Effects of alkyl chain length on properties of N-alkyl-N-methylpyrrolidinium fluorohydrogenate ionic liquid crystals

A series of N-alkyl-N-methylpyrrolidinium fluorohydrogenate salts (CxMPyr(FH)2F, x = 10, 12, 14, 16, and 18) have been characterized by infrared spectroscopy, thermal analysis, polarized optical microscopy, X-ray diffraction, and anisotropic ionic conductivity measurements. Liquid crystalline mesophase with a smectic A interdigitated bilayer structure is observed for the salt with x ≥ 14, showing a fan-like or focal conic texture. Temperature range of the mesophase increases with increase in alkyl chain length (from 26.7 °C for C14MPyr(FH)2F to 90.7 °C for C18MPyr(FH)2F). The layer spacing in the smectic structure monotonously increases with increasing alkyl chain length and decreases with increasing temperature. The liquid crystalline mesophase of C14MPyr(FH)2F exhibits anisotropy in ionic conductivity and the ionic conductivity parallel to the smectic layer is roughly ten times larger than that perpendicular to the smectic layer.