Lyotropic liquid crystalline phases formed by phyosterol ethoxylates in room-temperature ionic liquids

The phase behaviors of four phytosterol ethoxylates surfactants (BPS-n, n = 5, 10, 20, and 30) with different oxyethylene units in room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4), have been studied. The polarized optical microscopy and small-angle X-ray scattering techniques are used to characterize the phase structures of these binary systems at 25 °C. The structure and ordering of the liquid crystalline (LC) phases in such BPS-n/[Bmim]BF4 systems are found to be influenced by BPS-n concentration and the temperature. Due to the bulky and rigid cholesterol group, the phytosterol ethoxylates surfactants exhibit different properties and interaction mechanism from the conventional CnEOm type nonionic surfactant systems. The rheological measurements indicate a highly viscoelastic nature of these lyotropic LC phases and disclose a lamellar phase characteristic with a rather strong rigidity at high surfactant concentrations. The control experiment with Brij 97(polyoxyethylene (10) oleyl ether)/[Bmim]BF4 system and the FTIR measurements help to recognize that the solvophobic interaction combining with the hydrogen bonding are the main driving forces for the LC phases formation.

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► The phase diagrams of nonionic surfactant BPS-n in [Bmim]BF4 at 25 °C are mapped.
► The formed lyotropic liquid crystal (LLC) structure and ordering are influenced by BPS-n content and temperature.
► The interaction between steroid rings of BPS-n is the dominated factor to result in LLC.
► The LLC in such system exhibits highly apparent viscosity.