Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Aprotic

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Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Aprotic Liquid Mixture: 1‑Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile Min Liang,† Xin-Xing Zhang,‡,§ Anne Kaintz,† Nikolaus P. Ernsting,‡ and Mark Maroncelli*,† †

Departments of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Humboldt Universität zu Berlin, Berlin 10099, Germany § College of Physical Science, Nankai University, Tianjin 300071, China ‡

S Supporting Information *

ABSTRACT: Solvation energies, rotation times, and 100 fs to 20 ns solvation response functions of the solute coumarin 153 (C153) in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) + acetonitrile (CH3CN) at room temperature (20.5 °C) are reported. Available density, shear viscosity, and electrical conductivity data at 25 °C are also collected and parametrized, and new data on refractive indices and component diffusion coefficients presented. Solvation free energies and reorganization energies associated with the S0 ↔ S1 transition of C153 are slightly (≤15%) larger in neat [Im41][BF4] than in CH3CN. No clear evidence for preferential solvation of C153 in these mixtures is found. Composition-dependent diffusion coefficients (D) of Im41+ and CH3CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as D, τ ∝ ηp with values of p = −0.88, −0.77, and +0.90, respectively. Spectral/ solvation response functions (Sν(t)) are bimodal at all compositions, consisting of a subpicosecond fast component followed by a broadly distributed slower component extending over ps-ns times. Integral solvation times (⟨τsolv⟩ = ∫ ∞ 0 Sν(t) dt) follow a power law on viscosity for mixture compositions 0.2 ≤ xIL ≤ 1 with p = 0.79. With recent broad-band dielectric measurements [J. Phys. Chem. B 2012, 116, 7509] as input, a simple dielectric continuum model provides predictions for solvation response functions that correctly capture the distinctive bimodal character of the observed response. At xIL ∼ 1 predicted values of ⟨τsolv⟩ are smaller than those observed by a factor of 2−3, but the two become approximately equal at xIL = 0.2. Predictions of a recent semimolecular theory [J. Phys. Chem. B 2011, 115, 4011] are less accurate, being uniformly slower than the observed solvation dynamics. using the fluorescence probe coumarin 153 (C153). This work complements a closely related study of dielectric relaxation and solvation in the ionic liquid + protic mixture [Im41][BF4] + H2O.5 Since the first study seeking to characterize [Im41][BF4] + CH3CN mixtures in 2003,6 many groups have reported on basic physical properties such as densities,6−13 viscosities,6,10,14,15 and electrical conductivies.16,17,10,14,13,15 Other, less studied properties include refractive indices9 and surface tensions.18 Spectroscopic studies of [Im41][BF4] + CH3CN mixtures have been limited, consisting of some measurements of “intrinsic” luminescence and infrared spectra in the CH stretching region,18 a Brillioun scattering study,9 and most recently, measurements of broad-band dielectric spectra.19 The last of these studies, by Buchner and co-workers,19 is especially pertinent to the present work due to the close relationship between dielectric relaxation and time-dependent solvation. Stoppa et al. found that permittivity and loss spectra

1. INTRODUCTION The distinctive properties of ionic liquids have made them attractive candidates for many applications, among them being use as solvents for synthesis1 and as electrolytes in electrochemical devices.2−4 In these applications, the high viscosities of ionic liquids is typically a disadvantage as it limits the rate of reactant and charge transport. Conventional organic solvents are therefore often mixed with ionic liquids to enhance fluidity. Understanding and predicting the properties of such mixtures is clearly important to their application, yet compared to the situation with neat ionic liquids, efforts to characterize the properties of ionic liquid + organic mixtures has so far been limited, and relatively few mixtures have been systematically studied from multiple perspectives. An important exception is the system 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) + acetonitrile (CH3CN). The relative simplicity of its constituents, together with complete miscibility near to room temperature, have made the [Im41][BF4] + CH3CN system the chief prototype of an ionic + dipolar aprotic liquid mixture for use in a variety of studies. The present contribution adds to the growing database on this exemplar, primarily through measurements of solvation and solvation dynamics © 2014 American Chemical Society

Received: December 10, 2013 Revised: January 14, 2014 Published: January 14, 2014 1340

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expect such “clustering” even in a densely packed random mixture. Nevertheless, these observations tend to support the idea that mixtures with xIL > 0.2 have the character of a bulk ionic liquid expanded and fluidized by CH3CN. Finally, we are aware of only two prior studies involving solvation in [Im41][BF4] + CH3CN mixtures.16,27 Both studies measured the solvatochromism of the UV absorption of N,Ndimethyl-4-nitroaniline and deduced the Kamlet−Taft π* polarity/polarizability parameter28 based on these measurements. Li et al.16 concluded from such data that no pronounced preferential solvation of this solute occurs. Mancini et al.27 used additional probe solutes to measure the Kamlet−Taft parameters α and β of several ionic liquid + dipolar solvent mixtures and used these parameters in a linear free energy analysis28 of a nucleophilic aromatic substitution reaction. They observed a 35% rate acceleration in the [Im41][BF4] + CH3CN mixture compared to the rate in neat solvents and suggested that this effect is due to the basicity of the BF4− anion relative to CH3CN. No measurements of nonreactive solute dynamics have been reported to date, but some work similar to that described here has been reported in other mixtures and summarized in our previous work.5 In the present contribution we report measurements of equilibrium and time-dependent solvation of the fluorescence probe coumarin 153 (C153) in [Im41][BF4] + CH3CN mixtures. This study builds upon our prior work using this well-known probe to characterize solvation in a variety of neat ionic liquids29,30 and the related mixture [Im41][BF4] + H2O.5 As in the more recent of these papers30,5 we combined two techniques, broad-band fluorescence upconversion spectroscopy (FLUPS) and time correlated single photon counting (TCSPC) to measure the complete (100 fs to 20 ns) solvation response. Our primary goals are to characterize the timedependent solvation response in this prototypical ionic liquid + dipolar aprotic solvent system, relate the observed dynamics to those in the neat component liquids, as well as to the recent dielectric relaxation measurements by Buchner and co-workers.19 Also of interest are comparisons to the presumably more structured mixture [Im41][BF4] + H2O.5 The remaining sections of this paper are structured as follows. After describing the experimental methods in section 2, we discuss our results in five parts. Section 3A first summarizes available experimental data on several physical properties of [Im41][BF4] + CH3CN mixtures at 25 °C. In this section we include measurements of refractive indices and component diffusion coefficients not previously reported. Sections 3B, C, and D, respectively, describe our results on equilibrium solvation, rotation, and solvation dynamics of C153 in these mixtures. Finally, section 3E discusses the accuracy of two models of solvation for relating solvation dynamics and dielectric relaxation in these mixtures.

recorded over the range 0.2−89 GHz could be fit in terms of two relaxation processes, a broadly distributed low-frequency mode attributed to coupled ionic and CH3CN motions, and a high-frequency Debye mode associated primarily with “free” CH3CN reorientations.19 Similar bimodal dynamics are found in the solvation response, and these same interpretations may also apply to the dynamics measured here. Stoppa et al. also proposed a description of the structural evolution of the mixture as a function of ionic liquid mole fraction, xIL. In their words “the ILs [ionic liquids] retain their chemical character up to high dilution with AN [CH3CN] (xIL ≲ 0.2). At xIL ≲ 0.2 the ILs behave like conventional, rather weakly associated electrolytes in AN. Consistent with the relatively weak solvation of both ions by AN, only CIPs [contact ion pairs] appear to be formed, becoming dominant at xIL ≲ 0.05. In essence, IL + AN mixtures can be divided into two regions. At low IL concentrations they behave as conventional electrolyte solutions, whereas at higher concentrations they behave like an expanded (“lubricated”) IL. The transition region (xIL ≈ 0.2) is characterized by redissociation of the ion pairs and establishment of an IL-like structure.”19 The results presented here are in general accord with this characterization of structure and dynamics. Two groups have reported molecular dynamics simulations of the [Im41][BF4] + CH3CN system. In a series of early papers, Wang and co-workers developed an all-atom force field for simulating imidazolium ionic liquids20 and used mixtures with CH3CN to help validate this force field21 as well as a united atom variant22 of it. Results simulated using both variants indicated deviations from ideal mixing to be moderate and attractive, with minima in the excess volume (VE) and enthalpy of −3 cm3 mol−1 (−3%) and −2.5 kJ/mol (−1%) at 298 K.21 Comparison to experimental volumetric data6 showed the simulations to be realistic with respect to component densities and the composition dependence of VE, but the volume nonideality of the real liquid is only ∼1%, 3-fold smaller than in these simulations. Wang and co-workers also calculated transport properties and claimed that calculated shear viscosities were in good agreement with experiment.21 However, subsequent ionic liquid simulations by many groups suggest that these early simulations were of insufficient length to converge dynamical properties such as viscosity. In a more recent series of papers Chaban and coworkers23−26 re-examined the Wang force field in using much more extensive simulations. Chaban et al. found that transport in neat [Im41][BF4] simulated with the original force field was actually ∼10-fold slower than in the experimental liquid.23 They also found that a simple scaling of ion charges (by a factor of 0.81) sufficed to bring viscosities and conductivities of neat [Im41][BF4] and its mixtures with CH3CN into good accord with experimental data.24,26 Using this modified force field, Chaban et al. characterized structure in terms of cluster distributions for purposes of understanding how one might optimize electrical conductivity. For compositions 0.05 ≤ xIL ≤ 0.50 distributions of cluster sizes (number of ions, n) are approximately exponentially decreasing functions of n. Consistent with the interpretations of Stoppa et al.,19 isolated ions and ion pairs were found to be the dominant species (accounting for >40% of the ions) for xIL ≲ 0.1. At xIL = 0.25, clusters of n = 1 and 2 ions account for fewer than 15% of all ions present, whereas more than 50% are associated with clusters of size n ≥ 6. It should be noted that a mole fraction xIL = 0.25 corresponds to a volume fraction of 54% and one would

2. EXPERIMENTAL METHODS Coumarin 153 was purchased from Exciton and used without further purification. 1-Butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) was obtained from Iolitec and dried under vacuum at 45 °C overnight prior to sample preparation. Acetonitrile (anhydrous, spectrophotometric grade) was used as received from Sigma-Aldrich. Mixtures were prepared by weight inside a nitrogen-purged glovebox. The water contents of the mixtures were measured using a Mettler-Toledo DL39 Karl Fischer coulometer and were below 100 ppm by weight in all cases. 1341

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content could be kept to less than 200 ppm by weight using this setup.30 These experiments were performed at room temperature, 20.5 ± 1 °C. Time-evolving spectra over the full range 100 fs to 20 ns were generated by combining data from the TCSPC and FLUPS experiments as described in detail in ref 30. Briefly, peak frequencies ν(t) were compared over the range 100−600 ps where both techniques are expected to provide accurate results and the FLUPS frequencies shifted slightly (typically 600 ps indicates data from TCSPC measurements, data at t < 200 ps are from FLUPS measurements, and between 200 and 600 ps the data are a weighted average of the two. Successive curves in the bottom panel are vertically offset by 0.2 for clarity.

ion and dipole mechanisms. As discussed below, the dielectric relaxation results of Stoppa et al.19 provide some insight into mechanism but detailed interpretation must await simulations. The data in Figure 9 provide an indication of the likely precision of the response functions. At times t < 0.5 ps the Sν(t) curves in Figure 9a cross in an unpredictable manner. At times longer than a few picoseconds the functions do not cross, but the spacing between the various compositions is less systematic than might be anticipated. We view this behavior as indicative of precisions of roughly ±0.1 in Sν(t) at short times and ±0.05 at later times. These estimates are consistent with the few repeated measurements we have performed. Some comment is also necessary regarding the response observed in neat CH3CN (dashed curve in Figure 9a). This function was recorded using the same FLUPS instrument used in the present work and was previously reported in ref 61. Of interest is the small excess shift leading to negative values of Sν(t) between times of 1 and 10 ps. This type of retrograde shift is observed in C153 and other solutes when excited significantly above the vibronic origin.61 Its interpretation is still uncertain but the most likely candidate is vibrational energy relaxation in S1 causing variations in the widths and to a lesser extent the peak positions of emission spectra.61 We do not observe retrograde shifts at the other compositions studied, but it is possible that the Sν(t) at earlier times ( 0.3) ionic liquid compositions.63 For comparison, Figure 10 also shows solvation times measured in mixtures of [Im41][BF4] + H2O from our previous work (open red diamonds).5 At the higher viscosities η > 10 mPa s, corresponding to xIL > 0.5 (ΦIL > 0.8), solvation times in the two mixtures are approximately equal for equal values of viscosity. The slower components of solvation are also similar at equal viscosities down to even lower ionic liquid content (xIL ∼ 0.2 in the CH3CN mixture; see Figure SI-3, Supporting Information). This observation suggests that one might view the slower components of solvation in the mixtures as being similar to those in the neat ionic liquid, i.e., as primarily due to ion motions, with the influence of the dipolar cosolvent being primarily to fluidize (or “lubricate”19) these motions. E. Comparison to Dielectric Models of Solvation. In this final section we consider the solvation response functions measured using C153 in relation to solvation models based on 1347

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the broad-band dielectric spectra recently reported by Stoppa et al.19 We consider two models here, a simple dielectric continuum (“DC”) approach and a semimolecular theory (“SM”) recently described for ionic liquid + dipolar solvent mixtures by Daschakraborty and Biswas.64 (The extended Debye−Huckel approach of Song and co-workers65,66 could also, in principle, be compared to the present data, but given the complexity of the calculations required, we leave such comparisons to future work.) The dielectric continuum model used here assumes the solute to be a polarizable point dipole centered in a spherical cavity. Solvation dynamics within this model depend only upon the solute’s polarizability, represented by a cavity dielectric constant εu = 2, and the frequencydependent permittivity of the solvent, η̂(v). Details of these calculations are provided in the Supporting Information and ref 30. The semimolecular theory of Daschakraborty and Biswas uses only information about dielectric relaxation of the two pure liquids, mixture viscosities, and information about liquid structure in the mean spherical approximation to predict the solvation response in mixtures.64 Here we compare predictions already reported in ref 64 for the system C153 in [Im41][BF4] + CH3CN mixtures to our experimental results. Comparisons of the DC and SM model predictions to the experimental data are provided in Figures 11 and 12. Consider

Figure 12. Comparison of component solvation times observed (large connected symbols) and those predicted by the simple dielectric continuum model (DC; small open symbols) and semimolecular (SM; small filled symbols) theory of Daschakraborty and Biswas.64 Circles are the fast component times and triangles the slow solvation times.

to be 2−4 times faster than those observed. From Figure 11 it appears that agreement with the DC model is improved upon the addition of CH3CN. This finding is confirmed in Figure 12 where fast and slow component times are plotted. Figure 12 suggests a general trend in which the DC predictions for ⟨τslow⟩ approach observed values as xIL decreases. (See also Figure SI-4 (Supporting Information) for more detailed comparisons of Sν(t) times.) Turning to the semimolecular theory, the predictions of the SM approach do not provide a general improvement over the simple continuum predictions. Although the fast component time is better matched (Figure 12), the amplitude of this fast time is significantly underestimated (Figure 11). In addition, the slow component of solvation is predicted to be uniformly about 7-fold slower in the SM model relative to the DC model. Thus, where the DC model underestimates ⟨τslow⟩, the SM model overestimates ⟨τslow⟩ and by a larger amount. In part, the poorer agreement of the SM model can be traced to the facts that only neat component dielectric data were used and no correction for missing high-frequency components of ε̃(v) was applied in the original calculations.64 Daschakraborty and Biswas have very recently made improvements in their mixture theory to account for both of these shortcomings and have achieved better agreement with the present experimental results.67 An alternative way of viewing the comparison between the observed solvation response functions and dielectric continuum predictions is to assume the correctness of the DC model and ask what effective dielectric function η̂eff (v), composed of ε̃eff (v) and σeff (see Supporting Information), is needed to reproduce a given Sν(t). Such an inversion of solvation data is made possible by an exact one-to-one mapping of the fit parameters of a multiexponential representation of Sν(t) onto the parameters of a multi-Debye representation of ε̃eff(v).47 In the present case, using a 4-exponential representation of Sν(t), and a limiting high-frequency permittivity ε∞ = nD2 based on experimental refractive index data provides ε̃eff(v) as a sum of 3 Debye terms plus an effective static conductivity σeff. The results of this type of analysis are displayed in Figures 13 and 14. Figure 13 compares the imaginary parts of ε̃eff (v) to observed spectra, and Figure 14 compares the magnitudes of the effective static permittivities εeff and conductivities σeff to those measured by Stoppa et al.13,19 From Figure 13 one

Figure 11. Comparison of observed (solid black curves) and predicted (dashed red) solvation response functions at xIL = 0.2, 0.5, and 0.9. The top two panels show predictions of simple dielectric continuum (DC) theory, and the bottom two panels are predictions of the semimolecular (SM) model of Daschakraborty and Biswas.64

first the dielectric continuum predictions. The top panels of Figure 11 compare observed spectral/solvation response functions (solid black curves) to DC predictions (red dashed curves) at three mixture compositions, xIL = 0.2, 0.5, and 0.9. As illustrated in this figure, the relative amplitudes of the fast and slow components of solvation appear to be reasonably captured by the dielectric continuum model, as does the shape of Sν(t). What is not quantitatively captured is the time associated with the long solvation component, which tends to be underestimated by the DC calculations. Prediction of faster-thanobserved solvation times is a general shortcoming of the dielectric continuum description of ionic liquids. As shown previously,30 solvation times in neat ionic liquids are predicted 1348

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measured by Stoppa et al.13,19 One could interpret these effective dielectric data as reporting that the environment surrounding the C153 probe is different from the bulk, indicating, for example, that conductivities and therefore the rates of ion motions are reduced relative to the bulk liquid. The presence of a solute such as C153 undoubtedly has some effect on the dynamics in its vicinity. However, given the known inadequacies of dielectric continuum descriptions for treatment of solvation dynamics in ionic liquids,68 it is probably best to view differences as large as those in Figure 14 as being more a result of the shortcomings of the DC model rather than of changes to the local environment.

4. SUMMARY AND CONCLUSIONS The work described herein allows for the following general conclusions about the prototypical ionic liquid + dipolar aprotic liquid binary system [Im41][BF4] + CH3CN. 1. Some of the basic physical properties of this mixture, such as density6−13 and viscosity,6,10,14,15 have been reproduced by multiple groups and are reasonably well established. Some properties such as refractive index9 and especially electrical conductivity16,17,10,14,13,15 have been measured multiple times but with significantly discordant results. Finally, other properties such as dielectric constant,19 self-diffusion coefficients (this work), and surface tension19 have been measured by only a single group, whereas many others, for example, excess enthalpy and compressibility, remain to be determined. Thus, although this binary system has enjoyed a special status among ionic liquid + conventional organic solvent mixtures, much work remains before it is fully characterized. 2. Self-diffusion coefficients of the Im41+ and CH3CN components of these mixtures follow power laws on solution viscosity, D ∝ η−p with p = 0.88 and 0.77, respectively. These transport coefficients are reproduced to better than 30% by the recent simulations of Chaban et al.26 The departures from hydrodynamic predictions for D are comparable to those observed in neat ionic liquids and conventional solvents.32 3. Solvation energies deduced from the solvatochromism of C153 in these mixtures are unremarkable. As far as the dipolar S0 ↔ S1 transition of C153 is concerned, [Im41][BF4] is slightly (14%) more polar/polarizable overall (ΔsolvG) and has a slightly (8%) larger nuclear polarizability (λsolv) than CH3CN. Given these modest differences and the good solubility of C153 in both solvents, significant preferential solvation is not anticipated in these mixtures and no clear evidence for preferential solvation is observed. 4. Rotational correlation functions of C153 deduced from fluorescence anisotropies are nonexponential and correlation times follow mixture viscosity as ⟨τrot.⟩ ∝ η0.90. Hydrodynamic predictions using stick boundary conditions and an ellipsoid representation of the solute predict exponential rotational correlation functions and ⟨τrot.⟩ ∝ η1, contrary to what is observed. Nevertheless, rotation times remain within a factor of 2−3 of those predicted by such calculations. The rotation of C153 in these mixtures is in keeping with what is observed for C153 in neat ionic liquids 29 and conventional solvents.36,69

Figure 13. Comparison of measured (dashed purple and solid blue curves) and effective (red) dielectric loss spectra determined from Sν(t). The dashed purple curves are from parametrized fits to bulk dielectric data extending to 89 GHz whereas the solid blue curves have been approximately corrected for missing high-frequency components as described in the Supporting Information. In all cases the diverging conductivity term has been subtracted from the spectra.

Figure 14. Comparison of measured relative permittivity εr and conductivity σ (blue curves) and their effective values εeff and σeff derived from solvation response functions of C153 (red points).

observes that the breadth and locations of the peaks in ε″eff(ν) are similar to those measured experimentally, with the similarity increasing at lower ionic liquid contents. Figure 14 shows that for xIL < 0.5 εeff is close to εr but for xIL > 0.5, εeff ∼ 0.5εr. In the case of the conductivity, as would be expected on the basis of the inverse proportionality predicted by the DC model, the effective value σeff is 2−3 times smaller than the values 1349

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5. Spectral/solvation response functions are strongly bimodal in these mixtures, as they are in neat ionic liquids. The subpicosecond component decays on roughly the same time scale, ∼170 fs, at all compositions, but the amplitude of this component decreases with increasing x IL . The slow component is broadly distributed over picosecond−nanosecond times and the integral time associated with this component roughly follows solution viscosity. Overall solvation times appear to be more simply related to solution viscosity than they are to conductivity. 6. Solvation dynamics of C153 in [Im41][BF4] + CH3CN and [Im41][BF4] + H2O5 are remarkably similar, despite the fact that its solubility and solvatochromism differ in the two mixtures. At equal viscosities, solvation times and solvation response functions of C153 are within uncertainties in these two mixtures for xIL > 0.5 (and perhaps even lower). This similarity supports the notion that over much of the full composition range the slower liquid dynamics reflect ion motions lubricated19 and augmented by the presence of the dipolar component. 7. Use of recently reported broad-band dielectric spectra19 in a simple dielectric continuum model predict observed solvation response functions with reasonable fidelity. The overall shapes of the observed response are reproduced but the speed of the slow solvation component is overestimated by factors of 2−3 at the neat ionic liquid limit and by lesser amounts at low xIL. The observed solvation dynamics can be reproduced by assuming effective dielectric constants and conductivities that are significantly lower than experimental values. The predictions of the semimolecular model of Daschakraborty and Biswas, as originally implemented,64 are less successful than the simple continuum model, but improved agreement has very recently been achieved using an extended model.67 8. The qualitative accuracy of simple dielectric continuum predictions suggests that the molecular mechanisms proposed to account for the dielectric spectra19 also apply to solvation of C153. It should be noted, however, that the subpicosecond component of solvation is not directly associated with the high-frequency (∼50 GHz) dispersion, which Stoppa et al.19 attributed to CH3CN reorientations. The subpicosecond component of solvation is primarily due to inertial motions occurring in the low terahertz region of the spectrum not covered in the work of Stoppa et al. The ∼50 GHz component is instead associated with the fastest parts of the “slow” solvation component reported here. The work of Stoppa et al. suggests that the character of [Im41][BF4] + CH3CN mixtures changes from that of an expanded ionic liquid to a conventional electrolyte at xIL ∼ 0.2. It would therefore be of interest to extend the range of compositions examined in the present work to lower values of xIL to explore solvation dynamics in the second of these regimes. Computer simulations of dielectric relaxation and solvation dynamics in [Im41][BF4] + CH3CN mixtures would also be of considerable value for testing and refining the proposed molecular interpretations of these dynamics.

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ASSOCIATED CONTENT

S Supporting Information *

Viscosities measured at 20.5 °C, tabulated values of refractive indices, diffusion coefficients, and various other properties at 25 °C, steady state absorption and emission spectra of C153, a comparison of spectral response functions reported here to those in [Im41][BF4] + H2O mixtures, a discussion of dielectric continuum calculations of solvation dynamics, and an additional comparison of observed solvation times and DC predicted times. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. Maroncelli: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.L., A.K., and M.M. were supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through grant DE-FG02-12ER16363, and X.-X.Z. and N.P.E. were supported by the Deutsche Forschungsgemeinschaft (priority program “Ionic Liquids”). X.-X.Z. is also grateful for support by the Humboldt University and the Chinese Scholarship Council during earlier stages of her work.

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REFERENCES

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