Ionic liquid crystals with novel thermal properties formed by the gemini surfactants containing four hydroxyl groups

Modification of the molecular structures of surfactants is a feasible way to construct ionic liquid crystals (ILCs) with novel properties. Five quaternary ammonium gemini surfactants with different spacers, whose head groups were substituted by four hydroxyethyl groups, abbreviated as 12(2OH)-s-12-(2OH) (s = 3, 4, 5, 6, 8), were prepared. The mesomorphic behaviors of these gemini surfactants were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and temperature-dependent FT-IR spectroscopy. These surfactants form smectic A (SmA) phases, the region of which is dependent on the spacer length. The presence of suitable spacer length and multiple hydroxyl groups is helpful in stabilizing the mesophases. Noticeably, because of synergistic effect of the spacer and the hydroxyl groups, the phase enthalpy change of SmA to crystal transitions can be slowly released below the phase transition temperature. This interesting discovery, which has never been reported in ILC systems formed by small molecular weight surfactants, could pave the way for special applications of ILCs in the area of sustained-release materials of heat and could widen the application of ionic liquid crystals.