Structural Analysis of Aggregates Formed by Linear- and Star-type Quaternary Ammonium Salt-Based Trimeric Surfactants Using Rheology and Small-Angle X-ray Scattering.

We performed a structural analysis of aggregates formed by two types of trimeric surfactants based on quaternary ammonium salts─linear-type 3C n lin- s -Q and star-type 3C n tris- s -Q─featuring varying alkyl chain lengths ( n ) and spacer chain lengths ( s ) in aqueous solutions. We performed rheology, dynamic light scattering, and small-angle X-ray scattering measurements on the trimeric surfactants and investigated the effects of the alkyl chain length, spacer chain length, spacer skeleton structure, and surfactant concentration on their aggregation behavior. Linear-type 3C 12 lin-3-Q transitioned from gel solutions to worm-like micelles at high concentrations, and 3C 14 lin-3-Q became gel solutions over a wide range of concentrations. In contrast, all other studied surfactants formed ellipsoidal micelles. The minor and major axes of the ellipsoidal micelles formed by liner-type 3C n lin-3-Q increased with the increasing alkyl chain length. As the spacer chain length of 3C n lin- s -Q increased from 3 to 6, and as the spacer skeleton expanded from linear-type 3C n lin- s -Q to star-type 3C n tris- s -Q, the surfactants formed ellipsoidal micelles without the formation of aggregates with a high-order structure, demonstrating this behavior over a broad concentration range.