Thermal behavior and electrochemistry of protic ionic liquids based on triethylamine with different acids

Abstract

Protic ionic liquids (PILs) composed of the triethylammonium cation with dihydrogen phosphite, tosylate, and trifluoroacetate anions were synthesized. All samples were salts with melting points below 100 °C and were characterized via NMR spectroscopy, attenuated total reflection (ATR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetry (TG). The electrochemical characteristics of each protic ionic liquid were obtained using a combination of impedance spectroscopy and cyclic voltammetry. Moreover, the influence of water on the thermal behavior and conductivity of triethylammonium tosylate is studied. A linear correlation between the temperatures of melting and crystallization is established for a number of PILs, including both derivatives of triethylamine with different acids reported in the literature and the newly synthesized PILs. The optimal combination of thermal characteristics and electroconductivity was observed for triethylammonium trifluoroacetate and tosylate.

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Introduction

Ionic liquids (ILs), which are organic salts, have attracted a significant attention due to their unique physicochemical properties such as low vapor pressure, high thermal stability and ionic conductivity, wide electrochemical window, and good solvation capacity. Room temperature ionic liquids (T_m < 25 °C) and molten salts (T_m > 100 °C) are distinguished depending on their melting points.

There are two main families of ILs: aprotic ionic liquids (AILs) and protic ionic liquids (PILs). Aprotic ionic liquids have no active protons in their chemical structure. Typical AILs, such as imidazolium salts, are generally synthesized by the quaternization (alkylation) of an amine, followed by anion exchange. PILs are prepared by the proton transfer reactions from Brønsted acids to Brønsted bases and have exchangeable (active) protons that are able to form H-bonds. PILs have been less studied in comparison to AILs. Herein, our attention is focused on the PILs due to their remarkable properties, which make them potentially usable in many industrial and technical applications. Moreover, the wide variety of substances that are the potential components of PILs allows the synthesis of PILs with desired properties.

The incorporation of PILs into the confined geometry of polymers¹ makes these systems applicable as electrolytes in non-humidified fuel cell systems.^1–6 Polymer electrolytes based on ILs surpass the conventional electrolytes in some characteristics, viz. they are safer and highly thermostable, as well as have a wide range of operating temperatures. There are many studies reporting the synthesis of protic ionic liquids and ionic melts with high thermal stability in combination with high ionic conductivity under anhydrous conditions. For example, it was shown³ that anhydrous diethylmethylammonium trifluoromethanesulfonate ([dema]/[TfO] = 1/1) exhibited high thermal stability (360 °C), low melting point (−6 °C), and high ionic conductivity (4.3 × 10⁻² S cm⁻¹ at 120 °C). H. Nakamoto et al.⁵ succeeded in synthesizing a neutral salt composed of bis(trifluoromethanesulfonyl)imide and benzimidazole, which was hydrophobic and electrochemically stable. The decomposition temperature of this salt was higher than 350 °C and its proton conductivity was equal to 8.3 × 10⁻³ S cm⁻¹ at 140 °C. In⁷ two series of liquid PILs, which were composed of secondary and/or tertiary ammonium cations and trifluoroacetate, methanesulfonate, trifluoromethanesulfonate, and tosylate anions, were synthesized. These salts had high conductivity values (1.4–4.9 mS cm⁻¹ at 70 °C) and were thermostable up to 200–300 °C.

Herein, we report the first results of our investigation of the anion nature effect on the thermal behavior and conductivity of protic ionic liquids composed of triethylammonium cation and anions of different acids.

Experimental

Materials and ionic liquid preparation

Phosphonic acid (H₂PHO₃) (99%, Sigma-Aldrich), trifluoroacetic acid (TFA) (99%), p-toluenesulfonic acid monohydrate (PTSA) (99%, Tokyo Chemical Industries), and triethylamine (TEA) (99%, ACROS) were used without any further purification.

All PILs were prepared by the neutralization reaction between equimolar amounts of triethylamine (Brønsted base) and different Brønsted acids according to the following (Scheme 1):

Scheme 1 PILs synthesis.

The reaction was carried out under argon. A certain amount of TEA (approximately 0.07 mol) was placed in a 100 mL round-bottom flask equipped with a Teflon coated stir bar. Since the reaction is highly exothermic, the flask was immersed in an ice bath and each acid was added dropwise to TEA, separately. Then, the mixture was stirred for 8 h at 90 °C and was dried under vacuum. Triethylammonium tosylate contains a lot of water because p-toluenesulfonic acid monohydrate was used for its synthesis. Therefore, lyophilization of the salt was carried out to reduce its water content.⁸

Thus, the following resulting products were obtained: white transparent solid, yellow liquid, and white crystals in the case of triethylammonium dihydrogen phosphite, triethylammonium trifluoroacetate, and triethylammonium tosylate, respectively. The structural formulae of the PILs synthesized are shown below:

The water content in the synthesized ionic liquids was measured using a Karl Fischer titrator (V30, Mettler Toledo). The synthesized PILs were characterized via the ¹H NMR, ¹³C NMR and ATR spectroscopic techniques.

¹H and ¹³C NMR chemical shifts

The structures of the ionic liquids were confirmed via ¹H and ¹³C NMR spectroscopy. NMR spectra were obtained relative to tetramethylsilane (TMS) using a Bruker AVANCE-500 spectrometer.

[TEA]/[H₂PHO₃]. ¹H NMR (CDCl₃, TMS, δ, ppm): 1.22 t (9H, CH₃–, J = 7.32 Hz); 3.02–2.99 q (6H, –CH₂–, J = 7.32 Hz); 6.16 s (0.5H, P–H); 7.41 s (0.5, P–H), 9.84 s_br (2H, HOacid); 11.76 s_br (1H, NH).

¹³C NMR (CDCl₃, TMS, δ, ppm): 7.61 (CH₃–), 44.61 (–CH₂–).

[TEA]/[PTSA]. ¹H NMR (CDCl₃, TMS, δ, ppm): 1.23 t (9H, CH₃–, J = 7.32 Hz); 2.28 s (3H, –CH₃); 3.05–3.04 q (6H, –CH₂–, J = 7.32 Hz); 7.1 d (2H, Ph-H, J = 7.935 Hz); 7.69 d (2H, Ph-H, J = 7.935 Hz), 10.02 s (1H, NH).

¹³C NMR (CDCl₃, TMS, δ, ppm): 7.56 (CH₃–); 20.3 (–CH₃); 45.30 (–CH₂–); 124.87 (C–H); 127.76 (C–H); 138.87 (C–H); 141.68 (C–H).

[TEA]/[TFA]. ¹H NMR (CDCl₃, TMS, δ, ppm): 1.31 t (9H, CH₃–, J = 7.32 Hz); 3.13 m (6H, –CH₂–); 12.07 s (1H, NH⁺ COO⁻).

¹³C NMR (CDCl₃, TMS, δ, ppm): 9.74 (CH₃–), 46.98 (–CH₂–), 119.12–116.79 (–CF₃–), 163.35 (C=O).

ATR-spectroscopy

FT-IR spectra were obtained using a VERTEX 80v (Bruker) spectrometer at room temperature within the region of 4000–400 cm⁻¹ by averaging 128 scans at a resolution of 2 cm⁻¹.

The spectra of all the synthesized PILs, and pure TFA and TEA were obtained using the attenuated total reflection (ATR) technique with a MVP 2 Series™ (Harrick) instrument with a diamond crystal. The spectra of solid samples (H₂PHO₃ and PTSA) were measured under vacuum using the KBr pellet. Every sample was ground with spectral grade KBr to the state of a homogeneous powder. The mixtures were then pressed into disc-pellets and analyzed.

FT-IR measurements were conducted to characterize the products with respect to the interactions between the Brønsted acids and TEA. The ATR spectra of the PILs together with the spectra of their source compounds over the whole frequency range are shown in the ESI.† In general, the spectrum of each PIL can be considered as a superposition of the spectra of its pure components with some band displacements, observed as expected. However, the appearance of some new modes within the frequency range of 2500–2700 cm⁻¹ (Fig. 1) is an evidence for the formation of proton transferred salts (or the protonated amine (CH₃–CH₂)₃NH⁺).^9,10

Fig. 1 ATR spectra of the synthesized PILs and the spectra of their source substances in the range of 2000–3750 cm⁻¹. (a) Triethylammonium dihydrogen phosphite, (b) triethylammonium tosylate, and (c) triethylammonium trifluoroacetate. The spectrum of pure TFA was obtained from ref. 11.

Thermal properties

Thermal properties were measured via differential scanning calorimetry (DSC) and thermogravimetry (TG). DSC measurements were carried out on a NETZSCH DSC 204 F1 under N₂ atmosphere within the temperature range from −100 to 100 °C. The samples for DSC measurements were tightly sealed using Al pans in a glove box. The sample masses of 10–20 mg were used for analysis. The samples were cooled from room temperature to −100 °C at a cooling rate of 10 °C min⁻¹ and then heated back to 100 °C at a heating rate of 2 °C min⁻¹.

TGA of the PILs was performed using a NETZSCH TG 209 F1 analyzer in a flow of argon (20 mL·min⁻¹) at a heating rate of 10 °C min⁻¹.

Ionic conductivity

The conductivity of the ionic liquids was determined using the electrochemical impedance method by an impedance/gain-phase Solartron 1260A analyzer over the frequency range of 0.1 Hz to 1 MHz with the signal amplitude of 10 mV and with an accuracy higher than 0.2%. The resistance magnitude was found from the high-frequency cutoff corresponding to the bulk resistance of the sample. All measurements were carried out within a wide temperature range in the cell with platinum electrode, and a platinum wire was used to supply the current. The temperature was kept constant with a HAAKE DC50-K35 thermostat within ±0.01 K. The cell was calibrated with standards recommended by IUPAC.¹² The calibration was done at 298 K, and the cell constants at higher temperatures were calculated according to the reported method.¹³

Voltammetric studies

Polarization measurements were carried out on a platinum electrode in a hermetic three-electrode electrochemical cell using a PI 50-Pro-3 pulse potentiostat with an automatic registration, and the data were processed using the PS Pack 2 software. The measurements were performed within the temperature range from 50 to 120 °C within the accuracy of ±0.1 °C. The potential scan rate was 0.01 V s⁻¹ for all measurements. The electrochemical cell was designed and made specially for the measurement of both hydrophilic and hydrophobic ionic liquids.¹⁴ The counter and reference electrodes were made of platinum.

Results and discussion

One of the most important properties of ILs is their melting and decomposition temperatures because they are such parameters that determine the range of ILs liquid state existence. Since many ILs have the propensity for supercooling and glass formation, the accurate determination of their melting temperatures is not a trivial task.¹⁵ The differences in the values of melting points reported in various studies can be caused by both the high viscosities of the liquids and some experimental factors, such as cooling and heating rates, as well as by polymorphous crystalline states.¹⁶ Moreover, it has been suggested that dissolved CO₂ decreases the melting point of ILs.^17–19

Theoretically, the freezing temperature is equal to the melting point. However, for many ILs these two temperatures are not the same. Therefore, substantial supercooling is observed when PILs are cooled from relatively high temperatures and the supercooled melt devitrifies (crystallizes) at the temperature of so-called “cold crystallization” (T_c). This behavior is typical for all the salts synthesized. As an example, the DSC scans for [TEA]/[TFA] are shown in Fig. 2.

Fig. 2 DSC thermograms of the [TEA]/[TFA] ionic liquid for the second cooling and heating steps.

The thermochemical characteristics of the synthesized PILs are shown in Table 1. It should be noted that no glass transition was observed for the PILs under the experimental conditions. Also, one can see that the melting point (T_m) of triethylammonium tosylate increases but the conductivity decreases after salt lyophilization.

Table 1 Key properties of the synthesized PILs^a

PILs	Tc (°C)	ΔH (J g^−1)	Tm (°C)	ΔH (J g^−1)	Tdec (°C)	κ (mS cm^−1)	Water (wt%)
a Tc – crystallization temperature; Tm – melting temperature; Td – decomposition temperature; κ – ionic conductivity.
[TEA]/[H2PHO3]	−2.7	−29.1	42.8	23.4	106.1	3.92 (100 °C)	2
[TEA]/[PTSA]	12.0	−33.4	67.5	41.8	324.6	7.47 (105 °C)	3.4
[TEA]/[PTSA] after lyophilization	18.5	−55.6	80.2	65.1	311.3	5.28 (105 °C)	<1
[TEA]/[TFA]	−41.4	−51.7	8.6	59.5	186.3	10.22 (105 °C)	<1

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The TG curves of the PILs under investigation are presented in Fig. 3, which show their thermal stabilities; PILs decomposition temperatures, which are stated as the temperatures corresponding to 3% loss of the sample weight, are listed in Table 1.

Fig. 3 TG scans of the PILs synthesized.

The temperature corresponding to the beginning of weight loss of the PIL based on H₂PHO₃ was low due to the condensation reaction of phosphites.³

The thermal properties of the synthesized PILs along with the reported data for triethylammonium salts are listed in Table 2.

Table 2 Thermal properties of the synthesized PILs along with the reported data for triethylammonium salts^a

Salts	H2O	Tg, °C	Tm, °C	Tc, °C	Td, °C	κ, mS cm^−1 (25 °C)	Ref.
a Ac: acetic acid; TFSA: bis(tetrafluoromethylsulfonyl)imide; TFA: trifluoroacetic acid; TFSI: bis(trifluoromethanesulfonyl)amide; MsOH: methanesulfonic acid; BF: heptafluorobutyrate; PFBSu: perfluorobenzenesulfonic acid; TfOH: trifluoromethanesulfonic acid; H2PHO3: phosphonic acid; FBSu: fluorobenzenesulfonic acid; TFBSu: trifluorobenzenesulfonic acid; PFBu: perfluorobutanesulfonic acid; BA: benzoic acid; PTSA: p-toluenesulfonic acid; Sacc: saccharin; H2SO4: sulfuric acid; PFOc: perfluoroctane sulfonic acid; BF4: tetrafluoroborate; BSu: benzenesulfonic acid; N: nitric acid; F: formic acid; H3PO4: phosphoric acid; H2PO3F: fluorophosphoric acid; and (BuO)2POOH: di-n-butyl-phosphate.
TEA Ac	0.42%	−93	−18		48		20
TEA TFSA	600 ppm		−0.8	−29		5.64	21
TEA TFA	50 ppm	−83	2	−42	190		22
	0.42%		8.6	−41.4	186.3	2.45	This work
TEA TFSI	<1 ppm		3.5		350	32.3 (130 °C)	4
	10 ppm		3			4.4	23
TEA ms	100 ppm	−78.9	24.3	−31.6		1.91	24
	50 ppm		33	−24	305		22
	2 mol%		25	−31	218	2.03	16
		−62.1	17.4		225	16.3 (120 °C)	3
	<1 ppm	−96.5	21.6		269.7		25
TEA BF	0.3		25	−62		0.679	26
TEA PFBSu	50 ppm	−58	29	−5	370		27
TEA TfOH	50 ppm	−58	32	−20	376		22
			36
	2 mol%		26	−13	322	4.79	16
			34.3		358	2.76	3
	<1 ppm				312.5		25
TEA H2PHO3		−78.4			135		3
	2%		42.8	−2.7	106.1	3.92 (100 °C)	This work
TEA FBSu	50 ppm	−70	59	2	320		27
TEA TFBSu	50 ppm		60	3	340		27
TEA PFBu	50 ppm		61	50	383		27
TEA BA	50 ppm		62	30	202		22
TEA PTSA	3.4%		67.5	12.0	324.6	0.46	This work
TEA Sacc		−27.4	72	36			24
				59.4
TEA H2SO4			74.7		270	13.1 (120 °C)	3
	<1 ppm	−100.1	84.2		262.8		25
TEA PFOc	50 ppm	−48	82	71	390		27
TEA BF4	<1 ppm		104.3		296.7		25
TEA BSu	50 ppm		119	106	330		27
TEA N	0.84%		115				28
			113–114				29
TEA F	0.19%	−119				13.05	28
	0.51%				54		20
TEA H3PO4		−27.8			175		3
	<1 ppm	−34.4			349.2		25
		−32.2
TEA H2PO3F	<1 ppm	−59.7					25
TEA(BuO)2POOH	100 ppm	−91				0.23	24

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The thermal properties of the PILs synthesized on the basis of phosphorous and trifluoroacetic acids agree well with the reported data. The discrepancy observed can be explained, first of all, by the experimental conditions. The parameters presented in Table 2 for the salts based on methanesulfonic and trifluoromethanesulfonic acids show that the changes of thermal characteristics do not correlate directly with the different water contents in the salts.

However, some regularity in T_m variation with the nature of acids is observed. Thus, the melting temperatures of the PILs obtained on the basis of carbon acids are, in general, lower as compared to those of the PILs based on sulfonic acids. The substitution of one hydroxyl-group in sulfuric acid by a methyl-group, specifically going from sulfuric to methanesulfonic acid, results in a decrease in T_m. At the same time, a significant increase in T_m was observed when the aliphatic methyl-group in methanesulfonic acid (T_m = 17.4 to 33 °C) was replaced by an aromatic group (T_m = 119 °C for BSu). According to ref. 27, this phenomenon can be explained by the structure, rigidity, and high packing ability of the aromatic ring. There is a considerable increase in T_m when CH₃COO⁻ (−18 °C) is replaced by CF₃COO⁻ (8.6 °C). The melting point values also depend on the length of the perfluorinated alkyl anionic chain. Thus, the T_m of the salts increases from 61 to 82 °C when going from perfluorobutanesulfonic acid to perfluoroctane sulfonic acid.

Most ionic liquids based on the triethylammonium cation are thermostable up to 200–350 °C. Triethylammonium formate and acetate are the least thermostable salts. In ref. 30 it was also noted that the acetate based PILs are less stable compared to the protic ionic liquids based on other anions.

The onset of thermal decomposition for the ionic liquids based on the alkylimidazolium cation showed that the nature of the anion substantially determines this property, whereas the effect of the cation is much weaker.^31,32

In some studies^7,33–35 a correlation between T_m, T_d, and Δpκ values was established. For TEA-based salts (Table 2), no similar correlation is observed. The only correlation established is the correlation between the temperatures of crystallization and melting (Fig. 4).

Fig. 4 The correlation between the melting and crystallization temperatures of the triethylamine-based PILs.

The conductivity values of triethylamine-based ionic liquids (Table 2) are within 10⁻⁴ to 10⁻³ S cm⁻¹ at 25 °C. The largest conductivity value was obtained for the salt containing formic acid.

The ionic conductivity of the synthesized PILs was evaluated from their impedance spectra. As an example, the spectra of [TEA]/[TFA] at different temperatures are presented in Fig. 5. The spectra of other PILs were similar.

Fig. 5 Impedance spectra for [TEA]/[TFA] at some selected temperatures.

The temperature dependencies of the ionic conductivity of the synthesized PILs are presented in Fig. 6. The conductivity values increase according to the following order: [TEA]/[H₂PHO₃] < [TEA]/[PTSA] < [TEA]/[TFA]. At temperatures below 70 °C the protic IL synthesized on the basis of p-toluenesulfonic acid was in the supercooled state due to its high viscosity. Therefore, the temperature dependence of its specific conductivity is shown as a dotted line within the temperature range from 25 to 70 °C.

Fig. 6 Arrhenius plot for the PILs specific conductivity.

From the Arrhenius plots (Fig. 6), it can be seen that the temperature dependencies of the ionic conductivity are not strictly linear. Therefore, these dependencies can be fitted with the nonlinear Vogel–Tamman–Fulcher (VTF) equation:^36,37

where, κ_o is the conductivity at an infinitely high temperature, B is a coefficient related to the activation energy E_a, and T_o is the ideal glass transition temperature. The calculated values of T_o are 50 degrees lower than the experimentally determined glass transition temperatures.²

The VFT parameters calculated from the temperature dependencies of the ionic conductivities of the [TEA]/[TFA] and [TEA]/[PTSA] salts are summarized in Table 3.

Table 3 VFT parameters for the ionic conductivity of the [TEA]/[TFA] and [TEA]/[PTSA] salts (T_o = T_g − 50 K)

PILs	κo (S cm^−1)	B (K)	To (K)	R^2
[TEA]/[TFA]	0.07 ± 0.02	360 ± 104	193 ± 21	0.997
[TEA]/[PTSA]	0.06 ± 0.01	285 ± 37	246 ± 6	0.998

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The electrochemical window is an important parameter for the electrochemical stability of PILs. There are many factors that affect the ECW of ionic liquids. Among other things, the electrode material has a strong effect on the value of ECW. Since the synthesized PILs are expected for their application in electrochemical cells, in which Pt-electrodes are the most widely used, therefore, Pt was used as the electrode material for ECW measurements. Fig. 7 displays the cyclic voltammograms obtained using a platinum electrode at 50 °C for the three determined PILs.

Fig. 7 Cyclic voltammograms of the synthesized PILs at 50 °C.

The electrochemical window (ECW) value of TEA-based PILs considerably depends on the nature of the anion and temperature. It should be mentioned that the largest ECW values were obtained at 50 °C for all the PILs measured. The other important observation is that triethylammonium cation reduces at different potentials, subject to the counterion nature. Thus the cathodic limits for [TEA]/[TFA], [TEA]/[PTSA] and [TEA]/[H₂PHO₃] are equal to −0.75, −0.85, and −0.45 V, respectively, at 50 °C. A similar dependence of the cathodic potential on the nature of anion was observed for ILs based on metals halides³⁸ and for pyrrolidinium-based PILs.³⁹ This is attributed to changes in the medium acidity. In addition, water impurities, which are detected in all the PILs synthesized, probably contribute to the value of the cathodic response.^38,40 The influence of temperature on the electrochemical stability of the TEA-based PILs is shown in Fig. 8.

Fig. 8 Cyclic voltammograms of the synthesized PILs at different temperatures: (a) [TEA]/[PTSA]; (b) [TEA]/[TFA]; and (c) [TEA]/[H₂PHO₃].

For all the synthesized PILs, ECW narrowing with an increase in temperature is typical. Such behavior was also observed earlier for RTILs.^41,42 The ECW decreases both in the cathodic and anodic areas of potential that is probably connected to the influence of temperature on the kinetics of redox processes.³⁴ Here, it should be noted that the magnitude of ECW narrowing is not the same in the cathodic and anodic potential areas. Moreover, the ECW for all synthesized the PILs decreases by less than 300 mV in the cathodic area, whereas in the anodic area the decrease is about 100 mV for [TEA]/[TFA] and 400 mV for the other two salts. Thus, it is obvious that the [TEA]/[H₂PHO₃] salt is the most sensitive to an increase in temperature, and the potential range of its electrochemical stability decreases by about 700 mV upon heating from 50 to 120 °C.

Conclusions

PILs based on triethylamine with phosphonic, trifluoroacetic and p-toluenesulfonic monohydrate acids were synthesized. All the samples, in this work, are salts with melting points below 100 °C. Triethylammonium dihydrogen phosphite, tosylate and trifluoroacetate were compared with respect to their thermal and electrochemical properties. Comparison and analysis of the properties were carried out using the reported data on some other PILs based on triethylamine. The correlation between the melting and crystallization temperatures of the compounds under consideration was established. The ionic conductivity of the synthesized salts was found to be within the range from 10⁻² to 10⁻³ S cm⁻¹, which increased in the following order: [TEA]/[H₂PHO₃] < [TEA]/[PTSA] < [TEA]/[TFA]. The Vogel–Tamman–Fulcher equation was used to fit the temperature dependencies of the ionic conductivity of the PILs synthesized. The maximum electrochemical window values of 2.7, 1.7, and 1.4 V for [TEA]/[PTSA], [TEA]/[H₂PHO₃], and [TEA]/[TFA], respectively, were obtained at 50 °C. The electrochemical measurements of all the synthesized PILs demonstrate a decrease in the electrochemical window values with an increase in temperature. The optimal combination of the thermal characteristics and conductivity values was observed for triethylammonium trifluoroacetate and tosylate. Thus, triethylammonium tosylate and triethylammonium trifluoroacetate are expected to be promising PILs as potential electrolytes for the electrochemical applications.

Acknowledgements

This work was supported by the Russian Science Foundation (No. 16-13-10371). All thermal (DSC and TG) and spectroscopic (NMR, ATR, and impedance) measurements were carried out at the center for joint use of scientific equipment (the Upper Volga Regional Center for Physico-Chemical Research).

References

1. J. Lu, F. Yan and J. Texter, Prog. Polym. Sci., 2009, 34, 431–448.
2. W. Ogihara, H. Kosukegawa and H. Ohno, Chem. Commun., 2006, 34, 3637–3639.
3. H. Nakamoto and M. Watanabe, Chem. Commun., 2007, 24, 2539–2541.
4. M. A. B. H. Susan, A. Noda, S. Mitsushima and M. Watanabe, Chem. Commun., 2003, 8, 938–939.
5. H. Nakamoto, A. Noda, K. Hayamizu, S. Hayashi, H.-o. Hamaguchi and M. Watanabe, J. Phys. Chem. C, 2007, 111, 1541–1548.
6. A. Fernicola, B. Scrosati and H. Ohno, Ionics, 2006, 12, 95–102.
7. T. A. Siddique, S. Balamurugan, S. M. Said, N. A. Sairi and W. M. D. W. Normazlan, RSC Adv., 2016, 6, 18266–18278.
8. M. Gruzdev, U. Chervonova, T. Frolova and A. Kolker, Liq. Cryst., 2016 DOI:10.1080/02678292.2016.1202340 ..
9. J. Luo, J. Hu, W. Saak, R. Beckhaus, G. Wittstock, I. F. J. Vankelecom, C. Agert and O. Conrad, J. Mater. Chem., 2011, 21, 10426–10436.
10. K. Mori, S. Hashimoto, T. Yuzuri and K. Sakakibara, Bull. Chem. Soc. Jpn., 2010, 83, 328–334.
11. http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi.
12. E. Juhasz and K. N. Marsh, Pure Appl. Chem., 1981, 53, 1841–1845.
13. J. Barthel, F. Feuerlein, R. Neueder and R. Wachter, J. Solution Chem., 1980, 9, 209–219.
14. E. P. Grishina, A. M. Pimenova, N. O. Kudryakova and L. M. Ramenskaya, Russ. J. Electrochem., 2012, 48, 1166–1170.
15. H. Weingrtner, Angew. Chem., Int. Ed., 2008, 47, 654–670.
16. J. L. Lebga-Nebane, S. E. Rock, J. Franclemont, D. Roy and S. Krishnan, Ind. Eng. Chem. Res., 2012, 51, 14084–14098.
17. J. M. Lopes, F. A. Sanchez, S. B. R. Reartes, M. D. Bermejo, A. Martin and M. J. Cocero, J. Supercrit. Fluids, 2016, 107, 590–604.
18. A. Serbanovic, Z. Petrovski, M. Manic, C. S. Marques, G. a. V. S. M. Carrera, L. C. Branco, C. A. M. Afonso and M. Nunes da Ponte, Fluid Phase Equilib., 2010, 294, 121–130.
19. A. M. Scurto and W. Leitner, Chem. Commun., 2006, 35, 3681–3683.
20. Ch. Zhao, G. Burrell, A. A. J. Torriero, F. Separovic, N. F. Dunlop, D. R. MacFarlane and A. M. Bond, J. Phys. Chem. B, 2008, 112, 6923–6936.
21. L. Timperman, P. Skowron, A. Boisset, H. Galiano, D. Lemordant, E. Frackowiak, F. Beguin and M. Anouti, Phys. Chem. Chem. Phys., 2012, 14, 8199–8207.
22. M. Martinez, Y. Molmeret, L. Cointeaux, C. Iojoiu, J.-C. Lepretre, N. El Kissi, P. Judeinstein and J.-Y. Sanchez, J. Power Sources, 2010, 195, 5829–5839.
23. H. Matsumoto, H. Sakaebe and K. Tatsumi, J. Power Sources, 2005, 146, 45–50.
24. G. L. Burrell, I. M. Burgar, F. Separovic and N. F. Dunlop, Phys. Chem. Chem. Phys., 2010, 12, 1571–1577.
25. J.-P. Belieres and C. A. Angell, J. Phys. Chem. B, 2007, 111, 4926–4937.
26. Y. Shen, D. F. Kennedy, T. L. Greaves, A. Weerawardena, R. J. Mulder, N. Kirby, G. Song and C. J. Drummond, Phys. Chem. Chem. Phys., 2012, 14, 7981–7992.
27. M. Martinez, C. Iojoiu, P. Judeinstein, L. Cointeaux, J.-C. Lepretre and J.-Y. Sanchez, ECS Trans., 2009, 25, 1647–1657.
28. T. L. Greaves, A. Weerawardena, I. Krodkiewska and C. J. Drummond, J. Phys. Chem. B, 2008, 112, 896–905.
29. T. L. Cottrell and J. E. Gill, J. Chem. Soc., 1951, 1798–1800.
30. Z. Ullah, M. A. Bustam, Z. Man, N. Muhammad and A. Sada Khan, RSC Adv., 2015, 5, 71449–71461.
31. J. G. Huddleston, A. E. Visser, W. Reichert, H. D. Willauer, G. A. Broker and R. D. Rogers, Green Chem., 2001, 3, 156–164.
32. S. Aparicio, M. Atilhan and F. Karadas, Ind. Eng. Chem. Res., 2010, 49, 9580–9595.
33. M. S. Miran, T. Yasuda, M. A. B. H. Susan, K. Dokko and M. Watanabe, ECS Trans., 2012, 50, 285–291.
34. P. K. Chhotaray and R. L. Gardas, J. Chem. Thermodyn., 2014, 72, 117–124.
35. M. Horikawa, N. Akai, A. Kawai and K. Shibuya, J. Phys. Chem. A, 2014, 118, 3280–3287.
36. H. Vogel, Phys. Z., 1921, 22, 645–646.
37. G. Tammann and W. Hesse, Z. Anorg. Allg. Chem., 1926, 156, 245–257.
38. P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2003.
39. M. Anouti, M. Caillon-Caravanier, Y. Dridi, H. Galiano and D. Lemordant, J. Phys. Chem. B, 2008, 112, 13335–13343.
40. J. Zhang and A. M. Bond, Analyst, 2005, 130, 1132–1147.
41. A. S. Best, A. I. Bhatt and A. F. Hollenkamp, J. Electrochem. Soc., 2010, 157, A903–A911.
42. V. Lockett, R. Sedev and J. Ralston, J. Phys. Chem. C, 2008, 112, 7486–7495.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR graphs of triethylammonium trifluoroacetate, dihydrogen phosphite, and tosylate; ATR spectra of both PILs synthesized and initial compounds. See DOI: 10.1039/c6ra21360j

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