Phytosterol ethoxylates in room-temperature ionic liquids: excellent interfacial properties and gel formation

Physicochemical properties of phytosterol ethoxylates (BPS-n, where n is the oxyethylene chain length of 5, 10, 20, and 30) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6), have been characterized on the basis of static surface tension, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopic (cryo-TEM) data. The surface tension data clearly show that the BPS-n surfactants employed in this study exhibit the most excellent surface activity in BmimPF6 yet reported in the literature. The decreased chain length of the polyoxyethylene unit results in a greater surface activity (i.e., lower critical association concentration (cac) and lower surface tension measured above the cac), as is similarly reported for nonionic polyoxyethylene alkyl ether surfactants in aqueous solution. This suggests that the hydrophobic phytosterol groups are normally oriented toward the air phase, and the polyoxyethylene chains are present in the ionic liquid. Just above the cac, the BPS-n surfactants spontaneously form molecular assemblies in BmimPF6, depending on their critical packing parameters: less-curved vesicular assemblies are seen for BPS-10, whereas greater-curved (spherical) micelles are formed for BPS-20 and BPS-30. Indeed, an increased surfactant concentration results in a structural transformation of the molecular assemblies from micelles to discontinuous cubic, hexagonal, and lamellar structures. The current article explores the best combination of surfactants with ionic liquids to see excellent surface activity and characterize molecular assemblies in bulk solution.