A novel series of ionic liquids (ILs) based on N-alkyl- -caprolactam as cations [Cn-CP]+ (Cn ) 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, variabletemperature powder X-ray diffraction, etc., were conducted. The results showed that the caprolactambased ILs (n≥8) were enantiotropic thermotropic liquid crystals, except for C18-CPMS (monotropic), and displayed smectic A phases. Some properties, such as higher transition enthalpies (e.g., ΔH=83.1 kJ/mol for C18-CPTS), higher specific heat capacities (e.g., Cp=2.85 J/g·K for C16-CPTS) and higher heat storage densities (e.g., sensible heat storage density Es=262.8l MJ/m3 for C16-CPTS; latent heat storage density El=146.0 MJ/m3 for C18-CPTS), were observed. Fluorescence measurements showed that the homologous Cn-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]BF4 IL.

★★★★☆ Yang J,Zhang QH,Zhu LY,et al. Novel Ionic Liquid Crystals Based On N-alkylcaprolactam As Cations[J]. Chem. Mater.,2007,19:2544-2550.