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

• The vesicular and lamellar aggregates could be constructed by BPS-30/β-CD mixture systems.
• The β-CD does not act as a host molecule but as a cosurfactant.
• The main driving force of aggregation is a synergistic effect of H-bonding and hydrophobic interactions.

The self-assembling behavior of a nonionic phytosterol ethoxylate surfactant (BPS-30), mixed with β-cyclodextrin (β-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/β-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/β-CD mixture systems, β-CD appears to function as a cosurfactant in this research, as shown by the 1H–1H 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/β-CD in this paper further demonstrate the potential expanded applications of soft matter construction.

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