Physicochemical properties of branched-chain dicationic ionic liquids

Dicationic ionic liquids (DILs) typically have higher viscosities and thermal stabilities than most conventional monocationic ionic liquids. However, due to their coulombic interactions, they often demonstrate higher melting points than conventional ILs which limit their use in many applications. Utilizing branched alkyl spacer chains between two cationic terminal groups resulted in dicationic ILs with lower melting points and therefore extended liquid ranges. A systematic investigation was conducted to determine how the position and type of substituent groups may affect the physicochemical properties of branched DILs. When compared to their linear counterparts, an increase in the kinematic viscosities and lower densities were observed for branched DILs. Thermal stability was found to be highly dependent on the position of the alkyl substituent group along with the length of the spacer chain. While the substitution on the α‑carbon of a spacer chain resulted in a significant decline in the thermal stability of DILs, the effect of mid-chain substitution on their stability was negligible. The performance testing of branched DILs as gas chromatography (GC) stationary phase displayed excellent separation of geometrical isomers in BTEX mixture.