Quasi-Complex Coacervates: Single-Phase Viscoelastic Analogues of Complex Coacervates

Large-scale production of complex coacervates (CC), formed by the association of two oppositely charged polyelectrolytes (PEs), is complicated due to the occurrence of phase separation. Using a moderately hydrophilic model system based on two strong polyelectrolytes doped with salt, we introduce quasi-complex coacervates (quasi-CC). These are single-phase salty solutions of fully doped (i.e. presumably non-interacting) oppositely charged PEs that specifically mimic the viscoelastic response of their phase-separated CC counterparts. To find quasi-CC, we focused on a less explored window of the phase diagram by changing PEs concentrations (W_PEs), crossing the binodal boundary into the one-phase region at a fixed added salt concentration. We found that the dynamics of the polymer-rich materials first increases before decreasing at W_PEs beyond the binodal boundary. By applying a novel time-PEs superposition, we found two distinct power-law relationships between the horizontal shift factors (a_P) and W_PEs inside and outside the binodal region. These relationships allow finding the binodal limit of the phase diagram and identifying pairs of CC and quasi-CC over a wide range of W_PEs. This study shows the feasibility of mimicking the viscoelastic response of phase-separated CC without phase separation and paves the way for large-scale production of PEs-based materials for broader applications.