Dependence of Carboxylate Anions on Physicochemical Properties of Tributyloctylphosphonium-Based Ionic Liquids
Authors: Akamatsu Hyoma , Kubo Sakika , Kikuchi Seiya , Tsuchida Yusuke , Tsunashima Katsuhiko , Hotta Hidekatsu , Yokobiki Akina , Funasako Yusuke , Okuno Yoshiharu , Hamada Atsuyo , Yamada Hirohisa , Tani Atsushi 
Published: December 2024

Ionic liquids, i.e. organic molten salts composed of cations and anions with melting points below 100°C, have unique properties such as high ionic conductivity, high thermal stability, low vapor pressure, low flammability, and so on. Among the numerous ionic liquids, considerable ionic liquids based on carboxylate anions have been reported1). On the other hand, several published papers reported ionic liquids based on quaternary phosphonium cations, demonstrating relatively high transport properties and thermal stability2,3). Our research group has preliminarily attempted to design and synthesize several carboxylate-based ionic liquids based on quaternary phosphonium cations. This paper discusses the dependence of carboxylate anions on physicochemical properties of room-temperature ionic liquids based on tributyloctylphophonium (P4448) cation in combination with various carboxylate anions (Fig. 1).
The precursor hydroxides were synthesized from the corresponding phosphonium bromides by using an anion exchange resin (Amberlite IRN78 OH). Carboxylate-based phosphonium salts were obtained by adding equimolar amounts of carboxylic acids (formic, acetic, propionic, butyric, octanoic and lactic acids) into the hydroxide solution. The products were isolated by evaporation and then were dried under high vacuum at 50°C. The physicochemical properties of carboxylate-based ionic liquids, e.g. melting point, density, viscosity, conductivity and thermal decomposition temperature, were measured under argon atmosphere.
Table 1 shows the melting points, glass transition temperature, thermal decomposition temperature and physicochemical properties of the carboxylate-based phosphonium ionic liquids at 25°C. All P4448-based ionic liquids became viscous liquids at room temperature, showing considerably low-melting behavior. It is noted that the density of P4448-Lac was higher than P4448-EtCO2. This result seems likely to be attributed to hydrogen bonding of the hydroxyl group in the lactate anion. The viscosity was decreased with increasing the carbon numbers in the alkyl chains of lower carboxylate anions. Although the details of this relationship between measured viscosity and the carbon numbers of the alkyl chains in carboxylate anions still remain unclear at present, the charge distribution of the anions is thought to be one of the factors. The acetate-, propionate-, butyrate-, and octanoate-based ionic liquids showed thermal decomposition temperatures around 280°C; however, lactate-based ionic liquids exhibited relatively high thermal decomposition temperature above 310°C. This may be due to the delocalization of the charge distribution in the lactate anion, resulting in the high thermal stability.
References
1) M. T. Clough, et al, Phys. Chem. Chem. Phys., 15, 20480 (2013).
2) K. Tsunashima, et al, Electrochem. Commun., 9, 2353 (2007).
3) K. Tsunashima, et al, Electrochemistry, 75, 734 (2007).

Figure 1