILs has been investigated. Experimental Section Materials. All organic solvents (A.R. grade) were supplied by Beijing Chemical Reagent Factory and distilled before use. 1-Methylimidazole was purchased from Acros Organics. Potassium tetrafluoroborate, potassium hexafluorophosphate, and potassium trifluoroacetate were provided by Alfa Aesar. [bmim][BF4], [bmim][PF6], and [bmim][CF3CO2] were synthesized and purified following procedures reported in the literature.3,28,29 The ILs were dried under vacuum at 70 °C for at least 48 h prior to use. The water content in the ILs was below 50 ppm, as analyzed by Karl Fischer titration, while the chloride content was less than 0.016 mol kg-1, as determined by a chloride-selective electrode.11,30 Density Measurements. All of the solutions were prepared by mass using a DT-100 balance with an accuracy of (0.1 mg (Shanghai Science Instrument Company). At least 1.0 g of the studied compounds was weighed each time. The density, viscosity, and conductivity were determined using the same samples to ensure an identical composition for the studies. The densities of the neat ILs and their mixtures were measured using a capillary pycnometer which was calibrated with pure water.
10.1021/jp071051m CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
Ionic Dissociation of Ionic Liquids
J. Phys. Chem. B, Vol. 111, No. 23, 2007 6453
Figure 2. Viscosity of ILs as a function of temperature.
Figure 1. Density of ILs as a function of temperature.
It was estimated that the accuracy of the density data was better than (0.0002 g‚cm-3. Viscosity Measurements. The viscosity was determined by using Ubbelohde viscometers, a reliable and conventional method. The viscometers were calibrated using standard oils of different viscosities provided by the National Standard Bureau of China. The temperature of the water bath was controlled by a Haake D8 controller. The desiccant-filled tube was rapidly connected to the viscometer to prevent the ILs from atmospheric moisture. The dynamic viscosity of the liquid, η, was determined by the following equation:
η/F ) Ct
(1)
where F stands for the density of the liquid, C is a constant related to the viscometer and determined by calibration, and t is the flow time. The densities of the neat ILs and their mixtures were measured using a capillary pycnometer as described above. The estimated accuracy of the viscosity measurements was (1%. Conductivity and Density Measurements. The apparatus and procedures for conductivity measurements were similar to those reported previously.20 Briefly, the experiments were performed using a conductivity meter produced by Shanghai Precision Scientific Instrument Co., LTD (model DDS-307). The sample and the electrode were sealed in a glass cell and placed in a constant temperature water bath. The cell constant was determined with 0.0200, 0.0400, 0.0600, 0.0800, 0.1000, 0.1500, and 0.2000 mol‚L-1 aqueous KCl solutions according to IUPAC recommendations.31 The repeatability and the estimated accuracy of the measurements were (0.3 and (1%, respectively. Results and Discussion Temperature Dependency of Physical Properties of the Neat ILs. The density, viscosity, and specific conductivity of the three neat ILs were carefully measured between 298.15 and 353.15 K. The molar conductivity of the ILs was calculated from the specific conductivity and the molar concentration. The temperature dependence of the density, viscosity, and molar conductivity for the ILs is shown in Figures 1-3. The data are given in the Supporting Information (Table S1). The results determined in this work agree well with those reported in the literature.6 The density of the ILs decreased linearly with increasing temperature, as shown in Figure 1. The density of different ILs at a fixed temperature decreased with the molecular weight of the anions, [PF6] > [CF3CO2] > [BF4]. Figures 2 and 3 show that the viscosity and the conductivity of the ILs are sensitive to variation in temperature. With increasing temperature, the viscosity dramatically decreased while the molar conductivity increased, showing strong cor-
Figure 3. Molar conductivity of ILs as a function of temperature.
relation between the viscosity and the conductivity of the ILs. The empirical Vogel-Tammann-Fulcher (VTF) equations have previously been used to describe the dependency of the viscosity and the conductivity on the temperature for neat ILs.5-8 The viscosity of the ILs studied in this work follows the order of [bmim][PF6] > [bmim][BF4] > [bmim][CF3CO2] at a fixed temperature in contrast to that of the molar conductivity. The Stokes-Einstein equation and the Nernst-Einstein equation are widely used to relate the viscosity and conductivity of electrolyte solutions.
kT cπηr
(2)
Ne2 (D + D-) kT +
(3)
D) Λ)
where D is the self-diffusion coefficient of the ionic species, k denotes the Boltzmann constant, c is a constant determined by the boundary conditions, r is the Stokes radius of the ion, N is Avogadro’s number, and e represents the electric charge. When ionic association takes place, the Nernst-Einstein equation is modified by adding the dissociation factor, a.
Λ)
Ne2 (D + D-)a kT +
(4)
where a is the degree of dissociation. Researchers have used the equations to discuss the diffusing and ionic conducting behavior in neat ILs.5-8 These studies have revealed that the self-diffusion coefficients of cations and anions of neat ILs are approximately proportional to Tη-1, indicating that the ionic diffusion in the neat ILs obeys eq 2. In other words, the viscous friction is the major force in impeding the motion of ions. It has been demonstrated that the degree of dissociation for neat ILs is almost independent of temperature.5-9 Therefore, the ionic conductivity of ILs is only affected by the viscosity. Figure 4 shows the relationship of the molar conductivity to the inverse of the viscosity for the ILs studied in this work. A similar linear plot of the molar conductivity versus the inverse of viscosity has been found for 1-ethyl-3-methylimidazolium tetrafluoroborate.9
6454 J. Phys. Chem. B, Vol. 111, No. 23, 2007
Li et al.
Figure 4. Molar conductivity of ILs as a function of the inverse of the viscosity.
Figure 6. Viscosity of mixtures as a function of the volume fraction of solvents, Φs: (a) [bmim][PF6]; (b) [bmim][BF4]; (c) [bmim][CF3CO2].
Figure 5. Viscosity of mixtures as a function of the mole fraction of solvents: (a) [bmim][PF6]; (b) [bmim][BF4]; (c) [bmim][CF3CO2].
Viscosity, Density, and Conductivity of Mixtures. Experimental values for the viscosity, density, and conductivity of the mixtures at 298.15 K are presented in the Supporting Information (Table S2). [bmim][PF6] is totally miscible with dichloromethane, ethyl acetate, pyridine, acetone, and acetonitrile, while chloroform is only miscible up to 0.75 mole fraction. The mole fraction of water in [bmim][PF6]-water solution was only up to 0.21 due to the limitation of its solubility in the IL.32 All of the selected solvents were completely miscible with [bmim][BF4] and [bmim][CF3CO2]. Figure 5 shows the change of the viscosity of the solutions with the mole fraction of solvents, xs; apparently, addition of the solvents resulted in a drastic reduction in the viscosity. Addition of a 0.2 mole fraction of acetonitrile reduced the viscosity of [bmim][PF6] from 261 mPa s to 92.9 mPa s.
As shown in Figure 6, when the viscosity is plotted against the volume fraction of solvents, Φs, the difference of the effects of different solvents on the viscosity is more obvious. The solvents of low dielectric constant, such as chloroform, have a small effect on the viscosity of the ILs, while the decrease of the viscosity is greater for solvents of higher dielectric constant, such as pyridine, acetone, actetonitrile, and water. This may be due to the difference in the ion-dipole interactions between the ions and solvents. Generally, polar or dipolar solvents of high dielectric constant show better miscibility with ILs while nonpolar solvents demonstrate poorer miscibility,33,34 indicating that polar solvents have stronger molecular interaction with ILs. Thus, solvents of high dielectric constant are more effective in reducing the electrostatic attraction between the ions of ILs and consequently reduce the viscosity more effectively than the solvents of lower dielectric constant. Recently, it has been found that the addition of water may change the molecular structure of ILs,35,36 probably due to hydrogen bonding between the water molecules and the anions of the ILs.37-39 It is not unexpected that water has the largest effect on the viscosity of the ILs in the IL-rich region considering the high dielectric constant of water and the ability to form hydrogen bonds. Relationship between Viscosity, Conductivity, and Association of the ILs. The dependence of the molar conductivity of ILs in different mixtures on the fluidity, η-1, is depicted in Figure 7. For comparison, Λ versus η-1 for the neat ILs are also plotted in the figure. The data were obtained from the conductivity and viscosity data in Supporting Information.
Ionic Dissociation of Ionic Liquids
J. Phys. Chem. B, Vol. 111, No. 23, 2007 6455 different mixtures with the same viscosity is not distinct and the plots of the mixtures almost fall on the same line with those of neat ILs. However, when more solvent was added and the viscosity of the mixture further decreased, the differences of the effects of solvents on the ion conduction became significant. Interestingly, all mixtures containing organic solvents gave lower molar conductivities than the corresponding neat IL systems, even for the highly polar acetonitrile, while the molar conductivities of the aqueous solutions were considerably higher than that of the neat ILs. For mixtures with organic solvents, the molar conductivity was mainly dependent on the solvent dielectric constant. The addition of solvents of low dielectric constant resulted in greater deviation from the molar conductivity of the corresponding neat ILs. It seems that all organic solvents used in this work tend to promote ionic association, in agreement with the findings of Watanabe et al.26 Water, however, significantly increases the degree of dissociation of the ILs. Apparently, the high dielectric constant of water is favorable to the dissociation of the ILs. Besides, it has been recognized that ILs form a three-dimensional network of anions and cations linked by hydrogen bonds, both in the solid and in the liquid states.40,41 We believe that by forming strong hydrogen bonds with the anions water will tend to separate the anions and cations in the aqueous solutions and break the aggregate structures of the ILs and thus promote dissociation of the ILs. It should be emphasized that the effect of water or organic solvents on the association of ILs is much more complex than discussed above. This interesting topic will be further studied. Conclusion
Figure 7. Molar conductivity as a function of the inverse of the viscosity for mixtures: (a) [bmim][PF6]; (b) [bmim][BF4]; (c) [bmim][CF3CO2].
As expected, the molar conductivity of the mixtures was found to increase with the increase of the fluidity. It has been known that the ionic diffusion in neat ILs is basically obeying the Stokes-Einstein equation, indicating the viscous friction to be the major force to impede the motion of ions. Therefore, for the IL-rich mixtures, with viscosities comparable to those of the neat ILs, the Stokes-Einstein equation can be used to relate the ionic conductivity with viscosity. Substituting eq 2 into eq 4 leads to the following equation:
Λ∝
(
)
a 1 1 + η r+ r-
(5)
Equation 5 indicates that the molar conductivity is related to three factors, a, η, and r. In order to study the effect of the solvents on the degree of dissociation of the ILs, we will assume that the effect of the solvents on r is not considerable in the linear region. That is, the molar conductivity of the ILs is mainly affected by the fluidity and the dissociation degree of the ILs in the system. In addition, the degree of association of neat ILs does not vary significantly with temperature.5-9 Therefore, a comparison of the molar conductivities of neat ILs with those of the ILs in mixtures at fixed viscosity can provide qualitative information about the change in ionic dissociation of ILs caused by the addition of the solvents. The following discussion is based on these assumptions. From Figure 7, it can be seen that when the amount of added solvent is small, the difference in the molar conductivities of
The density, viscosity, and conductivity of the mixtures formed from [bmim][PF6], [bmim][BF4], and [bmim][CF3CO2] with several common solvents were studied under different conditions. It was demonstrated that the solvents affect the density, viscosity, and conductivity significantly. The solvents of higher dielectric constant seem to have a larger effect on the viscosity and the conductivity of the solutions. From a comparison of the molar conductivity of the mixtures with that of neat ILs of the same viscosity, one may conclude that organic solvents enhance the ionic association of the ILs, the effect being dependent on the solvent dielectric constant, while water promotes significantly dissociation. Apparently, the high dielectric constant of water and its ability to form strong hydrogen bonds with the anions of the ILs cause the aggregates of the ILs to be effectively broken. Acknowledgment. The authors thank the National Natural Science Foundation of China (20533010). Supporting Information Available: The data of density, viscosity, and specific conductivity of the three neat ILs between 298.15 and 353.15 K and the experimental values for viscosity, density, and conductivity of the mixtures at 298.15 K. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071. (2) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (3) Ionic liquids in synthesis; Wasserscheid, P., Welton, T., Eds.; WileyVCH: Weinheim, Germany, 2003. (4) Reichardt, C. Org. Process Res. DeV. 2007, 11, 105. (5) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105, 4603. (6) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593.
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