Comparative Study on Physicochemical Properties of Protic Ionic

Article pubs.acs.org/jced

Comparative Study on Physicochemical Properties of Protic Ionic Liquids Based on Allylammonium and Propylammonium Cations Tomohiro Yasuda,† Hiroshi Kinoshita,‡ Muhammed Shah Miran,‡ Seiji Tsuzuki,§ and Masayoshi Watanabe*,‡ †

Cooperative Research and Development Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan § National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Centre 5, Tsukuba 305-8565, Japan ABSTRACT: Protic ionic liquids (PILs) were prepared by neutralization of a series of allylamines (allyldimethylamine, allyldiethylamine, and diallylmethylamine) with trifluoromethanesulfonic acid, in order to achieve highly conductive PILs, and their physicochemical properties, including thermal and transport properties, were compared with PILs from a series of propylamines (dimethylpropylamine, diethylpropylamine, and methyldipropylamine). The melting point and ionic conductivity of the allylammonium-based PILs were lower and higher than those of the corresponding propylammonium-based PILs having cations with the same number of carbon atoms, respectively. The origin of the difference in the properties of these two series of PILs was investigated based on the evaluation of the molar concentration, self-diffusion coefficient of the cations and anions, and ionicity. The molar concentration and ionicity of the allylammonium-based PILs were higher than those of the corresponding propylammonium-based PILs, whereas the diffusion coefficient was comparable for both sets of PILs. Ab initio geometrical optimization and calculation of the stabilization energies (Eform) of the ion pairs of the PILs indicated higher stability of the conformations in which the N−H proton in the cation interacts with the SO oxygen in the anion via hydrogen bonding interaction, relative to conformations in which such interaction is absent. Further comparison with the ion pairs of tetra-alkylammonium-based aprotic ILs (AILs) having cations with the same number of carbon atoms demonstrated that the interactions in the ion pairs of the PILs were stronger and had higher directionality than those of the AILs. However, the magnitude of the attraction and the directionality of the interactions in the ion pairs of the allylammonium-based PILs did not differ appreciably from those of the propylammonium-based counterparts. It is concluded that the allyl group facilitates the formation of more compact structures of the PILs without enhancing the interionic interactions with consequently higher molar concentration, thereby increasing the ionic conductivity of the allylammonium-based PILs due to an increase in the number of carrier ions. The viscosity increased and the ionicity decreased with increasing numbers of carbon atoms in the alkyl chain of the ammonium cations, due to an increase in dispersion interactions. Thus, among the PILs explored in this study, allyldimethylammonium trifluoromethanesulfonate ([N11a][TfO]) exhibited the highest ionic conductivity of 75 mS·cm−1 at 150 °C, which is the highest conductivity of PILs reported to date, to the best of our knowledge.

■

INTRODUCTION Protic ionic liquids (PILs), generally prepared by the neutralization of a Brønsted acid and a Brønsted base,1 constitute one of the most important subclasses of ionic liquids. Even though the first ionic liquid ever reported (ca. 100 years ago) is a PIL,2 the systematic study of the structure− property relationship of these materials has only recently been initiated and remains in the developmental stage. It is generally believed that PILs are thermally unstable relative to conventional aprotic ionic liquids (AILs) such as 1-methyl-3ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]),3 due to the possibility of an acid−base equilibrium shift from the salt to the corresponding acid and base with an increase in temperature. However, recent studies revealed that PILs can exhibit all of the characteristic properties demonstrated by typical AILs, such as high thermal stability, © 2013 American Chemical Society

negligible vapor pressure, and high ionic conductivity, if the ΔpKa value of the constituent acids and bases is sufficiently high.4−11 Our prior studies have suggested that PILs based on a superstrong base (i.e., with ΔpKa ≥ 15) offer high thermal stability and high ionicity similar to AILs.11 One of the most important characteristics of PILs is the presence of active (reactive and/or transferring) protons.12,13 By exploiting this characteristic, we reported for the first time that fuel cell electrode reactions (i.e., hydrogen oxidation and oxygen reduction) can proceed in PILs under anhydrous conditions.14,15 The significance of this finding is that the nonhumidifying operation of fuel cells over a wide temperature Received: December 5, 2012 Accepted: August 2, 2013 Published: August 20, 2013 2724

dx.doi.org/10.1021/je301284x | J. Chem. Eng. Data 2013, 58, 2724−2732

Journal of Chemical & Engineering Data

Article

or N,N-diethylmethylamine via quaternization using 1bromopropane, followed by the Hoffman elimination reaction. The typical synthetic procedure is as follows: 1-bromopropane (98.4 g, 0.800 mol) and 150 mL of DMF were added to a 300 mL 4-necked flask. N-Ethyldimethylamine (48.8 g, 0.667 mol) was slowly dropped into the mixture with vigorous stirring, upon completion of which stirring was continued for several hours. DMF and volatile materials were removed from the resulting mixture by evaporation. The obtained white solid was washed with ethyl acetate and hexane several times and was dried in vacuo. A small amount of water (20 mL) and potassium hydroxide (46.0 g, 0.820 mol) were added to the solid. The mixture was heated at 130 °C with stirring and the evolved N,N-dimethylpropylamine was collected by distillation. Calcium chloride and calcium hydride were successively added to the N,N-dimethylpropylamine to remove water, and distillation was repeated. All of the amines were purified by distillation, and their purity was evaluated via gas chromatography and confirmed to be higher than 99.9 %. PILs were synthesized by slowly adding equimolar amounts of the amine to trifluoromethanesulfonic acid under neat conditions with sufficient cooling. The synthesized PILs were dried at 80 °C for 2 days under vacuum and stored in an Ar atmosphere glovebox (VAC, [O2] < 1 ppm, [H2O] < 1 ppm). The water contents, determined by Karl Fischer titration, were below 100 ppm for all of the PILs. Given that pKa of the protonated forms of these amines is roughly 10−11 and pKa of trifluoromethanesulfonic acid is −7, the ΔpKa value of the resulting PILs is 17−18.11 Thermal Properties. Differential scanning calorimetry (DSC) was carried out on a Seiko Instruments DSC 220C under a nitrogen atmosphere. The samples were tightly sealed in aluminum pans in a dry glovebox. The samples were heated to 150 °C, then cooled to −150 °C, and heated again to 150 °C at cooling and heating rates of 10 °C·min−1. The DSC thermograms were recorded during the reheating scans. Melting temperature (Tm) was determined as the onset temperature of an endothermic peak, and glass transition temperature (Tg) was determined as the onset temperature of a heat capacity change. Enthalpy change of melting (ΔH) was calculated from the endothermic peak area of melting transition, and entropy change of melting (ΔS) was calculated from ΔS = ΔH/Tm. Thermogravimetric analysis (TGA) was performed using a Seiko Instruments TG-DTA 6200C instrument under a nitrogen atmosphere. The samples were weighed and placed in aluminum pans and then heated from room temperature to 550 °C at a heating rate of 10 °C·min−1. Viscosity and Density. The viscosity of the PILs was measured in the temperature range from (30 to 150) °C under dry atmosphere using a rheometer (Physica MCR301, Anton Paar). The density measurement was conducted using a thermoregulated density/specific gravity meter DA-100 (Kyoto Electronics Manufacturing Co. Ltd.) in the temperature range from (15 to 40) °C. Ionic Conductivity. The ionic conductivities of the PILs were determined in the temperature range from (30 to 170) °C by employing the complex impedance method. Individual PILs were placed in a dip-type sealed glass cell with two Pt rods fixed at a constant electrode distance, in the glovebox. The cell constant was determined using a 0.1 M KCl standard aqueous solution at 30 °C. The cells were placed in an oven and thermally equilibrated at each temperature for 1 h before the measurements. The measurements were carried out with a

range from below the freezing point of water to intermediate temperatures (> 100 °C) can be realized by employing PILs as a proton conductor.16−20 Furthermore, the active proton and the ability to form hydrogen bonds open up a variety of applications for PILs such as protein13 and virus21 stabilization, nanostructure formation,22 pharmaceutical applications,23 and as a capacitor electrolyte.24 By screening a number of PILs, we found that diethylmethylammonium trifluoromethanesulfonate ([N122][TfO]) may be suitable as an intermediate temperature fuel cell electrolyte, given its favorable bulk properties (melting point (Tm) = −6 °C, decomposition temperature (Td) = 360 °C, ionic conductivity (σ) at 150 °C under anhydrous condition = 68 mS·cm−1) and excellent activity toward the H2/ O2 fuel cell electrode reactions (open circuit potential = 1.03 V at 150 °C under nonhumidifying conditions).25 Successful fabrication of solid thin films incorporating this PIL in sulfonated polyimides16,26,27 and nonhumidifying H2/O2 fuel cell operation at 120 °C using the composite membrane were also achieved. However, the determining factors underlying these promising properties remain ambiguous. Conductivity is an important property for electrolytes given that it often determines the performance of electrochemical devices. For instance, an IR drop derived from low conductivity is a serious issue in fuel cell operation.28 The introduction of allyl groups into the 1- and/or 3-position(s) of the imidazolium cation of AILs reportedly suppressed crystallization, lowered their viscosity, and enhanced their ionic conductivity.29 However, the chemistry of PILs is much more complicated than that of AILs due to the existence of an acid−base equilibrium and strong hydrogen bonds,12 in addition to the other interionic interactions such as Coulombic interactions and van der Waals forces that are also common to AILs.30,31 In this study, with the objective of understanding the properties of PILs having a similar ΔpKa but containing allyl groups in the cationic moiety, PILs were prepared from a series of allylamines and trifluoromethanesulfonic acid; the physicochemical properties of these species were compared with those of PILs from a series of propylamines (Figure 1) and [N122][TfO], which has

Figure 1. Chemical structures of PILs utilized in this study.

the highest reported conductivity of PILs to date. Particular focus was placed on the transport properties of the prepared allylammonium-based PILs such as ionic conductivity, viscosity, and ionicity, and an extensive comparison with the propylammonium-based PILs is presented herein.

■

EXPERIMENTAL SECTION Materials. Allylamines (N-allyldimethylamine, N-allyldiethylamine, and N,N-diallylmethylamine) and N,N-diethylmethylamine were purchased from TCI. Propylamines (N,Ndimethylpropylamine, N-methyldipropylamine, and N,N-diethylpropylamine) were synthesized from N-ethyldimethylamine 2725



Article

potentiostat (Autolab, PGSTAT30) at an amplitude of 10 mV in the frequency range of 1 Hz to 10 MHz under anhydrous conditions. Self-Diffusion Coefficient. Pulsed-gradient spin−echo (PGSE)-NMR measurements were conducted on a JEOL JNM-AL 400 spectrometer with a 9.4-T narrow-bore superconducting magnet equipped with a JEOL pulse field gradient probe and a current amplifier. The sine-gradient-pulse, providing a gradient strength up to 12 T·m−1, was used throughout the measurements. The self-diffusion coefficients were measured using a simple Hahn spin−echo sequence, (i.e., 90°−τ−180°−τ acquisition), incorporating a gradient pulse in each τ period. The free diffusion echo signal attenuation, E, is related to the experimental parameters by32 Figure 2. DSC curves of PILs, measured at a heating rate of 10 °C· min−1.

ln(E) = ln(S /Sg = 0) = −γ 2g 2Dδ 2(4Δ − δ)/π 2

where S is the spin−echo signal intensity, δ is the duration of the field gradient with magnitude g, γ is the gyromagnetic ratio, D is the self-diffusion coefficient, and Δ is the interval between two gradient pulses and it was fixed at (20 to 60) ms in this study. A recycle delay sufficient to allow full relaxation (i.e., > 5T1) was used between each transition. Measurements of the cationic and anionic self-diffusion coefficients for each PIL were conducted using 1H (399.7 MHz) and 19F (376.1 MHz) nuclei, respectively. The measurements were performed at different temperatures from (30 to 150) °C with the samples thermally equilibrated at each temperature for 1 h prior to the measurements. The samples were inserted into a 5 mm (o.d.) NMR microtube (BMS-005J, Shigemi) to a height of 5 mm, in the glovebox. Computational Methods. Ab initio molecular orbital calculations were performed with the Gaussian 03 program33 using the basis sets implemented in the Gaussian program. The geometries of the ion pair complexes were fully optimized at the HF/6-311G** level. The energy of the interaction between the cation and anion (Eint) was calculated at the MP2/6311G**//HF/6-311G** level by means of the supermolecule method.34,35 Our previous calculations for the [C2mim]BF4 complexes36 show that the effects of the basis set on the calculated interaction energies of the complexes are very small, if basis sets including polarization functions are used, and that the effects of electron correlation beyond MP2 are negligible. Therefore, calculations of the interaction energies of the complexes were performed at the MP2/6-311G** level in this work. The interaction energy calculations were corrected for basis set superposition error (BSSE)37 using the counterpoise method.38 The stabilization 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 increase in the energies of the monomers due to deformation associated with the formation of the complex.39 Edef was calculated at the MP2/ 6-311G** level.

the cation. For instance, the Tm of [N1aa][TfO] and [N133][TfO] are −19 °C and +17 °C, respectively (Table 1). Noticeably, the allylammonium-based PILs have relatively slower crystallization kinetics than the propylammonium-based counterparts, given that the glass transition and subsequent crystallization processes are apparent in the DSC profiles of certain allylammonium-based PILs. The TGA curves of the evaluated PILs were measured, and the corresponding onset-decomposition temperatures (Tds) are summarized in Table 1. A relatively high Td (ca. 360 °C) was observed for all of the PILs with no notable difference for the various PILs, indicating that the allyl-substituted ammonium structure is not deleterious to the thermal stability. The similarity of the experimentally determined Td values revealed that these PILs have the same ΔpKa (17−18) given that the thermal stability has good correlation with the ΔpKa.11 Density. Figure 3a shows the temperature dependence of the density of the PILs (numerical data are shown in Table 2). The allylammonium-based PILs have higher densities than the corresponding propylammonium-based PILs. The density of the PILs decreases with the alkyl chain length, similar to the behavior exhibited by AILs.3 The molecular mass of the allylammonium-based PILs (235 Dal) is 0.8 % smaller than that of the propylammonium-based PILs (237 Dal), whereas the molar volume of the allyammonium-based PILs, [N11a][TfO], at 303 K (1.83·102 cm3·mol−1) is 2.6 % smaller than that of the propylammonium-based PILs, [N113][TfO], (1.88·102 cm3· mol−1). This comparison shows that the smaller molecular volume of the allylammonium-based PILs is the primary cause of the higher density of these species relative to the propylammonium-based PILs. A clear difference in the molar volume is also observed in the temperature dependence of the molar concentration of both species (Figure 3b). Viscosity. Figure 4 shows the temperature dependence of the viscosity of the PILs (numerical data are shown in Table 3). All of the PILs exhibit Vogel−Fulcher−Tammann (VFT) temperature dependence, as is typically observed for AILs;3 the VFT parameters are listed in Table 4. The viscosity is roughly dominated by the molecular weight of the cations, in other words, cationic size. Moreover, the allylammonium-based PILs have a slightly lower viscosity than the corresponding propylammonium-based PILs. Ionic Conductivity. Figure 5 shows the temperature dependence of the ionic conductivity for the PILs (numerical data are shown in Table 5). The ionic conductivity of the PILs

■

RESULTS Thermal Properties. Figure 2 shows the DSC curves of the PILs, and the thermal properties are summarized in Table 1. The allylammonium-based PILs tend to have a lower melting point (Tm) than the propylammonium-based PILs having the same total number of carbon atoms in the N-substituents. Among the PILs with the same number of total carbon atoms, Tm varies by > 30 °C depending on the chemical structure of 2726



Article

Table 1. Thermal Properties of PILs at Pressure p = 0.1 MPaa PILs [N122][TfO] [N11a][TfO] [N113][TfO] [N22a][TfO] [N223][TfO] [N1aa][TfO] [N133][TfO] a

Tg/°C

Tm/°C

ΔH/kJ·mol−1

ΔS/J·K−1·mol−1

Td/°C

−99

−6 16 20 −14 −14 −19 17

16 21 20 21 21 11 14

58 72 67 82 81 42 48

360 361 363 358 359 344 354

−88

Standard uncertainties u are u(Tg, Tm) = 3 % and u(ΔH, ΔS) = 5 %.

Figure 4. Arrhenius plots of viscosity for PILs.

evaluated PILs (σ = 75 mS·cm−1 at 150 °C), which is higher than that reported for [N122][TfO] (σ = 68 mS·cm−1 at 150 °C).25

■

DISCUSSION As alluded to in the Results section, in the case of PILs having the same molecular weight, those with N-substituents (C1, C2, and C3) that have similar numbers of carbon atoms tend to have lower Tm than those with a different number of carbon atoms. However, the effect of the cationic structure was not well reflected in the thermodynamic parameters (ΔH and ΔS, Table 1). The allylammonium-based PILs have lower melting points, higher densities, and lower viscosities than the propylammonium-based PILs. It is assumed that the lower conformational freedom of the allyl group compared with that of the propyl group makes the PILs of the former group more compact without enhancing the interionic interactions. The effect of the allyl groups on the ionic conductivity can be discussed as follows: molar conductivity based on ionic conductivity is defined as follows: Λ imp = σ /c

Figure 3. Density and molar concentration of PILs as a function of temperature.

Table 2. Density of PILs at Pressure p = 0.1 MPaa d/g·cm−3

a

PILs

15 °C

20 °C

25 °C

30 °C

35 °C

40 °C

[N122][TfO] [N11a][TfO] [N113][TfO] [N22a][TfO] [N223][TfO] [N1aa][TfO] [N133][TfO]

1.299 1.318 1.291 1.270 1.237 1.268

1.292 1.311 1.284 1.263 1.229 1.261 1.218

1.284 1.303 1.276 1.255 1.222 1.253 1.211

1.277 1.295 1.269 1.247 1.215 1.246 1.203

1.270 1.288 1.262 1.240 1.208 1.239 1.196

1.262 1.280 1.254 1.232 1.201 1.231 1.189

Molar conductivity can also be calculated based on the diffusivities of the cation and anion as follows:

Standard uncertainties u are u(K) = 0.5 K and u(d) = 1 %.

ΛNMR = [NAe 2(Dcation + Danion)]/kT

is also characterized by VFT temperature dependence, consistent with the viscosity behavior. The VFT parameters are listed in Table 6. Based on the observed trends, the PILs consisting of cations with five carbon atoms exhibit higher conductivity than those having cations with seven carbon atoms. It is also apparent that the ionic conductivity of the allylammonium-based PILs is slightly higher than that of the corresponding propylammonium-based PILs. Thus, [N11a][TfO] exhibits the highest ionic conductivity among the

where c, NA, e, D, k, and T, are respectively the molar concentration determined from the density and formula weight of the ILs, Avogadro’s number, the electric charge on each ionic carrier, the diffusion coefficient, Boltzmann constant, and absolute temperature. Ionicity has been defined as Λimp/ΛNMR (= α), and may act as an indicator of the dissociativity and/or the interionic interaction of the ILs.40 Thus, the ionic conductivity (σ) can be re-expressed as follows: 2727



Article

Table 3. Viscosity of PILs at Pressure p = 0.1 MPaa η/mPa·s

a

PILs

30 °C

50 °C

70 °C

90 °C

110 °C

130 °C

150 °C


36.9 28.8 33.2 51.0 54.3 38.5 48.7

19.4 16.4 18.0 25.5 26.8 19.7 23.9

12.1 10.4 11.0 14.9 15.3 11.6 13.6

8.13 7.11 7.42 9.64 9.76 7.61 8.63

5.82 5.18 5.32 6.70 6.71 5.39 5.96

4.41 3.95 4.04 4.94 4.87

3.48 3.17 3.19 3.87 3.73

4.40

3.44

Standard uncertainties u are u(K) = 0.03 K and u(η) = 1 %.

Table 4. VTF Parameters for Viscosity of PILsa η/mPa·s

a

PILs

30 °C

150 °C

η0·10−1/mPa·s

B·102/K

T0/K

R2


36.9 28.8 33.2 51.0 54.3 38.5 48.7

3.48 3.17 3.19 3.87 3.73

2.35 1.35 1.52 1.73 1.27 1.50 1.31

7.05 9.13 8.39 8.22 9.18 8.06 8.62

164 133 147 159 152 158 157

1.000 1.000 1.000 1.000 1.000 1.000 1.000

3.44

η = η0 exp(B/(T − T0)).

the propylammonium PILs having cations of the same alkyl chain numbers was elucidated based on detailed comparison to these three parameters using [N113][TfO] and [N11a][TfO] as target analytes, and data for [N122][TfO] was also used as a reference, given that the latter species has the same number of carbon atoms in the cationic structure as the former two and that to date, it exhibits the highest reported ionic conductivity in its class. As shown in Figure 3b, the molar concentration of [N11a][TfO] is higher than that of [N113][TfO] and is also higher than that of [N122][TfO]. Figure 6 shows the temperature dependence of the cationic and the anionic diffusion coefficients obtained from PGSE-NMR measurements (numerical data are shown in Table 7). The data show that the cationic and anionic diffusion coefficients of both [N11a][TfO] and [N113][TfO] are almost the same, and the data for [N122][TfO] were not appreciably different from those of these PILs. The ionicity of these three PILs is plotted as a function of temperature in Figure 7. For the entire evaluated temperature range, the ionicity of [N122][TfO] and [N11a][TfO] is almost the same, and is a little higher than that of [N113][TfO]. The ionicity of all of the PILs decreased with increasing temperature, which is a commonly observed phenomenon for PILs, possibly due to a change in the acid−

Figure 5. Arrhenius plots of ionic conductivity for PILs.

σ = [αcNAe 2(Dcation + Danion)]/kT

It is clear from the equations that the ionic conductivity is determined by the ionicity, molar concentration, and diffusion coefficients of the cation and anion. The origin of the higher ionic conductivity of the allylammonium-based PILs relative to Table 5. Ionic Conductivity of PILs at Pressure p = 0.1 MPaa

σ·103/ S·cm−1

a

PILs

30 °C

50 °C

70 °C

90 °C

110 °C

130 °C

150 °C

170 °C


8.33 9.68 9.01 4.19 3.84 4.69 4.15

14.5 17.4 15.7 7.85 7.10 8.93 7.72

21.5 26.5 23.9 12.6 11.3 14.3 12.3

31.5 37.3 32.5 18.1 17.1 20.6 18.1

39.1 49.2 42.9 24.3 22.2 27.4 24.8

47.3 61.9 53.0 31.7 28.8 35.4 31.8

54.6 74.8 64.0 39.1 35.7 43.9 40.4

59.7 88.2 76.8 47.1 43.1 52.1 48.8

Standard uncertainties u are u(K) = 1 K and u(σ) = 1 %. 2728



Article

Table 6. VTF Parameters for Ionic Conductivity of PILsa σ/mS·cm−1

a

PILs

30 °C

150 °C

σ0·10−1/S·cm−1

B·102/K

T0/K

R2


9.43 9.68 9.01 4.19 3.84 4.69 4.15

68.0 74.8 64.0 39.1 35.7 43.9 40.4

7.14 6.66 7.11 5.11 4.53 4.90 6.54

6.22 5.43 6.45 6.66 6.54 6.09 7.49

159 174 154 164 165 171 154

1.000 1.000 1.000 1.000 1.000 1.000 1.000

σ = σ0 exp(−B/T − T0).

Figure 7. Temperature dependence of ionicity (Λimp/ΛNMR) for PILs with cations having total number of five carbon atoms.

Figure 6. Arrhenius plots of cationic and anionic self-diffusion coefficients based on PGSE-NMR measurements for PILs with cations having total number of five carbon atoms.

Figure 8. Walden plots of PILs. Molar conductivities (Λimp) were calculated by dividing the ionic conductivity by molar concentration. The diagonal line is the ideal Walden line determined for 1.0 M KCl aqueous solution.

11,12

base equilibrium with temperature. Figure 8 shows Walden plots for all of the PILs explored in this study, where the Table 7. Diffusion Coefficient of PILs at Pressure p = 0.1 MPaa

D·106/cm2·s−1 PILs [N122][TfO] [N11a][TfO] [N133][TfO]

a

cation anion cation anion cation anion

30 °C

50 °C

70 °C

90 °C

110 °C

130 °C

150 °C

0.479 0.365 0.493 0.424 0.501 0.420

0.886 0.720 0.939 0.813 0.955 0.810

1.53 1.30 1.61 1.40 1.63 1.40

2.34 2.07 2.57 2.29 2.29 2.26

3.61 3.51 3.93 3.52 4.05 3.52

4.91 5.14 5.77 5.13 5.95 5.11

7.62 6.90 7.75 6.87

Standard uncertainties u are u(K) = 0.2 K and u(D) = 5 %. 2729



Article

Walden line of 1 M KCl is shown as an ideal line. Angell and co-workers categorized ILs as good ILs, poor ILs, and non-ILs, depending on the agreement of their Walden plots with the ideal Walden line.41 In other words, ILs having Walden plots that are close to and far from the ideal line are good ILs and non-ILs, respectively. Furthermore, ILs with Walden plots that lie above the ideal line and are less dependent on viscosity are categorized as super-ILs. We also recently reported that there is a linear correspondence between the ratios of the molar conductivity to the ideal molar conductivity at a certain viscosity and the ionicity values, which are the ratios of the molar conductivities based on ionic conductivity and diffusivity measurements.42 Although several approaches have been reported for the estimation of ionicity based on quantum mechanical calculations, the PGSE-NMR method and the Walden plot are the most widely utilized.31,43−45 The Walden plots of all of the PILs in this study are located close to the ideal line, and thus are categorized as good ILs. Close inspection of the Walden plots reveals that the PILs can be classified into two classes. The plots of [N122][TfO], [N113][TfO], and [N11a][TfO] (group 1) are closer to the ideal line than those of [N223][TfO], [N22a][TfO], [N133][TfO], and [N1aa][TfO] (group 2), which indicates that the PILs in the former group have higher ionicity values than those of the latter group. The difference between these two groups is the difference in the total number of carbon atoms in their cationic structures; members of the former group have five carbon atoms and those of the latter group possess seven carbon atoms. Based on prior analysis of typical AILs, we reported that the ionicity values are affected not only by Coulombic interaction but also by van der Waals interaction.31 A simple decrease in the ionicity values with increasing alkyl chain length of the 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amides was observed therein.46 The difference between the Walden plots of the species in groups 1 and 2 appears to arise from the difference in their ionicity values, and the group 2 PILs have lower ionicity values due to increased van der Waals interactions. It is also implicit that the effect of the introduction of the allyl group into the cationic structures on the ionicity values is smaller than the effect of the difference in the total number of carbon atoms in the cations. On the basis of these results, the effect of the allyl group on transport properties is summarized in that the introduction of the allyl group may facilitate an increase in the molar concentration without decreasing the diffusivity and ionicity, thereby enabling an enhancement of the ionic conductivity. The minimum energy conformations determined for the [N113][TfO] and [N11a][TfO] complexes and the calculated stabilization energies are shown in Figure 9. Geometrical optimization of the complexes was carried out from 10 or more initial geometries. 1a and 2a are the minimum energy conformations for the [N113][TfO] and [N11a][TfO] complexes, respectively. The N−H of the cation has contact with one of the oxygen atoms of the TfO anion in the most stable geometries. The stabilization energies (Eform) of 1a and 2a are (−97.1 and −96.4) kcal·mol−1, respectively. The Eform calculated for the geometries in which the N−H has contact with an oxygen atom (contact geometries) varies from (−91 to −97) kcal·mol−1. The N−H···O contacts are absent in a few of the optimized geometries of the complexes (noncontact geometries). The most stable noncontact geometries, 1b and 2b, are shown in Figure 9. The Eform values calculated for 1b and 2b ((−83.5 and −82.7) kcal·mol−1) are significantly less

Figure 9. Optimized geometries and stabilization energies (Eform) calculated for the [N11a][TfO] and [N113][TfO] complexes.

negative than those for 1a and 2a, which shows that the N−H··· O contact increases the stability of the complexes significantly and the interaction between the cation and anion has strong directionality. We recently reported that hydrogen bonding interactions contribute greatly to the stabilization of PILs based on a superstrong organic base, and that such interactions increase with a decrease in the acidity (increase of pKa) of the constituent acids.12 In the case of AILs, we previously found that the ionicity tends to decrease with an increase in the interaction energies of the ion pairs as well as with an increase in the directionality of the interactions;36 the directionality of the interaction was defined as the magnitude of the maximum interaction energy difference among the optimized structures. For example, the BF4 anion has contact with the C2−H group in the most stable geometry of the [C2mim]BF4 complex, whereas the geometries in which the anion has contact with the C4−H and C5−H species are less stable.36 The ionicity of the AILs containing aliphatic ammonium cations such as pyrrolidinium and tetra-alkylammonium are higher than those with aromatic cations such as imidazolium and pyridinium, due to the lower directionality of the interaction in these aliphaticcation-based AILs resulting from the absence of acidic protons.47 Although the PILs studied herein contain aliphatic cations, they also have acidic N−H protons. The contact of N− H with the anion increases the attraction between the cation and anion via electrostatic interactions. The interactions between the ion pairs of the PILs have greater directionality than those of the AILs. The Eform calculated for the most stable N-ethyl-N,N,N-trimethylammonium tetrafluoroborate ([N1112]BF4) complex is −84.6 kcal·mol−1, the directionality of the interaction is as high as 9.9 kcal·mol−1, and the ionicity at room temperature is 0.67.36 In comparison, the corresponding Eform, directionality, and ionicity values for [N11a][TfO] are −97.1 kcal·mol−1, 21.1 kcal·mol−1, and 0.52; and those for [N113][TfO] are −96.4 kcal·mol−1, 18.7 kcal·mol−1, and 0.50, respectively. Eform and directionality of the ion pairs of the PILs are larger than those of the AILs. However, there is no appreciable difference between Eform of the most stable geometries of the [N113][TfO] and [N11a][TfO] complexes and the directionality of their interactions. 2730



■

Article

(13) Fumino, K.; Wulf, A.; Ludwig, R. Angew. Chem., Int. Ed. 2009, 48, 3184. (14) Susan, M. A. B. H.; Noda, A.; Mitsushima, S.; Watanabe, M. Chem. Commun. 2003, 938. (15) Noda, A.; Susan, M. A. B. H.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2003, 107, 4024. (16) Lee, S.-Y.; Ogawa, A.; Kanno, M.; Nakamoto, H.; Yasuda, T.; Watanabe, M. J. Am. Chem. Soc. 2010, 132, 9767. (17) Snyder, J.; Fujita, T.; Chen, M. W.; Erlebacher, J. Nat. Mater. 2010, 9, 904. (18) Belieres, J.-P.; Gervasio, D.; Angell, C. A. Chem. Commun. 2006, 4799. (19) Wang, L.; Advani, S. G.; Prasad, A. K. Electrochem. Solid-State Lett. 2012, 15, B44. (20) Ke, C.; Li, J.; Li, X.; Shao, Z.; Yi, B. RSC Adv. 2012, 2, 8953. (21) Byrne, N.; Wang, L. M.; Belieres, J.-P.; Angell, C. A. Chem. Commun. 2007, 2714. (22) Greaves, T. L.; Mudie, S. T.; Drummond, C. J. Phys. Chem. Chem. Phys. 2011, 13, 20441. (23) Stoimenovski, J.; Dean, P. M.; Izgorodina, E. I.; MacFarlane, D. R. Faraday Discuss. 2012, 154, 335. (24) Mayrand-Provencher, L.; Lin, S.; Lazzerini, D.; Rochefort, D. J. Power Sources 2010, 195, 5114. (25) Nakamoto, H.; Watanabe, M. Chem. Commun. 2007, 2539. (26) Lee, S.-Y.; Yasuda, T.; Watanabe, M. J. Power Sources 2010, 195, 5909. (27) Yasuda, T.; Nakamura, S.-I.; Kinugawa, K.; Lee, S.-Y.; Watanabe, M. ACS Appl. Mater. Interfaces 2012, 4, 1782. (28) Kruer, K. D. Annu, Rev. Mater. Res. 2003, 33, 352. (29) Mizumo, T.; Marwanta, E.; Matsumi, N.; Ohno, H. Chem. Lett. 2004, 10, 33. (30) Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 19593. (31) Ueno, K.; Tokuda, H.; Watanabe, M. Phys. Chem. Chem. Phys. 2010, 12, 1649. (32) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288−292. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (34) Mϕller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618−622. (35) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503−506. (36) Tsuzuki, S.; Tokuda, H.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2005, 109, 16474. (37) Ransil, B. J. J. Chem. Phys. 1961, 34, 2109. (38) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (39) Tsuzuki, S.; Hayamizu, K.; Seki, S.; Ohno, Y.; Kobayashi, Y.; Miyashiro, H. J. Phys. Chem. B 2008, 112, 9914. (40) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105, 4603. (41) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107, 6170. (42) Ueno, K.; Yoshida, K.; Tsuchiya, M.; Tachikawa, N.; Dokko, K.; Watanabe, M. J. Phys. Chem. B 2012, 116, 11323.

CONCLUSION The effect of introducing an allyl group into the cationic structures of PILs on the transport properties was studied herein. It was found that the PILs possessing allylammonium cations exhibit higher ionic conductivity than the corresponding PILs having propylammonium cations. Clarification of the origin of the high ionic conductivity of the allylammonium based PILs by the evaluation of the diffusion coefficient using the PGSE-NMR technique, molar concentration, and ionicity revealed that the molar concentration and ionicity were higher for the allylammonium-based PILs, whereas the diffusion coefficient was comparable to those of the corresponding propylammonium-based PILs. Ab initio geometrical optimization and calculation of the interaction energies of the ion pairs of the PILs indicated that the conformations in which the N−H proton in the cation interacts with the SO oxygen in the anion via hydrogen bonding are more stable than the conformations where such interaction is absent. However, there is no appreciable difference between the interaction energies of the most stable conformation and the directionality of the interaction for the allylammonium-based and propylammonium-based PILs. The presence of the allylgroup appears to facilitate more compact structures of the PILs without enhancing the interionic interaction, and thus without lowering the ionicity, with a consequent increase in the ionic conductivity. Increasing the number of carbon atoms in the cationic structures increases the viscosity and lowers the ionicity due to an increase in the van der Waals interactions. Thus, among the PILs evaluated, the highest ionic conductivity of [N11a][TfO] (75 mS·cm−1 at 150 °C) is the highest ionic conductivity ever reported.

■

AUTHOR INFORMATION

Corresponding Author

*Tele/Fax: +81-45-339-3955. E-mail: [email protected]. Funding

This work was financially supported by Grant-in-Aid for Scientific Research in the Priority Area “Science of Ionic Liquid” from the MEXT of Japan and by NEDO of Japan. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Greaves, T. L.; Drummond, C. J. Chem. Rev. 2008, 108, 206. (2) Walden, P. Bull. Acad. Imper. Sci. 1914, 6, 405. (3) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 16593. (4) Yoshizawa, M.; Xu, W.; Angell, C. A. J. Am. Chem. Soc. 2003, 125, 15411. (5) Greaves, T. L.; Weerawardena, A.; Fong, C.; Krodkiewska, I.; Drummond, C. J. J. Phys. Chem. B 2006, 110, 22479. (6) Belieres, J.-P.; Angell, C. A. J. Phys. Chem. B 2007, 111, 4934. (7) Anouti, M.; Caillon-Caravanier, M.; Floch, C. L.; Lemordant, D. J. Phys. Chem. B 2008, 112, 9406. (8) Luo, H.; Baker, G. A.; Lee, J. S.; Pagni, R. M.; Dai, S. J. Phys. Chem. B 2009, 113, 4181. (9) Burrel, G. L.; Burgar, I. M.; Separovic, F.; Dunlop, N. F. Phys. Chem. Chem. Phys. 2010, 12, 1571. (10) Zhao, C.; Burrel, G.; Torriero, A. A. J.; Separovic, F.; Dunlop, N. F.; MacFarlane, D. R.; Bond, A. M. J. Phys. Chem. B 2008, 112, 6926. (11) Miran, M. S.; Kinoshita, H.; Yasuda, T.; Susan, M. A. B. H.; Watanabe, M. Phys. Chem. Chem. Phys. 2012, 14, 5178. (12) Miran, M. S.; Kinoshita, H.; Yasuda, T.; Susan, M. A. B. H.; Watanabe, M. Chem. Commun. 2011, 47, 12676. 2731



Article

(43) Harris, K. R. J. Phys. Chem. B 2010, 114, 9572. (44) Kashyap, H. K.; Annapureddy, H. V. R.; Raineri, F. O.; Margulis, C. J. J. Phys. Chem. B 2011, 115, 13212. (45) MacFarlane, D. R.; Forsyth, M.; Izgorodina, E. I.; Abbott, A. P.; Annat, G.; Fraser, K. Phys. Chem. Chem. Phys. 2009, 11, 4962. (46) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103. (47) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833.

2732


Comparative Study on Physicochemical Properties of Protic Ionic

Recommend Documents