Theory and quantitative assessment of pH-responsive polyzwitterion–polyelectrolyte complexation

Abstract

We introduce a theoretical framework to describe the pH-sensitive phase behavior of polyzwitterion–polyelectrolyte complex coacervates that reasonably captures the phenomenon from recent experimental observations. The polyzwitterion is described by a combinatorial sequence of the four states in which each zwitterionic monomer can occupy: dipolar, quasi-cationic, quasi-anionic, and fully neutralized. We explore the effects of various modifiable chemical and physical properties of the polymers—such as, pK_a of the pH-active charged group on the zwitterion, equilibrium constant of salt condensation on the permanently charged group on the zwitterion, degrees of polymerization, hydrophobicity (via the Flory–Huggins interaction parameter), and dipole lengths—on the window of complexation across many stoichiometric mixing ratios of polyzwitterion and polyelectrolyte. The properties that determine the net charge of the polyzwitterion have the strongest effect on the pH range in which polyzwitterion–polyelectrolyte complexation occurs. We finish with general guidance for those interested in molecular design of polyzwitterion–polyelectrolyte complex coacervates and opportunities for future investigation.