Regulating mesogenic properties of ionic liquid crystals by preparing binary or multi-component systems

Abstract

In this study, a series of binary systems based ionic liquid crystals (ILCs) C₁₂MIm(BF₄)_yI_x (x + y = 1) are prepared by mixing ILCs 1-dodecyl-3-methylimidazolium iodide (C₁₂MImI) and 1-dodecyl-3-methylimidazolium tetrafluoroborate (C₁₂MImBF₄), aiming to combine the properties of both C₁₂MImBF₄ and C₁₂MImI. Their mesogenic properties are characterized by thermal analysis, polarized optical microscopy, X-ray diffraction as well as anisotropic ionic conductivity measurements. It is found that the smectic A phase with interdigitated bilayer structure is observed in the C₁₂MIm(BF₄)_yI_x series. The binary system C₁₂MIm(BF₄)_yI_x has a larger mesophase temperature range and interdigitated layer spacing than pure C₁₂MImBF₄. And the anisotropic ionic conductivity of the binary system, including parallel and perpendicular to smectic layer, is higher than pure C₁₂MImI. Moreover, in binary system C₁₂MIm(BF₄)_yI_x, with the increasing content of C₁₂MImI the ability of the anion to form three dimensional hydrogen bonds increases, resulting in the increase of the mesophase temperature range, interdigitated layer spacing and the decrease of anisotropic ionic conductivity. Furthermore, multi-component system based ILCs are obtained by introducing iodine to the binary system, and the mesophase temperature range is lowered, which is associated with the weak ability of I₃⁻ to form hydrogen bond with cations. All these results suggest that we can regulate the mesogenic properties of ILCs as we need by making binary or even multi-component systems.