Novel ionic liquid crystals based on N-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, 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., C_p = 2.85 J/g·K for C₁₆-CPTS) and higher heat storage densities (e.g., sensible heat storage density E_s = 262.81 MJ/m³ for C₁₆~CPTS; latent heat storage density E₁ -146.0 MJ/m³ for C₁₈-CRTS), 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]BF₄ IL.