(523i) Interfacial Crosslinking of Polyelectrolyte Complex Coacervate Droplets in Non-Equilibrium Supernatant

Polyelectrolyte coacervation is the associative phase separation of polyelectrolytes into a condensed polymer-rich phase, called coacervate, and a counterion-rich phase, called supernatant. Coacervates emulsions form liquid-like droplets and find uses in diverse applications, including dye encapsulation, food and personal care products, biotechnology applications such as drug delivery and pharmaceutical engineering. Although coacervate droplets have been proposed as model artificial cells to perform various biochemical reactions and synthesis, it is difficult to achieve this at a practical level due to the destabilization of their emulsions caused by natural droplet coarsening. Various innovative techniques have been proposed to stabilize these droplets using molecular stabilizers such as lipids, liposomes, and block copolymers. Although successful in preserving droplet identity, these techniques may sometimes mask the actual interfacial properties of 'membraneless droplets' and may impose a physical and chemical barrier against diffusion through the interface.

In this work, we focused on understanding the effects of non-equilibrium solvents on the coacervate-supernatant interface in order to stabilize droplets against coalescence. We found that the coalescence of droplets depends on the ionic strength of non-equilibrium solvents. Our proposed technique inhibits interfacial coarsening by partially crosslinking the interfacial polyelectrolyte chains while leaving the bulk unperturbed. Results from various characterizations such as bulk rheology, fluorescence microscopy (and FRAP), and thermogravimetric analysis support our observations. Moreover, we found that the interfacial cross-linking of coacervate droplets leads to the elastic deformation of droplets under compressional sheer. This deformation of coacervate droplets is unprecedented and provides robust interfacial stability even under practical material processing conditions. We have also found that interfacial stabilization does not alter the interfacial diffusion properties of these membraneless compartments, and the technique is quite suitable for the practical use of polyelectrolyte coacervates in modern biotechnological applications.