Influence of the counterion on the structure and stability of the smectic C phase in ionic liquid crystals

Ionic liquid crystals (ILCs) are built of ion pairs. These consist often of a small anion and a large mesogenic cation. The properties of the ILCs like e.g. the phase behaviour or structure parameters, depend not only on the large mesogenic cations but also on the small counteranions. Here the influence of the small anion on the formation and stability of the in ILCs rare smectic C (SmC) phase is investigated in a series of methylimidazolium azobenzene ILCs with the same cation but different anions. All ILCs synthesised show the rare SmC phase. The phase behaviour and the tilt angles of the compounds were studied using polarising optical microscopy and differential scanning calorimetry. Furthermore, layer spacing and layer shrinkage of the SmC phase were examined using X-ray scattering. We show that the structural parameters of the phase-like layer spacing and tilt angle are only weakly influenced by the anions, whereas the mesophase ranges are strongly influenced by the choice of the anions. The results are also compared to the Hofmeister series for anions and it is found that the anions stabilise the SmC phase more the more kosmotropic they are. [GRAPHICS]