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

Area of Science:

• Polymer science
• Materials science
• Biophysics

Background:

• Coacervate droplets are liquid-liquid phase-separated structures with potential applications in protocell research.
• Existing methods for creating multiphase coacervate droplets often involve complex synthesis or thermodynamic control.

Purpose of the Study:

• To report a novel, single-step method for forming stable, core-shell coacervate droplets.
• To investigate the formation mechanism and stability of these droplets using high-throughput microscopy and machine learning.
• To identify conditions favoring the formation of double emulsion (DE) coacervate droplets over single emulsion (SE) droplets.

Main Methods:

• Utilized the poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) polyelectrolyte system.
• Employed a scalable, simple mixing process with varying compositions (polyelectrolyte ratios, salt concentrations) and processing routes (mixing rate, thermodynamic path).
• Applied high-throughput microscopy and machine learning for droplet morphology classification and analysis.

Main Results:

• Achieved single-step formation of protocell-like, core-shell coacervate droplets with a polyelectrolyte-rich shell and solvent-rich core.
• Observed coexistence of DE and single emulsion (SE) droplets, indicating a kinetic formation mechanism.
• Found that DE droplets preferentially form over SE droplets at a wide range of compositions with a slow injection mixing rate, lower salt concentrations, and near 1:1 charge stoichiometry (favoring polycation excess).
• Demonstrated stability of DE droplets to the micron scale, even after coalescence, though they are metastable and stabilized by shell viscoelasticity and viscosity.

Conclusions:

• The developed scalable, simple mixing process provides a novel and orthogonal mechanism for producing multiphase coacervate droplets.
• This method bypasses the need for dropwise synthesis or thermodynamic tuning required by existing routes.
• The findings contribute to the understanding and scalable production of complex coacervate structures for potential applications.