Thermal and Structural Studies of Imidazolium-Based Ionic Liquids

Article pubs.acs.org/JPCB

Thermal and Structural Studies of Imidazolium-Based Ionic Liquids with and without Liquid-Crystalline Phases: The Origin of Nanostructure Fumiya Nemoto, Maiko Kofu, and Osamu Yamamuro* Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, Japan 277-8581 ABSTRACT: To clarify the origin of the nanostructure of ionic liquids (ILs), we have investigated two series of ILs 1-alkyl-3-methylimidazolium hexafluorophosphate (CnmimPF6, n = 4−16, n is an alkyl-carbon number) and 1-alkyl-3methylimidazolium chloride (CnmimCl, n = 4−14) using differential scanning calorimetry and X-ray diffraction techniques. The PF6 samples with n > 13 and the Cl samples with n > 10 exhibited the liquid-crystalline (LC) to liquid (L) phase transitions, as reported before. We found that both samples with smaller n also exhibited the LC to L transitions under supercooled states as far as the ionic motions were not frozen-in at the glass transition temperatures Tg. The Tg of the LC phase was close to that of the L phase, indicating that the characteristic length of the glass transition is shorter than that of the nanostructure. A low-Q peak due to the nanostructure in the L phase and a diffraction peak due to the layer structure in the LC phase appeared at almost the same Q positions in both samples. On the basis of the above results and some thermodynamic analysis, we argue that the nanostructures of ILs are essentially the same as the layer structures in the LC phases.

1. INTRODUCTION The ionic liquid (IL) is one of the most remarkable research subjects in current liquid science. ILs are intensively studied for the sake of various applications utilizing their low melting temperature, negligible vapor pressure, nonflammability, high ionic conductivity, amphiphilicity, high designability, and so on. Actually, green solvents, electochemical materials, and actuators using ILs are being put to practical uses recently. The most typical and intensively studied ILs are imidazolium-based ones, including our target materials 1-alkyl-3-methylimidazolium salts. They are usually abbreviated as CnmimX, where n is the alkyl-carbon number and X the anion. Recently, basic scientific studies of ILs are also in great progress. In particular, the structural investigations for imidazolium-based ILs with n < 10 have been performed by using X-ray and neutron diffraction techniques.1−10 Among these studies, a characteristic peak, which is called the low-Q peak in this paper, appeared at ∼0.3 Å−1 in addition to the peaks at ∼1 Å−1, corresponding to the local correlation among ions, and at ∼1.5 Å−1 among alkyl chains.3−10 These studies indicate that imidazolium-based ILs have some higher-order structures on the nanometer scale, which is called the nanostructure in this paper. The molecular dynamics (MD) simulations6,11−14 demonstrated that the nanostructures consist of aggregations of the charged parts of cations and anions and the neutral parts of alkyl chains. Our neutron diffraction work8 revealed that the low-Q peaks drastically grow upon cooling. Despite the many structural studies, the texture of the nanostructure, for example, a layer structure or micellar structure, is still unknown. © 2015 American Chemical Society

On the other hand, for imidazolium-based ILs with n > 10, differential scanning calorimetry (DSC), X-ray and neutron diffraction measurements,15−21 and MD simulations22,23 have shown that they exhibit the liquid-crystalline (LC) phases. The LC phase is a smectic A (SmA) phase with layer structures. The position of the lowest-Q diffraction peak in the LC phase, corresponding to the layer distance, is similar to that of the lowQ peak in the liquid (L) phases, suggesting the similarity of both phases.20 The main purpose of this study is to clarify the origin of the low-Q peak and nanostructure in imidazolium-based ILs by constructing their n−T phase diagrams (n: alkyl-carbon number). It is a related interest to investigate the n dependence of the glass transitions where the structure is frozen-in. We have performed the DSC and X-ray diffraction (XRD) measurements for CnmimPF6 (n = 4−16) and CnmimCl (n = 4−14).

2. EXPERIMENTAL SECTION 2.1. Samples. C4mimPF6 was purchased from Kanto Chemical Co. Inc. C6, C8, C10, C12, C14, and C16mimPF6 were from Iolitec Inc. The purities of C6 and C8mimPF6 were claimed to be 99%, and C10, C12, C14, and C16mimPF6 were above 98%. Other than the pure samples shown above, IL mixtures Cn*mimPF6 were measured not only as alternatives to the unobtainable samples with odd n but also to stabilize the supercooled states. The measured IL mixtures were as follows: Received: February 2, 2015 Revised: March 16, 2015 Published: March 19, 2015 5028

DOI: 10.1021/acs.jpcb.5b01080 J. Phys. Chem. B 2015, 119, 5028−5034

Article

The Journal of Physical Chemistry B

avoid the crystallization, C10 and C13*mimPF6 were cooled/ heated at 100 °C min−1, C11*mimPF6 at 20 °C min−1, and C12*mimPF6 at 70 °C min−1. After subtracting the baselines, the heat flow values were normalized to the value per 1 g of sample quantity and at a 5 °C min−1 heating rate. The LC to L phase transitions appeared in the samples from C10 to C16, as shown by green lines. We have confirmed the LC phase of C16mimPF6 by polarized optical microscopy (see Figure 2).

C9*mimPF6 (0.5C8 + 0.5C10), C9.5*mimPF6 (0.25C8 + 0.75C10), C11*mimPF6 (0.75C10 + 0.25C14), C12*mimPF6 (0.5C10 + 0.5C14), C13*mimPF6 (0.25C10 + 0.75C14), C14*mimPF6 (0.5C12 + 0.5C16), and C15*mimPF6 (0.5C14 + 0.5C16). The numbers in the parentheses denote the mole fractions of each component. All of the PF6 samples were used without further purifications. C4mimCl was purchased from Kanto Chemical Co. Inc. C6mimCl was from Tokyo Chemical Industry, Co., Ltd. C8, C10, C12, and C14mimCl were from Iolitec Inc. The purities of C6, C8, C10, C12, and C14mimCl were above 98%. For the Cl samples, the measured IL mixtures were as follows: C9*mimCl (0.5C8 + 0.5C10) and C11*mimCl (0.5C10 + 0.5C12). C8, C9*, C10, C12mimCl were dried under vacuum for 16 h at 60 °C and 2 h at 80 °C because they are hygroscopic. The other Cl samples were used without further purifications. 2.2. DSC. The DSC measurements were performed using a PerkinElmer Diamond DSC. The samples were sealed in aluminum pans. The mass of the samples used was 4−16 mg. The measured temperature range was between −150 and 200 °C. The normal scanning rate of this experiment was 5 °C min−1. In some cases, however, faster scanning rates as high as 100 °C min−1 were chosen to avoid crystallization. 2.3. XRD. The XRD measurements were performed using a powder diffractometer Rigaku Ultima III. The temperature was controlled with a custom-built high-temperature stage (room temperature to 150 °C) and cryostat (−270 to 50 °C). The precision of temperature control was better than 0.1 °C. A reflection geometry with a narrow parallel beam of 0.1 mm width was employed to obtain high-quality data in a low-angle region. The scanning angle range was 1 < 2θ < 30°. The sample thickness was 1 mm.

Figure 2. Polarized optical microscope photograph for C16mimPF6 at 123 °C. P and A are the directions of the polarizer and analyzer, respectively.

3. RESULTS AND DISCUSSION 3.1. DSC. Figure 1 shows the DSC curves of the PF6 samples upon heating. The cooling/heating rate was 5 °C min−1 for C4, C6, C8, C9*, C14, C15*, and C16mimPF6. To

The fan texture is characteristic of the SmA LC phase. On the other hand, glass transitions appeared in the samples from C4 to C9*, as shown by blue lines. In pure C12mimPF6, the L to LC transition could not be observed due to the crystallization even at a cooling rate of 100 °C min−1. The endothermic peaks whose intensities are much larger than those of the LC to L transition are due to the melting. In C4mimPF6 and C10mimPF6, the exothermic peak due to the crystallization appeared in a heating direction. Endothermic peaks below the melting temperatures in C14mimPF6 and C16mimPF6 are due to the phase transitions between crystalline phases. Figure 3 shows the DSC curves of the Cl samples upon heating. All samples were cooled and heated at 5 °C min−1. The

Figure 1. DSC curves for CnmimPF6 (n = 4−16). The LC to L phase transitions and glass transitions are indicated with green and blue colors, respectively. The heat flow values are normalized to the values per 1 g of sample quantity and at a 5 °C min−1 heating rate.

Figure 3. DSC curves for CnmimCl (n = 4−14). The LC to L phase transitions and glass transitions are indicated with green and blue colors, respectively. The heat flow values are normalized to the values per 1 g of sample quantity and at a 5 °C min−1 heating rate. 5029


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The Journal of Physical Chemistry B normalization of the data is the same as that for the PF6 samples. The LC to L phase transitions, shown by green lines, appeared in the samples from C9* to C14. Glass transitions, shown by blue lines, appeared in the samples from C4 to C10. The large endothermic peaks in C12mimCl and C14mimCl are due to the melting, and the small endothermic peak at ∼10 °C in C14mimCl is due to the phase transition among crystalline phases. The DSC curves of C9.5*mimPF6 are shown in Figure 4. In path (1) represented by blue lines, a glass transition from the

Figure 5. Alkyl-carbon number (n) dependences of transition temperatures for CnmimPF6 (n = 4−16). The symbols are explained in the text.

Figure 4. DSC curves for C9.5*mimPF6 and the schematic drawings for the Gibbs free energy curves to explain the thermal paths (1) blue curve and arrows and (2) red curve and arrows.

Figure 6. Alkyl-carbon number (n) dependences of transition temperatures for CnmimCl (n = 4−14). The symbols are explained in the text.

supercooled L phase to its glassy state occurred at (a) upon cooling at 5 °C min−1. Upon heating at 5 °C min−1, a glass transition (b), an irreversible transition from the supercooled L phase to the LC phase (c), and the melting of the LC phase (d) took place successively. This process is shown by the blue arrows in the schematic Gibbs free energy diagram (1). In path (2) shown by the red lines, the sample was annealed at −57 °C for 10 min to obtain the LC phase completely. Upon cooling at 5 °C min−1, the glass transition of the LC phase occurred at (a). Upon heating at 5 °C min−1, a glass transition of the LC phase (b) and the melting of the LC phase (d) occurred successively. This process is shown by the red arrows in the schematic Gibbs free energy diagram (2). It is noteworthy that in the IL mixtures, the phase and glass transitions took place clearly at single temperatures interpolated from the n-dependence of the transition temperatures shown later (Figures 5 and 6). This indicates that the IL mixtures were homogeneous and exhibited the interpolated thermodynamic properties. Similar effects have been found for the glass transitions in the IL mixture systems with two different cations24,25 and anions.25−27 It is also new evidence that the glass transition of the LC phase is almost the same as that of the L phase. This reason will be discussed later. The glass transition temperatures (Tg), the temperatures of the LC to L transition (Tc), and the melting temperatures of

the crystal (Tm) for all measured samples are listed in Tables 1 (PF6) and 2 (Cl). The values of Tc and Tm were determined as the onset of the endothermic peaks and that of Tg as the intersection of the DSC curves and bisector of the baselines above and below Tg on heating scans. The transition and melting enthalpies ΔH were determined from the area of the endothermic peak after subtracting the baseline. The transition and melting entropies ΔS were calculated from the relations ΔS = ΔH/Tc and ΔS = ΔH/Tm, respectively. ΔCp is the jump of heat capacity at Tg. The values of Tc, Tm, ΔH, and ΔS for the PF6 samples almost agree with the reported values.16,21,28−30 For the Cl samples, however, Tc, Tm, and ΔH for the melting were smaller than the previous values,7,15,18,20,28,30−33 while ΔH for the LC to L transitions almost agree with the previous values.20,32 This may be caused by some impurities such as waters, but it is not clear because of the absence of the detailed description for the samples and experimental conditions in the previous works. The transition entropies ΔS for the LC to L transitions were much smaller than ΔS for the melting of the crystals. As mentioned in the ref 19, alkyl chains in the LC phase may be melt and behave like in liquids. ΔCp increases with an increase of n. This tendency is consistent with the accurate data in the adiabatic calorimetry.7 These results indicate that the major 5030


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present study; that is, the ILs have locally ordered structures (nanostructures) even in the L phases. Figure 5 shows the alkyl-carbon number n dependence of the transition temperatures (n−T phase diagram) for the PF6 samples. Red circles indicate Tc of the LC to L transition; filled and open symbols denote the pure and mixture samples, respectively. Blue diamonds and squares represent the melting temperatures Tm’s to the LC and L phases, respectively. The Tm’s of the IL mixtures are not plotted because the phase separations occurred on the crystallizations. The green squares and diamonds represent the glass transition temperatures Tg’s in the L and LC phases, respectively; the filled and open symbols denote the data for the pure and mixture samples, respectively. The gray symbols denote the literature data of the melting temperatures for the samples with n < 4.37,38 Tc systematically increases with an increase of n for both the pure and mixture samples. The samples with n < 13 exhibit the LC to L phase transitions at Tc, which is lower than the Tm, implying that the LC phase exists in supercooled metastable states. Tg slightly increases with an increase of n and does not depend on whether the glass transition occurs in the L or LC phase. This indicates that the characteristic length of the glass transition, which may correspond to the cooperatively rearranging region (CRR),33 is smaller than the size of the layer structures of the LC phase, and the interaction dominating the glass transition is common for both L and LC phases. It should be noted that CnmimPF6 (n ≤ 8) do not have the LC phase because the molecular motions are frozen-in at Tg, which is above the hypothetical Tc of the L to LC phase transition. The n dependence of Tm is unusual compared with ordinary molecular crystals; Tm decreases for n < 6 and increases for n > 6 with an increase of n, implying that Tm has a minimum value at around n = 6, though Tm of C6mimPF6 could not be obtained experimentally because it did not crystallize even by a long annealing below hypothetical Tm. Similar phase diagrams have been reported in the experimental39 and MD40 works for different ILs. This common feature on the Tm curves of ILs is explained as follows. In the region n < 6, the distance between an imidazolium ring with a positive charge and an anion will increase with an increase of n. On the other hand, in the region n > 6, where the nanostructure is formed in the L phase, the ionic distance will not change much, and the interalkyl chain interaction will increase with an increase of n. Figure 6 shows the n−T phase diagram for the Cl samples; the symbols are common with the PF6 samples (Figure 5). The gray symbols denote the literature data of the melting temperature with n < 4.30 The Tm of the Cl sample decreases for n < 4 and increases for n > 12 with an increase of n. There is no Tm data in the region 4 < n < 12 because the liquid samples do not crystallize in this region. From the results of the PF6 sample and the previous works, we guess that the n−Tm curve of the Cl sample has a minimum at around n = 8, as shown by the dashed line. It is worth noting that the area of the LC phase in the Cl samples is much wider than that in the PF6 samples. This may be due to the stronger ionic interaction in the Cl samples that tends to stabilize the LC phase. The stronger ionic interaction should be related to the fact that the Tg line in the Cl samples is ∼30 °C higher than that in the PF6 sample. 3.2. XRD. The XRD measurements were performed to investigate the structures of the LC and L phases. Figure 7 shows the diffraction patterns of the LC and L phases for C16mimPF6 and C9.5*mimPF6. A low-Q peak in the L phase

Table 1. Thermodynamic Parameters for the Transitions of CnmimPF6 (n = 4−16)a n 4 6 8 9* 9.5* 10

11* 12 12* 13* 14 14* 15* 16

T (°C) Tm Tg Tg Tm Tg Tg Tc Tg Tc Tm Tg Tc Tm Tc Tc Tm Tc Tc Tc Tm Tc

9.1 −79.2 −75.8 −0.9 −73.4 −72.6 −45.1 −69.4 −29.0 35.2 −67.3 −4.9 53.0 16.6 39.8 66.4 72.1 62.8 97.0 74.3 123.1

ΔH (kJ mol−1)

ΔS (J mol−1 K−1)

20.9

73.5

ΔCp (J mol−1 K−1) 78.4 81.2

12.9

47.0 88.1 93.7

0.29

1.3

0.56 21.7

2.2 69.1

0.30 27.2 − − 31.5 0.71 0.30 0.51 36.5 0.82

1.1 82.9 − − 92.2 2.1 0.9 1.4 105 2.1

102

107

a Tm, Tc, and Tg denote the melting, LC−L transition, and glass transition temperatures, respectively. ΔH and ΔS represent the enthalpy and entropy of the transition (or melting), respectively. ΔCp is a jump of the heat capacity at the glass transition.

Table 2. Thermodynamic Parameters for the Transitions of CnmimCl (n = 4−14)a n 4 6 8 9* 10 11* 12 14

T (°C) Tm Tg Tg Tg Tc Tg Tc Tg Tc Tg Tm Tc Tm Tc

68.7 −42.4 −54.1 −51.2 −20.4 −51.1 −2.1 −46.0 61.1 −47.8 32.4 117.5 46.5 185.2

ΔH (kJ mol−1)

ΔS (J mol−1 K−1)

25.4

74.0

ΔCp (J mol−1 K−1) 89.8 95.7 100

0.16

0.6

0.17

0.6

0.28

0.8

29.2 0.74 30.1 1.3

94.7 1.9 92.9 2.9

112 118 125

a Tm, Tc, and Tg denote the melting, LC−L transition, and glass transition temperatures, respectively. ΔH and ΔS represent the enthalpy and entropy of the transition (or melting), respectively. ΔCp is a jump of the heat capacity at the glass transition.

component of the motions frozen at Tg is that of the alkyl chains. It is known that the entropies of the SmA LC to L transitions are 9−14 J K−1 mol−1 in cyanobiphenyl LCs34 and 15−17 J K−1 mol−1 in ferroelectric LCs.35,36 These values are much larger than ΔS = 0.5−3 J K−1 mol−1 for the LC to L transition of ILs. This indicates that the structural difference between the LC and L phases in ILs is much smaller than that in other liquid crystals. The reason should be related to the central issue of the 5031


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Figure 7. XRD patterns for C16mimPF6 and C9.5*mimPF6. The red curves represent the data of the L phases, while the blue ones are those of the LC phases.

and a stronger diffraction peak due to the layer structures in the LC phase were observed at almost the same positions, Q = 0.2 Å−1, for both samples. Similar results have been reported for C16mim[Otf] (Otf: trifluoromethanesulfonate).20 The broad peaks mainly corresponding to the ionic correlation and alkyl chain correlation appeared at around 1 and 1.4 Å−1, respectively. These peaks do not depend on the phases. In addition to these peaks, there are some higher-order peaks associated with the layer structure of the LC phase. The peak at around 0.1 Å−1 in C9.5*minPF6 may be due to some alternate layer structure between C8minPF6 and C10minPF6. Figure 8 shows the XRD results in the LC and the L phases for C10mimCl, C11*mimCl, and C12mimCl. They are essentially the same as the results in the PF6 samples. Figure 9 shows the XRD patterns at several temperatures near Tc for C16mimPF6. The peak position slightly and

Figure 9. Temperature dependence of the XRD patterns near the lowQ peak of C16mimPF6. The lower graph is enlarged in intensity by about 1000 times to show the low-Q peak in the L phase clearly.

continuously decreased with a decrease of temperature across Tc. This is a strong evidence suggesting the structural similarity between the L and LC phases. The intensity gradually increased with decreasing temperature in the L phase and drastically increased (by more than 100 times) at the L to LC transition. However, this jump is not due to the intrinsic structural property but due to the strong alignment effect characteristic to the LC phase. Therefore, the investigation on the peak intensity is left for the future diffraction work using bulk samples and a neutron beam with high transmission. 3.3. Origin of the Nanostructure. The present results for the nanostructures of ILs are (1) the existence of the LC phases in the supercooled state for ILs with n < 10 and (2) the similarity of the low-Q peaks in the L phase and the diffraction peaks in the LC phase. We argue that the nanostructures of ILs are essentially the same as the layer structures in the LC phases. The increase in peak intensity upon cooling in the LC or L phases (Figure 9) should be attributed to the growth of the layer structure. Here, we examine this conclusion based on the Gibbs free energy. The schematic drawings of the Gibbs free energies for (a) C16mimPF6 and (b) C8mimPF6 are shown in Figure 10. In the case of C16mimPF6, the L, LC, and crystalline phases appear in series following the lowest Gibbs free energy curve in a cooling direction. In the case of C8mimPF6, however, the L phase supercools over the melting temperature and is frozen-in at the glass transition temperature Tg. If the Tg was 20−30 K lower, the LC phase might be observed in a supercooled state. It is worth remembering that the transition entropy between the LC and L phases is small in ILs. The transition entropy is the slope difference in the Gibbs free energy curves at the transition temperature. Therefore, the small transition entropy provides the situation where the Gibbs energy difference between the L and LC phases does not become larger even at temperatures far above the transition temperature. This situation makes the structural fluctuation of the low-temperature phase survive in the high-temperature phase in a wide

Figure 8. XRD patterns for C10mimCl, C11*mimCl, and C12mimCl. The red curves represent the data of the L phases, while the blue ones are those of the LC phases. 5032



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-4-71363494. Fax: +81-4-7134-6069. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 10. Schematic drawings of the Gibbs free energy curves for (a) C16mimPF6 and (b) C8mimPF6. L, LC, and C denote the L, LC, and crystalline phases, respectively.

temperature range. In the case of ILs, the structural fluctuation of the LC phase, that is, the local layer structure, survives in the L phase. We argue that the nanostructure of ILs with n < 10 is a sort of structural fluctuation of the LC phase. There are several experimental results supporting our argument. Some LC materials (e.g., cyanobiphenyl-based molecules) with nematic (N) to L phase transitions exhibit the structural fluctuations of the N phase above the N to L phase transition temperatures.41 The entropies of these transitions34 are ΔS = 1−4 J K−1 mol−1, which is as low as those of the LC−L transition of ILs. The structural fluctuations have been reported not only in the N LCs but also in the SmA phase (e.g., cyanobiphenyl LCs42 and ferroelectric LCs43) by means of a nonlinear dielectric permittivity measurement, in the smectic CA* phase by an XRD measurement,44 and in the electric-field-induced smectic CP phase by an electrochemical technique.45

4. CONCLUSION We have constructed the n−T phase diagrams of typical imidazolium-based ILs, CnmimPF6 and CnmimCl, in wide n (4−16) and temperature (−150−200 °C) ranges. These phase diagrams demonstrated that the LC phase exists in supercooled (metastable) states even for the samples with small n (n < 14 for the PF6 samples and n < 10 for the Cl samples). The XRD patterns of these samples revealed the evident similarity of the low-Q peaks in the L phase and the diffraction peaks in the LC phase. On the basis of these results and some thermodynamic analyses, we concluded that the nanostructure of ILs is essentially the same as the layer structures in the LC phases. We are now preparing the neutron diffraction experiments in a wide Q range to investigate the temperature dependence of the peak intensities over the LC−L transition. Quasielastic neutron scattering experiments are also planed to investigate the LC−L transition from the viewpoint of dynamics. 5033


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