Lyotropic liquid crystal phases of CTAB in aqueous non-stoichiometric protic ionic liquids

Article Abstract

Protic ionic liquids (PILs) are the largest and most tailorable known class of solvents which possess the ability to support amphiphile self-assembly. In this study, the lyotropic liquid crystal phase (LLCP) behavior of the cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated in ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN) derived multi-component solvent systems to determine phase formation and diversity with changing solvent composition. The solvent systems were composed of water, nitric acid and ethylamine (or ethanolamine), with 26 unique compositions for each PIL covering the apparent pH and ionicity ranges of 0-13.5 and 0-11 M, respectively. The LLCPs were studied using cross polarized optical microscopy (CPOM) and small and wide-angle X-ray scattering (SAXS/WAXS). Partial phase diagrams were constructed for CTAB concentrations of 50 wt% and 70 wt% in the temperature range of 25 °C to 75 °C to characterise the effect of surfactant concentration and temperature on the LLCPs in each solvent environment. Micellar, hexagonal and cubic phases were identified at both surfactant concentrations, and from temperatures as low as 35 °C, with large variations dependent on the solvent composition. The thermal stability and diversity of phases were greater and broader in solvent compositions with excess precursor amines present compared to those in the neat PILs. In acid-rich solvent combinations, the same phase diversity was found, though with reduced onset temperatures of phase formation; however, some structural changes were observed which were attributed to oxidation/decomposition of CTAB in a nitric acid environment. This study showed that the ability of PIL solutions to support amphiphile self-assembly can readily be tuned, and we anticipate that this knowledge could be applicable to other IL-amphiphile systems. It also shows the robustness of the ability of PILs to promote amphiphile self-assembly, even with other solvent species present.