Phase behavior of phytosterol ethoxylates in an imidazolium-type room-temperature ionic liquid.

The temperature-concentration phase behavior of nonionic surfactants in an aprotic imidazolium-type room-temperature ionic liquid (RT-IL) was evaluated on the basis of a combination of visual appearance, polarized optical microscopy, and small angle X-ray scattering data. Phytosterol ethoxylates (BPS-n, where n denotes oxyethylene chain lengths of 5, 10, 20, and 30) were used as surfactants in the RT-IL, 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6). The two component mixtures yielded various phases such as discontinuous cubic, hexagonal, and lamellar phases. An increased tendency toward formation of lesser-curved molecular assemblies was observed at higher BPS-n concentrations, at lower temperatures, and for shorter oxyethylene chain surfactants. These trends are similar to those observed in aqueous BPS-n systems; however, notable differences in the phase states of the aqueous system versus the BmimPF6 system were evident. Comparison with the water system showed that the BmimPF6 system yielded fewer phases and generally required higher BPS-n concentrations to induce phase transitions. Evaluation of the effects of addition of a third component (e.g., 1-dodecanol and dodecane) to the binary system on the phase behavior showed that at a given composition ratio of BPS-20 to BmimPF6, the addition of 1-dodecanol generally results in the phase transition to lesser-curved assemblies whereas dodecane generated no significant effects. The observed phase change is satisfactorily rationalized by localized solubilization of the third component into the binary surfactant assemblies.

Nonionic surfactant mixtures in an imidazolium-type room-temperature ionic liquid.

The physicochemical properties of nonionic surfactant mixtures in an aprotic, imidazolium-type room-temperature ionic liquid (RT-IL) have been studied using a combination of static surface tensiometry, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The surfactants used in this study are phytosterol ethoxylates (BPS-n, where n is an oxyethylene chain length of either 5 or 30) and the selected RT-IL is 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF(6)). The shorter chain oxyethylene surfactant (BPS-5) exhibits greater surface activity in BmimPF(6) than BPS-30; hence, BPS-5 is a major component in driving the interfacial adsorption and molecular aggregation of the mixed system. The surface tension data demonstrate that an increased mole fraction of BPS-5 results in a decreased critical aggregation concentration (cac) and negatively increased Gibbs free energies estimated for molecular aggregation (ΔG(0)(agg)) and interfacial adsorption (ΔG(0)(ads)). Indeed, the compositions of the monolayer adsorbed at the air/solution interface and the molecular aggregate formed in the bulk solution are enriched with BPS-5. The combination of the DLS and cryo-TEM results demonstrates the spontaneous formation of multi-lamellar vesicles resulting from the BPS-5-rich composition of the molecular aggregates.

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.