Ternary phase behavior of phytosterol ethoxylate, water, and imidazolium-based ionic liquid systems – Lyotropic liquid crystal formation over a wide range of compositions

• Liquid crystal formation in ternary system of nonionic surfactant, water, and ionic liquid over wide composition.
• Effect of anion species of ionic liquid on the phase behavior.
• Structural parameters calculated from SAXS measurements and the rheological properties of lyotropic liquid crystal.

Lyotropic liquid crystal formation from ternary mixtures of nonionic surfactants [phytosterol ethoxylates (BPS-n)], water, and room-temperature ionic liquids [1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)] was evaluated using optical and polarized microscopic observations, small-angle X-ray scattering (SAXS), and rheological measurements. In this study, the number of POE units was varied (BPS-n, n = 10, 20, 30) to form lyotropic liquid crystalline phases over a wide composition range for all of the systems. The major liquid crystalline phases formed for BPS-10, -20, and -30 were lamellar, hexagonal, and cubic phases, respectively. The differences could be explained by the size of the hydrophilic moieties (i.e., a decrease in critical packing parameters through an increase in the number of surfactant POE units). In addition, structural parameters were calculated from SAXS data for the hexagonal liquid crystalline phase with 60 wt% BPS-20. The area occupied by the hydrophilic moieties in the hexagonally packed cylinder micelle decreased with increasing ratio of water to ionic liquid, which indicated that the surfactant molecules were packed more closely in the hexagonal liquid crystalline phase at greater water concentrations. Rheology measurements indicated that ternary mixtures exhibited more elastic behavior at greater water concentrations, which also supports the change in structural parameters suggested by SAXS measurements.

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