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.