Transport Properties of the 1-Hexyl-3-methylimidazolium Bis

Article pubs.acs.org/JPCB

Transport Properties of the 1‑Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide−Trichloromethane Binary System: Indication of Trichloromethane Segregation David R. Saeva, Joaõ Petenuci, III, and Markus M. Hoffmann* The College at Brockport, State University of New York, Brockport, New York 14420, United States S Supporting Information *

ABSTRACT: Self-diffusion coefficients and electrical conductivity were studied for the binary system 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide−trichloromethane ([C6mim][NTf2]−CHCl3) as a function of composition and temperature. Self-diffusion coefficients of cation and anion are identical for ionic liquid mole fractions xIL < 0.95. The self-diffusion coefficient of CHCl3 is consistently larger than that of the ions by a factor of 4. A double logarithmic plot for the ratio of self-diffusion coefficient and temperature versus viscosity is linear for ionic liquid mole fractions 0.1 < xIL < 0.9 indicating (a) a fractional Stokes−Einstein applies where self-diffusion is inverse proportional to some power b of viscosity (D ∼ η−b) and (b) single average length scales are associated with the mass transport of [C6mim][NTf2] and CHCl3. However, the obtained length scale for CHCl3 is unreasonably small, which is indicative of CHCl3 segregation. The molar conductivity displays a maximum near xIL = 0.2. Evaluation of the ionicity from molar conductivity and self-diffusion coefficients indicates a gradual speciation change from charged species to neutral species for xIL < 0.5. The temperature dependencies of self-diffusion and molar conductivity follow Arrhenius behavior. The obtained xILdependent activation energies are found to be linear for molar conductivity and largest for the cation and anion self-diffusion coefficients. The activation energies for the self-diffusion of CHCl3 appear to be identical with those obtained from fluidity data.

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range, especially volumetric properties,29 viscosity,30 acoustic properties,31 and mixing enthalpies.32 It has been attempted to classify and correlate some of these properties to the structure of the ILs and molecular solvents.30,33 While a fair number of conductivity studies over large composition ranges of IL− organic solvent exists,34−47 in the case of self-diffusion measurements, such studies have mostly focused on IL−water binary systems,48−53 and self-diffusion studies on IL−organic solvent binary systems that cover wide composition ranges are scarce.54,55 The presented study here focuses on the particular binary system 1-hexyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) amide ([C 6 mim][NTf 2 ]) in trichloromethane (CHCl3). The study was motivated based on findings from prior work that began with the observation of two proton resonance sets for the related IL 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) amide ([C2mim][NTf2]) in CDCl3. Initially, the two resonance sets were assigned to the freely dissolved ions and ion paired ions,56 and the presence of ion pairs was confirmed by IR spectroscopy and DFT calculations.57 However, later work showed that the equilibrium species are ion pairs and aggregated ions.58 While [C2mim]-

INTRODUCTION Ionic liquids (ILs) are entirely composed of ions and have been defined as salts that are liquid below 100 °C.1 Massive research efforts on the topic ILs are presently ongoing. For example, typing the phrase “ionic liquids” in common literature search engines reveals that more than 7000 articles per annum were published in recent years. Applications of ILs span indeed a wide range including for example chemical synthesis,2−4 electrochemistry,5,6 and catalysis.7−9 Much attention has recently been paid on IL−molecular solvent binary systems because in many of their applications ILs come in contact with molecular solvents. For example, ILs are considered for the extraction of aromatic hydrocarbons and sulfur containing compounds from aliphatic hydrocarbons10−18 and the separation of azeotropic mixtures.19−23 The addition of molecular cosolvents has been found to enhance dissolution of cellulose into ILs.24 It has also been recognized for quite some time that the presence of molecular solvents, even at small amounts, leads to much lower viscosities than for the neat IL, which is much desired for most applications.25−28 Consequently, there has been an increasing need to understand how physical properties of IL−molecular solvent binary systems depend on composition. Several thermophysical and transport properties have been measured for a significant number of completely miscible binary IL−molecular solvent systems for the entire composition © 2016 American Chemical Society

Received: July 12, 2016 Revised: August 19, 2016 Published: August 24, 2016 9745

DOI: 10.1021/acs.jpcb.6b06974 J. Phys. Chem. B 2016, 120, 9745−9754

Article

The Journal of Physical Chemistry B

opened and stored under a nitrogen atmosphere in a glovebox and used as received. NMR samples were prepared by mass using an analytical balance in an inert atmosphere glovebox. A portion of the prepared solution was transferred into a melting capillary tube that then, via Schlenk line techniques, was flame-sealed under reduced nitrogen pressure after freezing the sample with liquid nitrogen. The flame-sealed capillary was placed in a standard 5 mm NMR tube, filled with deuterated acetone as the lock solvent, and the NMR tube was also flame-sealed under reduced pressure after freezing the sample with liquid nitrogen. Overall, the standard uncertainty of the solution compositions is estimated to be within 0.005 mole fraction units. The NMR self-diffusion coefficients were obtained with a variable temperature broadband probe using an Avance 300 instrument from Bruker-Biospin. The NMR tubes were not spun during measurements and the sample was allowed to equilibrate with respect to temperature for at least 15 min. The NMR sample temperature was measured to a precision of 0.1 K and was calibrated against the known chemical shift temperature dependencies of methanol and glycol.63 The pulse sequence used for the self-diffusion measurements was based on the stimulated echo as originally published by Jerschow and Müller.64,65 Pulse sequences using the stimulated echo have been proven to reduce systematic errors from eddy currents and thermal gradients and, compared with pulse sequences based on the classic spin echo, result in more accurate selfdiffusion coefficients for measurements with neat ILs and binary systems with ILs.66 The strength of the applied sine shape pulsed field gradients was calibrated against the known self-diffusion coefficient of water.67,68 The magnetic field gradient strength, g, was varied linearly from 5.6 to 56 G· cm−1 in 16 increments. The acquired stimulated echo intensities, I(g), were fit to

[NTf2] possesses a solubility gap with CHCl3, [C6mim][NTf2] is completely miscible with CHCl3, which allowed the study of transport properties for this binary system to IL mole fraction of xIL = 0.1. It was observed that the average hydrodynamic solute radius undergoes a maximum near xIL = 0.01.59 Similar radii maxima were observed in a subsequent study of five other [Cn=2,4,6mim][NTf2]−molecular solvent systems.60 The radii maxima were interpreted as indicative of a qualitative change in IL transport mechanism from being dominated by individual species moving on their own through the molecular solvent medium to being dominated by individual ions and ion pairs jumping from aggregate to aggregate at higher concentrations.59,60 One major goal of this study is to inspect if there are any additional changes in the speciation and dynamics in the [C6mim][NTf2]−CHCl3 binary system for the compositions 0.1 < xIL < 1. For this purpose, self-diffusion coefficients of all three species, cation, anion, and CHCl3, were investigated and correlated with conductivity measurements to assess in how far mass transport is carried by neutral or charged species. A second major goal for this study is to inspect and compare the temperature dependence of mass-transport properties. With respect to viscosity, prior work has shown that the temperature dependence generally follows the Arrhenius law for more than 200 inspected IL−molecular solvent binary systems, including the present [C6mim][NTf2]−CHCl3 binary system.30 Interestingly, the derived composition-dependent Arrhenius fit parameters show in a large number of cases linearity over wide ranges if not the entire range of xIL.30 This is of general importance because it is practically impossible to measure composition-dependent transport properties for all conceivable combinations of IL and molecular solvent. Therefore, predictive schemes that allow estimation of transport properties from knowledge of the neat component properties are highly desirable. There are considerably fewer temperature-dependent studies that cover wide ranges of composition for conductivity and self-diffusion coefficients of IL−molecular solvent binary systems. Arrhenius behavior for large ranges of composition has been noted for the temperature dependence of conductivity for several IL−molecular solvent systems.34,50,61 As for selfdiffusion, we are only aware of three studies of imidazoliumbased ILs with water, all of which show Arrhenius behavior.48,50,62 Interestingly, nearly identical xIL-dependent activation energies, Ea(xIL), were observed for the self-diffusion coefficients of the ions and water as well as fluidity in the case of 1-ethyl-3-methylimidazolium methanesulfonate ([C2mim][MeSO3])−water binary system50 but not for the [C4mim][MeSO3])−water binary system.62 To the best of our knowledge, except for these two IL−water binary systems, a comparison Ea(xIL) of several transport properties over wide ranges of xIL has not be done for other IL−molecular solvent binary systems.

2 2 2

I(g ) = I0e−Dγ g

δ ((4Δ− δ)/ π 2)

(1)

where I0 is the stimulated echo intensity in absence of any gradient, γ is the gyromagnetic ratio for 1H or 19F, and δ is the length of the gradient pulse, which was adjusted to each sample and temperature condition, while the diffusion time, Δ, was set at 0.1 ms. The necessary delays for gradient recovery and eddy currents suppression were set at 0.1 and 5 ms, respectively. Given the presence of several distinct 1H resonances available for the 1H self-diffusion analysis, the standard uncertainty of the self-diffusion coefficient of the cation was directly obtained from the redundant analysis of each observable IL cation signal, and the reported values represent the obtained average. The standard uncertainty for the self-diffusion coefficient of anion and CHCl3 is assumed to be of similar magnitude, as obtained for the cation self-diffusion coefficients. Solution conductivities were measured using an Accumet 30 conductivity meter with platinum two-electrode conductivity probes compatible for measuring conductivity in organic solvents including CHCl3. The conductivity probes were of nominal cell constant of 0.1 or 10.0 cm−1 and used depending on the needed measurement range. Several commercial conductivity standards as well as the known conductivity temperature dependence of aqueous KCl solutions69 were used for calibrating the electrodes, and consistency of conductivity measurements from either electrode was verified. The temperature of the IL sample was kept constant to within 0.1 K using a

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EXPERIMENTAL SECTION Trichloromethane (CAS no.: 67-66-3) was obtained from Acros with a mass fraction purity of 0.998, which excludes the presence of 0.005 to 0.010 mass fraction of ethanol as stabilizer that was not removed. The [C6mim][NTf2] (CAS no.: 38215050-7) was obtained from Io-Li-Tec as a clear and colorless liquid with a mass fraction purity of greater than 0.99 including a mass fraction of smaller than 1.5 × 10−4 of water, as determined by Karl Fischer titration. The chemicals were 9746


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[NTF2 ]−CHCl 3 binary system over nearly the entire composition range. Although the self-diffusion coefficients in Figure 1 are monotonically decreasing with increasing xIL, there appear to be three distinguishable regions: a steep decrease at dilute IL compositions up to about xIL = 0.01 followed by a less steep decrease up to xIL near 0.5 beyond which the decrease is rather small. The self-diffusion of CHCl3 is consistently about four times faster than that of the cation (Table S2) and thus follows the same trend, as described for the IL cation and anion for xIL > 0.1. This approximate constancy of the ratio of self-diffusion coefficients of CHCl3 and IL cation is qualitatively different from the behavior of [C4mim][PF6] with the molecular solvents acetonitrile, N,N-dimethylformamide, dimethyl sulfoxide, propylene carbonate, ethanol, 2,2,2-trifluoroethanol, and toluene, which all showed an increase with increasing xIL except for the nonpolar toluene, where a decrease was observed.55 The authors reporting these findings interpreted this ratio of selfdiffusion coefficients of solvent and cation as a measure of aggregation and referred to it as “aggregation index”. The aggregation index of about 4 observed in this study for the [C6mim][NTf2]−CHCl3 binary system is larger than any of their reported aggregation index values at molecular solventrich compositions. Therefore, the aggregation in the [C6mim][NTf2]−CHCl3 binary system appears to be rather extensive for 0.1 < xIL < 0.9 compared with other IL−binary solvent systems. The observed three xIL regions in Figure 1 for the selfdiffusion in the [C6mim][NTf2]−CHCl3 binary system are also discernible for the xIL dependence of kBTη−1D−1 shown in Figure 2 for the cation and CHCl3, where kB is the Boltzmann

water bath that was temperature controlled by a Lauda circulating water bath. In a typical conductivity measurement series at constant temperature, a portion of the sample was withdrawn after each measurement for analysis by 1H NMR spectroscopy and CHCl3 added to obtain the next target xIL composition, briefly shaken, and replaced in a water bath. The conductivity reading was taken upon stability for at least 5 min. It was noted that water was absorbed by the sample during this typical measurement procedure. Therefore, 1H NMR spectroscopy of the withdrawn samples included careful determination of water content, as described in the literature.70 The reported solution compositions, including the water content, represent the solution composition immediately after completion of each conductivity reading. The standard uncertainty of the reported x IL composition is estimated to be 0.005. On the basis of prior work,60 the relative standard uncertainty of the reported conductivity readings is 2%.

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RESULTS AND DISCUSSION Self-Diffusion. The complete data set of self-diffusion coefficients of the [C6mim][NTf2]−CHCl3 binary system as a function of xIL and temperature is provided in Table S1 in the Supporting Information. As can be seen from Table S1, the selfdiffusion coefficients follow similar composition dependences for each constant temperature. Therefore, Figure 1 shows the

Figure 1. Self-diffusion coefficients for [C6mim][NTf2]−trichloromethane binary system at 298.2 K as a function of ionic liquid mole fraction: ○, cation (white, this study; gray Scharf et al.59); □, anion (white, this study; gray Scharf et al.59); ×, trichloromethane. Figure 2. Inspection of the IL mole fraction dependence of kBTη−1D−1 for [C6mim][NTf2]−trichloromethane binary system at 298.2 K: ○, cation; ×, trichloromethane.

exemplary composition dependence of the self-diffusion coefficients of cation, anion, and CHCl3 at 298.2 K. Figure 1 also includes data at dilute IL concentrations from Scharf et al.59 The self-diffusion of cation and anion is very similar, and the data points of these are overlapping in Figure 1. However, a closer look at the ratio of cation over anion self-diffusion coefficients in Table S2 for the neat IL and xIL = 0.95 reveals that this ratio is ≥1.05 for all measured temperatures at these two compositions. Indeed, the self-diffusion coefficients for neat [C6mim][NTf2] have been reported before to be greater for the cation, 3.4 × 10−11 m2·s compared with 2.6 × 10−11 m2·s for the anion at 303.2 K.71 Evidently, this differentiation vanishes upon some addition of CHCl3. In contrast, the self-diffusion coefficients of anion and cation have been observed to differ over wide xIL ranges in several IL−water binary systems, where the cation diffuses faster than the anion at high xIL and slower at low xIL.50,51,53 Furthermore, the [C6mim]+ cation and [NTF2]− anion either self-diffuse together (ion pair, aggregates) or experience the same transport mechanism in the [C6mim]-

constant, η the viscosity, and D the self-diffusion coefficient. As can be seen in Table S3, kBTη−1D−1 is essentially temperatureindependent, and Figure 2 shows the exemplary data for 298.2 K. Included in Figure 2 and Table S3 are values obtained from using self-diffusion coefficients from Scharf et al.,59 and the values for kBTη−1D−1 were evaluated with viscosity data from Haghani et al.30 According to the Stokes−Einstein equation, eq 2, kBTη−1D−1 is related to the average hydrodynamic radius, r

r=

kBT cπηD

(2)

In eq 2, c is a constant typically between 4 and 6 for the socalled slip and stick boundary conditions, respectively.72 It has been noted in several reports that the Stokes−Einstein equation may not be valid for ILs and IL-rich solutions.54,73−77 9747


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The Journal of Physical Chemistry B Nevertheless, eq 2 is certainly the most widely used means to correlate viscosity and self-diffusion data with the involved length scales of the diffusing species. Furthermore, the discussion here is not so much concerned with the accurate determination of hydrodynamic radii, for which it would be difficult to establish a value of c in eq 2 because c may vary with xIL. Instead, we rather wish to more qualitatively inspect how the average length scale kBTη−1D−1, representing the time and ensemble average of all diffusing respective species of cation, anion, and CHCl3, is dependent on the composition of the [C6mim][NTf2]−CHCl3 binary system. The sharp rise of kBTη−1D−1 observed in Figure 2 for the cation at dilute IL compositions reflects a speciation change from ion pair dominant to higher aggregate dominant.59,60 The decline of kBTη−1D−1 for xIL greater than ∼0.01, where a maximum of kBTη−1D−1 is observed, has been interpreted as a change in the mass-transport mechanism from being dominated by the individual motion of ion pairs and aggregates to a jumping of ions and ion pairs from aggregate to aggregate.59,60 Inspection of a derivative plot of Figure 2 for the cation, shown in Figure S1 in the Supporting Information, indicates that this transition in transport mechanism appears to be complete between 0.1 < xIL < 0.2, beyond which changes in slope with increasing xIL are small. For CHCl3, kBTη−1D−1 in Figure 2 remains essentially constant near 0.1 × 10−8 m−1, except for xIL = 0.230 and 0.105, where kBTη−1D−1 is only slightly larger. According to eq 2, a value of ∼0.35 × 10−8 m−1 observed for the cation in Figure 2 at IL-rich compositions would correspond to an average hydrodynamic radius of 1.76 to 2.79 Å between the limits of c = 6 and 4, respectively. These values are smaller than reported van der Waals radii for [C6mim]+ (3.53 to 3.85 Å)59,78−80 as well as reported van der Waals radius for [NTf2]− (3.17 to 3.29 Å).59,72,78,80,81 Radii smaller than van der Waals radii have been a typical indication for the invalidity of eq 2, as noted by Marekha et al.54 Thus, non-Stokes−Einstein behavior is indicated for the [C6mim][NTf2]−CHCl3 binary system. To further inspect this issue, we considered a modification to the Stokes−Einstein equation (eq 3), sometimes referred to as fractional Stokes−Einstein equation that was originally used to describe the behavior of glass-forming liquids.82 Specifically, the self-diffusion coefficient is inversely proportional to some power of the viscosity in eq 3. In fact, Shiflett and coworkers have used eq 3 and its rearranged logarithmic form of eq 4 to model the diffusivity of hydrofluorocarbons (HFCs)83,84 and CO2 in ILs.85 D=

Figure 3. Inspection of the IL mole fraction dependence of ln(DT−1) versus ln(η/η0) at 298.2 K: ○, cation; ×, trichloromethane. The lines are fits to the linear range of the obtained graphs according to eq 4.

Table 1. Parameters Obtained from Application of Equations 3 and 4 T/K

⎛η⎞ ⎛ D/m·s−1 ⎞ ln⎜ ⎟ = a − b ln⎜⎜ ⎟⎟ ⎝ T /K ⎠ ⎝ η0 ⎠

288.15 298.15 308.15 318.15

−27.22 −27.16 −27.12 −27.10

± ± ± ±

0.03 0.03 0.02 0.02

288.15 298.15 308.15 318.15

−25.81 −25.79 −25.63 −25.64

± ± ± ±

0.16 0.09 0.05 0.05

b/ln(m2·s−1·K−1) Cation 0.77 ± 0.01 0.75 ± 0.01 0.74 ± 0.01 0.71 ± 0.01 CHCl3 0.77 ± 0.06 0.76 ± 0.03 0.75 ± 0.02 0.73 ± 0.02

R2

r/Å

0.999 0.998 0.999 0.998

4.88 4.57 4.38 4.29

± ± ± ±

0.18 0.17 0.13 0.14

0.981 0.990 0.996 0.994

1.19 1.16 0.99 1.00

± ± ± ±

0.19 0.04 0.09 0.03

are between that of an individual cation and a contact ion pair. We remind that the self-diffusion coefficients of cation and anion in Table S1 are essentially identical except for xIL > 0.9, which indicates that over the course of the NMR diffusion measurement, cation and anion experience the same structural environments and dynamic transport processes. While the cation radii in Table 1 are large enough to make physical sense, this is not the case for the CHCl3 radii, which are too small by a factor of ∼2.5 compared with its van der Waals radius. In this regard, even more extreme deviations (a factor of 20) for the molecular solvent radius were observed for the binary system composed of hexane and the phosphonium-based IL [P14,6,6,6][NTf2].86 Water was also observed to diffuse faster than what Stokes−Einstein behavior predicts in [C4mim][NTf2], where the deviations were observed to be linear with respect to water mole fraction, which the authors interpreted by an increase in water domains where water molecules can move freely.52 Unreasonably small radii were also obtained for ferrocene- and ferrocene-derivatized compounds in ILs in an electrochemical study, and the obtained radii were associated with the length scale of self-diffusing holes through the IL matrix.76 Apparently, mass transport of the CHCl3 is much faster and probably involves a different mechanism than what is conceived with the Stokes−Einstein equation, where the diffusing particle is thought of as a large sphere moving through an unstructured continuum. Structural heterogeneity has indeed been a wellrecognized feature of ILs.87 MD simulations for the related binary system [C2mim][NTf2] with CHCl3 have resulted in snap shots for an IL mass fraction = 0.66 that indicate the presence of segregated CHCl3 domains.88 Microheterogeneity was also observed in a theoretical study for the binary system [C2mim][NTf2]−methanol.89 The presence of molecular

kBT 6πrη0(η /η0)b

a/ln(m2·s−1·K−1)

(3)

(4)

In eqs 3 and 4, η0 is a unit viscosity (1 mPa·s) and a and b are adjustable parameters with a = ln(kB/(6πrη0)). Figure 3 shows an exemplary plot with the results for the cation and for CHCl3 obtained for the data at 298.2 K. A linear correlation is evident in Figure 3 for a range of data points that corresponds to 0.1 < xIL < 0.9 for both the cation and CHCl3. The obtained parameters from fitting these ranges according to eqs 3 and 4 are shown in Table 1. The cation radii obtained from the a fit parameters are decreasing with temperature from 4.88 Å at 288.2 K to 4.29 Å at 308.2 K, which is reasonable as larger thermal energy should enhance dissociation. These cation radii 9748


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conductivity from the raw conductivity data shown in Table S4 were obtained from Cade et al.33 A maximum in molar conductivity is observed near xIL = 0.2 for all four temperatures. The molar conductivities increase sharply with increasing xIL to about xIL = 0.2 and then decrease more gradually with increasing xIL. Such composition dependence of the conductivity is quite common for IL−molecular solvent binary systems,37,38,40,92 and for some binary systems the conductivity maximum is observed at even lower xIL.35,42,93,94 The molar conductivity that would be expected if all self-diffusing species were charged, ΛNMR, is shown in Figure 4b and was calculated from the measured self-diffusion coefficients of cation and anion, D+ and D−, according to eq 5

solvent pools dispersed throughout the IL framework would allow for faster translational motion of the molecular solvent within these pools. This would explain the fast self-diffusion of CHCl3 in the IL, faster than expected based on the bulk viscosity.46 In summary, the main insights gained from the analysis of the self-diffusion and viscosity data according to eqs 3 and 4 are that (a) an adjustment of the viscosity dependence according to eq 3 can account for some of the deviations from Stokes− Einstein behavior, (b) CHCl3 is to some extent present in segregated form, and (c) the length scales of the self-diffusing species appear to be constant for the range of 0.1 < xIL < 0.9, which means that changes in the average size of the present speciation or changes in the mass-transport mechanism are only indicated for 0 < xIL < 0.1 and 0.9 < xIL < 1.0. We note that the persistence of the principle IL structure upon the addition of a second component to xIL well below 0.5 has been suggested several times.50,86,90,91

ΛNMR =

F 2(D+ + D−) RT

(5)

where F is the Faraday’s constant. The obtained values for ΛNMR were interpolated to the mole fractions of the molar conductivity data using polynomial fit functions, as shown in Figure S2. For all four temperatures, ΛNMR is monotonically decreasing with increasing xIL because D for cation and anion in Figure 1 and Table S1 is also monotonically decreasing with increasing xIL. The ratio Λ/ΛNMR, also referred to as ionicity, is shown in Figure 4c. The value for Λ/ΛNMR for neat [C6mim][NTf2] has been reported before as 0.57,71 which is in agreement with our results. While Λ/ΛNMR is nearly constant between 0.5 < xIL < 1 in Figure 4c, it drops relatively sharply with decreasing xIL for xIL < 0.5. Evidently, even though the average length scale of the IL species involved in the masstransport mechanism does not change according to the findings from the self-diffusion measurements, the speciation is changing continuously with decreasing xIL for xIL < 0.5. The proportion of neutral species relative to charged species clearly increases with decreasing xIL and becomes dominant at the lowest investigated xIL, which correspond to millimolar IL concentrations. This can be understood in terms of a decrease in the relative permittivity with increasing CHCl3 content, which favors the formation of neutral species such as ion pairs.95,96 Another graphical way for inspecting the ionicity is the socalled Walden plot shown in Figure 5. According to the Walden rule, the product of molar conductivity and viscosity is constant, as represented by the solid line in Figure 5. As has been observed for many neat ILs and IL-rich binary systems,50,51,97 the data for the neat IL and for IL-rich

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CONDUCTIVITY Because only the movement of charged species contributes to molar conductivity, it is insightful to inspect the molar conductivity, Λ, as a function of xIL, which is shown in Figure 4a. The solution densities needed to evaluate molar

Figure 4. Inspection of the IL mole fraction dependence of molar conductivity and related properties for the [C6mim][NTf2]−trichloromethane binary system at △, 288.5K; □, 298.2 K; ○, 308.2 K; ▽, 318.2 K: (a) molar conductivity, (b) molar conductivity calculated from NMR self-diffusion measurements of cation and anion according to eq 5, and (c) ionicity.

Figure 5. Walden plot for the [C6mim][NTf2]−trichloromethane binary system at Δ, 288.5K; □, 298.2 K; ○, 308.2 K; ▽, 318.2 K. The solid line corresponds to the behavior according to the Walden rule that the product of molar conductivity and viscosity is constant. 9749


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The Journal of Physical Chemistry B concentrations fall on a line parallel to and below the solid line representing the Walden rule. The distance from the line representing the Walden rule has been considered as a measure of ionicity, where MacFarlane and coworkers have shown that a rescaling of the solution fluidity with respect to the ion size is necessary for such analysis.98 The Walden plot adjusted in this way is shown in Figure S3 of the Supporting Information, and it can be seen that it is qualitatively the same as the Walden plot in Figure 5. In any case, deviations from the Walden rule clearly beyond that which can be explained by viscosity rescaling are observed for the data points corresponding to xIL of about xIL to decreasing for xIL > 0.1, while fluidity monotonically decreases with increasing xIL throughout the entire composition range.30 Similar observations have also been made for other dilute IL solutions in organic solvents.36,60 In summary, the main insight gained from the conductivity data is that there is an increase in neutral species with decreasing xIL starting from at least xIL < 0.3, as indicated from the Walden plot if not already from xIL < 0.5, as indicated by the xIL dependence of Λ/ΛNMR. This change in speciation is despite the evidence from the self-diffusion data for constant length scales associated with the mass transport for compositions 0.1 < xIL < 0.9. Activation Energies. The temperature dependence of the obtained transport properties of self-diffusion and molar conductivity, which is generically symbolized here with X(T), was inspected according to the logarithmic form of the Arrhenius equation ln(X(T )) = ln(A) −

Ea RT

Figure 6. IL mole fraction dependence of the Arrhenius activation energy of various transport properties for the [C6mim][NTf2]− trichloromethane binary system: ○, cation self-diffusion; □, anion selfdiffusion); ×, trichloromethane self-diffusion; ▽, fluidity from Haghani et al.;30 △, molar conductivity. The lines serve as guides to the eye.

for translational motion than neutral solute species. The higher activation barrier for neutral species also points toward two competing effects on the dynamics in the [C6mim][NTf2]− CHCl3 binary system with decreasing xIL, that is, increasing CHCl3. An increasing amount of CHCl3 penetrating the IL matrix should lead to a gradual breakup of the IL network, which should speed up mass transport and lower the average radius of all charged species. However, because CHCl3 is with a dielectric constant69 of 4.7 relatively nonpolar, the IL responds with the increased formation of neutral species (ion pairs) involved in mass transport, rather than charged species (individual ions), which should slow mass transport and increase the average radius. Thus, the single length scale observed from the linear relationship in Figure 3 corresponding to compositions of 0.1 < xIL < 0.9 might be the result of a fortuitous cancelation of these two competing effects. Clearly, additional studies, which are beyond the scope of this report, are needed to better understand the mass-transport mechanism in IL−molecular solvent binary systems. Marekha et al. suggested for that purpose studies with quasi-elastic scattering techniques and molecular dynamics simulations.54 Finally, we note that the linearity of the activation energy for molar conductivity with xIL is striking in Figure 6. Linear Ea(xIL) for conductivity has also been observed for several other binary IL−molecular solvent systems61,100,101 and for the addition of Li[NTf2] into ether- and allyl-functionalized ILs.102 Although not a universal feature of IL−molecular solvent binary systems, a linearity of the activation energy with xIL has been observed for a large number of IL−molecular solvent binary systems in the case of fluidity.30 Knowledge of which structural features of IL and molecular solvents cause linear Ea(xIL) for some or all mass-transport properties would be very powerful because it would potentially allow straightforward access to the transport properties at any xIL for some binary IL−molecular solvent systems just from knowledge of transport property values of the neat components. In summary, the main insights gained from inspection of the activation energies are that (a) shear stress appears to be mitigated by the motion of CHCl3 further supporting the notion of CHCl3 segregation and (b) there could be competing effects from increasing the CHCl3 content that might result in constant average length scales in the mass transport for 0.1 < xIL < 0.9 indicated from the analysis of the self-diffusion data.

(6)

where A is the pre-exponential factor, Ea is the activation energy, and R is the gas constant. Figure S4 shows corresponding plots for the self-diffusion coefficients of cation, anion, and CHCl3, as well as for the molar conductivity. Only the range xIL > 0.3 was considered for the molar conductivity data and values at fixed xIL obtained through interpolation using polynomial fits, as shown in Figure S5 of the Supporting Information, because too few conductivity data points were obtained to allow for interpolation for xIL < 0.3. The linearity of the graphs in Figure S4 is evident for all cases. The obtained activation energies (Table S5) are graphed in Figure 6 as a function of xIL, including, for comparison, the fluidity (the inverse viscosity) activation energies from Haghani et al.30 Because the self-diffusion coefficients of cation and anion were essentially the same except for xIL > 0.9 (Figure 1, Table S1), their activation energies in Figure 6 are also the same within measurement uncertainty. The activation energies for the CHCl3 self-diffusion are generally lower than those for the cation and anion and appear to overlap with the activation energies of the fluidity. This suggests that shear stress is mitigated in the [C6mim][NTf2] binary system through the motion of CHCl3 molecules, which is in further support of CHCl3 segregation. The activation energies for the molar conductivity are generally the lowest in Figure 6 for any given xIL, which indicates that the activation barrier for translational motion is lower for charged species (individual ions) than for neutral species (ion pairs). For dilute xIL compositions, this has been explained with that charged solute species are more able to disturb the solvent structure99 and thus face a lower barrier 9750


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(2) Chaturvedi, D. Ionic Liquids, a Class of Versatile Green Reaction Media for the Syntheses of Nitrogen Heterocycles. Curr. Org. Synth. 2011, 8, 438−471. (3) Chaturvedi, D. Recent Developments on Task Specific Ionic Liquids. Curr. Org. Chem. 2011, 15, 1236−1248. (4) Kuchenbuch, A.; Giernoth, R. Ionic Liquids Beyond Simple Solvents: Glimpses at the State of the Art in Organic Chemistry. ChemistryOpen 2015, 4, 677−681. (5) Shiddiky, M. J. A.; Torriero, A. A. J. Application of Ionic Liquids in Electrochemical Sensing Systems. Biosens. Bioelectron. 2011, 26, 1775−1787. (6) Abbott, A. P.; Frisch, G.; Hartley, J.; Ryder, K. S. Processing of Metals and Metal Oxides Using Ionic Liquids. Green Chem. 2011, 13, 471−481. (7) Betz, D.; Altmann, P.; Cokoja, M.; Herrmann, W. A.; Kuehn, F. E. Recent Advances in Oxidation Catalysis Using Ionic Liquids as Solvents. Coord. Chem. Rev. 2011, 255, 1518−1540. (8) Oppermann, S.; Stein, F.; Kragl, U. Ionic Liquids for Two-Phase Systems and Their Application for Purification, Extraction and Biocatalysis. Appl. Microbiol. Biotechnol. 2011, 89, 493−499. (9) Steinrück, H.-P.; Wasserscheid, P. Ionic Liquids in Catalysis. Catal. Lett. 2015, 145 (1), 380−397. (10) García de Dios, S.; Larriba, M.; García, J.; Torrecilla, J. S.; Rodríguez, F. Liquid-Liquid Extraction of Toluene from Heptane Using 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids. J. Chem. Eng. Data 2011, 56, 113−118. (11) Larriba, M.; Navarro, P.; García, J.; Rodríguez, F. Liquid−Liquid Extraction of BTEX from Reformer Gasoline Using Binary Mixtures of [4empy][Tf2N] and [emim][DCA] Ionic Liquids. Energy Fuels 2014, 28 (10), 6666−6676. (12) Larriba, M.; Navarro, P.; García, J.; Rodríguez, F. Extraction of Benzene, Ethylbenzene, and Xylenes from N-Heptane Using Binary Mixtures of [4empy][Tf2N] and [emim][DCA] Ionic Liquids. Fluid Phase Equilib. 2014, 380, 1−10. (13) Larriba, M.; Navarro, P.; García, J.; Rodríguez, F. Liquid−liquid Extraction of Toluene from N-Heptane by {[emim][TCM]+[emim][DCA]} Binary Ionic Liquid Mixtures. Fluid Phase Equilib. 2014, 364, 48−54. (14) Larriba, M.; Navarro, P.; García, J.; Rodríguez, F. Liquid−Liquid Extraction of Toluene from N -Alkanes Using {[4empy][Tf2N] + [emim][DCA]} Ionic Liquid Mixtures. J. Chem. Eng. Data 2014, 59 (5), 1692−1699. (15) Wlazło, M.; Ramjugernath, D.; Naidoo, P.; Domańska, U. Effect of the Alkyl Side Chain of the 1-Alkylpiperidinium-Based Ionic Liquids on Desulfurization of Fuels. J. Chem. Thermodyn. 2014, 72, 31−36. (16) Rodriguez-Cabo, B.; Francisco, M.; Soto, A.; Arce, A. Hexyl Dimethylpyridinium Ionic Liquids for Desulfurization of Fuels. Effect of the Position of the Alkyl Side Chains. Fluid Phase Equilib. 2012, 314, 107−112. (17) González, E. J.; Navarro, P.; Larriba, M.; García, J.; Rodríguez, F. Use of Selective Ionic Liquids and Ionic Liquid/salt Mixtures as Entrainer in a (Vapor+liquid) System to Separate N-Heptane from Toluene. J. Chem. Thermodyn. 2015, 91, 156−164. (18) Domańska, U.; Walczak, K.; Królikowski, M. Extraction Desulfurization Process of Fuels with Ionic Liquids. J. Chem. Thermodyn. 2014, 77, 40−45. (19) Zhang, Z.; Hu, A.; Zhang, T.; Zhang, Q.; Sun, M.; Sun, D.; Li, W. Separation of Methyl Acetate+methanol Azeotropic Mixture Using Ionic Liquids as Entrainers. Fluid Phase Equilib. 2015, 401, 1−8. (20) Dohnal, V.; Baránková, E.; Blahut, A. Separation of Methyl Acetate+methanol Azeotropic Mixture Using Ionic Liquid Entrainers. Chem. Eng. J. 2014, 237, 199−208. (21) Li, Q.; Zhang, S.; Ding, B.; Cao, L.; Liu, P.; Jiang, Z.; Wang, B. Isobaric Vapor−Liquid Equilibrium for Methanol + Dimethyl Carbonate + Trifluoromethanesulfonate-Based Ionic Liquids at 101.3 kPa. J. Chem. Eng. Data 2014, 59 (11), 3488−3494. (22) Orchillés, A. V.; Miguel, P. J.; Vercher, E.; Martínez-Andreu, A. Isobaric Vapor-Liquid and Liquid-Liquid Equilibria for Chloroform +

CONCLUSIONS The transport properties of the [C6mim][NTf2]−CHCl3 were studied over the entire composition range and for several temperatures. The self-diffusion coefficients for cation and anion were found to be equivalent for all mole fractions except for xIL > 0.9, and the self-diffusion coefficient for CHCl3 was consistently about four times larger than that of the cation. Conductivity maxima were observed near xIL = 0.2. The temperature dependence of the self-diffusion coefficients and the molar conductivity could be adequately described by the Arrhenius equation. The obtained composition-dependent activation energies for the various transport properties showed varied dependencies. Most notably, Ea(xIL) was found to be linear for the molar conductivity, and Ea(xIL) for the selfdiffusion of CHCl3 overlaps with Ea(xIL) of the fluidity. Overall, the principle structure of the neat IL, which is generally understood to be a supramolecular network,103 persists upon the addition of CHCl3 to at least xIL = 0.5. At lower xIL, the proportion of neutral species increases even though the overall average length scale of species participating in mass transport appears to stay unchanged to xIL = 0.1. Neutral species become dominant at very low xIL, corresponding to IL concentrations in the millimolar range, where it has been shown that equilibrium species consist primarily of individual ion pairs and aggregates.59 The fractional Stokes−Einstein equation, where the self-diffusion is proportional to some adjustable power of the fluidity, was found to be only adequate to describe the behavior of the IL but not for CHCl3. The presence of dispersed CHCl3 segregation could be a possible explanation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b06974. Tabulated values for self-diffusion coefficients, ratio of diffusion coefficients, kBT/η−1D−1, conductivity, molar conductivity, and Arrhenius fit parameters; derivative plot of kBT/η−1D−1 with respect to xIL; various fits for interpolation purposes; Walden plot; and Arrhenius plots. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: mhoff[email protected]. Tel: 585-395-5598. Fax: 585-395-5805. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This report is based on work supported by the National Science Foundation under RUI-Grant No. 0842960. M.M.H. also gratefully acknowledges a Mercator fellowship from the Deutsche Forschungsgemeinschaft (DFG) under grant Bu 911-24-1, which enabled him a sabbatical at the Technical University Darmstadt. We thank Torsten Gutmann and Gerd Buntkowsky, TU Darmstadt for valuable discussions.

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