Novel Ionic Liquid Crystals Based onN-Alkylcaprolactam as Cations

A novel series of ionic liquids (ILs) based on N -alkyl-ε-caprolactam as cations [C n − CP] + ( C n = alkyl with different numbers of C atoms; n = 6, 8, 10, 12, 16, or 18) containing toluene- p -sulfonate [TS] - and methanesulfonate [MS] - as anions have been synthesized via a one-step atom-economic reaction. Characterizations of these ILs by differential scanning calorimetry, polarizing optical microscopy, variable-temperature powder X-ray diffraction, etc., were conducted. The results showed that the caprolactam-based ILs ( n ≥ 8) were enantiotropic thermotropic liquid crystals, except for C 18 −CPMS (monotropic), and displayed smectic A phases. Some properties, such as higher transition enthalpies (e.g., Δ H = 83.1 kJ/mol for C 18 −CPTS), higher specific heat capacities (e.g., C p = 2.85 J/g·K for C 16 −CPTS) and higher heat storage densities (e.g., sensible heat storage density E s = 262.8l MJ/m 3 for C 16 −CPTS; latent heat storage density E l = 146.0 MJ/m 3 for C 18 −CPTS), were observed. Fluorescence measurements showed that the homologous C n − CPTS ILs exhibited strong fluorescence behavior. Finally, the test of acute toxicity toward rats showed that these new ILs were less toxic than the popular [BMIm]BF 4 IL.