Lyotropic Liquid Crystalline Phases of a Phytosterol Ethoxylate in Amide Solvents

Materials exhibiting unique aggregation behavior in nonaqueous solvents have attracted attention due to their wide applications. Motivated by this recent interest, the aggregation properties of a phytosterol ethoxylate surfactant, BPS-10, in three organic amide compounds, formamide (FA), <i>N</i>-methylformamide (NMF), and <i>N,N</i>-dimethyl- formamide (DMF), have been studied. Polarized optical microscopy and small-angle X-ray scattering techniques were used to investigate the lyotropic liquid crystalline (LLC) phases formed in these binary systems. Herein, we discuss the relationship between subtle intermolecular interactions and the aggregation behavior of BPS-10. As good proton donors or acceptors to form hydrogen bonding, FA molecules allow BPS-10 to show a richer phase behavior. Compared with the systems formed in water and ionic liquids, the LLCs constructed in FA have higher thermal stability. In addition, two kinds of lamellar phases could coexist in a narrow region. With the methyl replacement in formamide, however, the ability to form hydrogen bonds is reduced and the solvent bulk phase structure becomes less ordered from FA to DMF. Consequently, the solvophobic interaction of BPS-10 becomes weaker, and the LLCs are more difficult to form. In addition, the extra strong interactions between the steroid rings of BPS-10 may provide enough driving force to produce the hexagonal phase (H<sub>1</sub>) directly in NMF and DMF without micelle formation, thereby creating a novel sequence (isotropic → H<sub>1</sub> → Lα) of ordered phases with increasing surfactant concentration. The results discussed herein should prove to be a useful complement to the growing body of literature regarding steroid surfactant aggregation in polar organic solvents.