Protic Ionic Liquids Based on the Alkyl-Imidazolium Cation: Effect of

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Protic Ionic Liquids Based on the Alkyl-Imidazolium Cation: Effect of the Alkyl Chain Length on Structure and Dynamics Iqbaal Abdurrokhman, Khalid Elamin, Olesia Danyliv, Mohammad Hasani, Jan Swenson, and Anna Martinelli J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b01274 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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The Journal of Physical Chemistry

Protic Ionic Liquids Based on the AlkylImidazolium Cation: Effect of the Alkyl Chain Length on Structure and Dynamics Iqbaal Abdurrokhman†, Khalid Elamin†, Olesia Danyliv†, Mohammad Hasani†, Jan Swenson‡ and Anna Martinelli†* †Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 41296 Gothenburg, Sweden ‡Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden

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ABSTRACT

Protic Ionic Liquids are known to form extended hydrogen bonded networks that can lead to properties different from those encountered in the aprotic analogous liquids, in particular with respect to structure and transport behavior. In this context, the present paper focuses on a wide series of 1-alkyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [HCnIm][TFSI], with the alkyl chain length (n) on the imidazolium cation varying from ethyl (n = 2) to dodecyl (n = 12). A combination of several methods such as vibrational spectroscopy, wide angle X-ray scattering (WAXS), broadband dielectric spectroscopy and 1H NMR spectroscopy is used to understand the correlation between local cation-anion coordination, nature of nanosegregation and transport properties. The results indicate a propensity of the -NH site on the cation to form stronger H-bonds with the anion as the alkyl chain length increases. In addition, the position and width of the scattering peak q1 (or the pre-peak), resolved by WAXS and due to the nanosegregation of the polar from the non-polar domains, is clearly dependent on the alkyl chain length. However, we find no evidence from PFG-NMR of a proton motion decoupled from molecular diffusion, hypothesized to be facilitated by the longer N–H bonds localized in segregated ionic domains. Finally, for all protic ionic liquids investigated the ionic conductivity displays a Vogel-FulcherTammann dependence on inverse temperature, with an activation energy Ea that also depends on the alkyl chain length, although not strictly linearly.


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1. Introduction Ionic Liquids (ILs)1 can be designed to an enormous variety of types, in theory up to 1018 different types,2 by varying the molecular structure of the constituting cations and anions. Despite this tremendous opportunity there still exist challenges with respect to finding the most appropriate cation-anion combination for each specific application, which may be in catalysis, electrochemistry, separation processes, lubrification or green chemistry.3-6 In order to select such an appropriate cation-anion pair, it is essential to better understand the relation between structure and physicochemical properties that include, but are not limited to, the nature and strength of intermolecular interactions. In general, ILs can be divided into two major categories, i.e. protic (PILs) and aprotic (AILs) ionic liquids, the latter being by far the ones most intensively investigated. By contrast, the interest in PILs is more recent and related studies have mainly explored the physicochemical properties of those based on imidazolium, ammonium, or pyrrolidinium cations.7 PILs have an exchangeable proton, typically on the cation, that can form hydrogen bonds to one electronegative atom of the anion (e.g. N, O or F). The formation of such an H-bonded network is the prerequisite for proton hopping and hence for a long-range proton transfer decoupled from molecular diffusion, also known as the Grotthuss mechanism.8 Zentel et al.9 have studied the structure and dynamics of Ethylammonium Nitrate (EAN), focusing on the hydrogen bonded network, through a combination of experimental and theoretical methods that include molecular dynamic simulations, infrared and NMR spectroscopy. The occurrence of H-bonding was studied in the frequency region where N-H stretching modes contribute, i.e. in the range 2800-3250 cm-1, finding that EAN creates hydrogen bonds of moderate strength. Campetella et al.10 have investigated the effect of the alkyl chain length in three


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ammonium based PILs using ab initio molecular dynamics simulations, showing that H-bonds are stronger and more stable with longer chains. In a more recent work, Moschovi and co-workers11 have investigated the effect of the alkyl chain length in PILs based on the alkyl-imidazolium cation ([HCnIm][TFSI], n = 0–12) on the nature of intermolecular interactions. Infrared spectroscopy revealed, through a red shift of the N-H stretching frequency (3250 – 3270 cm-1), that the strength of H-bonds tends to increase with n. The nanostructural features in the PILs alkylammonium nitrate (with n < 4, ethyl and propyl) were studied by Atkin and Warr12 using small angle neuron scattering (SANS) techniques. Their results demonstrate that even for n < 4 nanosegregation occurs due to the electrostatic interaction between cations and anions and the concomitant solvophobic effect. Finally, Garaga et al.13 have revealed

the

presence

of

nanostructuration

in

the

PIL

octylimidazolium

bis(trifluoromethanesulfonyl)imide ([HC8Im][TFSI]) by wide angle X-ray scattering (WAXS). The segregation of the non-polar domains from the ionic regions was found to be very similar to that observed in the aprotic analogous C1C8ImTFSI, but with stronger scattering intensities due to the increased polarity in the ionic domains. To summarize, the influence of the alkyl chain length on intermolecular interactions and nanostructuration in PILs has been explored to some extent, however there is still a lack of knowledge concerning the correlation between nanostructure and transport properties. With the aim to fill this gap, this paper focuses on explaining how the nanostructuration observed in imidazolium based PILs is driven by the nature of intermolecular interactions and, in turn, affects the transport properties. Attention is thus paid to the self-diffusion of ionic species, viscosity and ionicity, and the occurrence of proton hopping is also investigated. More specifically, the protic ionic liquid series [HCnIm][TFSI] with n varying from 2 (ethyl) to dodecyl (12) is studied, the


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TFSI anion being the common anion. Ionic conductivity, viscosity and density data are used to estimate ionicity, while vibrational and 1H NMR spectroscopy methods are employed to study the nature of interactions established between cations and anions and to estimate self-diffusion coefficients. The phase behavior of these PILs is investigated by calorimetric methods that allow identifying characteristic temperatures related to crystallization (Tc), melting (Tm) and glass transition (Tg).

2.

Experimental Section Materials. The PILs alkylimidazolium bis(trifluoromethylsulfonyl)imide ([HCnIm][TFSI], n: 2,

4, 6, 8, 10, and 12) were custom made by, and purchased from, Io.Li.Tec (Ionic Liquids Technologies GmbH). The molecular structure of the TFSI anion as well as of two imidazolium cations chosen as representative cases are shown in Figure 1.

Figure

1.

Molecular

structure

of

the

cations

ethylimidazolium

([HC2Im+])

and

dodecylimidazolium ([HC12Im+]), and of the [TFSI-] anion.


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All the PILs are in the liquid state at room temperature and were stored in the inert atmosphere of a nitrogen-filled glove box prior to being investigated. The water content was determined using a Karl Fischer coulometric titration instrument from Mettler Toledo, and is reported in Table 1 together with other relevant properties. Given in mole fraction, the water content is below 0.03 in all PILs. Thermogravimetric analysis and Differential Scanning Calorimetry. The thermal stability of the PILs was determined by thermogravimetric analysis (TGA) using a Mettler Toledo thermogravimetric analyzer, scanning the temperature interval 25 – 400 °C at a heating rate of 10 °Cmin-1 and using air at a rate of 20 mLmin-1. Samples with a mass between 3 and 20 mg were placed into an open 100 µL aluminum crucible. Tonset is here defined as the intercept of two lines: the baseline at which the mass loss is zero and the tangent to the data recorded at high temperatures where the mass loss is large.14 An illustration of how Tonset is extrapolated is given in Figure S1 of the SI document. Differential scanning calorimetric (DSC) measurements were performed using a Mettler Toledo DSC2 calorimeter equipped with a HSS7 sensor and a TC-125MT intracooler. Samples of a mass between 3 and 5 mg were placed in closed 70 µL aluminum crucibles with a hole on the lid. The cooling and heating rates were set to 10 and 5 °Cmin-1 respectively, while the covered temperature window spanned from -100 °C to +60 °C. The melting (Tm), the crystallization (Tc) and the glass transition (Tg) temperatures could be determined from the recorded DSC curves. For the purpose of this paper, however, only the second heating scan is reported and discussed. For completeness, the second cooling and heating scans for all PILs here investigated are provided in Figure S2 of the SI document.


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Conductivity measurements. The conductivity data were obtained using a Broadband Dielectric Spectrometer from Novocontrol GmbH. The PILs were placed between two gold-plated brass electrodes of 13.5 mm in diameter. The thickness of each sample was controlled to be 1 mm using a spacer of silica. Measurements were conducted in the frequency range of 10-1 – 107 Hz while the temperature was varied between -60 oC and 250 oC, during both heating and cooling scans. The temperature was controlled using a nitrogen gas cryostat, with a stability of ± 0.5 oC. Conductivity was measured every 10 oC, and the stabilization time at each temperature was set to 600 seconds.

Vibrational Spectroscopy. Infrared spectra were collected at room temperature with a Perkin Elmer spectrometer in the Attenuated Total Reflection (ATR) mode, pouring the liquids over a single reflectance diamond crystal. 32 scans were recorded covering the spectral range 400 - 4000 cm-1, achieving a spectral resolution of 2 cm-1. An example of the peak fitting approach used to model the frequency region of C-H and N-H stretching modes is given in Figure S3 of the SI document. In addition, Figures S4 shows the regions of C-H stretching in the alkyl chains, while Figure S5 shows the region of SO2 stretching that is sensitive to the anion’s conformation (i.e. cis or trans). Raman spectra were recorded at room temperature with an InVia Reflex Renishaw spectrometer, using the 785 nm line as the excitation source. The laser was set to a power of 3 mW at the sample, and a grating with 1200 lines/mm was used. Spectra were recorded covering the frequency range 80 – 4000 cm-1, accumulating 10 scans with a duration of 10 s each. A 50x Leica objective with a numerical aperture of 0.50 was used. The Raman spectrometer was calibrated before all measurements to the first order mode of a Si wafer at 520.6 cm-1. An example of the


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peak fitting approach used to model the Raman feature at 743 cm-1 is given in Figure S6 of the SI document.

X-ray Scattering. Small and wide-angle X-ray scattering measurements were performed in the Chalmers Material Analysis Laboratory (CMAL) using a Mat:Nordic SAXS/WAXS/GISAXS instrument from SAXSLAB. The samples were loaded in capillary quartz tubes of 0.15 mm thick walls, and the top aperture was sealed with wax. The q-range 0.07 – 2.2 Å-1 was covered, and all measurements were performed at ambient conditions (i.e. at 25 °C). The q-range was calibrated using silver behenate (AgBeh) as a reference. An example of the peak fitting approach used to model the scattering pattern obtained by WAXS is given in Figure S7 of the SI document.

Diffusion NMR. 500 microliters of each sample were loaded in standard 5 mm NMR tubes. Spectra were collected using a Bruker Avance 600 spectrometer at a magnetic field of 14.1 T equipped with a diffusion probe delivering a maximum gradient field of 1200 Gcm-1. The magnetic field was shimmed for each sample using Topspin's topshim routine and fine-tuned manually after allowing the samples 20 minutes to reach thermal equilibrium inside the probe at 25 °C. All chemical shifts are referenced to tetramethylsilane (TMS) using an external chemical shift reference and pulse calibration was performed. A Pulse Field Gradient Stimulated Echo (PFG-STE) pulse sequence was employed to determine the self-diffusion coefficients by fitting the decay of the echo signal with the Stejskal-Tanner expression15 2

I = I0 e−(γδG) D (∆−δ/3)

(1)

where I is the signal intensity, I0 the signal intensity of spin-echo at zero gradient, G the gradient strength, D the self-diffusion coefficient, δ the length of the gradient pulse, and ∆ the diffusion


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time. The applied linear gradient was varied in the range 0–550 Gcm-1, while the diffusion time ∆ and the pulse duration δ were set to 100 and 2 ms for all samples, respectively. Each experiment was based on 32 acquisitions with a relaxation delay of 15 seconds to ensure full relaxation and quantitative results.

Viscosity and density. The viscosity of all PILs was measured at 25 °C by a TA instrument Rheometer DHR3 model, using a cone and plate geometry with a ∅ = 40 mm, an angle of 1° and a truncation gap of 26 µm. All measurements were conducted at atmospheric pressure. Density values were measured at 25 °C with an Anton Paar DMA 5000 digital vibrating U-shaped tube densitymeter. The measuring cell is equipped with a built-in thermostat for maintaining the temperature constant within 0.01 oC. The viscosity and density values are summarized in Table 1.

Table 1. Water content (in mole fraction, , and weight percent) and other properties measured at 25 °C for the PILs [HCnIm][TFSI]. Chain Water Water Conductivity length content content

Self-diffusion

Peak q1

Density Viscosity

(n)

()

(wt %)

(mS/cm)

(10-11 m2/s)

(Å-1)

(g/cm3)

(Poise)

2

0.029

0.142

2.441

2.511

-

1.56913

0.549

4

0.012

0.056

0.806

1.305

0.508

1.47194

0.680

6

0.013

0.053

0.600

0.830

0.416

1.40242

1.090

8

0.008

0.032

0.380

0.583

0.344

1.34633

1.555

10

0.025

0.098

0.238

-

0.298

1.28754

1.801

12

0.023

0.081

0.197

0.327

0.258

1.26068

2.333


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3. Results and Discussion Thermal properties. The phase behavior of the PILs [HCnIm][TFSI] was investigated by DSC, as shown in Figure 2a. Only the PILs with very short (n = 2) or very long (n = 10, 12) alkyl chains have clear solid-liquid transitions while for intermediate chain lengths (n = 4, 6, 8) the liquids behave as stronger glass formers. These results are in line with those previously reported by Moschovi and co-workers.11 However, the glass transition temperature Tg is observed in all liquids, with values that increase with the alkyl chain length, Figure 2b. This behavior is qualitatively similar to that observed by Molecular Dynamic simulations as reported by Khabaz and coworkers16 and by calorimetric methods as reported by Rodrigues et al.17 for the aprotic IL series [CnC1Im][TFSI]. The trend that we find for the protic series, however, seems to depend more smoothly on n.


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Figure 2. a) DSC heating scans recorded for the [HCnIm][TFSI] series of PILs and b) the glass transition temperature, Tg, as a function of the alkyl chain length n. The dashed line in b) is a simple guide to the eye.

The thermal stability of the PILs has been investigated by thermogravimetric analysis, which shows chemical decomposition temperatures around 350 oC, Figure 3a. This thermal stability, however, is not significantly affected by n as demonstrated by the Figure 3b. It is notable that the small but detectable mass loss observed up to Tonset is much greater than the measured water content, wherefore we attribute it to proton back-transfer events (from the imidazolium cation to


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the anion)18, 19 that result in volatile neutral species.20 These events reflect the chemical (in)stability of PILs under increased temperatures and hence represent one possible limitation for use as electrolytes/proton conductors. To better understand the dependence of Tonset and Tg on the alkyl chain length, the nature of local interactions has been investigated, as discussed in more details below.

Figure 3. a) Thermogravimetric curves for the PILs [HCnIm][TFSI]. b) Tonset values as a function of the alkyl chain length, n.

Cation-anion interactions. The infrared spectra recorded at room temperature for the PILs are presented in Figure 4a, showing the region where the aromatic C–H (3000–3200 cm-1) and the N– H (above 3200 cm-1) stretching modes are found. The spectral region where the C–H stretching modes in alkyl chains typically fall (i.e. the range 2800–3050 cm-1) is shown in Figure S4, which reveals a red shift of the frequencies as a function of n. This is typical of long chain molecules (e.g. alcohols or aprotic ionic liquids)21 due to close chain proximity. The assignment of the highest frequency mode to the N–H stretch has been demonstrated in previous works, with reported values for this vibration between 3250 and 3270 cm-1.11, 22, 23 In agreement with these values, our peak fit


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analysis of the recorded spectraa shows N–H frequencies in the range 3255–3270 cm-1. Considering the low water content in the PILs investigated (see Table 1) the contribution of O-H stretching modes from H2O in this frequency region falls below detectability.

Figure 4. a) Infrared spectra recorded at room temperature for the PILs [HCnIm][TFSI]. Inset: change of the N-H stretching frequency as a function of the alkyl chain length, n. b) Wavenumber of the expansion-contraction mode of the whole TFSI molecule as a function of alkyl chain length.

As shown in the inset of Figure 4a, the frequency of the N–H stretching mode red shifts with n, indicative of a slight but measurable elongation of the N–H bond. Since it is established that in protic ionic liquids based on imidazolium cations the –NH site is the most favorable to form hydrogen bonds with the anion,24, 25 the trend reveals that –NH+  TFSI– hydrogen bonds become stronger in PILs with longer chains. This agrees with the findings reported in some previous

a

Using a linear background and Gaussian type functions in the Igor Pro 6.37 software, see also Figure S3.


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works.11, 26 It is also interesting to note that the red shift is most evident for values of n up to 6, with marginal changes for chains longer than the hexyl, a trend that has previously been observed for other properties of aprotic alkylimidazolium-based ILs, such as the volumic heat capacity,27 the self-diffusion coefficient28 and the local rotational motion of ions.21 Additional information comes from the infrared range 1250–1400 cm-1 that is sensitive to the conformation adopted by the TFSI anion (i.e. cis or trans) through the intensity of the SO2 stretching modes,21 see Figure S5. The decrease in intensity of the lower frequency component with n reveals a decrease of the cis/trans ratio, which is more pronounced up to n=6 as previously also observed by Moschovi et al.11 The state of the TFSI anion has been also investigated by Raman spectroscopy, analyzing the frequency of the expansion-contraction mode of TFSI observed at ~743 cm-1, the most intense Raman mode in TFSI containing compounds. This mode has strong contributions from the symmetric bending modes of the CF3 group22 and is sensitive to the strength of interaction with the chemical surrounding. The measured spectra do not indicate a significant dependence of this frequency on the length of the alkyl chain, Figure 4b, and reveal that the proximity of the TFSI anion to the cation is unchanged throughout the [HCnIm][TFSI] series. To rationalize this finding (i.e. a constant Raman shift for the TFSI anion at ~743 cm-1 along with the red shift of the N–H stretching mode) we propose the occurrence of a reorientation that changes the relative position of cations and anions. From previously published studies it is well known that hydrogen bonds have an important yet complex role in ionic liquids,29-35 and that the TFSI anion is one of the most complex anions to investigate since, due to its bulkiness and charge delocalization, it is able to form multiple but very weak hydrogen bonds with both aromatic and aliphatic –CH sites, mainly through its O atoms.25, 36 In addition, the TFSI anion can adopt two conformational forms, known as the cis and the trans isomers.37 In the specific case of protic ionic liquids, the polar -NH site on


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the imidazolium becomes the most probable for interactions (much stronger than the C2H site in aprotic cations).24,25 However, also in these protic ionic liquids multiple imidazolium-TFSI configurations must be considered, the linear –NH+  O– hydrogen bonds not being exclusive.24 Hence, our hypothesis is that while for short chain cations (e.g. n = 2) the TFSI forms hydrogen bonds with the –NH group but to some extent also with the ethyl and methyl groups of the side chain, for longer chain cations (i.e. n > 6) the front type of imidazolium-TFSI orientation will be more probable, in particular as a consequence of the closer chain-chain proximity that competes with the co-presence of the anion. In other words, we propose that the probability of finding the TFSI in a linear hydrogen bonded configuration with the –NH of the cation increases with the chain length (although it is already significant with short chains). This hypothesis is illustrated in Figure 5. As we will discuss further down, this hypothetic orientation change finds some support from X-ray scattering data in the q-range of local ion-ion alternations, but we also recognize that future molecular dynamic simulations could be very useful in this context.

Figure 5. Our hypothesis of a cation–anion relative position change with n, here shown for the representative cases of a) ethylimidazolium–TFSI (n = 2) and b) octylimidazolium–TFSI (n = 8). We speculate that in the case of a short chain cation hydrogen bonds are formed with both the – NH and other H containing sites (e.g. aromatic and aliphatic CH sites), while for long chain cations the imidazolium–TFSI interactions involves stronger –NHTFSI hydrogen bonds. In this picture,


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we also include the fact that the population of the trans isomers increases with n, although cis and trans always co-exist in the liquid state of these investigated ionic liquids.

Figure 6. Wide angle X-ray scattering data for the PILs [HCnIm][TFSI], with n varying from 2 to 12.

Nanostructure. The wide-angle X-ray scattering (WAXS) patterns recorded for the PILs [HCnIm][TFSI] are reported in Figure 6 and show three distinct peaks. The first peak is observed in the q range 0.2–0.6 Å-1 (pre-peak or peak I) with intensity and position that clearly depend on n. The second peak is detected in the q range 0.7–0.9 Å-1 (peak II); and the third in the q range 1.2–1.6 Å-1 (peak III). In line with the work of Annapureddy et al.,38 who studied the origin of the pre-peak and other peaks in X-ray scattering spectra of imidazolium based aprotic ionic liquids using MD simulation, we assign the pre-peak or peak q1 to a longer separation distance between charged groups by the alkyl chain attached on the cation. Also, the intermediate peak or peak q2 that appears between 0.8 and 0.9 Å-1 is described as the distance between polar groups of the same charge (e.g. to anion-anion and cation-cation distances).


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In the present work, q2 is peaked at a higher q value (q = 0.86 Å-1) for n = 2 but shows a rather constant peak position (at approximately 0.83 Å-1) for the wide range from n = 6 to n = 12. By contrast, peak III shows very little dependence on n. Peak I, on the other hand, gets sharper and more intense as n increases. This behavior is similar to what has been observed for the aprotic IL series [C1CnIm][TFSI],28,39,40 where the q value for the second peak in n = 2 was 0.89 Å-1 while it had a rather constant q value, of about 0.85 Å-1, for n = 4 up to n = 12.39 Hence, in analogy to the aprotic ionic liquid series, also in the case of protic imidazolium cations the formation of a nanosegregated structure is observed already for the butyl chain, and despite the liquid state. To gain further insights into these structural features the scattering data have been fitted using a cubic background and Lorentzian functions, consistent with the approach of previous X-ray studies.28, 40, 41 This allows estimating, along with a more precise position of the scattering peaks, the size of the scattering domains and the dispersion of the correlation lengths associated to the nanostructure. The peak fit analysis was not employed to the data of [HC2Im][TFSI] due to the extremely weak intensity of peak I.


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Figure 7. a) Position of the diffraction peaks qn and b) size of the repeating distances dn. c) Full width at half maximum of the first diffraction peak q1 as a function of the alkyl chain length. The error bars in a) and b) are smaller than the symbols. The red dashed lines are fits to the experimental data.

Figure 7 summarizes the results from the peak fit analysis and reveals that while the shortrange domains (q2) are not strongly dependent on the length of the alkyl chain, the scattering peak arising from the non-polar domains of the cation (q1) shows a clear shift to lower q values, Figure


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7a, and a narrower shape, Figure 7c, as the alkyl chain is increased. As a consequence of these features, the size of the repeating units di, estimated from the Bragg’s diffraction law di = 2/qi, are clearly dependent on n for the case of d1, Figure 7b, as opposed to the case of d2 and d3. This behavior is analogous to that observed in aprotic ILs28, 39 and is also in good agreement with that observed in other alkylammonium based PILs.42 It reveals non-polar domains that increase in size linearly with n (at least up to n = 12) and tells that the protonation of the imidazolium cation at the N-site does not change the propensity to form nanosegregated domains. On the contrary, this tendency seems to be emphasized, as evidenced in a recent work where peak q1 appeared more intense in the case of [HC8Im][TFSI]13 as compared to the case of [C1C8Im][TFSI].39 This behavior, also known as the solvophobic effect,43, 44 can be attributed to the more polar character of the protonated imidazolium head.13 In addition, by fitting the d1 values given in Figure 7b with a line, we obtain the relation d1 (Å) = 6.17 + 1.52n, which tells that the nonpolar domains increase in size by 1.52 Å per CH2 unit. This value is comparable to, but smaller than, the one found for the analogous aprotic series by Martinelli et al. (1.7)28 and Russina et al. (1.96),39 respectively. An interesting additional information is obtained from fitting the dependence of the d2 values on n, see Figure S8. When these data are analysed in a close-up scale, it emerges that d2 does not follow a linear dependence on n, but rather increases up to n = 6 and then stays constant for higher values. This behaviour is consistent with other properties changing only up to n = 6, as a consequence of nanosegregation and the set in of structural heterogeneities on the nanometer scale. Figure S8 also reveals that when n is varied from 2 to 12, the d2 space increases by ca 0.4 Å, an apparently small number but not negligible in the context of inter-molecular interactions. Physically, this means that due to nanosegregation the overall same-charge repetition distance increases, which supports our hypothesis of a relative orientation change (as proposed in Figure 5) considering that a higher


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population of the front type imidazolium-TFSI configuration is consistent with an increased cation-cation distance.

Transport properties. Figure 8a shows the 1H NMR chemical shift of the NH nucleus moving from 10.99 to 11.51 ppm as n varies from 2 to 12. This shift is marginal for values of n greater than 6 and indicates the evolution of stronger hydrogen bonds involving the -NH site, in great agreement with the results obtained by infrared spectroscopy. One hypothesized consequence of the longer N-H bonds (infrared and 1H NMR) along with clear nanosegregated domains (WAXS) as n increases, is that the exchangeable protons on position -NH display a motion decoupled from the diffusion of the parent molecule. This situation is typically monitored by excess self-diffusion coefficients extracted from the 1H NMR resonance of the -NH nuclei, or in other words by a DNH/Dcation ratio significantly higher than one.13, 45 We have investigated whether this was the case, but could not detect any significant deviations from unity for the DNH/Dcation ratio, Figure 8b. This means that despite longer N-H bonds are established in the case of cations with longer chains, this is not enough to induce a decoupled, or Grotthuss type, motion of the exchangeable protons. Other strategies, such as for instance the addition of an amphoteric molecule to the PIL,13, 46 may thus be more appropriate for this purpose.


20

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Figure 8. a) 1H NMR chemical shift in [HCnIm][TFSI] and b) DNH and Dcation values as a function of alkyl chain length. The error bars are smaller than the size of the symbols.

Figure 9. a) Arrhenius plot of the ionic conductivity in [HCnIm]TFSI], including fitting curves using eq. (2) (i.e. the VFT behavior). b) Activation energy in HCnImTFSI as a function of alkyl chain length, where Ea is estimated assuming T0≠Tg and Ea’ is estimated assuming T0=Tg, the solid lines being simple guides to the eye. The error bars in both figures are smaller than the size of the symbols.


21


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The ionic conductivity data measured as a function of temperature are shown in an Arrhenius plot in Figure 9a. These data show a non-linear dependence on inverse temperature, which is a typical behavior in ionic liquids and ionic liquid derived material, such as gels or polymer electrolytes. This behavior is well described by the Vogel-Fulcher-Tammann (VFT) relation47 expressed as in eq (2): -B T - T0

σ = σ∞exp

(2)

where  is the ionic conductivity extrapolated for infinite temperatures, B is a parameter related to the curvature and to the activation energy Eab and T0 is a temperature close to the glass transition temperature Tg.47, 48 These parameters were obtained by fitting the experimental data with the VFT expression, eq (2), following two approaches: i) leaving T0 as a free parameter (T0 ≠ Tg) and ii) imposing T0 equal to the glass transition temperature found by DSC (T0 = Tg). The fit parameters so found are summarized in

b

Through B= Ea/kB, where kB is the Boltzmann’s constant


22

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Table 2. We see from both approaches that the activation energy increases with n, Figure 9b, but interestingly also that for n > 8 both Ea and Ea’ decrease slightly or at least do not increase at the same rate as for shorter chains. This may be a consequence of nanostructuration, which results in slowed down cations and segregated ionic regions where the anions may experience a facilitated motion. Indeed, the effect of nanomorphologies on the selective ionic motion has been demonstrated for related ionic liquid systems, e.g. those based on long-chained imidazolium and ammonium cations or anions.49-51 It is important to note, however, that the T0 values obtained when we use the T0 ≠ Tg approach, are greater than the Tg measured experimentally. This has little physical meaning since it would mean that conductivity goes to zero at a temperature above Tg. For this reason, we believe that the trend shown by Ea’ better describes our liquids. The ionic conductivity measured at temperatures higher than those presented in Figure 9a shows a significant and sharp drop. This occurs at 180 oC or above and is less pronounced in ILs with longer alkyl chain, which display higher maximum conductivity values at a higher temperature, see Figure S9 and Figure S10 in the Supporting Information document. Based on the fact that the investigated ionic liquids are truly dry, by which we exclude that the conductivity decrease is due to the loss of water, we attribute this behavior to proton back-transfer events that by forming neutral volatile species contribute negatively to the overall ionic conductivity. This finding is in qualitative agreement with the results from TGA, although one should keep in mind that conductivity and TGA experiments were not conducted at identical conditions, for instance with respect to the heating rate and the equilibration time. With respect to the intended use of these PILs as electrolytes in e.g. fuel cells, our results indicate an upper limit for the operational temperature, that here is identified to be 180 °C.


23


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Table 2. Fitting parameters obtained from using the VFT equation, eq. (2), to describe the dependence of ionic conductivity on inverse temperature. Fitting condition Chain length (n)

T0 ≠ Tg

T0 = Tg

σ0

B

T0

Ea

(mS/cm)

(K)

(K)

2

113.2

319

215

27

4

77.5

359

219

6

147.5

499

8

147.8

10 12

σ0’

B’

T0’

Ea’

(K)

(K)

(meV)

173.3

458

187

39

31

130.8

530

189

46

207

43

197.9

600

192

52

526

210

45

209.8

647

193

56

133.9

637

197

55

131.6

631

198

54

112.7

574

208

49

118.4

591

205

51

(meV) (mS/cm)

Among the PILs investigated in this study, [HC2Im][TFSI] exhibits the highest ionic conductivity, which then monotonically decreases with n, Figure 10a. This is a direct consequence of viscosity that increases with the length of the alkyl chains, Figure 10b.52 In addition, Figure 10b shows that viscosity is as expected higher in the PILs here investigated than in their aprotic analogous (as shown by the offset of the data), but display a steeper dependence on n. This steeper increase can be explained by the presence of hydrogen bonds in PILs, that has an additional effect to that of stronger van der Waals interactions53 and larger molecules as n is increased.


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Figure 10. a) Ionic conductivity measured at 25 °C as a function of n (standard deviation equal to 3.3%) and b) apparent viscosity in [HCnIm][TFSI] and [C1CnIm][TFSI]54 as a function of n, both measured at 25 °C. The dashed lines are simple guides to the eye.

From the measured conductivity, density and viscosity data, we could estimate the ionicity of the PILs [HCnIm][TFSI] from a Walden plot,55 as shown in Figure 11. The ionicity values fall below the ideal line, which is the typical case for a certain degree of ionic association and/or the presence of undesired neutral species in the liquid. Nevertheless, the estimated ionicity is comparable to that of other related PILs, e.g. those based on the trialkylphosphonium cation,56 also shown in the plot. Because conductivity changes more rapidly with n than viscosity does, the ionicity shows a deviation from the ideal line as the alkyl chain length increases, see arrow in Figure 11. This seems to also be the case of the PILs triethylalkylphosphonium-TFSI and trialkylphosphonium-TFSI, previously investigated by other authors.56, 57


25


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Figure 11. Walden plot for the PILs [HCnIm][TFSI] at focus in this study (●). For comparison, the ionicity of other related ILs is also shown: i.e. Triethylaklylphosphonium TFSI (△),57 and Trialkylphosphonium TFSI (◇).56

Figure 12. Tg-scaled Arrhenius plot of the ionic conductivity of [HCnIm][TFSI] (n = 4, 6, 8). The dashed line is best fit through eq. (3) to the conductivity data.

In order to get some deeper insight on the mechanism of ionic conduction, a Tg-scaled Arrhenius plot58 has been prepared, only treating the data of those ionic liquids that in the DSC curve lack of crystallization/melting transitions, Figure 12. This figure reveals that although different in an


26

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absolute temperature axis, when scaled to Tg the conductivity values fall onto one single curve, emphasizing that the conduction of ions is strongly and predominantly related to viscosity (through Tg). On a Tg-scaled Arrhenius plot, it is also possible to distinguish liquids according to their fragility, with ‘strong’ and ‘fragile’ materials manifesting a close to linear or curved dependence on Tg/T respectively.58, 59 By using the modified VFT equation: D.T0

ln σ = ln σ0 - Tg T

(3)

- T0

to fit our experimental conductivity data, the parameter D, also called the ‘strength factor’,60 could be estimated. We find that D increases with n, from a value of 1.47 for n = 2 to a value of 2.65 for n = 12. This is a moderate change compared to the wider range of possible values that can vary by one order of magnitude or more, but the small values found allow classifying these PILs as fragile. This was also elucidated in a broader context by Sippel et al.61 who also emphasized the advantage with respect to ionic conductivity of ionic liquids that combine high fragility with a low Tg.

4. Conclusions This paper focuses on a series of PILs with a common anion, i.e. [TFSI], and the alkylimidazolium cation [HCnIm] with the alkyl chain varying from ethyl (n=2) to dodecyl (n=12). The aim was trying to correlate local structure and interactions with transport properties. The study shows that the solvophobic effect is manifested also in these PILs, with a nanostructuration that is strengthened by the hydrogen bonds established in the ionic regions. As a consequence, cations are progressively slowed down, but a decoupled proton motion is not observed for any chain length. All measured properties, e.g. the strength of the N-H bond, the ionic conductivity and the


27


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FWHM of peak q1, show the same dependence on n, that is a rapid change up to n=6 and less marked changes for longer chains (for which nanostructuration is already set). This demonstrates a clear correlation between local structure and macroscopically measured transport properties. Judging from their thermal behavior, these PILs are chemically stable up to 180 °C, but beyond this temperature proton back-transfer events seem to occur, which impose an operational limit if intended to be used as electrolytes in proton exchange fuel cells. To summarize, our results indicate that other strategies than only varying the alkyl chain length are needed to achieve a long-range proton (H+) motion, many times argued to be an advantage of PILs in general. One strategy could be the addition of a co-solvent able to form hydrogen bonds to both the cation and the anion. Desirably this solvent will be able, although added at low molar fractions, to also improve the thermal stability of the liquid thus enabling high ionic conductivities at high temperatures without loss of ionic species.

ASSOCIATED CONTENT Supporting Information The Supplementary Information document contains the following: The method used to determine Tonset from TGA curves. The full second DSC cycles including both cooling and heating scans. Extended regions of the infrared spectra, showing the peak fit analyses and the wavenumber change with n of the C–H stretching modes of the alkyl chains. Examples of the peak fit analyses applied to Raman spectra and X-ray scattering data. Change of d2 as a function of n, in a close-up scale. Arrhenius plots of the conductivity displayed by all ionic liquids in an enlarged temperature window. Dependence on n of the temperature where the maximum conductivity (max) is reached.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID: Anna Martinelli: 0000-0001-9885-5901

Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions

Funding Sources Funding support from the Knut and Alice Wallenberg Foundation (Wallenberg Academy Fellows grant 2016-0220), the Swedish Foundation for Strategic Research (SSF, FFL15-0092) and from Formas (2016-01189) is kindly acknowledged.

Notes Any additional relevant notes should be placed here.


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ACKNOWLEDGMENT The authors also thank Prof. Johan Bergenholtz (Gothenburg University) and Dr. Anna Ström (Chalmers) for helping with the density and viscosity measurements. The Swedish NMR Centre in Gothenburg is kindly acknowledged for the use of the NMR spectrometers.

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Protic Ionic Liquids Based on the Alkyl-Imidazolium Cation: Effect of

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