Coacervates formed by cationic polyelectrolytes and anionic surfactants are utilized to improve the user's tactile perception of shampooing hair during washing and after drying. In this study, we investigated the formation and structure of coacervates in aqueous systems containing anionic amino acid surfactants. The phase behaviors at constant temperature were investigated in aqueous systems combining cationic polyelectrolyte JR-400 with potassium cocoyl glutamate (CoGluK) or potassium cocoyl glycinate (CoGlyK) for a qualitative depiction of coacervate formation. The composition range of coacervate formation varied with the hydrophilic group of the surfactant. The surface tension was measured at different surfactant concentrations and constant polyelectrolyte concentration. The surface tension behavior revealed the critical association concentrations and critical micelle concentrations, indicating that coacervate was produced via complex formation through electrostatic interaction between opposite charges. Optical microscopy and small-angle X-ray scattering measurements revealed that the coacervates were composed of fibrous aggregates, a few microns thick, and those formed in the CoGlyK system had thicker fibers.
Keywords: amino acid surfactant; coacervation; phase behavior; polymer-surfactant interaction; small-angle X-ray scattering.