Synthesis and Electrochemical Properties of Fluorine-Containing Hydrophobic Ionic Liquid Gels with Low-Molecular Gelators
Authors: Matsumoto Kenta , Fujita Yuki , Sugiura Takumi , Yamaguchi Junya , Suzuki Nonoka , Morita Yuki , Cao Ban-Peng , Xiao Qiang , Okamoto Hiroaki 
Published: December 2024

INTRODUCTION

Ionic liquid is a general term for salts with melting points below 100 °C. In particular, the substances that become liquid at room temperature and pressure are being considered for applications in electrochemical devices and CO2 sorption materials. [1] In previous studies, protic ionic liquids (PILs), which obtained by neutralization of Brønsted acid and Brønsted base, are expected to be used application for next-generated electrolytes for fuel cells, [2] however, they can dissolve in water (i.e., they have hydrophilicity). In contrast, we have reported that PILs for CO2 sorption materials were synthesized by neutralization of aromatic amine (e.g., dialkylpyridines and N,N-dimethylaniline) and Tf2NH. [3]

Furthermore, we have reported that some rod-shaped low-molecular compounds containing fluoroalkyl chains can gelatinize some organic solvents and/or ionic liquids. [3,4] The formed gels are physical gels that undergo sol–gel transition by heating and cooling. Interestingly, their compounds are self-assembled by intermolecular interaction without hydrogen bonds since hydrogen-bonding groups (H-groups) such as hydroxy (–OH) and amide (–CONH–) groups are not contained. Generally, low-molecular gelators contain H-groups, which cause a strong intermolecular interaction then easily cause self-assembly to form fibrous aggregates then trapped solvents, resulting in gelation. [5] H-groups have been considered difficult to apply to electrolytes of Li-ion battery and fuel cells because of theirs effect on electrochemical properties. Therefore, low-molecular gelators without H-groups are very interested studies.
In this study, hydrophobic PILs based on diethylanilines, and rod-shaped phenyl benzoates containing fluoroalkyl chains as gelators (Figure 1) were synthesized and evaluated physicochemical properties of PILs and their gels such as thermal and rheological properties, and ionic conductivity.

EXPERIMENTAL

Hydrophobic PILs were synthesized by neutralization of anilines and Tf2NH without solvents. Compounds 1 and 2 were synthesized by esterification of 1H,1H,2H,2H-tridecafluorooctyl 4-hydroxybenzoate and corresponding to 4-substitutedbenzoic acids. Thermal properties of PILs were analyzed with DSC and these of gels were evaluated visually as described in [4]. Rheological properties of gels were evaluated by a steady flow viscosity. Ionic conductivity was calculated from inverse of solution resistance, which was recorded on an AC impedance spectroscopy with a Solartron 1280C.

RESULTS AND DISCUSSION

Synthesized PILs exhibited hydrophobicity since these were phase-separated with water and 1-octanol–water partition coefficients, which was determined by a HPLC system, were more than 0. In addition, ionic conductivities at 120 ºC were ca. 20 mS cm–1, so ionic conduction may not be related to humidity. Figure 2 shows gel–sol transition temperatures (T
gel-sol) of [N,N-Et2AN][TFSA] and [N,N,2,6-Et4AN][TFSA] gels. T
gel-sol of 5wt% [N,N-Et2AN][TFSA] gels formed by compounds 1 and 2 were 63 ºC and 102 ºC, respectively. It is suggested that this is due to the fact that the electrostatic repulsion of fluorine atoms reduces solubility, making it easier for the self-assembly to exist as a metastable state rather than a stable state, which predominantly facilitates the construction of fibrous aggregates compared to crystallization. Meanwhile, compound 1 was not able to gelatinize [N,N,2,6-Et4AN][TFSA] rather than compound 2 was able to gelatinize.
Figure 3 shows steady flow viscosity of 5wt% [N,N-Et2AN][TFSA] gel formed by compound 2. The gel exhibited thixotropy because of same viscosities before and after shear. Generally, self-assembly phenomena of gelation are the interaction between compounds (i.e., the solvent does not relate to gelatinize) then the gels are pseudoplastic fluids (i.e., viscosity of gel is decreased after shere) [6]. The gelation mechanism of 5wt% [N,N-Et2AN][TFSA] gels formed by compounds 2 is specific one since exhibiting thixotropy may be caused by intermolecular interaction between gelators and solvents.
Figure 4 shows Arrhenius plots of ionic conduction for [N,N-Et2AN][TFSA] and its 5wt% gel formed by compounds 2. The activation energy of ionic conduction, obtained by the least-squares method using the Vogel-Fulcher-Tammann equation (σ(T) = AT
–1/2 exp [–B/R(T – T
0)], where σ(T), A, B, T
0, and R indicate ionic conducticvity at T, pre-exponential factor, activation energy, ideal glass transition temperature, and gas constant, respectively), showed a slight increase after gelation compared to before gelation, suggesting that cation or anion may be trapped in the gelator molecules.

Acknowledgement

This work is financially supported by JST SPRING [JPMJSP2111], JSPS KAKENHI [22H03781, 23K25035], Paloma environmental technology development foundation, and Iketani Science and Technology Foundation.

References

[1] C. Feldmann and M. Ruck, Z. Anorg. Allg. Chem., 643, 2 (2017). [2] H. Nakamoto and M. Watanabe, Chem. Commun., 24, 2539 (2007). [3] K. Matsumoto et. al., Tetsu-to-Hagané, in Press (2024). [4] T. Yoshida et. al., ECS Trans., 50, 95 (2013). [5] K. Hanabusa et. al., Langmuir, 21, 10383 (2005). [6] Y. Zhang and R. G. Weiss, J. Colloid Interface Sci., 486, 359 (2017).

Figure 1