Physicochemical Properties of Ether-Functionalized Ionic Liquids

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Physicochemical Properties of Ether-Functionalized Ionic Liquids: Understanding Their Irregular Variations with the Ether Chain Length Zheng Jian Chen, Yanan Huo, Jun Cao, Lin Xu, and Shiguo Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02875 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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Industrial & Engineering Chemistry Research

Physicochemical Properties of Ether-Functionalized Ionic Liquids: Understanding Their Irregular Variations with the Ether Chain Length Zhengjian Chen,†,‡ Yanan Huo,† Jun Cao,† Lin Xu,*,† and Shiguo Zhang*,‡ †

Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou

Education University, Guiyang 550018, China. ‡

College of Materials Science and Engineering, Hunan University, Changsha 410082, China.

KEYWORDS: Ionic Liquid, Ether Functionality, Physicochemical Property, Irregular Variation

ABSTRACT: Ether groups are well-known for their unique contribution to low viscosity and high conductivity, and hence ether-functionalized ionic liquids (ILs) have been widely studied and successfully employed in various applications. However, the ether chain length effect on physicochemical properties is complex and still lack of a systematic study. In this study, an attempt was made to provide a thorough understanding for such complexities. A series of ILs functionalized with various ether groups (CmOCn-, n, m =1, 2, or 3) were synthesized and characterized, and their properties with irregular variations along the ether chain length were recorded and systematically analyzed. Generally, the irregular variations are mediated by three interrelated factors: the Cm- tail length, the -Cn- spacer length and hydrogen bonding interaction.

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For example, though ROC2- is bulkier by a CH2 unit than ROC1-, ROC2-based imidazolium ILs are less viscous and more conductive than ROC1-based analogues, since ROC1- is apt to form intermolecular rather than the five-membered-ring intramolecular hydrogen bonding with the imidazolium ring H atoms, while for ROC2- the six-membered-ring intramolecular hydrogen bonding comes into prominence.

INTRODUCTION Ionic liquids (ILs), a class of non-molecular compounds that melt below 100 °C, are widely known for their distinctive physicochemical properties (such as negligible vapor pressure and low flammability) and wide-ranging applications.1,

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Recently ILs are evolving from general

solvents to novel functional materials with diverse applications such as lubricants or magnetic and optical fluids,3 especially for task-specific or functionalized ILs.4 Nevertheless, functionalized ILs are usually less chemically and electrochemically stable and more viscous than their non-functionalized analogues, since the incorporation of vulnerabilities and the increase in polarity and ion size caused by functional groups, such as -CN, -SO3H, -CO2- and NH2,4 except for ether groups5. Ether-functionalized ILs make an exception for their lower viscosity and higher conductivity and comparable (electro)chemical resistance, in comparison with alkyl ILs.5 The highly flexible ether chains are inclined to pack less efficiently and thus provide more available free volume to enable faster mass transfer rates, in comparison with alkyl chains,6-9 for example, at 25 °C [MOE-EDMAm]NTf2 (63.6 cP and 3.1 mS cm-1) vs. [Bu-EDMAm]NTf2 (99.3 cP and 1.86 mS cm-1),6 in which MOE, Bu, EDMAm and NTf2 stand for CH3OC2H4-, C4H9-, N-ethyl-N,Ndimethylammonium and bis(trifluoromethanesulfonyl)imide respectively. Ether functionalities


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involve a decrease in the electron density of the cationic nitrogen atom, and usually result in a decrease of the cathodic stability as well as the overall electrochemical stability.10 Fortunately, this effect is very limited, and ether-functionalized ammonium ILs also exhibit electrochemical windows up to 5.5 V.6,

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So far, the ether groups used for functionalization of ILs include

monoethers,11-18 polyethers,12-15, 18,

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multi-(mono or poly)ethers,20-22 crown ether23 and ether

bridge,24 which are mainly linked to organic cations as well as inorganic anions, for example [CF3OCF2CF2BF3-].14, 25 These ILs are extensively used for various applications, such as gas absorbents,14, 18 desulfurization,15 phase separation,16 catalysts,19, 23 and particularly electrolytes in supercapacitors17 and secondary batteries20-22. Undoubtedly, the research into the topic of ether-functionalized ILs is far from exhaustive, because of the infinite possibilities for designing new species and their potential applications.5 Meanwhile, as a result of the huge number of possible ILs,2 great efforts were made to develop predictive methods for IL design to specific applications.26-28 However, most of the methods are empirical or semi-empirical and require the input of experimental results,26, 27 such as group contribution methods.28 Since the great variety in structures, many influencing factors should be included and involved,1 such as electrostatic interaction,29 hydrogen bond,30 polarity,31 side chain length,32 functional groups,4 as well as the combination of anion and cation,33, 34 leading to less reliable predictions.27 This is particularly evident for ether-functionalized ILs, since the effect of ether functionalities on physiochemical properties varies strongly and is very sensitive to minor changes in structures.5 So the experimental study in properties is still primarily important for ILs to be properly applied, as well as for providing reference data for more reliable predictions. In this study, a series of ILs functionalized with various ether groups were prepared and characterized, and their properties with irregular variations along the chain length were recorded


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and interpreted by multivariate analysis of the ether chain structures and hydrogen bonding interactions. EXPERIMENTAL SECTION Synthesis and Characterization A schematic representation of the ether-functionalized ILs used in this study is presented in Figure 1. The ether groups are abbreviated as CmOCn-, where m stands for the number of carbon atoms in the alkyl tail and n stands for the number of carbon atoms of the spacer between the cationic nitrogen atom and the ether oxygen atom. These ILs were synthesized by a two-step procedure, as previously described.6, 12 Firstly, the Menshutkin reaction of 1-Methylimidazole (MIm), Pyridine (Py) or 1-Methylpyrrolidine (MPyr) with the respective ether bromides was carried out to prepare bromide salt precursors. After twice recrystallization, the bromide salts were then used to react with lithium bis(trifluoromethanesulphonyl)imide (LiNTf2) for anion exchange to form NTf2-based room temperature ILs. The structures of the as-prepared ILs were confirmed by 1H-NMR (JEOL ECX-500) and FT-IR (Nicolet 6700) (see details in Supporting Information). NMR purity was estimated by integration of related and unrelated signals (except solvent, TMS and water peaks) in 1H-NMR spectra, and to be > 98.0% for all ILs. Br- ion impurity was measured on a LeiCi PXSJ-216F ion meter, and to be < 80 ppm for all ILs. These purity-related data and molecular mass and water content (ppm) are presented in Table 3. All chemicals were purchased as reagent grade from J&K Scientific Ltd. or Sigma-Aldrich and used without further purification. Apparatus and Measurements


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Before each measurement, all ILs would be vacuum dried for 20 h at 80 °C and 10-2 ~ 10-3 mbar to reduce the impact of water, as done usually.6 Water content was determined on a WA3000 coulometric Karl Fischer titrator with uncertainty < ±1 ppm. The solid-liquid phase transition was recorded using a Mettler Toledo differential scanning calorimeter (DSC-822e) under N2 atmosphere, on a cooling and heating cycle from 50 to -110 °C at a scanning rate of 10 °C min-1. The thermal stability was evaluated using a Mettler Toledo TGA 1 under N2 atmosphere, and the resulting thermal decomposition temperature was recorded at 5% mass loss. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) of [C2OC1-MIm]NTf2 was determined by using a pyrolyzer equipped with Agilent GC-7890A and MS-5975C, by pyrolysis at its thermal decomposition temperature of 262.8 °C. Density (g cm-3) was determined using a DMA 35 portable density meter at 25 °C or a 10 mL pycnometer as a function of temperature from 10 to 80 °C. The densitometers were calibrated with ultrapure water, and the deviations are less than 0.0005 g cm-3. Surface tension (mN m-1) was investigated on a QBZY-2 automatic surface tension meter, with uncertainty less than 0.04 mN m-1. Viscosity (cP) was determined on a Haake RS6000 rheometer/viscometer with a cone-plate sensor system (C60/1° Ti L) with deviation < 1%, under N2 atmosphere. Conductivity (mS cm-1) was estimated using a Mettler Toledo FE30K conductivity meter, which was accurate within ± 1%. Refractive index was measured by an Atago NAR-1T liquid Abbe Refractometer with precision of 0.0002. During measurement, the temperature was controlled to be ± 0.1 °C by means of an external controller (Thermo Scientific Haake A40 Waterbath). Meanwhile, the influence of moisture absorption on properties was studied for 8 ILs. They were vigorously stirred for ~ 50 h in open air to absorb moisture until it reaches an equilibrium with ambient environment (15 ~ 25 °C, 60 ~ 80%RH). Then the absorption capacity, density (ρ),


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refractive index (n), surface tension (γ), conductivity (κ) at 25 °C, viscosity (η) from 10 to 80 °C and its activation energy (Eη) were measured, as shown in Table S1 and S2. RESULTS AND DISCUSSION Thermal Properties The thermal properties of ILs, such as glass-transition temperature (Tg), melting point (Tm), enthalpy of melting (∆Hm), crystallization temperature (Tc), enthalpy of crystallization (∆Hc) and thermal decomposition temperature (Td), are presented in Table 1. During the DSC cycle, the observed liquid-solid phase transitions are all simple and regular, except for [C1OC1-Mpyr]NTf2. In most cases, 12 of the total 18 ILs show only liquid-glass transition without crystallization and melting. These ILs prefer to solidify as a glass rather than crystallize, most likely because their crystallization is impeded by the existence of two co-conformations of NTf2 anion35 and the torsional flexibility of ether chains.5, 36 For the rest 6 ILs, they all crystallize during the cycle, and 5 of them undergo cold crystallization on heating between their glass transition point and melting point. Only [C1OC1-MPyr]NTf2 crystallizes on cooling, and undergoes polymorphic transitions, as shown in Figure S1. While there is no glass transition after the crystallization on cooling, since glass transition is a second-order transition between amorphous (liquid and glass) states, instead of crystalline state.37 All the studied ILs, except for [C1OC1-MPyr]NTf2, were shown to undergo a glass transition with Tg ranging from -77.3 to -98.7 °C. No clear relationship was found between the Tg value and the ether chain length, like alkyl chains.38 While MPyr-based ILs are all of lower Tg values than their MIm- and Py-based counterparts, for example, [C2OC1-MPyr]NTf2 (-98.7 °C) vs. [C2OC1-MIm]NTf2 (-78.6 °C) and [C2OC1-Py]NTf2 (-78.2 °C). This result likely relates to the


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presence of hydrogen bonding of the cationic ring H atoms with the ether O atom in MIm- and Py-based ILs,12, 13, 39 which enhances the total cohesive energy.30, 40 The obtained melting points are all less than 0 °C, ranging from -0.2 to -21.0 °C, while much higher than the corresponding crystallization temperatures in the range from -34.2 to -55.0 °C, indicating their strong tendency to supercool. The enthalpies of melting and crystallization of ILs are ~ 40 J g-1, close to common ILs,41 except for [C3OC2-Py]NTf2 of only around 7.3 J g-1. It appears that [C3OC2-Py]NTf2 does not crystallize completely and preserves amorphous portion. The weight loss curves obtained from TGA analysis are typically illustrated in Figure 2, and the temperature at 5% mass loss was recorded as Td, varying in the range of 262.5 ~ 400.5 °C. Generally, the thermolysis of ILs proceeds via reverse Menshutkin reaction (dealkylation), and involves two main paths: the cleavage of N+─C bond (SN1) or the nucleophilic attack of anion (SN2).42, 43 For the ether-functionalized ILs, their thermal stability strongly depends on the -Cnspacer length in CmOCn-. For ROC1-functionalized ILs with a short -C1- spacer, their Td values are all less than 300 °C, much lower than those with a longer -C2- or -C3- spacer, for example, [C1OC1-MIm]NTf2 (276.5 °C) vs. [C1OC2-MIm]NTf2 (400.5 °C) and [C1OC3-MIm]NTf2 (383.3 °C). In comparison with ROCn- (n > 1), the CH2 unit in ROC1- chain is adjacent to both the cationic N+ atom and the ether O atom, and their synergistic electron-withdrawing effect on CH2 unit makes ROC1- easier to cleave or more susceptible to nucleophilic attack, resulting in relatively lower thermal stability for the corresponding ILs. In addition, the thermolysis process of ROC1-functionalized ILs is also quite distinctive from others, as shown in Figure 2. ROCn-functionalized (n > 1) ILs decompose with a gradual loss of weight until to around 0%, similar to alkyl ILs.42 While for ROC1-functionalized ILs, there are two distinct stages of thermal weight loss within the ranges of 220 ~ 320 °C and 320 ~ 500 °C


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respectively. The weight loss within the first stage in TGA curves of ROC1-functionalized ILs accounts for about 13wt%, which is close to the weight of ROC1-, for example, 14.02wt% of C2OC1- to [C2OC1-MIm]NTf2. This result strongly suggests that the first thermolysis stage of ROC1-functionalized ILs is the loss of ROC1- moiety. This deduction was further confirmed by PY-GC-MS result, in which the dimer C2H5OCH2-CH2OC2H5 (m/z: 118, 74, 59, 45, 31, 29, 15) was observed by pyrolysis of [C2OC1-MIm]NTf2 at its Td temperature of 262.8 °C. The dimer should be the dimerization product of the C2H5OCH2· radical. As it were, the thermolysis of ROC1-functionalized ILs should start with the rupture of ROC1- moiety via homolysis and SN1 mechanism. Density and Packing Efficiency Bulk density is strongly correlated to the intermolecular interactions and packing efficiency, so the study of density will be helpful to allow an in-depth insight into the microstructures and other macroscopic properties, such as refractive index and free volume fraction and transport properties.44 The variation of refractive index (n) with density (ρ) follows the Lorentz-Lorenz equation: (n2 - 1)/(n2 + 2) = 4πρNα/3M,45, 46 where N is a constant, α is polarizability and M is molar mass. Based on Fürth hole theory, the liquid-state particles can’t pack closely and lots of unoccupied free volume comes into existence, and surface tension (γ, mN m-1) can be used to estimate the molar free volume of ILs (Vf, cm3 mol-1): Vf = 2N×0.6791(kT/γ)3/2, in which 2N denotes the number of ions per mole, k is Boltzmann constant (1.38×10-23 J K-1) and T is the absolute temperature (K), respectively.47-52 Free volume and viscosity (η) are connected by the Cohen-Turnbull equation: lnη ∝ Vf*/Vf,53, 54 where Vf* is the required minimum volume of the hole for its exchange with an adjacent molecule.


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The obtained density (ρ), molar volume (Vm), refractive index (n), surface tension (γ), as well as the molar free volume (Vf) estimated by the above-mentioned surface tension approach, at 25 °C are presented in Table 2. These density values vary from 1.3787 to 1.5670 g cm-3, and are mainly affected by the cationic structures and the ether chain length. On the one hand, density decreases in the order of [R-Py]NTf2 > [R-MIm]NTf2 > [R-MPyr]NTf2, accordant with previous results.55 For [R-Py]NTf2 and [R-MIm]NTf2, the former is of less delocalized positive charge than the latter, resulting in the stronger electrostatic interaction of the former than the letter.56 Thus, though Py (0.98 g cm-3 at 25 °C) is less dense than MIm (1.03 g cm-3 at 25 °C), Py-based ILs are a little more dense than their MIm-based counterparts, for example, [C1OC1-Py]NTf2 (1.5670 g cm-3) vs. [C1OC1-MIm]NTf2 (1.5512 g cm-3). On the other hand, density generally decreases with the ether chain length, due to the addition of the lighter CH2 unit, and however there is a visible and meaningful difference in density between the isomers, as seen in Figure 3. There are 6 pairs of isomers: [C2OC1-MIm]NTf2 vs. [C1OC2-MIm]NTf2, [C2OC2-MIm]NTf2 vs. [C1OC3-MIm]NTf2, [C2OC1-Py]NTf2 vs. [C1OC2-Py]NTf2, [C2OC2-Py]NTf2 vs. [C1OC3Py]NTf2, [C2OC1-MPyr]NTf2 vs. [C1OC2-MPyr]NTf2 and [C2OC2-MPyr]NTf2 vs. [C1OC3MPyr]NTf2. Though these isomers are structurally similar with only a minor difference in the position of the ether oxygen atom, the former are uniformly less dense than the latter, implying the less efficient packing of the former than the latter. This inference is supported by the calculated free volume data. Though the surface tension approach for free volume calculation is not always accurate,51 especially in the case of long chains,52 here the free volume results of the isomers with the same chain length are useful as reference. For example, the free volume difference of 2.87 cm3 mol-1 between [C2OC1-MPyr]NTf2 (35.46 cm3 mol-1) and [C1OC2MPyr]NTf2 (32.59 cm3 mol-1) is very close to their molar volume difference of 2.91 cm3 mol-1


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(294.89 - 291.98 cm3 mol-1), in favor of the different packing efficiency between the isomers. There also exists difference in refractive index between the isomers. The more dense ILs uniformly exhibit larger refractive index than their isomeric counterparts, for example, [C2OC1MPyr]NTf2 (1.4391 g cm-3, 1.4170) vs. [C1OC2-MPyr]NTf2 (1.4534 g cm-3, 1.4219) at 25 °C, in good agreement with the Lorentz-Lorenz relation. Actually, the amount of difference in density or molar volume between the isomers relies on the cationic structures, and follows the order of imidazolium < pyridinium < pyrrolidinium, for example, 0.71 cm3 mol-1 from [C2OC1-MIm]NTf2 (280.61 cm3 mol-1) and [C1OC2-MIm]NTf2 (279.90 cm3 mol-1) vs. 0.89 cm3 mol-1 from [C2OC1-Py]NTf2 (276.72 cm3 mol-1) and [C1OC2Py]NTf2 (275.83 cm3 mol-1) vs. 2.91 cm3 mol-1 from [C2OC1-MPyr]NTf2 (294.89 cm3 mol-1) and [C1OC2-MPyr]NTf2 (291.98 cm3 mol-1). This order should be related to the chain flexibility and hydrogen bonding strength in ILs. On the one hand, C2OC1- is more flexible than C1OC2- to casue a less dense packing, since the torsion of the Cm- tail around C─O bond is valid for the rod-like C2- tail in C2OC1- and invalid for the ball-like C1- tail in C1OC2-. As a result, [C2OC1MPyr]NTf2 appears bulkier than [C1OC2-MPyr]NTf2, in the case without hydrogen bonding. On the other hand, both intramolecular and intermolecular hydrogen bonding between the ether O atom and the cationic ring H atoms can (sterically) hinder the ether chain torsional flexibility. As illustrated in Figure S2, the hydrogen bonding strength is strongest for imidazolium ILs, followed by pyridinium ILs, and negligible for pyrrolidinium ILs,30, 57 in accordance with the above order of the difference in density. In other words, when the torsion of ether chains is hindered, they tend to contribute similarly to density, and the difference in density between MIm- or Py-based ILs containing C2OC1- group and their counterparts containing C1OC2- group is smaller, in comparison with pyridinium-based isomers without hydrogen bonding.


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The equilibrium absorption capacity of moisture in ambient environment is about 8000 ppm for these hydrophobic ILs. The absorption capacity generally decreases with the ether chain length, in accordance with the hydrophobicity of CH2 unit. Generally, the impact of moisture absorption on density (ca. -0.0060 g cm-3), refractive index (ca. -0.0015) and surface tension (ca. +0.50 mN m-1) is relative modest, by comparison of the data in Table 2 and Table S1. The temperature dependence of density, refractive index and surface tension was measured for all the ILs over a temperature range from 10 to 80 °C, as listed in Table S3, S4 and S5 respectively. All of the obtained values for density, refractive index and surface tension almost linearly decrease with increasing temperature (see Figure S3), as normal.45, 48-50 Viscosity and Conductivity The measured viscosity values at 25 °C are presented in Figure 4 for a comparative analysis. Generally, the variation trend of viscosity along with the ether chain length is not as regular as the alkyl chain length, with which viscosity increases regularly,55, 58 since there are three major influence factors involved: the Cm- tail length and the -Cn- spacer length in CmOCn- groups, as well as the hydrogen bonding between the cationic ring H atoms and the ether O atom. Firstly, when the Cm- tail extends from C1OC2- to C2OC2- to C3OC2-, viscosity first decreases slightly and then increases sharply, e.g., [C1OC2-MPyr]NTf2 (53.7 cP) vs. [C2OC2-MPyr]NTf2 (50.7 cP) vs. [C3OC2-MPyr]NTf2 (69.1 cP). A possible explanation is that the torsional flexibility of the rod-like C2- tail is more efficient than the spherical C1- tail to produce more free volume,6, 7 while the effect of the chain length (or molecular volume) on viscosity plays a dominant role for the longest C3- tail. Secondly, the effect of the spacer length (C1OC1-, C1OC2- and C1OC3-) on viscosity appears complicated. For MPyr-based ILs, viscosity simply increases with the spacer


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length, as usual as alkyl chains.55, 58 While for MIm- or Py-based ILs, hydrogen bonding should be taken into consideration. C1OC2- prefers to form intramolecular six-membered-ring hydrogen bonding with the cationic ring H atoms in a stable conformation, and C1OC1- prefers to form intermolecular hydrogen bonding, instead of a five-membered ring with excess ring tension,13, 39 as briefly shown in Figure S2. The intermolecular hydrogen bonding has contribution to higher viscosity. Hence, [C1OC2-MIm]NTf2 (46.6 cP) is less viscous than [C1OC1-MIm]NTf2 (58.1 cP), though the former is bulkier by a CH2 unit than the latter. While [C1OC2-Py]NTf2 (59.0 cP) is almost as viscous as [C1OC1-Py]NTf2 (58.3 cP), perhaps due to the relatively weaker hydrogen bond donor strength of the pyridinium-ring H atoms, relative to imidazolium. In the case that the hydrogen bonding interaction is weak to negligible, [C1OC2-MPyr]NTf2 (53.7 cP) is as usual much more viscous than [C1OC1-MPyr]NTf2 (40.4 cP), since here the chain length (molecular volume) dominates in determining viscosity. Besides, for the isomers, the more viscous are uniformly of higher density or less free volume, for example, [C1OC3-MPyr]NTf2 (77.5 cP, 1.4244 g cm-3) vs. [C2OC2-MPyr]NTf2 (50.7 cP, 1.4111 g cm-3) at 25 °C, conforming to the Cohen-Turnbull equation.53, 54 The temperature dependence of viscosity was measured over a temperature range from 10 to 80 °C, as presented in Table S6. The viscosity of these ILs follows an Arrhenius dependence on temperature: lnη = lnA + Eη/RT,24 in which the activation energy (Eη) is the energy barrier that has to be overcome by mass transport. The Eη values were calculated by using the Arrhenius dependence of viscosity on temperature and summarized in Table 2. As expected, Eη generally increases with viscosity, as displayed in Figure 5. However, MIm- or Py-based ILs are of higher Eη values than MPyr-based ILs, when they are the same viscosity, perhaps due to the hydrogen bonding in MIm- or Py-based ILs, which poses extra energy barrier for viscosity.


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Conductivity is of vital importance for ILs, especially for their use as supporting electrolytes in electrochemical devices.44 The conductivity values at 25 °C vary from 2.49 to 5.44 mS cm-1, higher than their isoelectronic alkyl counterparts, especially for MPyr-based ones, e.g. [C2OC1MPyr]NTf2 (5.14 mS cm-1) vs. [C4-MPyr]NTf2 (2.77 mS cm-1).59 According to the conclusions proposed by Grätzel et al.,11 the conductivity of an IL is mainly determined by the ion sizes and viscosity. Accordingly, our conductivity values generally decrease with the ether chain length. While for the isomers, viscosity turns into the only decisive factor and should be reciprocal to conductivity, for example, [C2OC1-MIm]NTf2 (3.87 mS cm-1, 57.0 cP) vs. [C1OC2-MIm]NTf2 (4.68 mS cm-1, 46.6 cP) at 25 °C. The temperature dependence of conductivity was measured from 10 to 80 °C and shown in Table S7. Due to the presence of ion clusters, the temperature-dependent conductivity follows fractional Walden's rule with viscosity: logɅ = logC + αlogη-1,60 where Ʌ is molar conductivity that was calculated by Ʌ = κ × Vm, C is a constant and the exponent α < 1. The Walden plots are typically depicted in Figure S4. The impact of water on viscosity and conductivity is highly significant. Though only ~ 8000 ppm of water was “added” by absorbing moisture, viscosity could be decreased to ~ 70% and conductivity could be increased to ~ 125%, by comparison of the corresponding data at 25 °C in Table 2 and Table S1. The Arrhenius activation energy was also calculated using the viscosity data from 10 to 80 °C in Table S2. Generally, the activation energy of ILs was reduced by ~ 2.5 kJ mol-1, after absorbing moisture. CONCLUSION


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In conclusion, a series of ILs functionalized with various ether groups (CmOCn-, n, m = 1, 2 or 3) were prepared and characterized. Unlike alkyl chains, the variation trends of the properties of ILs along the ether chain length appear irregular and complex, since there are three influencing factors mainly involved: the Cm- tail length, the -Cn- spacer length and hydrogen bonding. ILs with ROC1- group of the short -C1- spacer are much less thermally stable than others and start to decompose with the loss of ROC1- group. The rod-like C2- tail is more flexible than the spherical C1- tail with invalid rotation and the longest C3- tail with flexibility limited by length, and thus the C2- tail is more suitable to be used to cause low efficient packing and achieve low viscosity. If there are H atoms with donating ability on the cationic ring such as imidazolium, with them ROC1- prefers to form intermolecular rather than the five-membered-ring intramolecular hydrogen bonding, while for ROC2- the six-membered-ring intramolecular hydrogen bonding comes into prominence, and hence [ROC1-MIm]NTf2 is more viscous than [ROC2-MIm]NTf2, contrary to the case of MPyr-based ILs. These results and analysis should be helpful to design suitable ILs for specific applications. Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. DSC trace of [C1OC1-MPyr]NTf2; intramolecular hydrogen bonding strength in etherfunctionalized ILs; temperature dependence of density, refractive index and surface tension; Walden plots for three ILs; physical properties of ILs that absorb moisture in ambient environment; the data for density, refractive index, surface tension, viscosity, conductivity of ILs over a temperature range from 10 to 80 °C; as well as NMR and IR spectra. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected], (86)158-8503-1502. *E-mail: [email protected], (86)137-3817-4026. Author Contributions Z.C., L.X. and S.Z. designed the study and wrote the manuscript. Z.C. and Y.H. performed the experimental work and analyzed the results. J.C. performed the computational simulation and analyzed the results. All authors critically reviewed the manuscript. Funding Sources Natural Science Foundation of China (No. 21503050) Natural Science Foundation of China (No. 21303028) Construction Project of Key Laboratories from the Education Department of Guizhou Province (No. QJHKY[2015]329) The Start-up Fund from Guizhou Education University Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 21503050 and 21303028), the Construction Project of Key Laboratories from the Education Department of Guizhou Province (No. QJHKY[2015]329) and the start-up fund from


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Figure 1. The structures and abbreviations of ILs used in this study.


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Figure 2. TGA curves of six ILs.


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Figure 3. Density of ILs at 25 °C.


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Figure 4. Viscosity of ILs at 25 °C.


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Figure 5. The changing relationship of viscosity (η) at 25 °C with activation energy (Eη).


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Table 1. Thermal properties of ILs. Tga (°C)

Tmb (°C)

∆Hmc (J g-1)

Tcd (°C)

∆Hce (J g-1)

Tdf (°C)

[C1OC1-MIm]NTf2

-78.7

-0.2

54.1

(-40.3)

45.9

276.5

[C1OC1-Py]NTf2

-80.5

-4.4

41.4

(-55.0)

28.8

269.5

-21.0

49.9

-40.4

25.6

298.8

Sample

[C1OC1-MPyr]NTf2 [C2OC1-MIm]NTf2

-78.6

262.8

[C2OC1--Py]NTf2

-78.2

262.5

[C2OC1-MPyr]NTf2

-98.7

282.5

[C1OC2-MIm]NTf2

-83.9

400.5

[C1OC2-Py]NTf2

-77.3

386.7

[C1OC2-MPyr]NTf2

-90.7

393.2

[C2OC2-MIm]NTf2

-84.8

-7.6

32.4

(-37.1)

34.4

385.2

[C2OC2-Py]NTf2

-79.1

-14.0

45.0

(-35.7)

48.0

357.3

[C2OC2-MPyr]NTf2

-91.3

384.3

[C3OC2-MIm]NTf2

-86.7

358.8

[C3OC2-Py]NTf2

-81.9

[C3OC2-MPyr]NTf2

-91.4

379.8

[C1OC3-MIm]NTf2

-85.7

383.3

[C1OC3-Py]NTf2

-82.4

379.3

[C1OC3-MPyr]NTf2

-86.6

398.7

a

-16.8

7.4

(-34.2)

7.2

340.6

Glass transition temperature. b Melting point temperature. c Enthalpy of melting. d Crystallization temperature

(cold crystallization). e Enthalpy of crystallization. h Thermal decomposition temperature at 5% weight loss.


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Page 28 of 30

Table 2. Physical properties of ILs at 25 °C. Sample

ρa

Vmb

γc

Vfd

ne

(g cm-3) (cm3 mol-1) (mN m-1) (cm3 mol-1)

ηf

Eηg

κh

(cP) (kJ mol-1) (mS cm-1)

[C1OC1-MIm]NTf2

1.5512

262.58

36.73

30.65

1.4259

58.1

31.77

4.56

[C2OC1-MIm]NTf2

1.5015

280.61

34.24

34.06

1.4259

57.0

30.95

3.87

[C1OC2-MIm]NTf2

1.5053

279.90

35.57

32.17

1.4273

46.6

29.08

4.68

[C2OC2-MIm]NTf2

1.4577

298.66

33.61

35.02

1.4269

46.4

29.06

4.02

[C1OC3-MIm]NTf2

1.4684

296.48

35.27

32.58

1.4292

55.5

30.64

3.57

[C3OC2-MIm]NTf2

1.4210

316.24

32.43

36.95

1.4274

50.2

30.35

3.18

[C1OC1-Py]NTf2

1.5670

258.01

37.19

30.09

1.4419

58.3

31.02

4.58

[C2OC1-Py]NTf2

1.5117

276.72

34.46

33.73

1.4410

48.4

29.92

4.14

[C1OC2-Py]NTf2

1.5166

275.83

36.26

31.25

1.4436

59.0

30.93

3.79

[C2OC2-Py]NTf2

1.4662

294.87

33.77

34.77

1.4414

52.7

30.04

3.34

[C1OC3-Py]NTf2

1.4790

292.32

35.79

31.87

1.4446

63.5

31.22

3.14

[C3OC2-Py]NTf2

1.4305

312.03

31.93

37.82

1.4417

62.0

31.17

2.61

[C1OC1-MPyr]NTf2

1.4833

276.65

35.54

32.21

1.4175

40.4

25.76

5.44

[C2OC1-MPyr]NTf2

1.4391

294.89

33.33

35.46

1.4170

38.1

26.06

5.14

[C1OC2-MPyr]NTf2

1.4534

291.98

35.26

32.59

1.4219

53.7

29.00

3.88

[C2OC2-MPyr]NTf2

1.4111

310.67

33.07

35.88

1.4214

50.7

28.77

3.41

[C1OC3-MPyr]NTf2

1.4244

307.77

35.19

32.69

1.4253

77.5

31.21

2.73

[C3OC2-MPyr]NTf2

1.3787

328.14

32.12

37.49

1.4227

69.1

30.02

2.49

a

Density (uncertainty < ±0.0005 g cm-3). b Molar volume. c Surface tension (uncertainty < ±0.04 mN m-1). d

Molar free volume calculated by using surface tension approach. e Refractive index (uncertainty < ±0.0002). f Viscosity (uncertainty < ±1%). g Activation energy calculated using Arrhenius equation. h Conductivity (uncertainty < ±1%).


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Table 3. Purity analysis of ILs. Molecular mass (g mol-1)

Water contenta (ppm)

Br- contentb (ppm)

(NMR) Purityc (%)

[C1OC1-MIm]NTf2

407.30

62.2

< 80

99.0

[C2OC1-MIm]NTf2

421.33

57.3

< 80

98.4

[C1OC2-MIm]NTf2

421.33

59.0

< 80

98.3

[C2OC2-MIm]NTf2

435.36

64.1

< 80

99.3

[C1OC3-MIm]NTf2

435.36

77.3

< 80

99.0

[C3OC2-MIm]NTf2

449.38

55.9

< 80

99.2

[C1OC1-Py]NTf2

404.30

73.7

< 80

98.1

[C2OC1-Py]NTf2

418.33

63.0

< 80

98.5

[C1OC2-Py]NTf2

418.33

58.9

< 80

98.3

[C2OC2-Py]NTf2

432.35

65.5

< 80

98.4

[C1OC3-Py]NTf2

432.35

54.7

< 80

99.3

[C3OC2-Py]NTf2

446.38

52.6

< 80

99.4

[C1OC1-MPyr]NTf2

410.35

72.9

< 80

98.8

[C2OC1-MPyr]NTf2

424.37

65.0

< 80

98.3

[C1OC2-MPyr]NTf2

424.37

64.6

< 80

99.4

[C2OC2-MPyr]NTf2

438.40

61.2

< 80

99.6

[C1OC3-MPyr]NTf2

438.40

55.9

< 80

98.4

[C3OC2-MPyr]NTf2

452.43

64.6

< 80

98.8

Sample

a

Water content (uncertainty < ±1 ppm) was determined on a WA-3000 coulometric Karl Fischer titrator, after

dry for 20 h at 80 °C and 10-2 ~ 10-3 mbar. b Br- ion content was measured on a PXSJ-216F ion meter, which was calibrated with a series of standard KBr solutions. 100 mL solution containing ~ 0.5 g sample was prepared and tested. While all the measured values (ca. 5×10-7 mol L-1) are below the measuring limit, i.e. < 5×10-6 mol L-1. Thus Br- impurity < 80 ppm (0.1 × 5×10-6 × 80/0.5) was adopted. c Since the water content and Br- impurity are negligible, NMR purity of ILs was approximately regarded as the overall purity.


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Table of Content Graphic:


30

Physicochemical Properties of Ether-Functionalized Ionic Liquids

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