Combined effect of ether and siloxane substituents on imidazolium ionic liquids

Abstract

Ionic liquids (ILs) in combination with ether and siloxane substituents on the imidazolium cation, comprising of bis(trifluoromethylsulfonyl)imide (NTf₂) anions, have been synthesized due to their low viscosities, high thermal stabilities and low melting or glass transition temperatures(T_g). The structural flexibility of the ether and siloxane substituents on the imidazolium center overcomes the effect of van der Waals forces and gives them desirable and unique physical properties. These novel ILs show low viscosities (∼72 mPa s at 25 °C), high thermal stabilities up to 430 °C, and a wide liquid range over 500 °C. In addition, these ILs exhibit only an amorphous glassy state on cooling at T_g below −74 °C and cannot order properly to afford crystallization. The detailed thermal stability, phase transitions and heat capacity were studied by TGA, DSC and temperature-modulated DSC analysis. More importantly, we have also reported the large-scale (one kilogram) microwave-assisted synthesis of ILs [BMIM]Br and [1O2O2-Im-2O1]I as efficient and greener processes.