Physicochemical Characterization of Sulfonate-Based Phosphonium Ionic Liquids

Tsunashima, Katsuhiko

Ionic liquids (ILs) have unique physicochemical properties such as favorable solubility of organic and inorganic compounds, relatively high ionic conductivity, no measurable vapor pressure, high thermal stability, low flammability, etc. 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. Although many kinds of ILs have already been investigated, phosphonium cation based ILs have rarely been proposed. We have previously designed and synthesized the phosphonium ILs together with typical sulfonylamide-based anions such as bis(trifluoromethylsulfonyl)amide and bis(fluorosulfonyl)amide anions.^1,2) On the other hand, interests in phosphonium ILs consisting of the other anions is increasing for diverse applications. In this work, we design and prepare the ILs based on tetrabutylphosphonium cation (P₄₄₄₄⁺) together with various sulfonate-based anions (Fig. 1), characterizing their physicochemical properties as a new family of ILs.

The phosphonium IL was prepared by an aqueous neutralization reaction of tetrabutylphosphonium hydroxide with the stoichiometric amounts of various sulfonic acids. The obtained phosphonium ILs were isolated by water evaporation, and were dried in vacuo for at least 1 day. The physicochemical properties of ILs, e.g. density, viscosity, conductivity (ac impedance method) and thermal decomposition temperature (thermogravimetric analysis), were measured under argon atmosphere.

The phosphonium salts based on unsubstituted sulfonate anions such as SO₃CH₃, SO₃CH₂CH₃ and SO₃(CH₂)₂CH₃ were white crystalline solids at room temperature, whereas both amino- and hydroxy- substituted sulfonate salts were viscous liquids at room temperature. All phosphonium salts obtained were hydrophilic. Table 1 summarizes the physicochemical properties of phosphonium ILs based on amino- and hydroxy-substituted sulfonate anions. These phosphonium ILs exhibited considerably high viscosities when compared to the previously published viscosity value of P₄₄₄₄-lactate IL (415 mPa s at 25 °C)³⁾, which suggests that the electrostatic interaction between P₄₄₄₄ cation and the sulfonate anions is much stronger than those in the case of carboxylate-based phosphonium ILs. It should be noted that P₄₄₄₄-SO₃(CH₂)₃NH₂ obviously indicated not only the highest density but also the highest viscosity and the lowest conductivity, which might be due to the fact that the IL has the largest anion to give a significant van der Waals interaction.