The experimental and theoretical study on the influence of alkali metals on the electronic charge distribution in five-membered aromatic acids (2-thiophenecarboxylic, 2-furanecarboxylic and 2-pyrrolecarboxylic acids)

Effect of Aromaticity in Anion on the Cation-Anion Interactions and Ionic Mobility in Fluorine-Free Ionic Liquids

Ionic liquids (ILs) composed of tetra(n-butyl)phosphonium [P₄₄₄₄]⁺ and tetra(n-butyl)ammonium [N₄₄₄₄]⁺ cations paired with 2-furoate [FuA]⁻, tetrahydo-2-furoate [HFuA]⁻, and thiophene-2-carboxylate [TpA]⁻ anions are prepared to investigate the effects of electron delocalization in anion and the mutual interactions between cations and anions on their physical and electrochemical properties. The [P₄₄₄₄]⁺ cations-based ILs are found to be liquids, while the [N₄₄₄₄]⁺ cations-based ILs are semi-solids at room temperature. Thermogravimetric analysis revealed higher decomposition temperatures and differential scanning calorimetry analysis showed lower glass transition temperatures for phosphonium-based ILs than the ammonium-based counterparts. The ILs are arranged in the decreasing order of their ionic conductivities as [P₄₄₄₄][HFuA] (0.069 mS cm^–1) > [P₄₄₄₄][FuA] (0.032 mS cm^–1) > [P₄₄₄₄][TpA] (0.028 mS cm^–1) at 20 °C. The oxidative limit of the ILs followed the sequence of [FuA]⁻> [TpA]⁻> [HFuA]⁻, as measured by linear sweep voltammetry. This order can be attributed to the electrons’ delocalization in [FuA]⁻ and in [TpA]⁻ aromatic anions, which has enhanced the oxidative limit potentials and the overall electrochemical stabilities.