Soft aggregates formed by a nonionic phytosterol ethoxylate and β-cyclodextrin in aqueous solution

The self-assembling behavior of a nonionic phytosterol ethoxylate surfactant (BPS-30), mixed with beta-cyclodextrin (beta-CD) at a 1:1 molar ratio, has been investigated in aqueous solutions. The anisotropic and birefringent aggregates obtained with the mixtures present vesicle and lamellar morphologies, which are rarely found in other surfactant/beta-CD mixture systems. The observed aggregate morphologies are determined mainly by the surfactant concentrations as evidenced through observations using scanning or cryo-transmission electron microscopy. The vesicle aggregates appear in solutions of low BPS-30 concentrations (<12 wt%), while the lamellar structures are observed in solutions of high BPS-30 concentrations (>12 wt%). Additional aggregate structural details are explored through small-angle X-ray scattering measurements. Different with other surfactant/beta-CD mixture systems, beta-CD appears to function as a cosurfactant in this research, as shown by the H-1-H-1 NOESY nuclear magnetic resonance spectroscopy and the surface tension measurements. Control experiments with urea molecules were conducted to demonstrate that the main driving force of aggregation is a synergistic effect of the H-bonding and hydrophobic interactions. The obtained results on the nonionic mixture of biosurfactant/beta-CD in this paper further demonstrate the potential expanded applications of soft matter construction. (C) 2015 Elsevier B.V. All rights reserved.