Key factor governing the physicochemical properties and extent of proton transfer in protic ionic liquids: ΔpK_a or chemical structure?

Abstract

In order to identify the key factor governing the transport properties and extent of proton transfer in protic ionic liquids (PILs), a series of PILs were prepared by simple neutralisation of a super-strong acid, bis(trifluoromethanesulfonyl)amide acid (H[NTf₂]), with a range of amines comprising diverse structures, including secondary and tertiary amines. The bulk physicochemical properties of the resulting PILs with a wide variation in the ΔpK_a values of the constituent acid and protonated bases were compared to those of previously reported PILs derived from a super-strong base, 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), and different acids (M. S. Miran, H. Kinoshita, T. Yasuda, M. A. B. H. Susan and M. Watanabe, Phys. Chem. Chem. Phys., 2012, 14, 5178–5186) with emphasis on the thermal stability, vibrational spectroscopy, density, viscosity, conductivity, and ionicity characteristics. The thermal stability of all PILs, as analysed by thermogravimetric analysis (TGA), was found to linearly increase with the ΔpK_a values. However, isothermal TGA measurements for an extended duration indicated that [NTf₂]-based PILs exhibit higher thermal stability than [DBU]-based PILs at low ΔpK_a values (<15). PILs with secondary amines showed higher viscosity and lower ionic conductivity than those with tertiary amines because of the formation of multiple hydrogen bonds. The evaluated ionicity suggested that all [NTf₂]-based PILs are “good PILs”, irrespective of the basicity of the amines, and have higher ionicity than [DBU]-based PILs particularly at low ΔpK_a values (<15).

---

Introduction

Protic ionic liquids (PILs) are an important subclass of ionic liquids (ILs) that, by suitable molecular design, can be endowed with unique physicochemical properties similar to those of their aprotic counterparts, such as high ionic conductivity, low-vapour pressure, high thermal stability, and low flammability.^1–3 PILs have shown competence in versatile applications, such as media for sustainable cotton dyeing,⁴ natural product extraction,⁵ biomass processing,⁶ protein⁷ and virus stabilisation,⁸ biocatalysis,⁹ catalysis,¹⁰ CO₂ capture,¹¹ nanostructure formation,¹² pharmaceutical applications,¹³ fuel processing,¹⁴ and as capacitor electrolytes.¹⁵ However, the most relevant example is their possible use as fuel cell electrolytes^16–21 due to the presence of labile protons in their cationic structure. However, correlation of the physicochemical properties of PILs with suitable parameters underpinning their key properties remains a challenge. Thus, systematic studies on said physicochemical properties are an emerging need in current research.

Recent developments in PIL research have revealed that the most significant parameter to control the physicochemical properties of PILs is the ΔpK_a value resulting from the pK_a values of the constituent acid and protonated base in water.^22–24 Using pK_a data, a proton energy diagram can be constructed by analysis of the thermodynamics of proton transfer.²⁵ However, discrepancies exist on the conditions determining the extent of proton transfer in terms of the ΔpK_a value.^22,23,26 It has been reported that ΔpK_a > 10 is a requisite to ensure satisfactory proton transfer to produce highly ionised PILs²³ and that even ΔpK_a ≤ 4 is enough for complete proton transfer using primary amines.²⁶ Recently, we have determined the ΔpK_a threshold to obtain desirable physicochemical properties for a series of PILs based on an organic super-strong base, 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), with various Brønsted acids.²² For instance, PILs with ΔpK_a > 15 display high thermal stability and ionicity, comparable to those of typical aprotic ionic liquids (AILs). Recently, it has been shown that the inclusion of a hydroxyl functional group on a PIL cation significantly increases its ionic nature as compared to that of a di-amino or a non-functionalised cation despite having the same ΔpK_a value.²⁷ Primary and secondary ammonium carboxylate-based PILs exhibit significantly high apparent ionicity even at low ΔpK_a values.²⁶ We observed remarkable disparity in the ionic nature of two PILs based on water with a super-strong acid and a super-strong base.²⁸ High proton dissociation was also found when the PIL was used as a solvent.²⁹ It has also been shown that the dissociation constants of Brønsted acids in water and in the PIL, ethylammonium nitrate, barely vary.²⁹ Thus, determining the dominant factors governing the proton transfer in PILs based on various acids and bases is of great interest and importance, since the establishment of an equilibrium between ionic and non-ionic species is a critical issue for PILs. Particularly, the presence of free acids and bases is not desirable for applications under open atmosphere and elevated temperatures.^30,31 For instance, neutral species seriously affect the electrochemical activity of fuel cell electrolytes owing to their corrosive nature.^31–33 Our previous study was simply based on PILs obtained from an organic super-strong base with various acids (8 < ΔpK_a < 24).²² Thus, using a similar ΔpK_a range, a systematic study with super-strong acid-based PILs is quintessential to answer critical questions on whether the ΔpK_a value or the proton accepting/donating ability of the base/acid in water governs the thermal, transport, and ionic properties of PILs, which has not been clarified to date.

In this study, we prepared a novel series of PILs by neutralisation of a super-strong acid, bis(trifluoromethanesulfonyl)amide acid (H[NTf₂]), with different amines as the base (8 < ΔpK_a < 24),^34–41 including tertiary amines DBU, pyrazine (Pyra), pyridine (Pyri), 1-ethylimidazole (EIm), diethylmethylamine (dema), 5-methylisoxazole (MIox); and secondary amines pyrrolidine (Pyrr), morpholine (Morp), and 1,2,4-triazole (TZL); and investigated the factors governing the properties of PILs (Fig. 1) by comparison with those previously obtained for [DBU]-based PILs with different acids (8 < ΔpK_a < 24).²² The primary focus was on the thermal and transport properties, ionicity, and extent of the proton transfer for the PILs based on this super-strong acid and different bases so as to determine whether the ΔpK_a value or the structure of the acid/bases dominates the key characteristics of these promising materials.

Fig. 1 Chemical structure of the acid and bases used in this work. The values in parentheses indicate the pK_a values in aqueous medium.^34–41

Experimental

Reagents

For this study, DBU (98%), Pyrr (99%), EIm (98%), dema (98%), Morp (99%), Pyri (99%), TZL (98%), Pyra (98%), and MIox (95%) were purchased from Tokyo Chemical Industries. In addition, (CF₃SO₂)₂NH (99%) was obtained from Kanto Chemical. All chemicals were utilised as received without further purification.

Preparation of PILs

The PILs were prepared through simple, solvent-less neutralisation reactions at stoichiometric ratios of the acid and base in accordance with a previous report.²² The synthesised PILs, as shown in Table 1, were kept and handled in an inert atmosphere glovebox (VAC, [O₂] < 1 ppm, [H₂O] < 1 ppm). The moisture content of the PILs was below 100 ppm, as determined via Karl-Fischer titration.

Table 1 List of the ΔpK_a values of the PILs synthesised in this work^34–41

PILs	ΔpKa
[MIox][NTf2]	8.0
[Pyra][NTf2]	10.4
[TZL][NTf2]	12.3
[Pyri][NTf2]	15.2
[Morp][NTf2]	18.4
[EIm][NTf2]	18.5
[dema][NTf2]	20.5
[Pyrr][NTf2]	21.3
[DBU][NTf2]	23.4

---

Measurement of bulk properties

The bulk physicochemical properties, such as the density, viscosity, ionic conductivity, as well as the thermal properties and vibrational spectra, of the prepared PILs were evaluated in this study as described below. The corresponding data for [DBU]-based PILs were taken from our previous report.²²

The thermal properties, such as melting, crystallisation, and glass transition temperatures, were determined using a Seiko Instruments differential scanning calorimeter (DSC 6220C) with tightly sealed aluminium pans handled under Ar atmosphere. The samples were then heated to 150 °C, after which they were cooled to −150 °C and then heated again to 150 °C at heating and cooling rates of 10 °C min⁻¹. The DSC data were recorded during the reheating scans. For thermogravimetric analysis (TGA), a Seiko Instruments thermogravimetry-differential thermal analyser (TG-DTA 6200) was operated from 30 to 550 °C at a heating rate of 10 °C min⁻¹ under N₂ atmosphere in open aluminium pans. The temperature at which the mass loss began was considered the decomposition temperature (T_d). Isothermal TGA was conducted at 130 °C for 2 h under N₂ atmosphere in order to compare with our previous data on DBU-based PILs.²² Certain electrochemical devices (i.e. fuel cells) require electrolytes with extended thermal stability, hence ITG experiments for PILs were recorded at 130 °C.²

Fourier transform infrared (FT-IR) spectra were recorded on an Avatar 360 FT-IR spectrometer in the 1000–4000 cm⁻¹ range using CaF₂ cells and sampling inside an inert atmosphere glovebox. For solid samples at 25 °C, an aliquot of the solvent (CD₃Cl) was added to prepare dispersions. After placing a small drop of the sample on the surface of a CaF₂ cell, it was left at room temperature for evaporation of the solvent.

Ab initio molecular orbital calculations were performed with the Gaussian 09 program²⁰ using its implemented basis sets. The interaction energies between the cations and anion (E_int) were calculated at the MP2/6-311G** level for geometry optimisation of the ion pair at the HF/6-311G** level.^21–23 The basis set superposition error (BSSE)²⁴ was corrected by the counterpoise method.²⁵ The stabilisation energy derived from the formation of a complex from isolated species (E_form) was calculated as the sum of the interaction energy (E_int) and the deformation energy (E_def), where E_def is the sum of the increases in the energies of the monomers due to the deformation associated with the formation of complexes.²⁶ The E_def values were calculated at the MP2/6-311G** level.

A rheometer (Physica MCR 301, Anton Paar) was used for viscosity estimation in the range of 30–150 °C under dry air conditions. A steady pre-shear was applied at a shear rate of 1 s⁻¹ for 60 s, followed by a 120 s rest period before each measurement to eliminate any previous shear history.

The ionic conductivity was measured using a locally designed dip cell probe consisting of two platinum wires sheathed in glass. The cell constant was determined with a solution of 0.1 M KCl at 25 °C. The complex impedance spectra in the frequency range of 10 MHz to 0.01 Hz were used for the determination of conductivities on a potentiostat (Autolab, PGSTAT30). The conductance was estimated from the real axis intercept in the Nyquist plots. The temperature was controlled at 20 °C intervals in the range of 30–150 °C via thermal equilibration at each temperature for at least 1 h using a constant temperature oven (DKN 611).

A JEOL ECX400 nuclear magnetic resonance (NMR) spectrometer with a 9.4 T narrow bore superconducting magnet equipped with a JEOL pulse field gradient probe and current amplifier were used to perform pulsed-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) measurements to study the self-diffusion coefficients of each component in the [NTf₂]-based PILs (¹H, 399.7 MHz) and fluorinated anion (¹⁹F, 376.1 MHz). The detailed experimental procedure for the PGSE-NMR analysis has been previously reported.^22,34

Density measurements were performed using a thermo-regulated density/specific gravity meter DA-100 (Kyoto Electronics Manufacturing Co. Ltd) in the range of 15 to 40 °C. The density of PILs exhibiting high melting points was measured with a pycnometer.

Results and discussion

Thermal properties

Thermo-analytical techniques such as TGA and DSC were employed to investigate the thermal properties of the PILs. From the TGA, the thermal stability was assessed under both non-isothermal and isothermal conditions. First, the decomposition temperature, T_d, was determined from the onset of the weight loss in the TGA curves, as presented in Fig. S1 (ESI†). The T_d value appears to linearly increase at first with the increasing ΔpK_a value (Fig. 2a). However, the T_d values for the [NTf₂]-based PILs follow the order: [DBU]⁺ > [EIm]⁺ > [Pyrr]⁺ > [dema]⁺ > [Morp]⁺ > [TZL]⁺ ≈ [Pyri]⁺ > [Pyra]⁺ > [MIox]⁺, showing good correlation with the N–H stretching frequencies (vide infra) and coinciding with our previous results for a series of DBU-based PILs.²² Similar findings were obtained for another series of PILs through correlation of the boiling point with the ΔpK_a values.²³ However, non-isothermal TGA is a technique in which the temperature is ramped at a fast rate, thus affording an overestimation of the thermal stability of PILs.⁴² Therefore, harsher isothermal TGA were conducted to distinguish differences in the thermal stability of the [NTf₂] and [DBU]-derived PILs. Fig. 2b shows the correlation between the weight losses during isothermal TG measurements at 130 °C for 2 h with the ΔpK_a values. The [NTf₂]-based PILs exhibit no weight losses at ΔpK_a > 15, similarly to the case of [DBU]-based PILs. Surprisingly, [Pyra][NTf₂] and [TZL][NTf₂] also showed high weight retention even though their ΔpK_a values fall within 15 > ΔpK_a > 10. Note that, in a previous study,⁴³ high intermolecular energy densities were also observed in [TZL]-based AILs. Apparently, these TG measurements indicate that PILs with high T_d are those with low weight loss in the isothermal measurement. However, difference in weight loss in the low ΔpK_a regime becomes more apparent by the isothermal measurement, as seen Fig. 2b. One can see the chemical structure effect for [Pyra][NTf₂] and [TZL][NTf₂] against the [DBU][CH₃CO₂] and [DBU][CF₃CO₂], respectively, despite having similar ΔpK_a.

Fig. 2 (a) Variation of the onset-decomposition temperature and (b) weight loss by isothermal TGA at 130 °C for 2 h as a function of ΔpK_a for both [NTf₂] and [DBU]-based PILs. The lines are drawn as a guide to the eye.

Then, DSC measurements were performed to gain insight into the phase transition behaviour of the [NTf₂]-based PILs. The DSC thermograms under heating exhibited glass transition temperatures (T_g), cold crystallisation temperatures (T_cc) above the T_g, and subsequent melting of the crystallised samples at the melting temperature (T_m) (Fig. 3). Cold crystallisation peaks were mainly observed for [TZL][NTf₂] and [Morp][NTf₂] during the heating step, indicating that these PILs have relatively slow crystallisation kinetics under the same cooling conditions. Thermal heating provided the necessary energy to exert the conformational changes required for crystallisation, indicating strong intermolecular interactions^43,44 in the above PILs. These results support the earlier observations made by Nikolakis⁴⁵ and Krishnan et al.⁴⁶ for [TfO] and [NTf₂]-based PILs, respectively. The high glass transition temperatures also suggest the presence of high cohesive energies in these PILs.⁴⁶ The endothermic peaks corresponding to the melting points, T_m, were found at temperatures below 100 °C for all the samples. Note that [Pyra][NTf₂] (ΔpK_a = 10.4) exhibits the highest T_m value (70 °C), while [EIm][NTf₂] (ΔpK_a = 18.5) shows the lowest T_m (12 °C). It is well-known that H-bonding is prevalent in PILs compared to conventional AILs. In primary alkylamine-derived PILs with [NTf₂]⁻, dominance of hydrogen bonding contributed to the higher melting points of the PILs in contrast with their secondary and tertiary counterparts.⁴⁷ But, ion-pairing is not dominated by the cations only. For example, in a recent report on AILs, weaker effect of ion-pairing was clearly demonstrated for [EMIM][NTf₂], in contrast with [EMIM][OAc], where [OAc]⁻ stands for acetate anion by using glucose as the disruptive agents.⁴⁸ So, the depolarized anionic charge in [NTf₂]⁻ in comparison with [OAc]⁻, reduces the strength of ion-pairing owing to the weaker hydrogen bonding network.⁴⁸ So, the extent of H-bonding can strongly influence the ion-pair binding energy (IPBE) for PILs depending on the structure of the cations and anions, which in turn affects their melting points as evidenced in the phase behaviour of the [NTf₂]-based PILs.

Fig. 3 DSC thermograms of [NTf₂]-based PILs. T_m, T_cc, and T_g correspond to the melting, cold crystallisation, and glass-transition temperatures, respectively.

FT-IR spectra

Inter and intra-ionic interactions are easily identified by IR (vibrational) spectroscopy as information on the specific cation–anion bonding interactions in ILs. Fig. 4 displays the FT-IR spectra for [NTf₂]-based PILs at ambient temperature. The N–H bands were assigned in the spectra according to a previous report.²² N–H stretching was observed at 3154–3354 cm⁻¹ for the [NTf₂]-based PILs depending on the cationic structure. A closer look at the FT-IR spectra revealed that the N–H stretching frequency follows the order: [DBU]⁺ > [EIm]⁺ > [Pyrr]⁺ > [dema]⁺ > [TZL]⁺, in agreement with the order of the thermal decomposition temperature shown in Fig. 2a. However, the trend considering only the ΔpK_a values^22,41 could be expected to follow the order: [DBU]⁺ > [Pyrr]⁺ > [dema]⁺ > [EIm]⁺ > [TZL]⁺. Stronger H-bonding interactions, indicated by lower N–H stretching frequencies, result in easy dissociation into the constituent acid and base, which in turn leads to lower thermal stability.

Fig. 4 FT-IR spectra of [NTf₂]-based PILs at room temperature.

Here, it is important to consider the special “anionic-structure effect” of the [NTf₂]⁻ in terms of H-bonding interactions. Triflate anion, [TfO]⁻, is a suitable candidate to compare with [NTf₂]⁻ as they are derived from super strong monoprotic acids. Hence, both are quite distinct in terms of H-bond basicity and conformational flexibility of their structures. [NTf₂]⁻ is presumably a weaker proton acceptor than [TfO]⁻, as the negative charge in [NTf₂]⁻ is more distributed than in [TfO]⁻. However, the conformational variability⁴⁵ of [NTf₂]⁻ and multiple H-bond acceptor sites (N and O atoms) affords a configurational entropic contribution and thus, H-bonding may also be established in [NTf₂]-based PILs, as previously reported by Umebayashi and co-workers.⁴⁹ For example, ab initio calculations provided twice the number of stable geometries (local minima) for [DBU][NTf₂] with very similar and smaller ion-pair stabilisation energies (E_form) than for [DBU][TfO], indicating the variable spatial accessibility of [NTf₂]⁻ in PILs (Fig. S2, ESI†).

Moreover, the FT-IR results were further supported by ab initio molecular orbital calculations, demonstrating the decreasing N–H bond length in the order: [TZL]⁺ > [Pyrr]⁺ > [dema]⁺ > [EIm]⁺ > [DBU]⁺ (Fig. 5a). The E_form parameter follows the same trend, as shown in Fig. 5b–d. Once we switched from [NTf₂]-based PILs to [DBU]-based PILs, the N–H bond length and E_form were exclusively dictated by the ΔpK_a values (Fig. 5a–d). Thus, the extra stabilisation energy gain seems to stem from the unique structural features of the cations in [NTf₂]-based PILs. Multiple strong H-bonding is possible in [Pyrr][NTf₂] as the cation derives from a secondary amine. It is already well-known that ion-pair interactions are intrinsically stronger in PILs than AILs because of the presence of H-bonding,⁵⁰ which further increases from tertiary amine to primary amine-based PILs.⁴⁸ Next, the similarity in the stabilisation energy values (Fig. 5b) for [Pyrr][NTf₂] and [TZL][NTf₂] could be correlated with their similar H-bond density despite the large difference in their ΔpK_a values. Finally, [EIm][NTf₂] also exhibits rather weaker H-bonding compared to [Pyrr][NTf₂] and [dema][NTf₂], as displayed in Fig. 4. In [EIm][NTf₂], the probable protonated site (N-1) presents delocalisation of the positive-charge density owing to the sp²-configuration of N-1 in [EIm]⁺, whereas it is localised in [Pyr]⁺ and [dema]⁺.

Fig. 5 (a and b) Variation in the N–H bond length and stabilisation energy, and (c and d) most stable geometries for [DBU] and [NTf₂]-based PILs as a function of ΔpK_a.

Thus, the fluctuation frequency of N–H bond becomes higher, and blue-shifting (toward higher wavenumbers) is observed together with band broadening.^51,52 Moreover, a recent study by Nikolakis and colleagues⁴⁵ showed that amongst various imidazole (Im)-derived PILs, [HC_nIm][NTf₂] (where n represents the length of the alkyl chain at N-3) parallel π–π stacking facilitates H-bonding, which presents the lowest probability in the case of [HC₂Im][NTf₂]. Thus, it can be envisaged that, in our case, the density and strength of H-bonding in [EIm][NTf₂] (where n = 2) are considerably lower and weaker than those in [Pyrr][NTf₂] and [dema][NTf₂].

Viscosity

Fig. 6 displays the temperature dependence of the viscosity (η) for the [NTf₂]-based PILs after fitting with the Vogel–Tamman–Fulcher (VTF) equation^53–55 (see, ESI† for details). The best fit parameters are listed in Table S1 (ESI†).

Fig. 6 Viscosity as a function of the temperature for [NTf₂]-based PILs.

From Fig. 6, the viscosity values at 130 °C follow the order based on the cations: [TZL]⁺ > [Morp]⁺ > [Pyra]⁺ > [DBU]⁺ > [Pyri]⁺ > [EIm]⁺ ∼ [Pyrr]⁺ > [dema]⁺. Ignatiev et al.⁵⁶ reported that other than the influence of electronic, inter, and intra-molecular forces, the size of the ions, i.e. their mass, also dictates the viscosity of ILs. The high viscosities of [Morp][NTf₂] and [TZL][NTf₂] are indicative of strong H-bonding, which has also been observed in [Morp][F] (F = formate anion) and [TZL]-based AILs.⁵⁷

Conductivity

The temperature dependence of the ionic conductivity (σ) for [NTf₂]-based PILs is shown in Fig. 7 after fitting with the VTF equation^53–55 (see, ESI† for details and Table S2).

Fig. 7 Ionic conductivity as a function of ΔpK_a for [NTf₂]-based PILs.

All PILs exhibit very high ionic conductivities comparable to those of other previously reported highly conductive PILs.⁵⁹ Especially, [dema][NTf₂] exhibits the highest ionic conductivity at every temperature of the studied range. At 130 °C, the ionic conductivity values follow the order: [dema]⁺ > [Pyrr]⁺ ∼ [EIm]⁺ > [Pyri]⁺ > [Pyra]⁺ > [DBU]⁺ > [TZL]⁺ > [Morp]⁺, which is almost the opposite of the viscosity order (vide supra). From the viscosity data analysis (Fig. 6), [dema][NTf₂] was found to be the most fluidic PIL and thus the most conductive (Fig. 7). Conversely, ions in H-bonding-dominated fluids are less mobile, as observed by the low ionic conductivity values of [TZL][NTf₂] and [Morp][NTf₂]. Therefore, the E_a of the ionic conductivity is quite high (i.e. a large barrier) in these PILs.

Ionicity and extent of proton transfer

We now turn our focus onto the ionicity and extent of proton transfer, which is a challenging aspect of PILs. AILs entirely consist of ions whereas PILs exhibit dynamic acid–base equilibria. Therefore, the meaning of ionicity in PILs is rather more complex than that in AILs. As such, it is essential to differentiate PILs from acid–base mixtures to understand better the ionic nature of PILs, which may be governed by the extent of proton transfer, i.e. ΔpK_a.

Among many approaches,^22,60–64 Walden plots⁶⁵ are the easiest way to roughly distinguish qualitatively between good/poor ILs and non-ionic liquids, as proposed by Angell et al.⁶⁰Fig. 8 presents the Walden plots of the [NTf₂]-based PILs, whereby all the [NTf₂]-based PILs in this study can be considered “good PILs”. In general, the data points against the ideal line show significant negative deviation depending on the ΔpK_a values of the PILs. We have also estimated quantitatively the ionicity as the ratio of the molar conductivities measured by both impedance and PGSE-NMR analyses, (Λ_imp/Λ_NMR),⁶¹ which can be effectively applied for all types of ILs such as AILs,^61,62 solvate ILs,⁶⁶ and PILs.^22,26,48Λ_imp was calculated from the equation:

where σ is the ionic conductivity and M_PIL is the molar concentration of the PIL. The values of σ were obtained from Fig. 7, and M_PIL was calculated using the measured density data (Fig. S3 and Table S3, ESI†) and molar mass of the PILs. Then, Λ_imp is shown as a function of temperature (Fig. S4, ESI†), and the VTF fitting parameters are given (Table S4, ESI†).^53–55Λ_NMR was calculated using the Nernst–Einstein equation, as shown below:where F and R are the Faraday and gas constants, respectively, T is the absolute temperature (K), and D corresponds to the diffusion coefficients of the cation (D_cation) and anion (D_anion), which were experimentally determined by PGSE-NMR measurements. Thus, we present here the ionicity of the current [NTf₂]-based PILs using Walden plots (Fig. 9a) and PGSE-NMR data (Fig. 9b).

Fig. 8 Walden plots for [NTf₂]-based PILs. The dashed straight line corresponds to a 1 M KCl aqueous solution.

Fig. 9 ΔpK_a dependency of the ionicity for [DBU] and [NTf₂]-based PILs at 50 °C. The ionicity was estimated from (a) Walden plots and (b) PGSE-NMR data. The lines are drawn as a guide to the eye.

The ionicity values are less than unity for all the PILs considered herein. The Walden plot analysis provided more scattered ionicity values than the PGSE-NMR approach. In Coulombic fluids, the ionicity is mainly governed by Coulombic interactions. Yet, as reported in numerous studies,^34,60–63 the subtle balance of Coulombic and dispersion forces and H-bonding ultimately determines the ionicity of AILs. Moreover, we have reported that the directionality of interaction in the ion pairs of PILs is generally larger than that of AILs from ab initio MO calculations.^50,67 In the case of AILs, the ionicity tends to decrease with the increasing interaction energy of the ion pairs as well as with the increasing directionality of said interactions.⁶⁸ As shown in these figures, the ionicity of the [DBU]-based PILs increased up to ΔpK_a = 15 and then remained constant. Proton transfer is incomplete at ΔpK_a < 15 for [DBU]-based PILs. The maximum ionicity value (∼0.6) for the [DBU]-based PILs (Fig. 9) is lower than that for high-ionicity AILs.^69,70 Therefore, we concluded that stronger H-bonding in [DBU]-based PILs results in weaker ionic nature of the PILs as compared to that in AILs.²²

On the other hand, the ionicity of [NTf₂]-based PILs appeared to be higher at low ΔpK_a values and then became independent of the ΔpK_a values. Interestingly, [Pyra][NTf₂] and [TZL][NTf₂] (10 < ΔpK_a < 15) exhibited higher ionicity than [DBU]-based PILs with similar ΔpK_a values ([DBU][CH₃CO₂] and [DBU][CF₃CO₂]). Furthermore, it is fascinating that the ionicity of [Pyra][NTf₂] (ΔpK_a = 10.4) is comparable to that of [dema][NTf₂] (ΔpK_a = 20.5). Note that [NTf₂]-based PILs with primary alkyl ammonium cations exhibit lower ionicity owing to the dominance of H-bonding.⁴⁸ Therefore, by suitable choice of the cation, highly dissociative PILs at low ΔpK_a values can be obtained by tuning the balance between Coulombic and intermolecular interactions. In the case of [TZL][NTf₂], [TZL]⁺ possibly allows the rapid exchange of protons between the N-1 and N-3 sites,⁵⁷ a phenomenon also observed in guanidine-derived PILs,^71,72 which may weaken the directionality of the interactions. Also, in the case of [Pyra][NTf₂], [Pyra]⁺ is not only an H-bond donor but also a weak H-bond acceptor;⁵⁸ thus, the PIL displays a more dissociated nature. Such characteristics of [NTf₂]-based PILs of their ionicity would originate from the multiple proton donating and accepting sites within the structure of the certain cations and [NTf₂]⁻ anion, respectively.

While the ΔpK_a value is considered the key parameter determining the degree of proton transfer in PILs,²² the individual role of the proton donors and acceptors has not been considered in detail before.^22–24 Complete proton transfer from a Brønsted acid to a base is assumed for PILs with high ΔpK_a values owing to fast kinetics.⁷³ However, in Brønsted acid–base mixtures, neutral acids and bases do exist at low ΔpK_a values and, in such case, the proton transfer is incomplete. Such systems are not regarded as true PILs but pseudo-PILs.⁷⁴

Furthermore, the kinetics of the proton transfer process is also affected by the thermodynamics of a given system, which in turn depends on several factors such as the exothermic nature of the reaction, electron delocalisation in the base, and intra-molecular H-bonding.⁷⁵ Even at the same ΔpK_a values, the structural flexibility of [NTf₂]⁻ may also accelerate the proton transfer for [NTf₂]-based PILs, in contrast to the less flexible carboxylate anions such as [CH₃CO₂]⁻ and [CF₃CO₂]⁻ for [DBU]-based PILs. Therefore, the extent of proton transfer, which corresponds to the ionicity at low ΔpK_a values, depends not only on ΔpK_a values but also on the chemical structure of the acids and bases. At high ΔpK_a values, the degree of proton transfer is very high owing to the sufficient proton donating and accepting properties of the reactant acid and base, respectively. However, at relatively low ΔpK_a values, the effect of the chemical structure of bases/acids becomes significant. Quantitative analysis via potential energy surfaces⁷⁵ would provide further insight to clarify this phenomenon, but this type of analysis was beyond the scope of this manuscript.

Conclusions

In this study, we reported the physicochemical properties of PILs based on a super-strong acid, H[NTf₂], with a wide variety of amines in term of structures and basicities. In addition, we compared the properties of the prepared PILs with those of previously reported [DBU]-based PILs derived from a super-strong base and different acids. The thermal stability of PILs depended almost linearly with the ΔpK_a values of the reactant acid–base pair. However, under harsher conditions, even at low ΔpK_a values, [NTf₂]-based PILs exhibited superior thermal stability. According to the Walden plots, the [NTf₂]-based PILs are “good PILs”. Moreover, [NTf₂]-based PILs with low ΔpK_a values exhibited higher ionicity than [DBU]-based PILs. Such characteristics of [NTf₂]-based PILs in terms of their thermal stability and ionicity seem to originate from the multiple proton donating and accepting sites within the structure of the cations and [NTf₂]⁻ anion, respectively, especially in low ΔpK_a PILs. As a result, the threshold value of ΔpK_a for obtaining thermally stable and high ionicity PILs can be lowered from 15 to 10 for [NTf₂]-based PILs. To the best of our knowledge, this is the first demonstration about the effect of the chemical structure on the physicochemical features and extent of proton transfer in PILs with a systematic study via suitable selection of acids and bases.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by Grants-in-Aid for Scientific Research (S/15H05758) and Grants-in-Aid for Specially Promoted Research on “Iontronics” from the Japan Society for the Promotion of Science.

Notes and references

1. P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2003.
2. T. L. Greaves and C. J. Drummond, Chem. Rev., 2008, 108, 206–237.
3. P. Walden, Bull. Acad. Imp. Sci., 1914, 8, 405–422.
4. R. S. Andrade, D. Torres, F. R. Andrade, D. Torres, F. R. Ribeiro, B. G. C. Andreo, J. A. Oshiro Jr and M. Iglesias, ACS Sustainable Chem. Eng., 2017, 5, 8756–8765.
5. M. W. Sanders, L. Wright, L. Tate, G. Fairless, L. Crowhurst, N. C. Bruce, A. J. Walker, G. A. Hembury and S. Shimizu, J. Phys. Chem. A, 2009, 113, 10143–10145.
6. C. Chiappe, A. Mezzetta, C. S. Pomelli, B. Masciocchi, A. Gentile and G. Iaquaniello, Green Chem., 2016, 18, 4982–4989.
7. N. Byrne, L. M. Wang, J.-P. Belieres and C. A. Angell, Chem. Commun., 2007, 2714–2716.
8. N. Byrne, B. Rodoni, F. Constable, S. Varghes and J. H. Davis Jr., Phys. Chem. Chem. Phys., 2012, 14, 10119–10121.
9. R. A. Sheldon, Chem. – Eur. J., 2016, 22, 12984–12999.
10. Q. Zhang, H.-Y. Yuan, N. Fukaya, H. Yasuda and J.-C. Choi, Green Chem., 2017, 19, 5614–5624.
11. K. A. Mumford, S. J. Pas, T. Linseisen, T. M. Statham, N. J. Nicholas, A. Lee, K. Kezia, R. Vijayraghavan, D. R. Macfarlane and G. W. Stevens, Int. J. Greenhouse Gas Control, 2015, 32, 129–134.
12. T. L. Greaves, S. T. Mudie and C. J. Drummond, Phys. Chem. Chem. Phys., 2011, 13, 20441–20452.
13. J. Stoimenovski, P. M. Dean, E. I. Izgorodina and D. R. MacFarlane, Faraday Discuss., 2012, 154, 335–352.
14. Z. Li, J. Xu, D. Li and C. Li, RSC Adv., 2015, 5, 15892–15897.
15. P. L. Mayrand, S. Lin, D. Lazzerini and D. Rochefort, J. Power Sources, 2010, 195, 5114–5121.
16. S.-Y. Lee, A. Ogawa, M. Kanno, H. Nakamoto, T. Yasuda and M. Watanabe, J. Am. Chem. Soc., 2010, 132, 9767–9773.
17. J. Snyder, T. Fujita, M. W. Chen and J. Erlebacher, Nat. Mater., 2010, 9, 904–907.
18. M. Watanabe, M. L. Thomas, S. Zhang, K. Ueno, T. Yasuda and K. Dokko, Chem. Rev., 2017, 117, 7190–7239.
19. A. Noda, M. A. B. H. Susan, S. Mitsushima, K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2003, 107, 4024–4033.
20. D. R. Macfarlane, N. Tachikawa, M. Forsyth, J. M. Pringle, P. C. Howlett, G. D. Elliott, J. H. Davis Jr., M. Watanabe, P. Simon and C. A. Angell, Energy Environ. Sci., 2014, 7, 232–250.
21. A. Khan, C. A. Gunawan and C. Zhao, ACS Sustainable Chem. Eng., 2017, 5, 3698–3715.
22. M. S. Miran, H. Kinoshita, T. Yasuda, M. A. B. H. Susan and M. Watanabe, Phys. Chem. Chem. Phys., 2012, 14, 5178–5186.
23. M. Yoshizawa, W. Xu and C. A. Angell, J. Am. Chem. Soc., 2003, 125, 15411–15419.
24. H. Luo, G. A. Baker, J. S. Lee, R. M. Pagni and S. Dai, J. Phys. Chem. B, 2009, 113, 4181–4183.
25. J.-P. Belieres and C. A. Angell, J. Phys. Chem. B, 2007, 111, 4926–4937.
26. J. Stoimenovski, E. I. Izgorodina and D. R. MacFarlane, Phys. Chem. Chem. Phys., 2010, 12, 10341–10347.
27. E. S. J. Reid, C. E. S. Bernardes, F. Martins, S. Shimizu, M. E. M. Piedade and A. J. Walker, Phys. Chem. Chem. Phys., 2017, 19, 28133–28138.
28. M. S. Miran, T. Yasuda, R. Tatara, M. A. B. H. Susan and M. Watanabe, Faraday Discuss., 2018, 206, 353–364.
29. J. R. Kanzaki, H. Kodamatani, T. Tomiyasu, H. Watanabe and Y. Umebayashi, Angew. Chem., Int. Ed., 2016, 55, 6266–6269.
30. S. E. Goodwin, D. E. Smith, J. S. Gibson, R. G. Jones and D. A. Walsh, Langmuir, 2017, 33, 8436–8446.
31. C. A. Angell, Y. Ansari and Z. Zhao, Faraday Discuss., 2012, 154, 9–27.
32. M. S. Miran, T. Yasuda, M. A. B. H. Susan, K. Dokko and M. Watanabe, RSC Adv., 2013, 3, 4141–4144.
33. M. S. Miran, T. Yasuda, M. A. B. H. Susan, K. Dokko and M. Watanabe, J. Phys. Chem. C, 2014, 118, 27631–27639.
34. M. Hasani, J. L. Yarger and C. A. Angell, Chem. – Eur. J., 2016, 22, 13312–13319.
35. I. Hermecz, Adv. Heterocycl. Chem., 1987, 42, 83–202.
36. E. A. Braude, F. C. Nachod and W. D. Phillips, Determination of organic structures by physical methods, Academic Press, New York, 1971.
37. R. Linnell, J. Org. Chem., 1960, 25, 290.
38. B. Lenarcik, B. Barszcz and J. Kulig, Pol. J. Chem., 1979, 53, 963.
39. K. Hall Jr., J. Am. Chem. Soc., 1957, 79, 5441–5444.
40. S. Li, Z. Zhou, Y. Zhang and M. Liu, Chem. Mater., 2005, 17, 5884–5886.
41. Relative strength of acids and bases were maintained according to the results of FT-IR spectra of pure PILs.
42. P. Navarro, M. Larriba, E. Rojo, J. García and F. Rodríguez, J. Chem. Eng. Data, 2013, 58, 2187–2193.
43. C. Cadena and E. J. Maginn, J. Phys. Chem. B, 2006, 110, 18026–18039.
44. L. V. N. R. Ganapatibhotla, J. Zheng, D. Roy and S. Krishnan, Chem. Mater., 2010, 22, 6347–6360.
45. A. M. Moschovi, V. Dracopoulos and V. Nikolakis, J. Phys. Chem. B, 2014, 118, 8673–8683.
46. J. L. Lebga-Nebane, S. E. Rock, J. Franclemont, D. Roy and S. Krishnan, Ind. Eng. Chem. Res., 2012, 51, 14084–14098.
47. M. Hoque, M. L. Thomas, M. S. Miran, M. Akiyama, M. Marium, K. Ueno, K. Dokko and M. Watanabe, RSC Adv., 2018, 8, 9790–9794.
48. C. D’Agostino, M. D. Mantle, C. L. Mullan, C. Hardacre and L. F. Gladden, ChemPhysChem, 2018, 19, 1081–1088.
49. H. Watanabe, H. Doi, S. Saito, M. Matsugami, K. Fujii, R. Kanzaki, Y. Kameda and Y. Umebayashi, J. Mol. Liq., 2016, 217, 35–42.
50. S. Tsuzuki, W. Shinoda, M. S. Miran, H. Kinoshita, T. Yasuda and M. Watanabe, J. Chem. Phys., 2013, 139, 174504.
51. R. Janoschek, E. G. Weidemann, H. Pfeiffer and G. Zundel, J. Am. Chem. Soc., 1972, 94, 2387–2396.
52. P. A. Hunt, C. R. Ashworth and R. P. Matthews, Chem. Soc. Rev., 2015, 44, 1257–1288.
53. H. Vogel, Phys. Time, 1921, 22, 645–646.
54. G. Tammann and W. Hesse, J. Inorg. Gen. Chem., 1926, 156, 245–257.
55. G. S. Fulcher, J. Am. Ceram. Soc., 1925, 8, 339–355.
56. A. Noda, K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2001, 105, 4603–4610.
57. C. Brigouleix, M. Anouti, J. Jacquemin, M. Caillon-Caravanier, H. Galiano and D. Lemordant, J. Phys. Chem. B, 2010, 114, 1757–1766; D. D. Zorn, J. A. Boatz and M. S. Gordon, J. Phys. Chem. B, 2006, 110, 11110–11119.
58. M. Watanabe, D. Kodama, T. Makino and M. Kanakubo, J. Chem. Eng. Data, 2016, 61, 4215–4221.
59. H. Nakamoto and M. Watanabe, Chem. Commun., 2007, 2539–2541.
60. W. Xu, E. I. Cooper and C. A. Angell, J. Phys. Chem. B, 2003, 107, 6170–6178.
61. H. Tokuda, K. Ishii, M. A. B. H. Susan, S. Tsuzuki, K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2006, 110, 2833–2839.
62. K. Ueno, H. Tokuda and M. Watanabe, Phys. Chem. Chem. Phys., 2010, 12, 1649–1658.
63. D. R. MacFarlane, M. Forsyth, E. I. Izgorodina, A. P. Abbott, G. Annat and K. Fraser, Phys. Chem. Chem. Phys., 2009, 11, 4962–4967.
64. M. Shen, Y. Zhang, K. Chen, S. Che, J. Yao and H. Li, J. Phys. Chem. B, 2017, 121, 1372–1376.
65. P. Walden, J. Phys. Chem., 1906, 55, 207.
66. K. Ueno, K. Yoshida, M. Tsuchiya, N. Tachikawa, K. Dokko and M. Watanabe, J. Phys. Chem. B, 2012, 116, 11323–11331.
67. T. Yasuda, H. Kinoshita, M. S. Miran, S. Tsuzuki and M. Watanabe, J. Chem. Eng. Data, 2013, 58, 2724–2732.
68. S. Tsuzuki, H. Tokuda, K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2005, 109, 16474–16481.
69. M. S. Miran, H. Kinoshita, T. Yasuda, M. A. B. H. Susan and M. Watanabe, Chem. Commun., 2011, 47, 12676–12678.
70. M. Watanabe, Electrochemistry, 2016, 84, 642–653.
71. M. Kuroha, H. Gotoh, M. S. Miran, T. Yasuda, M. Watanabe and K. Sakakibara, Chem. Lett., 2014, 43, 649–651.
72. Z. Zhao, K. Ueno and C. A. Angell, J. Phys. Chem. B, 2011, 115, 13467–13472.
73. A. J. Kresge, Acc. Chem. Res., 1975, 8, 354–360.
74. H. Doi, X. Song, B. Minofar, R. Kanzaki, T. Takamuku and Y. Umebayashi, Chem. – Eur. J., 2013, 19, 11522–11526.
75. I. V. Fedorova, M. A. Krestyaninov and L. P. Safonova, J. Phys. Chem. A, 2017, 121, 7675–7683.

---

Footnotes

† Electronic supplementary information (ESI) available: TG curves of [NTf₂]-based PILs; optimised structures of [DBU][NTf₂] and [DBU][TfO]; VTF fitting parameters for the viscosity, conductivity, and molar conductivity; fitting parameters for the density and molar conductivity at various temperatures for the [NTf₂]-based PILs. See DOI: 10.1039/c8cp06973e

‡ Present address: Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh.

§ These two authors equally contributed to this work.

---