Effects of Molecular Weight, Charge Density, and Concentration of Cationic Cellulose on Coacervation Behavior in Model Shampoo Systems.

Coacervation is a liquid-liquid phase separation phenomenon driven by electrostatic interactions between oppositely charged polyelectrolytes or colloidal particles, and its behavior is known to depend strongly on polymer characteristics such as molecular weight and charge density. In cleansing formulations, coacervates are formed through interactions between cationic polymers and anionic/amphoteric surfactants and are considered to play important roles in ingredient deposition and sensory performance. However, studies that systematically and independently investigate the effects of polymer molecular weight, charge density, and concentration on coacervation behavior remain limited. In this study, model shampoo formulations containing cationized celluloses with different degrees of cationization and molecular weights with varying polymer concentration were prepared to evaluate the effects of polymer characteristics on coacervation behavior and the structure of the resulting coacervates. Turbidity, ζ-potential, rheological measurements, and polarized light microscopy were employed to characterize coacervate formation during dilution. The results showed that the critical relative concentration for coacervation was primarily governed by the degree of cationization of the polymer and not by molecular weight. In contrast, the mechanical properties of the resulting coacervates increased with increasing both degree of cationization and molecular weight. Furthermore, a structural transition of coacervate from a hexagonal liquid-crystalline phase to a lamellar liquid-crystalline phase was observed during dilution, accompanied by an increase in elasticity. The rheological and microscopic results collectively suggest that dilution induces the development of a more interconnected network structure within the coacervates. These findings indicate that the degree of cationization determines the conditions for coacervation, whereas both the degree of cationization and molecular weight play important roles in controlling the network structure and physical properties of the coacervates.