Effect of Alkyl Chain Branching on Physicochemical Properties of

Article pubs.acs.org/jced

Effect of Alkyl Chain Branching on Physicochemical Properties of Imidazolium-Based Ionic Liquids Published as part of The Journal of Chemical and Engineering Data special issue “Proceedings of the 6th International Congress on Ionic Liquids” Lianjie Xue,† Eshan Gurung,† George Tamas,† Yung P. Koh,‡ Michael Shadeck,§ Sindee L. Simon,*,‡ Mark Maroncelli,*,§ and Edward L. Quitevis*,† †

Department of Chemistry & Biochemistry, and ‡Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409, United States § Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: The branched ionic liquids (ILs) 1-(iso-alkyl)-3methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(N − 2)mCN‑1C1im][NTf2] with N = 3−7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(n-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CNC1im][NTf2] with N = 3− 7). For N = 4−7, the density of the branched IL [(N − 2)mCN−1C1im][NTf2] is the same as that of its linear analogue [CNC1im][NTf2] within the standard uncertainty of the measurements. In the case of the N = 3 [1mC2C1im][NTf2]/[C3C1im][NTf2] pair, the density of the branched IL is 0.13% higher than that of the linear IL. For a branched/linear IL pair with a given N, the glass transition temperature Tg, melting temperature Tm, and viscosity η are higher for the branched IL than for the linear IL. [2mC3C1im][NTf2] is an exception in that its Tm is lower than that of [C4C1im][NTf2]. Moreover, the viscosity of [2mC3C1im][NTf2] is anomalously higher than what would be predicted based on the trend of the other branched ILs. These trends in the viscosities of the linear and branched ILs are consistent with recent molecular dynamics simulations. Thermal gravimetric analysis indicates that linear ILs are thermally more stable than branched ILs. Pulsed-gradient spin−echo (PGSE) NMR diffusion measurements show that the self-diffusion coefficients of the ions vary inversely with the viscosities according to the Stokes−Einstein (SE) equation. The hydrodynamic radii of the cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs.

1. INTRODUCTION

relationships for the rational design of ILs for task-specific purposes. ILs differ from conventional solvents not only because of their ionic nature, but also because they are structurally heterogeneous on a nanoscale. This nanostructural heterogeneity was initially revealed in molecular dynamics (MD) simulations of ILs based on the 1-alkyl-3-methylimidazolium ([CNC1im]+) cation9−11 and confirmed experimentally by small-wide X-ray scattering (SWAXS) measurements on [CNC1im][Cl] and [CNC1im][BF4].12 The hallmark of this nanoscale heterogeneity is the presence of nonpolar domains (low-charge density regions) that arise from the nanosegregation of the CN alkyl chains that are embedded in or intertwined with polar domains (high-charge density regions).

Ionic liquids (ILs), which by definition are organic salts that melt at or below 100 °C, possess unique characteristics that make them attractive for a variety of scientific and technological applications. For many ILs, these characteristics include low volatility, high conductivity, wide electrochemical window, recyclability, and high miscibility with many liquids. ILs are alternatives to conventional solvents in numerous applications, including synthesis and catalysis,1 chemical separations,2 electrochemistry,3 CO2 capture,4,5 and biomass processing.6 Part of the attractiveness of ILs is that their physicochemical properties can be finely tuned for a specific application by using different cation/anion combinations and varying the structures of the ions. Thus, terms such as “designer” solvents or “taskspecific” solvents are often applied to ILs.7 It has been estimated, if one includes binary and ternary mixtures, that there are as many as 1018 possible ILs.8 Such an astronomical number necessitates a broad database of structure−function © XXXX American Chemical Society

Received: July 30, 2015 Accepted: February 11, 2016

A

DOI: 10.1021/acs.jced.5b00658 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Structures of the Linear [CNC1im][NTf2] and Branched [(N − 2)mCN‑1C1im][NTf2] ILs Investigated in This Studya

a

See Table S1 in the Supporting Information for a more complete sample description table.

decreases the boiling and melting points28 and increases the viscosities29−32 of alkanes. Andresova et al.33 recently examined the effect of branched and cyclic alkyl groups on the physicochemical properties of imidazolium-based ILs and found that the viscosities of the branched and cyclic ILs are greater than those of the analogous linear ILs. In contrast to alkanes, Erdmenger et al.34 found melting temperatures to be higher for branched ILs than for linear ILs. Kurnia et al.35 studied the effect of cation alkyl branching in ILs on the mutual solubility of water and found that branching decreased the solubility of water in the IL while increasing the solubility of the IL in water. Most recently, Pison et al.36 studied the effect of alkyl branching on the solubility of n-butane and isobutane in ILs. For both linear and branched ILs the solubility of butane was higher than that of isobutane, with the gases being more soluble in the linear IL than the analogous branched IL with the same number of alkyl carbon atoms. The difference in the solubilities of the gases was attributed to the more negative enthalpy of solvation for n-butane. Pison et al. showed through MD simulations that the difference in solubilities of the gases between branched ILs versus linear ILs is due to the slightly bulkier nonpolar domains in the branched ILs. Kashyap et al.37 used X-ray scattering measurements and MD simulations to investigate the differences in the structures of ILs based on the 1-alkyl-1-methylpyrrolidium cation with [NTf2]− as the anion and with linear, branched, and cyclic alkyl groups. The X-ray structure factor S(q) showed features characteristic of ILs: a low-q peak or prepeak associated with polarity alternations, an intermediate-q peak associated with charge

The degree to which ILs are structurally heterogeneous is mainly dictated by the length of the alkyl chains. As for the properties of ILs, Watanabe and co-workers13−15 used pulsed-gradient spin−echo (PGSE) NMR to determine the self-diffusion coefficients of cations and anions in a series of ILs that share a common cation or a common anion and correlated these coefficients with macroscopic properties such as viscosity and conductivity to describe the intrinsic relationships between molecular structure and physicochemical properties. Maroncelli and co-workers16 studied the physicochemical properties of ILs consisting of [C4C1im]+ and various anions and ILs consisting of [NTf2]− with various cations. Quitevis and Triolo and co-workers17−20 studied the effect of cation symmetry and cation alkyl chain length on physicochemical properties and nanoscale structural heterogeneity in ILs. Their results showed that asymmetric ILs tend to have a greater structural heterogeneity and higher viscosity than symmetric ILs of the same molecular weight. Martinelli et al.21 combined PGSE NMR and SWAXS to systematically investigate how local structure and diffusional motion vary with increasing alkyl chain length for 1-alkyl-3-methylimidazolium ILs. Their results showed that the self-diffusivity of the individual ionic species is correlated to the local structure in the corresponding ILs. The studies above as well as numerous other studies22−27 have focused primarily on the structure−property relationships of ILs based on cations with linear alkyl chains. In contrast, branching in the alkyl chains has not been extensively exploited as a means of systematically varying the physicochemical properties of ILs. Indeed, it is well-known that branching B



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alternations, and a high-q peak associated with intramolecular and intermolecular contact correlations. By examining the polar contributions to the cation−cation component of S(q) for cyclic and branched systems, they showed the polar−polar components scatter in a way similar to systems with linear tails of approximately the same spatial length but not the same number of carbons. We report herein, the synthesis and physicochemical properties of a homologous series of branched ILs based on the 1-(iso-alkyl)-3-methylimidazolium cation from 1-(1-methylethyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide to 1-(5-methylhexyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide. To our knowledge, this is the first systematic study of the effect of alkyl chain branching on the physicochemical properties of a homologous series of ILs. In this article, the chemical formula [(N − 2)mCN−1C1im]+ will be used to designate the branched cations, where (N − 2)mCN−1 represents an N-carbon atom alkyl chain, CN−1 being the backbone and m being the methyl group at the (N − 2) position relative to the carbon connecting the alkyl chain to the imidazolium ring. To understand branching effects in ILs, the physical properties of [(N − 2)mCN‑1C1im][NTf2] are compared to those of the corresponding [CNC1im][NTf2]. The structures of these ILs and the abbreviations used to identify them in this article are given in Table 1. This article is one of a series describing the effect of alkyl chain branching on the physicochemical properties, intermolecular dynamics (as probed by OHD-RIKES), and structure and transport properties (as revealed in MD simulations) of ILs. This article is organized as follows. In section 2, the synthesis and purification of the ILs and the methods used to measure the physicochemical properties are described. In section 3, the properties of the branched ILs (specifically, density ρ, viscosity η, glass transition temperature Tg, melting temperature Tm, thermogravimetric analysis onset temperature Tonset, and selfdiffusion coefficient D) are compared to those of the linear ILs. Differences in the properties of branched and linear ILs are discussed within the context of the structure and possible intermolecular/interionic interactions within these ILs. In particular, we will show how recent equilibrium and nonequilibrium molecular dynamics (EMD and NEMD) simulations38 are able to explain the trends in the viscosities in terms of local dynamics and molecular topology. We finish with conclusions and plans for future work.

Scheme 1. Generic Procedure for the Synthesis of the Branched 1-(iso-Alkyl)-3-methylimidazolium Bis[(trifluoromethane)sulfonyl]amide Ionic Liquids [(N − 2)mCN‑1C1Im][NTf2]

methylimidazole. To this closed system, an excess (1.05 equiv) of the iso-alkyl bromide was added. The reaction was left under constant stirring at 55 °C until complete consumption of the 1methylimidazole. The reaction times varied from 12 to 36 h, with longer times corresponding to the substitutions involving the shorter-chain branched halides. The iso-alkylmethylimidazolium bromide formed was then washed three times with 50 mL of a 4:1 v/v mixture of hexanes and dichloromethane to remove the unreacted excess alkyl bromide. The product was concentrated in vacuo using a rotary evaporator. For decolorization, 3 g of activated charcoal and 60 mL of dichloromethane were added to the viscous, slightly yellow bromide, and left under stirring for 96 h. The black slurry was then filtered through a gravitational column packed with 10 cm of aluminum oxide (activated, basic, 50−200 μm) and 3 cm of Celite 545. The removal of the solvent under reduced pressure afforded the iso-alkylmethylimidazolium bromide as a colorless, viscous liquid at room temperature. The last stage of synthesis was accomplished via a metathesis reaction. 4 0 An aqueous solution of lithium bis[(trifluoromethane)sulfonyl]amide (0.98 equiv) was added to the bromide IL and left to stir overnight. The two-phase system was then repeatedly washed with triple-deionized water until no bromide ions could be detected by the silver nitrate test. The final product, [(N − 2)mCN‑1C1im][NTf2], a colorless liquid at room temperature, was dried with a benzene azeotrope and the residual benzene removed under vacuum. The 1H NMR spectra showed only peaks of the ILs and deuteroacetone. The purity of the ILs synthesized was >99% as determined by electrospray ionization mass spectroscopy (ESI-MS) and elemental analysis. Yields: 83−91% (see Supporting Information for the sample description Table S2, elemental analysis, ESI-MS and 1H NMR spectra of the ILs). 2.2. Karl Fischer Coulometric Titration. The moisture contents of the samples were determined by Karl Fischer (KF) coulometric titration using a Mettler-Toledo DL36 KF coulometer with Aquastar Coulomat C as the catholyte and Aquastar Coulomat A as the anolyte. Sample volumes were 0.1 mL. The lowest amount of water that can be measured by this instrument is 10 μg. The moisture contents of the samples determined by KF coulometric titration are listed in Table 1. 2.3. Density Measurements. The densities of the ILs with branched and linear alkyl chains were measured as functions of temperature using an Anton Paar DMA 60/602 vibrating tube

2. EXPERIMENTAL SECTION 2.1. Ionic Liquid Synthesis and Purification. All reagents and solvents were acquired from commercial sources (Acros Organics, Alfa Aesar, and Sigma-Aldrich) and were used as received, without further purification. The purity of the reagents used in the synthesis of the ILs was >97%. (The purities of the reagents used in the synthesis are listed in Table S1 in the Supporting Information.) All reactions were run under nitrogen atmosphere using oven-dried glassware. Analytical NMR spectra were recorded on a JEOL 400 spectrometer and collected as solutions of deuterochloroform (bromide ILs) or deuteroacetone (bistriflate ILs). The synthesis of the branched ILs is summarized in Scheme 1. The synthesis of the linear ILs has been described previously.24,39 For reference, the scheme for the linear ILs is given in the Supporting Information. A one-neck round-bottom flask, under nitrogen, equipped with a magnetic stirrer and a condenser, was charged with 1C



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Table 2. Densities ρ (Corrected for Viscosity-Induced Errors) for Linear [CNC1im][NTf2] and Branched [(N − 2)mCN‑1C1im][NTf2] ILs with N = 3−7 as Functions of Temperature at 0.1 MPaa ρ (g cm−3)

a

IL

283.15 K

288.15 K

293.15 K

298.15 K

303.15 K

308.15 K

313.15 K

C3C1 C4C1 C5C1 C6C1 C7C1 1mC2C1 2mC3C1 3mC4C1 4mC5C1 5mC6C1

1.4902 1.4512 1.4175 1.3861 1.3584 1.4922 1.4503 1.4175 1.3859 1.3575

1.4852 1.4465 1.4129 1.3815 1.3541 1.4872 1.4456 1.4127 1.3813 1.3530

1.4806 1.4419 1.4084 1.3772 1.3498 1.4825 1.4411 1.4081 1.3770 1.3487

1.4756 1.4369 1.4036 1.3727 1.3454 1.4776 1.4363 1.4033 1.3724 1.3442

1.4708 1.4321 1.3990 1.3683 1.3410 1.4728 1.4316 1.3985 1.3679 1.3398

1.4658 1.4271 1.3939 1.3636 1.3364 1.4678 1.4266 1.3937 1.3633 1.3352

1.4606 1.4226 1.3893 1.3588 1.3316 1.4626 1.4219 1.3887 1.3585 1.3306

Standard uncertainties are u(T) = 0.02 K and u(ρ) = 0.0005 g cm−3.

work. Although Tf′ is measured on heating, we call this value the glass transition temperature in the remainder of the article since Tf′ is approximately equal (within 1 K) to the Tg-value that would be obtained on cooling at the same rate.46−48 One sample (1mC2C1) crystallized on cooling at 10 K/min and no Tg was observed on heating; for this sample, we employed a higher cooling rate to avoid crystallization with cooling performed by placing the sample on a heavy metal block in liquid N2. The cooling rate was found to be 600 K/min in the vicinity of Tg (190 K) and 2 times higher in the region of maximum crystallization (220−240 K). After cooling, the sample was measured at 10 K/min on heating. To compare all samples at the same rate, this same quench procedure was used on all samples. In addition to the glass transition, cold crystallization and melting behaviors were observed on heating and the onset of these thermal transitions and their associated enthalpy changes are reported. We report onset temperatures of melting because melting occurs at constant temperature for pure compounds. We adopt the same method for cold crystallization, although the crystallization is a kinetic transition and peak placement depends on heating rate. Enthalpies have the unit of joule per mole of material, not per mole of crystalline material, because of the potential presence of both amorphous and crystalline phase in the supercooled samples. The DSC temperature was calibrated with n-hexane, n-octane, mercury, and gallium at 10 K/min on heating. The heat flow of DSC was calibrated with n-octane. 2.5. Thermogravimetric Analysis (TGA). The thermal stability of ILs with branched and linear alkyl chains was examined using a Mettler-Toledo TGA/SDTA851e thermogravimetic analyzer under nitrogen atmosphere. The TGA sample mass ranged from 10 mg to 20 mg in a 70 μL standard alumina crucible. The dynamic method was employed, heating from 25 to 600 °C at 10 K/min. The onset temperature, Tonset, that is, the intersection of the zero mass loss baseline and the tangent line of mass loss, is reported for ease of comparison of the thermal stability. 2.6. Viscosity Measurements. The viscosities of the branched ILs were measured as functions of temperature by use of a TA Instruments AR2000 rotational rheometer with a Couette cell. The standard uncertainty in the viscosity of this rheometer was determined to be u(ρ) = 2 mPa·s using N35, S20, and S60 Cannon viscosity standards. These three standards cover the viscosity range of all the ILs in this study. The amount of IL sample for the measurement was approximately 10 mL. To confirm that moisture in the air has a

density meter. The density meter was calibrated in the standard way using air and tetrachloroethylene (Sigma-Aldrich, >99%). Tetrachloroethylene was chosen because of its density being in the same range as that of the ILs in this study. The measured densities were also corrected for the viscosity-induced error Δρ(η) following the procedure of Sanmamed et al.41,42 The density standards, Lube Oil Largo 32, Lube Oil A90, and Lube Oil Largo 100 (H&D Fitzgerald, Ltd.) were used to determine Δρ(η). In this study the temperature was varied between 283.15 and 313.15 K using a recirculating water bath. The IL samples were transferred to a gastight syringe in a nitrogenpurged glovebox and introduced into the inlet of the vibrating tube of the density meter, which was then capped. Over the temperature range 283.15−313.15 K, we estimate our vibrating tube density meter is capable of measuring densities with a nominal standard uncertainty u(ρ) = 0.00004 g cm−3. This estimate was determined by measuring the density of a dodecane standard (H&D Fitzgerald Ltd.) and comparing the measured values with the certified values. However, for the current work, the standard uncertainty in the density measurements was estimated from the deviation of the measured density of C6C1 from the IUPAC recommended value, as will be described below. This method yields a standard uncertainty that is more consistent with the standard uncertainties of literature density values from the NIST IL Database.43 Details of the density calibration and determination of the viscosity-induced error are described in the Supporting Information. 2.4. Differential Scanning Calorimetry. A Mettler Toledo DSC1 differential scanning calorimeter (DSC) was used with a liquid nitrogen cooling system maintained at −150 °C under nitrogen atmosphere. DSC samples were prepared in a glovebox to minimize absorption of adventitious water with samples of approximately 10 mg sealed in PerkinElmer hermetic DSC pans. Before and after DSC measurements, the DSC samples were stored in a desiccator. Prior to all DSC measurements, samples were heated to 120 °C and held for 3 min in the DSC to erase their thermal histories. Samples were then cooled to −120 °C at 10 K/min, and immediately heated again to 80 °C to obtain the limiting fictive temperature (Tf′), cold crystallization temperature (Tc), and melting temperature (Tm). The fictive temperature is determined by the Moynihan method44 as the intersection of the extrapolated liquid and glass enthalpy lines. We note that the fictive temperature is termed the limiting fictive temperature Tf′ when no isothermal aging occurs between the cooling and heating45 as in the case of this D



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minimal effect on the viscosity, the viscosity of a representative IL was measured at a specific temperature once and then again after approximately 1 h, during which time the viscosity was observed to be constant. The viscosities of C3C1, and C7C1 were also measured using this same rheometer. The viscosities of C4C1, C5C1, and C6C1 were obtained previously in our laboratories using a rheometer with a cone−plate geometry.19 The value of 73 ± 2 mPa·s for the viscosity for C6C1 at 298.15 K is in good agreement with the IUPAC-recommended value of 69.4 ± 1.4 mPa·s.49 Although IUPAC-recommended values have not yet been established for other ILs, literature values of viscosities in the NIST database43 were used as benchmarks for the measured viscosities of the other linear ILs in the current work, as will be discussed below. 2.7. Pulsed Gradient Spin−Echo (PGSE) NMR Diffusion Measurements. Self-diffusion coefficients were measured on a Bruker AV-III-850 MHz NMR spectrometer with a Di-30 probe using the longitudinal-eddy-current delay/bipolar gradient pulse pair sequence50 as described previously.51 Cation diffusion coefficients were from 1H measurements averaged over cation protons and anion diffusion coefficients from 19F measurements of the CF3 resonance. Temperatures were checked using the calibration of Riaford et al.52 Uncertainties of 10−15% in the diffusion coefficients are estimated from three independent measurements.

Table 3. Density Parameters of the Linear [CNC1im][NTf2] and Branched [(N − 2)mCN‑1C1im][NTf2] ILsa ρ0

a

αp

ionic liquid

g·cm−3

10−4 g·cm−3·K−1

10−4 K−1

C3 C1 C4 C 1 C5 C 1 C6 C 1 C7 C 1 1mC2C1 2mC3C1 3mC4C1 4mC5C1 5mC6C1

1.7682 1.7228 1.6841 1.6414 1.6097 1.7701 1.7191 1.6880 1.6435 1.6105

9.82 9.59 9.41 9.01 8.87 9.81 9.49 9.55 9.10 8.94

6.65 6.67 6.70 6.57 6.59 6.64 6.61 6.81 6.63 6.65

Parameters for the equation ρ = ρ0 − aT. αp, isobaric thermal expansion coefficient at T = 298.15 K and P = 0.1 MPa. Standard uncertainties are u(ρ0) = 0.0001 g·cm−3; u(a) = 7 × 10−6 g·cm−3·K−1; u(αp) = 3 × 10−6 K−1. a

Figure 2 shows the relative deviation of the density of C6C1 (uncorrected and corrected for viscosity-induced errors) from

3. RESULTS AND DISCUSSION 3.1. Densities. The densities of the linear CNC1 and branched (N − 2)mCN‑1C1 ILs at 0.1 mPa are listed in Table 2 and plotted as functions of temperature in Figure 1. Preliminary

Figure 2. Relative percent deviations of the experimental density ρ of [C6C1im][NTf2], uncorrected and corrected for viscosity induced errors Δρ(η), from the IUPAC-recommended density ρ of [C6C1im][NTf2]49 as a function of temperature. The dashed lines are reference lines. Also included in the plot for comparison purposes are the deviations for representative experimental densities obtained by Azevedo et al.,53 Jacquemin et al.,54 Widegren and Magee,55 Tariq et al.,56 and Kanakubo and Harris.57

Figure 1. Comparison of density data for [(N − 2)mCN‑1C1im][NTf2] and [CNC1im][NTf2] pairs with N = 3−7. See Table 2 for density values. Solid lines are linear fits of the equation ρ = ρ0 − aT to the data, with fit parameters given in Table 3.

the IUPAC-recommended density as a function of temperature. For comparison, the relative deviations of representative literature values53−57 of C6C1, for which the water content was reported to be less than 150 μg/g, are shown in this plot. The dashed lines are reference lines. As can be seen in this plot, the uncorrected density is ∼0.05(1)% greater than the IUPACrecommended density. Correcting for viscosity-induced errors brings this deviation down to ∼0.03(1)%. Although our density meter is capable of measuring densities with a nominal standard uncertainty u(ρ) = 0.00004 g cm−3, from the deviation plot in Figure 2, we estimate the standard uncertainty of the densities of the ILs in this current work to be u(ρ) = 0.0005 g cm−3. Figure 3 shows an analogous plot of the relative deviation of the density of C4C1 obtained in the current work from the density of C4C1 recently reported by Kanakubo and Harris57 with an uncertainty of 0.02%. The relative deviations of other literature values53,54,56,58 of C4C1 are also shown in this plot. As

values of the densities were reported in a previous publication.38 The densities decrease linearly with increasing temperature in the range 283.15 to 313.15 K, and at a given temperature, decrease with increasing N. Fit parameters to the equation ρ = ρ0 − aT are given in Table 3. A similar temperature dependence of the densities is observed for both the linear and branched ILs, with the value of the slope (∂ρ/ ∂T)P = −a increasing from ≈ −(9.8 ± 0.1) × 10−4 g cm−3 K−1 for N = 3 to ≈ −(8.9 ± 0.1) × 10−4 g cm−3 K−1 for N = 7 (Table 3). In contrast, the value of the isobaric thermal expansion coefficient −(∂ ln ρ/∂T)P = αP at 298.15 K is independent of N, with the average value of αp equal to 6.67(8) × 10−4 g cm−3 K−1 for the linear ILs and 6.64(5) × 10−4 g cm−3 K−1 for the branched ILs (Table 3). E



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Figure 4. Comparison of densities of [CNC1im][NTf2] with N = 3−7 at 298.15 K and P = 0.1 MPa in this current work with with the averages of literature values from the NIST database.43 Error bars as given by the 95% confidence limits in the plot are less than or equal to the size of the points. See Table 4 for densities and statistics.

Figure 3. Relative percent deviations of the experimental density ρ of [C4C1im][NTf2], uncorrected and corrected for viscosity induced errors Δρ(η), from the density ρ of [C4C1im][NTf2] reported by Kanakubo and Harris57 as a function of temperature. The dashed lines are reference lines. Also included in the plot for comparison purposes are the deviations for representative experimental densities obtained by Azevedo et al.,53 Jacquemin et al.,54 Tariq et al.,56 and Troncoso et al.58

the Supporting Information show that the densities in this current work are generally in good agreement with the densities from the NIST Database at other temperatures. Using MD simulations, Zhang et al.38 calculated that for N = 4−7, the density of the branched (N − 2)mCN‑1C1 IL to be higher than that of the analogous linear CNC1 IL by ∼0.5%. The case of N = 3 was an exception in that the density of 1mC2C1 was 0.07% lower than that of C3C1. As can be seen in Table 3, for N = 4−7 the density of (N − 2)mCN‑1C1 is lower than that of the analogous linear CNC1 on average by ∼0.05%. Moreover, in contrast to the simulations, the measurements indicate that the density of 1mC2C1 is 0.13% higher than that of C3C1. Within the experimental uncertainty of the measurements, the densities of branched and linear ILs at 298.15 K are the same, with the exception being the N = 3 linear/branched IL pair. These slight discrepancies between the measured and simulated densities are not unexpected, given that the force fields used in the simulations may not account for all the possible interactions in ILs. 3.2. Glass-Transition Temperatures and Melting Temperatures. The DSC heating curves of C6C1 and the branched sample 2mC3C1 are depicted in Figures 5a and 5b and show the transitions typically observed for the linear and branched ILs. The heating curve of C6C1 displays a glass transition at Tg = 187.4 K, an exothermic peak at 238.9 K corresponding to cold crystallization with an enthalpy ΔHcc = −15.4 J/g, and a sharp endothermic peak at Tm = 261.9 K corresponding to a melting transition with an enthalpy ΔHm = 15.4 J/g. The fact that ΔHcc = −ΔHm indicates that the sample was initially amorphous with no crystalline domains present at the start of the heating scan. Similarly for 2mC3C1, ΔHcc = −ΔHm. For both heating scans, the small change in slope between the exothermic crystallization and the endothermic melting peak may indicate the presence of multiple transitions. Other researchers have also observed a shoulder on the melting peak for C4C1 ILs having both hexafluorophosphate ([PF6]−) and ([NTF2]−) as the anion in slow heating experiments.58,59 The DSC traces for C4C1 samples from two batches are shown in Figure 6, where in one case (Figure 6a) a shoulder is observed, leading to a decrease in the temperature of fusion. Troncoso et al.58 indicated that the shoulder may be due to an impurity, and we suspect that this is the case. For the C4C1 sample, the crystallization kinetics are in an intermediate range, such that crystallization history affects the melting behavior. We

can be seen in this plot, the uncorrected density of C4C1 is ∼0.03(1)% greater than the density values of Kanakubo and Harris. Correcting for viscosity-induced errors reduces the deviation to ∼0.018(9)%, which is equal to or within the 0.02% uncertainty reported by Kanakubo and Harris for their density measurements. In their study, Kanakubo and Harris also measured the density of C6C1 as a function of temperature. Their data are one of the data sets included in the deviation plot in Figure 2. The excellent agreement of their data for C6C1 with the IUPAC-recommended densities suggests that their C4C1 densities are likely to be also accurate and therefore an appropriate reference system for determining the accuracy of the C4C1 densities in the current work. A comparison of the values of the densities of the linear CNC1 ILs with the average of values from the NIST database43 at 298.15 K and 0.1 MPa is given in Table 4 with a plot of the Table 4. Comparison of the Densities (g cm−3) of [CNC1im][NTf2] in the Current Work with the Averages of the NIST Database Values at 298.15 K and P = 0.1 MPa Ionic Liquid C3 C1

C4C1

C5C1

C6C1

C7C1

a

1.4756 Statisticsb average standard dev. 95% C.L.c a

1.4746 0.0007 0.0012

Current Work 1.4036 1.3727 NIST Database 1.4361 1.4045 1.3723 0.0012 0.0048 0.0007 0.0023 0.0023 1.4369

1.3454 1.344 0.008

−3 b

Standard uncertainty is u(ρ) = 0.0005 g cm . Values used to obtain the averages are provided in Table S10 in the Supporting Information. c 95% confidence limit.

data shown in Figure 4. Table S10 of the Supporting Information provides a complete list of the NIST database values used to compute the averages in Table 4. As can be seen from the table and the figure, the values of the densities in the current work, within the standard uncertainty of our measurements, are in good agreement with the average of the literature values from the NIST Database. The plots in Figures S3−S7 in F



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samples were also quenched at the rate of 600 K/min to measure their Tgs. This resulted in a systematic difference between Tg values measured on cooling at 600 K/min and those measured after a cooling rate of 10 K/min (see below). Using this systematic difference, we can estimate the glass transition temperature of 1mC2C1, which would be the hypothetically measured temperature for a cooling rate of 10 K/min in the absence of crystallization. DSC curves of the other ILs in the current work are given in Figures S8−S11 in the Supporting Information and in Figures 5 and 6 in our previous work.19 Table 5 summarizes the thermal properties of the branched (N − 2)mCN‑1C1 and linear CN‑1C1 ILs. With the exception of C3C1, which only has a glass transition, both the branched and linear ILs exhibit glass transitions, cold crystallizations, and melting transitions. For both linear and branched ILs, the melting transitions are strongest for the shorter alkyl chains, as evidenced by the high enthalpies of melting (Table 5). Also listed in Table 5 are values of the ratio of Tg to Tm (Tg/Tm). The value of Tg/Tm for 2mC3C1 is statistically an outlier based on Chauvenet’s criterion.62 As will be discussed below, the value of Tg/Tm for 2mC3C1 can be attributed to its anomalously low Tm. Without the outlier, the value of Tg/Tm for these ILs is 0.69 ± 0.01 (95%, N = 8), which is typical for glass-formers.63 Decomposition temperatures (Tonset) of both the branched and linear ILs obtained from TGA measurements (which will be discussed below) are also listed in Table 5. In Figure 8 are plotted the glass transition temperatures of the linear and branched ILs as functions of the alkyl carbon number N. In general for an (N − 2)mCN‑1C1/CNC1 pair with a given carbon number N, the branched IL has a higher Tg than its corresponding linear analogue. An even−odd alternation in Tg is quite evident for the branched ILs, with Tg for 2mC3C1 appearing to be anomalously high compared to the other branched ILs, whereas the even−odd alternation is less evident or nonexistent for the linear ILs. Note that even−odd alternation in Tg has been observed previously in networkforming ionic glass-formers.64 In Figure 9 the melting temperatures of the linear and branched ILs as functions of the carbon number N are plotted. For linear ILs, Tm is lowest for N = 5 and 6; a similar minimum has been observed in the data complied by Zhang et al.65 as well as in the review by Rooney et al.66 Literature values of Tm and the enthalpy fusion ΔHm for the linear CNC1 ILs from NIST database43 are compared with the values in this current work in Table S11 in the Supporting Information. Similar to what is observed in the case of Tg, Tm of the branched IL for a given (N − 2)mCN‑1C1/CNC1 pair is higher than Tm of its corresponding linear analogue, with the exception again being that of 2mC3C1. But surprisingly, in contrast to Tg, the Tm of 2mC3C1 is lower than the Tm of its linear analogue C4C1 (256.9 K versus 266.7 K). 3.3. Thermal Stability (TGA Results). The dynamic TGA curves at a heating rate of 10 K/min are shown in Figure 10 panels a and b for the linear and branched ILs, respectively. The linear ILs have the better thermal stability than the branched ILs as indicated by the higher value of Tonset not only for the same carbon number, but also for all the carbon numbers in the study. The Tonset of the linear C7C1 is 710 K, in good agreement with 710.2 K of our previous study.67 Although the linear ILs show no significant dependence on the carbon number N, the thermal stability of branched ILs increases with carbon number N from 2 to 5 and then is similar for N = 5 and 6. In summary,

Figure 5. DSC heating curves of (a) [C6C1im][NTf2] and (b) [2mC3C1im][NTf2] (10 K/min cooling, 10 K/min heating) showing glass transition, cold crystallization, and melting transition. See Table 5 for thermal properties.

Figure 6. DSC heating curves of [C4C1im][NTf2] (10 K/min cooling, 10 K/min heating) (a) with a shoulder and (b) without a shoulder showing glass transition, cold crystallization, and melting transition. See Table 5 for thermal properties.

performed isothermal crystallization for the C4C1 sample at 233.2 K for 1 h (after cooling first to 153.2 K) and the heat of fusion is slightly lower than that observed from dynamic scans (see the footnote in Table 5) and in good agreement with results from adiabatic calorimetry.60,61 Figure 7 shows DSC cooling and heating curves for 1mC2C1. For a cooling rate of 10 K/min, crystallization occurs during the cooling process (black curve in Figure 7), because the shorter alkyl chain in 1mC2C1 does not strongly hinder crystallization. For this IL, complete crystallization is achieved on cooling at 10 K/min and thus, neither a glass transition nor cold crystallization is observed in the heating curve (red curve in Figure 7). To determine the Tg of 1mC2C1, the sample was quenched through Tg at the rate of 600 K/min. The heating curve of the quenched sample (blue curve in Figure 7) exhibits a glass transition step, as well as cold crystallization and melting peaks. Because Tg depends on the cooling rate, all other G



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Table 5. Thermal Properties of the Linear [CNC1im][NTf2] and Branched [(N − 2)mCN‑1C1im][NTf2] ILsa ionic liquid b

C3C1 C4C1c C5C1b C6 C 1 C7 C 1 1mC2C1d 2mC3C1 3mC4C1 4mC5C1 5mC6C1

Tg

ΔCp

Tcc

ΔHcc

Tm

ΔHm

K

J·g−1·K−1

K

kJ·mol−1

K

kJ·mol−1

Tg/Tm

K

184.0 184.2 186.5 187.4 187.7 190.0 191.9 189.4 192.0 191.8

0.35 0.38 0.31 0.43 0.41 0.37 0.35 0.36 0.39 0.39

226.5 240.5 238.9 226.9 222.3 241.0 230.0 230.1 260.2

20.3 1.4 6.9 0.01 15.5 7.0 0.01 0.02 0.5

269.6 262.6 261.9 272.2 282.1 256.9 273.1 276.3 284.3

25.0 1.3 6.9 0.4 23.9 6.8 0.1 0.1 0.5

0.683 0.710 0.716 0.690 0.674 0.747 0.694 0.695 0.675

712 710 709 710 710 680 689 704 706 706

Tonset

Tg, glass transition temperature; ΔCp, heat capacity change at Tg; Tm, Tcc, cold crystallization temperature; ΔHcc, enthalpy of cold crystallization; Tm, melting temperature; ΔHm, enthalpy of melting. Tonset, the intersection of the zero-mass loss baseline and the tangent line of mass loss in TGA measurement. Maximum standard uncertainties are u(Tg) = 1.4 K, u(ΔCp) = 0.05 J g−1 K−1, u(Tcc) = 1.2 K, u(ΔHcc) = 3.2 kJ mol−1, u(Tm) = 0.3 K, u(ΔHm) = 0.3 kJ mol−1 with exception of C6C1 where u(ΔHm) = 3.2 kJ mol−1. bReference 19. cFor isothermal crystallization at 233.2 K for 1 h (after first cooling to 153.2 K), melting properties change: Tm = 268.5 ± 1.5 K and ΔHm = 24.1 ± 0.6 kJ mol−1. dProperties obtained on heating at 10 K/ min after cooling at 10 K/min except in the case of 1mC2C1 which was quenched at 600 K/min. a

Figure 7. DSC scans of [1mC2C1im][NTf2]: (black) 10 K/min cooling; (red) 10 K/min heating after 10 K/min cooling; (blue) 10 K/ min heating after quenching at 600 K/min. Inset image shows an enlarged view of the DSC scans in the region of glass transition. See Table 5 for thermal properties.

Figure 9. Melting temperatures Tm for branched and linear ILs vs alkyl chain carbon number N (see Table 5 for Tm values).

the branched ILs have weaker thermal stability than the linear ILs and only the branched ILs shows a dependence on carbon number, with thermal stability increasing from N = 2 to 5. 3.4. Viscosities. In Figure 11 are plotted the viscosities of the branched and linear ILs as a function of temperature with the measured values listed in Table 6. The values of the viscosities in Table 6 at 298.15 K were reported in a previous publication.38 The solid lines through the data in Figure 11 are fits of the Vogel−Fulcher−Tammann (VFT) equation68−70 ⎛ B ⎞ η = η0 exp⎜ ⎟ ⎝ T − T0 ⎠

(1)

with fit parameters η0, B, and T0 listed in Table 7. The viscosities of branched-linear IL pairs with the same carbon number N at 298.15 K are plotted in Figure 12. Figures 11 and 12 clearly show that for a given carbon number N, the branched IL is higher in viscosity than the corresponding linear IL. For the most part, the viscosities of these ILs increase monotonically with increasing alkyl chain length as found previously.15,19,20,71 However, in the case of 2mC3C1, its viscosity is anomalously higher than what would be predicted based on the viscosity trend of the other branched ILs.72

Figure 8. Glass transition temperatures for branched and linear ILs as a function of alkyl chain carbon number N: (black) Tg of linear ILs measured after 10 K/min rate of cooling; (red) Tg of branched ILs measured after 10 K/min rate of cooling; (blue) Tg of branched ILs measured after quenching at 600 K/min. See Table 5 for Tg values.

H



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Figure 10. Dynamic TGA curves (heating rate =10 K/min) for the linear (a) and branched (b) ILs. Figure 11. Comparison of viscosity data for [(N − 2)mCN‑1C1im][NTf2] and [CNC1im][NTf2] pairs with N = 3−7. (See Table 6 for viscosity values.) Solid lines are fits of the VFT eq (eq 1) to the data, with fit parameters given in Table 7.

A comparison of the values of the viscosities of the linear CNC1 ILs with the average of the literature values from the NIST database43 at 298.15 K and 0.1 MPa is presented in Table 8, and these data are plotted in Figure 13. Table S12 of the Supporting Information provides a complete list of the NIST database values used to compute the averages in Table 8. As can be seen from the table and figure, the values of the viscosities in the current work, within the standard uncertainty of our measurements, are in good agreement with the average of the values from the NIST database. The plots in Figures S12−S16 in the Supporting Information show that the viscosities of the linear CNC1 ILs in this current work are generally in good agreement with the viscosities of the linear CNC1 ILs from the NIST Database at other temperatures. In MD simulations of 29 ILs spanning a broad range of cations and anions, Zhang and Maginn73 showed that selfdiffusivities are linearly correlated to the inverse of the ion-pair (IP) lifetime. This linear correlation was independent of temperature and the nature of the IL. Because a given ion in an IL is surrounded by multiple counterions, the concept of a physical IP is not meaningful. Nonetheless, Zhang and Maginn found it useful to define an IP by the anion in closest contact with a cation and vice versa. The IP lifetime is then given by the time integral of the IP correlation function associated with the probability that an IP is present at time t. To see whether IP lifetimes could be used to explain the trends in the viscosities in the current work, Zhang et al.38 performed equilibrium MD simulations on the branched and linear ILs and calculated their viscosities from the time integral of the stress tensor

autocorrelation following the Green−Kubo equation. In agreement with experiment, the calculated viscosities of branched ILs were higher than those of the linear ILs. The calculated viscosities of both series were found to be linearly correlated to IP lifetimes. Zhang et al. also showed that the IP lifetime was correlated to the potential of mean force (PMF) associated with the more stable packing of the cations and anions. Moreover, Zhang et al. found that the abnormally high viscosity of 2mC3C1 is due to the specific shape and length of the alkyl chain, which blocks the approach of the anion to the side of the imidazolium ring. Because of the difficulty of simulating viscosities with equilibrium methods, viscosities were simulated at 400 K, and therefore could not be quantitatively compared to the experimental viscosities. However, nonequilibrium simulations performed by Zhang et al. at 298 K yielded results that were in good quantitative agreement with the experimental viscosities after normalizing with the viscosity of the shortest (N = 3) linear IL. The nonequilibrium simulations also reproduced the abnormally high viscosity of 2mC3C1. 3.5. Pulsed Gradient Spin Echo Diffusion. Figure 14 depicts and Table 9 summarizes the self-diffusion coefficients of cations and anions at 298.15 K as functions of the alkyl chain carbon number N for the two series. As seen in Figure 14, the I



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Table 6. Viscosities of the Linear [CNC1im][NTf2] and Branched [(N − 2)mCN‑1C1im][NTf2] ILs as Functions of Temperature at P = 0.1 MPaa η, mPa·s ionic liquid C3C1 C4C1b C5C1b C6C1b C7C1 1mC2C1 2mC3C1 3mC4C1 4mC5C1 5mC6C1 a

278.2 K

288.15 K

298.15 K

308.15 K

318.15 K

103 133 165 209 234 278.15 K

64 79 97 118 132 288.15 K

43 51 61 73 82 293.15 K

30 35 42 49 54 298.15 K

22 26 30. 35 37 303.15 K

122 264 195 237 306

75 139 112 137 164

59 111 86 107 124

48 81 69 85 96

39 63 56 69 76

328.15 K 17 20. 22 26 27 313.15 K 28 40 38 45 50

338.15 K 13 16 18 20 20 323.15 K

333.15 K

21 28 27 32 35

16 20 20 23 25

Standard uncertainties are u(η) = 2 mPa·s, u(T) = 0.02 K, and u(P) = 0.3 kPa. bReference 19.

Table 7. VFT Fit Parameters of Linear [CNC1im][NTf2] and Branched [(N − 2)mCN‑1C1im][NTf2] ILsa ionic liquid C3C1 C4C1 C5C1 C6C1 C7C1 1mC2C1 2mC3C1 3mC4C1 4mC5C1 5mC6C1

η0, mPa·s 0.150 0.161 0.200 0.170 0.19 0.122 0.11 0.131 0.203 0.132

± ± ± ± ± ± ± ± ± ±

0.004 0.082 0.081 0.052 0.03 0.086 0.05 0.064 0.121 0.02

B, K 849 841 792 836 824 889 853 885 817 883

± ± ± ± ± ± ± ± ± ±

8 152 108 82 40 207 19 140 149 42

Table 8. Comparison of the Values of the Viscosities (mPa·s) of [CNC1im][NTf2] in the Current Work with the Averages of the NIST Database Values at 298.15 K and P = 0.1 MPa

T0, K 148.1 152.6 160.1 160.5 162.7 149.4 168 157.0 162.4 164.1

± ± ± ± ± ± ± ± ± ±

Ionic Liquid

0.7 12.2 9.1 6.5 3.1 17.1 3 11.3 11.6 3.1

C3 C1 43 Statisticsb average standard dev. 95% C.L.c

44.7 1.2 1.8

C4C1

C5C1

Current Worka 51 61 NIST Database 50.6 60.6 0.7 2.4 0.5 6.0

C6 C1

C7C1

73

82

70.8 3.9 1.9

82.3 1.8 4.4

a

Standard uncertainty is u(η) = 2 mPa·s. bValues used to obtain the averages are provided in Table S12 in the Supporting Information. c 95% confidence limit.

a See eq 1 for definition of VFT parameters. Standard uncertainties u(η0), u(B), and u(T0) as given in table.

Figure 12. Viscosity of branched and linear ILs vs alkyl chain carbon number N at T = 298.15 K and P = 0.1 mPa (see Table 6 for viscosity values).

Figure 13. Comparison of viscosity values of [CNC1im][NTf2] with N = 3−7 at 298.15 K and P = 0.1 MPa in this current work with the averages of literature values from the NIST database.43 Error bars correspond to 95% confidence limit. See Table 8 for viscosity values and statistics.

self-diffusion coefficients of the linear cations are generally (except for N = 5) higher than those of the corresponding branched cations. The self-diffusion coefficient of the [NTf2]− anion is generally smaller than that of the cation in a given liquid. Measured values of the self-diffusion coefficients of the cations and anions are listed in Table 9. Like viscosity, the diffusion coefficients of the branched series are a more complex function of chain length than is the case with the linear series.

However, rather than the N = 4 case standing out as anomalous, an even−odd alternation is apparent in the diffusion coefficients of both components of the branched series. Any such dependence is much more subtle or nonexistent in the linear series. That the self-diffusion coefficient of the anion is generally smaller than that of the cation for a given liquid is interesting given that the size of the anions are smaller than that of the J



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To the extent that a hydrodynamic description is appropriate at molecular levels, diffusion and viscosity may be simply related via the Stokes−Einstein (SE) equation: D=

kBT CπηR hyd

(2)

where kB is the Boltzmann constant, T is the absolute temperature, C is a constant, which depends on the hydrodynamic boundary conditions (6 for stick and 4 for slip),49 and Rhyd is the effective hydrodynamic radius of the diffusor. In Figure 15 are plotted the values of Rhyd, with C set

Figure 14. Cation and anion self-diffusion coefficients vs alkyl chain carbon number N at T = 298.15 K and P = 0.1 MPa (see Table 9 for self-diffusion coefficient values).

cations. However, similar behavior has also been observed in MD simulations74−76 as well as other experiments.77 Urahata and Ribeiro78 were the first to give an explanation for this phenomena using MD simulations. The reason why imidazolium cations have higher self-diffusivities than the anions is because of the preferential displacement of the cation ring along the direction of the carbon at the 2 position of the ring, which is the same direction that exhibits the lowest frequency contribution to the vibrational density of states obtained by Fourier-transforming the velocity-time correlation functions. This suggests that other large cations may not display the same behavior. A combined MD and NMR study of various alkylpyridinium cations conducted by Cadena et al.79 showed that the 1-n-octyl-3-methylpyridinium ([C8mpy]+) cation and 1-n-hexyl-3,5-dimethylpyridinium([C6dmpy]+) cation had lower self-diffusion coefficients than the [NTf2]− anion, confirming that not all the cations have higher self-diffusion coefficients than anions, and that the imidazolium cations might have higher diffusion coefficients due to their structures and or intermolecular dynamics.

Figure 15. Hydrodynamic radii (Rhyd = kBT/4πDη) and van der Waals radii RvdW of ions vs alkyl chain carbon number N at T = 298.15 K and P = 0.1 MPa (see Table 9 for values of Rhyd and RvdW).

equal to 4, which previous studies have shown often provides reasonable estimates for the self-diffusion coefficients of imidazolium-based ILs21 and many other liquids.51 Figure 15 shows that the relationship between self-diffusion and viscosity in both series of lLs conforms reasonably to the hydrodynamic predictions. Rhyd increases linearly with the carbon number N, with Rhyd being just slightly larger for linear ILs than for branched ILs. The hydrodynamic radius of the [NTf2]− anion is approximately independent of N, whereas the radii of the

Table 9. Self-Diffusion Coefficients, Hydrodynamic Radii and Van Der Waals of Cations and Anions of Ionic Liquids at T = 298.15 K and P = 0.1 MPa.a ionic liquid C3C1 C4C1 C5C1 C6C1 C7C1 1mC2C1 2mC3C1 3mC4C1 4mC5C1 5mC6C1

D+

D−

Rhyd+

Rhyd−

RvdW+

RvdW−

m·s−1

m·s−1

10−10 m

10−10 m

10−10 m

10−10 m

± ± ± ± ± ± ± ± ± ±

3.17 3.30 3.42 3.53 3.64 3.17 3.30 3.42 3.53 3.64

3.36 3.36 3.36 3.36 3.36 3.36 3.36 3.36 3.36 3.36

4.5 2.9 2.2 2.0 1.5 4.2 2.3 2.3 1.6 1.4

± ± ± ± ± ± ± ± ± ±

0.4 0.3 0.2 0.2 0.1 0.4 0.2 0.2 0.2 0.1

3.1 2.1 1.9 1.8 1.4 2.9 1.8 2.0 1.5 1.4

± ± ± ± ± ± ± ± ± ±

0.3 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.1

1.7 2.2 2.4 2.2 2.7 1.6 1.8 2.0 2.4 2.4

± ± ± ± ± ± ± ± ± ±

0.2 0.3 0.3 0.3 0.4 0.2 0.3 0.3 0.3 0.3

2.4 2.9 2.8 2.5 2.8 2.3 2.3 2.3 2.5 2.5

0.3 0.4 0.4 0.4 0.5 0.3 0.3 0.3 0.3 0.4

a + D , self-diffusion coefficients of cations; D−, self-diffusion coefficients of anions; Rhyd+, hydrodynamic radius of cations; Rhyd−, hydrodynamic radius of anions, RvdW+, van der Waals radius of cations; RvdW−, van der Waals radius of anions. Standard uncertainties are u(T) = 0.02 K, u(P) = 0.3 kPa, and u(D+/D−) as given the table.

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cations increase with N approximately in proportion to their van der Waals radii (RvdW, dashed lines). Both components of these ILs diffuse more rapidly than predicted by the SE equation, with values of Rhyd/RvdW averaging 0.8 ± 0.1 for [NTf2]− and 0.6 ± 0.1 for the cations. The departure from hydrodynamic predictions is systematically larger (Rhyd/RvdW smaller) for the branched series compared to the linear series. Overall these data do not suggest any unusual behavior with respect to how viscosity influences diffusional motion in these systems.

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4. CONCLUSIONS Comparison of the physicochemical properties of the homologous series of branched ILs [(N − 2)mCN‑1C1im][NTf2] with those of their linear counterparts [CNC1im][NTf2] reveals a number of interesting differences. Whereas the densities of a branched/linear IL pair with a given alkyl carbon number N are equal to within experimental errors (except for the N = 3 pair), glass transition temperatures, melting temperatures, and viscosities of branched ILs are systematically higher than those of their linear analogues. An even−odd alternation is apparent in both the glass transition temperatures and self-diffusion constants of the branched series. Such an alternation is absent or greatly muted in the linear series. As expected from the SE equation, the selfdiffusion coefficients of cations and anions of branched ILs are lower than those of their linear analogues. The hydrodynamic radii of cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs. The values of Tg, Tm, and η for 2mC3C1 are anomalous when compared to the corresponding values of of Tg, Tm, and η for the other branched ILs, suggesting that the liquid structure and/or interactions in this IL may be different than in the other branched ILs. On the basis of TGA measurements, linear ILs are systematically more stable than branched ILs. Ultimately, a detailed molecular understanding of how alkyl chain branching affects the glass transition, viscosity, and ion diffusivities in alkylimidazolium ILs can only be gleaned from MD simulations on these systems. Indeed, the recent EMD and NEMD simulations of Zhang et al.38 have yielded mechanistic insights into why the viscosities of branched ILs are higher than those of linear ILs, and in particular, why the viscosity of 2mC3C1 is anomalously higher than what would be predicted on the basis of the other branched ILs. As discussed above, the simulations show that the higher viscosities in the branched ILs are due to the relatively more stable packing between the cations and the anion, whereas the abnormal viscosity of 2mC3C1 was found to be the result of the specific side chain length and molecular structure. MD simulations to understand the behavior of the self-diffusion constants and the hydrodynamic radii of the ions and glass transition temperatures in these ILs are currently in progress in our laboratories.

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and correction for viscosity-induced errors in the density; comparison of density values of linear ILs in the current work with literature values from NIST Database; comparison of melting temperatures and enthalpies of fusion in current work with literature values from NIST Database; comparison of viscosity values of linear ILs in the study with literature values from NIST Database; comparison of the viscosities of two different batches of 2mC3C1 synthesized using two different sources of 2methylpropyl bromide (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Funding

This research was supported by the National Science Foundation (CHE 1153077 to E.L.Q. and DMR-1006972 to S.L.S.) and the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DE-SC0008640 to M. M.). We also thank the National Science Foundation for funding the purchase of the JEOL 400 NMR spectrometer (CHE 1048553) used in this project. Notes

The authors declare no competing financial interest.

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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.jced.5b00658. Synthesis of the linear ILs; purity of the reagents used in the synthesis; sample description table; 1H NMR spectra of the branched ILs; elemental analysis of the ILs, ESIMS of the branched and linear ILs; density calibration L



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