Tuning the formation of thermotropic ionic liquid crystals through the spacer length in 14-s-14 gemini surfactants

Thermotropic ionic liquid crystals (TILCs) are ion-containing fluids forming upon heating between the crystalline and liquid phases of ionic liquid-like amphiphiles. Bis(quaternary ammonium) gemini surfactants with general formula n - s - n , where n and s are the main tail and the spacer lengths, respectively, are able to form TILCs, but systematic studies on the effect of the spacer length are still lacking. Here, we investigated the possibility of tuning TILC formation by varying s for 14- s -14 gemini surfactants using an unusually wide range of spacers, from 2 up to 20 methylene groups ( s = 2, 6, 8, 12, 14, 18 and 20). The thermal stability of these compounds was assessed by thermogravimetric analysis (TGA) and the thermodynamic parameters of the phase transitions (temperature, enthalpy and entropy changes) by differential scanning calorimetry (DSC), while TILCs were assigned by polarized light microscopy (PLM). X-ray diffraction (XRD) of the powder compounds was also carried out to provide insight into the solid phase packing. Notably, the isotropization temperature to the ionic liquid phase shows an inverted V trend with increasing s in the s = 2–12 range, and then increases linearly in the range s = 12–20. Smectic A liquid crystals form for all compounds, with 14-2-14, 14-12-14 and 14-14-14 displaying lower temperatures (approx. range 100–120 °C) than the rest. Overall, results show that varying incrementally the spacer length affects the phase behavior, thermal stability, thermodynamic parameters of the thermotropic phase transitions and solid-phase smectic d -spacings in a marked and complex manner, with several non-monotonic trends observed. Moreover, the spacer length can be selected to tune the formation of ionic liquid crystals, pointing to a fine balance between the main chain and spacer lengths. • Structure–property studies are key to designing functional ionic liquid crystals. • The thermal behavior of 14- s -14 across spacer sizes ( s = 2–20) is under study. • Surprising non-monotonic trends in the melting behavior are found upon increasing s. • SmA phases prevail before isotropization; solids have tilted, interdigitated layers. • Spacer length in gemini is crucial to tailoring ionic liquid crystal formation.