Spontaneous Formation of Core–Shell Microdroplets during Conventional Coacervate Phase Separation

We report the single-step formation and stability of core-shell droplets during poly(allylamine hydrochloride) (PAH)/ poly(acrylic acid) (PAA) coacervation. These double emulsion (DE) droplets comprise a polyelectrolyte-rich shell and a solvent-rich vacuole core without added surfactant. The DE droplets are stable to the micron scale and retain their core-shell structure even after coalescence. They coexist with classical single emulsion (SE) droplets, suggesting a kinetic mechanism of formation. We use high-throughput microscopy and machine learning to classify the dominant droplet morphology across various final polyelectrolyte ratios, salt concentrations, and mixing rates. DE droplet formation is enhanced at lower salt (NaCl) levels and near 1:1 charge stoichiometry, showing a preference for polycation excess. A slow rate of injection mixing rate is critical to preferential DE formation. Direct observations of coarsening phenomena indicate that the coacervate shell's viscoelasticity and high viscosity contribute to DE metastability. Overall, the scalable, simple mixing process used herein offers a novel mechanism to produce multiphase coacervate droplets and provides an orthogonal approach to existing routes which require either dropwise synthesis or thermodynamic tuning.