Tunable phase separation and interfacial behaviour of mixed surfactant micelle-polymer complex coacervates

Complex coacervation offers a versatile platform for designing stimuli-responsive materials. Among others, surfactant micelle-polymer complex coacervates provide the added advantage of encapsulating and delivering hydrophobic substances via micellar structures. In this study, we investigate the phase behaviour and interfacial properties of a model system composed of mixed surfactant micelles and a cationic polymer. We demonstrate that micelle composition, particularly the fraction of ionic surfactant, governs surface charge and conformation of micelles, which in turn drive phase transitions from soluble complexes to coacervates and ultimately to precipitates. Under iso-ionic dilution, we observe dilution-induced complex coacervation (DICC), with the coacervation window tunable by micelle charge and polymer-to-surfactant stoichiometry. Contact angle measurements reveal that the resulting coacervates spread efficiently on both hydrophilic and hydrophobic surfaces. Notably, they exhibit superior spreading on hydrophobic substrates with multiscale nano-micro structures compared to micelle solutions alone. This enhanced wetting behaviour is likely driven by the network structure formed by micelle-polymer complexes within the coacervates, which facilitates cooperative surfactant adsorption and promotes the transition from the Cassie-Baxter to the Wenzel wetting regime. These findings underscore the potential of micelle-polymer complex coacervates as tunable, stimuli-responsive materials for advanced surface coating applications.