Muhammed Shah
Miran‡§
a,
Mahfuzul
Hoque§
a,
Tomohiro
Yasuda
a,
Seiji
Tsuzuki
b,
Kazuhide
Ueno
a and
Masayoshi
Watanabe
*a
aDepartment of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. E-mail: mwatanab@ynu.ac.jp
bResearch Centre for Computational Design of Advanced Functional Materials (CD-FMat), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
First published on 4th December 2018
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 ΔpKa 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 ΔpKa values. However, isothermal TGA measurements for an extended duration indicated that [NTf2]-based PILs exhibit higher thermal stability than [DBU]-based PILs at low ΔpKa 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 ΔpKa values (<15).
Recent developments in PIL research have revealed that the most significant parameter to control the physicochemical properties of PILs is the ΔpKa value resulting from the pKa values of the constituent acid and protonated base in water.22–24 Using pKa 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 ΔpKa value.22,23,26 It has been reported that ΔpKa > 10 is a requisite to ensure satisfactory proton transfer to produce highly ionised PILs23 and that even ΔpKa ≤ 4 is enough for complete proton transfer using primary amines.26 Recently, we have determined the ΔpKa 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 ΔpKa > 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 ΔpKa value.27 Primary and secondary ammonium carboxylate-based PILs exhibit significantly high apparent ionicity even at low ΔpKa 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 < ΔpKa < 24).22 Thus, using a similar ΔpKa range, a systematic study with super-strong acid-based PILs is quintessential to answer critical questions on whether the ΔpKa 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 < ΔpKa < 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 < ΔpKa < 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 ΔpKa 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 pKa values in aqueous medium.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 |
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 (Td). 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 (Eint) 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 (Eform) was calculated as the sum of the interaction energy (Eint) and the deformation energy (Edef), where Edef is the sum of the increases in the energies of the monomers due to the deformation associated with the formation of complexes.26 The Edef 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.
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 (Tg), cold crystallisation temperatures (Tcc) above the Tg, and subsequent melting of the crystallised samples at the melting temperature (Tm) (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 interactions43,44 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, Tm, were found at temperatures below 100 °C for all the samples. Note that [Pyra][NTf2] (ΔpKa = 10.4) exhibits the highest Tm value (70 °C), while [EIm][NTf2] (ΔpKa = 18.5) shows the lowest Tm (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. Tm, Tcc, and Tg correspond to the melting, cold crystallisation, and glass-transition temperatures, respectively. |
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 (Eform) 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 Eform 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 Eform were exclusively dictated by the ΔpKa 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 ΔpKa 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 ΔpKa. |
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, [HCnIm][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].
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
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 Ea of the ionic conductivity is quite high (i.e. a large barrier) in these PILs.
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 ΔpKa 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, (Λimp/ΛNMR),61 which can be effectively applied for all types of ILs such as AILs,61,62 solvate ILs,66 and PILs.22,26,48Λimp was calculated from the equation:
(1) |
(2) |
Fig. 8 Walden plots for [NTf2]-based PILs. The dashed straight line corresponds to a 1 M KCl aqueous solution. |
Fig. 9 ΔpKa 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 ΔpKa = 15 and then remained constant. Proton transfer is incomplete at ΔpKa < 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 ΔpKa values and then became independent of the ΔpKa values. Interestingly, [Pyra][NTf2] and [TZL][NTf2] (10 < ΔpKa < 15) exhibited higher ionicity than [DBU]-based PILs with similar ΔpKa values ([DBU][CH3CO2] and [DBU][CF3CO2]). Furthermore, it is fascinating that the ionicity of [Pyra][NTf2] (ΔpKa = 10.4) is comparable to that of [dema][NTf2] (ΔpKa = 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 ΔpKa 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 ΔpKa 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 ΔpKa values owing to fast kinetics.73 However, in Brønsted acid–base mixtures, neutral acids and bases do exist at low ΔpKa 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 ΔpKa 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 ΔpKa values, depends not only on ΔpKa values but also on the chemical structure of the acids and bases. At high ΔpKa 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 ΔpKa 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.
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. |
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