Ellipsoidal micelle formation of quaternary ammonium salt-based gemini surfactants: structural analysis through small-angle X-ray scattering.

Gemini surfactants exhibit better adsorption and aggregation properties than those of monomeric surfactants. However, to enhance the functional properties of gemini surfactants, the effect of spacer structures on their aggregation behavior must be elucidated. Small-angle X-ray scattering (SAXS) has been performed to study the aggregate structures of monomeric surfactants, but its application has not been expanded to the analysis of gemini surfactants. Therefore, in this study, we performed a structural analysis of aggregates formed by quaternary ammonium salt-based gemini surfactants with different spacer structures in aqueous solutions through viscosity, dynamic light scattering (DLS), and SAXS measurements. We also investigated the effects of the spacer structure and surfactant concentration on the aggregation behavior. The DLS results indicated that gemini surfactants with spacers containing cyclic structures, such as diethylene and triethylene chains, formed small micelles (several nanometers in size) at the limiting concentration for dissolution in water. In contrast, gemini surfactants with nitrogen and oxygen atoms at the center of the spacer formed ellipsoidal micelles at low concentrations, as shown by SAXS results. The core and overall radii of the minor and major axes of the ellipsoidal micelles decreased with increasing surfactant concentration and were larger for spacers with two ethylene chains connected to central nitrogen and oxygen atoms than for spacers with rigid diethylene and ethylene chains connected to a central nitrogen atom.