What causes the low viscosity of ether-functionalized ionic liquids? Its dependence on the increase of free volume

Abstract

A series of 12 ether-functionalized room temperature ionic liquids (RTILs) and their alkyl counterparts (short of an oxygen atom) were prepared and characterized to investigate the reason for the alkoxy chain effect on decreasing viscosity and how to best apply it. In addition to the ability of the alkoxy chains to decrease viscosity (η_O/η, as low as 0.594) and its activation energy (ΔE_η, as low as -4.14 kJ mol⁻¹), they were also found to be able to increase the total free volume (ΔFV_O, up to 4.93 mL mol⁻¹), by using density and surface tension. Furthermore, both the obtained η_O/η and ΔE_η values for the 12 pairs of RTILs show a strong decreasing dependence on ΔFV_O. Accordingly, the reason for the alkoxy chain effect on decreasing viscosity was proposed to be due to the ability of the highly flexible alkoxy chains to increase the total free volume, which offers the convenience of transport for the adjacent molecules. To maximize this effect, the alkoxy groups were supposed to contain a rod-like alkyl tail and a short –CH₂– spacer between the ether O atom and the cationic N atom, and be in an environment without hydrogen bond donors. Generally, the ether-functionalized pyrrolidinium and ammonium RTILs possess high conductivity (up to 4.97 mS cm⁻¹ at 25 °C) and wide electrochemical windows (∼5.75 V), indicating their significant promise for electrochemical applications.

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Introduction

Over the past three decades, room temperature ionic liquids (RTILs) have been extensively studied and regarded as an environmentally preferable alternative to reaction media,¹ supporting electrolytes² as well as functional materials.³ Since RTILs are composed entirely of ions, they are endowed with some unique features in most cases, such as negligible vapor pressure, non-flammability, high thermal stability, wide liquid range and intrinsic ionic conductivity. These are all considered environmentally friendly characteristics. In addition, the properties of RTILs can be custom-designed by changing either the cation or anion species to meet specific requirements.⁴ As a result, RTILs have been applied successfully in various fields, such as organic synthesis,^1,5 transition metal catalysis,^1,6 biotechnology,^4,7 ionothermal synthesis,⁸ nano-materials,^8,9 separation techniques,¹⁰ lubricants,¹¹ lithium-ion batteries,¹² dye-sensitized solar cells,¹³ supercapacitors,¹⁴ fluorescent materials,¹⁵ magnetic fluids,¹⁶ and many more.

However, RTILs are generally tens and even hundreds of times more viscous than the traditional organic solvents. High viscosity not only decreases the possibility of its applications, such as heterogeneous catalysis and supporting electrolytes, in these fields that require fast mass and ion transport, but also leads to operational difficulties in filtration, extraction and decantation. Therefore, in most cases, the required viscosity of RTILs should be as low as possible. Generally, a low-viscosity RTIL is characteristic of well-delocalized positive and negative charges,¹⁷ low-symmetry ions¹⁸ and highly flexible side chains.¹⁹ These structural features are relevant to weak Coulomb interactions and low effective packing between ions, thereby leading to low viscosity.

Recently, ether groups have been well known for their ability to form low viscosity and high conductivity RTILs,^19a,20 as well as low crystallinity and high conductivity polymer electrolytes.²¹ For example, when the C₄H₉– group in 1-butyl-1-methylpyrrolidinium CF₃BF₃⁻ was replaced by an isoelectronic CH₃OC₂H₄– group, its viscosity was reduced from 137 to 87 cP, and its conductivity was increased from 3.3 to 4.3 mS cm⁻¹ at 25 °C.^20e Up to now, many ether-functionalized RTIL species have been developed and extensively studied, such as imidazolium cations,^20c,22 ammonium cations,^20d–g phosphonium cations,²⁰ⁱ guanidinium cations,^20j,k sulfonium cations,^20l pyrazolium cations,^20m as well as perfluoroalkylborate anions.²⁰ⁿ However, with regard to the mechanism of the alkoxy chain effect on decreasing viscosity, it was often only vaguely related to their high rotational flexibility,^20d,e which results from the smaller atomic radius of the O atom compared to a C atom, and that the ether O atom only forms bonds with two atoms rather than four atoms, as is the case for the C atom.²³ More recently, molecular dynamics simulations indicated that the more flexible alkoxy chains can cause a less effective assembly, and therefore decrease the viscosity, in comparison with alkyl chains.^19a However, the alkoxy chain effect on viscosity is highly variable and strongly depends on the RTIL and alkoxy chain structures. For example, the ability of ether groups to reduce viscosity is much more effective in ammonium RTILs than in imidazolium RTILs,^{20c–e,20j,20n} and is much more effective for the C₂H₅OC₂H₄– group than for the CH₃OC₂H₄– group.^20g,h These unusual phenomena provide access to conduct an exploratory study on the alkoxy chain effect on viscosity, and to see what exactly causes the effect and how to maximize it.

Recently, hole theory indicated that (1) the ions in RTILs cannot pack closely and lots of holes between ions may come into existence; (2) the bulk volume of a RTIL consists of the inherent volume and the total volume of holes; and (3) a large total hole volume is of advantage to a high rate of mass transport and low viscosity.²⁴ Accordingly, the alkoxy chain effect on viscosity was conceived to be relevant as alkoxy chains can increase the total free volume and provide more available holes for the convenience of mass transport and low viscosity. In order to test this assumption, 12 ether-functionalized RTILs and their alkyl counterparts short of an ether O unit were synthesized and characterized, including viscosity, the activation energy of viscosity, density, molar volume, surface tension and the total free volume, and the alkoxy chain effect on viscosity and its activation energy and the total free volume was also calculated and comparatively studied.

Results

Scheme 1 presents the structures and abbreviations of the RTILs employed in this study. In detail, MPyrr, EDMAm, MIm and Py stand for the cationic species of N-methylpyrrolidinium, N,N-dimethylethanammonium, N-methylimidazolium and pyridinium, respectively; and Pr–, Bu–, MOE–, EOM– and EOE– stand for the side chains of CH₃CH₂CH₂–, CH₃CH₂CH₂CH₂–, CH₃CH₂OCH₂–, CH₃OCH₂CH₂– and CH₃CH₂OCH₂CH₂–, respectively. ¹H-NMR and FT-IR spectra (see Experimental Section for details) were recorded to confirm the structures of the synthesized compounds. Several basic and desired physicochemical properties of the RTILs were characterized and comparatively studied, including viscosity (η), the activation energy of viscosity (E_η), density (ρ) and molar volume (MV), as listed in Table 1. In addition, the group contribution of the incorporated ether O atom to viscosity (η_O/η), its activation energy (ΔE_η), molar volume (ΔMV_O) and the total free volume (ΔFV_O) were also calculated and are summarized in Table 2. The total free volume (∑ν) in the isomeric RTILs, i.e., [MOE-MPyrr]NTf₂vs. [EOM-MPyrr]NTf₂ and [MOE-EDMAm]NTf₂vs. [EOM-EDMAm]NTf₂, was calculated by the surface tension method to confirm the rationality of the obtained ΔFV_O values, as shown in Table 3. Besides, the high conductivity (up to 4.97 mS cm⁻¹ at 25 °C) and wide electrochemical windows (5.68–5.79 V) of the ether-functionalized pyrrolidinium and ammonium RTILs, as shown in Table 4, imply their promising applications in the field of electrochemistry.

Scheme 1 The structures and abbreviations of the RTILs employed in this study.

Table 1 Some basic physicochemical properties of the alkoxy- and alkyl-substituted RTILs at 25 °C

RTIL	η ^a cP	E η ^b kJ mol^−1	ρ ^c g mL^−1	MW^d g mol^−1	MV^e mL mol^−1
a Viscosity. b Activation energy of viscosity. c Density. d Molecular weight. e Molar volume. f Literature value, 53 cP.^20e g Literature value, 1.4539 g mL^−1.^20e h Literature value, 76 cP.^20e i Literature value, 1.3931 g mL^−1.^20e j Literature value, 51.0 cP.^25 k Literature value, 1.4366 g mL^−1.^25 l Literature value, 108.3 cP.^26 m Literature value, 1.20143 g mL^−1.^26
[Pr-MPyrr]NTf2	62.4	28.83	1.4283	408.38	285.92
[MOE-MPyrr]NTf2	54.8^f	27.76	1.4546^g	424.38	291.75
[EOM-MPyrr]NTf2	38.7	25.29	1.4375	424.38	295.18
[Bu-MPyrr]NTf2	79.6^h	31.07	1.3950^i	422.41	302.80
[EOE-MPyrr]NTf2	51.8	28.27	1.4104	438.41	310.84
[Pr-EDMAm]NTf2	75.1	31.53	1.4039	396.37	282.33
[MOE-EDMAm]NTf2	63.6	29.96	1.4326	412.37	287.85
[EOM-EDMAm]NTf2	44.6	27.39	1.4134	412.37	291.76
[Bu-EDMAm]NTf2	99.3	33.49	1.3719	410.40	299.15
[EOE-EDMAm]NTf2	61.8	29.89	1.3870	426.40	307.43
[Pr-MIm]NTf2	45.3	27.68	1.4756	405.34	274.70
[MOE-MIm]NTf2	49.6	29.04	1.5088	421.34	279.26
[Bu-MIm]NTf2	51.7^j	28.60	1.4367^k	419.36	291.89^l
[EOE-MIm]NTf2	45.1	28.07	1.4584	435.36	298.52
[Pr-MIm]BF4	78.9	33.38	1.2398	212.00	171.00
[MOE-MIm]BF4	90.2	35.07	1.2996	228.00	175.44
[Bu-MIm]BF4	111.3^l	36.56	1.2025^m	226.02	187.96
[EOE-MIm]BF4	103.0	36.27	1.2485	242.02	193.85
[Bu-Py]NTf2	61.9	30.20	1.4490	416.36	287.34
[EOE-Py]NTf2	52.3	29.34	1.4667	432.36	294.78
[Bu-Py]BF4	170.7	40.77	1.2152	223.02	183.53
[EOE-Py]BF4	157.5	40.56	1.2566	239.02	190.21

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Table 2 Alkoxy chain effect on η (ηO/η), E_η (ΔEη), MV (ΔMVO) and the total free volume (ΔFV_O) of RTILs, by comparing the ether-functionalized RTILs with their alkyl counterparts short of an ether O unit

Alkoxy/alkyl RTIL pair	η O/η	ΔEη kJ mol^−1	ΔMVO mL mol^−1	ΔFVO mL mol^−1
[MOE-MPyrr]NTf2/[Pr-MPyrr]NTf2	0.878	−1.07	5.83	1.33
[EOM-MPyrr]NTf2/[Pr-MPyrr]NTf2	0.620	−3.54	9.26	4.76
[EOE-MPyrr]NTf2/[Bu-MPyrr]NTf2	0.651	−2.80	8.04	3.54
[MOE-EDMA]NTf2/[Pr-EDMA]NTf2	0.847	−1.57	5.52	1.02
[EOM-EDMA]NTf2/[Pr-EDMA]NTf2	0.594	−4.14	9.43	4.93
[EOE-EDMA]NTf2/[Bu-EDMA]NTf2	0.622	−3.60	8.28	3.78
[MOE-MIm]NTf2/[Pr-MIm]NTf2	1.095	1.36	4.56	0.06
[EOE-MIm]NTf2/[Bu-MIm]NTf2	0.872	−0.53	6.63	2.13
[MOE-MIm]BF4/[Pr-MIm]BF4	1.143	1.69	4.44	−0.06
[EOE-MIm]BF4/[Bu-MIm]BF4	0.925	−0.29	5.89	1.39
[EOE-Py]NTf2/[Bu-Py]NTf2	0.845	−0.86	7.44	2.94
[EOE-Py]BF4/[Bu-Py]BF4	0.923	−0.21	6.68	2.18

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Table 3 Surface tension and the total free volume of the isomeric RTILs

RTIL^a	γ ^a mN m^−1	∑ν^b mL mol^−1	Δ∑ν^c mL mol^−1
a Surface tension at 25 °C. b The total free volume calculated by eqn (3). c The difference of the total free volume between the isomeric RTILs.
[MOE-MPyrr]NTf2	38.06	29.06
[EOM-MPyrr]NTf2	35.51	32.25	3.19
[MOE-EDMAm]NTf2	37.04	30.27
[EOM-EDMAm]NTf2	34.47	33.72	3.45

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Table 4 Electrochemical properties of pyrrolidinium and ammonium RTILs

RTIL	κ ^a mS cm^−1	Ecathodic^b V	Eanodic^c V	EW^d V
a Conductivity at 25 °C. b Cathodic potential limit. c Anodic potential limit. d Electrochemical windows. e Literature value, 3.7 mS cm^−1.^20e f Literature value, 2.6 mS cm^−1.^20e g Literature value, 3.1 mS cm^−1.^20d
[Pr-MPyrr]NTf2	3.75	−3.05	2.76	5.81
[MOE-MPyrr]NTf2	3.72^e	−3.06	2.71	5.77
[EOM-MPyrr]NTf2	4.97	−3.05	2.71	5.76
[Bu-MPyrr]NTf2	2.62^f	−3.03	2.74	5.77
[EOE-MPyrr]NTf2	3.23	−3.05	2.65	5.70
[Pr-EDMA]NTf2	2.93	−3.07	2.76	5.83
[MOE-EDMA]NTf2	3.10^g	−3.05	2.74	5.79
[EOM-EDMA]NTf2	4.23	−3.08	2.71	5.79
[Bu-EDMA]NTf2	1.86	−3.05	2.75	5.80
[EOE-EDMA]NTf2	2.78	−3.01	2.67	5.68

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The viscosity of the alkoxy- and alkyl-substituted RTILs was determined using a BROOKFIELD DV-II+ Pro viscometer that is accurate within ±1.0%. The viscosity (η) values obtained at 25 °C fall in the range from 38.7 to 170.7 cP, as listed in Table 1. As can be expected, the addition of an ether O atom into an alkyl side chain exhibits a significant effect on the viscosity of RTILs. For example, by replacing a Pr– group with an EOM– group, the viscosity can be reduced from 62.4 cP of [Pr-MPyrr]NTf₂ to 38.7 cP of [EOM-MPyrr]NTf₂. In order to quantify the ether O atom's contribution to the viscosity of RTILs, the change rate of viscosity was calculated and expressed as η_O/η in Table 2. For example, the ratio of the viscosity of [EOM-MPyrr]NTf₂ (38.7 cP) to that of [Pr-MPyrr]NTf₂ (62.4 cP) was calculated to be 38.7/62.4 = 0.620, which represents the contribution of the O atom in the EOM– group to the viscosity of [R-MPyrr]NTf₂. The η_O/η values presented in Table 2 are highly variable and range from 1.143 to 0.594. Generally, the η_O/η values (0.878–0.594) in pyrrolidinium- and ammonium-based RTILs are all less than 1, indicating that the alkoxy chains in these RTILs are all helpful to decrease viscosity, especially for the EOM- group (0.620–0.594). However, the η_O/η values less than 1 in imidazolium and pyridinium RTILs are limited to the EOE- group (0.925–0.845), but still larger than those in pyrrolidinium and ammonium RTILs, and the MOE- group in imidazolium RTILs (1.095–1.143) even tends to increase viscosity.

The temperature dependence of the viscosity of the RTILs was also investigated over a temperature range from 10 to 80 °C. As typically shown in Fig. 1, the variation of the viscosity with temperature agrees well with an Arrhenius-type behaviour (eqn (1)),^25–27 where η is viscosity (cP), A is a constant, E_η is activation energy (kJ mol⁻¹), R is the universal gas constant (8.314 J K⁻¹ mol⁻¹), and T is the absolute T/K.

lnη = lnA + E_η/RT (1)

Fig. 1 Typical Arrhenius plots of viscosity for two RTILs (●: [Pr-MPyrr]NTf₂, ■: [EOM-MPyrr]NTf₂).

According to eqn (1), the E_η values for all of the studied RTILs were calculated and summarized in Table 1, with the linear fitting parameter (R²) larger than 0.995. In addition, the change in the E_η values of the RTILs by incorporating an additional ether O unit was also calculated as ΔE_η and summarized in Table 2. For example, the difference of the E_η values between [EOM-MPyrr]NTf₂ (25.29 kJ mol⁻¹) and [Pr-MPyrr]NTf₂ (28.83 kJ mol⁻¹) was calculated to be 25.29–28.83 = −3.54 kJ mol⁻¹, which represents the contribution of the ether O unit in EOM- group to E_η value of [R-MPyrr]NTf₂. In theory, viscosity flow can be treated as a process depending on the activation energy, and the E_η value can be regarded as the energy barrier to be overcome by mass transport.²⁸ As a consequence, the change in viscosity should be consistent with the change in the E_η value. As expected from Fig. 2, the ΔE_η value generally decreases with decreasing η_O/η value. As a result, the ΔE_η values can also be used, as well as η_O/η, to characterize the effect of an ether O atom on the viscosity of RTILs.

Fig. 2 The changing relationship between η_O/η and ΔE_η.

The density (ρ) of the studied RTILs was measured by a DMA 35 portable density meter at 25 °C, and the resulting data in Table 1 fall in the range from 1.2025 to 1.5088 mL mol⁻¹. The corresponding molar volumes (MV) were calculated as their molecular weight divided by density, as listed in Table 1. Theoretically, the molecular volume can be calculated using the group additivity method.²⁹ For example, a mean contribution per methylene to the molecular volume of RTILs was calculated to be 27.8 Å⁻³ (16.74 mL mol⁻¹) for [C_nMIm]alanine series (n = 2, 3, 4, 5, 6)³⁰ and 27.5 Å⁻³ (16.56 mL mol⁻¹) for [C_nMIm]BF₄ series (2, 4, 6, 8, 10) and [C_nMIm]NTf₂ series (1, 2, 4, 6, 8, 10, 12).³¹ These values are close to our results, i.e., [Bu-MPyrr]NTf₂vs. [Pr-MPyrr]NTf₂ (302.80–285.92 = 16.88 mL mol⁻¹), [Bu-EDMAm]NTf₂vs. [Pr-EDMAm]NTf₂ (299.15–282.33 = 16.82 mL mol⁻¹), [Bu-MIm]NTf₂vs. [Pr-MIm]NTf₂ (291.89–274.70 = 17.19 mL mol⁻¹) and [Bu-MIm]BF₄vs. [Pr-MIm]BF₄ (187.96–171.00 = 16.96 mL mol⁻¹). It was argued that the contribution per methylene to the molar volume behaves in a regular manner, and should be the same or at least close to a constant.^29a,31

Using the same method, the contribution of an additional ether O unit to molar volume was also calculated as ΔMV_O (mL mol⁻¹), as listed in Table 2. The ΔMV_O values of the 12 pairs of RTILs fall in a wide range 4.44–9.43 mL mol⁻¹, and yield the largest relative difference up to 112% = (9.43–4.44)/4.44. In other words, the ΔMV_O value for [EOM-EDMA]NTf₂/[Pr-EDMA]NTf₂ (9.43 mL mol⁻¹) is even larger than twice that for [MOE-MIm]BF₄/[Pr-MIm]BF₄ (4.44 mL mol⁻¹). Based on hole theory, the bulk volume of a liquid consists of inherent volume and the total volume of the holes between molecules.^24f,g Accordingly, the ΔMV_O values should contain excess free volume, especially for the larger ones. Although the inherent volume of the ether O unit is unclear, it should be a constant or at least an approximation, as noted above. Generally, the larger the ΔMV_O value is, the larger the excess free volume should be. In fact, the proposed excess free volume is consistent with “the less effective assembly” of the highly flexible alkoxy chains.^19a

Actually, the ΔMV_O values can be well correlated with the rotational flexibility of the alkoxy chains in different conditions. Fig. 3 shows the increasing dependence of the ΔMV_O values on the alkoxy chain rotational flexibility. Generally, around the ether O atom, there are two main rotational models,²³ which seem to be affected by hydrogen bonds, the O-terminal alkyl group, and the spacer length between the ether O atom and the cationic nitrogen atom. First of all, either intra- or intermolecular hydrogen bonds tend to restrict the rotational motion and increase the rigidity of the side chains.^22,32 As shown in Fig. 3, the [MOE-MIm]NTf₂/[Pr-MIm]NTf₂ pair (★) exhibits a lower ΔMV_O value of 4.56 mL mol⁻¹ than 5.83 mL mol⁻¹ of [MOE-MPyrr]NTf₂/[Pr-MPyrr]NTf₂ (◆), since the alkoxy chain rotational flexibility in imidazolium RTILs can be weakened by the hydrogen bond between the ether O atom and imidazolium ring hydrogen atom. Similar results could also be observed in pyridinium-based RTILs, and the ΔMV_O value of [EOE-Py]NTf₂/[Bu-Py]NTf₂ pair (7.44 mL mol⁻¹, ▲) is a bit larger than that of [EOE-MIm]NTf₂/[Bu-MIm]NTf₂ pair (6.63 mL mol⁻¹, ▼), which should be ascribed to the relatively weaker hydrogen-bond donor in the former than that in the latter. Secondly, the ΔMV_O values obtained from the MOE-substituted RTILs (e.g. 5.83 mL mol⁻¹ of [MOE-MPyrr]NTf₂/[Pr-MPyrr]NTf₂, ◆) are always smaller than those from EOE-substituted RTILs (e.g. 8.04 mL mol⁻¹ of [EOE-MPyrr]NTf₂/[Bu-MPyrr]NTf₂, ●). This result can be related to the structure of the terminal group connected with the ether O atom. In comparison with the rod-like CH₂CH₃ tail on the EOE- chain, the O-terminal alkyl group on the MOE- chain is the spherical CH₃, whose rotation is invalid for the chain flexibility. Thirdly, the EOM- moiety gives the largest ΔMV_O value, even larger than the EOE- moiety, for example, 9.26 mL mol⁻¹ of [EOM-MPyrr]NTf₂/[Pr-MPyrr]NTf₂ (■) vs. 8.04 mL mol⁻¹ of [EOE-MPyrr]NTf₂/[Bu-MPyrr]NTf₂ (●). The largest ΔMV_O value in EOM-substituted RTILs is perhaps due to the short spacer length of only one methylene between the ether O atom and the cationic N atom, and the side chain flexibility can promote the body's molecular motion through the short methylene spacer.

Fig. 3 The proposed alkoxy chain rotational motion for the ΔMV_O values. (★: [MOE-MIm]NTf₂/[Pr-MIm]NTf₂, ◆: [MOE-MPyrr]NTf₂/[Pr-MPyrr]NTf₂, ▼: [EOE-MIm]NTf₂/[Bu-MIm]NTf₂, ▲: [EOE-Py]NTf₂/[Bu-Py]NTf₂, ●: [EOE-MPyrr]NTf₂/[Bu-MPyrr]NTf₂, ■: [EOM-MPyrr]NTf₂/[Pr-MPyrr]NTf₂.)

According to the above analysis, the rotational flexibility of the MOE- group is totally restricted in imidazolium RTILs, and no excess free volume should come into existence. Consequently, the MOE-substituted imidazolium RTILs possess the lowest ΔMV_O values, i.e. 4.44 mL mol⁻¹ of [MOE-MIm]BF₄/[Pr-MIm]BF₄ and 4.56 mL mol⁻¹ of [MOE-MIm]NTf₂/[Pr-MIm]NTf₂. As a result, the average of the two ΔMV_O values, (4.44 + 4.56)/2 = 4.50 mL mol⁻¹, is approximately considered to be the inherent volume of the ether O unit in RTILs. Since the ΔMV_O value consists of the inherent volume of the ether O unit and the total excess free volume, and the inherent volume of the ether O atom was supposed to be 4.50 mL mol⁻¹, the total excess free volume derived from the ether O atom was calculated using eqn (2), and the resulting ΔFV_O values are listed in Table 2.

ΔFV_O = ΔMV_O − 4.50 mL mol⁻¹ (2)

Actually, the total free volume of a liquid can be roughly calculated by surface tension method (eqn (3)),^24f,g where ∑v is the total free volume (mL mol⁻¹), N is the Avogadro constant (6.02 × 10²³), k is the Boltzmann constant (1.38 × 10⁻²³ J K⁻¹), T is the absolute T/K, and γ is surface tension (N m⁻¹).

Σv = 2N × 0.6791(kT/γ)^3/2 (3)

However, this method is not applicable as the side chain length increases.^24b,33 In order to confirm the rationality of the above obtained ΔFV_O values, the surface tension method was used to calculate the total free volume (∑v) of two pairs of the isomeric RTILs, and the corresponding results are listed in Table 3. The surface tension of these RTILs was observed between 34.47 and 38.06 mN m⁻¹, which is close to the common NTf₂-based imidazolium RTILs.³⁴ Using the γ values, the ∑v values were calculated to be between 29.06 and 33.72 mL mol⁻¹. Generally, the major materials exhibit a 10–15% free volume fraction.^24f The free volume fraction of the studied RTILs falls within the expected range, for example, 32.25/295.18 = 10.9% for [EOM-MPyrr]NTf₂. In comparison, the surface tension of the lower-viscosity MOE-substituted RTILs is generally smaller than their analogous EOM-substituted RTILs, and the total free volume of the former is larger than the latter. The difference of the total free volume between the isomeric RTILs is 32.25–29.06 = 3.19 mL mol⁻¹ for [EOM-MPyrr]NTf₂/[MOE-MPyrr]NTf₂, and 33.72–30.27 = 3.45 mL mol⁻¹ for [EOM-EDMAm]NTf₂/[MOE-EDMAm]NTf₂. According to the ΔMV_O values, the corresponding two values were calculated to be 3.43 mL mol⁻¹ (4.76–1.33) and 3.91 mL mol⁻¹ (4.93–1.02), respectively. The approximation between 3.19 and 3.43 mL mol⁻¹ or between 3.45 and 3.91 mL mol⁻¹ defends the rationality of the ΔFV_O values in Table 2.

Discussion

The changes in the free volume and viscosity as a result of the incorporation of an oxygen atom in an alkyl side chain of RTILs were measured and recorded as ΔFV_O (mL mol⁻¹) and η_O/η or ΔE_η (kJ mol⁻¹), respectively. According to Fig. 4 and 5, both η_O/η and ΔE_η exhibit a nearly linear dependence on the ΔFV_O values. This dependence is consistent with the hole theory, where viscosity involves an exchange of position between a hole and its adjacent molecule.^24b,35 In detail, a larger ΔFV_O value means the alkoxy chain can provide more available hole locations to promote mass transport and to decrease viscosity, i.e., smaller η_O/η (<1) or ΔE_η (<0) values. It can be seen from Fig. 4 and 5 that both η_O/η and ΔE_η generally decrease with the increase of ΔFV_O values. The two observed trends clearly indicate that the increased free volume (ΔFV_O) plays a decisive role in the viscosity change of the ether-substituted RTILs (η_O/η or ΔE_η). According to the fitted linear trend in Fig. 4, when η_O/η = 1.00, where the alkoxy chain shows no effect on viscosity, the ΔFV_O value was found to be 0.71 mL mol⁻¹. Analogously, the corresponding ΔFV_O value for ΔE_η = 0.00 kJ mol⁻¹ is 1.07 mL mol⁻¹ as shown in Fig. 5. The two fitted values yield an average of 0.89 mL mol⁻¹, which could be taken as the key point of the ΔFV_O value for the alkoxy chain effect on viscosity. Generally, when ΔFV_O > 0.89 mL mol⁻¹, the addition of an ether O unit will decrease or increase viscosity. For ΔMV_O, the corresponding key point is (4.50 + 0.89) = 5.39 mL mol⁻¹.

Fig. 4 The changing relationship between η_O/η and ΔFV_O.

Fig. 5 The changing relationship between ΔE_η and ΔFV_O.

Is it reasonable for the quantitative relationship between ΔFV_O and η_O/η or ΔE_η values shown in Fig. 4 and 5? An alternative method is to decrease the free volume and increase viscosity of the RTILs by an external pressure.^25–27 For example, at 25 °C, when the pressure was raised from 0.1 to 24.3 MPa, [Bu-MIm]NTf₂ would be compressed by 3.44 mL mol⁻¹ from 291.90 to 288.46 mL mol⁻¹, and its viscosity would be increased from 51.0 to 68.1 (0.749 times).²⁵ According to the hole theory,^24b,24f,35 3.44 mL mol⁻¹ variation in free volume is responsible for the viscosity change of 0.749. Although this characterization cannot be conducted in our laboratory, the available literature data^25–27 can be used for comparative study. For the isomeric RTILs, i.e., [MOE-MPyrr]NTf₂vs. [EOM-MPyrr]NTf₂ and [MOE-EDMAm]NTf₂vs. [EOM-EDMAm]NTf₂, there are great differences in viscosity and free volume between them. Generally, the EOM-substituted RTILs have larger free volume and lower viscosity than their isomeric MOE-RTILs, for example, [MOE-MPyrr]NTf₂ (29.06 mL mol⁻¹, 54.8 cP) vs. [EOM-MPyrr]NTf₂ (32.25 mL mol⁻¹, 38.7 cP), and [MOE-EDMAm]NTf₂ (30.27 mL mol⁻¹, 63.6 cP) vs. [EOM-EDMAm]NTf₂ (33.72 mL mol⁻¹, 44.6 cP). If [MOE-MPyrr]NTf₂ and [EOM-MPyrr]NTf₂ were taken as two states of a liquid under different pressure, 3.19 mL mol⁻¹ (32.25–29.06) change in the total free volume should be responsible for the viscosity change of 0.706 (38.7/54.8), as well as 3.45 mL mol⁻¹ free volume (291.76–287.85) for 0.701 (44.6/63.6) viscosity change in the pair of [MOE-EDMAm]NTf₂ and [EOM-EDMAm]NTf₂. These change relationships between free volume and viscosity in the isomers are close to the previously reported value 3.44 mL mol⁻¹ free volume change for the viscosity change of 0.749 for [Bu-MIm]NTf₂.²⁵ As a result, the quantitative relationships between ΔFV_O and η_O/η or ΔE_η values shown in Fig. 4 and 5 were confirmed to be appropriate.

Actually, the η_O/η and ΔE_η values could be correlated with the alkoxy chain rotational flexibility via the ΔFV_O values. Generally, the highly flexible alkoxy chains trend to cause the less efficient packing between bulk ions^19a and some excess free volume come into existence, resulting in decreasing viscosity. In order to maximize the alkoxy chain effect on decreasing the viscosity of RTILs, three factors need to be considered, i.e. hydrogen bond, the terminal alkyl group in the ether, and the spacer length between the ether O atom and the cationic nitrogen atom, analogous to ΔFV_O (Fig. 3). First of all, the η_O/η and ΔE_η values in imidazolium and pyridinium RTILs are larger than those in pyrrolidinium and ammonium RTILs, in agreement with the reported literature,^{20c–e,20j,20n}e.g., [EOE-MIm]NTf₂/[Bu-MIm]NTf₂ (η_O/η = 0.872, ΔE_η = −0.53 kJ mol⁻¹) vs. [EOE-MPyrr]NTf₂/[Bu-MPyrr]NTf₂ (0.651, −2.80 kJ mol⁻¹). The hydrogen bonds between the ether O atom and the aromatic ring hydrogen in imidazolium and pyridinium RTILs should be responsible for the low efficiency of the alkoxy chain effect on viscosity, via restraining the rotational motion of the alkoxy chains. Secondly, the spherical CH₃ terminal group reduces the rotational flexibility of the MOE-ether chain, compared with its analogous EOE-ether, whose terminal group is the rod-like CH₂CH₃ and effective in increasing the chain flexibility via rotation. As a result, the η_O/η and ΔE_η values obtained from MOE-substituted RTILs are larger than those from EOE-RTILs, e.g., [MOE-EDMA]NTf₂/[Pr-EDMA]NTf₂ (0.847, −1.57 kJ mol⁻¹) vs. [EOE-EDMA]NTf₂/[Bu-EDMA]NTf₂ (0.622, −3.60 kJ mol⁻¹). In addition, the EOE-substituted RTILs can be even less viscous than their MOE-substituted counterparts, e.g. at 25 °C, [EOE-MPyrr]NTf₂ (51.8 cP) vs. [MOE-MPyrr]NTf₂ (54.8 cP), consistent with the reported literature.^20g,h It is argued that the EOE- group is longer but more flexible than the MOE- group, and flexibility dominates over length in reducing the viscosity of the ether derivatives. Thirdly, the short methylene spacer between the ether O atom and the cationic N atom in the EOM- group seems to be able to promote the structural molecular motion (flexibility), and can lead to the largest ΔFV_O value and the lowest η_O/η and ΔE_η values, in comparison with MOE- and EOE- groups, e.g., [EOM-MPyrr]NTf₂/[Pr-MPyrr]NTf₂ (0.620, −3.54 mL mol⁻¹) vs. [EOE-MPyrr]NTf₂/[Bu-MPyrr]NTf₂ (0.651, −2.80 mL mol⁻¹) and [MOE-MPyrr]NTf₂/[Pr-MPyrr]NTf₂ (0.878, −1.07 mL mol⁻¹).

Consequently, it could be argued that the alkoxy chain effect on decreasing viscosity is strongly dependent on the amount of the increased total free volume, which results from the alkoxy chain rotational flexibility and can facilitate mass transport. According to the above results, in order to maximize the alkoxy chain effect on decreasing viscosity, both the ether and cation structures should be considered. First of all, the tail group should not be the spherical methyl; secondly, the spacer length between the cationic core and the ether O atom should be as short as only one methylene; thirdly, there should not be the factors that weaken the rotational flexibility, such as hydrogen bonds.

Low viscosity allows high conductivity for RTILs to be better applied as electrolytes in electrochemical devices. Another major factor on the ionic conductivity of RTILs is the ion sizes.^36,37 The conductivity (κ) of the alkyl- and alkoxy-substituted pyrrolidinium and ammonium RTILs was determined, ranging from 1.86 to 4.97 mS cm⁻¹ at 25 °C, as shown in Table 4. As can be summarized, the obtained conductivity values show a significant dependence on the side chains, and generally increase in an order of Bu- < EOE- < Pr- ≈ MOE- < EOM-. Although the incorporation of an ether O unit tends to increase the cation volume and decrease conductivity, the rotational flexibility of the alkoxy chains dominates over the increase in the cation volume, and leads to high conductivity of the ether derivatives, e.g., [Bu-EDMA]NTf₂ (1.86 mS cm⁻¹) vs. [EOE-EDMA]NTf₂ (2.78 mS cm⁻¹). For the isomeric RTILs, viscosity is the only decisive factor and should be reciprocally dependent on the conductivity, e.g., [EOM-MPyrr]NTf₂ (4.97 mS cm⁻¹, 38.7 cP) and [MOE-MPyrr]NTf₂ (3.72 mS cm⁻¹, 54.8 cP).

A wide electrochemical window is also required for RTILs to be used as electrolytes in electrochemical devices with high energy and power density, such as supercapacitor and battery systems.^12,13,20f The electrochemical windows of the pyrrolidinium and ammonium RTILs were evaluated by cyclic voltammetry on a glassy-carbon electrode, as typically illustrated in Fig. 6. Table 4 summarizes the test results for the respective cathodic (E_cathodic) and anodic (E_anodic) limits, and electrochemical windows (EW = E_anodic − E_cathodic). Evidently, all of the measured RTILs possess wide electrochemical windows, ranging up to 5.68–5.83 V. Unlike some previous literature, where the alkoxy group sometimes can cause more than 0.5 V reduction in EW,^20d,e the alkoxy group in this study shows no significant negative effect on the electrochemical windows, which are larger than 5.68 V. Both the wide electrochemical windows and high conductivity of the ether-functionalized RTILs indicate their significant promise for electrochemical applications.

Fig. 6 Three typical cyclic voltammetry of RTILs.

Conclusion

An attempt to understand the reason for the low viscosity of the ether-functionalized RTILs was performed in an experimental method, based on a comparative study between 12 ether-substituted RTILs and their alkyl counterparts short of an ether O atom. The properties of the RTILs, including viscosity, the activation energy of viscosity, density, molar volume, surface tension, the total free volume, conductivity and electrochemical window, were investigated for this study. Replacing an alkyl chain by an alkoxy chain with an additional oxygen unit in RTILs shows a significant effect on viscosity (η_O/η) and its activation energy (ΔE_η). Both the obtained η_O/η and ΔE_η values for the 12 pairs of RTILs are highly variable and strongly dependent on the structures of the cation and ether moiety, and range 1.143–0.594 and 1.69–−4.14 kJ mol⁻¹, respectively. For the first time, the ether groups were found to be able to increase the total free volume of RTILs, with ΔFV_O ranging up to 4.93 mL mol⁻¹. For the 12 pairs of RTILs, both the η_O/η and ΔE_η values generally decrease with increasing ΔFV_O values, indicating that the increased free volume plays a positive key role in decreasing viscosity. Consequently, the alkoxy chain effect on decreasing viscosity was ascribed to the increased free volume, which results from the high conformational flexibility of the ether moieties and provides more available holes for the convenience of mass transport and low viscosity. In these RTILs, the alkoxy chain flexibility is generally governed by three factors. In order to better exert the alkoxy chain effect on decreasing viscosity and increasing the free volume, the alkoxy groups were supposed to contain a rod-like alkyl tail and a –CH₂– spacer between the ether O atom and the cationic N atom, and be in an environment without hydrogen bond donors. As a result of their low viscosity, the ether-substituted RTILs possess high conductivity up to 4.97 mS cm⁻¹ at 25 °C, and meanwhile the alkoxy chains can maintain the high electrochemical stability of pyrrolidinium and ammonium RTILs with electrochemical windows wider than 5.68 V, indicating significant promise for electrochemical applications. The results in this work should be very helpful to design and synthesize novel functionalized liquid materials, in particular those with highly flexible side chains to form low viscosity solvents and high conductivity electrolytes.

Experimental section

Materials and characterization

All the chemicals and solvents were reagent grade quality and obtained commercially and used without further purification, except for CH₃CH₂OCH₂Cl, which was redistilled under anhydrous conditions before use. ¹H-NMR spectra (ppm, Δ) were recorded on a BRUKER NMR spectrometer (300 MHz), with CD₃COCD₃ as solution and TMS as an internal reference. FT-IR spectra (cm⁻¹) were measured using diamond smart orbit ATR on a Nicolet 6700 FT- IR instrument.

Property testing

Prior to each test, the RTIL samples would be vacuum dried for 20 h at 80 °C and 10⁻²–10⁻³ mbar. The water content in RTILs could be reduced to be less than 150 ppm,^20h after such a vacuum drying process. The density (g mL⁻¹) of the RTILs was determined at 25 °C using a DMA 35 portable density meter, which was calibrated with ultrapure water (0.99704 g mL⁻¹ at 25 °C). Each density value represents the average of three measurements, with standard deviation less than 0.0002 g mL⁻¹. The viscosity (cP) was recorded with a BROOKFIELD DV-II+ Pro viscometer at nine temperatures ranging from 10 to 80 °C. The viscosity measurement was performed in a glove box with moisture concentrations below 1 ppm, and each value is based on the average of three measurements with standard deviation less than 2 cP. The surface tension (mN m⁻¹) was measured by a Kibron EZ-Pi plus Tensiometer using the du Noüy technique at 25 °C, and averaged by 6 tests (±0.05 mN m⁻¹). The conductivity (mS cm⁻¹) was obtained by using a JENCO EC330 (B) conductivity meter, and averaged by three tests (±0.05 mS cm⁻¹). The cyclic voltammetry testing was carried out on a CHI 660D electrochemical work station with a glassy carbon working electrode and an Ag/AgCl reference electrode at room temperature. The temperature for the measurements was maintained at ±0.1 °C by means of an external temperature controller.

General procedure for the synthesis of [Pr-MPyrr]Br, [Bu-MPyrr]Br, [MOE-MPyrr]Br, [EOE-MPyrr]Br, [Pr-EDMAm]Br, [Bu-EDMAm]Br, [MOE-EDMAm]Br, [EOE-EDMAm]Br, [Pr-MIm]Br, [Bu-MIm]Br, [MOE-MIm]Br, [Bu-Py]Br and [EOE-Py]Br

In a typical procedure, equimolar amounts (0.40 mol) of an amine and of an alkyl or alkoxyalkyl bromide were added into 80 mL anhydrous ethanol. After stirring at room temperature for 10 h and then at 50 °C for 48 h, the solvent was removed under vacuum at 80 °C, leaving a colourless or pale yellow solid. After twice recrystallization from a mixture of acetone and anhydrous diethyl ether, a white solid was obtained in a yield of about 80%.

General procedure for the synthesis of [EOM-MPyrr]Cl and [EOM-EDMAm]Cl

In a typical procedure, 0.40 mol CH₃CH₂OCH₂Cl was added dropwise into a mixture of 60 mL anhydrous dichloromethane and an amine (0.40 mol equiv.) in a round bottom flask fitted with a drying tube at 0 °C. After stirring for 4 h at 30 °C, the solvent was evaporated under vacuum at 50 °C, affording a pale yellow solid. After twice recrystallization from the mixture of acetone and anhydrous diethyl ether, a white solid was obtained in a yield of about 80%.

Synthesis of [EOE-MIm]Br

A mixture of 0.48 mol CH₃CH₂OCH₂CH₂Br, 0.40 mol N-methylimidazole and 80 ml anhydrous ethanol was vigorously stirred for 4 d at 50 °C. After the reaction, the solvent was removed under vacuum at 80 °C, leaving a colourless liquid. After extraction with anhydrous diethyl ether (3 × 50 mL) and drying under vacuum, a colourless liquid was obtained in a yield of 88%.

General procedure for the synthesis of NTf₂-based RTILs

In a typical procedure, an ammonium halide (0.10 mol) and lithium bis(trifluoromethylsulfonyl)imide (LiNTf₂) (0.11 mol) were added into 40 mL ultrapure water and then stirred for 4 h at room temperature. After the reaction, the upper water phase was decanted off. The bottom IL phase was washed with 0.1 M aqueous LiNTf₂ solution (20 mL) and ultrapure water (3 × 20 mL) in a separating funnel. Concentration under vacuum at 80 °C afforded a colorless or pale yellow liquid, with a yield of about 90%. By AgNO₃ test and halide titration with 0.5M AgNO₃ using 0.2 g of the ionic liquid, no halide content was detected (halide content < 200 ppm).³⁸

General procedure for the synthesis of BF₄-based RTILs

The anion exchange reaction was performed by using triethyloxonium tetrafluoroborate [Et₃O]BF₄, and in a glove box with moisture concentrations below 1 ppm. In a typical procedure, an ammonium halide (0.10 mol) and equimolar [Et₃O]BF₄ were added to 40 mL anhydrous dichloromethane in a round bottom flask. After stirring for 2 h at room temperature, the mixture was concentrated in vacuo at 80 °C to remove the solvent, yielding a colorless or pale yellow liquid. The halide content was evaluated by AgNO₃ test to be less than 200 ppm.

[Pr-MPyrr]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.726 (m, 4H), 3.504 (m, 2H), 3.256 (s, 3H), 2.328 (m, 4H), 1.959 (m, 2H), 1.109 (t, 3H); IR (cm⁻¹): 2980.64, 2947.26, 2888.93, 1472.23, 1347.14, 1328.94, 1225.78, 1173.45, 1131.65, 1049.81, 970.30, 938.77, 788.57, 761.91, 739.55, 653.80, 611.84, 599.63, 568.62, 509.44.

[Bu-MPyrr]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.727 (m, 4H), 3.548 (m, 2H), 3.263 (s, 3H), 2.328 (m, 4H), 1.916 (m, 2H), 1.440 (m, 2H), 0.984 (t, 3H); IR (cm⁻¹): 2969.37, 2943.26, 2880.85, 1467.06, 1347.03, 1329.10, 1225.71, 1173.96, 1131.91, 1050.19, 964.88, 928.14, 788.42, 762.07, 739.38, 653.72, 612.32, 599.59, 568.69, 509.67.

[MOE-MPyrr]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.948 (m, 2H), 3.783 (m, 6H), 3.400 (s, 3H), 3.301 (s, 3H), 2.321 (m, 4H); IR (cm⁻¹): 2991.65, 2942.42, 2902.29, 2825.53, 1475.20, 1463.33, 1347.29, 1328.80, 1226.08, 1174.44, 1130.49, 1050.15, 996.65, 933.40, 788.56, 762.25, 739.68, 653.79, 611.83, 599.70, 568.72, 509.17.

[EOE-MPyrr]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.990 (m, 2H), 3.792 (m, 6H), 3.584 (q, 2H), 3.315(s, 3H), 2.325 (m, 4H), 1.193 (t, 3H); IR (cm⁻¹): 2981.90, 2938.53, 2885.34, 1474.61, 1462.78, 1347.60, 1329.11, 1226.01, 1175.07, 1130.54, 1050.11, 998.47, 935.29, 788.43, 762.14, 739.60, 653.71, 612.48, 599.68, 568.78, 509.39.

[EOM-MPyrr]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 4.866 (s, 2H), 3.960 (q, 2H), 3.769 (m, 2H), 3.628(m, 2H), 3.274 (s, 3H), 2.313 (m, 4H) 1.288 (t, 3H); IR (cm⁻¹): 2986.90, 2940.15, 2903.54, 1464.10, 1391.98, 1347.52, 1329.13, 1225.88, 1174.23, 1131.03, 1050.34, 998.61, 927.07, 788.49, 762.24, 739.67, 653.86, 612.11, 599.61, 568.69, 509.36.

[Pr-EDMAm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.607 (q, 2H), 3.454 (m, 2H), 3.262 (s, 6H), 1.934 (m, 2H), 1.466 (m, 2H), 1.019 (t, 3H); IR (cm⁻¹): 3049.95, 2982.40, 2949.71, 2888.72, 1487.73, 1462.82, 1422.98, 1346.78, 1328.30, 1226.16, 1174.44, 1131.84, 1050.22, 976.34, 950.43, 788.85, 761.92, 739.85, 653.99, 612.36, 599.97, 568.86, 509.72.

[Bu-EDMAm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.609 (q, 2H), 3.494 (m, 2H), 3.265 (s, 6H), 1.880 (m, 2H), 1.459 (m, 5H), 0.989 (t, 3H); IR (cm⁻¹): 3050.48, 2969.54, 2942.50, 2881.87, 1487.84, 1464.15, 1422.78, 1346.67, 1328.46, 1225.99, 1174.87, 1132.02, 1050.43, 983.09, 955.74, 920.14, 788.71, 762.20, 739.70, 653.90, 612.58, 599.91, 568.88, 510.05.

[MOE-EDMAm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.950 (m, 2H), 3.749 (t, 2H), 3.678 (q, 2H), 3.394 (s, 3H), 3.314 (s, 4H), 1.474 (m, 3H); IR (cm⁻¹): 3048.85, 2997.09, 2941.45, 2894.82, 2831.37, 1484.11, 1473.31, 1458.08, 1346.77, 1328.26, 1226.22, 1175.01, 1131.95, 1050.42, 960.88, 918.31, 788.88, 762.42, 739.90, 653.93, 612.32, 599.99, 568.85, 510.01.

[EOE-EDMAm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 3.983 (m, 2H), 3.736 (t, 2H), 3.678 (q, 2H), 3.576(q, 2H), 3.314 (s, 4H), 1.473 (m, 3H), 1.188 (t, 3H); IR (cm⁻¹): 3048.77, 2983.02, 2936.81, 2882.65, 1486.63, 1470.48, 1347.19, 1328.64, 1226.13, 1175.81, 1131.92, 1050.46, 960.37, 914.72, 788.70, 762.25, 739.80, 653.97, 612.82, 600.08, 568.93, 510.41.

[EOM-EDMAm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 4.860 (s, 2H), 3.977 (q, 2H), 3.585 (q, 2H), 3.213(s, 6H), 1.461 (m, 3H), 1.294 (t, 3H); IR (cm⁻¹): 3047.79, 2989.48, 2939.74, 2906.19, 1473.93, 1452.94, 1347.31, 1328.63, 1226.16, 1175.55, 1132.12, 1050.46, 981.91, 862.68, 788.90, 762.40, 739.91, 654.16, 612.36, 600.02, 568.91, 510.22.

[Pr-MIm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.022 (s, 1H), 7.768 (t, 1H), 7.721 (t, 1H), 4.336 (t, 2H), 4.072 (s, 3H), 1.976 (m, 2H), 0.966 (t, 3H); IR (cm⁻¹): 3157.29, 3121.85, 2975.01, 2943.75, 2885.26, 1573.24, 1459.42, 1346.55, 1329.09, 1169.06, 1131.80, 1050.16, 843.23, 788.95, 739.40, 651.98, 611.10, 598.71, 568.50, 508.64.

[Bu-MIm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.062 (s, 1H), 7.791 (t, 1H), 7.735 (t, 1H), 4.384 (t, 2H), 4.079 (s, 3H), 1.940 (m, 2H), 1.391 (m, 2H), 0.949 (t, 3H); IR (cm⁻¹): 3156.43, 3121.11, 2966.71, 2940.25, 2879.33, 1573.07, 1466.69, 1346.72, 1329.19, 1176.16, 1131.91, 1050.45, 843.02, 788.82, 739.35, 652.32, 611.36, 598.76, 568.55, 508.97.

[MOE-MIm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.001 (s, 1H), 7.734 (t, 1H), 7.701 (t, 1H), 4.536 (t, 2H), 4.080 (s, 3H), 3.815 (t, 2H), 3.353 (s, 3H); IR (cm⁻¹): 3159.31, 3122.12, 2943.12, 2902.05, 2840.13, 1566.93, 1453.35, 1346.57, 1329.05, 1174.34, 1131.83, 1050.22, 835.08, 788.91, 761.92, 739.57, 652.38, 611.35, 598.53, 568.50, 508.10.

[EOE-MIm]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.013 (s, 1H), 7.746 (t, 1H), 7.702 (t, 1H), 4.532 (t, 2H), 4.083 (s, 3H), 3.854 (t, 2H), 3.532 (q, 2H), 1.136 (t, 3H); IR (cm⁻¹): 3158.11, 3122.62, 2980.72, 2937.76, 2876.07, 1566.90, 1450.78, 1347.18, 1329.42, 1175.40, 1131.39, 1050.48, 831.58, 788.81, 761.84, 739.53, 652.27, 611.79, 598.68, 568.65, 508.05.

[Pr-MIm]BF₄

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 8.987 (s, 1H), 7.752 (t, 1H), 7.706 (t, 1H), 4.315 (t, 2H), 4.044 (s, 3H), 1.959 (m, 2H), 0.955 (t, 3H); IR (cm⁻¹): 3161.46, 3121.73, 2971.68, 2942.52, 2883.47, 1574.29, 1460.75, 1431.64, 1389.97, 1347.71, 1338.78, 1285.31, 1171.08, 1044.91, 1030.62, 1016.40, 846.57, 754.93, 649.64, 622.65, 520.21.

[Bu-MIm]BF₄

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 8.987 (s, 1H), 7.754 (t, 1H), 7.699 (t, 1H), 4.349 (t, 2H), 4.038 (s, 3H), 1.921 (m, 2H), 1.383 (m, 2H), 0.944 (t, 3H); IR (cm⁻¹): 3161.03, 3121.41, 2963.67, 2938.17, 2876.77, 1573.75, 1466.54, 1431.73, 1385.07, 1339.08, 1285.07, 1169.32, 1044.99, 1032.56, 1015.72, 847.41, 753.54, 651.04, 622.68, 520.26.

[MOE-MIm]BF₄

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 8.961 (s, 1H), 7.715 (t, 1H), 7.686 (t, 1H), 4.512 (t, 2H), 4.052 (s, 3H), 3.802 (t, 2H), 3.345 (s, 3H); IR (cm⁻¹): 3161.94, 3122.15, 2942.41, 2902.30, 2837.07, 1574.94, 1567.15, 1453.93, 1431.16, 1385.29, 1357.10, 1339.88, 1285.89, 1224.93, 1171.10, 1113.52, 1045.10, 1028.26, 1011.45, 832.24, 753.26, 703.41, 652.61, 622.87, 520.20.

[EOE-MIm]BF₄

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 8.968 (s, 1H), 7.724 (t, 1H), 7.687 (t, 1H), 4.504 (t, 2H), 4.052 (s, 3H), 3.841 (t, 2H), 3.526 (q, 2H), 1.131 (t, 3H); IR (cm⁻¹): 3162.14, 3122.12, 2977.05, 2934.68, 2874.18, 1574.74, 1567.24, 1451.46, 1431.39, 1382.25, 1352.27, 1339.80, 1285.13, 1216.36, 1171.51, 1114.05, 1044.59, 1028.32, 1014.87, 846.89, 754.12, 703.02, 652.43, 622.92, 520.20.

[Bu-Py]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.191 (d, 2H), 8.753 (t, 1H), 8.293 (t, 2H), 4.861 (t, 2H), 2.129 (m, 2H), 1.463 (m, 2H), 0.982 (t, 3H); IR (cm⁻¹): 3138.32, 3094.50, 3073.79, 2969.06, 2940.56, 2880.26, 1636.24, 1503.33, 1489.34, 1468.75, 1346.49, 1329.60, 1226.10, 1174.22, 1030.85, 1050.15, 956.07, 787.48, 769.11, 739.20, 682.89, 653.93, 611.48, 598.99, 568.49, 508.96.

[EOE-Py]NTf₂

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.146 (d, 2H), 8.778 (t, 1H), 8.295 (t, 2H), 5.024 (t, 2H), 4.041 (t, 2H), 3.527 (q, 2H), 1.093 (t, 3H); IR (cm⁻¹): 3140.77, 3095.29, 3072.50, 2981.96, 2936.37, 2877.27, 1636.82, 1501.86, 1489.82, 1448.59, 1347.04, 1329.61, 1226.20, 1174.73, 1030.21, 1050.34, 929.35, 788.23, 774.52, 739.70, 682.12, 653.92, 611.67, 599.25, 568.52, 507.94.

[Bu-Py]BF₄

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.163 (d, 2H), 8.731 (t, 1H), 8.263 (t, 2H), 4.832, (t, 2H), 2.102 (m, 2H), 1.451 (m, 2H), 0.972 (t, 3H); IR (cm⁻¹): 3139.76, 3096.64, 3075.23, 2963.96, 2937.70, 2876.97, 1635.96, 1503.72, 1489.15, 1468.04, 1386.37, 1322.87, 1285.25, 1221.43, 1173.24, 1044.03, 1031.06, 1020.92, 770.83, 737.18, 683.09, 520.17.

[EOE-Py]BF₄

¹H-NMR (CD₃COCD₃, 300 MHz, ppm): 9.109 (d, 2H), 8.757 (t, 1H), 8.265 (t, 2H), 4.986 (t, 2H), 4.016 (t, 2H), 3.519 (q, 2H), 1.085 (t, 3H); IR (cm⁻¹): 3141.10, 3097.34, 3074.15, 2978.47, 2934.43, 2874.72, 1636.41, 1501.47, 1489.15, 1448.95, 1382.19, 1352.67, 1285.29, 1219.28, 1178.93, 1043.56, 1019.83, 928.81, 824.06, 776.51, 682.45, 520.22.

Acknowledgements

This work is supported by Startup Grant of Nanyang Technological University and Academic Research Fund (RG21/09) of Ministry of Education in Singapore.

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