phytosterols and 1-butyl-3-methylimidazolium-hexafluorophosphate

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.

Topics: Imidazoles; Ionic Liquids; Oils; Phase Transition; Phytosterols; Temperature

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.

Topics: Adsorption; Ethylene Glycols; Imidazoles; Ionic Liquids; Microscopy, Electron, Transmission; Phytosterols; Scattering, Radiation; Surface Tension; Surface-Active Agents; Temperature