Linear- and star-type quaternary ammonium salt-based trimeric surfactants: Effect of structure on adsorption and aggregation properties

Oligomeric surfactants with multiple hydrophobic and hydrophilic groups are an attractive novel surfactant with improved properties and functionalities compared with those of conventional monomeric surfactants. In this study, linear- and star-type quaternary ammonium salt-based trimeric surfactants, 3Cnlin-s-Q and 3Cntris-s-Q, were synthesized with different alkyl chain lengths (n), spacer chain lengths (s), and spacer skeletons (linear or star) and their adsorption and aggregation properties were characterized by electrical conductivity, surface tension, fluorescence, viscosity, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). Trimeric surfactants exhibited high water solubility despite containing three alkyl chains. Conversely, the lineartype trimeric surfactant 3C14lin-3-Q, composed of rigid short spacer chains and long alkyl chains, exhibited low water solubility. The adsorption and orientation of trimeric surfactants at the air/water interface depended on the alkyl chain length, spacer chain length, and spacer skeleton, and the effect of alkyl chain length was dominant. The star-type trimeric surfactant 3C12tris-6-Q was efficiently adsorbed and oriented at the air/water interface, despite its long spacer and bulky skeleton, which extended from the center of the amino group in the spacer. Therefore, trimeric surfactants with high performance and advanced functionalities were successfully achieved. This study provides insights into the molecular design and synthesis of trimeric surfactants for future application in various industrial fields.