Unique lamellar lyotropic liquid crystal phases of nonionic phytosterol ethoxylates in glycerol

Abstract

As one type of biocompatible surfactant, phytosterol ethoxylates (BPS-n, n is the oxyethylene chain length) have attracted more and more attention for their characteristic molecular structure. In this paper, the aggregation behaviors of BPS-5 or BPS-10 with five or ten oxyethylene units were investigated in glycerol to explore the solvent molecular structure effect, especially at higher surfactant concentration regions. Surface tension measurements were adopted to analyze the thermodynamic process for micelle formation. Polarized optical microscopy and small-angle X-ray scattering were then used to identify and characterize the formed lyotropic lamellar liquid crystalline (LLC) phase structures. An interesting coexistence of two kinds of lamellar phases in the BPS-5/glycerol system was found, which was attributed to different hydrogen bonding interactions between BPS-5 and glycerol. More intermolecular interaction information was mapped through the rheological measurement and Fourier transformed infrared (FTIR) spectroscopy. The relatively high self-assembling capability of glycerol was recognized to result from its molecular structure with more hydroxyl groups. Its higher Gordon parameter benefitting from the stronger hydrogen-bonded networks made the glycerol exhibit obvious enhancement on the LLC phase formation of BPS-5 or BPS-10, compared to those in water or amide solvents (wider concentration range of LLC phase and higher viscoelasticity). Such a stronger organized solvent structure in glycerol might originate from the special three-dimensional hydrogen-bonding patterns and much higher lifetimes compared to those for water.