Synthesis of Thermally Stable Geminal Dicationic Ionic Liquids and Related Ionic Compounds: An Examination of Physicochemical Properties by Structural Modification

Geminal dicationic ionic liquids (ILs) often display higher thermal stabilities, viscosities, and densities compared to traditional monocationic ILs. Also, dicationic ILs have advantages in terms of tuning of their physicochemical properties by different structural modifications. They can be considered as a combination of three structural moieties: (1) cationic head groups, (2) an alkane linker chain, and (3) the associated anions. Two types of each imidazolium, pyrrolidinium, and phosphonium cations were joined by different alkane linkages (C₆, C₉, and C₁₂) to develop 18 different dications. These dications were paired with two different anions (NTf₂^– and PFOS^–) to synthesize 36 different dicationic ILs. The effect of variations in the structural moieties of these related ILs on their physicochemical properties, including melting points, densities, viscosities, solubilities, and thermal stabilities, were evaluated. ILs synthesized in this study displayed TGA thermal stabilities in the range 330–467 °C. Also, nine ILs with high TGA stability and low melting points were tested with inverse gas chromatography, and some of them displayed stabilities up to 400 °C.