Preparation and Physicochemical Characterization of Phosphonium-Based Ionic Liquids Containing Carboxylate Anions

Abstract

Ionic liquids (ILs) are organic salts composed of cation and anion with melting points below 100°C. They have unique characteristic features.1) ILs are also promising liquid materials available for various applications because their function can be easily controlled by changing the combination of cations and anions and by introducing substituents. Among the numerous ILs, considerable ILs based on carboxylate anions have been reported. On the other hand, several published papers reported ILs based on quaternary phosphonium cations, demonstrating relatively high transport properties and thermal stability2). We report here the preparation and physicochemical properties of ILs based on triethyloctylphosphonium (P2228) and tributyloctylphosphonium (P4448) cations together with various carboxylate anions.The intermediate hydroxides were obtained by adding an anion exchange resin (Amberlite IRN78 OH) into the precursor bromide aqueous solutions. Equimolar amounts of carboxylic acids (formic, acetic, propionic, butyric and octanoic acids) were added to the hydroxide solutions to form carboxylate-based phosphonium salts. The salts obtained were isolated by evaporation and then were dried under high vacuum at 50 °C. The physicochemical properties of ILs, e.g. density, viscosity, conductivity and thermal decomposition temperature were measured.Table 1 shows the physicochemical properties of the ILs at 25 °C. P2228-PrCO2 became a solid at room temperature, while P2228-HCO2, P2228-MeCO2, and P2228-EtCO2 became viscous liquids at room temperature. Furthermore, all P4448-based salts became viscous liquids at room temperature. The density of each IL was increased with decreasing the carbon numbers of the alkyl chains in carboxylate anions. The number density also showed similar tendency to the density. These results indicate that the number density of the formate was the highest since the anion size was the smallest. The viscosity of each ILs was decreased with increasing the carbon numbers of the alkyl chains in carboxylate anions. Although the details of the relationship between measured viscosity and the carbon numbers of the alkyl chains in carboxylate anions still remain unclear at present, it is thought to be the charge distribution of the anions is one of the factors. The acetate-, propionate-, butyrate-, and octanoate-based ILs showed high thermal decomposition temperatures around 270 to 300°C. However, the formate-based ionic liquid showed lower thermal stability, which might be due to the decarboxylation of the formate anion. Figure 1